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i

NASA/CP—2012–217466

Materials Research in Microgravity 2012

R. Hyers, Editor

University of Massachusetts, Amherst, Massachusetts

V. Bojarevis, Editor

University of Greenwich, London, United Kingdom

J. Downey, Editor

Marshall Space Flight Center, Huntsville, Alabama

H. Henein, Editor

University of Alberta, Edmonton, Alberta, Canada

D. Matson, Editor

Tufts University, Medford, Massachusetts

A. Seidel, Editor

EADS Astrium, Friedrichshafen, Germany

D. Voss, Editor

ESA-ESTEC, Noordwijk, The Netherlands

M. SanSoucie, Compiler

Marshall Space Flight Center, Huntsville, Alabama

September 2012

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812

Proceedings of a symposium held at the 

141st TMS 2012 Annual Meeting and Exhibition, 

Orlando, Florida, March 11–15, 2012
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076–1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>
iii

TABLE OF CONTENTS

1. MATERIALS RESEARCH IN MICROGRAVITY: SESSION I ....................................... 1

1.1 Materials Science Experiments Under Microgravity—A Review of History, 

 Facilities, and Future Opportunities

 C. Stenzel .................................................................................................................. 1

2. MATERIALS RESEARCH IN MICROGRAVITY: SESSION II ..................................... 12

2.1 Studies of Thermophysical Properties of Metals and Semiconductors by

 Containerless Processing Under Microgravity

	 	 	 A. Seidel • W. Soellner • C. Stenzel ........................................................................... 12

2.2 Advanced Measurement Devices for the Microgravity Electromagnetic Levitation

 Facility EML

	 	 	 J. Brillo • H. Fritze • G. Lohöfer • M. Schulz • C. Stenzel ......................................... 21

2.3 Electrostatic Levitation Furnace for the ISS

	 	 	 K. Murakami • N. Koshikawa • K. Shibasaki • T. Ishikawa • J. Okada •

	 	 	 T. Takada • T. Arai • N. Fujino • Y. Yamaura ........................................................... 29

3. MATERIALS RESEARCH IN MICROGRAVITY: SESSION III .................................... 33

3.1 Liquid Structures and Physical Properties—Ground Based Studies for

 ISS Experiments

	 	 	 K. Kelton • J. Bendert • N. Mauro ............................................................................ 33

3.2 Thermophysical Properties Measurement of High-Temperature Liquids Under 

 Microgravity Conditions in Controlled Atmospheric Conditions

	 	 	 M. Watanabe • S. Ozawa • A. Mizuno • T. Hibiya • H. Kawauchi •

	 	 	 K. Murai • S. Takahashi ........................................................................................... 41

3.3 Microstructure Formations in the Two-Phase Region of the Binary Peritectic 

 Organic System TRIS-NPG

	 	 	 J. Mogeritsch • A. Ludwig ......................................................................................... 48

4. MATERIALS RESEARCH IN MICROGRAVITY: SESSION IV .................................... 56

4.1 ISS—Experiments of Columnar-to-Equiaxed Transition in 

 Solidification Processing

	 	 	 L. Sturz • G. Zimmerman • C. Gandin • B. Billia • N. Mangelinck •

	 	 	 H. Nguyen-Thi • D. Browne • W. Mirihanage • D. Voss • C. Beckermann •

 A. Karma ................................................................................................................. 56
iv

TABLE OF CONTENTS (Continued)

4.2 Liquid Droplet Dynamics in Gravity Compensating High Magnetic Field

	 	 	 V. Bojarevics • S. Easter • K. Pericleous .................................................................... 63

4.3 Measurements of Dendritic Growth Velocities in Undercooled Melts of Pure 

 Nickel Under Static Magnetic Fields

	 	 	 J. Gao • Z. Zhang • Y. Zhang .................................................................................... 72

5. MATERIALS RESEARCH IN MICROGRAVITY: SESSION V ..................................... 80

5.1 X-Ray Radiographic Observation of Directional Solidification Under 

 Microgravity: XRMON-GF Experiments on MASER12 Sounding Rocket Mission

	 	 	 G. Reinhart • H. Nguyen-Thi • A. Bogno • B. Billia • Y. Houltz • K. Löth •

	 	 	 D. Voss • A. Verga • F. de Pascale • R. Mathiesen • G. Zimmermann......................... 80

5.2 Innovative Video Diagnostic Equipment for Material Science 

	 	 	 G. Capuano • D. Titomanlio • W. Soellner • A. Seidel ............................................... 88

5.3 Containerless Measurements of Density and Viscosity of Fe-Co Alloys

	 	 	 J. Lee • P. Choufani • R. Bradshaw • R. Hyers • D. Matson ...................................... 97

5.4 TEMHD Effects on Solidification Under Microgravity Conditions

	 	 	 A. Kao • K. Pericleous .............................................................................................. 105

6. MATERIALS RESEARCH IN MICROGRAVITY: SESSION VI .................................... 113

6.1 Multi-Scale Modeling of Liquid Phase Sintering Affected by Gravity: 

 Preliminary Analysis

	 	 	 E. Olevsky • R. German ............................................................................................ 113

6.2 Self-Assembly of Ni Nanoparticles in Aerosols Produced Thermally 

 on Ground and Under Microgravity Conditions 

	 	 	 S. Lösch • D. Nolle • E. Goering • B. Günther .......................................................... 120

6.3 Crystallographic Stability of Metastable Phase Formed by Containerless 

 Processing in REFeO3 (RE: Rare-Earth Element)

	 	 	 K. Kuribayashi • M. Kumar ...................................................................................... 128

6.4 On-Line, Real-Time Diagnostics of a Single Fluid Atomization System

	 	 	 P. Khatibi • A. Ilbagi • H. Henein ............................................................................. 136

6.5 Electrodeposition of Metals in Microgravity Conditions

	 	 	 K. Nishikawa • T. Homma • E. Chassaing • M. Rosso • Y. Fukunaka ...................... 144

6.6 Propagation Regime of Iron Dust Flames

	 	 	 F. Tang • S. Goroshin • A. Higgins ............................................................................ 157

7. MATERIALS RESEARCH IN MICROGRAVITY: POSTER SESSION ......................... 163

7.1 Containerless Processing on ISS: Ground Support Program for EML

	 	 	 A. Diefenbach • S. Schneider • R. Willnecker ........................................................... 163
v

TABLE OF CONTENTS (Continued)

7.2 High-Precision Temperature Control of a Crystal Growth Furnace at 1,500 °C

	 	 	 C. Stenzel • A. Hess • A. Cröll • D. Bräuer • H. Sauermann ..................................... 170

7.3 Investigation of Thermocapillary Convection of High Prandtl Number Fluid 

 Under Microgravity

	 	 	 R. Liang • G. Duan ................................................................................................... 177

7.4 Real Time In Situ Observations of Equiaxed Dendrite Coherency in Al-Cu Alloys 

 Using High-Brilliance, 3rd Generation Synchrotron Sources

	 	 	 A. Murphy • D. Browne • W. Mirihanage • R. Mathiesen ......................................... 185

7.5 Truncated Dual-Cap Nucleation Site Development

	 	 	 D. Matson • P. Sander .............................................................................................. 193

MATERIALS SCIENCE EXPERIMENTS UNDER MICROGRAVITY - A 

REVIEW OF HISTORY, FACILITIES, AND FUTURE OPPORTUNITIES 

Ch. Stenzel1, 1

Astrium GmbH, D-88090 Immenstaad 

Keywords: Diffusion, solidification, crystal growth, levitation 

Abstract 

Materials science experiments have been a key issue already since the early days of research 

under microgravity conditions. A microgravity environment facilitates processing of metallic and 

semiconductor melts without buoyancy driven convection and sedimentation. 

Hence, crystal growth of semiconductors, solidification of metallic alloys, and the measurement 

of thermo-physical parameters are the major applications in the field of materials science making 

use of these dedicated conditions in space. In the last three decades a large number of successful 

experiments have been performed, mainly in international collaborations. In parallel, the 

development of high-performance research facilities and the technological upgrade of diagnostic 

and stimuli elements have also contributed to providing optimum conditions to perform such 

experiments. A review of the history of materials science experiments in space focussing on the 

development of research facilities is given. Furthermore, current opportunities to perform such 

experiments onboard ISS are described and potential future options are outlined. 
Introduction 

A microgravity environment facilitates processing of fluids without buoyancy driven convection 

and sedimentation. Ideally, the absence of gravity-driven buoyancy convection features a purely 

diffusive transport; the absence of sedimentation leads to homogeneous particle distribution in 

mixtures. For materials science these effects are mainly relevant for experiments aiming at 

crystallisation of bulk materials from the fluid state (melt or vapour phase) and on the 

determination of thermo-physical parameters in the under-cooled regime. 

Materials science experiments have been therefore a prominent topic, already since the early days 

of research under microgravity (μg) conditions. The first experiments under reduced gravity were 

reported during the fly-back phase of the Apollo 14 moon mission in 1971 [1]. A couple of 

demonstration experiments have been performed by the astronauts A. Shephard, S. Roosa, and E. 

Mitchell to experimentally assess the advantage of materials processing in space. Composite 

casting has been selected as one reference experiment for materials science. 11 different samples 

were melted and re-solidified in order to permit evaluation of the particle re-distribution. For 

example, it could be shown that a mixture out of paraffin, Be-Cu particles and Ar gas displayed a 

somewhat homogeneous distribution after re-solidification under μg conditions. Fig. 1 shows a 

photograph of the mixture after cool-down and the simple setup to perform this experiment. 

Figure 1. Composite casting experiment on Apollo 14. Left part: Re-solidified mixture out 

of paraffin, Be-Cu particles, and Ar gas bubbles; right part: Experimental 

apparatus 

These experiments represented a pioneering work in terms of a first proof-of-principle and hence 

opened the door for a long series on investigations under space conditions on various carriers and 

missions, starting with Skylab, the Apollo/Soyuz mission, Spacelab, other shuttle experiments, 

sounding rockets, free flyers, and finally on space stations like Mir or ISS. 

2
In this paper a review on the objectives, performance and results of high-lighted experiments in 

the main fields for materials science research is presented 

- Crystal growth 

- Solidification 

- Diffusion processes 

- Measurement of thermo-physical properties. 

Furthermore key technology improvements which have lead to innovative scientific research are 

reported and finally an overview on currently available facilities, opportunities, and recognizable 

trends is given. 

Materials Science Experiments 

During the last three decades the general objectives that materials science experiment in space 

have thus been focussed 

- To understand fundamental mechanism and processes 

- To validate numerical models 

- To provide benchmark data for industrially important materials 

- To measure accurately thermo-physical properties which can not easily reached on earth. 

In the following sections the different areas of materials science are reviewed and the results of 

high-lighted experiments are described. 

Crystal growth

Crystal growth of single-crystalline materials from the liquid and gas phase as well is performed 

very close to the thermodynamic equilibrium, i. e., with very low growth rates and having a flat 

or only smoothly curved solid/liquid interface. Hence, these processes are rather sensitive to 

small perturbations of all kinds, maybe caused by short-term fluctuations in the temperature field 

of the furnace, variations in the ganging of the mechanical drive of the heater or sample, or by 

different types of time-dependent convective flows in the melt [2]. The first two issues must be 

addressed by the design and performance of the facility, processing under μg conditions could 

help to reduce the latter one. 

In the early days the main target for crystal growth experiments in space - especially for 

semiconductors - was the improvement of material quality caused by the absence of buoyancy 

effects. The first experimental results however, were quite disillusioning: A. Eyer et al. 

discovered during the float-zone growth of Si that Marangoni convection is the dominant factor 

in the melt flow, more than outbalancing the absence of buoyancy-driven convective motions [3]. 

In a comprehensive survey K. W. Benz has described the consequences drawn by the community 

from Eyer's early experiment and the resulting metamorphosis of objectives for semiconductor 

crystal growth in space [4]. The most striking results for performing crystal growth experiments 

under reduced gravity conditions which have been obtained so far are summarised below: 

- To attain larger diameter for float-zone crystals [5]

- To measure Marangoni numbers without overlaying buoyancy flows [6, 7] 

- To study the influence of soluto-capillary flow [8, 9] 

3
- To study the influence of fluid flow stimuli like magnetic fields or mechanical vibrations 

[10, 11, 12] 

- To allow for numerical calculations under well-defined conditions [13] 

- To study detached solidification [14] 

As example the float zone growth of Ga-Sb [5] which yielded a significant reduction of striations 

in the space grown crystal compared to its earth-grown counterpart, is illustrated in Fig. 2. 

These investigations were also fostered by a steady improvement in the technical capabilities of 

the respective facilities. For example, mirror furnaces were the working horses in the early days 

on S/L, sounding rocket, and EURECA missions. They were robust, easy to operate, and allow 

for a direct optical control of the solid/liquid [15]. But the thermal profile was fixed and the 

temperature gradient at the S/L IF could not be adjusted. Nowadays multi-zone furnaces built up 

by several individual heaters with a high-stability temperature control system are available 

onboard ISS. With these furnaces the thermal gradient at the phase boundary can be adjusted 

over a wide range, and perform various types of experiments can be performed [16, 17]. 

Furthermore, in order to suppress stochastic flows which are caused by residual accelerations, the 

implementation of magnet systems inducing a well-defined flow in the melt into these new 

facilities has been pursued in the last decade. First experiments performed on sounding rocket 

missions revealed indeed that the melt flow can be substantially affected by a rotating magnetic 

field [18] in microgravity, thus providing ideal reference cases for testing and improving 

numerical models on the melt flow. 

Figure 2. Crystal growth of GaSb on Spacehab-4 mission. Left part: Size comparison 

between earth grown and μg crystals. The right part illustrates the significantly 

reduced micro-segregation under μg (less striations in upper photograph) 

Actually, two major experiments from field of the crystal growth are envisaged for the ISS: 

RDGS and SISSY. The first one is targeted to determine the effects of “detached” Bridgman 

growth in comparison to regular Bridgman growth and float-zone (FZ) growth on the defect 

formation and distribution in Ge-Si crystals [19]. The international cooperation SISSI aims at 

4
investigating the gravitational effects on heat and mass transport phenomena in directional 

solidification of upgraded metallurgical silicon for photovoltaic applications [20]. 

Solidification

One key aspect of the solidification that influences the microstructure is the shape of the solidliquid interface in a solidifying material. At low growth rates the interface is planar like for semiconductors. Industrially relevant metallic materials crystallise at much higher growth rates. As 

the rate of growth increases, the interface develops a corrugated texture until three dimensional 

cells form in the solid. Even higher growth rates cause the formation of dendrites. The 

development of these different interface shapes and the transition from one shape to another is 

controlled by the morphological stability of the interface. Gravity represents one of the most 

influencing factors to this stability. In particular, buoyancy-driven convection can influence the 

stability and, thus, the shape of the solidifying interface. Hence, processing under reduced gravity 

imposes therefore well-defined conditions for crystallisation. Thus, the results obtained so far 

from microgravity experiments about the conditions under which certain types of solidification 

boundaries appear could help to explain the formation of the crystalline structure of a material, to 

validate numerical models, and to improve terrestrial applications [21]. 

In the recent decades the subsequently listed most striking results have been obtained in 

microgravity experiments: 

- Study of morphological stability in immiscible systems [22] 

- Processing of eutectics [23] 

- Melt interface stability studied by Seebeck coefficient variation [24] 

- Dispersed systems [25] 

In addition to the mentioned issues dendritic growth is the common mode of solidification 

applied for metals and alloys under low thermal gradients. The growth of dendrites in pure melts 

depends on the transport of latent heat from the moving crystal–melt interface and the influence 

of weaker effects like the interfacial energy. Experimental data for critical tests of dendritic 

growth theories remained limited because dendritic growth can be complicated by convection. A 

fundamental understanding of dendritic solidification is necessary to correct mathematical 

models that will provide a basis for improved industrial production techniques. Prof. M. 

Glicksman and his research team have performed a series of pioneering and quite successful 

experiments on the mechanisms of dendrite formation by eliminating buoyancy-induced 

convection in the Isothermal Dendritic Growth Experiment (IDGE). This facility has been flown 

three times as part of the United States Microgravity Payload (USMP) series. During the first two 

flights of IDGE on the shuttle, the IDGE flight hardware grew and photographed individual 

dendrites of the material succino-nitrile (SCN) as they solidified at various temperatures (see Fig. 

3). The third flight of IDGE on USMP–4 has used a different sample material, pivalic acid 

Dendrite growth velocities could be measured and the researchers were able to compare the 

results with theoretical predictions and to answer questions on fundamental characteristics on 

dendrite growth [26, 27, 28]. 

5
Figure 3. Example of two image sequences of dendritic growth for SCN at two 

supercoolings: Left: 0.1 K (photographs taken every 700 s); right: 1 K (photos 

every 6.75 s) 

The modular experimental facility MSL (Materials Science Laboratory) [16] which is 

implemented in the US-module destiny of the ISS is successfully being operated since 2009. It 

enables in its current condition to perform investigations on 

 Columnar to Equiaxed Transition in Solidification Processing (CETSOL) [29] 

 Microstructure Formation in Casting of Technical Alloys under Diffusive and 

Magnetically Controlled Convective Conditions (MICAST) [30] 

 Solidification Along A Eutectic Path in Ternary Alloys (SETA) [31] 

In the near future it is considered to develop an MSL insert for in-situ diagnostic of dendritic 

growth by radiography [32]. 

6
Diffusion

The determination of diffusion constants in liquids allows for validating theoretical models on 

diffusive transport phenomena at one side. Furthermore, if a purely diffusion-controlled flow 

regime is established under μg conditions the exact knowledge of the diffusion constants is 

essential for modelling the fluid flow in the melt during crystal growth and solidification 

processes. 

Under terrestrial conditions the measurement of diffusion constants in the liquid state is always 

overlaid by gravity-driven convective flows. For this reason the measurement of diffusion 

constants was already one of the major topics for performing experiments under microgravity 

conditions in the early years. The first measurements of diffusion constants were performed 

during S/L missions or on the MIR station using the long-capillary (LC) technique [33-35]. For 

single-component systems the experiments indeed disclosed much lower values for the diffusion 

constants than obtained in earth-bound experiments [33]. This technique, however, leads for 

multi-component systems to segregation effects resulting in an additional mass transport which 

give rise to a systematic measurement error. This drawback can be overcome by a stable density 

layering of the sample materials. Thereby the density has to increase parallel to the gravity vector 

resulting in a density gradient corresponding to the concentration gradient. 

By the implementation of the shear-cell technique other drawbacks (free surface convection, 

diffusion during heat-up and cool-down) of the LC technique can be avoided: A rapid 

partitioning the sample material into several small volumes after processing but before cooldown by means of a dedicated mechanical mechanism leads to a quasi-frozen concentration 

distribution for off-line analysis. On the un-manned FOTON-12 mission pioneering 

measurements of diffusion constants employing the shear-cell technique up to a temperature of 

1200°C have been successfully carried out [36, 37]. The diffusion constants of several multicomponent systems have been successfully measured on the FOTON-M2 mission in 2005 [38]. 

A further innovation has moved in the field during the recent years, the in-situ determination of 

diffusion profiles via radiography. DLR has initiated the development of the DIXI heater insert 

for the MSL facility onboard ISS. This facility would allow performing in-situ diagnostics for 

high-temperature diffusion experiments as illustrated in Fig. 4 [32]. 

Figure 4. The concentration profiles of a Al-Cu alloys can be deduced from the grey values 

of an in-situ radiographic image (right) in the DIXI facility on ground [32] 

7
The measurement of diffusion constants of self-diffusion in elements like Si and of multicomponent systems where no stable density layering can be establish (Si-Ge) represents the 

actual trends for future experiments. Today, all the envisaged experiments on the ISS from 

European- of Japanese-coordinated research groups favour the combination of the shear-cell 

technique with in-situ radiography. 

Levitation techniques

Experiments on crystal growth and solidification are generally bound to a crucible containing the 

sample material. The crucible is integrated into a cartridge if a closed containment to the process 

environment has to be provided. This design affects the measurement of surface-sensitive 

properties like viscosity and surface tension and prevents to reach significant undercooling of the 

liquid sample as well. Furthermore, the chemical compatibility between the crucible and sample 

material at elevated temperatures imposes often severe problems leading in the worst case to a 

complete exclusion from processing for rather critical metals than Ti where no appropriate hightemperature resistant crucible materials is available. 

To overcome these obstacles levitation methods have been established into in materials science 

research facilities. Two main techniques have been successfully applied in the past: 

- Electrostatic levitation 

- Electromagnetic levitation 

In electrostatic levitators the sample is electrically charged, levitated in a static electric field, and 

heated by a laser. This enables to process preferably non-metallic elements and oxide materials. 

Both, JAXA and NASA are fostering the development and operation of an Electrostatic Levitator 

(ESL) on ground for a potential later implementation on the ISS [39, 40]. 

Electromagnetic levitation allows processing of electrically conductive samples including pure 

metals, alloys and semiconductors. The sample is thereby placed in the center of a coil system 

which is part of an oscillating RF circuit and generates an RF electromagnetic field. The 

interaction of eddy currents induced in the sample with the electromagnetic field leads to a 

positioning of the sample at the center of the coil system. Heating is achieved by ohmic losses of 

the eddy currents flowing in the sample [41, 42]. 

For ground-based experiment strong levitation fields are needed for both techniques in order to 

counteract gravity. This leads to a number of drawbacks like large power consumption, difficult 

control of sample positioning, convective flows in the melt, and a deformed geometrical sample 

shape. These drawbacks can be eliminated if levitation is performed under microgravity 

conditions where only small levitation forces are required to compensate for the residual 

accelerations. Moreover, convective flows are significantly reduced and the sample shape is 

nearly spherical. 

Electromagnetic levitation experiments in space have already a long-lasting successful history: 

The DLR facility TEMPUS (see Fig. 5) was operated on three Spacelab missions (IML-2 in 

1994, MSL-1 and MSL-1R in 1997) and provided a wealth of scientific data [41, 43]. 

8
Figure 5: TEMPUS facility (left). In the upper right part the levitated molten sample is 

shown, the lower right part displays the undercooling of a levitated Zr sample as 

measured during the MSL-1 (see [44]). 

One of the prominent scientific topics of these missions was the investigation on the kinetics of 

solid nucleation in Zr. Bayuzick and co-workers could perform 122 cycles of undercooling with a 

ΔT=333°C below the nominal solidification temperature and could study the influence of fluid 

flow regimes on the nucleation kinetics via variation of the cooling rate [44]. 

Compared to crystal growth or solidification experiments the duration of a levitation run is short 

and the sample can be re-processed several times enabling to perform systematic investigations 

even on one single mission. For the purpose of extending the experimental capabilities even 

further, currently the ISS version of this facility the Electromagnetic Levitator (EML) is being 

developed and will be operational in space from 2014 [42]. 

In addition to the performed Spacelab experiments a number of campaigns have been conducted 

on sounding rockets and on parabolic flights as well in the recent decades. This makes 

electromagnetic levitation the most prevalent technique for materials science investigations in 

space. 

9
Outlook 

Microgravity has to be regarded as tool which can be employed to provide dedicated 

experimental conditions like the application of external fields or forces does. In all relevant 

research areas of materials science it could be shown that a microgravity environment is 

advantageous for the investigation of specific subjects. Numerous distinguished experiments 

have been performed and were well received within the science community. The character of 

these results, however, can be portrayed in most cases as proof-of-principle or benchmarks. Due 

to the lack of continuous experiment opportunities so far and also due to the long duration of 

experiments mainly single-shot experiments have been conducted - experiments using 

electromagnetic levitation being an exception here because of the reasons mentioned above. 

The next coherent step in this context would be to convert the nature of material science research 

from primarily benchmarking to a more systematic research. Utilization of the ISS offers exactly 

this perspective and gives the unique possibility to initiate and perform intense research programs 

embedded in international frameworks especially if emphasizing the extended utilization period 

up to 2020. The facilities to perform the above described class of experiments are either already 

implemented (MSL, GHF, DECLIC), in preparation (EML), planned (ESL, TRANSPARENT 

ALLOYS), or addressed in several outstanding science proposals and consequently envisaged by 

the Agencies. The extended capabilities leave certainly much room for an extension of the 

science community resulting in a higher science output. 

References 

[1] R. J. Sharpe, M. D. Wright, "Analysis of Microgravity Experiments Conducted on the 

Appollo Spacecraft", NASA/TM-2009-215905 

[2] Müller, G., Convection and Inhomogeneities in Crystal Growth from the Melt, in: 

Crystal Grwoth, Properties and Applications, Vol. 3, III-V Semiconductors, SpringerVerlag, Berlin, Heidelberg, New York, 1980 

[3] A. Eyer, H. Leiste, R. Nitsche, Proceedings of the Fifth European Symposium on 

Materials Science and Microgravity, ESA-SP222, Schloß Elmnau, 1984, p. 173 

[4] K. W. Benz, P. Dold, J. Cryst. Growth 237-239, 2002, p. 1638-1645 

[5] A. Cröll, T. Kaiser et al., J. Cryst. Growth 191, 1998, p. 365 

[6] A. Cröll, W. Müller-Sebert et al., Microgravity Sci. Technol. III/4, 1991, p. 204 

[7] G. Müller, R. Rupp, Cryst. Properties Prep., 35, 1991, p.138 

[8] P. Tison, D. Camel et al., Proceedings of the First International Symposium on 

Hydromechanics and Heat/Mass Transfer in Microgravity, Perm/Moscow, 1991. p. 121 

[9] K. Arafune, A. Hirata, J. Cryst . Growth 197, 1999, p. 811 

[10] F. M. Herrmann, J. Baumgartl et al., Proceedings of the VIIIth European Symposium on 

Materials and Fluid Sciences in Microgravity, ESA SP-333, Brussels, 1992, p. 57 

[11] F. R. Szofran, K. W. Benz et al., NASA Microgravity Materials Science Conference, 

NASA/CP-1999-209092, Huntsville, AL, 1998, p. 617 

[12] A. Cröll, F. R. Szofran et al., J. Cryst. Growth 183, 1998, p. 554 

[13] T. Kaiser, A. Cröll et al., Proceedings of the Second European Symposium: Fluids in 

Space, Napoli, 1996, p. 245 

10
[14] L. L. Regel, W. R. Wilcox , 1998, Microgravity Sci. Technol. 14, 1999, p.152 

[15] H. Lenski, Microgravity Quarterly 1, 1990, p. 47 

[16] H. Lenski et al., Proceedings of the 4th International Symposium on Physical Sciences 

in Space, Bad Godesberg, 2011 

[17] A. Hess, A. Cröll et al., Floating Zone Furnace with Rotating Magnetic Field. Poster 

Presentation DGKK, 2010 

[18] P. Dold, A. Cröll et al., J. Cryst. Growth 231, 2001, p. 95 

[19] M. P. Volz, A. Cröll, RDGS, http://issresearchproject.nasa.gov/MSFC/RDGS/index.php 

[20] SISSI project, AO-2009/ILSRA-09 Results and Selection, ESA/PBHME/EUB(2010)28, May 2010 

[21] B. K. Dhindaw, Sadhana, Vol. 26, Parts 1&2, 2001, p. 59 

[22] J. B. Andrews, A. C. Sandlin et al., Advances in Space research 11, 1991, p. 291 

[23] D. J. Larson, NASA Conference Publication 3342, 1996, p. 337 

[24] G. Cambon, G. Cadet et al., Acta Astronautica 17, Issues 11-12, 1988, p. 1229 

[25] F. R. Juretzko, B. K. Dhindaw et al., Met. Trans. A29, 1997, p. 1691 

[26] M. E. Glicksman, M. B. Koss et al., Phys. Rev. Lett. 73, 1994, p.573 

[27] J. C. LaCombe, M. B. Koss et al., Phys. Rev. Lett. 83, 1999, p. 2997 

[28] M. B. Koss, J. C. LaCombe et al., Metallurgical and Materials Transactions A, 30A, 

1999, p. 3177 

[29] http://www.spaceflight.esa.int/users/materials/research/solid/cetsol/cetsol.html 

[30] http://www.spaceflight.esa.int/users/materials/research/solid/micast/micast.html 

[31] http://www.spaceflight.esa.int/users/materials/research/solid/seta/seta.html 

[32] F. Kargl, M. Balter et al., Proceedings of the 4th International Symposium on Physical 

Sciences in Space, Bad Godesberg, 2011 

[33] Y. Malmejac, G. Frohberg, in: Fluid Science and Materials Science in Space, Springer, 

1987, p. 159-190 

[34] G. Frohberg, Diffusion in solids: Scientific results of the German Spacelab mission D-2, 

WPF/DLR, 1995, p. 275-287 

[35] R.W. Smith, Microgravity Sci. Technol. XI (2) 78-84 (1998) 

[36] T. Masaki, T. Fukuzawa et al., Meas. Sci. Technol. 16, 2005, p. 327-335 

[37] A. Griesche, K.-H. Kraatz et al., Proceed. First Intern. Symp. on Microgr. Res. and 

Appl. in Phys. Scienc., ESA-SP-454, 2000, p. 985 

[38] S. Suzuki, K.-H. Kraatz, G. Frohberg, Micrograv. Sci. and Technol. 16-1, 2005, p. 120-

126 and 18(3/4), 2006, p. 82 

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[40] T. Ishikawa, J. T. Okada et al., ISSN 1349-1113, JAXA-RR-08-003E2009 ESL-JAXA 

[41] R. Knauf R et. al., Containerless Processing Facility TEMPUS for Materials Science 

Research onboard Spacelab: 6th International Symposium on Experimental Methods for 

Microgravity Material Science (San Francisco), 1994 

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[43] Team TEMPUS, in: Materials and Fluids under low gravity, L. Ratke H. Walter, B. 

Feuerbacher, Springer-Verlag, Berlin, Heidelberg, New York, 1996 

[44] W. Hofmeister, J. Rogers, N. Singh, S. Marsh, P. Vorhees, eds., Solidification 1999, 

1999, Warrendale, TMS 

11
Studies of Thermophysical Properties of Metals and Semiconductors by 

Containerless Processing under Microgravity

A. Seidel, W. Soellner, C. Stenzel 

Astrium Space Transportation 

Claude Dornier Strasse, Immenstaad, 88090, GERMANY 

Keywords: Electromagnetic Levitation, Microgravity, Materials Science, Thermophysical 

Properties 

Abstract 

Electromagnetic levitation under microgravity provides unique opportunities for the 

investigation of liquid metals, alloys and semiconductors, both above and below their melting 

temperatures, with minimized disturbances of the sample under investigation. The opportunity to 

perform such experiments will soon be available on the ISS with the EML payload which is 

currently being integrated. With its high-performance diagnostics systems EML allows to 

measure various physical properties such as heat capacity, enthalpy of fusion, viscosity, surface 

tension, thermal expansion coefficient, and electrical conductivity. In studies of nucleation and 

solidification phenomena the nucleation kinetics, phase selection, and solidification velocity can 

be determined. Advanced measurement capabilities currently being studied include the 

measurement and control of the residual oxygen content of the process atmosphere and a 

complementary inductive technique to measure thermophysical properties. 

Introduction 

Electromagnetic levitation provides unique opportunities for the investigation of electrically 

conductive liquid metals, alloys and semiconductors (if pre-heated or doped), both above and 

below their melting temperatures. In particular, the undercooled regime where the sample is 

liquid at temperatures below its equilibrium melting temperature is accessible through 

electromagnetic levitation thus allowing the determination of thermo-physical properties of 

materials in this particular state as well as studying of nucleation and solidification phenomena. 

Thermo-physical properties that can be measured comprise heat capacity, enthalpy of fusion, 

thermal transport coefficients in the liquid phase, viscosity, surface tension, thermal expansion 

coefficient, hemispherical emissivity, fraction solid and electrical conductivity. In studies of 

nucleation and solidification phenomena the nucleation kinetics and solidification velocity can 

be determined. [1], [2] 

Ingredients 

In order to prepare the undercooled state and to perform experiments aiming at the determination 

of the thermo-physical properties without adverse impacts related to chemical reactions, 

heterogeneous nucleation, or lack of visibility, the following ingredients are essential: 

2.1. Electromagnetic levitation 

Electromagnetic levitation allows processing of the samples without contact to a container. A 

wide range of electrically conductive samples including pure metals, alloys and semiconductors 

can be processed with this technique. The sample is placed in the center of a coil system which is 

part of an oscillating RF circuit and generates an RF electromagnetic field. The interaction of 

eddy currents induced in the sample with the electromagnetic field leads to a displacement force 

which keeps the sample at the center of the coil system. Heating is achieved by ohmic losses of 

the eddy currents flowing in the sample. In ground-based experiments the required levitation 

force and therefore electromagnetic field strength to counteract gravity is so large that many 

materials are melted just by applying the field to position them. Heating and positioning of the 

12
sample are therefore not independent and the undercooled regime of samples with low melting 

temperatures is not accessible. Furthermore, the electromagnetic pressure exerted by the strong 

fields leads to strong convection and a deformation of the liquid samples which are not 

compatible with many experiment objectives such as the determination of viscosity and surface 

tension. The need for lower electromagnetic field strengths thus becomes evident which means 

that gravity has to be largely eliminated in order to perform experiments with undisturbed 

samples over a wide temperature range [4-8]. 

g

μg

P P H H P P

Figure 1. Principle design of levitation coils for levitation under 1g (left) and μg (right). In a 1g 

levitation coil the positioning force has to counteract gravity, therefore strong electromagnetic fields 

are needed just for the positioning of the sample, which leads to a strong heating and deformation of 

the sample (left). In contrast, the forces required to position a sample in a μg environment are 

orders of magnitude smaller, therefore the heating and deformation of the sample due the 

positioning field are negligible. As a consequence, heating and positioning of the sample can be 

controlled largely independently, e.g. by using dedicated coils for heating and positioning, 

respectively (right). Note that this illustrative sketch does not show the current EML flight coil 

geometry.

2.2. Ultraclean process environment: 

In order to reduce the potential for heterogeneous nucleation as much as possible an ultra-high 

vacuum or ultra-pure noble gas process atmosphere is necessary. In particular, the amounts of 

reactive trace gases, like for example oxygen, need to be minimized. 

2.3. Contactless diagnostics 

The diagnostic instruments have to be compatible with the containerless processing principle as 

well, such as pyrometers for measurement of the sample temperature and various dedicated video 

cameras for specific purposes, e.g. cameras with high spatial or temporal resolution. 

2.4. Sample handling 

Even though the samples are processed without contact to a container wall they need to be 

carefully prepared and handled during the experiment. Typical experimental set-ups provide the 

possibility to store several samples, to select a dedicated sample for processing and to transfer 

the selected sample into the coil. Therefore, the samples are placed in so-called sample holders 

made of chemically inert thermal shock resistant material which are connected to a transfer 

mechanism and provide enough free space for the sample such that the containerless principle is 

respected during processing. 

3. Undercooling at Work 

Driven by the size of the RF coils which are optimized for positioning and heating efficiency 

under microgravity the samples are spherical with diameters typically ranging from 5 to 8 mm. 

The process environment for the samples is either UHV or ultra pure noble gas (He or Ar) up to 

400 mbar. 

13
A typical experiment consists of several melt cycles. Starting from the solid state the sample is 

heated up above the melting temperature (overheating of up to 300 K), and then the heater power 

into the sample is reduced. During the subsequent cooling phase, the majority of the 

experimental data are gathered. Depending on its properties the sample undercools to 

temperatures below the melting temperature until solidification sets in rapidly and the heat of 

fusion is released in a very short time such that the sample temperature rises again to the melting 

temperature, a phenomenon known as recalescence. During the following cool down of the solid 

sample the cooling rate can be enhanced by additional convective cooling with the process gas 

being circulated through a pipe and filter system and released through a nozzle at the sample's 

vicinity. A typical melt cycle is shown in figure 2. 

Due to the small size and related low heat content of the sample the typical duration of a melt 

cycle is in the order of seconds to minutes, depending on the sample material and the definition 

of the process, such that many such melt cycles can be performed during a the time frame 

available for an on-orbit experiment which is in the order of hours. 

At high temperatures samples processed in ultra-high vacuum may evaporate some material 

which leads to a deposition of evaporated material on the inner surfaces of the experiment set-up 

in a physical vapor deposition process. Components and assemblies in the vacuum chamber that 

are sensitive to deposition of evaporated sample material like the components of the optical path 

from the sample to the pyrometer and video cameras have to be protected from deposition by e.g. 

exchangeable double mirror systems. If samples are processed in a noble gas atmosphere the 

evaporated material does coagulate into microscopic dust particles which are toxic and need to 

be removed from the experiment setup by a dedicated gas cleaning system. 

After processing samples can be retrieved from the facility and returned to ground for further 

evaluation. 

time t

sample temperature T

T overheating 800°C - 2100°C

Tmelting 600°C - 2000°C

positioning processing positioning

Tundercooling 

undercooling 

0 - 500K

Melt Cycle 20 sec - 30 min

recalescence

Experiments conduction

overheating

melting

 

Class A: 

Undercooling 

and nucleation 

Class B: 

Modulation 

Calorimetry 

Class C: Surface 

oscillations 

Class D: Size 

Measurement 

Class E: 

Electrical 

Coupling 

Figure 2. Principle sketch of the temperature-time profile of a typical melt cycle in the EML 

facility. Experiments are typically performed on the cooling flank when the sample is in the 

liquid state. Experiments in 5 different classes can be performed with EML, these classes are 

schematically introduced on the right side of the figure, and explained in detail in the text below. 

4. EML on ISS: Experiment Classes and Measurement Capabilities 

Based on a long and successful heritage of electromagnetic levitation facilities on various 

carriers and missions, the electromagnetic levitation facility EML for the International Space 

Station ISS is currently being developed by Astrium under contracts to ESA and DLR. A 

comprehensive discussion of the performance and design features of EML is provided in [8]. 

14
4.1. Experiment Class A: Undercooling and nucleation 

The scientific objectives for this experiment are to: 

1. measure solidification front speed 

2. perform nucleation statistics (maximum undercooling) 

3. study morphology and phase selection depending on undercooling and induced fluid flow [9] 

 

An experiment of class A type is being executed as follows: The molten sample will be cooled 

by turning off the heater, whereas the cooling rate may be enhanced by dedicated convective 

cooling per adjustable gas flow. Recalescence may occur either spontaneously or it can be 

triggered by touching the undercooled sample with a custom-made trigger needle at a predefined 

undercooling temperature. The recalescence of the undercooled sample is detected automatically 

by an algorithm that detects the sudden increase of the pyrometer signal upon recalescence. A 

high-speed video camera is used to observe the sample from a direction perpendicular to the 

trigger needle such that the nucleation front passing from the nucleation point (trigger needle tip) 

across the surface of the sample can be observed. Once recalescence has been detected the highspeed video acquisition is stopped. Pre- and post-event storage of video data is facilitated by the 

use of a ring memory for the video data. See also the first pictogram in figure 2. 

Typical duration of one melt cycle is in the range of 30-60 seconds. The temperature is recorded 

with a frequency of 100 Hz; video images are taken with up to 30.000 frames/s @ 256x256 pixel 

spatial resolution. An example of high-speed video recording is shown in figure 3. 

The following equipment is needed for the execution of a class A experiment: 

• high-speed and high resolution camera to resolve solidification process, this includes high 

digitalization of the camera chip to support good thermal resolution of the phases solidifying 

• trigger needle to induce nucleation at desired temperatures of the undercooled liquid 

• precision pyrometer 

Figure 3. Class A Experiment: Sequence of a NiAl sample solidifying from the undercooled liquid 

captured by high-speed video camera @ 10.000 frames/s during parabolic flight. The total duration of 

the recalescence event was 29.1ms.

4.2. Experiment Class B: Modulation Calorimetry 

The scientific objectives for this experiment class are to measure thermo-physical properties and 

if applicable their temperature dependence [10] such as: 

1. heat capacity 

2. enthalpy of fusion 

3. total hemispherical emissivity 

4. thermal transport coefficients 

5. fraction solid 

15
The class B experiment is carried out as follows: At a constant base temperature of the molten 

sample the heater power is modulated sinusoidally and the sample's temperature response is 

evaluated. The modulation can be repeated at different sample temperatures with different 

modulation frequencies in order to obtain several data points within one melt cycle. The typical 

duration of a melt cycle ranges from 10 to 20 minutes. The temperature data will be acquired 

with 100 Hz; furthermore input data of RF coil system (voltage, current) are needed. 

The following equipment is mandatory to perform a class B experiment: 

• high precision pyrometer 

• calibrated modulated power input source (amplitude or power modulation is possible) 

4.3. Experiment Class C: Surface Oscillations 

The measurement of surface tension and viscosity and their temperature dependence are the main 

scientific objectives for these experiments. 

The experiment starts with heating up the sample slightly above its melting point. During 

subsequent cooling of the molten sample a short heater pulse is applied which excites surface 

oscillations due to the dipole nature of the heating field, and the radial force of the field 

squeezing the sample. The surface oscillations are imaged by two video cameras in orthogonal 

views, as shown in figure 4. From the oscillating frequency the surface tension can be derived 

and from the oscillations decay the viscosity can be derived since this is the only damping 

mechanism involved for these oscillations. The temperature as well as the stimulus (pulse 

amplitude & shape) can be altered to achieve various measurement points during one melt cycle. 

For increasing viscosity the amplitude can be increased to observe a high enough oscillation 

signal. Instead of an excitation pulse also a frequency sweep can be applied to find the resonance 

frequency of the sample's surface oscillations. The typical duration of a melt cycle is in the range 

of 5-15 min. 

The video images are usually taken with the radial camera @ 200 frames/s and 600x600 pixel 

spatial resolution and with axial camera @ 150 frames/s and 350x350 pixel spatial resolution. An 

example is shown in figure 4. The temperature is recorded with 100 Hz. 

To perform a class C experiment the following equipment must be provided: 

• fast and high resolution video cameras to resolve shape oscillations such that offline Fourier 

analysis on the sample diameter can be performed to derive the oscillation frequency and decay 

time 

• high precision pyrometer 

• free programmable power input source to apply either short pulses or a user defined signal 

shape as e.g. frequency sweep 

4.4. Experiment Class D: Size Measurement 

The scientific target for this class of experiments is to measure the thermal expansion coefficient 

and its temperature dependence. It is carried out as follows: The diameter of the solid and molten 

sample is precisely determined by the two video cameras in orthogonal view along the changing 

temperature of a melt cycle. For this purpose a high spatial resolution of the camera is necessary 

which will be even enhanced by a sub-pixel resolution algorithm which is applied to the video 

images off-line during evaluation. The algorithm requires the image of the sample's edge to be 

smeared out over a small number of camera pixels such that the algorithm can fit a 50% intensity 

value, as assumed sample edge, with a precision higher than the pixel resolution. The slightly 

blurred imaging of the sample's equator is achieved by setting the cameras focal plane slightly in 

front of it still achieving a sharp picture of the sample's surface. The typical duration of a melt 

cycle is in the range of 1-5 min. Video images captured by the axial camera are taken with 

typically 25 frames/s at up to 1000x1000 pixel resolution, whereas the radial camera offers 

16
Figure 4. Class C experiment: Induced surface oscillations of a CuSnP sample (TEMPUS, 600x600x 

pixels @ 200 fps)

600x600 pixel spatial resolution. The selected frame rate depends on the temperature gradient 

and the desired temperature resolution of individual measurement points. The temperature data 

are recorded with 100 Hz. 

The following equipment is needed for the execution of a class D experiment: 

• high resolution video cameras to resolve sample diameter 

• high precision pyrometer 

4.5. Experiment Class E: Electrical coupling 

The measurement of the electrical conductivity and its temperature dependence are the scientific 

targets for these investigations. 

A class E experiment is carried out as follows: The electrical data of the RF oscillating circuit are 

used to derive the electrical conductivity of the sample since the sample itself is changing 

slightly the characteristics of the RF oscillating circuit by its inductance, which is determined by 

its geometry and electrical conductivity. Some calibration measurements on the empty coil have 

to be performed to distinguish between the coil's inductance and the one introduced by the 

sample. The relevant housekeeping data are measured by the levitation power supply which uses 

them mainly for internal control of the high RF power output. The basic resolution and accuracy 

can be further enhanced by using dedicated measurement equipment for this task- the so-called 

SCE which is described later. 

The measurement can be performed together with other experiment classes since it requires only 

the high precision pyrometer as diagnostics, as all the other experiments too, and the RF 

oscillating circuit data which are part of the regular EML housekeeping data anyway. The 

acquisition frequency for the temperature data is 100 Hz, the RF oscillating circuit data (voltage, 

current, frequency) are recorded with 10 Hz. 

5. Experiment preparation 

Experiment preparation involves all aspects of transforming a scientific objective or idea into a 

fully developed flight experiment. Since EML provides a wide range of functions related to the 

definition of the sample processing including diagnostics, a deep understanding of the associated 

systems, their capabilities, functions, interrelations and constraints is needed in order to develop 

a successful flight experiment. Support to the scientists for the preparation of their experiments is 

given in the frame of the Ground Support Program GSP by a joint team consisting of Astrium 

and the DLR Microgravity User Support Center MUSC. Thereby the experience made in former 

TEMPUS missions is fully provided to the scientific community. A brief outline of the support 

tasks is given hereafter. The GSP is described in more detail in [12]. 

17
From the EML facility operation point of view the experiments are organized into melt cycles 

which in turn are decomposed into process steps. Each process step is characterized by a number 

of EML facility settings such as heater power, camera speed, etc. These settings are called 

Experiment Parameters or EP. In order to perform an experiment in the EML facility all EPs 

belonging to this experiment are needed since the EML facility is being operated using these 

parameters. 

One of the major tasks of the GSP is therefore to translate an experiment idea into an EP set, a 

task that involves multiple steps and activities (see also figure 5), such as 

• Definition of a rough outline of the experiment, in a so-called "science protocol" 

• Characterization of sample properties relevant for processing in EML in a ground 

levitator 

• Development of the Experiment Parameter (EP) set 

• Verification of the EP set in a representative EML facility on ground 

• Sample Integration and Launch 

Science 

Objective

Sample

Characterization

Experiment

Outline

Script

Simulator-assisted

Parameter 

Development

Verification

Experiments

1 128

2 9888

3 223

4 23,5

5 111

6 597

7 36985

8 23

9 75

Flight Sample 

Integration Launch

Figure 5. Preparation tasks for conducting experiments in EML onboard the ISS. 

5. Mission Scenario 

The EML facility will be launched and transported to the International Space Station ISS, where 

it will be installed in the European Drawer Rack (EDR). During its service lifetime of 5 years 

EML will be operated in the EDR. In order to be able to process a large number of scientific 

samples during the service lifetime, samples can be uploaded and downloaded between ground 

and EML on ISS according to the following scenario (see also figure 6): 

• The flight samples which include pure metals, alloys and semiconductors, are spherical in 

shape with diameters ranging from 5 to 8 millimeters and are manufactured by the participating 

scientists in their labs. 

• These samples are then integrated into dedicated sample holders and subsequently into a 

Sample Chamber (SCH) under very strictly controlled environmental conditions since the 

scientific results of the levitation experiments depend critically on the purity of the samples. 

Each Sample Chamber has a capacity of 18 samples which are stored in individual sample 

holders mounted on a magazine wheel in order to manipulate and select each sample individually 

for processing. 

• The fully loaded Sample Chamber is uploaded to the ISS where it is attached to the EML 

facility. 

• Sample processing is performed according to a sophisticated procedure which not only involves 

the settings of the levitator but also of diagnostics elements. The data delivered by the diagnostic 

18
elements, mainly high-speed and high-resolution video cameras and pyrometers are the main 

scientific raw data gathered during the mission. Since the duration of a typical melt cycle of an 

EML sample is in the order of seconds or minutes and the processing procedure is very complex 

with hundreds of individual settings, the procedure is developed before the mission in a ground 

based experiment development program, converted into a parameter set for the facility, and is 

then automatically executed by the facility. Interactive control of running experiments can be 

performed e.g. to optimize parameters based on the results of the previous runs by telecommanding from the user center on ground which is permanently monitoring and controlling 

the running experiments. 

• After all samples in a Sample Chamber have been processed they are transferred to a Sample 

Download Container (SDC) by the flight crew for download to ground. The design of SDC has 

been optimized for the tight constraints of the download in terms of mass and size. 

• After landing the processed flight samples are removed from the SDC and returned to the 

scientists for further analysis. 

5 - Sample Download

4 - Flight Experiments

up to 180 samples

in 10 batches

Payload Development Payload Launch

2 – Sample Integration

1 - Experiment Development

Sample Preparation

6 - Turnover to Scientist

7 - Ground Evaluation

18

3 – Sample Launch

Figure 6. EML Mission Scenario, showing the complete life-cycle of the experiments from 

experiment preparation to post-flight evaluation. 

According to the mission scenario up to two Sample Chambers per year will be uploaded to the 

EML facility, which translates to a total of 36 samples that can be uploaded, processed, and 

downloaded per year. Therefore, a maximum of 180 samples can be processed during the 5 year 

service lifetime of EML. 

6. Advanced experimental techniques 

The scientific outcome of the experiments as described herein may be significantly improved by 

implementing the following two upgrades of the EML facility [1], [9], [13], [14]: Oxygen 

Control System (OCS) & Sample Coupling Electronics (SCE). A description of these 

instruments and their capabilities is provided in [15]. 

19
Acknowledgement 

TEMPUS and EML are developed by Astrium GmbH under programs of and funded by the 

European Space Agency (ESA) and by the Space Agency of the German Aerospace Center 

(DLR e.V.) with funds of the German Federal Ministry of Economics and Technology. 

References 

[1] Soellner W, Seidel A, Stenzel C, Dreier W and Glaubitz B 2010 EML - Containerless 

Processing Facility for Material Science Research onboard The ISS J. Jpn. Soc. Microgravity 

Appl. 27 No. 4 183 

[2] Egry I and Voss D 2010 Present Activities fo the Investigators Working Group (IWG) for 

the Electromagnetic Levitator (EML) on ISS - A Status Report J. Jpn. Soc. Microgravity Appl. 

27 No. 4 176 

[3] Team TEMPUS 1995 In Materials and Fluids Under Low Gravity: Proc. IXth Symp. on 

Gravity-Dependent Phen. in Physical Sciences ed L Ratke et al. (Berlin: Springer) 

[4] Piller J et. al. 1986 Electromagnetic Positioning and Inductive Heating under micro-g: 6th 

European Symposium on Materials Sciences under Microgravity (Bordeaux) 

[5] Knauf R, Piller J, Seidel A and Dreier W 1992 TEMPUS - An Electromagnetic 

Levitation Facility for Containerless Processing under Micro-g in Spacelab: Proc VIIIth 

European Symposium on Materials and Fluid Sciences in Microgravity (Brussels, Belgium) ESA 

SP-333 

[6] Geiger R and Thomas J 1993 Resonant High Power Supply for Electromagnetic 

Levitation and Melting of Metallic Samples European Space Power Conference (Graz) 

[7] Knauf R et. al. 1994 Containerless Processing Facility TEMPUS for Materials Science 

Research onboard Spacelab: 6th International Symposium on Experimental Methods for 

Microgravity Material Science (San Francisco) 

[8] Seidel A, Soellner W and Stenzel C 2011 EML - An Electromagnetic Levitator for the 

International Space Station, Proc. 4th International Symposium on Physical Sciences in Space 

(Bonn) 

[9] Matson D, Hyers R and Volkmann T 2010 Peritectic Alloy Rapid Solidification with 

Electromagnetic Convection J. Jpn. Soc. Microgravity Appl. 27 No. 4 238 

[10] Fecht H J, Wunderlich R, Ricci E, Etay J, Egry I, Seetharaman S and Battezzati L 2010 

The Thermo-Lab Project: Thermophysical Property Measurement in Space for HighTemperature Alloys J. Jpn. Soc. Microgravity Appl. 27 No. 4 190 

[11] Study data package of ESTEC TRP study "Oxygen Sensing and Control" - ESTEC 

contract no. 19963/07/NL/PM 

[12] Diefenbach A, Schneider S, Willnecker R, Containerless Processing on ISS: Ground 

Support Program for EML, this proceedings 

[13] Ozawa S, Suzuki S, Hibiya T and Fukuyama H 2011 Influence of oxygen partial pressure 

on surface tension and its temperature coefficient of molten iron J. of Appl. Phys. 109 014902 

[14] Lohöfer G 2009 EML Sample Coupling Electronics Test Model Final Report EML/SCETM-Rep 1 (DLR, Cologne) 

[15] Brillo J, Fritze H, Lohöfer G, Schulz M, Stenzel C, Advanced Measurement Devices for the 

Microgravity Electromagnetic Levitation Facility EML, this proceedings 

20
ADVANCED MEASUREMENT DEVICES FOR THE MICROGRAVITY 

ELECTROMAGNETIC LEVITATION FACILITY EML 

Jürgen Brillo1, Holger Fritze2, Georg Lohöfer1, Michal Schulz2, Christian Stenzel3

1

Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 

51170 Köln, Germany 

2

Clausthal University of Technology, Institute of Energy Research and Physical Technologies, 

38640 Goslar, Germany 

3

Astrium – Space Transportation, Department: TO 5, 88039 Friedrichshafen, Germany 

Keywords: Electromagnetic levitation, micro gravity, inductive measurement technique, 

electrical resistivity, surface tension, liquid metals, oxygen sensing and control 

Abstract 

This paper reports on two advanced measurement devices for the microgravity electromagnetic 

levitation facility (EML), which is currently under construction for the use onboard the 

“International Space Station (ISS)”: the “Sample Coupling Electronics (SCE)” and the “Oxygen 

Sensing and Control Unit (OSC)”. 

The SCE measures by a contactless, inductive method the electrical resistivity and the diameter 

of a spherical levitated metallic droplet by evaluating the voltage and electrical current applied to 

the levitation coil. The necessity of the OSC comes from the insight that properties like surface 

tension or, eventually, viscosity cannot seriously be determined by the oscillating drop method in 

the EML facility without knowing the conditions of the surrounding atmosphere. In the 

following both measurement devices are explained and laboratory test results are presented. 

Introduction 

The study of liquid metals and alloys is often in company with major challenges and difficulties, 

which is due to the high temperatures and, consequently, high reactivity of these materials. An

elegant way to bypass these problems is the use of containerless methods. Among these, 

electromagnetic levitation is a well established and robust technique for electrically conducting 

melts [1]. By application of alternating magnetic fields of high frequency (~375 kHz), metallic 

samples are heated and positioned in electromagnetic levitation inductively without external 

contact by inductive means. 

In order to exploit this containerless environment for thermophysical property measurements of 

liquid metals, electromagnetic levitation must be combined with non-contact diagnostic tools [1]. 

Generally, these tools are based on optical techniques like pyrometry, which applies Planck's law 

of radiation for temperature measurement, and videometry, which uses high-speed cameras for 

an analysis of the static and dynamic shape of the levitated drop. 

However, in electromagnetic levitation under earthbound conditions, the sample droplets are not 

force free. The gravity deforms the droplet to a non-spherical shape and distorts the measurement 

of certain properties, like surface tension. Furthermore, the magnetic fields used to heat and 

levitate the sample will also generate turbulent fluid flows inside the melt, which strongly disturb 

measurements of e.g. growth velocities, or melt viscosities [2]. 

These problems are almost completely removed when electromagnetic levitation is performed in 

the essentially forceless microgravity environment. Here, the used positioning fields are strongly 

reduced and, consequently, also the side effects. This has led to the development of the 

21
microgravity electromagnetic levitation facility “EML”. It is currently under construction for the 

use onboard the “International Space Station (ISS)”, and will provide an ideal processing 

environment for undisturbed measurements of thermophysical properties of electrically 

conductive melts.

In this paper, we report on two proposed extensions of the EML: One is the “Sample Coupling 

Electronics (SCE)” and the other is the “Oxygen Sensing and Control Unit (OSC)”. Both 

measurement devices have been proposed for integration into the EML facility. 

The SCE measures by a contactless, inductive method the electrical resistivity and the diameter 

of a spherical levitated metallic droplet by evaluating the voltage and electrical current applied to 

the levitation coil. Any change in the electrical or volumetric properties of the sample can be 

measured this way. The SCE is therefore a comparatively simple but highly effective diagnostic 

tool which complements the existing video diagnostics and elegantly expands the spectrum of 

properties that can be measured. 

The necessity of the OSC comes from the insight that properties like surface tension or, 

eventually viscosity cannot seriously be determined without knowing the conditions of the 

surrounding atmosphere. Oxygen is one of the most important surface active elements for liquid 

metals. Only a few ppm can decrease the surface tension by more than 10%. Such a dramatic 

effect can also lead to a sign reversal of its temperature coefficient and, hence, to a reverse 

Marangoni flow. In order to study and control these effects, not only the measurement, but also 

the control of the oxygen partial pressure plays an important role. Both can be achieved by the 

OSC. 

Electromagnetic Levitation under Microgravity 

For the stable electromagnetic positioning of the sample, the EML facility uses a magnetic 

quadrupole field, generated by two parallel and coaxial “positioning coils” carrying alternating 

currents of the same strength but opposite directions. Since the alternating electromagnetic 

levitation fields repel electrical conducting samples from areas of high field strengths, the 

weightless metallic sphere is fixed in the center of the two coils, where the magnetic field 

strength is weakest, see Fig. 1. 

~~ ~~

positioning circuit heating circuit

150 kHz

power amplifier

400 kHz

power amplifier

Fig. 1: Principal sketch of the electrical oscillating circuits of a microgravity 

levitation facility including the positioning coil (black, near the equator of the 

sample) generating an alternating (~150 kHz) magnetic quadrupole field (curved 

arrows), and the heating coil (gray, above and below the sample) generating an 

alternating (~375 kHz), homogeneous magnetic dipole field (straight arrows). Each of 

the circuits is powered by its own current supply. The actual EML circuits, basing on 

the superposition principle, differ in details from those shown above. 

Since, moreover, the residual accelerations under microgravity, which have to be compensated 

by the positioning field, are relatively small, the remaining magnetic field strength around a 

droplet can be decreased under microgravity to values that are considerably weaker than those 

22
necessary to position the same sample against earth gravity. 

On the other hand, the positioning of the sample in a region, where the magnetic field strength is 

weakest, prevents an efficient heating and melting by the quadrupole field. Therefore, an 

additional, independent, highly efficient "heating coil", connected with its own power generator 

for the alternating current supply, has been provided in the EML facility. This coil arrangement 

has several advantages: 

• The very poor heating efficiency of the positioning (quadrupole) field (see Fig. 1 left) and the 

very low center of gravity forces applied on the sample by the nearly homogeneous heating 

(dipole) field (see Fig. 1 right), enables an almost independent positioning and heating of the 

sample under microgravity. 

• The high efficiency and the homogeneity of the heating field allows also its use as a detection 

field for a contactless, inductive determination of electrical and volumetric sample properties 

Sample Coupling Electronics (SCE) 

The “Sample Coupling Electronics (SCE)” uses simultaneously the high frequency magnetic 

heating field of the EML also for the inductive, contactless determination of the electrical 

resistivity of the levitated metallic droplet and its thermal expansion [3]. 

Inductive Measurement Principle 

Voltage

drop

Current

signal

~

Heating Circuit

375 kHz

Sample

Heater Power Supply

Current Sensor

Tempus,

EML

Fig. 2: Schematic sketch of the EML heating circuit with the electrical signal outputs. 

A power supply feeds a resonant current (~375 kHz) in the parallel oscillating circuit, 

the inductivity of which consists just of the heating coil and the inductively coupled 

metallic sample. The signal of the alternating current I through the circuit as well as 

the alternating voltage drop U across the circuit is conveyed to the outside. 

As already indicated in Fig. 1 and shown in Fig. 2, the relation between the measurable 

quantities, i.e., U0: amplitude of the alternating voltage drop across the levitation circuit, I0: 

amplitude of the alternating current through the levitation circuit, φ: phase difference between 

voltage drop and current, as well as ω: angular frequency of voltage and current, and the sample 

properties, i.e. ρ: sample resistivity and a: sample radius, is given by the simple complex 

impedance relation 

),,()(

1

)(

1

0

0

ZZZ a

e

U

I

cap coil sample

i

ωω ρω

φ

+

+= − , 

23
where 1 )()(

−

cap = ωω CiZ is the capacitor impedance, coil )( += ωω LiRZ is the impedance of the 

empty coil and Z a),,( sample ρω is that part of the coil impedance resulting from the inductively 

coupled sample. By a measurement of I0, U0, φ, and ω, real and imaginary part of the complex 

sample impedance Z a),,( sample ρω , depending on the sample properties ρ and a, can be 

determined, after a preceding calibration measurement without sample provided the circuit 

parameters C, L and R. 

For a spherical sample shape and a homogeneous magnetic induction field, two conditions which 

are largely satisfied in the EML facility under microgravity, Z a),,( sample ρω assumes a relatively 

simple mathematical form [4] allowing its inversion for a calculation of ρ and a from the 

“measured” values of Re{ } Zsample and Im{ } Zsample . Both quantities are, however, relatively small 

compared to the real and imaginary part of Zcoil and Zcap , respectively. Therefore, to obtain the 

temperature dependent electrical resistivity ρ(T) of liquid samples with reasonable accuracy, the 

necessary resolution of the measurement quantities has to be correspondingly high: 

ΔU0/U0 < 8⋅10-4, ΔI0/I0 < 8⋅10-4, Δφ < 0.15°, Δω/ω < 2⋅10-5.

Test Results 

A development model of the so called “Sample Coupling Electronics (SCE)” has been designed 

and constructed by DLR for the EML facility onboard the International Space Station (ISS). The 

SCE supplies dc voltages proportional to the measurement quantities: U0, I0, and φ with the 

above mentioned resolution from the alternating voltage and current signals of the levitation 

facility, see Fig. 2. These signals are then sent to the EML “Data Acquisition (DAQ)” unit, 

which converts the analogue dc voltages with a rate of 10 Hz into 16bit digital values. Finally, all 

digitized SCE data are stored synchronously with the sample temperature data, provided by the 

pyrometer of the levitation facility, to enable the calculation of the temperature dependent 

resistivity ρ and radius a of the sample with the help of the theoretical model of Z a),,( sample ρω

given in Ref. [4]. 

To visualize the resolution of the SCE, Fig. 4 shows results of the electrical resistivity ρ and the 

radius a of a solid, spherical tungsten test sample for different sample temperatures. 

390 395 400 405 410 415 420 425 430 435 440

6.5x10-7

7.0x10-7

7.5x10-7

8.0x10-7

8.5x10-7

Measured at: Uctr

heat = 0.6V

Temperature dependent el. resistivity of solid tungsten sample

ρfit(T) = 7.33·10-7 · (1 + 21.8·10-4 · (T - 415K)/[K]) [normalized]

"SCE Development Model" within TEMPUS Parab. Flight facility

Uncorrected el. sample resistivity: ρ

Estimated sample temperature [K]

Fig. 4: Plot of the uncorrected electrical resistivity (left) and the radius (right) of the 

solid tungsten test sample against the estimated sample temperature. The open circles 

show the values evaluated from the measured SCE data. The straight lines present a 

linear fit of these data points. 

24
The scatter of the resistivity data (Fig. 4 left) is extremely low. This is also due to the fact, that 

the sample itself is solid and fixed within the coil during the test. The fitted absolute values of 

the resistivity are arbitrary, because there was no previous calibration with a sample of known 

properties providing a correct “coupling constant”. 

Due to the very low increase of the sample radius with the temperature: ≈ .3 1076 ][ −− 16 ⋅ K (see 

Fig. 4 right) and the necessarily much higher resolution of this diagram compared to that of Fig. 

4 left, the data scatter of the radius values is higher. Nevertheless, the radius increase of the test 

sample with increasing sample temperature is clearly visible, proving that the above listed data 

resolution of the SCE is sufficient. 

Oxygen Sensing and Control Unit (OSC) 

Using the OSC, the oxygen partial pressure in the atmosphere of the EML experiment chamber 

can be measured and controlled. This enables a measurement of critical properties, such as 

surface tension and viscosity in dependence of an adjusted oxygen partial pressure. 

Technical Realization 

The OSC consists of an oxygen loading system (OLS), a solid state potentiometric sensor (SS1) 

and a gas circulation pump. The SS1 is located at the outlet of the EML chamber and the OLS at 

the inlet. Due to the design constrains in the EML facility, the OLS and SS1 are mounted outside 

the chamber and thus require gas circulation. 

Both, the oxygen loading system (OLS) and the potentiometric solid state sensor (SS1) are based 

on an ionic conductor, yttrium-stabilized zirconium (YSZ), operated at the temperature 

sufficiently high to enable the transport of O2- ions throughout the electrolyte, but low enough to 

suppress electronic conduction. The oxygen partial pressure adjusted by the pump depends on 

the electric current I, total pressure ptot, total gas flow Jtot and the initial oxygen partial pressure 

in the carrier gas 0

O2 p as follows: 

0

O2 O2 4

tot

tot

p I

p p J F= + , 

whereas F is the Faraday constant. As shown in Fig. 5, the tube of the oxygen ion pump is 

covered with platinum paste at three regions i.e. the large pumping area, the reference electrode 

and the counter electrode. The reference electrode is used to monitor the electromotive force, and 

thus pO2, which is given by the Nernst equation. Due to the large distance between the pumping 

and reference electrode, no impact of the pumping current on the Nernst potential is observed. 

The oxygen partial pressure is also measured by an additional potentiometric solid state sensor 

(SS1) located at the outlet of the chamber. 

Fig. 5: The schematic view of oxygen ion pump. The carrier gas is delivered through 

a tube into the active area, where electric current I is applied and, therefore, oxygen 

ions are transferred throughout the electrolyte. An additional electrode is provided 

where the Nernst potential and, therefore oxygen partial pressure is measured. 

25
Both, the oxygen ion pump and the sensor are operated at 870 K. This relatively low temperature 

ensures sufficiently high oxygen ion mobility and long-term stability of the system as required 

for the electromagnetic levitation experiments. 

Since the system is operated in an environment, where strong electromagnetic noise occurs, 

commercial temperature controllers do not fulfill the stability requirements under such 

conditions. Therefore, a robust custom device is built. In order to reduce the cross-feed between 

the power supply and monitored temperature, commonly used thermocouples are replaced with 

resistive temperature sensors. Moreover, shielding for control electronics and for entire OLS is 

provided. The pumping current I of at most 40 mA is adjusted by a pulse width modulation 

control based on a PID algorithm. Thereby the polarity of the current can be changed. The 

controller achieves a resolution of about 1 μA. At maximal current about 8 cm3 of O2 can be 

transferred throughout the electrolyte within an hour. In order to adjust the oxygen partial 

pressure at two different potentiometric sensors, the system features a cascade PID controller. 

Test Results 

The OLS system is tested in two different environments. Laboratory conditions, free of strong 

electromagnetic fields and dust particles in carrier gas due to evaporation of liquid samples, 

enable precise determination of oxygen pump's performance, response time and signal stability. 

In case of the experiments, where the oxygen partial pressure is adjusted during levitation, strong 

electromagnetic fields impact potentially the stability of the control system. 

The result of oxygen ion pump controlling pO2 at the carrier gas (Ar 99.999 %, pO2=1.6×10-4 bar) 

is shown in Fig. 6. Here, the oxygen partial pressure is varied stepwise between 10-7 and 10-3 bar. 

Remarkable is the very short response time of less than 90 s and signal stability of 

Δlog(pO2/bar) < 0.02. Since the oxygen control system features switching of the pumping current 

polarity, it is possible to control pO2 below and above oxygen's partial pressure in the carrier gas. 

Fig. 6: The oxygen partial pressure adjusted by the OLS. 

The results of OSC operated in parallel with a ground based electromagnetic levitator are shown 

in Fig. 6. The system proves to operate stable under these conditions, even if the levitator is 

operated at full power required to levitate and heat the sample in 1g conditions. Thanks to the use 

of resistive Pt100 sensors, the fluctuation of the temperature is negligible. The electromagnetic 

interference the OSC was exposed to in this test is much stronger than expected in EML (under 

μg conditions) since the ground levitator uses fields that are approximately three orders of 

26
magnitude larger than in the EML under μg. Therefore, this was a very harsh test providing a 

large confidence level that the OSC will work well in an μg-EML environment 

 The response time of OSC varies between several seconds and an hour. It depends strongly on 

the experimental conditions like e.g. the gas flow rate, kind and purify of levitated material as 

well as presence of unintended buffer mixtures like H2/H2O, CO/CO2 or CxHy/CO2. In particular, 

the liquid sample acts an oxygen buffer and its influence on the measurement and control of pO2

cannot be ignored. 

Fig. 7: Surface tension of liquid Ni at 1990 K versus pO2,OLS together with data from 

Ref. [8] (dotted line). 

During levitation, the liquid sample performs spontaneously excited surface oscillations. The 

sample is observed by a fast digital C-MOS camera (400 frames per second) and the frequency 

spectra of the surface oscillations are obtained from image analysis software. Using the sum 

formula of Cummings and Blackburn [6] the surface tension γ can be calculated from a 

spectrum. This is shown in Fig. 7 for a liquid Ni sample of 99.999 % purity at 1990 K where γ is 

plotted versus pO2,OLS. 

For pO2 < 10-6 bar, γ remains constant at a value of 1.6 N/m. It decreases with increasing pO2

when the oxygen partial pressure becomes larger than 10-6 bar. The results can be described by 

the Belton-equation [7] and the agreement is very good, as shown in Fig. 7b. 

At the corresponding temperature of 1990 K, the surface tension of pure Ni is 1.68 N/m, Ref [8], 

which is slightly larger than the value of 1.6 N/m that was found in this experiment. However, 

this discrepancy is still within the error bar of ±5 %, so that it can be stated that a good 

agreement with the reference data is obtained. 

Summary 

Two advanced measurement devices for the microgravity “electromagnetic levitation facility 

(EML)”, which is currently under construction for the use onboard the “International Space 

Station (ISS)”, have been presented. One is the “Sample Coupling Electronics (SCE)” and the 

other is the “Oxygen Sensing and Control Unit (OSC)”. 

The SCE measures by a contactless, inductive method the electrical resistivity and the diameter 

of a spherical metallic droplet, levitated in the EML under microgravity conditions, by 

evaluating the voltage and electrical current applied to the levitation coil. A development model 

27
of the SCE has been designed, constructed and tested under laboratory conditions by DLR. These 

tests show a sufficiently well resolution of the electronics for a temperature dependent 

determination of those measurement quantities. 

The OSC measures the oxygen partial pressure of the atmosphere in the EML experiment 

chamber. The necessity of the OSC comes from the insight that properties like surface tension or, 

eventually viscosity by the oscillating drop method in the EML facility cannot seriously be 

determined without knowing the conditions of the surrounding atmosphere. A technology 

demonstrator of an OSC system for the use at the ISS-EML has been developed and successfully 

tested. The tests were carried out under ideal laboratory conditions as well as in real levitation 

environment under gravity conditions. In both test it was shown that the device works reliably. 

An example experiment, the measurement of the surface tension of liquid Ni as function of the 

oxygen partial pressure, succeeded convincingly. The obtained data could be described by the 

Belton equation. 

References 

[1] Brillo J., Lohöfer G., Schmidt-Hohagen F., Schneider S. and Egry I., Int. J. Materials and 

Product Technology 26 (2006), 247-273. 

[2] Schneider S., Viskositäten unterkühlter Metallschmelzen (PhD Theses, RWTH Aachen 

2002). 

[3] Lohöfer G., Meas. Sci. Technol. 16 (2005), 417-425. 

[4] Lohöfer G., Int. J. Engng Sci. 32 (1994), 107-117. 

[5] Ellingham, H. J. T., J. Soc. Chem. Ind. 63 (1944) 125. 

[6] Cummings D.L. and Blackburn D.A., J. Fluid Mech. 224 (1991) 395. 

[7] Belton G.B., Metall. Trans. 7B (1976) 35. 

[8] Brillo J and Egry I., J. Mater. Sci. 40 (2005) 2213. 

28
Electrostatic Levitation Furnace for the ISS 

Keiji Murakami1, Naokiyo Koshikawa1, Kohichi Shibasaki1, Takehiko Ishikawa1, Junpei Okada1

2

Tetsuya Takada, 2Tatsuya Arai, 2Naoki Fujino, 2Yukiko Yamaura 

1

JAXA, 2-1-1, Sengen, Tsukuba, Ibaraki,305-8505, Japan

2

IHI Aerospace, 900,Fujiki, Tomioka,Gunma,370-2398, Japan

Keywords: ISS, JEM, Multiuser Facility 

Abstract 

JAXA (Japan Aerospace Exploration Agency) has just started the development of Electrostatic 

Levitation Furnace to be launched in 2014 for the ISS. 

This furnace can control the sample position with electrostatic force and heat it above 2000 

degree Celsius using semiconductor laser from four different directions. The announcement of 

Opportunity will be issued soon for this furnace. 

In this paper, we will show the specifications of this furnace and also the development schedule. 

Introduction 

As for microgravity material science research, JAXA has started the development of Gradient 

Heating Furnace in 1993. The GHF has been launched by H-II Transfer Vehicle #2 last 

year(2011). At that time, Electrostatic levitation Furnace (ELF) was also the candidate of the 

JAXA multi-user facility for the ISS. Because of the technical difficulty of ELF, the start of the 

development of ELF has delayed, and also the ISS assembly schedule has delayed. Since then, 

many circumstance conditions has changed (such as rack location availability, crew time 

availability, maximum development cost and etc.), so the specification and requirements have 

also changed. Through these hard time, JAXA started the development of ELF in February 2011. 

The ELF will be integrated into Multi-purpose Small Payload Rack (MSPR) in KIBO. The 

launch target year is 2014. 

History 

In 1993, at Furnace Workshop which was held at ESTEC among the each space agency (CSA, 

ESA, NASA and JAXA), JAXA presented the intent of developing ELF and also preliminary 

specifications was presented. At that time, DLR has presented Electro-magnetic Levitator 

Furnace, based on the result of Shuttle equipment TEMPUS. In 2000, Levitation Furnace 

Workshop was held at San Diego, USA. There NASA, ESA and JAXA has discussed and came 

to some conclusion, that each agency’s furnace contribution. The image of the contribution is 

shown in Table 1. 

Specification 

ELF has been developed based on a design by Rhim et al.1) from which several modifications 

were made. Fig. 1 depicts schematically the ELF and its optical diagnostic instruments. The 

electrostatic levitation method utilizes the Coulomb force between the sample and the 

surrounding electrodes. A positively charged sample was levitated between a pair of electrodes 

which were utilized to control the positions of the specimen. 

Table 2 is the latest specification of the ELF. The heating lasers hit the sample from the top of 

regular tetrahedron to center, so that the sample temperature could be isothermal. 

29
Low Temp High Temp

Conductors

(Metal, Alloy)

Insulators

(Oxides) ELF(JAXA)

EML(ESA)

Space

DRUMS

(NASA)

Table 1 Levitation Furnace Contribution of each space agencies 

To integrate the ELF into MSPR, vacuum pump was deleted, so the samples which requires high 

vacuum condition are impossible with ELF. The maximum environment pressure is 0.2 Mpa. 

And the best vacuum condition is worse than 0.13 Pa. ELF can control the environment pressure 

between those values with N2 or Ar. We plan to conduct the oxides sample in the maximum 

pressure so that the outgas from the sample become minimum. When we process metal samples 

with ELF, we will choose Ar environment in which we can not levitate samples in 1-G condition. 

The cleanliness of the supply gas are shown in also table 2. 

Figure 2 shows the rough images of ELF sample chamber. ELF experiment chamber is 

polyhedron shape so that the chamber accommodates many optical windows in minimum 

chamber size and enables us to set various functions(such as heating, sample position detection, 

observations and etc.) . 

ELF is equipped an UV lamp (peak 

wavelength;365nm) to assist the 

electric charge of the sample so that 

ELF electrostatic positioning control 

can maintain the sample position to 

correspond to the sample charge 

decrease, caused by such as outgas 

from the sample. 

ELF usually control the sample 

positions at 1 kHz frequency. This 

position control frequency can be 

reduced by ground commanding as 

well as PID control parameters. 

Figure 1 Schematic drawing of the ELF 

Utilization Concept of ELF 

To demonstrate the ELF capability, we are now planning to process the oxide samples, whose 

melting temperature are over 2000 degree Celsius. The thermophysical properties such as density, 

30
surface tension, and viscosity will be measured over wide temperature range including under 

cooled region. The density can be calculated using magnified image of the molten sample2), 

while surface tension and viscosity can be measured by oscillating drop method3). We will 

superimpose the oscillation signals to position control voltage. Then measure the sample 

oscillation with photo detectors with vertical slit. 5 kHz of oscillating data would be down linked 

to ground for the measurement. The processed samples will be retrieved to the ground. These 

process would be conducted as ELF initial check. Some thermophysical properties of molten 

oxides have not been measured on the ground and will be the first measurement in the world. 

Also we plan to investigate the detailed structure of the processed sample on ground. 

By using ELF, we can obtain the cooling curve of the sample, with which we may calculate the 

heat capacity of the sample if we can estimate the conductive heat transfer by surrounding gas. 

The processing sample would be installed into cartridge (up to 15 samples per cartridge), and 

then launched by HTV(H-II Transfer Vehicle) or other launch system. After installing the 

cartridge into ELF chamber, each sample can be processed by ground commanding. The images 

of the sample and equipment telemetry data would be down linked to ground automatically, 

when we oscillate the sample, we plan to get the resonance frequency and attenuation factor of 

the sample. After the experiment, the sample cartridge would be retrieved by Space-X or Soyuz 

to ground. 

The ELF chamber equipped double seal and around the chamber connectors, there is 

decompression environment, so that ELF can provide three protecting method for the sample 

vapor gas. After each measurement, the inner gas could be replaced by ground commanding. 

Table 2 Specification of ELF 

*SN-C-0005C 

Development Schedule 

Figure 2 shows the development schedule of ELF. Launch target year of ELF is 2014. Using the 

engineering model, we plan to verify the sample control logic with parabolic flight. After that 

verification, the engineering model would be modified to training/ground model. 

Items Requirements

Sample Size (diameter) 1 ~5mm

Accuracy of Positioning ±100micro meter

Heater

Laser (Wave 

Length/Power) 980nm(TBD) /40W x 4 directions

Target Temperature Melting Point of Zirconia (2710 degC)

Observation

Temperature 

Measurements

one thermometer (ORU)(range;1450-1800 nm(TBD))

Magnified Image higher than 140

pixels/ half diameter (1mm)

Observation of Overview One camera with zoom (NTSC)

Measurements of 

Thermo-physical 

Properties

Density through Image Analysis

Surface Tension Oscillating Droplet Method

Viscosity Oscillating Droplet Method

Others Ambient N2 Gas,(Level300A)*

Ar Gas(Level300A)*, Vacuum with Evacuation Line

One camera with zoom (NTSC)

Sampling Rate (more than 100Hz) equipped with one camera(NTSC)

31
Figure 2 Sample chamber images of ELF 

fy22(fy2010) fy23(fy2011) fy24(fy2012) fy25(fy2013) fy26(fy2014)

schedule

Critical Design 

EM PFM

Launch PDR MDR/SDR CDR

Project

Start

Preliminary

Design

Figure 3 Development Schedule of ELF 

Conclusion 

Announcement of Opportunity (AO) will be issued for the ISS experiment (including the usage 

of ELF) at Japan in February 2012. We hope many researchers are interested in ELF and apply 

to the AO. 

References 

1) Rhim, W.-K., Chang, S. K., Barber, D., Man, K. F., Gutt, G., Rulison, A., and Spjut, R. E.: 

Rev. Sci. Instrum. 64 (1993), pp. 2961. 

2)S. K. Chung, D. B. Thiessen, W.-K. Rhim,Rev. Sci. Instrum.67 (1996), 3178.

3) W.-K. Rhim, K. Ohsaka, P.-F. Paradis, R. E. Spjut, Rev. Sci. Instrum.70 (1999), 2796.

32
LIQUID STRUCTURES AND PHYSICAL PROPERTIES – GROUND 

BASED STUDIES FOR ISS EXPERIMENTS

K. F. Kelton, J. C. Bendert, and N. A. Mauro 

Department of Physics, Washington University, St. Louis, MO 63130 

Keywords: electrostatic levitation, liquid structure, short-range order, medium-range order, 

icosahedral order 

Abstract 

Studies of electrostatically-levitated supercooled liquids have demonstrated strong short- and 

medium-range ordering in transition metal and alloy liquids, which can influence phase 

transitions like crystal nucleation and the glass transition. The structure is also related to the 

liquid properties. Planned ISS experiments will allow a deeper investigation of these results as 

well as the first investigations of a new type of coupling in crystal nucleation in primary 

crystallizing liquids, resulting from a linking of the stochastic processes of diffusion with 

interfacial-attachment. A brief description of the techniques used for ground-based studies and 

some results relevant to planned ISS investigations are discussed. 

I. Introduction 

Fahrenheit [1] first demonstrated that under the right conditions water can be cooled far below its 

melting temperature without freezing, indicating a barrier to crystallization. That metallic liquids 

resisted supercooling was believed to indicate a small nucleation barrier, arising from similarities 

in the atomic structures of the liquid and crystal, a view supported by their similar densities and 

coordination numbers. However, if heterogeneous sites for nucleation are removed it is possible 

to deeply supercool liquid metals [2, 3]. This can be explained if undercooled liquids develop 

icosahedral short-range icosahedral order (ISRO), which is incompatible with crystalline 

periodicity [4]. Experimental data from liquids and glasses support this view [5-9]. This and 

subsequent work show that nucleation in liquids is more complicated than generally thought. It 

is well known that it can be coupled to other first-order processes, such as phase separation prior 

to crystallization and the precipitation of nano-sized heterogeneous sites, but these studies 

demonstrate that local order in a liquid can also couple to the nucleation barrier. Dynamical 

coupling effects can occur. For example, when the nucleating phase has a different chemical 

composition than that of the original phase, models predict that the interfacial flux, of central 

concern in the classical theory of nucleation, can couple to the long-range diffusion field [10, 

11]. Ordering can also couple to atomistic dynamics, significantly influencing the nucleation 

kinetics and possibly giving rise to dynamical phase transitions, such as the fragile/strong 

transition. A more thorough investigation of these processes requires a microgravity 

environment to minimize stirring effects and allow key thermophysical properties to be 

measured. As a member of the THERMOLAB and ICOPROSOL teams, we will carry out these 

studies on the International Space Station. To support those activities, we are making extensive 

ground-based studies of liquid structures and some thermophysical properties. In this 

33
manuscript, a brief overview of our ground-based techniques and activities and a brief review of 

some of the results obtained, are provided. 

II. X-Ray Diffraction of Supercooled Liquids 

Electrostatic levitation (ESL) is used for containerless investigations of the high temperature 

liquids of interest; most of our studies are now made using the recently constructed Washington 

University Beamline ESL, or WU-BESL (Fig. 1). In ESL, charged samples (2.0 – 4.0 mm 

diameter) are levitated in an electrostatic field (0 to 2 MV/m) under high vacuum (≈ 10-7torr). 

Three pairs of orthogonal electrodesand a robust control algorithm maintain the sample position 

during processing to better than 50μm. The samples are initially charged by induction and the 

charge is maintained during processing with an external UV source. Samples are melted using a 

50 W diode laser and the sample temperature is measured across a range of 160 to 2300o

C using 

a combination of two infrared pyrometers with overlapping ranges. In addition to structural 

studies, WU-BESL is capable of making coordinated density, viscosity and surface tension 

measurements. More details on WU-BESL can be found elsewhere [12]. 

Figure 1 –Schematic drawing of the WU-BESL: (a) vertical and (b) horizontal cuts through the 

sample. The top and bottom electrode and two side electrodes are schematically indicated in (a) 

(solid black). Components: TP - turbo pump; HV - high voltage feed through; X-EN –X-ray 

entrance window; X-EX –X-ray exit window; PL - positioning laser; PSD - position sensitive 

detector, PY – pyrometer; DC – density camera; HL - heating laser; UV - ultra violet light 

source; VC - visualization camera; DB – backlight for density measurements. 

X-ray scattering experiments are made in a transmission geometry (Fig. 2); a series of 

collimators and a beam-stop reduce the background to acceptable levels. For a typical study (E = 

129keV, λ = 0.09611Å), useful data can be obtained to 15 Å-1 using a GE Revolution 41-RT 

amorphous Si flat panel X-ray detector at a sampling rate of up to 10 Hz. 

The scattering data are used to obtain the total X-ray static structure factor, S(q), from which 

structural information is inferred using either single cluster [7, 8] or Reverse Monte Carlo 

(RMC) [13, 14] approaches. When constrained with the measured liquid density and atomic 

sizes, an RMC fit yields a reasonable topological model for the liquid; it does not, however, yield 

realistic chemical correlations (such as the partial pair distribution functions). If the RMC can be 

34
further constrained with information from molecular dynamics (MD) structural studies, elastic 

neutron diffraction, EXAFS, or anomalous scattering data, it becomes possible to obtain more 

accurate partial pair distribution functions. The order in the RMC structures is characterized 

using the Honeycutt-Andersen (HA) indices [15], bond orientation order parameters [16, 17] and 

Voronoi polyhedra (VI) construction [18]. The HA indices typically indicate a large amount of 

five-fold symmetry, which may not, however, signal complete icosahedral clusters. Consistent 

with this, the VI construction tends to show much smaller amounts of icosahedral order [19]. 

Figure 2 – Schematic of the X-ray optics in WU-BESL 

Many studies indicate that icosahedral short-range order (ISRO) in metallic liquids increases 

with supercooling [7, 8, 20-22]. A developing shoulder on the high-q side of the second peak in 

S(q) often correlates with ISRO. This is observed in the scattering data from liquid 

Zr59Ti3Cu20Ni8Al10, abulk metallic glass former (Fig. 3.a). An increase in the number of 1551 

HA indices from atomic structures obtained from a RMC fit to the scattering data (Fig. 3.b) 

supports this conclusion in this case. However, it is important to point out that a developing 

shoulder should only be used as a rough guide. Other local structures can produce a similar 

shoulder, and in some cases ISRO may be dominant with no evidence of the shoulder. The latter 

is the case for liquid Al, which shows no asymmetric second peak, yet an HA analysis of the 

RMC structures indicates a high degree of ISRO [23]. When present, ISRO can couple to the 

nucleation barrier [8] and may play a role in the glass transition in some cases [9]. 

Many reports exist of medium-range order (MRO) in glasses, i.e. ordering beyond nearest 

neighbors, [24-28]. In some case, evolution of MRO has been correlated with changes in 

plasticity [25] and viscosity [29]. Since reports of medium-range ordering in metal-metal liquids 

have been rare, it is of interest to determine under what conditions it can be observed. Binary 

liquids, such as Zr-Pd and Zr-Pt, provide good systems for such studies. The two alloys are very 

similar. Both have deep eutectics at similar Zr-rich compositions and both form glasses that 

devitrify first to the icosahedral quasicrystal phase [30]. Further, Pd and Pt have similar bonding 

properties with Zr and are of similar sizes. However, recent WU-BESL studies of the two 

eutectic liquids show qualitatively different X-ray static structure factors [31, 32]. As shown in 

Fig. 4, a pre-peak is clearly observed in the S(q) data at all temperatures for Zr80Pt20 liquids 

(indicated by an arrow), while it is not observed in Zr75.5Pd24.5 liquids. Given the many 

similarities between these two systems, the presence of a pre-peak in one and not in the other is 

 γ

Electrode 

Assembly and 

Sample 

Beryllium 

Windows 

X-Ray 

Entrance 

Lead 

Collimator 

Tungsten 

Collimator 

Lead Beamstop 

Detector 

35
at first difficult to understand. An analysis of constrained RMC fits to the data, made using 

partial pair-correlation functions obtained from MD simulations [31, 32] revealed a solute-solute 

(Pd-Pd or Pt-Pt) correlation in both cases, which manifests itself as a prepeak at ~1.7Å-1. 

Because Pd has a much smaller atomic form factor than Pt, however, the correlation is not 

evident in the experimental X-ray data for Zr-Pd, suggesting that MRO may be much more 

common in metal-metal liquids than previously thought. 

Figure 3 – (a) Experimental S(q) data for liquid Zr59Ti3Cu20Ni8Al10 as a function of temperature. 

(b) Number of 1551 HA indices (indicating ISRO) obtained from RMC fits to the S(q) data as a 

function of temperature. 

2 4 6 8 10

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

S(q)

q (Å-1)

Liquid X-ray Static Structure Factors

Zr-Pd

Zr-Pt

Figure 4 – Liquid X-ray static structure factor for Zr-Pt and Zr-Pd eutectic liquids. For each 

alloy, a series of S(q)’s are plotted corresponding to the temperature range over which the data 

were collected. 

36
III. Thermophysical Property Measurements using ESL 

Employing non-contact techniques, WU-BESL can be used to measure thermophysical 

properties such as density, thermal expansion, specific heat, viscosity and surface tension.A brief 

discussion of the techniques used for measurements of the density, thermal expansion coefficient 

and specific heat in WU-BESL is provided here. 

Following techniques developed by others [33, 34], the density is measured from video data of 

the shadow of a levitated sample. A schematic layout of the WU-BESL density measuring 

system is shown in Fig. 5. Video data are taken at 25 frames per second (fps) with an exposure 

time of 0.8 ms using a 1.6 megapixel CMOS monochrome camera (pixeLINK®) and a K2/SC 

long distance video microscope lens, with a close focus objective that is capable of 

magnifications of 2.13 - 0.71 and a working distance of 222 - 418 mm. A 450 nm band pass 

having a 40 nm full-width-at-half-max is placed between the camera and the lens system to 

exclude interference from thermal radiation [34]. The samples are backlit through a transparent 

port that is opposite to the camera and lens with a 455 nm collimated microscope LED, having a 

total beam power of approximately 240 mW and a beam diameter of 37 mm. The pixel 

dimensions are calibrated between studies of samples of each composition using 3/32” diameter 

grade 3 (± 3 x 10-5

” diameter) tungsten carbide standards (Industrial Tectonics Inc). 

Figure 5 – Schematic diagram for density and volume expansion measurements in WU-BESL. 

The video are analyzed to identify the edges of the 2D sample silhouette and then integrated 

about an axis of symmetry to calculate sample volume, following a method described by 

Bradshaw et. al. [35]. The coefficient of thermal expansion (β = ( ln / ) ∂ ∂ V T P ) is determined 

from linear fits to regions of the volume-temperature data. The error in the density is dominated 

by the uncertainty in the volume and mass calibrations (± ~0.4%). These uncertainties cancel in 

the thermal expansion calculations. There, the dominant contribution to the error are the 

uncertainty in the temperature calibration (~ ±1%) and the uncertainty in the linear fit to the data 

(~ ± 1%), giving a total uncertainty in β of ~ ± 2%. 

For illustration, the measured density of the best glass-forming Cu-Zr liquid (Cu64Zr36) during 

free cooling is shown in Fig. 6.a. The density increases approximately linearly with decreasing 

temperature (see inset in Fig. 6.a) and can be described by 

( ) ([ ] )( ) -3 5 -1 o ρ T T 7.316 .023 gcm 1. 8.57 0.15 10 K 994 C − = ± +± # $# $ ⋅ − & '& '. (1) 

37
Since the sample is levitated in vacuum, when laser power is removed, the sample will cool by 

radiation loss described by the Stefan-Boltzmann equation, giving 

( ) 4 4 / mC A T P R dT dt = − − εσ T , (2) 

whereTR is the effective room temperature of the chamber, A is the sample surface area, ε is 

the hemispherical emissivity, σ is the Stefan-Boltzmann constant, m is the sample mass, and CP

is the sample specific heat capacity. By determining the temperature derivative from the 

measured temperature as a function of time during cooling, the ratio / CP ε is readily obtained. 

As an example, the / CP ε values measured as a function of temperature for Cu64Zr36 are shown 

in Fig. 6.b.As observed in other alloy liquids, / CP ε increases approximately linearly with 

decreasing temperature, with the rate of increase becoming slower at lower temperatures. Were 

the emissivity approximately constant across this temperature range, this would suggest a rise in 

the specific heat, which would be consistent with the ordering of the liquid. 

Figure 6 – (a) Temperature as a function of time during free cooling of the sample and the 

corresponding change in density. The density as a function of temperature is shown in the inset 

to the figure. (b) Measured / CP ε as a function of temperature with the estimated error. 

IV. Summary and Future Directions 

New methods for obtaining structural information have been applied to supercooled metallic 

glass-forming liquids. These studies have demonstrated that these liquids typically develop 

significant short-range order, often with an icosahedral character. Some liquids also develop 

medium-range ordering, extending beyond nearest-neighboring atoms; our studies indicate that 

this may occur in more metallic liquids than is generally thought. Structural ordering has an 

impact on phase transitions occurring in the liquid and likely on liquid properties. We have 

developed capabilities that allow the density, volume expansivity, specific heat, viscosity and 

surface tension to be measured simultaneously with the structural studies to investigate this. 

Since neutrons scatter differently from X-rays, they provide additional information about 

chemical ordering in a sample. Further, since the neutron scattering cross section differs among 

38
different isotopes, it is possible to obtain additional information from elastic neutron scattering 

studies on samples made with isotopic substitution. These data can be used to constrain RMC 

fits to the X-ray scattering data, allowing structures with realistic topological/chemical order to 

be inferred. Neutron scattering studies can also provide information about dynamics. Due to 

their lack of periodicity, only long wavelength phonons are present in liquids; the strong 

temperature dependence of the liquid structure makes it difficult to predict the vibrational 

properties. Inelastic neutron scattering experiments allow these and related issues in liquids, 

such as atomic diffusion and the existence of dynamical transitions, like mode-coupling and the 

glass transition, to be quantitatively investigated. We are currently constructing an ESL for 

neutron scattering studies (Neutron ESL, NESL). NESL has been designed take full advantage 

of the high neutron flux and the exceptionally high solid angle detector coverage at several of the 

beamlines at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, including 

VULCAN, ARCS, CNCS, BASIS and NOMAD. 

Using the quiescent microgravity environment on the ISS, experiments are planned to investigate 

the role of stirring on the amount of supercooling possible in metallic liquids that crystallize to 

phases of different composition. This will allow the effects of diffusion on the nucleation rate to 

be studied. Modulated specific heat and other thermophysical property measurements in metallic 

glass-forming liquids will enable studies of possible liquid/liquid phase transitions and their 

relation to structure, determined from the ground-based WU-BESL and NESL studies. The 

results of these studies will improve our knowledge of phase transitions that occur in liquids and 

help develop methods that can lead to improved microstructural control. 

Acknowledgements 

The research presented here has been supported by NASA (NNX07AK27G & NNX10AU19G) 

and the National Science Foundation (DMR-08-56199). We thank R. W. Hyers for his 

collaboration on many of the past and ongoing ESL studies; he is a co-investigator on the 

upcoming ISS experiments designed to investigate the role of diffusion on nucleation. 

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40
Thermophysical Properties Measurement of High-Temperature Liquids 

under Microgravity Conditions in Controlled Atmospheric Conditions 

Masahito Watanabe1

, Shumpei Ozawa2, Akitoshi Mizuno1,Taketoshi Hibiya3, 

Hiroya Kawauchi1

 ,Kentaro Murai1 ,Suguru Takahashi2 

1

Department of Physics, Gakushuin University; 

1-5-1 Mejiro, Tokyo, 171-8588, JAPAN 

2

Department of Aerospace Engineering, Tokyo Metropolitan University; 

6-6 Asahigaoak, Hino, Tokyo, 191-0065, JAPAN 

3

SDM Research Institute, Keio University; 

4-1-1 Hiyosi, Yokohama, 223-8526, JAPAN 

Keywords: Electromagnetic Levitation, Surface Tension, Viscosity. 

Abstract 

Microgravity conditions have advantages of measurement of surface tension and viscosity of 

metallic liquids by the oscillating drop method with an electromagnetic levitation (EML) device. 

Thus, we are preparing the experiments of thermophysical properties measurements using the 

Materials-Science Laboratories ElectroMagnetic-Levitator (MSL-EML) facilities in the 

international Space station (ISS). Recently, it has been identified that dependence of surface 

tension on oxygen partial pressure (Po2) must be considered for industrial application of surface 

tension values. Effect of Po2 on surface tension would apparently change viscosity from the 

damping oscillation model. Therefore, surface tension and viscosity must be measured 

simultaneously in the same atmospheric conditions. Moreover, effect of the electromagnetic 

force (EMF) on the surface oscillations must be clarified to obtain the ideal surface oscillation 

because the EMF works as the external force on the oscillating liquid droplets, so extensive 

EMF makes apparently the viscosity values large. In our group, using the parabolic flight 

levitation experimental facilities (PFLEX) the effect of Po2 and external EMF on surface 

oscillation of levitated liquid droplets was systematically investigated for the precise 

measurements of surface tension and viscosity of high temperature liquids for future ISS 

experiments. We performed the observation of surface oscillations of levitated liquid alloys 

using PFLEX on board flight experiments by Gulfstream II (G-II) airplane operated by DAS. 

These observations were performed under the controlled Po2 and also under the suitable EMF 

conditions. In these experiments, we obtained the density, the viscosity and the surface tension 

values of liquid Cu. From these results, we discuss about as same as reported data, and also 

obtained the difference of surface oscillations with the change of the EMF conditions. 

Introduction 

 Microgravity conditions have advantages of measurement of surface tension and viscosity of 

metallic liquids by the oscillating drop method with an electromagnetic levitation (EML) device 

[1]. Thus, we are preparing the experiments of thermophysical properties measurements using 

the Materials-Science Laboratory ElectroMagnetic-Levitator (MSL-EML) in the international 

space station (ISS). Recently, it has been identified that dependence of surface tension on oxygen 

partial pressure must be considered for industrial application of surface tension values [2]. Effect 

of oxygen partial pressure (Po2) on surface tension would apparently change viscosity from the 

damping oscillation model. Because viscosity values are obtained from damping time of surface 

oscillations, the change of surface tension with oxygen partial pressure would change the 

41
damping time of surface oscillation. However, viscosity is bulk properties, so the values affected 

by oxygen partial pressure are not real properties. Therefore, surface tension and viscosity must 

be measured simultaneously in the same atmospheric conditions. In our project using the 

parabolic flight levitation experimental facilities (PFLEX), the effects of Po2 and external forces 

by EML are systematically investigated for the precise measurements of surface tension and 

viscosity of high temperature liquids. From the above purpose, we performed the observation of 

surface oscillations of levitated liquid Cu and Ag in the controlled Po2 atmosphere and various 

electromagnetic forces (EMF) under the microgravity conditions by parabolic airplane flight in 

order to decide the suitable conditions of thermophysical properties measurements by EML. On 

the suitable conditions, we measured the surface tension and viscosity of liquid Cu. From the 

results, we propose the refinement of the surface oscillation measurement under the microgravity 

for precise measurements of thermophysical properties of high-temperature liquid, such as 

industrial application materials and scientifically interested materials, on the future ISS 

experiments using MSL-EML facilities. 

Experiments 

a. Oxygen partial pressure control 

 To control Po2 conditions in microgravity, we used Ar+H2 gas as atmospheric condition gas 

around the levitated liquid samples. The control of Po2 by Ar+H2 gas is used the reaction of 

H2+1/2O2=H2O, so Po2 values has the temperature dependence. However, surface tension of 

liquid Ag is weak dependence on Po2 in wide range of Po2 (10-2-10-20Pa) [3], so we can separate 

between temperature dependence and Po2 dependence of surface tension values. On the other 

hand, liquid Cu has large dependence of surface tension in the range from 10-10 to 105

Pa [4]. In 

the rage from 10-20 to 10-10Pa, the surface tension of liquid Cu does not depend on the Po2 values. 

For liquid Cu we can perform the measurements of surface tension under the controlled Po2

conditions below 10-10. Therefore, the method using Ar+H2 gas is not best way to control Po2, 

so for the future experiments we must use oxygen pomp using electrochemical reaction based on 

the solid-state electrolyte. 

b. Electromagnetic levitation experiments on board parabolic flight 

 We performed electromagnetic levitation experiments under microgravity by parabolic flight 

of airplane (Gulfstream-II) operated by Diamond Air Service Co. (DAS) using PFLEX. PFLEX 

has electromagnetic levitation coil in high vacuum chamber, 2kW radio frequency (RF) power 

supply with 300Hz, control system for electromagnetic levitation conditions, water cooling 

system for coil cooling, and sample droplet observation system. The observation system has 

single color pyrometer for temperature measurement of samples and high-speed camera with 

sampling rate of 250Hz and shutter speed of 100msec and also we monitor oxygen partial 

pressure in vacuum chamber by the zirconia based solid electrolyte sensor. The surface 

oscillation of levitated liquid droplets was observed from the top view by high-speed camera 

with the frame rate of 250kHz by the recording system of the flash solid-state drive (SSD) 

system. For electromagnetic levitation experiments under microgravity, TEMPUS facilities have 

been used for long time in European research group [5]. TEMPUS has two coils system for 

separation between heating and positioning. Thus, since the magnetic field shape is different in 

the one coil system used in ground experiments, we cannot identify the effect of electromagnetic 

force on modification of liquid droplet shape. From the above reason, we adopted the one coil 

system same as the ground experiments. Using one coil system, we can levitate a sample before 

microgravity conditions during parabolic flight, so the samples can be melted just at the start of 

microgravity conditions. However, for the one coil system, weak electromagnetic force applies 

42
to liquid droplet even in microgravity conditions in order to keep position of droplets, so we 

cannot reduce the electromagnetic force to completely zero. The effect of the weak 

electromagnetic force on liquid droplet oscillation, we checked the effect of electromagnetic 

force on the surface oscillation of liquid droplets before thermophysical properties measurements. 

For this purpose, we changed electrical current applying to the coil from 0 to 10A. 

Experimental Results 

 We observed surface oscillation of liquid Cu in Ar+H2 atmospheres under microgravity with 

different electromagnetic force conditions. The surface oscillation was obtained from the 

analysis of the change of liquid droplet shape with time. Figure 1 shows the result of the 

dumping of surface oscillation of liquid Cu droplet with change of electric current applying to 

levitation coil. From the results, on the large EMF case surface oscillation was long damping 

time and was not completely damped. This is due to that the EMF continuously generated the 

surface oscillation during sample levitation. For the case of long time dumping, we cannot obtain 

correct viscosity. Also, the EMF affects on the surface oscillation frequency shown in Fig.2. 

Figure 2 shows the power spectrum of surface oscillation of both conditions. From Fig.2, we 

found that the small EMF had single peak of surface oscillation but for the large EMF case multi 

peaks found and the main peak position was shifted. This means that the electromagnetic force 

by applying small EMF case did not modify the surface oscillation of liquid droplets. However, 

for the case of large EMF, the electromagnetic force modified the surface oscillation. The effects 

of gravity and electromagnetic force cause the modification of the surface oscillation of 

electromagnetically levitated liquid droplets in the ground. For the case of electromagnetically 

levitated liquid droplets under microgravity, only electromagnetic force caused for the effect of 

modification of the surface oscillation. Thus, details of the modification of surface oscillation are 

different between microgravity and ground conditions. The modification of surface oscillation of 

liquid droplets by gravity and EMF must be clarified for the preparation of ISS experiments in 

near future. Based on the conclusion we selected the levitation condition for precise 

measurement of surface oscillation for thermophysical properties measurements. Figure 3 shows 

the results of viscosity of liquid Cu measured with the change of EMF. In Fig.3, previous 

reported viscosity of liquid Cu [6] was also plotted. From the results, on the small EMF case 

viscosity values almost the same as the previous reported values. From our results, temperature 

dependence of liquid Cu viscosity was fitted by Arrhenius type formula with the activation 

energy of 25kJ/mol. From these measurements, in order to obtain correct viscosity we found the 

critial EMF conditions for keeping the sample position was below 3% of maximum EMF for 

heating. 

Fig.1 Surface oscillation dumping of liquid Cu at 1620K under microgravity conditions with 

different electric current applying to RF coil, (a) 0.2A and (b) 7A 

(a)

-12

-8

-4

0

4

8

12

0 2 4 6 8 

Amplitude, [pixel]

Time, t/sec

-12

-8

-4

0

4

8

12

0 2 4 6 8 

Amplitude, [pixel]

Time, t/sec

(b)

43
Fig.2 Power spectrum of surface oscillations of liquid Cu shown in Fig.1. 

 In order to find the affect of EMF on the surface oscillation frequency fro surface tension 

measurements, we calculated the surface tension values of liquid Cu from the surface oscillation 

frequency shown in Fig.2 with different EMF conditions. For these measurements of surface 

oscillations of liquid Cu, we controlled Po2 condition in 10-20Pa. Figure 4 shows the results of 

temperature dependence of surface tensions liquid Cu [7] with previous reported values. Figure 4 

exhibits the surface tension of liquid Cu calculated from the single frequency peak of the droplet 

under small EMF case using the Rayleigh equation [8] (■) and that calculated from the 

frequencies of the m = 0, ±1, and ±2 oscillations for the l = 2 mode of the droplet on the 

ground using the Cummings and Blackburn calibration (▲) [9]. The surface tension is also 

calculated from the frequency of the main peak (●) in the spectrum which shows three peaks as 

shown in figure 4a. Even though the droplet oscillation does not degenerate completely under 

microgravity in the EMF conditions shown in figure 8a, the main peak may correspond to the 

frequency of the l = 2 mode if it is predominant in the oscillation of the droplet. For the case, the 

surface tension is calculated from the frequencies assigned same as the m = 0, ±1, and ±2 

oscillations observed in the terrestrial experiment. The temperature dependence of surface 

tensions of liquid Cu obtained from single peak of surface oscillation agrees well with the 

literature values [10,11,12] and the surface tension decreases from about 1300mN/m to 

1200mN/m as the sample temperature rises from 1515K to 1815K. However, the calculated 

surface tension shows a large scatter when it is calculated from the frequency of the main peak in 

the frequency spectrum that also has small two additional peaks. 

Fig.3 Viscosity of liquid Cu measured under microgravity with different EMF conditions. 

44
This suggests that the main peak does not correspond to the frequency of the l = 2 mode. The 

surface oscillation of the droplet would not degenerate as long as a few additional peaks are 

observed in the frequency spectrum even if it is very small. 

When the surface tension of the droplet is calculated from the single frequency peak of the 

surface oscillation observed under the microgravity experiment, it is almost the same as that 

measured in the terrestrial experiment though it shows a small scattering. This implies that the 

Cummings and Blackburn equation can calibrate the effect of the deformation of the droplet due 

to the gravitational acceleration and the external electromagnetic force. 

Fig.4 Surface tension of molten copper calculated from the single frequency of the surface 

oscillation observed under microgravity (■) and that calculated from the frequencies of m =0 , 

±1, and ±2 oscillations under 1G (▲). The calculated surface tension from the main peak in 

the frequency spectrum which shows three peaks as shown in figure 6a is also exhibited (●). 

Discussion 

 As mentioned above, we succeeded in a significant decrease in the electromagnetic force given 

to the droplet during the viscosity and the surface tension measurements by the oscillating 

droplet method using EML under microgravity. Furthermore, the oxygen partial pressure of the 

ambient atmosphere was monitored and controlled simultaneously. The surface tension of the 

liquid Cu calculated from the single frequency peak of the surface oscillation of l = 2 mode 

observed under the microgravity condition became almost the same as that calculated from the 

frequencies of the m = 0, ±1, and ±2 oscillations on the ground. However, the surface tension 

calculated from the frequency of the single peak for the microgravity experiment showed a small 

scattering due to the incorrect estimation of the sample mass because the sample could not be 

collected after the experiment. Furthermore, the calculation result of the surface tension of liquid 

Cu under microgravity included the influence of the temperature variation of the sample due to 

the decrease in the electromagnetic force. Since a large electromagnetic force was applied to the 

sample to levitate it under the high gravity condition of 1.5G before the microgravity condition 

starts, in many cases the surface oscillation did not degenerate within the short microgravity 

period prepared by the parabolic flight of the airplane. A long microgravity experiment without 

any gravity change is expected in the International Space Station (ISS). In this case it is easy to 

1100

1200

1300

1400

1500

1600

1700

1200 1300 1400 1500 1600 1700 1800 1900

Calculated surface tension 

Temperature T, K

, mN/m

Multi peaks

Single peak

1G case

Nogi et al.

Rhee et al.

Kasama et al.

▲ Measured under 1G

● main peak under G

■ single peak under G

Melting Temp.

10)

11)

12)

45
levitate the sample in a stable state. Furthermore, there is a lot of flexibility for the design of the 

RF coil because a large electromagnetic force is not necessary to levitate the sample. As a result, 

the sample would be heated and kept at certain temperature easily. Since the sample can be 

collected after its solidification due to a sufficient experiment time, it would become easy to 

estimate the change of the sample mass during the measurement. Further investigation is 

required under a long microgravity environment in the ISS so that the oscillation behavior of the 

droplet, which is used for calculation of surface tension, can be understood more clearly. We are 

planning the microgravity experiment in the ISS as collaboration of ESA (European Space 

Agency), JAXA (Japan Aerospace Exploration Agency), and NASA (the National Aeronautics 

and Space Administration). For ISS experiments, the present status of our planning was shown 

below. Our project of thermophysical properties measurements of high-temperature liquid using 

the materials-science-laboratory electromagnetic-levitator (MSL-EML) in ISS initially focus on 

the semiconducting materials liquid, so our project name is SEMITHERM. SEMITHERM is 

concerned with the THERMophysical properties of SEMIconductors, specifically Ge, and Si-Ge. 

Conclusion 

 Using the parabolic flight levitation experimental facilities (PFLEX), the effects of Po2 and 

external EMF on surface oscillation of levitated liquid droplets were investigated for the 

measurements of surface tension and viscosity of liquids Cu. The surface oscillations of 

levitated liquid Cu using PFLEX on board flight experiments by G-II were observed in Ar+H2

and pure Ar gas atmospheric conditions. From these observations, we obtained the density, the 

viscosity and the surface tension values as same as reported data, and also obtained the 

difference of surface oscillations with the change of the Po2. For these thermophysical 

properties measurements, we investigated the effect of EMF on the surface oscillations in 

controlled Po2 conditions. This clarified the effect of EMF on the surface oscillation of levitated 

under microgravity. This is different from the ground conditions. Therefore, if large EMF 

applying cases, we need modified the surface oscillation analysis method from the present 

method based on the Cummings and Blackburn analysis. We future more systematically perform 

precise measurements surface oscillation in different Po2 conditions. On the basis of present 

experimental results, we are discussing about ISS experiments of MSL-EML with controlled 

atmospheric Po2 conditions with Investigator Working-Grope of EML (IWG-EML) in order to 

progress high temperature liquid science. 

Acknowledgments 

This work was performed in JAXA research-working group of “Measurement of Oxygen 

Partial Pressure Dependence of Surface Tension for High Temperature Melts under 

Microgravity”, and also supported by SENTAN, JST. We appreciate these supports for this 

work. 

References 

[1] Egry, I., Lohoefer, G. and Jacobs, G., Phys. Rev. Lett., 75(1995) pp. 4043- 4046. 

[2] Mito, M., Tsukada, T., Hozawa, M, Yokoyama, C., Li, Y-R., and Imaishi, N., Meas. Sci. 

Technol., 16 (2005) pp.457-466. 

[3] Lee, J., Tanaka, T., Yamamoto, M. and Hara, S., Materials Tran., 45 (2004) pp.625-629. 

[4] Lee, J., Tanaka, T., Asano, Y. and Hara, S., Materials Trans., 45 (2004) pp.2719-2722. 

[5] Fecht, H.-J., Wunderlich, R., Battezzati, L., Etay,J., Ricci, E., Seetharaman, S., Egry, I., 

Europhysics News, 39 (2008) pp.19-21. 

46
[6] Sato,Y et al., Proc.24th Jpn. Symp. Thermophysical. Properties, 24 (2003) pp.68-71. 

[7] Load Rayleigh, Proc. Roy. Soc. London, 29 (1879) pp.71-97. 

[8] Ozawa, S., Watanabe, M., Kiyamura, K., Morohoshi, K., Aoyagi, T., Tanno, M., Matsumoto, 

T., Adachi, M., Mizuno, A., Fujii, H., and Hibiya, T., J. Jpn. Soc. Microgravity Appl., 27 

(2010)pp.215-219 

[9] Cummings, D. L., and Blackburn, D. A., J. Fluid Mech., 224 (1991) pp.395-416. 

[10] Kasama, K, Iida, T., and Morita, Z., J. Jpn. Inst. of Metals, 16 (1976) pp.787-790 (in 

Japanese) 

[11] Rhee, S. R., J. American Ceramic Soc., 53 (1970) pp.639-642. 

[12] Nogi, K., Oishi, K., Ogino, K., J. Jpn. Inst. of Metals, 52 (1988) pp.72 -75 (in Japanese). 

[13] Ozawa, S., Koda, T., Adachi, M., Morohoshi, K., Watanabe, M., and Hibiya, y., J. Appl. 

Phys., 106 (2009) 034907-1. 

47
MICROSTRUCTURE FORMATIONS IN THE TWO PHASE REGION OF 

THE BINARY PERITECTIC ORGANIC SYTEM TRIS-NPG 

Johann Mogeritsch, Andreas Ludwig 

University of Leoben, Department Metallurgy, 

Franz-Josef-Strasse 18, 8200 Leoben, Austria 

Keywords: Peritectic solidification, in-situ observations, microgravity 

Abstract 

In order to prepare for an onboard experiment on the International Space Station (ISS), systematic directional solidification experiments with transparent hypoperitectic alloys were carried out 

at different solidification rates around the critical velocity for morphological stability of both 

solid phases. The investigations were done in the peritectic region of the binary transparent organic TRIS-NPG system where the formation of layered structures is expected to occur. The 

transparent appearance of the liquid and solid phase enables real time observations of the dynamic of pattern formation during solidification. The investigations show that frequently occurring nucleation events govern the peritectic solidification morphology which occurs at the limit 

of morphological stability. As a consequence, banded structures lead to coupled growth even if 

the lateral growth is much faster compared to the growth in pulling direction. 

Introduction 

Experimental investigations under reduced gravity condition lead to a better understanding of the 

influence of gravitational related phenomena as natural convection and sedimentation on the solidification morphologies of peritectic alloys. For an alloy of peritectic composition the liquid 

[L], the primary solid [α] phase, and the peritectic [β] phase are in equilibrium at the peritectic 

temperature, Tp. The focus of the present investigation is put on the solidification morphologies 

at growth rates where one or both phases might reveal a planar solid/liquid interface. Such morphologies reveal a complex dynamic which is still not fully understood in detail. 

Trivedi [1] published a conceptual model which predicts cyclic nucleation and overgrowth under 

purely diffusive growth conditions in the hypoperitectic region where both [α] and [β] phases 

would grow independently as planar fronts. Evidence for such bands were found in experiments 

where convection is suppressed by using thin samples [2,3]. Hunzinger et al. [4] developed a microstructure map with the nucleation and constitutional undercooling criterion under the assumption of infinitely high nuclei density and steady-state growth. Chalmers [5] predicted the simultaneous growth of both phases in form of lamellae or fibers and Lo et al. [6] show by simulation 

that bands are formed only for approximately equal volume fraction of the two phases otherwise 

islands are formed. 

Directionally solidification experiments with metal alloys were carried out and published in [2, 

3,7-17], but in-situ observations of peritectic solidification with transparent organic compounds 

were done recently only by the authors. Organic alloys with a high temperature non-faceted 

phase solidify in a metal-like manner and thus enable the detailed observation of the dynamic of 

the solid/liquid interface morphology during solidification. The transparent non-facetted/non48
facetted peritectic system TRIS–NPG [18,19] was selected for in-situ observations of solidification experiments with near peritectic compositions. The peritectic region of the TRIS-NPG phase 

diagram is shown in Fig. 1. Preliminary studies of the phase diagram show a thermal sensibility 

of the organic substance TRIS at temperatures close to the melting point which constrains the 

experimental process conditions. The diffusion coefficient in the liquid within the peritectic region could be measured as DL ≈ 1.5⋅10-11 m2

/s. 

The observation was realized with a micro Bridgman-furnace and a light microscope in combination with a CCD camera. For the selected process conditions growth competition between the 

primary phase and the peritectic phase is expected and the growth of discrete bands are predicted 

[20-23]. Our experimental results show oscillatory and isothermal coupled growth morphologies 

within the peritectic region, published and discussed in details in [24-27]. 

In the present paper, we report the results of investigations for a hypoperitectic alloy with x = 0.5 

mol fraction NPG at a fixed temperature gradient and variable solidification rates. Furthermore, 

the impact of different initial solid/liquid interfaces morphologies on the dynamic of peritectic 

solidification near the morphological stability limit of both phases was evaluated. 

hyperperitectic

 region

hypoperitectic

 region

Cα CL NPG

TP

Cp

[α] [α+β] [β]

[L+β]

[L]

[L+α]

Temperature 

x

peritectic region

TRIS

Figure 1. Detail of the TRIS –NPG phase diagram. At the peritectic temperature Tp and the peritectic concentration Cp the primary [α] phase, the 

peritectic [β] phase, and the liquid phase [L] are in equilibrium. 

Experimental procedure 

The organic substances TRIS and NPG were delivered as powder from Adrich [28] with an indicated purity of 99% and 99.9+%, respectively. Both substances are highly hydroscopic [29], 

whereby, the water content of the organic substance NPG was reduced by a drying process at 

310 K for 24 hours. TRIS, sensitive to long time annealing and delivered with high-purity, was 

used without further purification. All operations were done in an argon atmosphere within a 

glove box. 

49
The alloy preparation for the hypoperitectic concentration was done by mixing the powder of 

both organic substances in a glass container. Subsequently, the powder was heated up to the liquid state and cooling down to room temperature and storied for further use. 

Extra large but thin rectangle glass samples (0.1 x 2.0 mm2 inner dimensions with 100 μm wall 

thickness) were filled with liquid alloy by capillary force on a heater within the glove box. The 

sample itself has a length of more than 20 cm and 4 cm observation length can be used within the 

micro Bridgman-furnace. The movement of the glass sample through the furnace is PCcontrolled which allows the application of different cooling rates. 

The furnace itself consists of a heating and a cooling zone departed by a small adiabatic observation gap of 3.6 ± 0.1mm, see Fig. 2. 

Figure 2: Sketch of the micro Bridgman-furnace. 

observation gap 

hot brass block 

cold brass block 

upper ceramic plate 

pulling direction illumination 

cold zone 

thermo sensors 

hot zone 

bottom ceramic plate 

sample ZEISS microscope 

The hot parts of the Bridgman-furnace are controlled by electrical resistant heaters and the cold 

parts are cooled by circulating water. The temperatures of the hot zone were measured with Pt 

100 temperature sensors placed inside each brass block and regulated independently by a EUROTHERM 2408 controller [30] with an accuracy of ± 0.1 K. The temperature of the cold zone 

was measured with a Pt 100 temperature sensor placed between the brass blocks and regulated 

by a water cooling system with an accuracy of ± 0.5 K measured within the water tank of the 

pump. A slot of 0.4 x 2.5 mm2

 is milled in the hot and cold brass block of the bottom plate to 

host the sample. 

The sample are illuminated from the top and observed from the bottom with the used ZEISS microscope. The construction of the Bridgman-furnace enables a field of view of 3180 x 2760 μm2

. 

50
For the optical investigations, the microscope was equipped with a 1⁄2” Pulnix TMC-6700 digital 

camera with a resolution of 648 x 484 pixels. During solidification the taken images were recorded simultaneously with relevant temperatures from the furnace. 

For the experiments, the micro Bridgman-furnace is preheated to bring the entire system, furnace 

and microscope, in a thermally equilibrated stage. After 1 hour the desired temperature gradient 

of GT = 14.5 K/mm in the furnace is stable and a sample is placed into the furnace. The sample 

was held at least 30 minutes to prepare a planar solid/liquid interface. Now, the sample is ready 

for the solidification experiment and the optical system is started to record pictures and temperatures. Solidification experiments were carried out in two different ways: (i) at a constant pulling 

rate close to the constitutional undercooling limit with a corresponding concentration profile in 

front of the planar solid/liquid interface, or (ii) the sample was moved with a high pulling rate 

above the morphological stability limit (here Vp = 115 μm/s) for 30 seconds to create localized 

concentration fields around dendrites tips. Afterwards, the sample was moved with a pulling rate 

clearly below the morphological stability limit (here Vp ≤ 0.89 μm/s) through the furnace. 

Results and discussion 

The investigations of a hypoperitectic alloy with x = 0.5 mol fraction NPG with solidification 

rates from 1.28 μm/s to 0.06 μm/s and two different initial solid/liquid interfaces show a wide 

range of solidification morphologies. We are able to observe simultaneous growth of [α] and [β]

phases in form of an oscillating coupled growth and isothermal coupled growth. The results are 

published and discussed in [25, 27]. 

(a) (b) (c) (d) 

 t = 0 min t = 10 min t = 12 min t = 25 min 

Figure 3. Development of layered solidification structure. 

600 μm 

[β]

[α] 

[β] [β] 

[L] 

[α] 

[β] 

In some cases only the change from [α] planar growth to [β] planar growth or vice versa was observed. In other cases we found the formation of layers by the mechanism shown in Fig.3a-d. 

51
(a) (b) (c) (d) 

 t = 0 min t = 34 min t = 55 min t = 70 min 

(e) (f) (g) (h) 

 t = 72 min t = 82 min t = 87 min t = 89 min 

 

(i) (j) (k) (l) 

 t = 132 min t = 177 min t = 222 min t = 257 min 

Figure 4. Evolution of the solidification morphology from a planar solid/liquid 

interface to an isothermal coupled growth. Note that the dark and bright grey 

shadows visible in Fig. 4i-k result from an uneven sample illumination. 

600 μm 

[β]

300 μm 

[α]

[L] 

[[ββ]]

[α]

[L] 

[β]

800 μm 

Here, the solidification starts with a planar interface of the [β] phase and the formation of an [α] 

band along the solid/liquid interface (Fig. 3a). Now, the solid/liquid interface goes to a lower 

temperature and became unstable (Fig. 3b). In this moment, the [β] phase nucleates multiple on 

52
the liquid/[α] interface and start to propagate as dendrites (Fig. 3c). As soon as the dendrite tips 

reach the solidus line of the [β] phase the solidification morphology transforms to a planar 

solid/liquid interface, see Fig. 3d. The cycle from Fig. 3b to Fig. 3d was observed 3 times with 

different time intervals in between. 

0.44 0.46 0.48 0.50 0.52 0.54 0.56

404

406

408

410

412

414

416

TP

c[L] c[β]

TRIS NPG

[α] [α+β] [β]

[L+β]

[L]

[L+α]

c[α]

hypoperitectic hyperperitectic

Temperature [K]

x

0.44 0.46 0.48 0.50 0.52 0.54 0.56

0.0

5.0x1010

1.0x1011

1.5x1011

2.0x1011

2.5x1011

G/V [K s/m2

]

x

 planar

 only 1 band

 dendrites

 bandes

isothermal PCG

 oscillation

 coupled growth

 

Figure 5: Peritectic region of the phase diagram and calculated microstructures depending on the applied G/V and the mol fraction x as full lines. The 

solidification morphologies for a hypoperitectic concentration are given as 

symbols. Experiments with a initial planar front are in the left column and 

results with a dendritic/cellular initial interface are on the right one. 

Another form of layered solidification structures is shown in Fig. 4. Here, the initial solid/liquid 

interface is dendritic/cellular. During the fast movement of the sample the [α] phase nucleates on 

the wall or on the wall-solid-liquid junction, see Fig 4a. Following this, the pulling rate is 

changed from fast movement with Vp = 115 μm/s to a slow movement with Vp = 0.13 μm/s. 

53
Within the first 10 minutes the interface changes into a planar solidification morphology. The [α]

phase, visible on the left corner of the picture in Fig. 4a, grows laterally with Vα = 0.34 ± 0.05 

μm/s along the solid/liquid interface of the [β] phase, see Fig. 4a-c. The entire [β] phase is overgrown in Fig. 4d and only the [α] phase grows further on. After the growth of an [α] phase band 

of 133 ± 22 μm the [β] phase nucleates on the solid/liquid/wall junction and starts to grow from 

the right side to the left side of the sample with Vβ = 1.11 ± 0.1 μm/s (see Fig. 4e-f). Whereby, 

the [β] phase is overgrown by the [α] phase, with Vα = 0.69 ± 0.1 μm, before the [β] phase 

reaches the other side of the sample, as visible in Fig. 4g-h. Only a section with a maximum wide 

of 166 ± 22 μm is created. The [α] phase grows stable for further 200 ± 22 μm before the [β]

phase nucleates again. Now, nucleation takes place on the solid/liquid interface with a distance 

between the nucleation events of 500 ± 120 μm (Fig. 4i). The [β] phase overgrows with 

Vβ = 0.46 ± 0.05 μm the [α] phase along the entire solid/liquid length and creates a new [β]

phase band with a maximum of 535 ± 20 μm and a minimum of 20 μm. Further on, the [α] phase 

nucleates at the solid/liquid interface of the [β] phase (Fig. 4j) and starts to grow laterally with 

Vα = 0.51 ± 0.05 μm. Before the [α] phase overgrows the [β] phase multiple nucleation events 

happen on the solid/liquid interface (Fig. 4k). The distance between the nucleation events is now 

gradually reduced and the solidification morphology changes to isothermal coupled growth. 

It is important to notice that all lateral growth velocities are faster than the solidification velocity 

in pulling direction. Further more, the small thickness of the sample suppresses convection. Nevertheless, only parts of bands were grown. Instead, the layered structure of bands changes to isothermal peritectic coupled growth [22]. It seems that in the present NPG-TRIS system nucleation 

dominates against lateral growth. The results of our investigations for a hypoperitectic concentration with x = 0.5 are gathered in the microstructure map shown in Fig. 5. For a better visualization experiments with an initial planar front are put on the left side and the ones with an initial 

dendritic/cellular interface are put on the right side. 

Conclusions 

We have investigated the relationship between solidification velocity and the solid/liquid interface morphology during solidification of an alloy of hypoperitectic composition at x = 0.5. Experiments were carried out in a micro Bridgman-furnace at different solidification rates and a 

constant temperature gradient. For pulling rates above the morphology stability limit of both 

solid phases we found an oscillatory solidification dynamic of dendritic/cellular kind [25]. For 

pulling rates below these limits we observed (i) a change of planar growth from on to the other 

solid phase, (ii) isothermal coupled growth, and (iii) lateral bands which finally also turn into 

isothermal coupled growth. The results show no significant dependence on whether the initial 

solid/liquid interface was planar or dendritic/cellular. Although we found that the lateral growth 

of the respective other phase is much faster than the forwards solidification rate, multiple nucleation promote the formation of isothermal couple growth. 

Acknowledgements 

This work was supported in part by the European Space Agency ESA and in part by the Austrian 

Space Agency ASA through means of the ESA map project METCOMP. 

54
References 

[1] R. Trivedi, Metall. Mater. Trans., 26A (1995) 583-90. 

[2] J.S. Park, R. Trivedi, J. Cryst. Growth, (1998) pp. 511. 

[3] H. Yasuda, I. Ohnaka, K. Tokieda, Solidification processing. University of Sheffield 1997, 

44-8. 

[4] O. Hunziker, M. Vandyoussefi, W. Kurz, Acta Mater, 46 (1998) 6325-36. 

[5] B. Chalmers, Physical Metallurgy, New York Wiley (1959) 

[6] T.S. Lo, A. Karma, M. Plapp, Phys Rev E, 63 (2001) pp. 031504. 

[7] Y. Li, S.C. Ng, H. Jones, Scripta Mater., 39 (1998) 7-11. 

[8] P. Busse, F. Meissen, Scripta Mater., 36 (1997) 653-8. 

[9] L.S. Luo, Y.Q. Su, J.J. Guo, X.Z. Li, S.M. Li, H. Zhong, L. Liu, H.Z. Fu, J. Alloys Comp., 

461 (2008) 121-7. 

[10] Y.Q. Su, L.S. Luo, X.Z. Li, J.J. Guo, H.M. Yang, H.Z. Fu, Appl. Phys. Lett., 89 (2006) 

2319181-3. 

[11] L.S. Luo, Y.Q. Su, X.Z. Li, J.J. Guo, H.M. Yang, H.Z. Fu, Appl. Phys. Lett., 92, (2008) 

0619031-3. 

[12] T.S. Lo, S. Dobler, M. Plapp, A. Karma, W. Kurz, Acta Mater., 51 (2003) 599-611. 

[13] S. Dobler, T.S. Lo, M. Plapp, A. Karma, W. Kurz, Acta Mater., 52 (2004) 2795-808. 

[14] J.H. Lee, J.D. Verhoeven, J. Cryst. Growth, 144 (1994) 353-66. 

[15] H.W. Kerr, W.Kurz, Int. Mater. Rev., 41 (1996) 129-64. 

[16] W.Luo, J.Shen, Z.Min, H.Fu, J. Cryst. Growth, 24 (2008) 5441-6. 

[17] W.Z. Luo, J. Shen, Z.X. Min, H.Z. Fu, Mater. Lett., 63 (2009) 1419-21. 

[18] L. Sturz, V.T. Witusiewicz, U. Hecht, S.Rex, J. Crystal Growth, 270 (2004) 273-82. 

[19] M. Barrio, D.O. Lopez, J.L. Tamarit, P. Negrier, Y .Haget, J. Solid State Chem., 124 (1996) 

29-38. 

[20] W.J. Boettinger, Metall.Trans., 5 (1974) 2023-31. 

[21] R. Trivedi, J.S. Park, J. Cryst. Growth, 235 (2002) 572-88. 

[22] F. Kohler, L. Germond, J.D. Wagniére, M.Rappaz, Acta mater, 57 (2009) 56-68. 

[23] R. Trivedi, Scripta Mat.53 (2005) 47-52. 

[24] J. Mogeritsch, A. Ludwig, S. Eck, M. Grasser, B. McKay, Scripta Mater., 60 (2009) 882-5. 

[25] A. Ludwig, J. Mogeritsch, M. Grasser, Trans. Indian Inst. Met., 62 (2009), 433-6 

[26] J. Mogeritsch, S. Eck, M. Grasser, A. Ludwig, Mater. Sci. Forum, 649 (2009) 159-64. 

[27] J. Mogeritsch, A. Ludwig, John Hunt International Symposium, London (2011), in print. 

[28] http://www.sigmaaldrich.com (access date: 30.10.2011). 

[29] NPG CAS-No 126-30-7 and TRIS CAS-No 77-86-1. 

[30] http://www.eurotherm.com (access date: 30.10.2011). 

55
ISS-EXPERIMENTS OF COLUMNAR-TO-EQUIAXED TRANSITION IN 

SOLIDIFICATION PROCESSING

Laszlo Sturz1; Gerhard Zimmermann1; Charles-Andre Gandin2; Bernard Billia3; Nathalie 

Mangelinck3

; Henry Nguyen-Thi3; David John Browne4; Wajira U. Mirihanage4; Daniela Voss5; 

Christoph Beckermann6; Alain Karma7

1

Access e.V.; 

2

MINES ParisTech CEMEF Sophia Antipolis; 

3

Université P. Cézanne, Marseille; 

4

University College Dublin; 

5

European Space Agency - ESA/ESTEC; 

6

University of Iowa, USA; 

7

Northeastern University, USA;

Keywords: Solidification, Microgravity, Grain structure 

Abstract 

The main topic of the research project CETSOL in the framework of the Microgravity 

Application Promotion (MAP) programme of the European Space Agency (ESA) is the 

investigation of the transition from columnar to equiaxed grain growth during solidification. 

Microgravity environment allows for suppression of buoyancy-driven melt flow and for growth 

of equiaxed grains free of sedimentation and buoyancy effects. This contribution will present 

first experimental results obtained in microgravity using hypo-eutectic AlSi alloys in the 

Materials Science Laboratory (MSL) on-board the International Space Station (ISS). The 

analysis of the experiments confirms the existence of a columnar to equiaxed transition, 

especially in the refined alloy. Temperature evolution and grain structure analysis provide 

critical values for the position, the temperature gradient and the solidification velocity at the 

columnar to equiaxed transition. These data will be used to improve modeling of solidification 

microstructures and grain structure on different lengths scales. 

Introduction 

Casting of metallic alloys often results in a structure which consists of several dendritic grains. 

This grain structures is the result of a competition between the growth of several arrays of 

dendrites that develop under constrained and unconstrained conditions. In case of unidirectional 

solidification typically columnar dendritic grains exist. At high cooling rates or low temperature 

gradients nucleation and growth of equiaxed grains in the undercooled melt may occur. The 

often observed change in grain structure is described as a columnar-to-equiaxed transition (CET) 

[1-4]. 

The effect of CET was intensively investigated in the last decades because it’s of high relevance 

in industrial application. Therefore, several computational models of the CET were developed. 

Volume-averaged multi-phase/-scale models [5-8] calculate the transport phenomena on the 

scale of an entire casting. Here, the CET can be determined based on the volume fractions of 

columnar and equiaxed grains. Meso-scale models track the growth of the envelope of each 

individual grain. Examples are the cellular-automaton finite-element (CAFE) model [9] or the 

front-tracking models [10-12]. Micro-scale models and phase field models resolve details of the 

56
solid-liquid interface and, thus, do not require a separate expression for dendrite tip growth or a 

criterion for the CET [13, 14]. In spite of the fact that these numerical models are able to 

describe the CET quite well, there are some shortcomings in the modeling studies. Especially the 

treatment of nucleation and growth of equiaxed grains is the purpose of numerical studies. In the 

presence of a gravitational field, settling or floatation of solid particles in the melt and their 

interaction with the development of the columnar grain structure as well as natural convection of 

the melt itself must be taken into account in order to predict the grain structure of a casting. 

Here, a microgravity environment allows for suppression of buoyancy-driven melt flow and so 

for growth of equiaxed grains free of sedimentation and buoyancy effects. Therefore, 

experiments in microgravity provide unique data for testing fundamental theories of grain 

structure formation. To carry out such experiments and to model the process of columnar to 

equiaxed transition is the topic of the research project Columnar-to-Equiaxed Transition in 

SOLidification Processing (CETSOL) in the framework of the Microgravity Application 

Promotion (MAP) programme of the European Space Agency (ESA). Some results of 

experiments performed in microgravity on the International Space Station (ISS) are shown in the 

paper. 

Set-up of the microgravity experiments 

To investigate the columnar-to-equiaxed transition under diffusive conditions for heat and mass 

transport, experiments in microgravity were performed in the Materials Science Laboratory 

(MSL) with the Low Gradient Furnace (LGF) module onboard the International Space Station 

(ISS). For a first batch, six samples from Al-7wt%Si alloy (refined and non-refined) were 

processed. Rod-like samples of diameter 7.8mm and length 245.0mm were integrated in 

tantalum cartridges, which were also equipped with 12 thermocouples to measure the axial 

temperature distribution along the sample. Figure 1 shows such a fully integrated MSL-SCA 

cartridge. More details are given in [15]. For sample processing the MSL-SCA set-up is inserted 

into the furnace, which consists of a ‘cold zone’ with 3 heaters, a ‘hot zone’ with 4 heaters, 

separated by an “adiabatic zone”. By controlling the temperatures of the cold and the hot zone a 

temperature gradient along the sample axis is applied. Melting or solidification of the metallic 

alloy is realized by a movement of the furnace insert along the axis of the fixed sample with a 

defined speed. 

Figure 1: Fully integrated MSL-SCA cartridge to be processed in the MSL-LGF furnace on ISS. 

57
The first phases of the experimental procedure were identical for all six samples FM#1 to FM#6 

and result in an initial temperature gradient in the sample of 0.9K/mm. In samples FM#3 and 

FM#4 non-refined binary Al-7wt%Si alloys were used, while all other samples contain 0.5wt% 

of TiB2 for grain refinement (AlSi7 + g.r.). The main process parameters are summarized in 

Table 1. The experiments differ mainly in the homogenization time tH and in the parameters for 

the solidification phase 2. In this phase a transition from columnar dendritic growth to equiaxed 

growth (CET) should be triggered either by increasing the furnace velocity to v2=200μm/s or by 

decreasing the temperature gradient [15]. 

Table 1: Process parameters of the CETSOL1 flight experiments 

Sample 

No. 

Alloy Homoge

nization 

time

tH

(min) 

Solidification 

phase 1 

Solidification 

phase 2 

Fast 

movement 

v1

(μm/s) 

z1

(mm) 

v2

(μm/s) 

z2

(mm) 

dT/dt 

(K/min) 

v3

(μm/s) 

FM#1 AlSi7+g.r. 10 10 20 200 50 -4 3000 

FM#2 AlSi7+g.r. 300 10 20 200 50 -4 3000 

FM#3 AlSi7 300 10 20 200 50 -4 3000 

FM#4 AlSi7 300 10 20 10 20 -4 3000 

FM#5 AlSi7+g.r. 10 10 20 10 20 -4 3000 

FM#6 AlSi7+g.r. 300 10 20 10 20 -4 3000 

Grain structure evaluation 

As an example, Figure 2 shows the processed sample CETSOL1 FM#2. The maximum melting 

position at z=68mm can be identified, which separates the non-molten region and the mushy 

zone region. The adjacent solidification phases are also shown qualitatively (see also Table 1). 

Figure 2: Image of the processed sample CETSOL1 FM#2 with indication of the different 

regions related to the solidification process (top) and markings for metallographic preparation of 

longitudinal cross-sections (below). 

58
For analyzing the microstructure and the grain structure the samples have to be sectioned. First, 

the samples were cut into pieces of length 30mm to analyze the transversal cross-sections. 

Second, each of these pieces was sawed along the axis to get two halves for analysis of the 

longitudinal cross-sections L1 to L6 (see also Figure 2). 

To determine the microstructure the samples were polished, slightly etched and observed with a 

microscope. Figure 3 shows the longitudinal cross-section L4 of FM#2 sample. The length of 

the cross-section is 30mm, the position values are referred to the non-molten end of each sample. 

The left part of the cross-section shows a structure with rather large primary dendrites which 

corresponds to the solidification phase 1 with furnace velocity v1=10μm/s. The increase in 

furnace velocity to v2=200μm/s results in the development of a much finer dendritic 

microstructure. 

To identify the grain structure qualitatively the cross-sections were electrolytically etched and 

analyzed in a polarized light microscope. Then, different colors represent different 

crystallographic orientations of the dendritic grains [16]. As a result, in cross-section L4 of 

sample FM#2 two large columnar grains can be identified on the left side (Figure 4). The 

increase of the furnace velocity from v1=10μm/s to v2=200μm/s results in the development of 

many smaller grains with different orientations. 

Figure 3: Microstructure in the longitudinal cross-section L4 (z =120mm to 150mm) of FM#2 

flight sample; direction of solidification from left to right.

59
Figure 4: Grain structure in cross-section L4 (z =120mm to 150mm) of FM#2 sample obtained 

from electrolytical etching. 

For quantitative evaluation of the grain structure the cross-section was vibratory polished for 

many hours. This allows for a surface which is exceptional flat and nearly free of stress. Grain 

orientation measurements were performed in a scanning electron microscope using an electron 

backscatter diffraction device (SEM-EBSD) [17]. The size of the field of view is typically 4mm 

x 8mm. From 8 subsequent measured fields of view the grain structure in the cross-sections L4 

of FM#2 sample was analysed quantitatively. For each grain the fibre direction (pole 100) in 

transverse direction was determined. Related to the rod-like sample this direction corresponds to 

the sample axis. Figure 5 shows the resulting grain structure in cross-sections L4 of FM#2 

sample. The grey level indicates the deviation of the crystallographic axis of each grain from the 

sample axis. Black colour corresponds to 0° deviation angle and white colour to the maximum 

deviation angle of 54.74°. The different grey levels indicate a variety of grains with different 

orientations, which is characteristic for equiaxed grain growth. 

Figure 5: Grain structure in cross-sections L4 (z =120mm to 150mm) of FM#2 sample obtained 

from EBSD measurements with crystallographic orientation related to the sample axis (white: 

deviation angle 0°; black: maximum deviation angle 54.74°). 

Determination of CET 

For determination of the columnar-to-equiaxed transition in FM#2 flight sample the grain 

structure was evaluated quantitatively from the electrolytically etched longitudinal cross-sections 

L3 to L5 (Figure 6). The grain sizes were measured using digital image analysis. Averaging was 

performed over all grains being totally or partly within a sheet of ±2mm around the actual 

position. The maximum value corresponds to columnar growth of just two large grains in the 

first solidification phase. Equiaxed growth is characterized by a significant decrease of the 

average grain size. Thus, based on this criterion and this type of evaluation, the region of CET is 

determined and is centered at z=126±2mm. 

The critical parameters for CET, i.e. a critical temperature gradient of about Gc=0.75K/mm and a 

critical velocity of the isotherm of about vc=87μm/s, were determined from the temperature 

measurements along the sample axis and were already given in [15]. 

60
Figure 6: Grain structure identified from longitudinal cross-sections L3 to L5 (z =90mm to 

180mm) of FM#2 sample (below) and determination of averaged grain size (top). 

Conclusions 

This paper reports on results obtained during metallic alloy solidification experiments onboard 

the International Space Station in the Materials Science Laboratory using the Low Gradient 

Furnace module. Within a first batch six experiments with Al-7wt%Si alloy were performed 

successfully to investigate columnar-to-equiaxed (CET) solidification behaviour in microgravity. 

Qualitative and quantitative grain structure analysis using electrolytical etching and EBSDtechnique show that CET is observed in the grain refined samples. Here, the microgravity 

environment allows for pure diffusive conditions for heat and mass transfer in the melt and 

therefore for investigation of CET without buoyancy convection and sedimentation of equiaxed 

grains in the melt. The critical parameters for CET were determined from analysis of thermal 

data and taking into account the grain structure. These data basis will be used for calibration and 

further development of numerical model predicting CET. 

Acknowledgments 

This work was conducted within the ESA MAP programme ‘CETSOL’, the ESA PRODEX 

programme, and also funded by the German Space Agency DLR and the French Space Agency 

CNES. 

References 

1. J.D. Hunt, ”A numerical analysis of time dependent isolated dendritic growth for conditions 

61
near the steady state,” Acta Metall. Mater., 38 (1990), 411-418. 

2. Ch.-A. Gandin, ”From constrained to unconstrained growth during directional solidification,” 

Acta Materialia 48 (2000), 2483-2501. 

3. D.J. Browne and J.D. Hunt, ”A fixed grid front-tracking model of the growth of a columnar 

front and an equiaxed grain during solidification of an alloy,” Numerical Heat Transfer, Part B: 

Fundamentals 45 (2004), 395-419. 

4. L. Sturz and G. Zimmermann, ”Investigations on Columnar-to-Equiaxed Transition in Binary 

Al Alloys with and without Grain Refiners,” Materials Science Forum 508 (2006), 419-424. 

5. C. Beckermann and C.Y. Wang, ”Multi-Phase/-Scale Modeling of Transport Phenomena in 

Alloy Solidification," Annual Review of Heat Transfer VI, ed. C.L. Tien, Begell House, New 

York, NY, (1995), 115-198. 

6. M.A. Martorano et al., ”A solutal interaction mechanism for the columnar-to-equiaxed 

transition in alloy solidification,” Metall. Mater. Trans. A, 34A (2003), 1657-1674. 

7. A. Ludwig and M. Wu, ”Modeling the columnar-to-equiaxed transition with a three-phase 

Eulerian approach,” Mater. Sci. Eng., A413-414 (2005), 109-114. 

8. A. Noeppel et al., ”Numerical modelling of columnar to equiaxed transition – application to 

microgravity experiments,” Int. Journal of Cast Metals Research 22 (2009), 34-38. 

9. Ch.-A. Gandin and M. Rappaz, ”A coupled finite element-cellular automaton model for the 

prediction of dendritic grain structures in solidification processes,” Acta Metall. Mater. 42, 

(1994), 2233-2246. 

10. I. Steinbach et al., ”Three-dimensional modeling of equiaxed dendritic growth on a 

mesoscopic scale,” Acta Mater. 47, (1999), 971-982. 

11. P. Delaleau et al., ”Mesoscopic Simulation of Dendritic Growth Observed in X-ray Video 

Microscopy During Directional Solidification of Al–Cu Alloys,” ISIJ International 50 (2010), 

1886-1894. 

12. J. Banaszek et al., ”Natural Convection and Columnar-to-Equiaxed Transition Prediction in a 

Front-Tracking Model of Alloy Solidification,” Metallurgical Materials Transact. A38 (2007), 

1476-1484. 

13. H.B. Dong and P.D. Lee, ”Simulation of the columnar-to-equiaxed transition in directionally 

solidified Al–Cu alloys,” Acta Mater. 53 (2005), 659-668. 

14. A. Badillo and C. Beckermann, ”Phase-field simulation of the columnar-to-equiaxed 

transition in alloy solidification,” Acta Mater. 54 (2006), 2015-2026. 

15. G. Zimmermann et al., “Investigation of columnar-to-equiaxed transition in solidification 

processing of AlSi alloys in microgravity – The CETSOL project”, accepted in IOP Conference 

Series Materials Science and Engineering, ISPS4.

16. E. Schaberger, F. Grote and A. Schievenbusch, “Farbätzung und Farbbildanalyse - Ein Weg 

zur Charakterisierung von Gefügen innovativer Gusswerkstoffe,“ Prakt. Metallographie 37

(2000), 419-434. 

17. A.J. Schwartz et al., Electron Backscatter Diffraction in Materials Science (Springer 

Science+Business Media, 2nd edition, 2009) 

62
Liquid droplet dynamics in gravity compensating high magnetic field

V. Bojarevics, S. Easter and K. Pericleous 

University of Greenwich, Park Row, London SE10 9LS, UK 

Keywords: Magnetic levitation, droplet oscillations, free surface dynamics, 

magnetohydrodynamics 

Abstract 

Numerical models are used to investigate behavior of liquid droplets suspended in high DC 

magnetic fields of various configurations providing microgravity-like conditions. Using a DC 

field it is possible to create conditions with laminar viscosity and heat transfer to measure 

viscosity, surface tension, electrical and thermal conductivities, and heat capacity of a liquid 

sample. The oscillations in a high DC magnetic field are quite different for an electrically 

conducting droplet, like liquid silicon or metal. The droplet behavior in a high magnetic field is 

the subject of investigation in this paper. At the high values of magnetic field some oscillation 

modes are damped quickly, while others are modified with a considerable shift of the oscillating 

droplet frequencies and the damping constants from the non-magnetic case. 

Introduction 

The electromagnetic (EM) and electrostatic levitation experiments with liquid metal droplets 

show difficulties related to confinement stability and a need for complex correction functions to 

establish a correlation between the measurements and the droplet material properties [1,2]. 

Intense internal fluid flow is visually observed, apparently being in the turbulent regime for 

earthbound conditions. The combination of AC and DC magnetic fields was recently recognized 

as an efficient tool for the thermo-physical property measurements without a contact to 

contaminating walls [3,4]. The intense AC magnetic field required to produce levitation results 

in turbulent large-scale toroidal recirculation within the droplet, which prevents accurate 

measurements. The use of a homogenous DC magnetic field allows the toroidal flow to be 

damped. However the turbulence generated in these conditions makes the effective viscosity to 

behave in a non-linear fashion depending on the DC and AC magnetic field intensity [4]. The 

flow in a typical droplet is approaching the conditions with laminar viscosity and heat transfer 

when a uniform DC magnetic field exceeds about 4-5 T. For even higher DC magnetic field the 

levitation, using para- and dia-magnetic properties of the materials, can be used for advanced 

material research [5,6]. The vertical field gradient permits to compensate gravity along the 

central axis, while the radial variation acts to centre it on the axis for stable levitation. In practice 

a small vibration of the droplet as a whole remains even in carefully conducted experiments [6]. 

Frequency measurements using the oscillating drop technique have been conducted for water 

droplets by Beaugnon et al. [5], where estimates were made as to the frequency modification due 

to the magnetic field. More recent experiments have been conducted by Hill & Eaves [6], in 

which a derivation of the frequency modifications due to the magnetic field are made and 

compared with the experimental results. 

The oscillations in a high DC magnetic field are quite different for an electrically conducting 

droplet. In a recent publication [7] an asymptotic solution for very high magnetic field shows 

damping of the even axisymmetric modes, but the odd modes are not damped or damped 

63
moderately. The asymptotic linear theory predicts a considerable shift of the oscillating droplet 

frequencies from the non-magnetic case. The droplet behavior in high magnetic field is the 

subject of investigation in this paper. Numerical models of the flow coupled with the moving 

free surface give an insight to the dynamics of levitated droplets of various sizes and magnetic 

field intensities. The realistic gradient fields, as in the solenoidal coils of superconducting 

magnets, are used for the modeling experiments with electrically non-conducting (water) and 

conducting (liquid silicon) droplets. 

Droplet oscillation numerical model 

The free surface shape of the droplet is defined by a deviation from a sphere of radius R0. The 

surface position can be represented in the spherical co-ordinates by a function S of time and the 

angular coordinates: 

rR S t s = + 1 , , 0 # $ ( ) θ φ % &. (1) 

The mode dependent oscillation frequency and damping constants are generally derived by 

following a perturbation approach. The S function can be represented as a series of spherical 

harmonics: 

( ) ( ) ( )

0 0

, , cos .

L ML

M M iM

L L

L M

S t A tP e φ θ φ θ

=∞ =

= =

=∑ ∑ (2) 

The Y0

0 = P00 mode is included to account for conservation of mass (volume) of the sphere with 

radius R0 if an initial perturbation of the surface is given. The time-dependent surface 

coefficients are assumed to be harmonic with exponential damping: 

( ) ( ) M M

L L M i t At e L

ω γ− = . (3) 

The zero order (in powers of the perturbation amplitude) result for the frequency was obtained 

by Rayleigh, and is given for a droplet with density ρ and surface tension Γ by: 

( )( ) 3

0 1 2 /( ) M ω ρ L = LL L R − + Γ . (4) 

The damping coefficient for a droplet of viscosity ν was determined by Lamb: 

( )( ) 2

0 1 2 1 / M γ ν L = LL R − + . (5) 

The zero order approximation for both the frequency and the damping constant are independent 

of the azimuthal number M. These formulae provide a good first approximation for low 

amplitude oscillations of droplets in microgravity conditions where the effects of any external 

forces used to position the droplet are negligible compared with surface tension and viscous 

forces. However it is worth noting that these formulae are restricted and are only valid in the 

limit that the amplitude of oscillation and also non-linear effects tend to zero. Modifications to 

these have been derived by Tsamopoulos & Brown [8] for ‘moderate amplitude’: the frequency 

decreases with increasing amplitude and a coupling of modes appears at the second order. 

In a recent paper Priede [7], considers the effect of a constant high intensity magnetic field on the 

oscillation frequency and damping rate of an electrically conducting drop. The magnetic field is 

shown to significantly alter the dynamics of the droplet with some interesting results. There are 

two distinct cases: the longitudinal modes when (L-M) is odd and the transversal modes when (LM) is even. The frequencies for the odd modes including the axisymmetric (M=0, of the greatest 

interest for this paper) are given by: 

( )( ) 3

0 1 2 / ( ) M ω ρ L = LL R − + Γ , (6) 

which is √L less than the normal mode frequencies (4). In the following section an outline is 

given of an axisymmetric and a 3D numerical model that will be used to investigate these effects. 

Following results are presented that demonstrate quantitatively the theoretical predictions and the 

64
deviations from the zero order asymptotic theory due to the presence of the gravity field and 

finite amplitude oscillations. 

The numerical model uses a grid point formulation of the spectral collocation method [10] with 

the Chebyshev grid for the radial direction and Fourier in the angular directions. The model uses 

a coordinate transformation for the free surface, which allows the problem to be solved on a unit 

sphere. The equations solved by the numerical model are the momentum and mass conservation 

equations with the modified pressure, 2

mod 0 cos 2 /( ) P P gR B = + ρ θχ− v μ :

( ) ( ) 1 1

mod ; 0 Tt ρν ρ P − − ∂ VV V + ⋅∇ =− ∇ + ∇⋅ ∇V V JB V +∇ + × ∇⋅ = . (7) 

The total force due to the magnetic field consists of two components, the Lorentz force due to the 

conducting properties, which is added to the momentum equations as a body force and the 

diamagnetic force, which is potential and is implemented in the model as boundary condition to 

the pressure equation along with the gravitational force, which is also potential. The full 

boundary conditions for the equations (7) relate the normal stress to the surface tension: 

e e n n ⋅ ⋅ Π = ΓK, (8) 

continuity of the velocity field and the tangential stress conditions: 

∇ ⋅V 0 = , Π 1 0 τ e e n ⋅ ⋅ = , Π 2 0 τ e e n ⋅ ⋅ = , (9) 

where 1 2 , , n τ τ ee e are the unit vectors normal and tangential to the free surface, Π is the stress 

tensor, K is the surface curvature, and is the χv volumetric magnetic susceptibility. The free 

surface shape given by (1) is updated at each time step using the kinematic condition: 

 t s ∂ r e Ve ⋅ ⋅ n n = . (10) 

The magnetic field in a solenoid is modelled as a superposition of the magnetic field generated 

by axisymmetric coil filaments of finite cross-section (see Figure 1 as an example). The 

axisymmetric magnetic field is obtained from the analytical formulae [9]. 

The stable levitation conditions can be established by adjusting the electric current in the coils to 

form a minimum in the magneto-gravitational potential: 

2

0 cos (2/ ) U gR = θχ ρ − v B μ . (11) 

The electromagnetic body force requires in general an additional equation to be solved for the 

electric potential (only in the 3D case). The electric current density is given by: 

J VB = σ φ (−∇ E + × ). (12) 

The equation for the electric potential is obtained from the charge conservation ∇ ⋅ J =0 . Taking 

the divergence of the electric current density gives: 

( ) 2 ∇ φE = VB ∇ × ⋅ . (13) 

This equation is solved subject to the condition at the instantaneous drop surface: ⋅ = 0 n J e . 

Results & Discussion 

The following section contains the results of numerical simulations made using both the 2D 

axisymmetric [10] and 3D modes compared with experimental results [6] and theoretical 

asymptotic results [7]. The material chosen for the numerical test cases is either water 

(diamagnetic, electrically non-conducting) or molten silicon (diamagnetic, electrically 

conducting) having the material property values given in the Table 1. 

65
Table 1. The material property values used for the numerical test cases. 

The numerical simulation uses the superposition of the modes L = 2,3,4,5,6,7 for axisymmetric 

surface shape, or L=2, M=1 for the 3d test as the initial condition. The amplitude coefficient is 

chosen to give a deformation of 1% of the unperturbed radius (R0 = 5mm or 10 mm). A 

spherically symmetric constant component (L=0) is also required to ensure the initial surface 

shape conserves the mass of the droplet based on the equilibrium spherical radius. 

 

Figure 1. Water droplet R0 = 10 mm in the bore of solenoid magnet and the initial velocity field 

(color contours for vertical velocity component W and the scale vector in m/s). 

Water Droplet in Solenoid Magnet 

Figure 1 shows the droplet in the solenoid magnet and the zoom-in view to the initial velocity 

field after 41 ms, generated by the axisymmetric surface deformation condition in the simulation. 

The droplet is positioned in the upper part of the magnet 8 cm above the magnet centre. We 

attempted to preserve dimensions similar to the experiment [6], however the exact design of the 

magnet is a proprietary information not available for this modelling work. The magnetic field 

and the required magneto-gravitational potential (11) are updated at each time step of maximum 

length 0.000125s. The instantaneous distributions of the magnetic field are computed only in the 

volume occupied by the droplet as shown in the Figure 2. The magnetic field at the top part of 

the solenoid is reduced in magnitude relative to the central part where it is about 16.5 T. 

Symbol Quantity Water Silicon Units 

Surface Tension Coefficient 0.0733 0.865 N m-1

Density 999.0 2510. Kg m-3

Kinematic Viscosity 1.11e-6 1.0e-6 m

2

 s

-1

Volumetric Magnetic Susceptibility -9.0e-6 -4.2e-6 N/A

66
 Figure 2. Magnetic field in T, force (N/m3

) and the potential U (m2/s2) in the water droplet R0 = 

10 mm at the top part of the solenoid magnet. (The reference vector scales in respective units.) 

 Figure 3. Oscillations of the surface top position Rt and Rt-Zc (difference to centre of mass

position), and power (relative units) spectra computed for water droplet R0 = 10 mm.

The computed oscillation pattern appears to be of similar nature as observed in the experiments 

[6], i.e., the frequency spectra peaks are slightly shifted to increase the mode frequencies relative 

to the non-magnetic Rayleigh values (4). The oscillation pattern shown in the Figure 3 indicates 

a considerable centre of mass position influence on the surface variation. The droplet position as 

a whole vibrates with a very small amplitude, typically less than 1 mm [6]. The numerical results 

in the Figure 3 give the oscillation of the surface top position difference to the centre of mass 

position: Rt-Zc (t). This function clearly shows the decay of the surface oscillation, while the Zc 

continues to move without an apparent damping. 

Figure 4 illustrates the centre of mass oscillation for a smaller size water droplet R0 = 5 mm. In 

this numerical run we additionally modified the electric current in the solenoid so that the droplet 

is stably levitated in a region of higher magnetic field (max B = 15.46 T). In this case the centre 

of mass oscillates with a small amplitude of about 0.01 mm. The surface oscillation is damped at 

different rate for various amplitudes, apparently the mode L = 3 is the dominant as the power 

spectra for this case shows (Figure 4). The other remarkable feature for this case is the 

systematic reduction of all the frequencies relative to the non-magnetic values (4) and contrary to 

the observations in [6]. The explanation for this discrepancy is the effect of the finite amplitude 

shifting the frequency to smaller values [8] as supported by our tests running the same simulation 

with B = 0 ( not shown due to space restrictions). 

67
 Figure 4. Oscillations of the surface top Rt and the centre of mass Zc, and Rt power spectra (in 

relative units) computed for the water droplet R0 = 5 mm. 

3d Water Droplet Oscillations in Solenoid Magnet 

A 3d test case generally supports the more accurate axisymmetric simulations. A numerical 

procedure is used to analyze the surface shape at each time-step in order to determine the relative 

contribution of each mode. This results in time dependent coefficient for each of the spherical 

harmonic modes. The dominant components are the L = 2, M =1 mode, which corresponds to the 

initial condition, axisymmetric modes L = 2, M = 0, and L = 3, M = 0, which are introduced by 

the external forces, and the L = 1, M = 0 translational mode. Other small components are also 

present that ensure the conservation of mass. Figure 5 shows the computed instantaneous 3d 

velocity field and the oscillations corresponding to the dominant mode with the respective power 

spectra. The theoretical value for the normal mode, derived for the magnetic modification to the 

first order [6], is indicated in the power spectra with dashed line. 

Figure 5. The 3d instantaneous velocity and pressure on the surface, and the oscillations of the 

top with the power spectra computed for the water droplet R0 = 5 mm. 

Table 2 compares the theoretical frequencies and the frequencies calculated numerically. There 

is a reasonable agreement between the theoretical and numerical values. The frequency shifts are 

small, but not insignificant when considered in the context of surface tension measurements. 

68
Y0

1 Y02 Y12 Y03

Normal Mode, see (4) N/A 1.09e+1 1.09e+1 2.11+1 

Theoretical [6] N/A 1.12e+1 1.11e+1 2.14e+1 

Numerical 1.62e+0 1.11e+1 1.11e+1 2.13e+1 

Table 2. The frequency calculations for 3d water droplet oscillations. 

Liquid Silicon Droplet in Solenoid Magnet 

An electrically conducting levitated droplet motions are expected to be damped in strong DC 

magnetic field, however some types of movement are not affected in a particular field 

configurations. In the microgravity case (g = 0) and a uniform vertical magnetic field the 

asymptotic solution [7] shows that odd axisymmetric oscillation modes are very moderately 

damped and the frequencies are reduced by a factor of √L (6). It is not immediately clear if this 

behavior will stay in the presence of terrestrial gravity and the gradient magnetic field required 

for the magnetic levitation. The numerical experiments with liquid silicon appear to support the 

general conclusions of the asymptotic solution as demonstrated by the Figure 6. The L = 3 (see 

the velocity field in the Figure 7) and the longitudinal centre of mass L = 0 modes are the only 

ones not damped immediately in the 17 T magnetic field required to levitate the 5 mm radius 

liquid silicone droplet. The frequency of the L = 3 mode is reduced about √3 times relative to the 

non-magnetic case (4) and closely matches the theoretical value [7]. The small deviation can be 

explained by the presence of the gradient part of the magnetic field. By readjusting the electric 

current in the solenoid the gradient part can be reduced at the expense of increased uniform Bz

component. When the magnitude of the field is 28 T the computed frequency is practically 

coinciding with the theoretical value to the accuracy of 0.01%. However in this case the droplet 

weight is balanced only at the initial time, then vigorous oscillations of the modes L = 3 and L = 

1 (centre of mass) follow (Figure 7), and the droplet very slowly slides downwards (Figure 8). 

The magneto-gravitational potential on the surface 

0

( ) ( ) (cos ) s nn

n

UR r U tP θ == =∑ experiences 

similar oscillations to the free surface shape as shown in the Figure 8. 

 

 Figure 6. Oscillations of the surface Rt and the centre of mass Zc, and Rt power spectra 

computed for the liquid silicon droplet R0 = 5 mm in 17 T field. 

69
 

 Figure 7. Magnetic potential, force and velocity field in the liquid silicon droplet R0 = 5 mm. 

 Figure 8. Oscillations of the surface Rt, centre of mass Zc, and the magnetogravitational 

potential expansion linear coefficient U1 for the liquid silicon droplet R0 = 5 mm in 28 T field. 

References 

[1] Egry I, Lohofer G, Seyhan I, Schneider S and Feuerbacher B 1999 “Viscosity and surface tension 

measurements in microgravity” Int. J. Thermophys. 20 (4) 1005-1015

[2] Cummings D L and Blackburn D A 1991 “Oscillations of magnetically levitated aspherical 

droplets” J. Fluid Mech. 224 395-416

[3] Kobatake H, Fukuyama H, Minato I, Tsukada T and Avaji S 2007 “Noncontact measurement of 

thermal conductivity of liquid silicon in a static magnetic field” Applied Physics Letters 90 

094102

[4] Bojarevics V, Easter S, Roy A and Pericleous K 2009 “Levitated Liquid Droplets in AC and DC 

Magnetic Field” Proc. Int. Symp.Liquid Metal Processing and Casting, Santa Fe, TMS, ed-s Lee 

P, Mitchell A, Williamson R, 319-326

[5] Beaugnon E, Fabregue D, Billy D, Nappa J and Tournier R 2001 "Dynamics of magnetically 

levitated droplets" Physica B 294-295 715-720

[6] Hill R J A and Eaves L 2010 "Vibrations of a diamagnetically levitated water droplet" Phys. Rev E 

81 (5) 056312

[7] Priede J 2010 “Oscillations of weakly viscous conducting liquid drops in a strong magnetic field” J. 

Fluid Mech. 671 399-416

[8] Tsamopoulos J A and Brown R A 1983 “Nonlinear oscillations of inviscid drops and bubbles” J. 

Fluid Mech. 127 519-537

[9] Smythe W R 1950, Static and Dynamic Electricity, McGraw-Hill, London

[10] Bojarevics V and Pericleous K 2003 “Modelling electromagnetically levitated liquid droplet 

oscillations” ISIJ International 43 (6) 890-898

70

MEASUREMENTS OF DENDRITIC GROWTH VELOCITIES IN 

UNDERCOOLED MELTS OF PURE NICKEL UNDER STATIC 

MAGNETIC FIELDS

Jianrong Gao1, Zongning Zhang1, Yingjie Zhang1

1 Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education) 

3-11 Wenhua Road; Shenyang 110004, China 

Keywords: Pure metal, Dendritic growth velocity, Fluid flow, Impurity 

Abstract 

Dendritic growth velocities in undercooled melts of pure Ni have been intensively studied over 

the past fifty years. However, the literature data are at marked variance with the prediction of the 

widely accepted model for rapid dendritic growth both at small and at large undercoolings. In the 

present work, bulk melts of pure Ni samples of high purity were undercooled by glass fluxing 

treatment under a static magnetic field. The recalescence processes of the samples at different 

undercoolings were recorded using a high-speed camera, and were modeled using a software to 

determine the dendritic growth velocities. The present data confirmed the effect of melt flow on 

dendritic growth velocities at undercoolings below 100 K. A comparison of the present data with 

previous measurements on a lower purity material suggested an effect of impurities on dendritic 

growth velocities at undercoolings larger than 200 K as well. 

Introduction 

Many studies have been carried out to measure dendritic growth velocities in undercooled melts 

for an understanding of the mechanism of dendritic growth [1]. Pure nickel has attracted constant 

attention over the past fifty years [2-16]. The main reason is because its materials properties both 

in the liquid and in the solid states are well known so that the measured growth velocities can be 

referred to for a rigorous test of physical models for rapid dendritic growth. A review of previous 

measurements on pure nickel could be found elsewhere [12,16]. The latest studies [9,15,16] 

showed reduced scatter as well as an improved agreement with the widely accepted dendritic 

growth model (known as LKT model [17]) owing to the use of a series of advanced techniques 

including those for undercooling and those for in-situ observations. However, the measured data 

were yet at marked variance with the predictions of the LKT model. A positive and a negative 

deviations were observed for the small (below 100 K) and for the large undercooling regions 

(above 200 K), respectively [9,15]. Efforts were already made to account for these discrepancies. 

Eckler et el. [8,9] first proposed two alternative explanations for the positive deviation: impurity 

and fluid flow effects. Galenko et al. [18] extended the LKT model by taking into account the 

contribution of fluid flow and showed a reduction of the discrepancy. Herlach and Galenko [19] 

further incorporated the impurity effect and explained the residual discrepancy. Using a phase 

field method, Bragard et al. [20] and Nestler et al. [21] explored effects of capillary and kinetic 

anisotropy on dendritic growth to understand the deviation in the large undercooling region. The 

former group suggested that the deviation might also be related to fluid flow or impurities in 

liquid samples. Thus, it is of great interest to determine the effects of fluid flow and impurities 

on dendritic growth velocities in undercooled Ni samples experimentally. 

72
A fluid flow effect on dendritic growth velocities has been observed for electromagnetically 

levitated samples of few intermetallic compounds under reduced gravity conditions [22-23]. 

However, no experimental data for pure nickel samples processed under similar experimental 

conditions are available. Recently, a few studies [24-26] have shown that a static magnetic field 

of few Tesla is able to damp convective flow in electromagnetically levitated samples under 

terrestrial conditions. Such advancement opens a new opportunity for investigations of fluid flow 

effects on dendritic growth velocities in undercooled metals. Very recently, the present authors 

designed a novel glass-fluxing facility, which combined an inducting heating furnace with a 

superconducting magnet. A preliminary study on nickel samples of 99.99% purity (referred to 

4N Ni below) showed evidence for a detectable effect of fluid flow on dendritic growth 

velocities in the small undercooling region [16]. In the present work, the measurements were 

extended to samples of higher purity, 99.999% purity (referred to 5N Ni below) to investigate the 

effects of fluid flow and impurities simultaneously. 

Experimental Details 

The experimental facility used for measurements of dendritic growth velocities comprised a 

vacuum induction melting furnace, a superconducting magnet, a shielding tube, an Ultima-APX

type digital high-speed camera (HSC), a METIS MS09 type single-color pyrometer, and two 

computers. The details of the facility can be found elsewhere [16]. In the present measurements, 

individual samples of 1.0 g were cut from a 5N purity Ni rod of 5 mm in diameter offered by 

Alfa Aesar. Under each condition, the measurements were made on one sample only. The cut 

samples were polished using an abrasive paper and cleaned in an ultrasonic bath for few minutes. 

Before melting, a single sample was placed onto an alumina crucible containing soda lime glass 

lumps. The crucible was positioned in a heating coil. After evacuation to a vacuum pressure of 5 

×10-3 Pa, the chamber of the facility was back-filled with argon of 99.999% purity to a pressure 

of 5 ×104

 Pa. The superconducting magnet was ramped to a magnetic field of 1.0 T. Then, the 

sample was heated and melted inductively. The sample temperature was measured using a 

single-color pyrometer at a frequency of about 100 Hz. The glass powders were fused by 

conducted heat. The sample was overheated and soaked for a few minutes. After soaking, the 

power to the coil was switched off. The sample was cooled and solidified spontaneously. A 

recalescence event took place during solidification of the undercooled sample due to rapid 

release of latent heat. For measurements of dendritic growth velocities, the recalescing sample 

surface was in-situ observed using a high-speed camera with a resolution of 256 × 32 pixels at a 

frame rate of 87600 fps. A pixel corresponded to an area of 100 μm × 100 μm. The sample was 

subjected to more than 40 melting-solidification cycles. In order to determine the nucleation site 

and therefore the growth velocity of each recalescence process, the recorded images of the 

sample surface was modeled using computer software named POV Ray 3.7 [23]. The 

recalescence front was assumed to travel through the bulk volume of the undercooled sample at a 

constant rate like a spherical wave. Thus, the three-dimensional recalescence front traveling 

through the sample volume could be reconstructed by a continuous analysis of the recorded loci 

of the recalescence front sweeping across the sample surface with the aid of the software. The 

dendritic growth velocity was calculated by dividing the total traveling distance of the threedimensional recalescence front by the consumed time in a given recalescence process. 

Results and Discussion 

Figure 1 illustrates cooling curves of a Ni sample of 5N purity. The recalescence temperature, 

i.e., the temperature reached at the end of recalescence, often showed a deficit with respect to a 

73
well-determined melting temperature of 1728 K. The deficit generally increases with increasing 

undercooling. A similar phenomenon was observed by the present authors for the Ni sample of 

4N purity [16] and by other researchers for glass-fluxed Ni-Cu single-phase alloys [27] and NiSn eutectic alloys [28]. The most probable reason for the deficit is a thermal loss of the sample to 

the crucible through the molten glass during recalescence. Other reasons include creation of 

numerous defects and stresses in the solid phase, which might reduce the energy difference 

between the solid and the liquid phases during recalescence [27]. Because of this deficit, the 

measured sample temperature was calibrated with respect to a maximum recalescence 

temperature derived from a statistical analysis of all data. 

Figure 1 Illustration of cooling curves of 5N Ni sample at different undercoolings 

Figure 2 illustrates the corresponding sequential images of the top surface of the recalescing 

sample recorded by the high-speed camera. In agreement with many previous studies (e.g. Refs. 

[12] and [15]), the observed recalescence front shows an angular morphology and a smooth 

morphology for small and large undercoolings, respectively. The transition does not correspond 

to a sharp undercooling, but occurs over a span of 15 K. The imposition of the static magnetic 

field did not produce any visible changes either in the morphology or in the transition. It is 

Figure 2 Illustration of HSC images of the 5N Ni samples at different undercoolings 

74
worthy of mentioning that the irregular morphology of the recalescence front for small 

undercoolings brought about a difficulty in modeling of the recalescence process. However, this 

difficulty could be overcome by modeling the major contour of the recalescence front instead of 

the whole front. In this sense, the derived traveling distance of the recalescence front represented 

a mean value, rather than a specific value for local regions. 

Figure 3 shows the measured dendritic growth velocities of the 5N Ni samples as a function of 

undercooling. As seen in Figure 3a, the data measured under the static magnetic field show a 

reduced scatter with respect to those measured without the static magnetic field. Apart from this 

aspect, it is hardly to see a difference between the two sets of the data in the normal-scale plot. In 

agreement with many previous studies, the present data show a power-law behavior and a linear 

law behavior for small and large undercoolings, respectively. Such a transition of the growth 

velocities, however, is sharper than the morphological transition of the recalescence front is. The 

former occurs at a well-defined undercooling of 200 K. If compared with the previous data for 

the 4N purity samples [16], one can find a striking difference in the large undercooling region. 

The velocities measured on the 5N purity samples are considerably higher than those measured 

on the 4N purity sample. Such a difference highlights that the effect of unknown impurities on 

growth velocities of undercooled nickel melts is tremendous. In the following, attention is 

focused onto the data measured at small and medium undercoolings. 

 

Figure 3 Growth velocity – undercooling relationship measured on the 5N Ni samples with and 

without imposition of a static magnetic field of 1 T. For comparison, two sets of the previous 

data measured on 4N purity samples [16] are also shown. 

Due to relatively small values in magnitude, the differences of the growth velocities in the small 

undercooling region cannot be readily distinguished on a normal-scale plot. For this reason, the 

data are re-plotted on a dual-logarithm scale. However, the data are too few to allow us to 

directly extract the effect of impurities and residual fluid flow on the growth velocities in the 

small undercooling region of high interest. Thus, the data are all truncated at 200 K and fit to 

different power laws. As seen in Fig. 3b, each power law is represented by a straight line. Two 

clear tendencies are shown by these lines. One is that the imposition of the static magnetic field 

decreases the slope of the power law fitting line independent of the sample purity. The other is 

that the improvement of sample purity increases the slope of the power law fitting line under 

identical magnetic field conditions. The first tendency means that damping of fluid flow in the 

undercooled samples lowers the dendritic growth velocities. Reversely, the introduction of fluid 

flow increases the dendritic growth velocities as expected. Such a tendency agrees reasonably 

75
with the prediction of the extended LKT model [18]. The second tendency means that the 

reduction of impurities increases the dendritic growth velocities. Reversely, the presence of the 

impurities lowers the dendritic growth velocities. This tendency, however, is in contrast to the 

theoretical prediction for a strongly partitioning impurity [8,19]. Probably the species or the 

amount of the impurity is different from that assumed in the previous studies [8,19]. 

Consequently, the reversed effect of the impurities occurs to the present samples.

It is also noted in Figure 3b that two power law fitting lines for the samples of different purities 

intersect at an undercooling of 168 K in the absence of the static magnetic field. However, the 

other lines never intersect. This difference between the fitting lines implies that the lower purity 

sample can reach a growth velocity comparable to that of the higher purity sample with the aid of 

fluid flow, but never can reach the same velocity without the aid of fluid flow. For the higher 

purity 5N sample, the two power law fitting lines intersect at an undercooling of 145 K. In 

contrast, the two power law fitting lines of the lower purity 4N sample intersect at an 

undercooling of 405 K, where the power laws do not apply any longer. Such a difference 

suggests that the fluid flow effect becomes negligible at a medium undercooling for the higher 

purity 5N sample, but does not vanish for the lower purity 4N samples for all accessible 

undercoolings. 

In order to produce more insight into the effects of impurities and fluid flow, dendritic growth 

velocities of the Ni samples are calculated for five different undercoolings in terms of the above 

power law fittings. Relative changes of the growth velocities are determined. The results are 

listed in Table 1. At an undercooling as small as 10 K, the imposition of the static magnetic field 

decreases the growth velocities by 76% and by 85% for the 4N and 5N purity samples, 

respectively. With increasing undercooling, the effect of the static magnetic field on the 4N 

purity sample shows a continuous decrease. For the 5N purity sample, the effect of the static 

magnetic field decreases faster and reaches a minimum at an undercooling of 150 K. After that, 

the effect resumes but changes its direction. On the other hand, the impurities produce the 

strongest effect on the growth velocities at small undercoolings. In the absence of the static 

magnetic field, the effect of the impurities decreases quickly with increasing undercooling and 

changes its direction at 200 K. Under the action of the static magnetic field, the effect of 

impurities decreases slightly with increasing undercooling. Such different behavior highlights 

that the impurities interact strongly with fluid flow during dendritic growth at small 

undercoolings. In many of the previous studies, these two quantities were not controlled strictly. 

Thus, it is not surprising that a large discrepancy had arisen between the measured growth 

velocities and the prediction of the LKT model. A detailed analysis of the present data within the 

framework of the LKT model is under way. In the following, the present data measured under 

the static magnetic field are further discussed with respect to two recent studies and one phase 

field modeling. 

Table 1 Relative changes of dendritic growth velocities of pure nickel in response to altering of 

the experimental conditions 

Sample conditions Relative change of growth velocity (%) 

Undercooling 10 K 50 K 100 K 150 K 200 K 

0T→1T 4N -76 -55 -42 -32 -24 

5N -85 -53 -23 2 26 

5N→4N 0T -67 -38 -19 -5 7 

1T -47 -41 -38 -37 -35 

76
As shown in Figure 4a, the present data measured under the static magnetic field are in good 

agreement with the previous measurements by Lum et al. [12] for undercoolings larger than 180 

K. However, there is a large discrepancy in the medium undercooling region. Since those 

researchers employed the same glass-fluxing method for undercooling, the convection in their

samples is assumed to be comparable to that in the present samples. Under this assumption, the 

large discrepancy may be ascribed to a difference in the method for calibration of the 

temperature measurements. Actually, if the deficit of the recalescence temperature of the present 

samples with respect to the melting temperature was eliminated deliberately, the derived growth 

velocity-undercooling relationship would show an excellent agreement with those researcher’s 

data over the whole undercooling regime. However, such calibration may not be necessary. Note 

that the present data coincide with the lower limits of the latest data measured by Funke et al. [15] 

for electromagnetically levitated samples. As concluded from the above discussion, residual fluid 

flow in the present samples can enhance growth velocities of pure nickel. Convective flow in the 

electromagnetically levitated samples is much stronger [24,29] and should make a larger 

contribution to the growth velocities than in the present samples. From this point of view, the 

present method for calibration of the sample temperature is reasonable. More importantly, the 

present data show an excellent agreement with the recent phase field modeling [21] up to an 

undercooling of 220 K (see Fig. 4b). The contributions of fluid flow and impurities were not 

incorporated in the modeling. Therefore, the conditions hypothesized therein were well satisfied 

through the present experiments under the static magnetic field. The discrepancy remaining at 

extremely large undercoolings can be ascribed to the impurity effect because it is smaller for the 

higher purity sample. It will vanish if the material purity can be improved further. In other words, 

the deviation of the growth velocities from a power law at large undercoolings is largely due to 

the impurities dissolved in the pure Ni samples. 

 

Figure 4 Comparison of the present data with two sets of literature data [12,15] (a) and the 

results of the recent phase-field modeling [21] (b). For clearance, the data measured in the 

present of the static magnetic field are shown only. 

Conclusions 

Dendritic growth velocities in undercooled melts of high purity nickel samples have been 

measured with and without the imposition of a static magnetic field of 1 T. Similar to the latest

measurements on the samples of lower purity, the present data have confirmed the long-term 

suspected effect of fluid flow on growth velocities in the small undercooling region. The present 

data have also shown that the impurity effect plays a role both in the small and in the large 

77
undercooling regions. Although the species of the impurities are unknown, they tend to lower the 

dendritic growth velocities under the present experimental conditions. Additionally, the present 

data measured under the static magnetic field show an excellent agreement with the prediction of 

the recent phase field modeling [21] up to an undercooling of 220 K. The remaining discrepancy 

at larger undercoolings has been attributed to the impurity effect. 

Acknowledgements 

This work is financially supported by the National Natural Science Foundation of China (Grant 

No. 51071043) and by the Fundamental Research Funds for the Central Universities (Grant Nos. 

N090509001 and N090109001).The authors thank D. M. Herlach, P. Galenko, T. Volkmann, A. 

Kao for helpful discussions. The authors thank S. Binder for offering the computer code and X. 

T. Deng, H. F. Li, P. Zhang and Y. Xiao for their assistance in the experimental work. 

References 

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[15] O. Funke, G. Phanikumar, P. K. Galenko, L. Chernova, S. Reutzel, M. Kolbe and D. M. 

Herlach, J. Cryst. Growth 297 (2006) 211. 

[16] J. Gao, Z. N. Zhang, Y. K. Zhang and C. Yang, in: Solidification of Containerless 

Undercooled Melts, ed. D. M. Herlach and D. Matson, Chapter 13, Wiley, in press. 

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488. 

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[20] J. Bragard, A. Karma, Y. H. Lee and M. Plapp, Interface Sci. 10 (2002) 121. 

[21] B. Nestler, D. Danilov and P. Galenko, J. Comput. Phys. 207 (2005) 221. 

[22] S. Reutzel, H. Hartmann, P. K. Galenko, S. Schneider and D. M. Herlach, Appl. Phys. Lett. 

91 (2007) 41913. 

[23] S. Binder, Ph.D. Thesis of Ruhr University of Bochum, 2010. 

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[25] H. Yasuda, I. Ohnaka, Y. Ninomiya, R. Ishii, S. Fujita and K. Kishio, J. Cryst. Growth 260 

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79
X-RAY RADIOGRAPHIC OBSERVATION OF DIRECTIONAL 

SOLIDIFICATION UNDER MICROGRAVITY: XRMON-GF 

EXPERIMENTS ON MASER12 SOUNDING ROCKET MISSION

G. Reinhart1, 2, H. Nguyen Thi1, 2, A.Bogno1, 2, B. Billia1, 2, Y. Houltz3, K. Löth3, 

D. Voss4

, A. Verga4, F. de Pascale4, R.H. Mathiesen5, G. Zimmermann6 

1

Aix Marseille Univ & (2) CNRS, IM2NP UMR 6242 

Campus Saint-Jérôme, Case 142, 13397 Marseille Cedex 20, France 

3

Swedish Space Corporation, P.O. Box 4207, SE-171 04 Solna, Sweden 

4

European Space Research & Technology Centre, Keplerlaan 1 Postbus 299, 2200 AG 

Noordwijk, the Netherlands 

5

Dept. of physics, NTNU, N-7491 Trondheim, Norway 

6

ACCESS e.V, Intzestrasse 5, 52072 Aachen, Germany 

Keywords: X-ray radiography, Microgravity, Sounding Rocket, Convection, Fragmentation, 

Solidification, Aluminium 

Abstract 

 The European Space Agency (ESA) - Microgravity Application Promotion (MAP) 

programme entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes 

under microgravity and terrestrial conditions) aims to develop and perform in situ X-ray 

radiography observations of metallurgical processes in microgravity and terrestrial environments. 

The use of X-ray imaging methods makes it possible to study alloy solidification processes with 

spatio-temporal resolutions at the scales of relevance for microstructure formation. 

XRMON has been selected for MASER 12 sounding rocket experiment, scheduled in 

autumn 2011. Although the microgravity duration is typically six minutes, this short time is 

sufficient to investigate a solidification experiment with X-ray radiography. This communication 

will report on the preliminary results obtained with the experimental set-up developed by SSC 

(Swedish Space Corporation). Presented results dealing with directional solidification of Al-Cu 

confirm the great interest of performing in situ characterization to analyse dynamical phenomena 

during solidification processes. 

Introduction 

Many phenomena involved during solidification are dynamic, such as dendrite fragmentation [1; 

2], columnar-to-equiaxed transition [3; 4], and equiaxed growth [5-7]. The study of such 

phenomena by classical approaches like quenching or decanting is not well adapted. Therefore, 

in situ and real-time X-ray imaging should be retained as the method of choice for investigating 

the solidification front evolution of metallic alloys grown from the melt [8-10]. In addition, for 

most real solidification processes involving a liquid phase, natural buoyant convection plays an 

important role and may interact with the microstructure. An important pending issue is to clarify 

the role of convection on macrosegregation and on microstructure formation of the solidification 

process [11; 12]. It has been shown that microgravity (μg) environment is an efficient way to 

eliminate buoyancy and convection to provide benchmark data for the validation of models and 

numerical simulations [13-17]. In addition, a comparative study of solidification experiments at 

1g and μg can also enlighten the effects of gravity. Therefore, in the frame of the XRMON 

80
project, an experimental set-up has recently been developed to perform directional solidification 

in microgravity conditions with in situ X-ray radiography observation. The first solidification 

experiment in microgravity using this facility is scheduled during MASER 12 sounding rocket 

mission, in autumn 2011. 

This paper is divided into two parts. The first part will present a brief review of some gravityinduced effects on solidification process, investigated by means of synchrotron X-ray 

radiography. Experiments were carried out at ESRF (European Synchrotron Radiation Facility) 

in thin Aluminium-based alloys. In the second part of this paper, we will describe the breadboard 

tests carried out in Stockholm (Sweden) with a prototype of the XRMON-Gradient Furnace setup. Preliminary results show the high capabilities of this experimental set-up in terms of thermal 

behaviour, as well as X-ray observation. 

1. Influence of convection on solidification process 

The synchrotron experiments are carried out at the European Synchrotron Radiation Facility 

(ESRF), inside a Bridgman furnace [18]. In situ and real time observation of the solidification 

process is possible by the use of synchrotron X-ray radiography. The main surface of the sheetlike sample (40 mm x 6 mm x 0.2 mm) is set perpendicular to the incident monochromatic X-ray 

beam, with energy in the range 14-18 keV. Absorption is the main source of the image contrast 

and depends on the atomic number of the elements and the solute content. In all radiographic 

pictures, the solid aluminum microstructures appear in white and the copper-rich liquid in dark 

grey after an image processing [19]. The optics is chosen to obtain a good compromise between 

a large field of view (15 x 6 mm2

) and a sufficient resolution (pixel size: 7.46 μm). 

Initial Solidification Transient 

It is well known that even when solidification is performed under both thermal and solutal 

stabilizing configuration, convection in the melt can nevertheless be strong due to residual 

transverse gradient, in particular at low growth rates [16; 20]. Indeed, in real experimental 

configurations there are generally small departures of the initial interface from planarity, even 

after a long thermal stabilization phase. As solidification proceeds, the solute is rejected at the 

solid–liquid interface. Due to gravity, the denser Cu-enriched liquid flows towards depressed 

regions of the front, which results in solute accumulation in those regions. According to the 

phase diagram, solute accumulation in depressed regions retards solidification and further 

deepens the solid–liquid interface modulations. This leads to a large macroscopic deformation of 

the solidification front (steepling phenomena). In addition, the transverse solute gradient gives 

rise to a transverse microstructure gradient (clustering phenomena), as already shown by postmortem analysis [16]. 

Figure 1 shows a sequence of images recorded during the initial transient solidification of Al4wt% Cu. The temperature of the hot zone of the furnace was lowered at a given cooling rate RH

= 1 K/min, while the temperature of the cold zone of the furnace was maintained constant RC = 0 

K/min. The average temperature gradient G is about 35.5 K/cm. In this experiment an initially 

slightly curved smooth interface is observed (figure 1a), owing to a residual transverse 

temperature gradient Gy of about 1 K/cm [21]. When the cooling of the sample begins the 

growth rate gradually increases as solidification proceeds, leading to the destabilization of the 

interface first in the center of the sample. The propagation of the morphological instability 

towards the two sides of the sample is visible in figure 1, as well as the transverse gradient of 

microstructure (i.e. clustering phenomena). Concomitantly, macroscopic distortion of the solid–

liquid interface becomes larger and larger with time, due to fluid flow in the melt (i.e. steepling 

phenomena). In figure 1, it can be seen that the liquid phase in the two channels becomes darker 

as solidification proceeds, which confirms an increase in the liquid solute concentration there. 

81
Figure 1: Sequence of 4 images showing the progressive deformation of the solid-liquid 

interface during the initial transient of solidification for Al-4wt% Cu (RH = 1 K/min; RC = 

0K/min, G=35.5 K/cm). (a) t≈ 20 min; (b) t≈ 40 min; (c) t≈ 50 min; (d) t≈ 60 min 

Sedimentation and buoyancy in Al-Cu alloys 

Sedimentation or floating of solid parts (i.e. dendrite fragments or equiaxed grains) are two key 

issues to understand the final microstructure or grain arrangement and can only be analyzed by in 

situ observation [22-25]. In the case of Al-Cu alloys, Ganesan and Poirier [26] calculated the 

densities of the solid and the liquid phases as function of temperature and composition. They 

found that below approximately 10 wt% Cu, solid density is larger than liquid density so that we 

may expect to have solid sedimentation. Above this threshold value, the opposite behavior is 

anticipated and solid floating assumed. The gravity-settling behavior of equiaxed grains is shown 

in figure 2, with a sequence of radiographs recorded during an Al – 4 wt% Cu solidification 

experiment. In this experiment, a few equiaxed grains nucleated above the columnar front 

(Fig.2a). Some of them remained stuck on crucible wall, but others can fall down due to gravity 

like the grain indicated by the white circle on the right side of the sample. 

Figure 2: Sequence of 3 radiographs showing sedimentation of an equiaxed grain under the 

effect of gravity during directional solidification of Al-4wt% Cu 

(RH = 1.5K/min, RC = 0, G = 35.5 K/cm) (t0 is the beginning of the cooling). 

(b) 

unmelted

Solid 

Liquid 

(a) (c) (d) 

1 mm 

G

(a) t = t (b) t = t0 + 1336s 0 + 1320s (c) t = t0 + 1368s 

1 mm 

82
Live monitoring showed that equiaxed grains grew faster and faster during sedimentation, which 

is in agreement with the fact that they moved in a richer and colder liquid (Fig.2b and 2c). In 

addition, they may rotate because of uneven lengthening of their dendrite arms. Finally, the 

grains stopped to move when they met the columnar front or when their size was comparable to 

the crucible thickness (Fig.2c). A rough estimate of the sedimentation velocity, typically about 

25 μm/s, could be obtained based on successive radiographs. It should be stressed that the falling 

down of equiaxed grains is enhancing grain packing on the columnar front and may lead to the 

Columnar – to – Equiaxed Transition when their number and size were large enough [3; 4]. 

According to this, CET is expected to occur earlier on earth than in microgravity. 

The floating behavior of dendrite fragments is shown in figure 3, with a sequence of radiographs 

recorded during an Al – 10 wt% Cu solidification experiment. In this experiment, fragmentation 

phenomena of the dendritic columnar microstructure were visible on the left side of the dendritic 

microstructure (Fig.3a and 3b). The dendrite fragments moved upward because it was buoyant 

with respect to the surrounding fluid. On contrary to the sedimentation case, the size of the 

dendrite fragments slightly decreased during their upwards movement because they were 

gradually melted. A rough estimate of the sedimentation velocities between two successive 

radiographs gave an approximate value of 60 μm/s. It is worth noticing that these dendrite 

fragments could not promote Columnar-to-Equiaxed Transition, because they were carried up far 

into the liquid where they were re-melted. Our conclusions are in agreement with Griffiths et al 

[27], who analyzed in details the influence of initial solute composition on the formation of 

equiaxed regions in Al-Cu and Al-Si alloys but during downwards solidification. 

 

Figure 3: Sequence of radiographs showing dendrite fragments moving upwards under the effect 

of buoyancy during columnar growth of Al-10wt% Cu 

(RH = 0.1K/min, RC = 0.1K/min, G = 23.5 K/cm). 

2. XRMON – GF project 

XRMON-GF device 

The conceptual design of the breadboard setup is given in figure 2. It consists of a gradient 

furnace system for solidification of Al-Cu based alloys and an attached high-resolution X-ray 

diagnostic system. The metallic samples had a sheet-like geometry with a length of 50mm, a 

width of 5mm and a roughly constant thickness of less than 0.2mm along the sample. Each 

sample was mechanically polished down to the desired thickness and then sandwiched between 

two rectangular glassy carbon plates. To investigate the dynamics of solidification processes, the 

field of view for the X-ray diagnostics was larger than 5 x 5 mm2 with a spatial resolution of 

(a) t = t0 (b) t = t0 + 44s (c) t = t0 + 615s (d) t = t0 + 700s

1 mm

83
(d) (e) (f)

(a) (b) (c)

Figure 5. Sequence of radiographs showing the formation of a columnar structure of Al10wt% Cu (cooling rate = 6 K/min; G ≈ 43 K/cm) (a) t=t0+150s; (b) t=t0+170s; (c) 

t=t0+200s; (d) t= t0+260s; (e) t=t0+300s; (f) t=t0+400s. t0 is the reference time, just 

before the beginning of the solidification. Image width=4.5 mm. 

about 5 μm and a temporal resolution of 2-3 Hz. The X-ray source was based on a microfocus 

transmission target in molybdenum using polychromatic radiation with a peak at 17.5 keV.

Figure 4. (a) Design of the breadboard set-up, (b) picture of the breadboard set-up showing the 

X-ray source (left), the furnace (very close to the microfocus source) and the detector (right).

Initial transient during directional solidification of Al - 10 wt% Cu alloy 

Fig.5 shows a sequence of radiographs recorded during the initial solidification transient of an Al

- 10 wt% Cu alloy for a temperature gradient of 43 K/cm and a cooling rate of the hot heater 

element of 6 K/min. In Fig.5a (t = t0 + 150s, with t0 the reference time, just before the beginning 

of the solidification) the first disturbances are visible along the whole interface, following the 

planar front breakdown induced by the Mullins–Sekerka instability [28]. One can already see in 

Fig.5a that side branches begin to grow on most of the perturbations, which indicates the 

inception of the formation of dendrites. At this stage, it is possible to determine an average 

wavelength λi for the microstructure, λi ≈ 180 μm. 

Further development of the pattern occurs with a progressive increase of the amplitude of the 

disturbances (Fig. 5b, t = t0 + 170s), while the liquid ahead of the columnar front becomes darker 

and darker, due to the solute rejection during the liquid-solid transformation. In the subsequent 

stages, both the amplitude and lateral size of dendrite increase concomitantly, with solute 

Microfocus

tube

Heater/sample

system

Sample

translatio

n Furnace

Insulation 

2D detector 

Sample (a) (b) 

X-ray source 2D camera

Furnace

84
(a) (b) (c) (d)

Figure 6. Sequence of radiographs during columnar growth of Al – 20 wt% Cu showing several 

dendrite fragmentations (cooling rate = 20K/min; G = 400 K/cm) (a) t=t0+50s; (b) t=t0+100s; (c) 

t=t0+150s; (d) t=t0+168s. t0 is the reference time, just before the beginning of the solidification. 

Image width=5 mm. 

screening causing a strong decrease of the growth of the neighboring dendrites. The last 

observed microstructure is mainly composed of several dendrites protruding markedly into the 

liquid phase with an average final primary spacing λf ≈ 1 260μm (Fig.5f). 

Fragmentation phenomenon during solidification of Al – 20 wt% Cu alloy 

One mechanism for the formation of equiaxed grains during the Columnar-to-Equiaxed 

Transition is the dendrite fragmentation, which is believed to be at the origin of the central 

equiaxed core region in casting processes [1; 2]. The details of the fragmentation phenomenon are 

insufficiently understood and controlled. Thus, it is compulsory to confirm the predominance of the 

fragmentation phenomenon in CET and also to improve the characterisation and understanding of its 

mechanism, in particular the role of gravity at each step of CET. To achieve this goal, an efficient 

approach is to perform a comparison between solidification experiments carried out at 1g and in μg, in 

both cases with in situ characterization by means of X-ray radiography. In Fig.6a one can see the 

gradual establishment of columnar microstructure, as described in the previous section. In this 

experiment, an equiaxed grain nucleated ahead of the columnar front, on a small heterogeneity. 

This grain remained fixed and simply continued to grow in interaction with the columnar front. 

In Fig.6b and Fig.6d, the black circles pinpoint the presence of several dendrite fragments. For 

Al – 20 wt% Cu, the solid being much lighter than the surrounding liquid, dendrite fragmentation 

was promoted by buoyancy forces acting on secondary dendrite arms. After the fragmentation, 

the dendrite pieces were immediately carried away to the upper part of the sample. In this 

experiment, fragmentation occurred continuously during the growth process. Radiographs made 

it possible to have a rough estimation of the fragments velocity, about 60μm/s. It is worth 

noticing that these dendrite fragments could not promote CET, because they were carried up far 

into the liquid where they were re-melted. 

 In Fig.6c, the fragmentation of a dendrite primary trunk can also be noticed, indicated by 

a black ellipse. Its size being close to the sample thickness, the primary trunk fragment could not 

move far into the liquid phase. This phenomenon was rarely observed compared to the secondary 

dendrite arm fragmentation, certainly because of the difference in diameter between primary 

trunk and secondary arms. It has recently been observed by using combined radiography and 

topography that the rotation of some tenth of degrees around the growth axis of the entire 

primary trunk during the growth process can occur [29]. This rotation has been interpreted as a 

consequence of a torque induced by shear stress that builds up with the growth [30]. In our 

experiments, it was also observed that this phenomenon occurred mainly for a specific 

crystallographic dendrite direction <110>, which suggests an effect of dendrite morphology. 

85
Conclusions 

This paper reports on a selection of results obtained during metallic alloy solidification 

experiments characterized in situ and in real time by X-ray radiography. Experiments were 

carried out with both a synchrotron source and a microfocus tube, with a dedicated novel 

experimental set-up developed in the frame of the ESA-MAP XRMON. These results emphasize 

the great impact of gravity and convection on the dynamics of the solidification front by showing 

the influence of convection on the initial solidification transient, the sedimentation/buoyancy of 

equiaxed grains or dendrite fragments. The next step of the project is to perform an experiment 

in microgravity environment to obtain purely diffusive transport conditions and then to provide 

benchmark data for the validation of models and numerical simulations. In addition, a 

comparative study of solidification experiments at 1g and μg will also enlighten the effects of 

gravity. This is the goal of the solidification experiment scheduled during MASER 12 sounding 

rocket mission, in autumn 2011. 

Acknowledgments 

This research work is supported by the XRMON project (AO-2004-046) of the MAP program of 

the European Space Agency (ESA) and by the French National Space Agency (CNES). 

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87
"Innovative Video Diagnostic Equipment for Material Science" 

G. Capuano1, D. Titomanlio1, W. Soellner2, A. Seidel2

1

TSD - Techno System Dev. s.r.l.; 

Via Provinciale Pianura, 2 - Z.I. S. Martino, 23 - Pozzuoli, 80078, ITALY 

2

ASTRIUM Space Transportation 

Claude Dornier Strasse, Immenstaad, 88090, GERMANY 

Keywords: 

High-Resolution High-frame rate imaging, Real-time Image Compression and Storage 

Abstract 

Materials science experiments under microgravity increasingly rely on advanced optical systems 

to determine the physical properties of the samples under investigation. This includes video 

systems with high spatial and temporal resolution. The acquisition, handling, storage and 

transmission to ground of the resulting video data are very challenging. Since the available 

downlink data rate is limited, the capability to compress the video data significantly without 

compromising the data quality is essential. 

We report on the development of a Digital Video System (DVS) for EML (Electro Magnetic 

Levitator) which provides real-time video acquisition, high compression using advanced Wavelet 

algorithms, storage and transmission of a continuous flow of video with different characteristics 

in terms of image dimensions and frame rates. The DVS is able to operate with the latest 

generation of high-performance cameras acquiring high resolution video images up to 

4Mpixels@60 fps or high frame rate video images up to about 1000 fps@512x512pixels. 

Introduction 

Microgravity provides a number of unique advantages for performing materials science 

investigations. For example, electromagnetic levitation experiments with undercooled melts can 

be performed under microgravity conditions with very weak levitation fields compared to 

experiments on ground leading to a situation where the disturbances induced in the samples by 

the levitation fields are very small and the temperature range accessible during the experiments 

can be extended to much lower values compared to ground-based experiments. This allows 

performing experiments that are a targeted to measure a wide range of solidification phenomena 

and thermophysical properties, such as the direct observation of the solidification front upon 

recalescence, as well as the measurement of the heat capacity, surface tension, viscosity, and 

thermal expansion [1]. 

Due to the nature of the experiments and the measurement objectives, non-contact diagnostics 

systems are needed to perform such measurements. Among others very often high-resolution and 

high-frame rate video cameras are used producing a very high data amount. Therefore, advanced 

video compression and handling systems are essential for many diagnostic systems of materials 

science payloads. An example of such a video system and its application is presented in this 

paper. 

Measurement Objectives and System Requirements 

The main scientific data provided by electromagnetic levitation experiments are data on the size, 

shape, brightness distribution and temperature of the sample and their evolution over time. 

Physical data that can be extracted by analyzing the shape of the sample include surface tension 

and viscosity, which are derived from sample shape oscillations (see figure 2), and the thermal 

88
expansion which is directly derived from the diameter of the sample. Other physical data such as 

the solidification speed is derived from direct observation of the solidification front (see figure 

1). To perform these measurements with the required accuracy either a high spatial or a high 

temporal resolution is needed or intermediate settings of both, whereas these settings may need 

to be drastically changed within single measurements to cope with various scientific goals. 

Typically digital video cameras with advanced optics are used meeting these needs due to their 

high flexibility in spatial and temporal resolution [2]. 

Figure 1 – Solidification (bright

area) of undercooled sample

Figure 2 – Induced sample shape oscillations of a liquid sample, oxide 

particles are observed as bright white structures visualizing the fluid flow

The samples in both figures are levitated in an EML-like facility for parabolic flights; the 4 dark patterns in 

front are parts of the sample holder preventing the hot sample from leaving the confined area for levitation. 

Moreover, monitoring of the levitation process needs a real-time downlink of the captured video, 

within the limitations of the currently available downlink capacity, for process control purposes 

and near real-time evaluation. The limited downlink capacity also motivates the need for storage 

of data on-board the spacecraft of the high amounts of captured video data. Data compression 

applied due to constraints on the on-board storage resources shall not impair the video images' 

quality because of the scientific need of image evaluation accuracy on pixel scale. Playback of 

these high quality video data stored on-board is needed for non real-time evaluation on ground 

allowing improving experiment settings for upcoming experiment runs. 

Technological Challenges 

The operation scenario demands from the video acquisition system not only a high data 

acquisition rate but also flexibility and fast reaction time on changed camera settings. Moreover 

the system has to duplicate the video into two streams, real-time and playback, which puts an 

even higher requirement on the data throughput of the system. Both streams have to be handled 

differently in terms of compression, frame rate adjustment and routing to meet the downlink 

requirements for the real-time video on one hand and the requirements for video storage onboard on the other hand. Further, the entire handling of the acquired video stream needs to be 

performed without delay since the camera does not provide for any video buffer and no image 

captured shall be lost. Provided this, the video system must fit within a small available volume 

and a tight mass and power budget. 

Design Solution and Technology 

TSD (Techno System Dev.) has been working since 1991 on the development of digital video 

systems for space applications, exploiting different hardware and software architectures. The 

DVS (Digital Video System), subject of the present paper, is based on a compact and very 

powerful platform, named H2VMU (High Resolution/High Frame Rate Video Management 

Unit), developed by TSD, and whose first generation implementation flew on board a Sounding 

Rocket on May 2005 [3]. The H2VMU has been designed for real-time video acquisition, lossy 

and/or loss-less compression and transmission/storage of a continuous video data flow. The 

89
H2

VMU platform is periodically enhanced and in the current third generation it is able to operate 

with very high performance cameras (both high resolution and high frame rate) with an input 

data throughput up to 240Mpixels/s. The H2

VMU is based on a modular architecture comprising 

compact and low power modules implementing a wide set of functions: 

• Camera interface & video acquisition 

• Wavelet lossy and/or loss-less compression 

• Custom image processing 

• Volatile or no volatile video data storage on different technologies 

• Command reception and decoding for unit configuration and control according to 

CCSDS (Consultative Committee for Space Data Systems) standard 

• Video Data packetization and encoding according to CCSDS standard 

• Low power management 

The modules can be arranged in different configurations in order to fulfill different and specific 

user requirements. The modules are also available with different qualification levels, including 

radiation tolerant versions. Table 1 reports the maximum specifications that can be achieved. 

Since budgets strongly depend on the configuration, the values reported in the table 2 are 

provided for three typical configurations. 

Table 1 - H2VMU maximum specifications

Video input data rate: up to 240 [Mpixel/s] for each video input module

Image resolution up to 4[Mpixel] @ 60[frame/s] 

Frame rate 1000[fps] @ ≈512x512 [pixel] (higher frame rate at lower resolution

available) 

Pixel depth: up to 12[bit]

Compression rate: - up to 60[Mpixel/s] in loss-less mode for each compression 

module

- up to 300[Mbyte/s] in lossy mode for each compression module

Storage rate and capacity: - 60[Mbyte/s] and 64[Gbyte] for each custom high rel solid state 

storage module

- 100[Mbyte/s] and 250[Gbyte] for each COTS (Commercial Off 

The Shelf) removable SATA hard disk

Table 2 - H2VMU Budgets

Configuration

60 [Mpixel/s] single 

Channel & 32 GB 

custom storage module

240 [Mpixel/s] four 

Channels & 148 GB

custom storage module

60 [Mpixel/s] single 

Channel & Removable 

SATA disk (250 GB)

H2VMU HDD

Mass [kg] 2.7 4.7 5.8 2.2

Dimensions [mm] 150x140x150 149x252x140 186x114x254 58x140x220 

Power@28V [W] 13.4 40 46 (including HDD)

Internal Communication Architecture 

The DVS is based on a novel approach for board interconnection and data communication. 

Depending on the unit configuration, an application-specific back-plane is designed based on: 

! high-speed point-to-point SerDes (Serializer/Deserializer) links with multi-drop bus 

configuration for data transfer 

! low-speed redundant CAN (Controlled Area Network) network for control and 

configuration 

90
The SerDes links are based on the National ChannelLink technology that provides high speed 

data transfer (up to 1,785 Gbps) over a reduced size interconnection (a reduction up to 80% is 

possible with respect to the classic parallel bus approach), thus allowing the implementation, in a 

single back-plane PCB (printed circuit board) of a significant number of those links working at 

the same time. The physical level is LVDS (low voltage differential signaling) that provides 

differential signaling with small swing, low noise and good noise tolerance. The unit implements 

ChannelLink communication channels between modules with multi-drop configuration, thanks to 

a careful PCB design that requires to follow demanding guidelines to ensure signal integrity; 

these include, for instance, hand routing, impedance control, electrical length matching, proper 

track separation and loading balance. 

The source module requiring transmitting data acts as driver of the communication channel while 

destination nodes are on the receiver side. As these modules support different input data rates 

(either as sustained or peak rate) a simple request/grant scheme is then implemented to manage 

the communication flow. The driver asserts the request line whenever it has valid data to 

transfer. A dedicated module generically acts as router (implemented as a stand-alone module or 

embedded in one of the already existing others) moving those requests to the destination nodes. 

The receivers, whose input request lines are set, acknowledge the request by asserting their 

relevant grant lines whenever they’re available to accept the incoming data. They can release the 

grant line during the data transmission, as well, to add wait state to the communication. The 

router implements the logical and of the grant lines of the receivers that are involved in the 

communication and asserts thereafter the grant line towards the driver to start the data 

transmission. It can also implement the masking of the request and grant lines to/from one or 

more receivers. So, according to the chosen configuration, data can be moved to all the receivers 

physically connected to the multi-drop bus or only to a subset of them. All the same, faulty 

receivers that are no more capable of receiving data can be isolated to prevent any dead-lock in 

the communication. 

Video Compression 

Video data reduction techniques are of utmost importance in space applications, due to the often 

reduced on-board resources devoted to data storage and transmission to ground. The importance 

for data reduction is increasing over time as modern diagnostic instruments like high resolution 

digital cameras provide increasingly large amount of data. 

The choice of compression factors to be applied to video data obviously depends on the use to be 

made of the images. As a rule of thumb one can say that for scientific images that have to be 

stored on-board a lossless or quasi-lossless compression is often required in order to guarantee 

the maximum scientific return. This imposes the use of compression factors in the range 2÷5. On 

the other hand, for images to be transmitted to ground in real-time to be used for control and 

monitoring purposes it is often possible to achieve much higher reduction levels. By applying 

different techniques like image lossy compression, image binning, frame rate reduction it is 

possible to have overall reduction factors in the range 100÷300. 

Most of the image compression techniques use the signal expansion as major component. 

Generally this is achieved via digital filter banks. The reasons for expanding the video signals 

and process them in the transform domain are numerous. Usually the signal processing in the 

transform domain allows us to achieve a better data compression for a prefixed encoder 

complexity. The signal in the transform domain can be submitted to a more efficient quantization 

that takes into account also criteria about the human vision perception. 

The most common transform is the DCT (Discrete Cosine Transform); it is still used for image 

compression standard like MPEG and JPEG. This type of compression technique exhibits several 

limitations from the implementation point of view. The algorithm needed to perform the DCT 

91
requires a great amount of calculations to obtain the transformed image. This is due to the nature 

of the basis function used for signal expansion that has not a finite time extension (Cosine 

function is periodic). Therefore, in order to limit the amount of calculations to a reasonable level, 

the image is decomposed in 8x8 block of pixels. When high compression ratio is applied, this 

results in block artifacts that are typical of this compression technique. 

In the recent years the image compression technique based on Wavelet transform has been 

considered an extremely interesting alternative due to the several advantages that it offers with 

respect to the DCT. This technique also uses signal expansion, but the basis function employed 

appears more appropriate in order to achieve the following goals: 

! high quality image compression 

! considerable reduction of transform computational complexity 

As clearly visible in figure 3, the block artifacts are present, at high compression factors, only in 

the image compressed with JPEG; the same image compressed with the same factor using 

Wavelet, exhibits only a very small degradation. 

Figure 3 – Image compressed (170:1) by Wavelet (left) vs. JPEG (right)

The reduction of the amount of calculation to perform the image transformation is due essentially 

to an important property of the wavelet basis functions: their finite time extension. 

A compression system based on the discrete transform consists mainly of 3 blocks: 

! mapper ! quantizer ! entropy encoder 

The mapper transforms the image from a space/time domain in a new domain where the 

redundant information between the pixels is reduced. In the case of a wavelet compressor, the 

mapper is represented by the discrete wavelet transform that transforms the image from a 

space/time domain in a time/frequency domain. The quantizer reduces the redundant 

psychological-visual information; the entropy encoder reduces the redundant numerical 

information. The solution that TSD has tuned over several years is a Wavelet compressor based 

on an EZW (embedded zerotree wavelet) encoder. The EZW is an algorithm for data 

compression derived from a scheme designed by J.M. Shapiro [4] especially to work with the 

discrete wavelet transform. The EZW coder is progressive in nature. This means that as more 

data is added to the compressed stream, the more detailed the reconstructed image will be. 

Progressive coding is also known as embedded coding - thus the E in EZW. EZW utilizes the 

observation that in natural images most of the energy (or information) is in the low frequency 

spectrum. After a wavelet transform has been calculated, the coefficients in the lower frequency 

subbands tend to be larger (in absolute value) than the coefficients in the higher frequency 

subbands. This allows for the creation of structures called zerotrees. Another observation used by 

EZW is that larger valued coefficients are more important than small value coefficients. 

Therefore EZW codes the larger coefficients first. The TSD solution offers significant higher 

quality especially at high compression factors than other Wavelet compressors, as visible in the 

example pictures of figure 4 using a factor of 160:1. 

92
Original photo TSD Wavelet JPEG2000 Wavelet

Figure 4 – Comparison between TSD Wavelet and JPEG2000 Wavelet at extreme compression factor 160:1. 

All the functional blocks needed to carried out the above described image compression 

algorithm, have been implemented, by TSD, in a hardware solution using high-density FPGAs, 

employed as complete System-On-Chip solutions. The newest implementation, running on 

Xilinx Virtex4 FPGA, is able to perform lossy compression at a rate of 300 Mbytes/s. 

Video Storage 

The video data storage functionality can be implemented with either custom solutions based on 

an array of flash memory devices or with commercial solutions like SATA disks. Custom arrays 

of flash devices are to be preferred in applications requiring high levels of reliability, while 

SATA mechanical disks offer higher storage density and better radiation tolerance. 

TSD has developed products in both categories, reaching high levels of storage rates. For custom 

arrays of flash devices the latest implementation allows to store up to 240Mpixel/s, thanks to a 

proprietary pipelined architecture, with storage capacity up to 64GByte per module, while in the 

case of SATA disks it is possible to store more than 100MByte/s per disk with up to 4 disks 

running in parallel. The disk is used as a raw device, i.e. without a file system, so to maximize 

the performances. 

Usability 

One of the most appealing characteristics of the DVS for the end user is its high degree of 

flexibility. The on board software provides a rich set of commands and house keeping data that 

allows the facility controller to configure the DVS on-the-fly according to the user needs. 

Among the various features, the most remarkable are: 

• Possibility to modify various parameters while a video acquisition is running, like 

compression factors, binning, frame skipping, test patterns generation 

• Possibility to route the different video streams (loss-less science video, lossy real-time 

video, etc.) to the on board storage devices and to the payload data transmitter in a 

number of different configurations 

• Possibility to perform the software update during flight 

• Possibility to interrogate any processing node (either main CPU or FPGAs) by means of 

low level CAN bus commands in case of troubleshooting 

• Possibility to power off the video processing modules in case a low power mode is 

needed 

Apart from its flexibility, the DVS is remarkable for the responsiveness: all the operations are 

executed in “real-time”; no latency is expected when a command is issued, regardless of the 

operations that are being performed in that moment. This is due to the hardware and software 

architecture of the DVS that features a powerful CPU, a highly deterministic real-time operating 

system, a number of highly specialized FPGAs and very fast internal communication links. 

93
Applications 

Main task of the H2VMU is to compress huge amounts of acquired data in real-time to make 

them accessible to users whenever the data transmission rate between facility and control station 

is limited. This feature is often employed to scientific systems using video diagnostics but also 

for data of X-ray detectors. The H2VMU has been successfully employed on-board several 

MASER Sounding Rockets [3], [5] and Foton Russian capsule [6], providing video diagnostic 

capabilities to different microgravity experiments. Figure 5 refers to the CDIC-2 experiment 

(Chemically Driven Interfacial Convection) while figure 6 refers to the SOURCE experiment 

(SOUnding Rocket Compare Experiment) which studies heat and mass transfer in a heated tank 

with a liquid pressurized by non-condensable gas. Both performed on board MASER 11. 

Figure 5 – Interferometric image of fluid 

interface, Compression factor 48:1

Figure 6 – Shadowgraph image of fluid science 

experiment cell, Compression factor 68:1

 

Other H2VMU based specific configurations are currently flown on board the ISS and on the 

main spacecraft of the PRISMA formation flying mission [7]. Next applications are foreseen for 

Maser 12 and for the ISS, supporting the EML Material Science Experiment [2]. 

Electro Magnetic Levitator (EML) on the International Space Station (ISS) 

For EML the DVS plays a central role in the chain of optical diagnostics acquiring the digital 

video stream of a highly flexible camera wrt spatial and temporal resolution [2]. Since the 

acquired video stream is used twofold, for scientific and for process control purposes, the 

H2

VMU (see figures 7 and 8) has to duplicate the video stream and have the capability to treat 

both streams (scientific & process control) in different ways wrt compression, pixel reduction 

and frame rate reduction. All the above actions have to be performed in real-time, storing the 

scientific data on the DVS owned hard drive and provide the process control video at the video 

interface for downlink to ground control. Moreover, playback of the scientific data from HDD 

via the downlink interface is possible offline, i.e. between two experiment runs, and the DVS 

configuration can be quickly changed to match the current scientific needs and camera settings. 

Figure 7 – The engineering model of the H2VMU 

for EML (internal view of the boards)

Figure 8 – The engineering model of the H2VMU 

for EML (rear view showing the connectors)

94
H2

VMUs on board Sounding Rockets 

The H2VMU represents the core of the DVS employed in all the MASER Sounding Rocket 

Program since 2005. A number of H2

VMUs are used on board the MASER supporting all the 

Experiment Modules requiring video functionalities such as real-time image processing, on 

board storage, compression and downlink. Figure 9 below shows typical concept of DVS as 

applied to the MASER missions. The performance requirements of the experiments are more and 

more increasing and currently culminating to 313Mpixel/s (pixel depth up to 12 bit) on 

MASER12. The DVS architecture is decentralized and each Experiment Module is provided 

with a local H2VMU with multichannel capability in accordance with the requirement of the 

specific video diagnostic systems; a three channel H2VMU used on board MASER 12 is 

hereinafter shown in the insert of figure 9. 

Figure 9 – DVS architecture for Sounding Rocket missions as e.g. MASER

Insert: 3 channel H2

VMU for BIOMICS-2 experiment on MASER 12

 

ERB-2 Stereoscopic Camera 

The Erasmus Recording Binocular 2 (ERB-2) is a digital stereoscopic camera, based on the 

H2

VMU platform, allowing the simulation of human binocular vision. The ERB-2 has been 

proposed by ESA-ESTEC as a technology experiment to be flown on board the ISS able to 

capture and send to ground 3D films and related audio of the scenes filmed by the camera (see 

figure 10). 

The ERB-2 is presently on-board the ISS. The first tests were successfully performed in July and 

August 2010. During March 2011 the Astronaut Paolo Nespoli conducted a fly-through filming 

program, starting in Kibo JPM (JEM Pressurized Module) and proceeding through Node-2, US 

Lab, and Node-3 with Cupola to COLUMBUS Laboratory (see figure 11). ERB-2 is also 

provided with two motorized objectives, driven by the H2VMU electronics, providing auto-iris 

and auto-focus functionalities. 

95
Figure 10 – The flight model 

of ERB-2 at TSD premises

Figure 11 – Astronaut P. Nespoli 

with ERB-2 on board the ISS

Conclusions 

Data compression has become a key technology for applications which produce high amounts of 

data and also suffer from a low data transmission rate. The technique introduced in this paper 

using EZW Wavelet compression is capable of maintaining the original data information by 

applying medium compression factors and achieves an unparalleled image quality for high 

compression factors compared to other Wavelet techniques. 

The compression algorithm, as well as other customized features, is embodied in a hardware 

solution, called H2

VMU, using state of the art and radiation hardened components. This modular 

and very flexible platform allows for real-time video acquisition, customized processing, lossy 

and/or loss-less compression and transmission/storage of a continuous video data flow. 

The successful application of the H2

VMU on board different space platforms (Sounding Rockets, 

Capsule, ISS, and Satellites) has demonstrated firstly its maturity to be now widely employed on 

board of various space platforms and secondly that it is best suited for material science facilities 

utilizing demanding image diagnostics. 

References 

1. “Materials Science Investigations using Electromagnetic Levitation”, A. Seidel, W. Soellner 

and C. Stenzel, Proceedings to 4th ISPS conference – Bonn, July 2011 

2. “EML - Containerless Processing Facility for Material Science Research onboard The ISS”, 

W. Soellner, A. Seidel, C. Stenzel, W. Dreier and B. Glaubitz, J. Jpn. Soc. Microgravity 

Appl. 27 No. 4 183, 2010 

3. “Digital Video System on-board Maser10”, G. Capuano et. al., 17th ESA Symposium on 

European Rockets and Balloon related research – Sandefjord, Norway 30 May – 2 June 2005 

4. “Embedded Image Coding Using Zerotrees of Wavelet Coefficients”, Jerome M. Shapiro, 

IEEE Transactions on Signal Processing, December 1993 

5. “Digital Video System on-board Maser11”, G. Capuano et. al., 18th ESA Symposium on 

European Rockets and Balloon related research – Visby, Sweden 3 - 7 June 2007 

6. “Data Handling Unit for Foton-M3 Mission”, D. Titomanlio et. al., DASIA 2007, June 2007, 

Naples (Italy). 

7. “Video System for PRISMA Formation Flying Mission”, G. Capuano, M. Severi, F. Cacace, 

et. al., 4th Symposium on Small Satellite, Systems and Services – Rhodes (Greece), May 

2008 

96
Containerless Measurements of Density and Viscosity of Fe-Co Alloys

Jonghyun Lee1

, Paul Choufani1, Richard C. Bradshaw2, Robert W. Hyers3, Douglas M. Matson1

1

Tufts University; 

Department of Mechanical Engineering; 

200 College Avenue; Medford, MA 02155, USA

2

Bradshaw Consulting 

108 Harnden Avenue; Watertown, MA 02453, USA 

3

University of Massachusetts; 

Department of Mechanical Engineering; 

160 Governors Drive; Amherst, MA 01003, USA 

Keywords: Fe-Co Alloys, Density, Viscosity, Containerless measurements, Double recalescence 

Abstract 

During the past years, extensive collaborative research has been done to understand phase 

selection in undercooled metals using novel containerless processing techniques such as 

electrostatic and electromagnetic levitation. Of major interest is controlling a two-step 

solidification process, double recalescence, in which the metastable phase forms first and then 

transforms to the stable phase after a certain delay time. The previous research has shown that 

the delay time is greatly influenced by the internal convection velocity. In the prediction of 

internal flow, the fidelity of the results depends on the accuracy of the material properties. This 

research focuses on the measurements of density and viscosity of Fe-Co alloys which will be 

used for the fluid simulations whose results will support upcoming International Space Station 

flight experiments. 

Introduction 

Molten metallic alloys can be undercooled if the preferred sites for nucleation such as crucibles 

and impurities are effectively removed. One effective way of achieving undercooling is 

containerless processing. When the undercooled liquid solidifies, the heat of fusion is rejected 

and the temperature increases suddenly. This phenomenon is referred to as recalescence. The 

solidification of undercooled alloys can occur in two ways: either the stable phase nucleates 

directly from the liquid (single recalescence) or the metastable phase forms first followed by the 

growth of the stable phase into the mushy zone (double recalescence). The metallography reveals 

that alloys which have undergone double recalescence show fine and globular grains, which can 

result in better mechanical properties. Since the growth rate of the metastable phase is lower than 

that of the stable phase, it is important to attain a certain amount of delay time between the 

97
nucleation events. If there is enough delay time, most part of the alloy will double recalesce, 

otherwise, the stable phase begins to grow and overtakes the metastable phase solidification front 

as shown in Figure 1 [1]. Previous research has shown that the delay time is greatly affected by 

the internal convection in the molten alloy. If the convective velocity is large, the primary arms 

of dendrites can be bent by the drag forces and eventually collide with each other. The collision 

between neighboring dendrite arms can serve as a preferred site for the unwanted early 

nucleation of stable phase [2]. Therefore characterization of fluid flow in the molten alloys is 

important in understanding the phase selection during solidification. 

Figure 1. Microstructure of steel alloy: The solidification proceeded from top to bottom. 

The upper portion of the figure underwent double recalescence and thus shows globular 

and fine microstructures. As the solidification proceeded, the stable phase overtook the 

metastable solidification resulting in the dendritic structure shown in the lower portion of 

the figure. This corresponds to single recalescence behavior. 

In the near future, a series of solidification experiments will be performed in space over a wide 

range of internal convective velocities to acquire data for comprehensive understanding of 

solidification processes. For the success of the International Space Station (ISS) mission, the 

identification of the correlation between test parameters and internal fluid flow is of great 

importance. 

Hyers et al [3] have developed mathematical models which describe the convection inside the 

droplet while processed in both electromagnetic (EML) and electrostatic (ESL) levitators. These 

models can be solved using appropriate numerical techniques. To obtain satisfactory numerical 

results, it is important to secure reliable material properties. Using containerless measurement 

techniques [4], material properties of Fe-Co alloys, one of the candidate alloys for the ISS 

mission, are measured. These values will be used for the computational fluid dynamics 

simulations in support for the ISS mission. 

98
Positi

on+

senso

r+

Position sensor

Pyrometer

Vacuum

chamber

D2 arc 

lamp

Electrodes

Long-range 

microscope 

CCD camera

Positioning 

lasers

Turbo 

molecular 

pump

Fiber 

optic 

light 

source Heating 

laser

Experimental Apparatus 

The experiment was performed with the electrostatic levitation (ESL) facility at NASA Marshall 

Space Flight Center (MSFC) in Huntsville, AL. The weight of the sample was cancelled by 

Coulomb forces exerted by a pair of vertical electrodes. Two additional pairs of electrodes on the 

horizontal plane were used to stabilize the sample using the PID (Proportional, Integral, and 

Derivative) control scheme. After levitation and stabilization, the sample was heated by a 200 W 

YAG laser and the temperature was monitored by optical pyrometers (Mikron Infrared Inc.) 

which detects the wavelength of emitted light from the sample. The processing was achieved in a 

high vacuum chamber (=10-8 torr) to avoid chemical reactions. Figure 2 shows a schematic of 

ESL apparatus in NASA MSFC. 

During the processing, images of the sample were captured by a high speed camera (Redlake 

Motion Pro 1000) and analyzed to obtain measurement of density and viscosity as a function of 

temperature. For analysis of images, the edges of the sample in the images were detected and 

fitted with sixth order Legendre polynomials for the calculations of projected areas and volumes 

of samples. The details of image processing techniques can be found in [5]. 

Figure 2. A schematic of ESL apparatus in NASA MSFC. 

99
Measurement of Density 

To estimate the density of the sample, it is essential to know the volume and mass of the sample 

for each time step in a thermal cycle. As the molten sample free cooled video images were 

acquired at a rate of 25 Hz and edge-detection software was used to tract sample shape. While 

the volume can be easily obtained by integrating the fitted functions (6th order Legendre 

polynomials), a care must be taken in dealing with the mass. Throughout the density test, the 

reduction in mass due to evaporation was observed. Adopting the Langmuir’s equation for 

evaporation, the loss of mass for a given time was calculated. 

( )

kT

pp

A

m vapor atm

s π

α

2

− = 

!! ! ! ! !

 (1) 

where m is the rate of loss of mass due to evaporation, As is the surface area, pvapor is the vapor 

pressure, patm is the partial pressure of the atmosphere, α is the correction factor to account for 

the mass transport diffusion layer resistance, k is the gas constant, and T is the absolute 

temperature. Having the volume and mass as functions of time, the density of the sample could 

be estimated as a function of time. Combining the estimated density and temperature as functions 

of time t, density-temperature plots were generated. 

( )( ) ( )( )

volume ( )( ) tT

tTm

ρ tT = (2)

Measurement of Viscosity 

Since the viscosity acts as a resistance to fluid motion, the decay time for an oscillating sample 

can be used to measure the viscosity. The sample was excited by applying periodic changes in 

the vertical electrostatic field. The excitation started with the lower frequencies and was 

gradually increased until the sample showed noticeable oscillations; the excitation frequency was 

near the sample natural frequency (≈ 200Hz). Once the desired level deformation is achieved, the 

excitation was stopped and the sample was allowed to dampen out freely. The sample

oscillations were captured at the rate of 1000 Hz. 

100
Figure 3. Shapes of an oscillating sample. 

By analyzing the captured images, the areas of the oscillating sample were calculated for each 

frame. After the area of the oscillating sample was plotted in the time domain, the Fast Fourier 

Transform was applied to extract the natural frequency of the sample and then a sinusoidal curve 

as expressed in Equation (3) was fitted to the decaying portion of the amplitude y(t). 

( ) ( ) !

"

# $

%

& +=

τ

φω t

cos tty exp (3)

where ω is the natural frequency, φ is the phase angle, and τ is the damping coefficient which 

characterizes how fast the sample dampens out. The viscosity can be determined using its

relationship with the damping coefficient as below. 

( )( ) τ

ρ μ

1

121

2

0 ⋅ − + = ll

R (4)

where ρ is the density, R0 is the radius of the undeformed droplet, l is the mode of oscillation 

(l=2). 

Results and Discussion 

The density of Fe45Co55 was measured as a function of temperature and plotted in Figure 4. As 

indicated above, during the experiment 6.86% of mass was lost due to the mass evaporation. 

According to the temperature-time plot, the sample was melted and superheated up to Tm+130K 

for 66 seconds and cooled for 6 seconds during which the density was measured. Considering the 

fact that the time duration for the density measurement is only 9% of that for melting and 

superheating, it is reasonable to assume that the majority of mass evaporation had occurred 

before the measurement of density. Thus, the post-mass was used for calculation of density and 

this technique should result in a slight underestimation of density results. For comparison, the red 

line in the graph indicates the predicted values for density using rule of mixtures (ROM). For the 

101
prediction, the density and the rate of change in density with respect to temperature ( ρ ∂∂ T ) of 

Fe and Co shown were taken from the literature [6] and these values are shown in Table 1. 

Figure 4. Variation of density as a function of temperature. The black line is a leastsquares line fit to the measurements. 

Table 1. Densities and the slopes of density with respect to temperature of Fe, Co, and 

Fe45Co55. 

Fe Co Fe45Co55

Tm (K) 1808 1765 1747 

ρ at Tm (kg/m3

) 7030 7750 7458 

ρ ∂∂− T (kg/m3

·K) 0.880 1090 0.996 

The density at the melting point was estimated to be 7405 kg/m3

 and it was slightly greater than 

the predicted value by 0.7%. The rate of change in density with respect to the temperature at the 

melting point was estimated to be -0.9741 kg/m3·K and the difference from ROM predicted slope 

was thus 2.1%. It can be stated that the experimental results and ROM predictions agree well 

with each other and. 

Two samples of Fe45Co55 were used for the measurements of viscosity. Figure 4 shows the 

viscosity of Fe45Co55. The viscosity data was fitted with an Arrhenius relation. 

!

!

"

#

$

$

%

& = Tk

E

b

μμ 0 exp (5)

102
where E is the activation energy, kb is the Boltzmann constant (8.61730×10-5 eV/molecule-K). 

The fitted values are 6

0 .4 10912 − μ = × Pa·s and E = 24891.1 eV/molecule. The viscosity at the 

melting point was estimated to be 0.0197 Pa·s. 

Figure 5. Variation of viscosity as a function of temperature. 

Summary 

As support and preparation for ISS mission, the density and viscosity of Fe45Co55 were measured 

with the novel containerless processing technique. The measured density and its rate of change 

with respect to the temperature at the melting point ( ρ ∂∂ T ) were 7,405 kg/m3

 and -0.9741 

kg/m3

·K and these values show an excellent agreement with the predicted values by the rule of 

mixtures and their differences were to be 0.7% and 2.1% respectively. The viscosity of Fe45Co55

was measured and fitted with the Arrhenius relation as .4 10912 exp( ) 14,493 T −6 μ = × . The 

measured density and viscosity values will be used for the simulations of convection inside the 

droplets. 

Acknowledgements 

This project is sponsored by NASA under grants NNX08AL21G and NNX10AV27G. 

Experimental portions of this research were performed at NASA Marshall Space Flight Center. 

The authors wish to thank Trudy Allen and Glenn Fountain for extensive technical advice. 

.4 10912 exp( ) 14,493 T −6 μ = ×

103
References 

1. A.B. Hanlon, R.W. Hyers, and D.M. Matson, “Modeling the Effects of Internal 

Convection on Dendritic Evolution in Stainless Steel Alloys,” EPD Congress – Proceedings of 

the Symposium by the Extraction and Processing Division of the Minerals, Metals and Materials 

Society, 2005, pp. 501-507. 

2. A.B. Hanlon, D.M. Matson, and R.W. Hyers, “Internal Convective Effects on the 

Lifetime of the Metastable Phase Undercooled Fe-Cr-Ni Alloys,” Philosophical Magazine 

Letters, vol. 86, no. 3, 2005, 165-174. 

3. R.W. Hyers, Douglas M. Matson, Kenneth F. Kelton, and Jan R. Rogers, “Convection in 

Containerless Processing,” Annals of New York Academy of Science, 2004, pp. 474-494. 

4. R.C. Bradshaw, “Automatic Containerless Measurements of Thermophysical Properties 

of Quasicrystal Forming Metals,” PhD Dissertation, The University of Massachusetts Amherst, 

2006. 

5. R.C. Bradshaw, D.P. Schmidt, J.R. Rogers, K.F. Kelton, and R.W. Hyers, “Machine 

Vision for High-Precision Volume Measurement applied to Levitated Containerless Material 

Processing,” Review of Scientific Instruments, vol. 76, no. 12, 2005. 

6. T. Iida and R.I.L. Guthrie, “The Physical Properties of Liquid Metals,” Oxford University 

Press, Oxford, 1988. 

104
TEMHD Effects On Solidification Under Microgravtiy Conditions

Andrew Kao, Koulis Pericleous

University of Greenwich, Old Royal Naval College, Park Row, Greenwich, London, SE10

9LS, United Kingdom

Keywords: Dendritic Growth, Solidification, Thermoelectricty, Magnetohydrodynamics,

Undercooling

Abstract

An unexplored potential exists to control microstructure evolution through the use of external

DC magnetic fields. Thermoelectric currents form during solidification and interact with

this external field to drive microscopic fluid dynamics within the inter-dendritic region. The

convective heat and mass transport can lead to profound changes on the dendritic structure.

In this paper the effect of high magnetic fields is demonstrated through the use of both

3-dimensional and 2-dimensional numerical models. The results show that the application of

a magnetic field causes significant disruption to the dendritic morphology. Investigation into

the underlying mechanism gives initial indicators of how external magnetic fields can either

lead to unexpected growth behaviour, or alternatively can be used to control the evolution

of microstructure in undercooled melts as encountered in levitated droplet solidification

Introduction

Levitation/confinement is often achieved through the combined use of both AC and DC

magnetic fields [8]. Under these conditions it is possible to undercool the droplet into a

metastable state, where the droplet is still liquid but its temperature is below the solidus

temperature. Upon nucleation, rapid dendritic solidification occurs. These conditions are

of scientific interest; to understand the fundamental behaviour of solidification in both pure

materials and alloys. A common addition is to apply a high DC magnetic field to damp

macroscopic fluid flow and create a purely diffusion driven situation. However under certain

conditions this magnetic field will interact with microscopic thermoelectric currents that are a

natural and inherent part of solidification; generating a Lorentz force and driving microscopic

fluid flow. This fluid flow can be sufficient to disturb the heat and solute boundary layers

significantly altering the solidification process. In the absence of gravity a weak AC field can

be used purely for positioning, while the DC field can be chosen to be significantly large to

interact with the thermoelectric currents. The focus of this work is to explore what effect

this may have and to provide the first study of how this phenomenon will affect droplet

solidification. To do this a dimensionless 3D enthalpy based model is used and scaled to

represent an AlSi hypo-eutetic alloy.

Theory

The Seebeck effect [1] describes how an e.m.f (∆Ψ) forms when two materials with varying Seebeck coefficients (S) are placed in thermal contact and that at the boundary this

e.m.f varies with the surface temperature. The generalised form of Ohm’s law includes a

contribution from the Seebeck effect.

105
J = σ (E + u × B − S∇T) (1)

At the interface the potential difference can be written as

∆Ψ = ∆ST (2)

The evolution of the dendrite morphology is calculated by an enthalpy-based method [2].

The interfacial temperature (T

i

) is defined by the Gibbs-Thompson condition

T

i = Tm −

γ (θ, φ) Tm

L

κ − m



C0 − C

i

l



(3)

where the second term accounts for local free energy through surface energy anisotropy

and curvature and the final term accounts for solute partitioning in binary alloys. This

equation leads to a variation in surface temperature and the formation of thermoelectric

currents irrespective of an external magnetic field.

Results

Free Growth of a Single Crystal

To understand the mechanism that will lead to morphological changes a simplified situation

of a single crystal freely growing in the bulk is considered. Previous work [3–7] has shown

that the thermoelectric currents circulate between the cold tips and the hot roots of the

crystal. In figure 1 a similar situation can be observed for a developing dendrite. In this

example an additional positive curvature exists at the base of each arm near to the root.

This is analogous to the formation of a secondary branch and the resulting undercooling gives

rise to a relatively higher surface potential and the formation of another current circulation.

As a dendrite continues to develop and many secondary branches form, this can have a

significant impact on magnitude and direction of the thermoelectric currents. A more in

depth description of this effect is presented in [6]; in summary a larger variation in surface

temperature (i.e. an increase in hot and cold regions along the interface due to secondary

arms) gives a higher average magnitude of J. In the presence of a low magnetic field aligned

along the tip in the +z-direction, such that Re → 0 and P e → 0 the growth can be considered

to be diffusion driven; giving an idea of the direction and relative magnitude of the fluid flow.

The flow is given in figure 2, where circulations form around each of the tips and circulations

around the entire dendrite. For the tips tangential to the field a pair of vortices form, passing

from the tip, under and over the arm. For the tips parallel to the field the circulation forms

around the arm. The highest velocities can be seen in the root of the dendrite, passing over

the tangential arms and at the z tips.

As the magnetic field is increased to 10T convective transport becomes important. The

incident flow onto the tangential arm perturbs the interface causing secondary growth and a

deflection of the tip. This is highlighted in figure 3, showing the transient growth from both

an isometric perspective and also the perspective along the magnetic field. The secondary

branches have a direct impact on the formation of thermoelectric currents; as the tips evolve

from the surface of the arm the local curvature increases resulting in a decrease in the surface

temperature and consequently a change of the potential boundary condition. The net effect

106
is the formation of a potential sources (tips) and sinks (local roots) where thermoelectric

currents will begin to circulate around the secondary tips. This is highlighted in figure 4,

which shows the surface potential. The currents formed from this potential will also interact

with the magnetic field producing Lorentz forces and the situation becomes increasingly

complex.

Thermoelectric currents also exist in the inter-dendritic region and the 3-dimensional

simulation shows that a high velocity can also exist in this region. To highlight this, a crosssection in the x, z plane through the centre of the dendrite is given in figure 5, showing the

y component of velocity. The flow between the first secondary branch and the tangential

primary arms persists all the way into the root. A second cross section, in the x, y plane

and in between these two branches shows the velocity and how it is not simply confined

between the two branches, but forms a global circulation. However this is expected due to

the rotational symmetry of the problem.

Figure 1: Stagnant growth. Left: Electric potential. Centre: Direction of J. Right: |J|.

Figure 2: Stagnant growth velocity. Left: Direction of u. Right: Normalised |u|.

107
Figure 3: 10T Transient Growth. Top: Isometric perspective. Bottom: Along magnetic field

direction.

Figure 4: Electric potential of growth in the presence of a 10T magnetic field. Left: x, y

plane. Right: x, z plane.

108
Figure 5: Velocity of growth in a 10T field. Top left: Location of planes. Top right: uy in

the x, z plane. Bottom: |u| in the x, y plane.

109
Droplet Solidification

Under microgravity conditions, with no external body forces a droplet of liquid metal will

form a spherical equilibrium shape. Consider a droplet with a diameter of 0.4mm. The

surface of the droplet is assumed to be adiabatic as the droplet is placed in a vacuum and

due to the short duration of solidification heat loss through radiation may be neglected.

The droplet begins in a supercooled state and nucleation is triggered on the surface at the

positive x pole. Figures 6 and 7 show the evolution of the solidification within the droplet

for cases without a magnetic field and with a 10T transverse magnetic field (perpendicular

to the plane of growth). In the absence of a magnetic field the dendrite grows statistically

symmetric exhibiting 4-fold symmetry. In the case where a magnetic field is present the

solidification path takes a preferential route around the surface of the droplet. The change

to the morphology is a consequence of the fluid dynamics generated through the interaction

of thermoelectric currents and the magnetic field. At the tips of the dendrite the direction

of the Lorentz force has the same sense, where each tip forms a clockwise micro circulation

of flow. As multiple tips grow these circulations will interact forming a single circulation.

The net effect is a larger mesoscopic circulation traversing multiple tips and transporting

solute rich fluid downstream of the nucleation site; resulting in a plume of solute ejected

from the bulk. The velocity is given in figure 8, where flow that appears to exist within the

solid regions has entered a quasi 3-dimensional plane above and below the structure. This is

analogous to the inter-dendritic flow of the 3D single crystal shown in figure 5. Figure 9 shows

the magnitude of the thermoelectric currents; it is important to note that the normalised

scale is logarithmic, highlighting the short range in which these currents act under these

conditions. The highest J is located at the tips growing into the bulk (along the top of the

droplet), where micro circulations are perturbing the interface encouraging secondary growth

and increasing J. This give a large Lorentz force, but acting over a short distance.

This model suggests two plausible mechanisms. The first is a consideration to the extent

of the plume of solute. Through continuity the localised high velocity forces solute into the

bulk, however in this region electromagnetic damping becomes dominant and the velocity

decreases sharply away from the microstructure. For low magnetic fields, where bulk flow

can be significant the extent of this plume is governed by the inertia generated by the Lorentz

force and the time for the flow to accelerate under a lower driving force. Ultimately this

could lead to macro segregation, while for high magnetic fields the effect is limited to micro

segregation. The second mechanism is a change to the inter-dendritic structure. Consider an

inter-dendritic region with many secondary arms. Surface energy shows that thermoelectric

currents and fluid flow will persist. The flow will act to homogenise the liquid destabilising

the equilibrium of the secondary arms, causing some regions to re-melt and others to solidify.

This will continue until the velocity becomes zero, which if it is assumed occurs when the

region becomes devoid of thermoelectric currents then the surface temperature must become

constant. For a homogeneous solute concentration and neglecting surface energy anisotropy,

this occurs when the local curvature becomes constant. Thus over an extended period of

time, it is anticipated that this region will slowly minimise its surface area; this is highlighted

in figure 7, where the absence of secondary branches within the microstructure is evident.

Therefore, initially secondary branching may be encouraged and later suppressed.

Conclusion

The focus of this paper is to look at the complex nature of the interaction of thermoelectric

currents and a magnetic field in undercooled liquid metal droplets in the absence of gravity.

110
Figure 6: Solute field of droplet solidification with no external magnetic field.

Figure 7: Solute field of droplet solidification with a 10T external transverse magnetic field.

Figure 8: Relative |u| in an external magnetic field.

Figure 9: Relative |J| in an external magnetic field.

111
The 3-dimensional model highlights a variety of fluid microcirculations that all contribute

to altering the convective transport and the evolution of dendritic growth. Perturbations

from the flow of the thermal and mass boundary layer at the interface initiate secondary

growth, altering the formation of thermoelectric currents, the Lorentz force and the velocity.

Under a high magnetic field this intimate coupling results in a significant change to the

dendritic structure and the numerical results indicate that an unexpectedly strong convection

can develop within the inter-dendritic region. Using a quasi 3-dimensional approximation

solidification of a droplet under a high magnetic field is also considered. The additional

geometric constraints of the droplet surface give rise to a mesoscopic rotational flow, causing

the dendrite to preferentially grow around the surface of the droplet. Downstream of this

flow the thermal and mass boundary layers are extended as hot solute is ejected, stunting

growth. As the solidification front propagates the microstructure continues to evolve through

inter-dendritic flow and a hypothetical mechanism of the effect of this flow has been proposed.

References

[1] J. A. Shercliff, “Thermoelectric magnetohydrodynamics”, Journal of Fluid Mechanics,

1978, 91 part 2, 231-251.

[2] V. R. Voller, “An enthalpy method for modelling dendritic growth in a binary alloy”,

International Journal of Heat and Mass Transfer, 2008, 51, 823-834.

[3] A. Kao, G. Djambazov, K. A. Pericleous and V. Voller, “Thermoelectric MHD in Dendritic Solidification”, Magnetohydrodynamics, 45 No.3, 305-315, 2009.

[4] A. Kao, K. A. Pericleous, M. K. Patel and V. Voller, “Effects of magnetic fields on

crystal growth”, International Journal of Cast Metal Research, 22 No 1-4, 147-150,

2009.

[5] A. Kao “Thermoelectric magnetohydrodynamics in dendritic solidification”, (Ph.D. thesis, University of Greenwich, 2010)

[6] A.Kao and K. Pericleous “The Effect of Secondary Arm Growth on Thermoelectric

Magnetohydrodynamics”, (Paper presented at the 8th Internatioal PAMIR Conference

on Fundamental and Applied MHD, Borgo, Corsica, France, 5-9 September, 2011)

[7] A.Kao and K. Pericleous, “A numerical model coupling thermoelectricity, magnetohydrodynamics and dendritic growth”, Journal of Algorithms and Computational Technology, 2012, Vol 6 (1), 50-77

[8] T.Tsukada, H.Fukuyama and H.Kobatake, “Determination of Thermal Conductivity

and Emissivity of Electromagnetically Levitated High-Temperature Droplet based on

the Periodic Laser-Heating Method: Theory.”, International Journal of Heat and Mass

Transfer, 2007, 50, 3054-3061

112
MULTI-SCALE MODELING OF LIQUID PHASE SINTERING 

AFFECTED BY GRAVITY: PRELIMINARY ANALYSIS 

Eugene Olevsky and Randall M. German 

San Diego State University 

Mechanical Engineering Department 

5500 Campanile Dr., San Diego, CA 92182-1323, USA 

Keywords: Sintering, Microgravity, Finite-Element Method, Multi-Scale 

Abstract 

A multi-scale simulation concept taking into account impact of gravity on liquid phase sintering 

is described. The gravity influence can be included at both the micro- and macro-scales. At the 

micro-scale, the diffusion mass-transport is directionally modified in the framework of kinetic 

Monte-Carlo simulations to include the impact of gravity. The micro-scale simulations can 

provide the values of the constitutive parameters for macroscopic sintering simulations. At the 

macro-scale, we are attempting to embed a continuum model of sintering into a finite-element 

framework that includes the gravity forces and substrate friction. If successful, the finite 

elements analysis will enable predictions relevant to space-based processing, including size and 

shape and property predictions. Model experiments are underway to support the models via 

extraction of viscosity moduli versus composition, particle size, heating rate, temperature and 

time. 

Introduction 

One of the most important findings from the earlier conducted combined ground-based and 

microgravity studies [1,2] is that the rules isolated by mankind over many years of Earth-based 

processing do not carry over into the microgravity environment. New pathways are needed to 

obtain densification and satisfactory mechanical properties while minimizing distortion and 

attaining the desired dimensional precision for operating components. In this connection, the 

understanding of the link between the events occurring at the level of microstructure and the 

macroscopic behavior of powder specimens during liquid phase sintering (shape distortion, 

porosity spatial distribution) is of vital importance. This understanding can be gained through the 

development of a multi-scale model framework taking into account microscopic phenomena and 

macroscopic shape evolution. 

Modeling of micro-scale evolution 

At the microscale, simulations should be constructed to show how individual grains bond during 

sintering by the combined motion of thousands of atoms. Three types of simulations can be 

applied to this problem in liquid phase sintering: 

 atomistic level (molecular dynamics) - applied to the study of sintering based on a large 

ensemble of individual vibrating atoms, providing a means to study time dependent particle 

coalescence, neck growth and bonding between grains, dihedral angle stabilization, and the 

roles of factors such as dopants or temperature on sintering [3];

 viscous models (surface energy minimization) - wherein an initial configuration is input with 

prescribed volume fractions, surface energy, and other boundary conditions and allowed to 

113
morph as a viscous solid through a sequence of small steps to a lowest energy configuration, 

providing a means to identify terminal configurations in multiple grain and multiple phase 

systems [4];

 hybrid model (fine grained Monte Carlo) - relies on computational cells larger than atoms but 

still involves discrete jumps based on energy minimization rules and appropriate boundary 

conditions; provides time dependence to the microstructure transformation, enables 

extraction of important constitutive parameters relating to grain size, grain growth, grain 

agglomeration, neck growth, and liquid or pore distribution and sizes [5].

The atomistic level simulations provide a means to study the particle coalescence and 

redistribution of phases, including how additives change particle-particle sintering [3].

Unfortunately, to be relevant these simulations require accurate atomic interaction potentials to 

compute the total energy of the system. First-principles calculations can provide the most reliable 

interatomic potentials. An alternative is to use empirical interaction potentials such as the 

embedded atom method. However, a simulation of sintering requires a very large number of 

atoms and considerable computational time, making these methods rather slow. 

To address these challenges, a hybrid solution to the micro-scale aspects can be employed. The 

discrete atom level calculation can be replaced by a coarser grid to expedite the simulation, but 

the time based discrete simulations should enable identification of morphology versus time 

needed for the mesoscale and component scale models. 

Modeling of meso-scale evolution 

Kinetic Monte-Carlo (KMC) models have been used extensively to simulate problems of 

microstructure evolution in multiple grain and multiple phase materials. The first KMC models 

were used to simulate normal grain growth in single-phase materials [6,7]. It has been shown [8] 

that very good representation of grain structure, topology and kinetic data could be obtained 

from KMC simulations. Since then, KMC models have been used to study many other types of 

microstructure evolution including abnormal grain growth, recrystallization [9], phase 

transformations, Ostwald ripening [10], pore migration [11] and final-stage sintering [12]. The 

earliest attempt to simulate sintering using KMC models was limited to final stage sintering and 

implemented densification by expanding the area of the simulation as densification proceeded. 

The criterion for densification was to reduce porosity in proportion to the mean distance between 

pores in the simulation. 

While KMC models are fundamentally stochastic, the results of the studies cited above are very 

stable and have been found to always converge to the same topological and kinetic solutions 

[13].

Starting from the late 1940’s, many researchers, including Kuczynski [14], Kingery [15], Exner 

[16] Johnson [17], Coble [18], Ashby [19] and others [20,21,22], have studied microstructure 

evolution during sintering. These studies initially considered sintering of idealized powder 

compacts consisting of 2 or 3 spherical particles of equal size, subjected to sintering by various 

diffusion mechanisms. The main accomplishment of these early models was an understanding of 

the driving forces, transport mechanisms and densification processes for sintering of crystalline 

materials. Next, sintering models consisting of repeating unit cells of the same geometry were 

proposed by DeHoff [23], Bouvard and McMeeking [24], Riedel and co-workers [25,26,27], 

Slaughter and Nettleship [28], and others [29,30,31]. In these models, researchers extended the 

two and three particle systems to idealized repeating unit cell models where each cell consists of 

a grain and the porosity around it. Molecular dynamics simulations used to study the sintering of 

nano-particles [32,33] showed that additional mechanisms such as particle rotation is active, as 

previously observed in tungsten heavy alloy LPS. 

114
Continuum mechanics modeling of sintering of two [34], three [35] and a row [36] of particles 

have given very accurate particle shapes evolution. The application of continuum 

thermodynamic principles by the finite difference method to simulate the sintering of two 

particles [37], a row and close-packed particle [24], and unit cells of different packing [29] have 

given accurate data on the shrinkage kinetics during sintering. A cellular model [38] was used to 

study sintering by surface diffusion in a multiple particle system and an MC model [12] was used 

to simulate final-stage sintering of multiple grains. Some of these idealized geometric 

simulations [24] have been used to obtain the sintering stress, or capillary stress, necessary for 

modeling sintering at the continuum level. 

When including a link to the deterministic micro-scale stress analysis, Monte Carlo model can be 

used to obtain more general and accurate thermodynamic (sintering stress, bulk and shear 

moduli) and kinetic data (densification rate, etc.) for liquid-phase sintering taking into 

consideration the gravity influence. 

Modeling of macro-scale shape distortion during sintering 

Sintering kinetics of real porous bodies is determined by temperature, the properties of the 

powder particles, and the nature of property interactions during microstructure evolution. Among 

some of the important aspects are kinematic constraints (e.g., adhesion of porous component’s 

end face and its support surface, which depends on gravity and the component mass), externally 

applied forces and, also, inhomogeneity of properties in the volume under investigation (for 

example, inhomogeneity of the initial density distribution in the green body). 

The complex problems can be solved in terms of a macro-scale description if the local events at 

the micro-scale are known from a parallel set of local simulations. An approach capable of 

solving these problems is the continuum theory of sintering [39].

The main constitutive relationship of the continuum theory of sintering is represented below: 

( ) 1

3

ij ij ij L ij

W

e P

W



     

   

           

 (1) 

where 



 and 



 are the normalized shear and bulk viscosity moduli, which depend on porosity 



and other structure parameters such as grain size, relative interparticle neck radius, etc.; 

ij 

is a 

Kronecker symbol; 

e



 is the first invariant of the strain rate tensor. The effective equivalent strain 

rate W is connected with temperature, the current porosity, and with the invariants of the strain 

rate tensor. The effective equivalent stress 

 )W(

 determines the constitutive behavior of a 

porous material. PL is an effective Laplace stress (sintering stress), which depends on the local 

sintering stress PLo, porosity and various pore structure parameters such as relative interparticle 

neck radius. The porosity



is the ratio of the pore volume to the volume of a porous body. 

The formulation (1) is used for the description of a wide range of powder processing techniques. 

Parameter ij corresponds to the macroscopic stress applied to a representative volume element. 

The first term of the right-hand part characterizes the porous material’s flow resistance, and the 

second term corresponds to the influence of capillary (sintering) stresses. In the case where the 

macroscopic stress ij is equal to zero, Eq. (1) describes free sintering. In the case when the 

effective sintering stress PL is equal to zero, Eq. (1) describes treatment by pressure without 

sintering. In general, Eq. (1) describes sintering under pressure. 

It should be noted that existing model concepts and numerical codes, in particular those based 

upon the continuum theory of sintering, depend on the accurate determination of the constitutive 

parameters such as the bulk and shear moduli ( and ) and the sintering stress PL. This 

determination, relying on realistic pore-grain structures, can be provided by the microscale 

simulations (see above). A new synergistic combination of the macroscopic continuum model, 

115
meso-scale Monte Carlo model, and microscale hybrid model simulations should enable a 

coupled description of liquid phase sintering at different scale levels with effective integration of 

gravity as an adjustable parameter. 

The gravity influence 

Studies on microstructural and configuration changes in liquid phase sintering have provided 

insight into the sintering and composition factors that influence distortion [40-48]. Most of the 

research in this area analyzed microstructure gradients as influenced by gravity. The influence of 

gravity-induced grain settling was examined by Kohara and Tatsuzawa [40], Niemi and 

Courtney [41], and German et al. [42-49]. The last group also studied gravity effects on the grain 

structure under liquid-phase sintering, with emphasis on grain packing [46] and grain coarsening 

[48]. Experimental evidence of gravity-induced shape distortions is given in several studies 

[43,47]. A geometry described as an “elephant foot” shape was frequently observed for distorted 

W-Ni-Fe powder compacts. 

A publication by Raman and German [46] provided a first mathematical model for gravityinduced shape distortion during liquid-phase sintering. In this study, the evolution of the shape of 

a “top hat” (cylinder with flange) was modeled and experimentally observed. However, one of 

the model assumptions is constant density during the distortion process. This is based upon the 

hypothesis that bulk densification occurs during heating and is essentially complete prior to 

liquid formation [49]. In light of this idea, shape change is calculated for an incompressible 

viscous material in a gravity field [46]. Although qualitative successful for liquid phase sintered 

tungsten heavy alloys with excessive liquid quantities, such an assumption substantially restricts 

the model basis and excludes shrinkage and its interplay with gravity forces. The first study 

which included the analysis of a coupled impact of sintering stresses and gravity has been 

conducted by Olevsky and German [50,51]. This work, however, was a purely macro-scale 

analysis, focused on specimen shape distortions without taking into account the evolution of the 

micro- and meso-structure. 

For a full-scale analysis, the gravity influence should be taken into account at both micro- and 

macro-scales. At the micro-scale, the respective bond energies are to be directionally modified to 

include the impact of gravity on the site exchanges. The buoyancy and grain settling has to be 

taken into account by introducing an additional Hamiltonian-based term in the bond energy 

expressions. In addition, the Potts model should be linked to the finite-element code calculating 

stresses and strains at the micro-scale in order to determine the gravity stress-effected state of the 

microstructure and to obtain the final form of the sintering body. The next step in the 

implementation of these ideas is the utilization of finite-element calculations allowing viscoplastic stress-strain analyses. 

At the macro-scale, the continuum model of sintering can be embedded in the following finiteelement framework: 

           1 1    T T

n L T g

B D B d V B P d g N n d

  

    

 

     

    

 (2) 

where [B] is the matrix correlating the strain rates with the nodal velocities Vn; [D] is the matrix 

correlating the stresses with the strain rates (“matrix of viscosities”); 



is a macroscopic porous 

volume under investigation; g is the gravity acceleration; T is the theoretical density of a fullydense material; N is the matrix of the finite element shape functions; {1} designates the unit 

column-vector; and {ng} is a unit vector oriented in the direction of the gravity forces’ action. 

116
The development of the respective computer codes based on the described multi-scale modeling 

concepts is a part of the on-going research. Model experiments are underway to support the 

models via extraction of viscosity moduli versus composition, particle size, heating rate, 

temperature and time.

Conclusions 

A multi-scale modeling framework for simulation of liquid phase sintering at micro-, meso-, and 

macro-scale structure levels is described. Both micro- and meso-scale modeling levels can rely 

on Monte-Carlo simulations, while macro-scale modeling is based on the finite-element 

approach. The development of the respective computer codes based on the described multi-scale 

modeling concepts is a part of the on-going research. Model experiments are underway to 

support the models via extraction of viscosity moduli versus composition, particle size, heating 

rate, temperature and time. 

Acknowledgements 

The support of the National Aeronautics and Space Administration, Grant G00008863, is 

gratefully acknowledged. 

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119
SELF-ASSEMBLY OF Ni-NANOPARTICLES IN AEROSOLS 

PRODUCED THERMALLY ON-GROUND AND UNDER 

MICROGRAVITY CONDITIONS 

Stefan Lösch1, Daniela Nolle2, Eberhard Goering2, Bernd H Günther1 

1

 Fraunhofer Institute for Manufacturing and Advanced Materials (IFAM) 

Wiener Straße 12, D-28359 Bremen, Germany 

2

 Max-Planck-Institut for Intelligent Systems (former Metals Research), 

Heisenbergstraße 3, D-70569 Stuttgart, Germany 

E-mail: loesch@ifam.fhg.de 

Keywords: magnetic nanoparticles, aerosol, microgravity, sounding rocket, parabolic flight, 

agglomeration 

Abstract 

The self-assembly behavior in Ni-aerosols was studied on-ground and under microgravity 

conditions by using the European sounding rocket MAXUS 8 and the parabolic flight aircraft 

A300 Zero-G. In microgravity, convection within the thermally produced aerosols could be 

effectively suppressed. It is shown that due to magnetic dipole interaction the Ni-particles 

agglomerate in a chain-like morphology both on-ground and in parabolic flights. It is shown 

that the chain length increases with increasing agglomeration time. This time scale could be 

extended in the range of minutes by using the sounding rocket microgravity platform. In 

general, under microgravity condition the intrachain size distribution of the Ni-particles is 

narrower then gets narrower. This is reduced to the ideal flow condition in microgravity. 

Surprisingly the sounding rocket platform did not result in chain-like Ni-agglomerates. 

Instead irregular morphology of fluffy agglomerates resulted similar to those observed for 

non-magnetic nanoparticles. These results are explained by considering the thermal history 

during particle synthesis in this process. 

1. Introduction 

The agglomeration of gas-borne particles usually occurs under the combined effect of 

magnetic, electrostatic and Van der Waals interactions [1-4]. In particular for Ni, Co, and Fe, 

this process is clearly dominated by magnetic dipole interparticle forces [5]. As a result, often 

a chain-like agglomeration of the magnetic particles is observed. For applications as sensors 

or in magnetic data storage, these “necklaces” should be as stretched as possible and contain 

particles with a narrow size distribution. The evaporation/condensation method is a wellunderstood physical method to produce nanosized particles from inorganic material. 

Unfortunately the flow conditions are difficult to control for such thermally produced aerosols 

due to convection and sedimentation effects. 

120
In this work we investigate the relationship between carrier gas pressure, dwell time and 

size/shape of the resulting agglomerates of nickel particles produced either on-ground or 

under microgravity, where thermally induced convection effects can be suppressed 

effectively. In microgravity the dwell time for agglomeration in thermally produced aerosols 

can be extended to the minutes range (sounding rocket). 

2. Experimental Procedure 

The process of evaporation/condensation in inert gases is illustrated in Figure 2. The ingot 

material is melted in a suitable crucible by thermal heating. Nanosized particles are formed 

from the evaporated material by vapour cooling in the (cold) carrier gas, particle nucleation 

and coalescence growth. After solidification the resulting nanoparticles stop growing and 

instead undergo agglomeration due to Brownian motion in the carrier gas. A laminar flow 

carries the particles and 

agglomerates towards the 

sampling stage. The coated 

substrates are removed after each 

experiment for ex-situ inspection. 

μ-g compatible experimental setups have been designed separately 

for parabolic flight (max. 22 s of 

μ-g condition) and the sounding rocket platform, respectively. Figure 1 shows a schematic of 

the modules used in both cases. The distance between the heating source and the sampling 

stage is 44 cm in both cases. However, due to technical constraints 2 different heater systems 

had to be used for the 2 μ-g platforms: (a) a 6 kW rf-induction heater for parabolic flight (PF) 

experiments, and (b) a Joule-heater for sounding rocket (SR) experiments. Using the 

induction coil heater a Ni-ingot (5g, purity 

3N5) in an alumina crucible is heated to the 

evaporation temperature of 1850°C within 1 

minute. The inert gas (Argon) carries the 

developing Ni aerosol downstream. At the 

exit of the flow tube the solid fraction is 

deposited on electro-polished austenitic 

stainless steel substrates with a sampling 

time of 6s. Technical details on this set-up 

can be found in [7]. In the Joule-heater set-up 

100mg Ni-wire (4N purity) is wound on a 

tungsten filament and evaporated at 1850°C. 

With this method a steady evaporation rate 

for max. 12 minutes could be achieved. The 

morphology of the deposited Niagglomerates was observed via electron 

microscopy (SEM/TEM). The magnetic 

analysis of the Ni-particles was done via 

superconducting quantum interference device 

(SQUID) measurements. 

Figure 2: principle of evaporation/condensation in inert gas flow

Figure 1: schematic of Ni evaporation/condensation 

device

121
3. Results 

3.1 Agglomeration Of Ni-Nanoparticles On-Ground (1-g) 

Figure 3 shows typical deposits of Ni-agglomerates obtained on-ground (1-g). There is no 

obvious difference in morphology between agglomerates obtained with Joule-heating and 

those obtained with induction coil heating. TEM-inspection reveals the particles’ crystallinity 

(Figure 4) and a ca. 2.5nm thick coating layer – probably Ni-oxide. 

Figure 3: Ni-agglomerates obtained on ground (1-g) 

with induction coil heater (left) and Joule heater (right) 

Figure 4: TEM image of 2 Ni-particles with singlecrystalline structure; the oxide layer is 2,5nm thick

In general, the length of agglomerates increases with increasing residence times as shown in 

Figure 5. 

Figure 5: Ni-agglomerates obtained on-ground at different residence times A: 1s, B: 3.7s, C: 5s, D: 7.5s 

The size distribution of Ni particles was determined as a function of gas pressure and velocity. 

The fairly broad intra-agglomerate size distribution of the Ni-particles made on-ground 

(Figure 6) is ascribed to convection effects in the vicinity of the evaporator, which promotes a 

mixing of particles having different growth histories. 

122
Figure 6: Intra-agglomerate size distribution of Ni-particles produced in on-ground for 2 different dwell times 

at a background pressure p of 100mb; insert: SEM image of a typical Ni-agglomerate

3.2 Agglomeration Of Ni-Nanoparticles In Microgravity (μ-g) 

3.2.1 Parabolic Flight Platform (PF)

Figure 7 illustrates the morphology of Ni-agglomerates obtained from PF-experiments. 

Obviously the particle chains are stretched much more perfectly compared to agglomerates 

produced on-ground (cf. Figure 3). With increasing “free-flow” residence time, (up to 16 sec), 

the agglomerates increased in length up to 30μm and contain several hundred particles. It is 

interesting to note that the intrachain particle size distribution obtained in μ-g (Figure 8) are 

much narrower compared to chains produced on-ground (Figure 6). We ascribe this to the 

absence of convection and sedimentation in the vicinity of the heating source in μ-g, since 

mixing of different zones of the aerosol is absent [11]. 

Figure 7: Ni-chains obtained in micro-gravity (PF); left: p=100mb, tres=16sec; right: p=100mb, tres=8sec 

123
Figure 8: Intra-chain particle size distribution for 2 dwell times at a background pressure p of 100mb in μ-g (PF); 

insert: typical Ni-aggregate

3.2.2 Sounding Rocket Campaign MAXUS (SR) 

With sounding rockets (e.g., MAXUS) the achievable residence time is extended to the 

minute range. We have used residence times of 7.5, 22.5, 37.5, and 55s, each at 2 pressure 

settings for the carrier gas, i.e.100 and 300mbar. The μ-g quality was approximately 10-5g. 

The laminar flow behavior of the aerosol could be followed with a CCD-camera. 

Surprisingly, from the MAXUS-8 experiments in none of the sampled deposits chain-like 

agglomerates were found. Instead the particle aggregates’ shape is similar to those usually 

observed in non-magnetic materials. Furthermore, no correlation was found between the 4 

settings of residence time and the agglomerates’ mean size (cf. Figure 9). 

Figure 9: SEM-images of Ni-agglomerates produced on sounding rocket MAXUS 8 flight at 300mbar and 

residence times of 7.5s, 22.5s, 37.5s and 55s (set for samples #1,3,5,7) 

Similar to the results from PF experiments the agglomerates’ morphologies do not change as a 

function of pressure (100 vs. 300 mbar) (cf. Figure 10). Furthermore, the intra-agglomerate 

particle size distribution is shifted to smaller values by a factor 2 (cf. Figs.8,10). 

124
 

Figure 10: Intra-agglomerate size distribution of Ni-particles obtained in μ-g (SR) at different dwell times and 

two different background pressures; insert: typical Ni-aggregate morphology

4. Discussion 

The formation of extended chains of nanoparticles is a well-known phenomena observed as a 

result of agglomeration within magnetic aerosols. Usually the magnetic dipole interaction is 

considered responsible for this type of self-organization (cf. [5]). Li et al. [8] observed 

morphologies of Ni-agglomerates depending on the external magnetic field strength: 

Applying a strong magnetic field would result in longer particle chains. Unfortunately in all 

these experiments the magnetic structure of the nanoparticles remains unclear. In our 

experimental set-up the Earth magnetic field (cf. Table 1) might influence the orientation of 

the magnetic dipoles and/or the chain fragments on their way towards the sampling stage. 

Table 1: earth magnetic field strength |B| at different locations 

Bremen (lab) Parabolic flights 

(during parabola)

MAXUS 8 

(at launch)

MAXUS 8 

(start of μ-g phase)

MAXUS 8 

(at apogee)

48.5 μT1 ca. 47.8 μT2 52.2 μT1 46.1 μT3 37.2 μT3

1

http://www.pimath.de/magnetfeld/gesamtfeld.html, 2[10]; 3values during MAXUS flight have been calculated 

Figure 11 illustrates the proposed agglomeration mechanism in the presence of dipolar 

magnetic interaction in lab-, PF- and SR-experiments. In Figure 12 is shown the mean chain 

length as a function of dwell time obtained in lab- and PF-experiments. No such data could be 

derived for the SR-samples, since no particle chains had formed. 

To elucidate the reason for the non-chainlike Ni-aggregates obtained in the SR experiments, 

we had performed SQUID and MFM (Magnetic Force Microscopy) measurements on the 

agglomerates. Due to the size distribution evaluated via TEM-measurements, we assumed 

125
blocked superparamagnetic behavior with related single domain particles at room 

temperature. Unfortunately, MFM investigations failed due to interactions between the MFM 

Figure 11: proposed model for agglomerate 

formation in 1-g and μ-g induced by dipolar 

interaction between Ni particles 

Figure 12: length vs. dwell time of Ni-agglomerates 

obtained on-ground and in PF experiments 

tip and the agglomerates which moved the Ni-chains during the measurements and therefore 

prevented usable images. SQUID analysis on samples assembled under 1-g and μ-g 

conditions, respectively, clearly show blocked superparamagnetic behaviour (Figure 13). We 

would like to mention that a small fraction of the coercive behavior could be related to the 

NiO induced exchange bias, provided from a small surface related contribution [9]. However, 

both samples show a superposition of ferromagnetic and paramagnetic behavior. We assume 

that due to the size distribution a mixture of blocked and unblocked superparamagnetic 

particles is present. Because the size of the particles assembled during the SR flight is smaller 

than in lab and during PF, this sample exhibits a smaller blocking temperature. Due to this 

and the relative low Curie temperature of Ni of about 631K one expects a stronger effect of 

magnetization reduction with increasing temperature at the SR made particles. Therefore it is 

most likely, that Ni-particles produced under μ-g conditions (SR) are not magnetic during the 

particle assembly phase, due to the particle size related smaller blocking temperature, which is 

below the material temperature after evaporation. 

 To summarize, the most likely explanation for the different behavior of the μ-g Ni-particles 

(SR) is the reduced blocking temperature due to the size dependency, yielding hot 

nonmagnetic Ni-clusters during the agglomeration phase. Nevertheless, the exact reason for 

this phenomenon is still under investigation. 

Figure 13: SQUID measurements of Ni-agglomerates developed on-ground (left) and on sounding rocket 

(right) 

126
5. Conclusions 

With this work we have presented new results on the agglomeration behavior of Ninanoparticles in aerosols produced under normal gravity and microgravity conditions. In 

general, necklace-like chains are obtained with primary Ni-particles being larger than about 

20 nm. Agglomerates obtained in μ-g during parabolic flights are similar in morphology with 

those obtained on-ground. The chain length slightly correlates with the time available for 

agglomeration. Furthermore, the intra-chain particle size distribution is much narrower 

compared to those obtained on-ground. Surprisingly, the results obtained from a sounding 

rocket campaign (MAXUS-8) do not fit into this scheme: no particle chains were found even 

with Ni-particles much larger than 20 nm. This might be due to a different temperature 

dependent magnetic behavior of the nanoparticles and will be part of future investigation on 

the particles’ magnetic properties. 

Acknowledgements 

The authors thank ESA, SSC, and Novespace for providing microgravity facilities, sounding 

rocket launch support, and assistance during parabolic flights, respectively. This work was 

partially funded by the European Commission under contract-No NMP3-CT-2004-500635. 

REFERENCES 

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magnetic applications – a review, J. Aerosol Sci. Vol. 29, No. 5/6, 1998, 511-535 

[2] S.P. Parkin et al, Magnetic Domain-Wall Racetrack Memory, Science 320, 2008, 190 

[3] H. Wang et al, Size-controlled synthesis, microstructure and magnetic properties of Ni 

nanoparticles, Materials Research Bulletin 43, 2008, 3529-3536 

[4] A. Oberholz et al, Winnacker-Küchler: Chemische Technik, 5. Auflage, (2003-2005), ISBN: 

978-3-527-30430-1, K. Wegner et al, Band 2, Kapitel 9, 821-905 

[5] V. Maceira et al, One-dimensional assemblies of silica-coated cobalt nanoparticles: Magnetic 

pearl necklaces, Journal of Magnetism and Magnetic Materials 303, 2006, 163–166 

[6] C. G. Granqvist et al, Ultrafine metal particles, J. Appl. Phys. Vol. 47. No.5, 1976, 2200-2219 

[7] S. Lösch et al, Microgravity compatible equipment for inert gas condensation of metals during 

parabolic flights, Rev. Sci. Instrum. 80, 2009, 083907 

[8] P. Lie et al, Well-aligned Nickel Nanochains Synthesized by a Template-free Route, Nanoscale 

Res Lett 5, 2010, 597–602 

[9] M. Fraune et al, Size dependence of the exchange bias field in NiO/Ni nanostructures, Appl. 

Phys. Lett., Vol. 77, 2000, 3815 - 3817 

[10] M. Rost, Aggregation of micron-sized magnetic dust particles in low magnetic field 

conditions, Copernicus, 2006 

[11] G.W.Webb, U.S. Patent No. 4758267, 1988 

127
Crystallographic Stability of Metastable Phase formed by 

Containerless Processing in REFeO3 (RE: Rare-earth Element) 

Kazuhiko Kuribayashi1, 2, M. S. Vijaya Kumar2

1

Shibaura Institute of Technology, 

Toyosu, Koto, Tokyo 135-8548, Japan 

2

 Institute of Space and Astronautical Science, JAXA, 

Yoshinodai, Sagamihara, Kanagawa 252-5210, Japan 

Keywords: Containerless processing, Undercooling, Metastable phase, 

Abstract 

Undercooling a melt often facilitates a metastable phase to nucleate preferentially. Although the 

classical nucleation theory shows that the most critical factor for forming a metastable phase is 

the interface free energy, the crystallographic stability is also indispensable for the phase to be 

frozen at ambient temperature. In compound materials such as oxides, authors have suggested 

that the decisive factors for forming a critical nucleus are not only the free energy difference but 

also the difference of the entropy of fusion between stable and metastable phases. In the present 

study, using REFeO3 (RE: rare-earth element) as a model material, we investigate the formation 

of a metastable phase from undercooled melts with respect to the competitive nucleation and 

crystallographical stabilities of both phases. 

Introduction 

A metastable phase is a phase that does not exist in thermal equilibrium state and, although 

thermodynamically unstable, can temporarily exist when some conditions are fulfilled. Research 

into the metastable phase began with Ostwald’s prediction that a phase formed first from 

supersaturated liquid is not always thermodynamically stable but is close to liquid in energy [1]. 

This prediction is called “step rule”. Later on, Stranski and Totomanov [2] suggested that the 

step rule is a consequence of preferential formation of a critical nucleus of the metastable phase. 

That is, the activation energy required to form a critical nucleus, Gn

*

, controls the nature of the 

process. Regarding this point, the classical nucleation theory [3] states that Gn

*

 can be 

understood in terms of the interfacial free energy  between the liquid and solid phases. Turnbull 

[4] and Spaepen [5], assuming that  of a simple material such as metal is related not to the 

enthalpy change but to the entropy change at the solid–liquid interface, formulated  as

 

 



  

S fT

N AV m

2

 

1

3

, (1) 

where Sf, T, NA and Vm are the heat of fusion, the temperature of material, Avogadro number 

and the molar volume, respectively. Furthermore, Spaepen and Meyer [6] derived 

dimensionless solid-liquid interfacial energy, as 0.86 for fcc or hcp crystals and 0.71 for bcc 

structures, respectively. The factors, which strongly depend on the structure of both solid and 

liquid phase, are to be a critical parameter to determine Gn

*

. In fact, it has been reported that the 

phase selection of the stable -phase or the metastable -phase in Fe–Ni–Cr alloys is controlled 

by  [7-9]. However, almost the metastable phases formed at the first recalescence, which 

128
change into the stable phases at the second recalescence, are not frozen into ambient temperature. 

In order that the metastable phase may be frozen into ambient environment, the nucleation 

criterion of metastable phase as well as the crystallographical stability must be fulfilled. Then, in 

the present investigation, using REFeO3 as the model material, where RE means rare-earth 

elements, the phase selections not only in the nucleation stage but also the growth stage are 

discussed. 

Entropy-undercooling regime criterion of phase selection 

Before taking up the main subject of this paper, we mention again the hypothesis that, in ionic 

crystals, Sf is to be a dominant factor in the determination of 

Spaepen [10] and Granasy [11], almost at the same time, developed rather similar models that 

 at equilibrium state is given by 

 



    H r  TS  r

V

 dr , (2)

where H(r) and S(r) are cross-interfacial enthalpy and entropy. Figure 1 shows a schematical

illustration of Eq. 2, where (a) shows the change of the atomic order and of the order parameter 

in solid and liquid, and (b) schematic representation of the change in the enthalpy H(r) and the 

product TES(r) of melting temperature and entropy at the solid-liquid interface. The colored area 

corresponds to the approximate value of the interfacial energy, showing that  is not a 

dimensionless interfacial energy but a dimensionless interface thickness. This model can 

qualitatively infer the temperature dependence of the solid-liquid interfacial free energy. 

However, we need analytical or numerical forms of H(r) and S(r) in order to evaluate the 

interface thickness that is the key parameter of the model. 

On the other hand, in a material having faceted interface, the order parameter representing the 

regularity of the atomic arrangement rapidly changes as the interface is crossed as shown in Fig. 

2. In this case, H(r) can be approxiomated by the near-step function as shown in Fig. 2(b). 

Consequently, the interfacial energy is approximated with a triangle if we assume S(r) as a linear 

function of r at the cross-interface region. This result also means that  is a dimensionless 

interface thickness rather than a dimensionless interfacial energy. The recent numerical 

calculation of  based on the molecular dynamics and the density functional analysis suggests 

Figure 1. Schematic representation of the nonfaceted interface between solid and liquid. (a) 

change of the atomic order and of the order parameter in solid and liquid. (b) schematic 

representation of the change in the enthalpy H(r) and the product TES(r) of melting 

temperature and entropy at the solid-liquid interface. The colored area corresponds to the 

approximate value of the interfacial energy, showing that  is not a dimensionless interfacial 

energy but a dimensionless interface thickness.

129
the interface thickness is approximately half of the atomic layer spacing [12], which implies that 

the model shown in Fig. 2 is qualitatively valid even in the nonfaceted interface. 

Figure 3 schematically shows the thermodynamic relation when considering the step rule [13]. 

The figure depicts temperature and free energy of both liquid and solid phases (stable and 

metastable phases). The reason why liquid phase changes to solid phase is that the free energy of 

the liquid phase becomes larger than that of solid phase. The energy-balance point of both phases 

is the melting point. Comparing the metastable phase to the stable solid phase in terms of free 

energy, the free energy of the metastable phase, Gms, is larger than that of stable phase, Gs, (the 

subscripts s and ms mean stable phase and metastable phase, respectively). Therefore, the melting 

point of the metastable phase, TE,ms, becomes lower than that of stable phase, TE,s. Meanwhile, 

Figure 2. Schematic representation of the faceted interface between solid and liquid. (a) 

change of the atomic order and of the order parameter in solid and liquid. (b) schematic 

representation of the change in H(r) and TES(r). The colored area corresponds to the 

approximate value of the interfacial energy, suggesting the interface thickness is 

approximately half of the atomic layer spacing.

Figure 3. Schematical image of temperature dependency of free energy in liquid and solid 

phases (stable and metastable phases). The entropies of three phase are related SL>Sms>Ss. 

Therefore, as for entropy of fusion Sf caused by solidification, the relation Sf,s>Sf,ms

becomes valid. 

130
the absolute value of the gradient of each curve (temperature coefficient of Gibbs free energy) in 

Fig. 2 corresponds to the entropy when pressure is constant. From the figure, we can see the 

relation SL>Sms>Ss between entropies of liquid phase SL, stable phase Ss and metastable phase Sms. 

Therefore, for the change of entropy, ΔSf, caused by melting, we can find a relation ΔSf,s >ΔSf,ms

(i.e., the change of the entropy is smaller when the liquid phase changes to metastable phase). 

From the relative relations of the three phases above, we can see that the metastable phase is to 

be a higher entropy phase than the stable phase [14]. 

Factors determining the entropy of material are first, density of material and secondly, 

symmetry of arrangement of atoms and/or molecules making up the material. Therefore, high 

entropy phase is liquid rather than solid, and gas rather than liquid. Among solid phases, it is 

guessed that the low-density phase becomes higher-entropy phase. In conclusion, we can say that 

the metastable phase is lower density, higher symmetric material than the stable state. 

Experimental procedure and results

Spherical samples of REFeO3 were prepared from high purity (99.99%) RE2O3 and Fe2O3 

powders. Levitation and melting of samples were carried out by an aerodynamic levitator, ADL, 

which was designed in order to solidify undercooled melts under the precisely controlled Po2. 

Details of the sample preparation and experimental facility are shown elsewhere [15, 16].

Goldschmidt [17] discussed the stability of the perovskite (ABO3) structure using the 

tolerance factor, TF: 

 



TF 

R

A

 R

O

2 R

B    RO

. (3) 

In the present investigation, ionic radii of RA, RB and RO correspond to those of rare-earth 

element, iron and oxygen, respectively. From the systematic investigation, he summarized that 

the perovskite structure is stable at TF > 0.8 and contrary unstable at TF < 0.8. Using Shannon 

ionic radii from La (0.1216 nm) to Lu (0.1032 nm), TF’s for the REFeO3 system were calculated 

to be 0.905 for LaFeO3 to 0.841 for LuFeO3. Therefore, the perovskite structure is expected to be 

stable in the REFeO3 system. 

Figure 4 shows SEM micrographs of samples of REFeO3 solidified in containerless conditions. 

As shown in the photographs, their surface profiles vary according to the type of rare-earth 

elements. The surface of LaFeO3 is nonfacetted and spherical while that of LuFeO3 is facetted and 

polyhedral [18]. Note that the different surface features result from differences in crystal structure, 

not differences in rare-earth element. Specifically, the lattice structure of the LaFeO3 sample is 

orthorhombic, the space group of which is Pbnm (o-REFeO3), while the LuFeO3 sample is a 

hexagonal-symmetric of P63cm (h-REFeO3). As the h-REFeO3 phase has a 10%~20% smaller 

Figure 4. Surface profiles of REFeO3 (RE=La, Lu) formed by containerless process. LaFeO3

with large ionic radius has a smooth and spherical surface while LuFeO3 with small radius 

has a rugged and polyhedral surface.

131
density than that of perovskite [19], it is estimated that the hexagonal crystal is a higher entropy 

phase than perovskite. In other words, the hexagonal crystal should have intrinsically become 

stable perovskite. However, having been largely undercooled to below TE,ms indicated in Fig. 2 by 

the containerless process, hexagonal crystals of high-entropy phase grew as metastable phase 

(Fig. 5). In fact, when we forced it to solidify at a temperature of around TE,s even by the same 

containerless process, stable-phase perovskite appears. 

Figure 5 shows typical images taken sequentially during recalescence in samples of LaFeO3, 

GdFeO3 and YFeO3, each of which is processed at oxygen environment. The elapsed time 

indicated in each image was set to 0 s for one frame before the nucleation. At oxygen 

environment, although single recalescence that can be ascribed to the phase transition from 

undercooled melt to equilibrium perovskite phase was observed in LaFeO3 samples, double 

recalescences were observed in GdFeO3 and YFeO3 samples, where a primary phase was 

solidified from the undercooled melt and then the secondary phase with higher brightness was 

initiated at the interface between melt and the primary phase. The high brightness implies that 

the melting temperature of the secondary phase was much higher than that of the primary phase. 

This result indicates that decrease of TF facilitates the undercooled melt to solidify into the 

metastable h-REFeO3 phase rather than the stable o-REFeO3 phase. According to this result, 

reduction of the oxygen partial pressure Po2 is expected to extend the range of TF for metastable 

h-REFeO3 phase to be formed, because decreasing Po2 increases the amount of Fe2+ (0.078 nm 

for CN=6) with larger ionic radius than that of Fe3+ (0.0645 nm for CN=6). 

In addition, YbFeO3 was used to study the effect of ionic radii of RE elements on the 

formation of metastable phases, because the ionic radius of Yb3+ (0.1042 nm) was slightly larger 

than that of Lu3+ (0.1032 nm). Figure 6 shows the XRD patterns of YbFeO3 samples processed at 

controlled Po2. At 105Pa of Po2, the stable orthorhombic phase (o-YbFeO3) was formed at the 

second recalescence as in the cases of GdFeO3 and YFeO3. At 104

Pa of Po2, however, the 

Figure 5. Sequence photographs of HSV images taken during recalescences in the REFeO3 (R = La, 

Gd and Y) samples processed at oxygen environment. Although in LaFeO3 single recalescence was 

observed, in GdFeO3 and YFeO3 double recalescences indicating the formation of metastable phases 

were observed.

132
metastable hexagonal phase (h-REFeO3) remained, forming the dual phase with o-YbFeO3, and 

at 9103Pa, the o-YbFeO3 phase did not appear [20]. 

These results suggest that the decrease of Po2 facilitates the undercooled melt to solidify to 

metastable h-REFeO3 phase rather than the stable o-REFeO3 phase, particularly in samples with 

RE3+ of relatively small ionic radius. 

Figure 7 shows the relation between the recalescence results and Po2 as a function of the ionic 

radii of RE3+, in which Shannon ionic radii for CN=9 were used. Decrease of Po2 extends the 

Figure 6. XRD patterns of the YbFeO3 samples processed at Po2=105Pa, Po2=104Pa and 

Po2=9103Pa, respectively. At 105Pa of Po2, the stable orthorhombic phase (o-YbFeO3) was 

formed at the second recalescence as in the cases of GdFeO3 and YFeO3. At 104Pa of Po2, 

however, the metastable hexagonal phase (h-REFeO3) remaind, forming the dual phase with oYbFeO3, and at 9103

Pa, the o-YbFeO3 phase did not appear.

Figure 7. Relation between the recalescence results and Po2 as a function of the ionic radii 

of RE3+, in which Shannon ionic radii for CN=9 were used.

133
range of TF for metastable h-REFeO3 phase to be formed. 

Discussion

Bertaut et al. [21] and Yakel et al. [22] have first reported the two hexagonal modifications in 

the ABO3 systems, the space groups of which are P63/mmc and P63cm, respectively. In the hREMnO3 system, the P63cm type modification was formed as a low temperature phase for RE3+

with small ionic radius (Ho-Lu, Y and Sc), whereas the P63/mmc type modification was reported 

as a high temperature phase. In our experiment, although the space group of the metastable hREFeO3 phase belonged to P63cm, the high temperature phase can be deduced to belong to the 

P63/mmc space group because the ionic radius of Fe3+ is as same as that of Mn3+ (0.0645 nm for 

6 coordination) [23]. Hence, in this investigation, the geometrical analysis of the atomic 

configuration in h-REFeO3 is developed on the assumption that the space group of the primary 

phase is P63/mmc. 

The atomic configuration of the P63cm modification in ABO3 system can be described as a 

dense oxygen-ion packing (ABCACB) with B3+ ions having coordination number CN=5 (fivefold distorted trigonal bipyramidal coordination), and A3+ with CN=7 (seven-fold monocapped 

octahedral coordination), forming a noncentrosymmetric structure. On the other hand, 

centrosymmetric P63/mmc is assumed to be described simply with B3+ ions of undistorted CN=5 

and A3+ of CN=6 (octahedral coordination). 

Figure 8 shows the geometrical configuration among RE3+(CN=6), Fe3+(CN=5), and O2- in a 

space group of P63/mmc, in which the constituent ions are packed without any spacing between 

neighboring ions. As shown in this figure, the relation among the ionic radii of constituent ions 

of h-REFeO3 is expressed as

 



R

RE

 R

O



6

2

R

Fe    RO

. (4)

Therefore, the h-REFeO3 phase will be ideal when the next equation is fulfilled, 

 



TF 

3

2

 0.87

 (5) 

The experimental results showed that the h-GdFeO3 phase is formed because TF of which is 

0.867. Whereas, the h-EuFeO3 is not formed because of slightly high TF of 0.871. This suggests

Figure 8. Geometrical configuration among RE3+ (CN=6), Fe3+ (CN=5), and O2-in a space 

group of P63/mmc, in which the constituent ions are packed with no space between 

neighboring ions. The h-REFeO3 phase will be the ideal at TF = 0.87.

134
that the aforementioned condition expressed by Eq. 5 is to be the criterion for the metastable hREFeO3 phase to be formed. 

Conclusion

Using REFeO3 (RE: rare-earth element) as a model material, containerless solidification for 

forming a metastable phase from undercooled melts was carried out as a function of Po2 (oxygen 

partial pressure). Based on the geometrical consideration on the ionic radii of constituent ion, 

RE3+, Fe3+ and O

2-

, tit was derived that the criterion for metastable hexagonal phase is also 

expressed by tolerance factor, TF: 

 TF < 0.87.

Experimental result well agreed with this criterion under reduced Po2 as well as for ambient 

conditions.

Acknowledgement 

This work was financially supported by a grant-in aid for Scientific Research from the Ministry 

of Education, Culture, Sports, Science and Technology. 

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135
ON-LINE REAL TIME DIAGNOSTICS OF A SINGLE FLUID 

ATOMIZATION SYSTEM 

P. Delshad Khatibi, A. Ilbagi, H. Henein 

Chemical and Materials Engineering Department, University of Alberta; 

Edmonton, Alberta, T6G 2V4, Canada 

Keywords: Online measurement, Undercooling, Radiant energy 

Abstract 

A drop tube-Impulse Atomization technique was used to produce copper droplets. In this 

method, energy is transferred to a liquid by plunger movement resulting in spherical droplets 

emanating from orifices. A mathematical model of the evolution of droplet velocity and 

temperature at various heights for different sized droplets was developed. A two-color 

pyrometer, DPV-2000, and a shadowgraph were used to measure droplets radiant energy, 

diameter and velocity. The temperature values from the model were used to assess the two color 

pyrometer assumption over the temperature range of measurement. The DVP 2000 

measurements were found to be dependent of droplet size wavelength and position of droplets 

below the atomizing nozzle. By calibrating the instrument for effective emissivity over the range 

of measurements, the thermal history of droplets may be recorded using a single color pyrometer 

approach. 

Introduction 

The variation of undercooling or cooling rate in solidification provides the possibility to control 

morphology and size of crystal structures. A number of models have been reported for predicting 

or estimating the undercooling temperature of droplets in gas, single fluid atomization systems 

and drop tubes [1-3]. A systematic effort on the investigation of thermal history of falling 

droplets in-situ during atomization is rare [4]. In addition to the thermal history, in-situ 

measurement of the droplet diameter, initial velocity and instantaneous velocity is crucial in 

validating solidification models. 

Delshad Khatibi et al. [5] used a two-color pyrometer, DPV-2000, to measure droplets effective 

radiant energy for different droplet sizes. It was also shown that the acceleration of falling 

droplets near the melting point is close to gravitational acceleration and as a result the falling 

droplets do not reach their terminal velocity at their melting point. 

The DPV-2000 is a high-speed two colors pyrometer that measures the effective radiant energy 

of the particles. According to Planck’s radiation law, the total energy radiated by a spherical 

particle can be expressed as: 

   

1

2

5

1







T

C

ii

i

i

e

C

E







(1) 

136
where, C1= 3.74×108 W.μm4

/m2and C2=1.4387×104μm.K [6]. ε(λi) is emissivity of the particle 

at λi, wavelength, and T is temperature. In two color pyrometry a measure of the energy radiated 

from a body is taken at two wavelengths. It is assumed, for a two color pyrometer, that over the 

measured temperature range the emissivity is not a function of wavelength. In this work, we aim

to test this assumption for molten copper droplets atomized into an argon gas atmosphere. Under 

these conditions, equation (1) would be re-written in the following form for the DVP 2000:

   

1

2

52

1







T

C

i i

i

i

e

dC

Q







(2) 

where Q(i) is proportional to E(i) and is the quantity measured by the DVP 2000, (i) is

proportional to ε(λi) and d is the droplet diameter in μm. The DVP 2000 reports measurements of 

Q at two specific wavelengths designed into the instrument. For these experiments these 

wavelengths are, 1 is 0.787 m and 2 is 0.995 m. A mathematical model was also utilized to 

predict the temperature of falling copper droplets at different heights. Thus, for a given 

measurement of Q for a known droplet size and distance below the nozzle, (i) at each 

wavelength is the only unknown in Equation (2) and will be compared for values determined at 

1 and 2. 

Experimental 

Copper with 99.99% purity (Alfa Aesar) was melted in a graphite crucible using a 20kW 

induction furnace located at the top of a drop tube-impulse atomization (IA) tower. An argon 

atmosphere of 90ppm oxygen was maintained in the tower during melting and atomization. 

After melting the copper, it was superheated to 1400°C and the molten metal was atomized 

through orifices located at the bottom of the crucible, forming ligaments, which spherodized into 

droplets in the inert argon atmosphere. The falling droplets were cooled in free fall by the argon 

atmosphere. 

DPV-2000 (Tecnar Automation Ltée) was used to measure effective radiant energy, Q(1) and 

Q(2) , and a shadowgraph (Sizing Master Shadow from LaVision GmbH in Gottingen, 

Germany) was used to measure velocity and droplet size of the falling copper droplets. These 

sensors were installed on a translation stage inside the drop tube that was capable of moving in 

all three directions. DPV-2000 is an optical sensing device based on a patented technology 

developed by the National Research Council of Canada. DPV-2000 uses a dual slit optical sensor 

that can measure the effective radiant energy of up to 800 droplets per second with the depth of 

field of 1.9 mm, and it can measure radiant energy of droplets at two different wavelengths [7].

Sizing Master Shadow from LaVision uses the backlighting technique to visualize droplets for 

image analysis. A light source, in this case a pulsed laser combined with diffuser optics, 

illuminates in-flight droplets 5 times per second. Backlight of droplets inside the measurement 

volume of 666 mm is captured by a high resolution high-speed imaging system. It is possible 

to investigate droplet sizes down to 5μm using the shadowgraph device. 

The focal point of both shadowgraph and DPV-2000 was set to an imaginary vertical line going 

through the center of the bottom of the crucible; therefore, any unfocused droplets would be 

rejected by the criteria set by the operator in the respective software of each instrument. Particle 

137
size distribution of falling droplets (D50 and D90), velocity and effective radiant energy were 

measured at three distances of 10, 30 and 50 cm from the orifices. 

Modeling 

A mathematical model was developed to calculate droplet velocity and temperature for different 

sized copper droplets at different heights. 

Droplets begin their downward trajectory with an initial velocity. The subsequent trajectory of 

the droplets depends on this initial velocity, and the forces of gravity, buoyancy and drag as 

shown schematically in Figure 1. 

Figure 1. Schematic of the forces acting on a falling droplet. 

Applying Newton’s Second Law on the droplet, the instantaneous acceleration can be found as 

follows [8]: 

 

 

 

 

 

 

 

 

 (3) 

where v is the relative velocity between the droplet and the atomization gas, (which in the case of 

IA vgas=0) and ρp and ρg are the densities of the droplet and atomization gas, respectively. The 

gravitational acceleration is given by g and d is the droplet diameter. The discharge coefficient, 

Cd, for droplets in laminar flow is given by [9]: 

 

 

 (4) 

The Reynolds number, Re, in Equation (4) is given by: 

 

 

 

(5) 

where μg is the gas viscosity. 

To calculate the thermal history of a falling droplet, the heat energy loss from the surface of the 

droplet to the surrounding gas is given by: 

 (6) 

138
where heff is the effective heat transfer and consisting of the additive contribution of convection, 

conduction and radiation heat transfer mechanisms, A is the surface area of the droplet, Tm is the 

droplet surface temperature and T0 is the gas temperature. It is assumed that the surface 

temperature of the droplet represents the entire droplet temperature, since the internal 

temperature gradient within the droplets are negligible (Biot <0.1) [10]. It is also assumed that 

the temperature increase due to surface oxidation is negligible [5]. The effective heat transfer 

was calculated using the modified Whitaker correlation introduced by Wiskel et al. [10], as 

shown in Equation 7. 

 

   

















 











 

s

g

gs

m

g

m

s

g s

eff

TT

TT

mk

C

k

dh

Nu



 2/1 4/03/2

1 1

4.0 Re .0 06 Re Pr

)1(

2

(7) 

In Equation 7, Nu is the Nusselt Number, Pr is the Prandtl number and d is the droplet diameter. 

kg and ks are conductivity of gas and droplet, respectively while μg and μs are viscosity of gas and 

the droplet, respectively. From the variation of gas conductivity with temperature (kg=CT

m

) for 

argon C = 1.86×10-4 and m = 0.7915 [4], the model was able to closely predict the range of time 

and distance in which the droplets completely solidified under the condition that ks be evaluated 

at the metal droplet surface temperature and the Re and Pr numbers evaluated at the free stream 

gas temperature. Table 1 lists the properties of pure copper and argon which were used in the 

model. 

Table I. Thermophysical properties of copper and argon at 1400°C and 40°C, respectively [11] 

Temperature

(°C)

Thermal 

conductivity

(W/K.m)

Specific heat

(J/kg.K)

Density

(kg/m3)

Viscosity

(m

2

/s)

Copper 1400 325 532 7722 -

Argon 40 0.01853 20.80 0.287 0.021

Results and Discussion 

During atomization, the shadowgraph was continuously collecting droplet size and velocity data 

at 5Hz frequency. A total number of 6100 droplets were measured during the entire atomization 

time of two minutes. Using the shadowgraph, D10=456 μm and D50=565 μm were measured. 

These two particles sizes will be used as examples to show the process of droplet temperature 

calculations. 

In our previous work [5], it was shown that the particles exit the orifices with an average initial 

velocity of 1 m/s. Using this initial average velocity, the temperature of droplets with diameters 

of 456 and 565 μm was determined using the model described above and the results are plotted 

as a function of distance from the nozzle plate (Figure 2). These predicted temperatures will be 

used below to estimate values for (1) and (2).

139
Figure 2. Temperature vs. distance from nozzle plate (plotted using the model) for two different 

particle sizes. Dashed line shows distance from the nozzle plate. 

While the shadowgraph was collecting the velocity and diameter of falling droplets at the center 

of the spray, the DPV-2000 was measuring the effective radiant energy of the droplets from the 

same location. Figure 3(a) shows the signal counts of each droplet that DPV-2000 measured at 

two wavelengths. The area under the curves shown in Figure 3(a) represents the effective radiant 

energy and as such it is a dimensionless value. The effective radiant energies measured for 

different droplet sizes at two different wavelengths are shown in Figure 3(b). The measurement 

shown in Figure 3 was done at 10cm below the nozzle plate. It can be seen that larger droplets 

have higher effective radiant energy. The same trend was also observed when measurements 

were performed at 30 and 50cm distances from the nozzle plate. 

Figure 3. (a) The signal counts measured at two different wavelengths for a single droplet, (b) 

Radiant energy vs. droplet diameter measured by DPV-2000 at 10cm below the nozzle plate. 

140
From Figure 3, the effective radiant intensity values for the particles with diameters of 456 and 

565 μm were averaged and were plotted at three different distances in Figure 4. The error bars in 

this figure represent the standard deviation from the averaged radiant energy measured for each 

particle size. The lines connecting the data points of same distances are only for better 

visualization of the results. 

Figure 4. The radiant energies measured by DPV-2000 for 456 and 565 μm particle sizes at three 

different distances from the nozzle plate. 

From Figure 4, it is evident that larger particles have emitted higher effective radiant energies at 

each wavelength. 

To find effective emissivity, (λ1) and (λ2), values at λ1 and λ2 for different particle sizes at 

different heights, the effective radiant energies shown in Figure 4 and the respective 

temperatures from Figure 2 were used in Equation 2. The results are shown in Figure 5. 

From Figure 5 it can be seen that there is a difference between (λ1) and (λ2) and by extension, 

ε(λ1) and ε(λ2) over the temperature range of interest in these experiments. This difference 

decreases with increasing temperature. Based on the two-color pyrometer’s theory, it is assumed 

that the wavelengths are chosen so close that the change in emissivity as a result of wavelength 

change is insignificant. This assumption may be more precise at high temperatures where the 

radiant energy as a function of wavelength does not significantly change, above 1600K in Figure 

5 for copper. However, below 1600K for copper, having the effective emissivity values as a 

function of temperature for each wavelength will allow the calibration of DPV-2000 as a onecolor pyrometer. Further work is required to determine if the values of ) and ) are a 

function of material and to test this approach over a wider range of droplet sizes and 

temperatures. 

141
Summary 

Diameter, velocity and radiant energy of falling droplets of copper were measured using a 

shadowgraph and a two-color pyrometer, DPV-2000. The measurements were performed at 10, 

30 and 50 cm from the nozzle. DPV-2000 measured radiant energy at two different wavelengths 

on atomized copper droplets. A mathematical model of droplet cooling was developed and used 

to predict the particle temperatures at different heights. The temperature values from the model 

were used with the measured values of effective radiance intensity to estimate values of )

and ). Both of these values are proportional to the respective emissivities. It was found that 

the values of α were a function of temperature and wavelength. Using these values it is feasible 

to calibrate the DVP instrument and use it as a single color pyrometer. 

0.000E+00

2.000E-03

4.000E-03

6.000E-03

8.000E-03

1.000E-02

1.200E-02

1.400E-02

1.600E-02

1.800E-02

1200 1300 1400 1500 1600

()

Temperature (K)

456 μm - λ1

456 μm - λ2

565 μm - λ1

565 μm - λ2

Power (456 μm - λ1)

Power (456 μm - λ2)

Figure 5. Effective emissivity, () as a function of temperature 

Acknowledgments 

The authors would like to thank Natural Sciences and Engineering Research Council of Canada

(NSERC) and Canadian Space Agency (CSA) for the financial support and Nils Ellendt for the 

modeling. 

References 

[1] A. Prasad et al., ISIJ International, Vol. 49 (2009), No. 7, 992.

[2] A. Prasad and H. Henein, Journal of Material Science, Vol. 43 (2008), 5930

[3] N. Zeoli, S. Gu and S. Kamnis, International Journal of Heat and Mass Transfer, Vol. 51, 

(2008), 4121 

[4] S. Whitaker, AIChE, Vol. 18(2), (1972 ), 361. 

[5] P. Delshad Khatibi et al., (Paper presented at ISPS 2011 Conference, Bonn, Germany, 2011). 

[6] J.P. Holman, Heat Transfer, (8

th Edition, McGraw Hill Inc., 1997).

[7]Tecnar Automation Ltée. DPV-2000 Calculation Principles, http://www.tecnar.com

/images/stories/TSS/DPV/pdf/dpv_calculation_principles.pdf, Accessed: August 22, 2011 

142
[8] Y. Ding, H. Henein and Fallavollita, US Patent 5,609,919 (1997); Canadian Patent 2,147,407 

(2003) 

[9] J.B. Wiskel et al., Canadian Metallurgical Quarterly, Vol. 41 (2), (2002), 193. 

[10] J.B. Wiskel, H. Henein and E. Maire, Canadian Metallurgical Quarterly, Vol. 41(1), 

(2002), 97. 

[11] J. Brillo and I. Egry, International Journal of Thermophysics, Vol. 24, (2003), 1155. 

143
Electrodeposition of Metals in Microgravity Conditions 

K. Nishikawa1

, T. Homma2, E. Chassaing3, M. Rosso1and Y. Fukunaka2,4 

1Laboratoire de Physique de la Matière Condensée, UMR CNRS 7643, Ecole 

Polytechnique, 91128 Palaiseau Cedex, France 

2Dept. of Applied Chemistry, Waseda University, Japan 

3

IRDEP, UMR CNRS 7174, 6 quai Watier, 78401 Chatou Cedex, France 

4

JAXA, ISS Science Project Office, Tsukuba, Japan 

E-mail: fukunaka@energy.kyoto-u.ac.jp

Abstract. Metal electrodeposition may introduce various morphological variations 

depending on the electrolytic conditions including cell configurations. For liquid 

electrolytes, a precise study of these deposits may be complicated by convective motion 

due to buoyancy. Zero-gravity (0-G) condition provided by drop shaft or parabolic 

flight gives a straightforward mean to avoid this effect: we present here 0-G 

electrodeposition experiments, which we compare to ground experiments (1-G). Two 

electrochemical systems were studied by laser interferometry, allowing to measure the 

concentration variations in the electrolyte: copper deposition from copper sulfate 

aqueous solution and lithium deposition from an ionic liquid containing LiTFSI. For 

copper, concentration variations were in good agreement with theory. For lithium, an 

apparent induction time was observed for the concentration evolution at 1-G: due to this 

induction time and to the low diffusion coefficient in ionic liquid, the concentration 

variations were hardly measurable in the parabolic flight 0-G periods of 20 seconds. 

1. Introduction 

Electrochemical interfacial phenomena in microgravity environments have not 

been paid much attention by electrochemical scientists and engineers. It is simply 

because of the apparently successful operated energy as seen even in the case of Apollo 

the 13th. However, the energy storage and power generation, as well as materials 

processing, may be planned in the international space station (ISS) and in the projects 

beyond ISS project after the year 2020. The study of electrochemical interfacial 

phenomena under microgravity conditions will be likely focused in the future. 

Electrodeposition or electrochemical dissolution of metal in aqueous solution is a 

good subject for primary stage of microgravity electrochemical research. Its reaction 

mechanism is relatively well understood. Moreover, the electrochemical reaction rate 

can be easily controlled by changing the current density or potential. A transition from 

144
smooth metal film to dendrite morphology is sometimes found. From the viewpoint of 

fundamental electrochemistry, dendritic electrodeposits may provide simple model 

systems to study Diffusion Limited Aggregation (DLA) phenomena. Furthermore, 

dendrite growth was intensively studied for practical reasons, because of its detrimental 

role in battery technology[2]. The surface flatness is a key issue to guarantee a longer 

reversibility of secondary battery during charging/discharging repetition. Thus, the 

coupling behavior between the morphological variations and ionic mass transfer rate 

must be indispensably understood. 

However, for liquid electrolytes, and whatever the electrochemical system under 

concern, a precise study of dendrite growth mechanisms may be complicated by 

convective motion due to buoyancy: even in thin, quasi-two-dimensional horizontal 

cells with vertical electrodes, electrodeposition is accompanied by a gravity-induced 

fluid flow at the electrodes. This effect is due to the electrolyte stratification near both 

electrodes: it has been extensively studied in the recent literature, both theoretically 

[4-6] and experimentally [7-11]. Convective motion mixes the electrolyte and tends to 

homogenize the concentration. The effect depends on cell configuration, salt 

concentration, and current density [4]. This convection driven by buoyancy was shown 

to increase the instability of system [11]. Obviously, zero-gravity experiments [12] 

should give a straightforward mean to avoid this effect: apart from gravity, no other 

parameter is altered. However, these experiments are relatively difficult to work out and 

of short duration (around 2-25 s in the most affordable facilities: drop tower and 

parabolic flight). 

JAMIC experiments are described at first in this paper. Then, we present a study 

of electrodeposition in the zero-gravity environment provided by parabolic flights. This 

project is supported by the Centre National d'Etudes Spatiales (CNES): it plans copper 

electrodeposition on copper electrode and lithium electrodeposition on Ni electrodes in 

the Airbus A300 from Novespace. During these flights, almost zero-gravity conditions 

are available over 20 to 25 seconds. The project includes the concentration 

measurements by laser interferometry, and similar but longer experiments performed at 

1-G in the cathode over anode configuration. Both series of experimental results are to 

be compared with numerical calculations. 

2. Experimental 

2.1 JAMIC Drop Tower 

A quasi two-dimensional electrolytic cell(Figure1) was used in JAMIC. A 1 mm 

145
diam. disk cathode was placed at the center of the cell and a flat ring-shaped anode was 

placed at the outer edge. The thickness of the cathode made of 1 mm diam. copper wire 

was adjusted to 100μm by a polishing technique. The circular periphery area of this 

disk was used as the effective surface of the cathode. Copper foil of 100μm thickness 

was perforated to manufacture a flat ring-type anode with a 20 mm inner diam. Both 

electrodes were sandwiched by two sheets of slide glass. This electrolytic cell was filled 

with 0.9 M CuSO4 aqueous solution. Electrolysis was carried out at relatively high and 

constant current densities in order that the interference fringe pattern would appear soon 

after starting the electrolysis. Thus, the diffusion layer grows in the radial direction.

Figure 1. Quasi Two Dimensional Cell with Figure 2 2-D Cell with Linear 

Radial Diffusion Field Diffusion Field 

A common path microscopic interferometry was installed in JAMIC drop capsule. 

Laser diodes of 5 mW with 680 nm wavelength and a luminous light-emitting diode 

were used to observe the interference fringe pattern and the field image, respectively. 

The electrolytic cell was horizontally installed in this common path interferometry 

inside a drop capsule. The thickness of electrolyte solution in the cell was measured to 

be 200 μ m, because both sides of cathode were coated for insulation with 

polyvinylchloride (PVC). The concentration dependence of refractive index of the 

electrolyte solution is 0.028 M-1and the resolution power of concentration was, 

therefore, 5.7 x 10-2M CuSO4 per fringe.

JAMIC drop capsule started to descend by gradually reducing the electromagnetic 

current. It was evacuated around the inner capsule, and the outer capsule was 

accelerated by injecting gas at an appropriate rate to compensate the friction of air 

inside the drop shaft. A microgravity level less than 10-4G was thus attained 1 s after 

disengagement of the capsule. The descending capsule started to decelerate in the 

braking zone 10 s after free fall. It experienced about 8G as a maximum value. The 

146
electrolysis was, therefore, initiated 1 s after the capsule had descended the shaft. 

Electrolysis was then terminated at 9 s in order to recover the sample electrodeposited 

only under microgravity conditions.

2.2 CNES Parabolic Flight 

The project presented here consists in carrying out electrochemical experiments 

during zero-g periods provided by parabolic flights. The experiments are carried out in a 

pseudo 2 D-cell (Figure 2). Care is taken to obtain a well defined cell geometry: see Ref. 

[19] for details. Copper electrodes are used for the copper electrodeposition and nickel 

electrodes are for lithium electrodeposition: they are held between two glass plates 

enabling to observe in-situ the electrochemical cell. 

The cell has a parallelepipedic shape, with dimensions L, the active length of the 

electrodes, l, the inter-electrode distance, and d, the thickness of the cell, respectively. 

Practical values are: L =0.8-1.5 cm, l ≈ 0.09 - 0.3 cm, d ≈ 0.012 - 0.1 cm. The lateral 

faces of the electrodes are coated with a hydrophobic film which limits the invasion of 

the electrolyte between the electrodes and the glass plates. After filling the cell with the 

electrolyte, it is sealed with a two-component resin before taking off: this allows to keep 

it under almost constant conditions for several hours [19]. During the flight, the 

temperature in the plane is kept at 18°C. Two electrochemical systems are studied: 

copper deposition from 0.1 and 0.2 mol L-1 aqueous CuSO4 solutions and lithium 

deposition from an ionic liquid.The ionic liquid is

N-methoxymethyl-N-methylpyrrolidinium bis (trifluoro-methane-sulfonyl) imide 

containing 1.0 mol L-1 LiTFSI. 

In these microgravity experiments, a concentration variation of the electrolyte 

induced a variation of the refractive index which is measured by a common path laser 

interferometer. Laser interferometry is a well known technique for measuring the 

concentration changes in electrochemical processes [20-24]. This study use the 

interferometer described in Ref. [12] to measure in-situ the variations of ionic 

concentration in the electrolyte. Because a thin cells (0.1 or 1 mm) is used, we expect 

negligible light deflection due to refractive index gradients [25]. The laser source is a 

laser diode with 685 nm wavelength. The change in the refractive index is given by 

 dΔn=Sλ (1)

where d: the optical path length in the electrochemical cell (here the cell thickness), Δn: 

147
the change of refractive index, S: the shift of interference fringes, and λ: the laser 

wavelength. The relationship between refractive index and concentration is given by: 

C

C

n

n 















 (2)

We use (∂n/∂C) = 0.0279 for a CuSO4 aqueous solution and (∂n/∂C) = 0.005 for LiTFSI 

in ionic liquid. The transient behaviour of this shift is recorded with a CCD camera. 

Electrolytic condition is conducted at a constant current or a constant potential. 

3. Results 

3.1. Copper Electrodeposition in JAMIC Drop Tower 

Figure 3 compares the time variation of interferograms recorded during the 

electrodeposition of copper at 0.2 A cm-2 from 0.9 M CuSO4 solutions obtained under 

1- and 0-G conditions. At 1 s after the start of electrolysis, a single fringe appears 

around the cathode under 1-G field, whereas no clear image of interference fringe is 

seen under 0-G. At 4 s, the annular fringe pattern under 1-G recedes from the cathode 

surface, and a fringe appears in the vicinity of the cathode surface under 0-G. After 8 s, 

two clear interference fringes with broader width are observed in 1-G experiments. At 

the same time, several fringes with narrower width appear within a distance of 100μm 

from the cathode surface under 0-G. It is difficult to count exactly the number of 

interference fringe with naked eye. The growth of annular interference fringe along the 

radial direction indicates the development of the diffusion layer of Cu2+ions, when 

convection is not induced. The interferogram recorded at 15 s after the start of 

electrolysis is also illustrated. The electrolysis has already been terminated, and the 

capsule has started to decelerate. The less concentrated solution stays near the cathode 

and slightly concentrated electrolyte does near the anode (with larger surface area). The 

acceleration force starts to act upon the electrolyte. Then, the lighter electrolyte, 

constrained inside the diffusional layer adjacent to the cathode, begins to flow to induce 

a convection loop along two glass sheets. This is a reason why the radius of the annual 

interference fringe pattern suddenly increased. 

In 1-G experiments, the fringes are observed at nearly constant intervals along the 

radial direction. The time variation of Cu2+ ion concentration at the cathode surface 

is compared with a transient diffusion model. It decreases much slower than the 

calculated value. The ionic mass-transfer rate from the bulk electrolyte to the cathod 

surface superimposed by the fluid flow induced even in such a quasi two-dimensional 

shallow electrolytic cell may partly contribute such a deviation as well as the optical 

148
deflection error. 

Figure 3 Transient Variations of Interference Fringes and Diffusion Layer 

Thickness with Progress of Cu Electrodeposition(0-G and 1-G) 

Followed by a Kind of Natural Convection Confined in Quasi-2D Cell 

in Deccelerated Zone 

The laser beam propagates in a straight line in the bulk electrolyte. Considering 

the resolution power of concentration in the present optical arrangement (0.057 M per 

fringe), the outer periphery of the diffusion layer may be conventionally defined as the 

position where the concentration is decreased by 3% of the bulk value. Thus, the time 

variation of the diffusion layer thickness can be reasonably measured. At 0.2 A cm-2, the 

diffusion layer thickness increases proportionally to the square root of time in 0-G. 

The measured diffusion layer thickness in 1-G begins to deviate from this linearity at 1 

or 3.2 s after the starting electrolysis, most probably due to induced natural convection. 

149
The development of diffusion layer thickness under microgravity is analyzed 

based on the one-dimensional transient diffusion model including the migration effect. 

Although the physical properties of diffusivity and the transfer number vary with 

electrolyte composition, constant physical properties of diffusivity and the transfer 

number are assumed as a first-order approximation. A diffusivity of 5 x 10-6 cm

2

 s

-1 and 

a cation transference number of 0.3 are found to give good agreement with measured 

thicknesses, which are very close to the reported values.22 

Figure 4 illustrates the potential difference between both electrodes. It almost 

stays constant at 620 mV for the first 3 s. It suddenly increases by 200 mV around 4 s, 

followed by a slight increase of potential difference in 0-G experiment, while the 

measured potential difference is slightly raised to 680 mV at 4 s and remained afterward 

under 1-G. The abrupt jump in the potential difference may be referred to the increase in 

concentration overpotential caused by depleted cupric ion at the cathode surface, since 

the calculated surface concentration of Cu2+ ion reached zero at 3 s. This behavior has 

been partly observed in the ground level experiment with a horizontal cathode over 

anode configuration. After the considerable concentration overpotential develops, 

copper grains grow in a dendritic manner (see Appendix below). 

Figure 4 Time Variations of Potential 

 Difference 

 (0.9 M CuSO4, 0.3Acm-2) 

The effect of microgravity on the morphology of copper electrodeposited at 

0.05 and 0.3 A cm-2 is seen in Figure 5. The electrolysis was conducted over 8 s under 

both environments. If copper could be precipitated densely and uniformly on the 

copper substrate without any voids, the average film thickness would be 0.17 and 1.0 

μm, respectively. At ground level, a number of 0.5 μm diameter grains is uniformly 

precipitated at both current densities. Since the averaged size of grains is close to the 

resolution power of the scanning electron microscope (SEM), it is difficult to 

distinguish any difference on the crystallographic aspect of each grain obtained at both 

current densities. These most likely grow in a three-dimensional nucleation manner.

150
Surprisingly, the significant difference of morphology of copper obtained at the 

same current density is noticed between 0-G and 1-G level experiments. Some grains 

grow with preferential growth of the lower index planes at both current densities. A 

similar difference is also noticed at 0.1 A cm-2. At the low current density, the size of 

larger grains reaches about 2 μm, which is about four times larger than that for the 

sample obtained at ground level. The preferential growth of lower index planes is more 

evident at the lower current density.

It is not clear why such a morphological difference is induced by the different 

level of gravitational acceleration. As shown above, the surface concentration of 

Cu2+ ion is more quickly lowered with time under microgravity, so long as the 

electrolysis conducted at the same current density. The main part of the cathode surface 

is thus exposed to the electrolyte with lower concentration of Cu2+ ion under 

microgravity. If we assume that the three dimensional nucleation rate becomes low with 

decreasing in the surface concentration of Cu2+ ion, we obtain fewer nuclei on the 

cathode under microgravity. When the electrolysis is conducted to produce the same 

amount of charge, more coulombic charge should be distributed on the particular grain 

to introduce the larger-sized grains. 

Figure 5. Comparison of Morphology of 

Copper Electrodeposited in Terrestrial and 

Microgravity Experiment: Scratched Traces 

by Sandpaper are Visible on Cu Disk 

Substrate Surface with 100μm Thickness.

(0.9M CuSO4 Aqueous Solution, 

Electrodeposition Duration Period of 8s, 

0.05 and 0.3A cm-2) 

However, the electrical conductivity is simultaneously reduced with lowered 

concentration of Cu2+ ions near the cathode. It may considerably influence the ionic 

mass transfer rate due to the migration effect.25 Thus, the concentration gradient of Cu2+

ion near the cathode is no longer the same as that observed at the initial stage of 

151
electrolysis. Further measurement of Cu2+ ion concentration profile is necessary to 

understand the morphological variations introduced by the different level of the 

gravitational acceleration force. 

3.2. Electrodeposition of Metals in Parabolic Flight 

3.2.1 Copper Electrodeposition

The concentration profile in the electrolyte for various current density and bulk 

concentrations were measured. Figure 6 shows an example of interference fringes 

recorded before and 15 seconds after the beginning of cell polarization. The fringes are 

bent in the vicinity of the cathode, at the bottom of the Figure, because of the variation 

of concentration. These concentration variations were also numerically calculated, 

using general equations describing transport phenomena in electrochemical systems 

[30], and taking into account the variation of the diffusion coefficients with 

concentration in CuSO4 aqueous solution [31]. The time variation of surface 

concentration during and after one parabola is shown in Figure 7, and compared with 

theory. A good agreement between experimental and theoretical results was found. 

After the end of the parabola, the concentration at the cathode rapidly returned to its 

initial value: this was due to the large G (~ 1.5G) imposed at this time in order to drive 

back the plane to its normal flying conditions. 

We have reported elsewhere [28] the concentration measurements obtained in 1-G 

conditions in a C/A cell. A similar agreement with theory was found for measurements 

performed before the onset of dendritic growth. On the other hand, in experiments 

performed in horizontal cells [4, 7-10], the concentration variations were markedly 

different. 

We did not observe dendritic growth of copper, however, because the dendrites 

only appear after an induction time ts the so-called Sand time [29]. Sand time varies as 

2

2

0

2

1( )

J

De C

t

aC

S

  

 (3)

where e is the electronic charge, D the diffusion constant, μc and μa the cationic and 

anionic mobilities respectively, C0 is the initial ionic concentration and J the current 

density. To obtain a Sand time shorter than 10 seconds for example, would require a 

high current density (larger than 20 mA cm-2) and still, the time available for observing 

dendritic growth would be very short. Experimental environment did not provide such 

conditions in this parabolic flight campaign. Further experiments are thus necessary to 

explore dendritic growth itself. 

152
 

Figure 6. Interference Fringes before Figure 7. Surface Concentration Variations 

of and 15 s after Cu Electrodeposition. Cu2+ Ion during Parabolic Flight 

(0.1 mol L-1; 8 mA cm-2; (Closed Circles: Measured, 

Cell Thickness is 0.1 cm.) Dashed Line: Calculated) 

3.2.2. Lithium Electrodeposition

As mentioned earlier, a series of experiments were performed at 1-G. A typical 

result is given in Figure 8, showing the variation of the cathode surface concentration as 

a function of the square root of time. In this experiment, the applied current density is 

1 mA cm-2 and cell thickness is d = 0.1 cm. One clearly sees a first stage, where the 

concentration variation is very small. Then, after 90 s, the variation is much more rapid. 

Such an induction time was already reported [16]. In the present case, the observed 

induction time might be due to side reactions of the electrodeposited lithium metal. 

The diffusion coefficients in the ionic liquids are very low: from the variation of the 

diffusion layer thickness in the experiment illustrated in Figure 8, we estimated a 

diffusion coefficient D ~ 10-7 cm

2

s

-1. This value is in good agreement with that reported 

by other methods [32]. The concentration variations are compared in Figure 8 with 

theoretical values [30], which are calculated without taking into account the 

concentration dependence of the diffusion coefficient. The result obtained for the above 

value of D and a transport number of 0.35 for Li+ ion is shown in Figure 8 as a straight 

continuous line. 

Due to the very low diffusion coefficient and to the induction time mentioned 

above, the concentration variations due to Li deposition during the first 20 seconds are 

very small, and the concentration boundary layer thickness is at most 10 μm: thus 

concentration variations are hardly detectable by our interferometer. The Li deposition 

experiment would thus require longer zero-g times. 

153
Figure 8. Li Electrodeposition in Ionic Liquid: Time Variations of Cathode Surface 

Concentration with Electrolysis(Experimental: Circles, Calculated: Straight Line). 

4. Conclusion 

A common path microscopic interferometry was used in a drop shaft experiment. 

The growth of the diffusion boundary layer of Cu2+ ions associated with 

electrodeposition of copper in CuSO4 solution was successfully measured in situ under 

microgravity. When the electrolysis is conducted at 1-G, natural convection is induced 

even in such a shallow electrolyte layer of 200 μm thick. Meaurements of voltage 

drop betwen both electrodes show the abrupt increase in potential difference, just after 

the surface concentration of Cu2+ ion reaches zero under 0-G. A significant difference 

of morphology appears after only 8s of electrodeposition at constant current density. 

Larger grains with preferential growth of lower indexes are obtained in 0-G. The 

coupling effect of the diffusion and migration mechanism to the electrocrystallization 

phenomena under 0-G must be further examined. It is necessary to monitor the 

transient variation of cathodic overpotential with the progress of electrodeposition. 

Although of limited duration, parabolic flight experiments can provide interesting 

results on the early stage of electrodeposition. The evolution of concentration profiles is 

consistent with the theoretical calculation curves. However, a more complete study 

would require further zero-g experiments: for studying dendritic growth of copper, or 

for deposition of lithium from ionic liquids, longer times would be necessary (such as 

those available in sound rockets or in the International Space Station). These 

improvements are considered in the framework of a project developed by an 

“Electrochemical Nucleation & Growth” ESA Topical Team.

5. References 

[1] Witten T A and Sander L M 1981 Phys. Rev. Lett. 47 1400

[2] Tarascon J-M and Armand M 2008 Nature 451 652

154
[3] Laitinen H A and Kolthoff I M 1939 J. Am. Chem. Soc. 61 3344 

[4] Chazalviel J-N, Rosso M, Chassaing E and Fleury V 1996 J. Electroanal. Chem. 

407 61 

[5] Marshall G, Mocskos P, Swinney H L and Huth J M 1999 Phys. Rev. E 59 2157

[6] Kawai S, Nishikawa K, Fukunaka Y and Kida S 2007 Electrochim. Acta 53 257

[7] Rosso M, Chazalviel J-N, Fleury V and Chassaing E 1994 Electrochim. Acta 39 507

[8] Barkey D P, Watt D, Liu Z and Raber S 1994 J. Electrochem. Soc. 141 1206 

[9] Huth J M, Swinney H L, Mc Cornmick W D, Kuhn A and Argoul F 1995 Phys. Rev. 

E 51 3444 

[10] Argoul F, Freysz E, Kuhn A, Léger C and Potin L 1996 Phys. Rev. E 53 1777

[11] Marshall G, Mocskos E, González G, Dengra S, Molina F V and Iemmi C 2006 

Electrochim.Acta 51 3058 

[12] Fukunaka Y, Okano K, Tomii Y, Asaki Z and Kuribayashi K 1998 J. Electrochem. 

Soc. 145 1876

[13] González G, Marshall G, Molina F and Dengra S 2002 Phys. Rev. E 65 051607

[14] Marshall G, Perone E, Tarela P and Mocskos P 1995 Solitons and Fractals 6 315

[15] Sagués F, López-Salvans M Q and Claret J 2000 Phys. Reports 337 97 

[16] Nishikawa K, Fukunaka Y, Sakka T, Ogata Y H and Selman J R 2007 Electrochim. 

Acta, 53 218

[17] González G, Rosso M, Chassaing E and Chazalviel J-N 2007 Electrochim. Acta 53 

141 

[18] Morisue M, Fukunaka Y, Kusaka E, Ishii R and Kuribayashi K 2003 J. Electroanal. 

Chem. 559 155

[19] Rosso M, Chassaing E, Chazalviel J-N and Gobron T 2002 Electrochim. Acta 47 

1267

[20] McLarnon F R, Muller R H and Tobias C W 1976 Electrochim. Acta 21 101

[21] Denpo K, Okumura T, Fukunaka Y and Kondo Y 1985 J. Electrochem. Soc 132, 

1145

[22] Eriksson R 1995 Electrochim. Acta 40 725 

[23] Konishi Y, Tanaka Y, Kondo Y and Fukunaka Y 2000 Electrochim. Acta 46 681

[24] Fukunaka Y, Nakamura Y and Konishi Y 1998 J. Electrochem. Soc. 145 3814

[25] Beach KW, Müller R H and Tobias C W 1973 J. Opt. Soc. Am. A 63 559 

[26] Hawkes J B, Astheimer R W 1948 J. Opt. Soc. Am. 38 804 

[27] Nishikawa K, private communication 

[28] Nishikawa K, Chassaing E and Rosso M 2011 Electrochim. Acta 56 5464

[29] Sand H J S 1901 Phil Mag 1 45

155
[30] Marciano S L and Arvia A J 1983 Diffusion in the absence of convection: steady 

state and nonsteady state Comprehensive Treatise of Electrochemistry Ed Yeager E, 

Bockris J O'M, Conway B E, Sarangapani S (New York: Plenum Press) vol.6, pp. 

65-132, 

[31] Miller D G, Rard J A, Eppstein L B and Robinson R A 1980 J. Solution Chem. 9 

467 

[32] Nishikawa K, Naito H, Kawase M and Nishida T 2011 Lithium Batteries 

Discussion Arcachon (France) 

6. Acknowledgements 

The authors thank Drs. H. Yasuda (Osaka University), Y. Inatomi (Japan Aerospace 

Exploration Agency - JAXA), K. Kinoshita (JAXA), M. Takayanagi (JAXA), S. Yoda 

(JAXA) and T. Maki (Olympus Corporation), for providing the interferometer 

especially designed for zero-g experiments that we used in this study. The authors also 

wish to thank Dominique Clément for the mechanical realization of the experimental 

setup and for the many technical solutions which he brought to the problems involved in 

the operation of this experiment in parabolic flight. Centre National d'Etudes Spatiales 

is acknowledged for its financial support and Novespace for help in preparing the 

experiments.

156
PROPAGATION REGIME OF IRON DUST FLAMES

François-David Tang1, Samuel Goroshin2

, Andrew J. Higgins2

1

European Space Agency, ESTEC; Kaplerlaan 1, Noordwijk, The Netherlands 

2McGill University, Department of Mechanical Engineering 

817 Sherbrooke St. West, Montreal, Quebec, Canada 

Keywords: Iron powder, Combustion, Reduced gravity 

Abstract 

A flame propagating through an iron-dust mixture can propagate in two asymptotic regimes. 

When the characteristic time of heat transfer between particles is much smaller than the 

characteristic time of particle combustion, the flame propagates in the continuum regime where 

the heat released by reacting particles can be modelled as a space-averaged function. In contrast, 

when the characteristic time of heat transfer is much larger than the particle reaction time, the 

flame can no longer be treated as a continuum due to dominating effects associated with the 

discrete nature of the particle reaction. The discrete regime is characterized by weak dependence 

of the flame speed on the oxygen concentration compared to the continuum regime. The discrete 

regime is observed in flames propagating through an iron dust cloud within a gas mixture 

containing xenon, while the continuum regime is obtained when xenon is substituted with 

helium. 

Introduction 

While the combustion of powdered metals is typically associated in solid-fueled rockets, metallic 

fuels have recently been suggested as a potential energy carrier for transport vehicles as a 

sustainable alternative to fossil fuels [1]. To exploit these applications, a fundamental 

understanding of the metallic dust flames is essential, but experimental results on fundamental 

flame parameters are scarce. As a result, current models of laminar flames propagating in 

heterogeneous media are simply based on a continuum assumption, which underlines the fact 

that the interparticle spacing lp is much smaller than the flame width ld . This assumption permits 

the function describing the heat release in the reaction zone to be described as a continuous 

function in space and the governing equations to be expressed as a set of partial differential 

equations [2,3]. 

However, if the interparticle distance lp is on the same order of magnitude as the flame width ld, 

then the continuum fails to describe the propagation of the flame. The flame exhibits so-called 

discrete characteristics where the propagation of the front is dominated by local ignition 

interactions between neighboring particles. The heat release function becomes dependent on the 

spatial distribution of the particles and the flame can no longer be described by the continuum 

theory [4,5]. The parameter quantifying the regime of propagation is the dimensionless 

combustion time defined as: 

 

 

 

 

, (1) 

where  is the characteristic particle reaction time, α is the thermal diffusivity of the gas mixture 

and l is the interparticle spacing. The discrete regime is realized when , while the 

continuum is when for [5]. 

157
In this work, the discrete regime is realized in an experiment where flames propagating through 

clouds of iron particulates are observed in low-thermal diffusivity, oxidizing environment, 

namely xenon-oxygen. In contrast, the continuum regime is achieved in a helium-oxygen 

mixture, which is characterized by a high thermal diffusivity. The continuum and discrete 

regimes are differentiated using the response of the flame speed to variations of the oxygen 

concentration. 

Experimental Details 

The dust combustion experiment uses a flame in an open-ended tube methodology that allows 

observation of a constant-pressure laminar flame. Experiments were performed onboard a 

reduced-gravity parabolic flight aircraft to minimize particle settling and buoyancy-induced 

convective flows. The schematics of the apparatus are shown in Fig. 1. 

Figure 1: Schematic of the dust combustion experiment. The top left 

inset illustrates the design of the dust dispersion system and the 

bottom right inset the design of the first stage cooling and filtering 

systems. 

The powder dispersion unit (see top inset in Fig. 1) consists of a dust feeder and a disperser. The 

initially compacted dust is fed via a small syringe-type device. As it is fed by the piston, the dust 

column is continuously dispersed via the impact of a transverse sonic gas jet produced by a 40-

100 m circular slot at the base of the conical dispersion chamber (see top inset in Fig. 1). The 

conical diffuser connecting the dust dispersion unit with the combustion tube provides expansion 

and laminarization of the initially highly turbulent flow containing the dispersed dust. The 

combustion tube is a Pyrex glass tube with an internal diameter of 48 mm and 70 cm in length. 

The fuel suspension was ignited by an electrically heated, 100-m-diameter tungsten wire at the 

open end of the tube, with the flame propagating toward the closed end (dispersion system). 

The fuel concentration in the suspension is controlled by varying the piston speed via an 

electromechanical linear actuator. The fuel concentration in the suspension can be calculated 

from the feeding rate of the piston, the packing density of the powder and the flow rate of the 

dispersing gas. The mass concentrations of iron powder varied in a range of 900-1,200 g/m3.

The exhaust from the combustion tube is passed through a series of cooling and filtering systems 

and ultimately discharged into the aircraft cabin to accommodate the absence of overboard vents. 

The combustion products were drawn into the inlet of the cooling and filtering system by a 

vacuum blower. A wide annular opening between the combustion tube and the filtering system 

158
(see Fig. 1, right bottom inset) ensures unobstructed access of the ambient air to maintain 

ambient pressure inside the combustion tube. 

The combustion rack installed onboard the airplane contains four replaceable combustion tube 

assemblies (see Fig. 2). Each assembly is comprised of a combustion tube integrated with an 

individual powder dispersion unit. Four tube assemblies are mounted horizontally parallel to 

each other on the combustion rack, providing an unobstructed view for the optical diagnostics. 

Figure 2: Photograph of the dust combustion apparatus installed on 

board parabolic flight aircraft.

The propagation of the flame was recorded by a high-speed digital camera viewing the entire 

length of the combustion tube at 300 frames per second. 

The use of iron, that burns entirely in the condensed phase was motivated to ensure that the 

reaction occurred in the absence of the formation of vapors in the attempt to reflect the 

heterogeneous characteristics of the modeling. Five different iron powders were used in this 

investigation and are designated A to D in the Table I. Scanning electron microscope 

photographs of the iron powders are shown in Fig. 3. 

Table I: Characteristics of iron powders used. 

Powder Particle shape Purity d10 (m) d32 (m)

(a) Spherical 99.9 3.3 4.3

(b) Spherical 99.5 7.0 9.6

(c) Spherical 99.5 9.9 13.7

(d) Spherical 99.9 26.8 44.7

Figure 3: Scanning Electron Microscope (SEM) pictures of iron 

particles. 

159
The experiments were performed using four separate gas mixtures containing helium or xenon at 

21% and 40% oxygen concentration. The mixtures were supplied by a commercial gas supplier 

and were certified to be within an accuracy of 2%.

Results 

The measured flame speeds from experiments performed in xenon and helium with 21% and 

40% oxygen concentration are shown in Fig. 4. The flame speeds were obtained from the 

analysis of the high-speed videos. The error bar represents the standard deviation from 5 to 11 

repeated measurements performed with the same gas mixture. 

Figure 4: Flame speed measurements for iron powders A, B, C and 

D in He-O2 and Xe-O2 in 21% and 40% oxygen concentrations. 

The flame speed is consistently larger in mixtures containing 40% compared to 21% oxygen. 

However, the difference is more pronounced in mixtures containing helium, compared to 

mixtures containing xenon. 

Discussion 

The ratios between the flame speeds in 40% and 21% oxygen concentration are shown in Fig. 5 

for helium- and xenon-balanced mixtures and for the different particle sizes. The ratios 

corresponding to measurements performed in xenon mixtures are consistently lower than those 

for helium mixtures for all particle sizes. These measurements highlight the fact that flames 

propagating in xenon mixtures are less sensitive to variations of the oxygen concentration 

compared to flames in helium mixtures. 

Using the shrinking core model of particle combustion, the oxygen concentration is related 

linearly to the particle combustion time ) [6]. Hence, flame speeds measured in 

xenon mixtures are less sensitive to changes of the particle reaction time compared to 

mixtures containing helium.

160
Figure 5: Ratio of the flame speeds between 21% and 40% O2 in 

helium and xenon mixtures. 

The stronger dependence of the flame speed on the reaction time  in mixtures containing 

helium compared to mixtures balanced with xenon can be attributed to a transition of the 

propagation regime from continuum to discrete. When substituting the balance inert gas from 

helium to xenon in 21% O2, the dimensionless combustion time  decreases from approximately 

3.4 (continuum) to 0.6 (discrete). 

Previous numerical work has shown that the discrete regime is characterized by a front speed 

that is independent of the reaction time  [7]. In other words, variations of the oxygen 

concentration do not affect the flame speed in the discrete regime. The flame speed ratio 

characteristic of the discrete regime reflects the insensitivity of the flame speed on the reaction 

time : 

 

 

|

 

 . (2) 

The distinctive relationship between the flame speed and the reaction time  in the discrete 

regime is in sharp contrast with the front speed predictions in a continuum. Using thermal theory, 

the flame speed is expected to vary with the reaction time 

 as √ ⁄ 

[8]. This behavior 

results in a ratio of the flame speeds 

 

⁄ : 

 

 

|

 

 √

 

 

 √

 

 

 . (3) 

In the discrete regime, the fact that flames exhibit a different behavior to change of the oxygen 

concentration from the predictions of the thermal theory highlights the breakdown of the 

continuum assumption. In the discrete regime, the heat release around particles becomes 

161
localized to the point that the propagation of the flame becomes limited by the heat diffusion 

between neighboring particles. When the flame is controlled mainly by heat diffusion, the 

increase of the reaction rate of particles does not cause an increase of the flame speed. 

In contrast, in a continuum, the limiting mechanism is assumed to be the particle reaction rate. 

This assumption underlines the fact that the reaction rate is treated as continuous function in 

space, where particles form a dense cloud of particles characterized by an interparticle spacing 

much smaller than the flame width. 

Conclusion 

The regime of propagation was investigated by varying the transport properties of the gas 

mixture as a means to change the dimensionless combustion time. Reduced experiments were 

performed in xenon- and helium-balanced mixtures to observe discrete and continuum flames, 

respectively. It was found that flames propagating in xenon-balanced mixtures were less 

sensitive to the oxygen concentration than flames propagating in helium-balanced mixtures. 

This observation reflects the distinct behavior of the flame propagation between the discrete and 

the continuum regimes. 

Acknowledgement 

This work was supported under Canadian Space Agency contract 9F007-052073/001/ST. The 

authors would like to thank the Flight Research Laboratory of the National Research Council of 

Canada. 

References 

1. K. Kleiner, “Powdered Metal: The Fuel of the Future,” New Scientist, 2522 (2005), 34- 

2. S. Goroshin, M. Bidabadi, J.H.S. Lee, “Quenching Distance of Laminar Flame in 

Aluminum Dust Clouds,” Combustion and Flame, 105 (1996), 147-160. 

3. A.G. Merzhanov, E.N. Rumanov, “Physics of Reaction Waves,” Reviews of Modern 

Physics, 71 (1999), 1173-1211.

4. J.M. Beck, V. Volpert, “Nonlinear Dynamics in a Simple Model of Solid Flame 

Microstructure,” Physica D, 182 (2003), 86-102. 

5. S. Goroshin, J.H.S. Lee, Y. Shoshin, “Effect of the Discrete Nature of Heat Sources on 

Flame Propagation in Particulate Suspensions,” Symposium (International) on 

Combustion, (27) 1998, 743-749. 

6. J.C. Yang. “Heterogeneous Combustion,” Environmental Implications of Combustion 

Processes, ed. I.K. Puri (Boca Raton, FL: CRC Press, 1993), 127-128. 

7. S. Goroshin, F.D. Tang, A.J. Higgins, “Reaction-diffusion fronts in media with spatially 

discrete sources,” Physical Review E, 84 (2011), 027301. 

8. L.P Yarin and G. Hetsroni, Combustion of Two-Phase Reactive Media (Berlin: SpringerVerlag, 2004), 301.

162
Containerless Processing on ISS: Ground Support Program for EML 

Angelika Diefenbach1, Stephan Schneider2, Rainer Willnecker1

1DLR Microgravity User Support Centre, Linder Hoehe, 51149 Cologne, Germany

2DLR, Institute for Material Sciences in Space, Linder Hoehe, 51149 Cologne, Germany

Keywords: electromagnetic levitation, containerless processing, reduced gravity, experiment 

preparation 

Abstract 

EML is an electromagnetic levitation facility planned for the ISS aiming at processing and 

investigating liquid metals or semiconductors by using electromagnetic levitation technique 

under microgravity with reduced electromagnetic fields and convection conditions. Its 

diagnostics and processing methods allow to measure thermophysical properties in the liquid 

state over an extended temperature range and to investigate solidification phenomena in 

undercooled melts. 

The EML project is a common effort of The European Space Agency (ESA) and the German 

Space Agency DLR. The Microgravity User Support Centre MUSC at Cologne, Germany, has 

been assigned the responsibility for EML operations. For the EML experiment preparation an 

extensive scientific ground support program is established at MUSC, providing scientific and 

technical services in the preparation, performance and evaluation of the experiments. Its final 

output is the transcription of the scientific goals and requirements into validated facility control 

parameters for the experiment execution onboard the ISS. 

Motivation for electromagnetic processing in space 

Containerless processing in the earth laboratory is an attractive way to provide high-purity 

environment to high-temperature or highly reactive materials. It is particularly suited to give 

access to the meta-stable state of an undercooled melt. In the absence of container walls, the 

nucleation rate is greatly reduced and high levels of undercooling can be obtained. 

Electromagnetic levitation is also suited for the in-situ study of properties of metallic melts. It 

allows to levitate bulk samples of several grams at high temperatures (well above 2000°C)and to 

measure the thermophysical properties of freely suspended liquid melts. 

However, there are limitations to terrestrial electromagnetic levitation, due to sample 

deformation, turbulent currents and high process temperatures caused by the required high 

electromagnetic fields. Most of these effects can be avoided if performing electromagnetic 

levitation under reduced gravity conditions. 

For the electromagnetic positioning of a sample against external residual forces in the 

microgravity environment, only weak field strength is required. Therefore, disturbances of the 

liquid sample by the magnetic field such as stirring or shape deformation are avoided. It becomes 

possible to reach very low process temperatures and to access deep levels of undercooling, 

respectively. Furthermore, thermophysical data of materials can be derived over a large 

temperature range including the undercooling regime. 

EML Payload and Experiment Overview 

The electromagnetic levitation Facility EML is planned to fly in the Columbus Module of the 

International Space Station in ESA’s European Drawer Rack. The EML accommodation in EDR 

163
is depicted in Fig. 1. EML is a successor of the German payload TEMPUS, which has been 

successfully flown on the Spacelab Missions IML-2, MSL-1 and MSL-1R. In recent years, EML 

development models have repeatedly been flown on parabolic flight and TEXUS missions. 

The EML facility is equipped with the so called SUPOS (Superpositioning) coil system, which 

allows to decouple heating and positioning of the sample. Several diagnostics are available for 

the experiment performance. From the top, the temperature is measured with a pyrometer and 

two digital video cameras are used for the process control and scientific aims. The side view 

camera is a high speed camera and will be used for frames rates up to 30 kHz. 

Fig. 1: EML Accomodation in European Drawer Rack (figures provided by Astrium) 

Fig. 2: EML coil and CuCo sample in a sample holder 

(TEXUS campaign) 

The planned experiments cover two 

major groups measurement of thermophysical properties (viscosity, surface 

tension, density, thermal expansion, 

etc.) and solidification. For investigation of the sample solidification the 

solidification can be triggered at an 

aimed undercooling level, the growth 

velocity of the solidification front can 

be measured via the high speed camera 

and after the experiment the microstructure of the in microgravity 

solidified sample can be analyzed. 

EML is able to process liquid metals or doped semiconductors containerless by the 

electromagnetic levitation technique in the range of 600 to 2000°C. The absence of gravity 

allows to decouple heating and positioning and to work under an environment with reduced 

164
electromagnetic fields and convection conditions. The experiments are divided into the following 

classes: 

Class A: Undercooling and solidification speeds 

Measurement of undercooling temperature (with pyrometer) 

Measurement of solidification velocity with optional high-speed video 

Triggering of solidification by touching the undercooled sample with a nucleation trigger 

needle (part of sample holder) 

Class B: Heat capacity, effective thermal conductivity, enthalpies, solid fraction 

Modulation of the heating input power into the sample, measure temperature response 

Measurement of the electrical power loss to the sample 

Class C: Surface tension and viscosity 

Pulsing the heating field to induce surface oscillations in the molten sample to observe 

surface oscillation frequencies and decays by video cameras 

Class D: Thermal expansion 

Measure the sample size with video cameras using subpixel resolution techniques 

Class E: Electrical conductivity 

Measure electrical data of the levitation system and optionally additional electrical 

diagnostic circuits to detect the changes of the coupling between the RF magnetic fields 

with sample temperature. 

All experiment classes require an ultra clean processing environment in form of either ultrahigh 

vacuum or high purity noble gas atmosphere at reduced pressure inside the hermetically tight 

process chamber. 

EML Operations Overview 

As an ESA payload, EML will be operated by the Microgravity User Support Center in Cologne, 

Germany, which is one of the nine User Support and Operation Centers (USOCs) in Europe. The 

European USOCs are responsible for the different ESA racks and experiments onboard the 

International Space Station. The tasks of the USOCs are to prepare on-orbit operations, conduct 

these and also act as a link between the responsible investigators and the experiments. Before the 

experiments are operated the ground segment has to be prepared and procedures need to be 

developed and validated. These depend on the experiment and can also involve the scientists as 

to what they want to achieve. Another important task is to support the crew training that has to 

happen on ground long before the actual experiment takes place. During on-orbit operations it is 

the USOCs responsibilities to receive telemetry from “their” payloads and send the necessary 

telecommands to perform the experiments. Also the scientists get their data usually from the 

USOCs. 

EML will be integrated in Columbus Rack EDR. This drawer rack will be operated from the 

ERASMUS USOC in Noordwijk. Both USOCs are part of the established ESA Ground Segment 

in which MUSC acts as the Center responsible for EML operation. As such MUSC is responsible 

for planning and preparation of the EML on-orbit activities and for executing these and 

ERASMUS, as Facility Responsible Center for the EDR Rack, provides the single point-ofcontact for all EDR real-time operations. 

EML is planned for an in-orbit lifetime of 6 batches with 18 samples each, shared between a 

broad international scientific community. For the first batch of experiments, ten research teams 

are planning for 36 individual investigations, all requiring multiple melt and solidification cycles 

on one sample. 

165
Data services to and from EML (Commanding, Telemetry and file uplink) are based on the 

utilization of the standardised systems available to all European USOCs - customised for EML 

requirements. In that scenario, MUSC will be responsible for the operation preparation for the 

experiments in EML, e.g. the development of crew and ground command procedures and all 

additional operational products (e.g. ground displays) needed to perform the experiments 

onboard the ISS. 

For the operation of the latest parabolic flight and sounding rocket campaigns, MUSC has 

developed an EML specific evaluation software for science telemetry and video which is already 

in use during current campaigns. With this software – TeVi – the scientists can monitor the video 

and housekeeping data of his experiments in parallel. For the process analysis the video data can 

be displayed in slow motion or frame by frame synchronously with the sample and facility data. 

A subroutine for the oscillation analysis of oscillating samples was added. It allows to measure 

the edge of the sample and to calculate two perpendicular radii, the area, center of mass etc. of 

the sample for each frame. From this data viscosity and surface tension data can be derived. A 

FFT from a selected region of the data can be performed and the oscillation frequency measured. 

In a further step the surface tension is calculated from the oscillation frequency. Fig. 3 shows 

screen shots of the TeVi tool for a parabolic flight experiment. 

The EML data are archived in a data archive based on the Hypertest platform. This archive was 

adapted to the structure of EML data and archives, video and facility data and documents from 

the historic missions. The data are stored together with metadata, which enables the user to 

search the data after sample material, principal investigator, campaign etc. The data can be 

distributed easily and fast via this archive and can be downloaded into the TeVi tool. The archive 

serves also as long term data archive fulfilling ESA and DLR data holding policies. 

The existing tools will be adapted to serve the science teams during EML ISS operations. 

 

Fig. 3: Screenshot of the TEVI software. Left: synchronous display of video and facility 

data, Right: FFT analysis and oscillation frequency 

Experiment Preparation: Ground Support Program 

The experiment preparation is embedded in the overall operations preparation at MUSC.The so 

called ground support program covers all the activities needed to transcribe the individual 

scientific ideas into verified experiment and facility control parameters and files. It provides the 

166
scientific users with technical and scientific services in the preparation, performance and 

evaluation of the experiments. 

The ground support program consists of three major parts: 

Sample Characterization: 

The determination of certain sample properties which are mandatory for the individual EML 

experiment execution process are provided as a service to the science teams. 

The knowledge of the amount of sample material evaporating from the sample surface during 

processing is essential because the evaporated metal dust is a resource in the EML. The 

deposited metal layer growing on the coil is limited and the evaporated metal must not exceed 

certain limits.Therefore the evaporation rate of the each flight sample composition is measured 

by MUSC in a dedicated ground model on sample material provided by the science teams. 

The set-up consists of a UHV chamber in which a sample is heated to specified temperature. The 

mass loss is determined by an oscillating quartz crystal, refer to Fig. 4 and Fig. 5. 

The obtained data are later used for experiment planning purposes and finally the experiment 

execution in-orbit. During the real-time experiment operation, the evaporation rate is used to 

observe that the element specific limit values are not exceeded and to monitor the layerthickness 

growing on the coil. 

For the detailed planning of the experiments the coupling of each sample to the RF field has to 

be known. Since EML is not designed to levitate samples under terrestrial conditions all ground 

based measurements are performed on samples suspended in the coils. Due to the longstanding 

history of EML, a number of experimental data stemming from parabolic flight and sounding 

rocket campaigns are available and will support the experiment development. 

Fig. 4 (left): UHV Chamber with oscillating quartz crystal for mass loss/evaporation rate determination 

Fig. 5 (right): Sketch of the vacuum chamber; the lower part shows the sample placed in the ceramic crucible 

and the graphite cylinder within the HF coil. The oscillating quartz crystal is placed in the upper part. 

167
The obtained coupling data are later used in order to simulate the required temperature time 

profile of the sample with an EML simulator software. 

For a correct temperature measurement with the pyrometer onboard, the sample emissivity is 

measured in the EML ground model. The sample is heated until melting sets in and by a

comparison of the measured and literature melting point the emissivity can be derived. The 

optical setup and used pyrometer of the ground model is comparable to the EML facility. 

Fig. 6: EML Bread Board for Coupling Efficieny Measurements

Experiment development 

The first step in the experiment development is the definition of the individual operational 

process flow for each experiment, yielding the outline of all nominal and contingency operation 

For that purpose, so called science protocols are prepared on behalf of and in cooperation with 

the science teams, describing key temperatures of the samples to be reached, facility settings 

(camera, pressure, etc.) and a detailed experiment planning. These science protocols are the base 

for the further development of the parameter sets. 

EML experiments consist of two sets of parameters. The experiment parameters EPs define the 

experiment as planned. The so called limit parameters LPs provide an independent guard rail 

envelope of RF values for each experiment that can never be exceeded. 

The core parameters for both EP sets are developed with the EML experiment simulator, which 

currently predicts temperature time profiles from applied RF parameters and the coupling 

behaviour of the sample to the RF field. The EML experiment simulator will be upgraded by a 

facility part and enables MUSC to simulate the facility behaviour for an experiment run and to 

schedule the experiments accordingly. 

Experiment verification 

After development of the parameter sets they must be validated in a representative EML model. 

For this purpose, the Operational Model (OM), which also acts as flight spare will be used. 

During the validation the parameter sets will be processed on a suspended high melting sample 

and thus the parametersets and the experiment performance can be validated. Especially the 

timing and interleaving of the two independent sets of parameters will be checked. This task will 

be performed in close cooperation with the involved scientists, using the verification runs for 

168
user training on the operational environment they will use later during on-board experiment 

performance. 

Summary and outlook 

The EML facility is currently under development and is planned to be launched in 2013. The 

scientific users of the EML are supported by the DLR MUSC, which performs an extensive 

scientific ground support program. This covers the sample characterisation, experiment 

preparation and validation of experiment parameters. 

The experiments onboard the ISS are controlled and operated from the control room at DLR 

MUSC in Cologne via a dedicated ground segment. 

After the experiments the scientists are supported by the data evaluation and long term archiving 

of the data. 

Acknowledgements 

The EML Ground Support Program is funded by the European Space Agency (ESA). 

References 

[1] Electromagnetic levitation in microgravity: EML on parabolic flights, TEXUS and ISS, 

Schneider, Engelhard, Heuskin, Diefenbach, 4th International Symposium on Physical Sciences 

in Space (ISPS 4), 11. - 15. Juli 2011

[2] Support tools for TEMPUS / EML campaigns, Schneider, Engelhard, Heuskin, Diefenbach, 

4th International Symposium on Physical Sciences in Space (ISPS 4), 11. - 15. Juli 2011

[3] Structuring and Visualizing Microgravity Material Science Data in a Topic Maps Ontology.

Grimbach, Alois und Wever, Philipp und Schneider, Stephan und Willnecker, Rainer PV 

International Conference 2009, 1. - 3. Dezember 2009, Castillo de Villafrance, Spanien. 

[4] Measurement of thermophysical properties of liquid metallic alloys in a ground- and 

microgravity based research programm. The Thermolab Project. Fecht, Schneider, Wunderlich, 

Battezzati, Papandrea, Palumbo, Egry, Mills, Quested, Brooks, Giuranno, Novakovic, Passerone, 

Ricci, Seetharaman, Aune, Vinet, Garandet, J.-P. (2005) Metallurgica Italiana, 3, p 47-51.

[5] Preparation for Containerless processing on ISS: parabolic Flights with the TEMPUS facility, 

Diefenbach, Lohöfer, Dreier, Piller, 53rd IAC,2002 

169
HIGH-PRECISION TEMPERATURE CONTROL OF A CRYSTAL GROWTH 

FURNACE AT 1500°C 

Ch. Stenzel1, A. Hess2, A. Cröll2, D. Bräuer3, H. Sauermann3

1

Astrium GmbH, D-88090 Immenstaad 

2

Kristallographie, Institut für Geowissenschaften, Universität Freiburg, D- 79104 Freiburg 3

Institut für Automatisierungstechnik, TU Bergakademie Freiberg, D- 09596 Freiberg 

Keywords: Temperature control, Crystal growth, High-temperature furnace 

Abstract 

For crystal growth of semiconductor materials a short-term temperature stability of 0.1°C at 

1500°C is one of the essential parameters to be addressed for achieving high-quality crystals. 

Hence, for temperature monitoring and control with high precision in a floating zone furnace two 

sets of thermo-sensors, type B thermocouples and optical fibre thermometers, have been 

implemented and successfully operated in the furnace for more than 2000 h. The optical fibre 

thermometers consist of an optical system made of sapphire (two fibres plus a prism in between 

for deflection) and transmit the infra-red radiation of the heater to the outside of the hot core of 

the furnace for pyrometric temperature measurement. 

A dedicated control algorithm has been set up which controlled the power settings to the 

individual heaters. Both sensor types showed no degradation after this period and yielded a shortterm stability at 1200°C of 0.05°C (optical fibre thermometers), respectively 0.08°C 

(thermocouples). 

170
Introduction 

For float zone crystal growth experiments onboard the International Space Station (ISS) the 

implementation of a dedicated heater insert for the Materials Science Laboratory (MSL) is 

envisaged - the Float Zone Furnace with Magnetic Field (FMF) [1]. One of the essential 

parameters to control inhomogeneities in the crystal growth of semiconductor materials is the 

short-term temperature stability (in the range of several minutes) of the furnace [2]. To set up a 

high-precision temperature control system requires an appropriate combination of fast actuators 

(heaters) and sensors as well as the implementation of a sophisticated temperature control 

system. This furnace is especially designed to provide enhanced short-term temperature stability 

in addition to the ability of adjusting the temperature gradient at the solid-liquid interface over a 

wide range. The specification on the short-term stability of the furnace was set to T < 0.05°C. In 

a laboratory model of this furnace the above mentioned features have been addressed in the 

design and the performance has been verified in an experimental test programme. 

In the following sections of this paper the detailed design of the respective subsystems of the 

FMF furnace to achieve a short-temperature stability - the heating system, the thermometry and 

the control system - will be introduced, the results of the performance tests reported, and a 

summary given. 

Heating system 

In order to provide a zone profile at 1500°C with a variable gradient at the solid/liquid interface, 

the FMF is laid out as a furnace with 7 individually controlled heating zones. In the area of the 

central zone the melt zone is established. Figure 1 shows a conceptual sketch of the FMF. 

The single heaters consist of encapsulated graphite heaters where a meandered graphite layer 

(50 μm thick) is deposited onto a pBN substrate. The heating layer is covered by a second pBN 

CVD layer to protect it against mechanical, electrical, and chemical injuries. The resulting 

thickness of wall of less than 2 mm provides a rather low thermal mass enabling fast heating rates 

and temperature changes. Due to the excellent material compatibility between the different 

materials and similar values for thermal expansion this type of heater can bear up a power density 

of more than 30 W/cm2

 leading to a compact design for high-temperature applications and also to 

long operation times of more than 2000 h. Figure 2 shows a photograph of the central heater of 

the FMF. 

171
Figure 1. Conceptual sketch of the FMF exhibiting the inner structure of the furnace: The 

furnace provides a symmetrical thermal profile with seven individual heating 

zones. Six zones are placed symmetrically at both sides of the central heater (on 

each side a cooling zone, a support, and a plateau heater is placed). 

Figure 2. Encapsulated graphite heater for the central zone of the FMF: The thermal load of 

1000 W provides a maximum temperature of 1500°C in this area 

To improve the radial homogeneity of the temperature profile in the FMF tubular thermal 

diffusors for each zone consisting of a graphite tube have been employed; thereby the heaters are 

placed directly around the diffusors tubes. 

172
Thermometry 

For temperature measurement and control each zone is equipped with two thermocouples. 

Additionally, one optical sensor is employed in the three inner zones of the furnace. The 

temperature control system of the furnace could thereby use signals of both sensor types as input. 

Thermocouples

The application of sheathed thermocouples represents the standard technique for temperature 

measurement and control in furnaces. The achieved stability amounts to about 0.1 K [3]. For 

temperature above 1200°C only type C (W-Re) or type B (Pt-Rh) thermocouples can be applied. 

Because of the better drift behaviour type B thermocouples with the following specifications 

have been selected: 

 Type B (Pt-PtRh) 

 Insulation material: MgO 

 Sheath Pt (: 1,0 mm) 

 Protection tube Ta (: 1,6 x 0,1 mm) 

The thermocouples were implemented into axial borings in the diffusors. In order to prevent a 

degradation of the Pt sheath due to a reaction with the graphite diffusor they were inserted into 

protection tubes of Ta. The compatibility of this protection tube with a graphite environment has 

been tested in advance for several hours at 1600°C in vacuum (1 x 10-5 mbar). No drifts or 

degradation have been observed. 

Optical Sensors

According to Planck's law higher temperature resolution can be obtained by utilising optical fibre 

thermometry with sapphire fibres at elevated temperatures [4, 5]. These sensors consist of an 

optical waveguide to transmit the IR radiation emitted from a small spot of the object to be 

measured out of the hot region to a photo detector which converts the radiation signal into a 

temperature reading following Planck's law. For application in high-temperature facilities these 

waveguide are made of sapphire. Using straight fibres in a 4-zone laboratory furnace a short-term 

stability of 0.015°C has been achieved [6]. 

Due to constructional boundary conditions of the MSL facility the sensors may not be routed in a 

straight line within the furnace, one 90° bend of the sensors has to be included. For the FMF 

optical sensors this is realised by the Optical Deflection System (ODS) which is built up by 

straight sapphire waveguides. The path of rays is being deflected by a sapphire prism which is 

embedded in a support structure out of ceramic. To enhance transmission by focussing a sapphire 

lens is implemented behind the prism. Similarly to the thermocouple design the sapphire fibres 

are integrated into protection tubes out of alumina. The distance to the heater surface amounts to 

2 mm. The final transmission for the ODS was measured to amount to 40%. Three out of 7 

heating zones of the FMF have been equipped with an ODS as illustrated in Fig. 3. 

173
Figure 3. Example of an ODS sensor: The short arm is directed to the hot side, the long arm is 

routing the IR signal out of the hot core; at its end a conventional quartz fibre leads 

the signal to a Si photo diode located outside the furnace. 

Temperature Control Algorithm 

The developed high-precision temperature control system is based on a holistic view of the 

furnace, reflecting sensors, heaters, software, and control algorithm. Since the individual zones 

are not adiabatically separated from each other, an initial temperature difference between the 

zones causes thermal fluxes which affect the temperatures of the zones again. Hence, there exists 

a strong thermal coupling between zones and the furnace is to be regarded as a multiple-input 

multiple-output (MIMO) plant. In order to develop an optimised feedback controller the thermal 

properties of each zone as well as the couplings have to be known exactly. In a first step a 

theoretical furnace model based on conduction and radiation between the zones is established, 

and then the unknown parameters are experimentally determined by process identification. This 

means measuring the dynamic temperature changes as response to well-defined power signals 

(Pseudo Random Binary Signal). These semi-empirical data represent the input for the parameter 

estimation algorithm which finds optimised model parameters. The obtained model is basis for 

feedback controller synthesis [7]. The final controller consists of a separate, standard PID 

(Proportional-Integral-Differential)-algorithm for each zone with pre-filter, pilot control, 

decoupling and anti-reset-wind-up. This structure and the V-canonical plant are depicted in 

Fig. 4. A V-canonical structure means that the loop interactions are regarded as feedback 

couplings, hence modifying the input signal. 

174
Figure 4. Structure of the feedback controller and the V-canonical MIMO plant as example 

for two zones: Thereby the power setting to zone I depend on the temperature of 

this zone and of its neighbouring zones via the thermal coupling which have to be 

experimentally determined. 

Tests 

At 1200°C the short-term stability has been investigated for both type of sensors control 

(thermocouples or ODS). The central heaters were set to 1200°C and the cooling zone to 1100°C 

to establish a temperature gradient of 50 K/cm. The test was performed over a time period of 18 h 

(ODS) and 20 h (TC). In the central zone temperature fluctuations of T = 0.05°C could be 

achieved for control with the ODS sensors, and T = 0.051°C for control with thermocouples as 

shown in Fig. 5. In the cooling zones even better values of T = 0.026°C (ODS) and T = 

0.05°C (TC) could be obtained. 

Figure 5: Measured short-term temperature stability for control with ODS sensors (left) 

and with thermocouples (right)

Pilot control

PrefilterPIDcontrol

Pilot control

PrefilterPIDcontrol

Static decoupling

Static decoupling

-

-

-

+ -

+

Saturation

Saturation

-

-

Thermal 

coupling

Thermal 

coupling

Zone i

Zone i+1

Plant

175
Conclusion 

Temperature control within a multi-zone furnace for semiconductor crystal growth has been 

realised with thermocouples and optical sensors as input signals for the control algorithm. With 

both systems a superior short-term stability of less than T < 0.05°C could be achieved in a 

multi-zone high-temperature furnace. No significant performance difference between the two 

types of sensors could be detected in the investigated temperature range. However, for 

environments with strong EMC perturbations the optical sensors reveal some advantage. 

References 

[1] Hess, A., Cröll, A., Stenzel, C., Bräuer, D., Sauermann, H., Uhlig, V., Floating Zone 

Furnace with Rotating Magnetic Field. Poster Presentation DGKK 2010 

[2] Müller, G., Convection and Inhomogeneities in Crystal Growth from the Melt, in: 

Crystal Grwoth, Properties and Applications, Vol. 3, III-V Semiconductors, SpringerVerlag, Berlin, Heidelberg, New York, 1980 

[3] Sauermann, H., Schneider, K., Janda, O., Gröll, L.,Bretthauer, G., Linn, H., Das 

Gradient-Freeze Verfahren und seine regelungstechnische Umsetzung. 39. IWK der TU 

Ilmenau, 1994, p. 809-817 

[4] Schietinger, C.: Temperature measurements using optical fibers, in: Thermal 

Measurements in Electronics Coolings, ed. K. Azar CRC Press, 1997, Boca Raton 

[5] Dils, R. R., J. Appl. Phys, 54, 1983, p. 1198 

[6] H. Sauermann, Ch. Stenzel, S. Keesmann, B. Bonduelle, Cryst. Res. Technol. 36, 2001, 

p. 1329-1343 

[7] Hermann, K., Rehkopf, A., Sauermann, H., Betrachtungen zur Identifikation einer 

Gradienz-Freeze-Anlage zur Einkristallzüchtung, Automatisierungstechnik, Vol. 54 

2006, p. 566-573 

176
Investigation of Thermocapillary Convection of High Prandtl Number Fluid 

Under Microgravity 

Ruquan Liang, Guangdong Duan

Key Laboratory of National Education Ministry for Electromagnetic Processes of Materials, 

Northeastern University, No.3-11, Wenhua Road, Shenyang 110819, China 

Keywords: Liquid Bridge, Thermocapillary Convection, Velocity, Temperature

177
Abstract 

Thermocapillary convection in a liquid bridge, which is suspended between two coaxial disks 

under zero gravity, has been investigated numerically. The Navier-Stokes equations coupled with 

the energy conservation equation are solved on a staggered grid, and the level set approach is 

used to capture the free surface deformation of the liquid bridge. The velocity and temperature 

distributions inside the liquid bridge are analyzed. It is shown from this work that as the 

development of the thermocapillary convection, the center of the vortex inside the liquid bridge 

moves down and reaches an equilibrium position gradually. The temperature gradients in the 

regions near the upper center axis and the bottom cold corner are higher than those in the other 

regions. 

1. Introduction 

Thermocapillary flow is often encountered in the production of single crystals using the 

floating-zone method, and the oscillatory thermocapillary convection is responsible for the 

periodic variations in striations in the production process. The oscillatory thermocapillary 

convection becomes significant and causes detrimental striations in the chemical composition of 

the finished crystal. Schwabe and Scharmann1 and Chun and Wuest2 observed, for the first time, 

the three-dimensional time-dependent state in thermocapillary convection in their experiments. 

Since the first observation, many studies have been done on the half-zone model where a liquid 

drop is held between two coaxial rods remained at different temperatures to impose an axial 

temperature gradient on the free surface, and the liquid drop is held by surface tension force 

between two rods. The studies found in the literature mainly concerned with the following 

aspects, 

1. The transition and the successive mode structures of the flow field3-6

, 

2. The flow structures at a region close to the critical point of the transition, 

3. The information regarding the flow fields far beyond the critical point7.

In the high Pr range (Pr>10), although many experimental data can be found in literature, no 

accurate stability analyses nor numerical simulations of the oscillation phenomenon are currently 

available. One important aspect is that the dynamic free surface deformation was not considered, 

although it may play an important role in the oscillation mechanism. In the present paper, the 

temperature and velocity fields in the liquid bridge under zero gravity are investigated 

numerically considering the free surface deformation and the ambient air by using the level set 

method. The present study aims at understanding the time dependent thermal fluid phenomena 

with dynamic free surface deformation in the half-zone liquid bridge by a direct nonlinear 

numerical simulation.

2. Governing Equations 

The half-zone model consists of an initially cylindrical liquid bridge suspended between the hot 

and cold circular disks as shown in Figure 1. The ambient air is included in the computational 

region. The diameter of the top and bottom disks (

D

) is 5.0 mm, and the aspect ratio of the 

liquid bridge (H/D) is 0.7, where 

H

is the height of the liquid bridge. The temperature 

difference between two disks is defined as 

 T . The general governing equations of the problem 

178
for Newtonian incompressible flow conditions under zero gravity are given by the following 

non-dimensional Navier-Stokes, the continuity and the energy conservation equations8.

Interface Liquid 

Region

Symmetry 

Axis Air 

Region

Tt

Tb

R

H

2R

z

O x

Figure 1. Schematic of a thermocapillary convection model 

1 1 1 C a

( ) (2 ) ( )

R e W e W e t

p d  



                      

u u u D n (1) 

   u 0 (2) 

  1 2

t Re Pr

 

      



u (3) 

where 

u  ( , ) u v

 is the fluid velocity, 

   ( , ) x t

 is the fluid density, 

   ( , ) x t

 is the fluid 

viscosity, 

D

 is the viscous stress tensor, 



 is the curvature of the interface, 

d

 is the normal 

distance to the interface, 



 is the Dirac delta function, 

n

 is the unit normal vector at the 

interface, t is the time, 

p

 is the pressure. The key parameters are 

Re   l l U D





, Reynolds 

number, where

D

 is the initial diameter of the liquid bridge and 

U



 is the characteristic 

velocity defined as 

/ U T



  T l  

 in microgravity conditions, where 

 T

 is the temperature 

dependency of surface tension, 

t b    T T T

 is the temperature difference between the top and 

bottom disks; 

2 W e U D l





 

, Weber number; 

Pr l l

   a

, Prandtl number, where 

a

 is the 

thermal diffusivity; 

Ca   l U





, Capillary number; 

Ma  Re Pr

, Marangoni number. We 

denote 



 as the surface tension, 

  c T b

      T T , where 

c



 is a reference value of 

surface tension, and 



 as the excess temperature, 

( ) b     T T T . Here 

l



 and 

l



 are the 

dimensional liquid density and viscosity, respectively. 

g



 and 

g



 are the dimensional density 

and viscosity of ambient air, respectively. 

3. Level Set Function and Its Formulation 

The level set method was originally introduced by Osher and Sethian (1988)9to numerically 

predict the moving interface 

 ( )t

between two fluids. Instead of explicitly tracking the 

179
interface, the level set method implicitly captures the interface by introducing a smooth signed 

distance from the interface in the entire computational domain. The level set function 

 ( , ) x t

is 

taken to be positive outside the liquid bridge, zero at the interface and negative inside the liquid 

bridge. The interface motion is predicted by solving the following convection equation for the 

level set function of 

 ( , ) x t

given by10, 

t

      0 u (4)

( ) ( )

x y

u       u v (5) 

 

1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 ,

, 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2

( ) ( ) ( ) ( ) / (2 ) ( ) ( ) / (2 )

( ) ( ) / (2 ) ( ) ( ) / (2 )

x y i j i j i j i j i j i j i j i j

i j i j i j i j i j i j i j i j

u v u u h u u h

v v h v v h

     

   

       

       

      

     

For smooth data, we have 

1/ 2 , 1/ 2 , , 1/ 2 , 1/ 2 ( ) ( )i j i j i j i j        

   . In addition, we have 

1/ 2 , 1/ 2 , , 1/ 2 , 1/ 2 ( ) ( )i j i j i j i j u u v v    

  

 because 

u

 is numerically divergence free. Thus, 

1/ 2 , 1/ 2 , 1/ 2 , 1/ 2 ,

, 1/ 2 , 1/ 2 , 1/ 2 , 1/ 2

( ) ( ) ( )( ) / (2 )

( )( ) / (2 )

x y i j i j i j i j

i j i j i j i j

u v u u h

v v h

   

 

   

   

   

  

In addition, the model of Continuum Surface Force (CSF) is employed to treat the surface 

tension force on the interface, which interprets the surface tension force as a continuous body 

force across the interface rather than as boundary conditions in normal and tangential directions 

to the interface. By using the level set function, body force due to surface tension can be 

expressed as, 

       1 1

W e W e

 d n       

The curvature of the interface is evaluated from 

        n 





  

          n

The Dirac delta function is defined as 

    

1

1 cos ,

2

0 ,



    

 



  

 





i f

otherw ise

4. Boundary and Initial Conditions 

The flow in the liquid bridge is assumed to be axis-symmetric initially, and the hot and cold 

disks are maintained at constant temperature 

t

T

 and 

b

T

, respectively. 

180
   0 ( 0) z 

   1.0 ( 1.0) z 

The adiabatic condition is adopted on the all other boundary walls expect for the hot and cold 

disks. The initially stationary liquid bridge and the ambient air are considered with the initial 

condition 

u( 0) 0 t   

The non-slip condition is used for all walls of the computational domain 

u  0 

The present strategy is further summarized below. The mass conserving level set methods is 

applied to analyze the free surface motion of the liquid bridge. The Navier-Stokes equations in 

primitive variable formulations and energy conservation equation are solved on a staggered grid 

by the method of lines. The advection terms are discretized by the quadratic upstream 

interpolation for convective kinematics (QUICK) method and the other terms by the central 

finite difference method except for the body force, and a second-order Adams-Bashforth method 

is used as the time integration scheme. The higher essentially non-oscillatory (ENO) scheme is 

also adopted to solve the convection term of the level-set function. The Poisson equations are 

solved by means of the Successive Over Relaxation (SOR) approach. The Continuum Surface 

Force (CSF) model is employed to treat the surface tension force at the interface. 

5. Code Validation 

In present study, the numerical model and code have been carefully tested again by comparing 

the isotherms and velocity vectors in the liquid bridge of 2cSt silicone oil obtained from the 

present study with those reported by Kawamura et al.11. In the comparison, the initial liquid 

bridge and the ambient air are denoted to be stationary and axis-symmetric. The radii and height 

of the liquid bridge are 

R  5 mm , H  5 mm , respectively. The temperature difference 

between the top and bottom disks is 

   T C 30 . The Marangoni number is 

4 Ma   4 10

, and 

the Prandtl number is 

Pr 28.0  . The temperature contour and velocity vectors obtained from 

the present work under zero gravity is shown in Figure 2, and the agreement between the present 

results and those from Kawamura et al. can be qualified as quite acceptable (see Fig. 5(b) of 

Kawamura et al. (2007)). 

6. Results and Discussion 

The mechanism of thermocapillary convection in a liquid bridge of high Prandtl number under 

zero gravity is investigated. The key parameters used are as follows: 

Pr 50  , Ma  5000 , 

We  15 , 328 t

T K  , 298 b

T K 

. Diameter of the top and bottom disks is

D mm  5.0 , and the 

aspect ratio (

H D/

) is 0.7. In this section, we study the flow structure inside the liquid bridge 

under the present computational conditions. 

181
1 1.5 0

0.25

0.5

0

0.2

0.4

0.6

0.8

1

 

Figure 2. Temperature contour and velocity vectors (2cSt silicone oil, 

4 Ma   4 10 , Pr 28.0 

) 

0.1

0.3 0.5 0.7 0.9

1 1.5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

 

0.1

0.3

0.5

0.7 0.9

1 1.5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

 

0.1

0.2

0.3

0.4

0.5

0.6 0.7 0.8 0.9

1 1.5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

t=2.0 t=15.0 t=200.0 

Figure 3 Time evolution of isothermals (

Pr 50  , Ma  5000 , We  15 ,  328 t

T K , 

 298 bT K , D mm  5.0 , H D/ 0.7 

) 

Figure 3 illustrates time evolution of isothermals inside the liquid bridge, where only half of the 

liquid bridge is plotted. Since the temperature of the top disk is higher than that of the bottom 

disk, the heat flux moves from the top disk toward the bottom disk, and the flow is driven in the 

hot corner. On the other hand, the surface flow is developed due to the temperature gradient on 

the free surface, which results in a return flow inside the liquid bridge. The return flow generates 

a radial convection, which tends to make the bulk fluid temperature distribution rather uniform 

near the free surface. It can be found from Figure 3 that the temperature gradients in the regions 

near the upper center axis and the bottom cold corner are higher than those in the other regions. 

t=2.0 t=50 t=100.0 

Figure 4 Time evolution of velocity vectors (

Pr 50  , Ma  5000 , We  15 ,  328 t

T K , 

 298 bT K , D mm  5.0 , H D/ 0.7 

) 

182
Figure 5 Time evolution of surface velocity in axial direction (

Pr 50  , Ma  5000 , We  15 , 

 328 tT K ,  298 b

T K , D mm  5.0 , H D/ 0.7 

) 

The velocity vectors in the liquid bridge and ambient air at t=2.0, t=50.0, and t=100.0 are 

presented in Figure 4. Note that only half of the computational domain is plotted in Figure 4 due 

to the symmetric property. The free surface remains straight shape due to the zero gravity. Two 

vortexes are generated due to the thermocapillary convection inside the liquid bridge and 

ambient air, respectively, and the vortex center inside the liquid bridge locates near the hot 

corner initially (t=2.0) as shown in Figure 4. As the development of the thermocapillary 

convection, the vortex center inside the liquid bridge moves down and reaches an equilibrium 

position gradually (t=50.0, t=100.0). The larger velocity vectors can be found mainly around the 

free surface because the flow is driven by the surface flow generated near the free surface. Figure 

5 shows the surface velocities in axial direction. It can be seen that the values of the surface 

velocities in axial direction are all negative, showing that the direction of the surface flow is 

from the top disk toward the bottom disk. The surface velocities in axial direction at the top disk 

and the bottom disk are zero due to the no-slip conditions in there. As the time proceeds (t=8.0, 

t=10.0, t=25.0, t=100.0), the levels of the surface velocities in axial direction rise, showing the 

enhancement of the surface flow gradually. After that, the profiles at t=100.0, t=150.0, and 

t=200.0 are very close, showing the surface flow approaches the equilibrium state after t=100.0. 

The maximum of the surface velocities in axial direction can be obtained on the surface position 

of about z=0.65 at every time, where z is the non-dimensional distance from the cold disk. 

7. Conclusions 

Thermocapillary convection in a liquid bridge of 

Pr 50 

 under zero gravity has been 

investigated numerically. From this work, the following conclusions can be drawn. 

1. The temperature gradients in the regions near the upper center axis and the bottom cold 

corner are higher than those in the other regions. 

2. As the development of the thermocapillary convection, the vortex center inside the liquid 

bridge moves down and reaches an equilibrium position gradually. 

3. As the time proceeds, the levels of the surface velocities in axial direction rise, showing the 

enhancement of the surface flow gradually. After that the surface flow approaches the 

equilibrium state. The maximum of the surface velocities in axial direction can be obtained on 

0 0.2 0.4 0.6 0.8 1

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

Distance from cold disk

Surface velocity V

t=200

t=150

t=100

t=25

t=10

t=8.0

0.595 0.6 0.605

-0.0138

-0.0136

183
the surface position of about z=0.65 at every time, where z is the non-dimensional distance from 

the cold disk. 

 Acknowledgement 

The present work is supported financially by the National Natural Science Foundation of China 

under the grant of 11072057 and the Natural Science Foundation of Liaoning Province under the 

grant of 20092002. 

References 

1. D. Schwabe, A. Scharmann, “Steady and oscillatory Marangoni convection in floating zones 

under microgravity,” J. Cryst. Growth, 6(1979), 125. 

2. C. H. Chun, W. Wuest, “Experiments on the transition from the steady to the oscillatory 

marangoni-convection of a floating zone under reduced gravity effect,” Acta Astronaut., 

6(9)(1979), 1073-1082. 

3. F. Preisser, D. Schwabe, and A. Scharmann, “Steady and oscillatory thermocapillary 

convection in liquid columns with free cylindrical surface,” J. Fluid. Mech., 126(1983), 545-567. 

4. Y. Kamotani, S. Ostrach, and M. Vargas, “Oscillatory thermocapillary convection in a 

simulated floating-zone configuration,” J. Cryst. Growth, 66(1984), 83-90.

5. R. Velten, D. Schwabe, and A. Scharmann, “The periodic instability of thermocapillary 

convection in cylindrical liquid bridges,” Phys. Fluids, 3(A) (1991), 267-280. 

6. J. Masud, Y. Kamotani, and S. Ostrach, “Oscillatory thermocapillary flow in cylindrical 

columns of high Prandtl number fluids,” J. Thermophys. Heat Transfer, 11(1997), 105. 

7. S. Frank , D. Schwabe, “Temporal and spatial elements of thermocapillary convection in 

floating zones,” Exp. Fluids, 23(1997), 234.

8. R. Q. Liang , Z. Chen, “Dynamics for droplets in normal gravity and microgravity,” 

Microgravity Sci. Technol., 21(2009), 247-254. 

9. S. Osher, J. Sethian, “Fronts propagating with curvature-dependent speed: Algorithms based 

on hamilton-jacobi formulations,” J. Comput. Phys., 79(1) (1988), 12-49. 

10. M. Sussman, P. Smereka, and S. Osher, “A level set approach for computing solutions to 

incompressible two-phase flow,” J. Comput. Phys., 114 (1994), 146-159. 

11. H. Kawamura, E. Tagaya, and Y. Hoshino, “A consideration on the relation between the 

oscillatory thermocapillary flow in a liquid bridge and the hydrothermal wave in a thin liquid 

layer,” Int. J. Heat Mass Tran., 50(2007), 1263-1268. 

184
REAL TIME IN-SITU OBSERVATIONS OF EQUIAXED DENDRITE 

COHERENCY IN AL-CU ALLOYS USING HIGH BRILLIANCE 3rd

GENERATION SYNCHROTRON SOURCES 

Andrew G. Murphy1, David J. Browne1, Wajira U. Mirihanage2, Ragnvald H. Mathiesen2

1

Engineering and Materials Science Centre, University College Dublin, Ireland 

2Deptartment of Physics, Norwegian University of Science and Technology, N-7491 Trondheim 

Norway 

Keywords: Aluminum alloys, Dendrite coherency, Solidification, Synchrotron radiation 

Abstract 

In the last decade synchrotron X-ray sources have fast become the tool of choice for performing 

in-situ high resolution imaging during alloy solidification. This paper presents the results of an 

experimental campaign carried out at the European Synchrotron Radiation Facility, using a 

Bridgman furnace, to monitor phenomena during solidification of Al-Cu alloys - specifically the 

onset of equiaxed dendrite coherency. Conventional experimental methods for determining 

coherency involve measuring the change in viscosity or measuring the change in thermal 

conductivity across the solidifying melt Conflicts arise when comparing the results of these 

experimental techniques to find a relationship between cooling rate and coherency fraction. It has 

been shown that the ratio of average velocity to the average grain diameter has an inversely 

proportional relationship to coherency fraction. In-situ observation therefore makes it possible to 

measure these values directly from acquired images sequences and make comparisons with 

published results.

Introduction 

Many casting defects, e.g. microsegregation, hot tearing, gas porosity and solidification 

shrinkage, develop in mush after the point of coherency is reached. Traditionally dendrite 

coherency has been determined by one of two methods, namely rheological [1–4] and thermal 

analysis [5]. Rheological methods are based on measuring the increasing viscosity of the melt, as 

solid becomes coherent, and ultimately the shear strength of the dendritic network. Experimental 

details of the rheological setup have been well described by others [6] so it will not be discussed 

here. Thermal analysis is based on the principle of differing thermal conductivities between solid 

and liquid in the solidifying melt. In both rheological and thermal experimental methods the 

coherency fraction solid is calculated from the thermal data and compared to various alloy and 

process parameters for correlation [7]. Early coherency studies compared both methods for 

industrial grade aluminum alloys, ultimately showing good agreement [8, 9]. Increasing either 

cooling rate or solute content exhibited earlier coherency and thus lower fraction solid. The 

addition of grain refiners decreased grain size, delaying coherency and thus increasing coherency 

fraction solid. Chai [6] et al. performed a detailed theoretical analysis, based on diffusioncontrolled equiaxed growth models, predicting coherency fraction solid, which was found to 

depend on the competition between nucleation and growth. Typically, for alloys with solute 

concentrations between zero and the solubility limit, as solute concentration increases both grain 

size and growth rate decrease. It is, however, the faster of the two decreasing variables, i.e. grain 

size or growth rate, which dictates final coherency fraction solid. An inversely proportional 

185
relationship is found for cooling rate, growth restriction faction and coherency time, the time at 

which the coherency-torque curve first deviates from steady state, as well as varying 

proportionally with d/V, where d is the grain diameter at coherency and V is the average dendritic 

growth rate. Although these theoretical predictions have been shown to underestimate the 

coherency fraction solid observed, they still display qualitative agreement with experimental 

results [9]. The most recent study of Veldman [10] et al. showed significant deviation between 

both experimental methods for Al-Si-Cu ternary alloys. In rheological experiments cooling rate 

was observed to have little effect on coherency fraction solid as opposed to thermal analysis 

which suggested the reverse. Similar results were observed when comparing solute additions 

with increasing silicon content observed to reduce coherency fraction solid. Silicon content is 

also observed to dominate, over cooling rate, the microstructure morphology, with lower levels 

exhibiting small globulitic structure and higher levels, larger orthogonal dendritic grains. A 

fullness morphological parameter, which is defined as the ratio of internal grain volume solid 

fraction, VGS, to grain envelope volume, VGE, or more traditionally the internal solid fraction, fS,

(equation (2)) [11, 12] showed high correlation with coherency fraction solid and solute 

additions indicating grain morphology as a controlling factor of coherency. 

Many detailed solidification studies have now been conducted using binary alloy systems and 

synchrotron radiation and much quantitative data has been gathered and analyzed [13–15]. 

Following on from previous X-ray studies, this paper compares the experimental rheological and 

thermal coherency analysis with preliminary real time in-situ observations of equiaxed grain 

coherency, showing qualitative agreement with recent rheological studies. 

Experimental Methods 

This experimental campaign was carried out on the ID15A beamline at the European 

Synchrotron Radiation Facility (ESRF) in Grenoble, France. The experimental, Bridgman 

furnace, set up was principally the same as described in previous campaigns [16–18] and thus 

has been well documented. Specific parameters unique to this campaign and salient features of 

the rig are herein described. Two alloy sample types, Al-15wt%Cu and Al-25wt%Cu, measuring 

30 x 15 x 0.2 mm, were used in this study. Grain refinement was achieved by the addition 0.5 

g/kg of Al-Ti-B as described previously. Samples, once oxidized and coated with a boron-nitrite 

spray, are encased between two 150 mm quartz glass plates. This assembly is fitted to a 

translation device allowing the sample to be pulled through an imposed thermal gradient. The 

thermal gradient is supplied by two heater elements surrounding the crucible and separated by an 

adiabatic zone, where the incident X-ray beam intersects the sample. Temperature readings from 

the heaters are continuously measured by embedded thermocouples during the course of 

solidification, allowing for mid-run manual adjustment of the temperature gradient. The field of 

view (FOV) for captured images is set to 1.3 x 1.3 mm. Image capture was performed by a 

Sarnoff fast readout 512 x 512 pixel CCD, with image acquisition every 0.045 seconds. The 

ID15A is a high energy beamline delivering incident photons in the range [30 keV, 750 keV]. 

For this particular experiment the full polychromatic incident beam was employed. In terms of 

X-ray absorption contrast in the radiograms, however, only the lower photon energies will give a 

measurable contribution. 

Solidification was performed by translating the sample through a constant or varying thermal 

gradient at a constant velocity, anti-parallel to gravity. Post experimental image enhancement 

and analysis is carried out using MATLAB from MathWorks, ImageJ, a java based open source 

image manipulation toolbox and the GNU Image Manipulation Program. Adaptive, linear and 

186
nonlinear spatial filtering algorithms are used in conjunction with adaptive contrast enhancement 

and flat fielding techniques augment visible contrasts. Figure 1 shows a schematic layout of the 

heater/sample section of the Bridgman furnace used in the experiments. Also indicated is the 

location and scale of the field of view in contrast with the sample dimensions. 

Figure 1. Schematic layout of the Bridgman type gradient furnace and sample orientation 

Results & Discussion 

Examples of the images obtained are shown in figure 2. Figure 2 (a) shows the original captured 

image, before image enhancement, for an Al-15wt%Cu sample run. The poor image contrast is 

found to be indicative of the entire experimental run. This discrepancy is attributed to the 

extremely high energy of the ID15 beamline. Even for the lowest energies, between 30 and 90 

keV, the majority of the photons pass through the sample unabsorbed giving approximately 1.5 –

2% in total integrated absorption contrast between 200 mm of a-Al solid and Al-Cu liquids 

between 15-25wt%Cu. This puts the contrast close to the intrinsic noise level of the image 

acquisition system. 

 

Figure 2. Al-15wt%Cu image acquisition data; (a) Original image; (b) Contrast enhanced image; 

(c) Envelope and Grain masking 

Figure 2 (b) shows (a) post image enhancement through, flat fielding, noise cancellation, 

adaptive histogram adjustment and spatial convolution filtering. The spatial filtering method 

used was designed and implemented specifically for these images. While noise is still evident, 

a b c

187
grains structures are clearly visible. Figure 2 (c) shows the process of envelope definition and 

solid a-Al extraction, with solid aluminum in black, coherency envelope in grey and outside 

envelope as white. In the case of rheological and thermal analysis [6], the point of coherency is 

determined at a discrete time based on changes in measured parameters, i.e. torque and 

temperature. In reality however, grain coherency is a transient process strongly governed by 

local undercoolings, solutal constitution and gravity-induced thermosolutal convection. 

Therefore degrees of coherency are occurring in any non-isothermal melt throughout the 

solidification process with no absolute time. It is also understood that once impingement occurs, 

primary growth ceases and dendritic coarsening dominates. With real time in-situ image 

acquisition it is possible to observe grain growth after nucleation, grain motion and finally 

impingement at the point of coherency. It is observed from these captured sequences that once 

individual grains become coherent, no further growth, either coarsening or preferential, occurs 

while grains are passing through the FOV. It is certain that post transport through the FOV, 

predictable solidification processes occur, however whilst visible, grains appear frozen in time at 

the point of coherency. It is proposed, therefore, to define a coherency envelope from which 

coherency fraction solid (fcoh) will be determined, based on the ratio of internal envelope solid 

area, VS, to internal envelope total area, VEV, equation (1). 

EV

S

coh V

V

f 

(1)

Figure 3 illustrates the coherency concept. Isolated nucleated grains grow and eventually 

impinge, creating coherent network clusters, as was apparent in this study. In cases where solid 

growth encompasses the entire FOV, as in figure 2(b), the FOV will serve as the envelope 

boundary. 

Figure 3. Coherency envelope definition 

Due to the quasi-2D nature of the sample certain factors must be taken into consideration in the 

analysis. For instance, calculated solid fractions are based on area ratios, rather than volume 

ratios. This introduced unavoidable error into measurements as grains were observed to have 

varying thicknesses, less than or equal to the sample thickness. This was evidenced by instances 

of grain superposition observed during solidification. This error can be minimized in some 

respects by virtue of the ability to track grains from nucleation to impingement, continuously 

monitoring individual grain evolution. Also grains were observed to impinge mechanically on 

the crucible walls, i.e. get trapped between quartz slides, possibly distorting coherency events. 

Measurement reproducibility error was also evident due to manual definition of grain envelopes. 

To quantify this error a number of grain measurements were repeated several times to determine 

188
average variation. This was calculated to be approximately 2.5%. Qualitatively, however, using 

consistent analytical methods the data showed minimal variation. Figure 4 shows a graph of the 

cooling rate across all samples versus coherency fraction solid. Cooling rate, in the case of a 

Bridgman furnace, is calculated as the product of thermal gradient and translation velocity. It is 

apparent from the data that change in cooling rate has no significant effect on fcoh, which 

supports the most recent published rheological/thermal comparisons [10]. Al-15wt%Cu samples 

were solidified initially at a low cooling rate, increasing step wise and then gradually increased 

to higher cooling rates. Al-25wt%Cu samples were solidified initially at low cooling rates and 

gradually decreased. Statistical correlation (r) was performed in the cooling rate/fcoh data and low 

values (<0.5) were observed for both alloy types suggesting a low degree of linearity between 

both variables. The coefficient of determination (r2) for both samples was calculated at 

approximately 20% again supporting negative interdependence. 

Figure 4. Cooling rate versus coherency 

fraction solid

Figure 5. Grain diameter versus coherency 

fraction solid

Figure 5 shows the relationship between grain size and coherency fraction solid. Larger, and thus 

more dendritic, grains exhibit lower coherency fraction solid than the smaller cellular-equiaxed 

structures. This, again, supports previous experimental data as it is suggested smaller more 

numerous grains, exhibit slower growth thus impingement is delayed, increasing fcoh. It is also 

evident from figure 5 that there is significant deviation in grain size measured in individual 

coherency envelopes. Solidification sequences show grain nucleation at various locations and 

times ahead of the coherent network. It has already been suggested that movement of growing 

solid through the melt changes the local undercooling and composition, affecting subsequent 

growth. Evidence has also been presented of effects of so called solutal poisoning [19] caused by 

sedimenting solute ejected from growing grains. The combination of these solidification effects 

appears to be the cause of the observed grain size deviation. Finally figure 6 shows the so called 

fullness parameter, fS, in relation to fcoh. It is not expected that this value should show any 

significant difference in trend however, it is expected to be higher than fcoh, as observed. The 

fullness, equation (2), parameter itself appears to be an appropriate measure of grain 

morphology, with large highly dendritic grains showing lower fullness and small globular grains 

exhibiting a large value of fullness. 

GE

GS

S V

V

f 

(2)

189
Figure 7 shows an example of the differences observed in morphological evolution. In figure 7 

(a), a more elongated dendritic growth morphology is observed, with well defined secondary 

dendrite arm spacings. Figure 7 (b) shows, on average, a more circular and smaller grain size 

exhibiting the so called equiaxed-cellular morphology. Apparent from figure 7 is the higher 

granular solid fraction in (b), which stems from smaller grains allowing for more efficient 

coherent packing ultimately increasing the coherency fraction solid. 

Figure 6. Fullness versus coherency fraction solid 

 

Figure 7. Al-15wt%Cu (a) and Al-25wt%Cu (b) morphological comparison 

A first order approximation method of calculating the volume coherency fraction solid is 

proposed. Assuming the 2D solid fraction can be modeled as a spherical volume of solid 

proportional to the solid area and 2D coherency envelope can be modeled as a cubic volume 

element proportional to envelope area, the conversion is derived as follows: 

  2

3

3

4

V S AS





(3)

where VS is the equivalent spherical solid volume based on the total captured solid area, AS. 

Similarly VEV, the cubic equivalent coherency envelope is determined using equation (4). 

  2

3

EV AV EV



(4)

a b

190
where AEV is the total area of the defined coherency envelope. The 3D coherency fraction, 

Df

coh 3

, is thus defined as the ratio of VS to VEV as expressed in equation (5). 

EV

S

coh V

V

3Df 

(5)

Figure 8 shows the newly calculated volume coherency fraction solid for the captured data. As 

expected these values are consistently lower than the values calculated for equivalent areas 

however, they are still higher than published data. It should be noted, however, in-situ studies 

show direct observation of solidification phenomena in real time and there have been significant 

inconsistencies between rheological and thermal data suggesting deviation may be systematic. 

Figure 8. Volume based coherency fraction solid 

Conclusions & Future Work 

This document presents a preliminary study into the determination of fraction solid at the point 

of coherency. Qualitative agreement was found between this work and the work of Veldman et 

al. [10] in relation to cooling rate independence and increasing coherency fraction solid with 

decreasing grain size. It is observed here however, that decreasing grain size follows an increase 

in solute concentration which contradicts previous work. Previous solute concentrations analyzed 

however have been limited to low concentration and industrial compositions [6, 8], which may 

account for the discrepancy. Based on this study, further work is a required with alloy 

composition more consistent with previous rheological and thermal experimental campaigns in 

order to make quantitative comparisons. Correlations between coherency fraction solid and other 

grain attributes, i.e. aspect ratio, grain rotation and grain motion, are proposed for future 

investigation. The 2D to 3D conversion described here also requires further investigation and 

definition. The nature of the thin sample has the effect of distorting grain growth, as growth in 

the plane of the beam is suppressed while grain cross-sections become artificially enlarged. 2D 

to 3D conversions can therefore result in cumulative errors in predicting volumetric solid 

fractions. Microgravity experiments are also warranted to determine the significance of 

thermosolutal convection and buoyancy on coherency fraction solid. 

191
Acknowledgements 

The authors wish to acknowledge the financial support of the European Space Agency (ESA) 

under the PRODEX program, (Contract No. 90392). This work is part of the ESA-MAP 

(Microgravity Applications Promotion) project XRMON. Also the authors wish to thank Dr. 

Marco Di Michiel (Exp. Division, ESRF) for his assistance with the experiment, Dr. Shaun 

McFadden (now at Dublin Institute of Technology, Ireland) and Dr. Paul Schaffer (now at Hydro 

Aluminium, Norway) for their efforts in capturing the experimental data. 

References 

1. S. A. Metz and M. C. Flemings, AFS Trans., (78) (1970), 453. 

2. J. Claxton, J. Metal., (1975), 14. 

3. D. B. Spencer, R. Mehrabian, and M. C. Flemings, Metallurgical Transactions, 3 (1972), 

1925-1932. 

4. D. Brabazon, D. J. Browne, and A. J. Carr, Materials Science and Engineering: A, 356 (1-

2) (2003), 69-80. 

5. L. Backerud and B. Chalmers, Trans Met Soc AIME, 245 (2) (1969), 309-318.

6. G. Chai et al., Metallurgical and Materials Transactions A, 26 (1995), 965-970.

7. J. Tamminen, Chem. Comm., Stockholm, 2 (1988), 8-33. 

8. L. Arnberg, G. Chai, and L. Backerud, Materials Science and Engineering: A, 173 (1-2) 

(1993), 101-103. 

9. A. K. Dahle and L. Arnberg, Materials Science and Engineering: A, 225 (1-2) (1997), 38-

46. 

10. N. L. M. Veldman et al., Metallurgical and Materials Transactions A, 32 (2001), 147-155. 

11. I. Dustin and W. Kurz, Z. Metallkd, 77 (1986), 265-73. 

12. M. Rappaz and P. H. Thevoz, Acta Metallurgica, 35 (12) (1987), 2929-2933. 

13. R. H. Mathiesen et al., Physical Review Letters, 83 (24) (1999), 5062. 

14. H. N. Thi et al., Journal of Physics D: Applied Physics, 36 (10A) (2003), A83-A86. 

15. H. Yasuda et al., Journal of Crystal Growth, 262 (1-4) (2004), 645-652. 

16. R. H. Mathiesen et al., Metallurgical and Materials Transactions B, 33 (4) (2002), 613-623. 

17. R. H. Mathiesen and L. Arnberg, Materials Science and Engineering: A, 413-414 (2005), 

283-287. 

18. R. H. Mathiesen and L. Arnberg, Acta Materialia, 53 (4) (2005), 947-956. 

19. A. Bogno et al., Transactions of the Indian Institute of Metals, 62 (4-5) (2009), 427-431. 

192
TRUNCATED DUAL-CAP NUCLEATION SITE DEVELOPMENT 

Douglas M. Matson and Paul J. Sander 

Tufts University; 

Department of Mechanical Engineering; 

200 College Avenue; Medford, MA 02155, USA 

Keywords: critical nucleus shape, heterogeneous nucleation, mushy-zone 

Abstract 

During heterogeneous nucleation within a metastable mushy-zone, several geometries for 

nucleation site development must be considered. Traditional spherical dual cap and crevice 

models are compared to a truncated dual cap to determine the activation energy and critical 

cluster growth kinetics in ternary Fe-Cr-Ni steel alloys. Results of activation energy results 

indicate that nucleation is more probable at grain boundaries within the solid than at the solidliquid interface. 

Introduction 

Undercooled hypoeutectic ternary Fe-Cr-Ni steel alloys solidify in a two-step process known as 

double recalescence [1]. The metastable ferritic bcc-phase forms first with subsequent 

transformation to the stable austenitic fcc-phase. The delay between these two nucleation events 

represents the incubation time for the stable phase and this delay is a strong function of melt 

convection. In classical nucleation theory [12-14], the steady-state nucleation rate, IS, and time 

dependent nucleation rate, I, are related by the equations 











 

 













 

Tk

G

IIfor

t

II

B

S OS

*

exp exp



(1) 

where τ is a characteristic incubation time, t the observed delay time, IO a pre-exponential factor, 

ΔG* the Gibbs free energy for formation of a critical nucleus of n* atoms of a pure material, and 

T the transformation temperature. The Boltzmann constant has a value of kB = 1.38 x 10-23

J/atomK. Steady-state nucleation does not become appreciable until ΔG* > 60 kBT while 

transient nucleation does not become appreciable until t >> τ. Turnbull estimated that for 

condensed phases the incubation time was a function of the size of the cluster and the rate at 

which atoms cross the interface, β*, between the surrounding matrix and the cluster [2].

 

*

*

2





n



(2) 

193
The attachment rate β* has variously been related to the jump frequency [2] and the lattice 

diffusivity [3-5]. By invoking the principle of time reversal, where the statistical fluctuations in 

cluster size follow the same path during growth and decomposition, Feder et al [6] determined 

the incubation time to be 

 

*

2

2

*

4

n

B

n

G

Tk













(3) 

Russell [7] evaluated condensed phase nucleation for binary systems to account for the influence 

of solute partitioning and found that clusters approaching the critical size were surrounded by an 

enriched solute shell, contrary to expectation, and that diffusivity of the slower moving species 

controls the attachment rate with an interchange frequency β'. 





















A B

BA

o

DD

DD

a

x 2

'

2



for 



















2

o

S

A

a

A

Cx

(4) 

where x is the number of atoms jumping a jump distance ao and the subscripts on the diffusivity 

D represent solute A and solvent B. The number of atoms jumping can be evaluated from the 

surface concentration CA, the interface area AS, and the area per atom. A key finding for multicomponent systems is that the rate controlling step for linked flux evaluations is replacement of 

shell atoms of the controlling species and not the interfacial jump frequency. 

In an effort to identify the mechanism for stable-phase nucleation in ternary Fe-Cr-Ni steel 

alloys, Koseki [1] showed that a dual cap cluster forming within the metastable solid along a 

bcc-bcc boundary was more probable than (1) homogeneous nucleation of a sphere in the melt, 

(2) heterogeneous nucleation as a single spherical cap protruding from the bcc-solid and into the 

melt, (3) heterogeneous nucleation of a spherical cap protruding into the solid, and (4) 

heterogeneous nucleation of a dual cap along the solid-liquid interface. In these analyses, the 

time to form a potential nucleation site is taken as zero. Homogeneous nucleation in the melt 

requires no site development time. Homogeneous nucleation within the solid and heterogeneous 

nucleation in the form of a spherical cap at the solid-liquid interface requires only that the preexisting solid exist. Since the solid/liquid interface forms immediately during primary 

recalescence there is no delay for site formation. 

A transient site, such as the solid-solid grain boundary dual cap, is one that forms sometime after 

primary solidification. Two processes become important in the evaluation of transient site 

development. First, we must assess how long it takes to form a suitable heterogeneous nucleation 

site and then, second, we must define how long it takes for a critical nucleus to grow after a 

suitable site has formed. The sum of these two events determines the incubation period which 

corresponds to the delay time between primary metastable recalescence and subsequent initiation 

of the conversion to the stable phase. 

Hanlon [8] proposed that fluid drag and dendrite bending can lead to secondary arm collision 

which will create the required bcc-bcc grain boundary. Matson [9] extended these analysis by 

showing that nucleation of the stable phase could only occur once a suitable site had formed on, 

194
or in, the pre-existing metastable solid thus explaining how convection can influence predictions 

using classical nucleation theory. Sites that form due to collision include the bcc-bcc grain 

boundary and the liquid-filled crevice formed by the intersection of two parallel cylindrical arms. 

Since the predicted delay time required for cluster growth for both the dual cap and crevice 

clusters was several orders of magnitude faster than the observed delay, the formation of the 

grain boundary crevice, and not cluster growth, was identified as the controlling process. 

One additional geometry needs to be considered. The current paper looks at the potential for 

interaction between a dual cap and the liquid due to growth near the grain boundary/liquid 

interface. Dual cap cluster formation was primarily limited due to diffusion along the grain 

boundary since diffusion through the solid was sluggish and thus grain boundary diffusion 

dominated. The activation energy was low due to the favorable low surface/volume ratio. 

Crevice cluster formation was significantly more rapid due to enhanced diffusion through the 

surrounding liquid but the activation energy was higher due to an increase in surface/volume. 

The intersection of a dual cap with the liquid surface combines these processes and must be 

evaluated to complete the picture. The geometry that forms is known as a truncated dual cap. 

Geometry of a truncated dual cap 

A dual cap consists of two identical spherical sections with a characteristic intersection angle, 2θ, 

defined by the relative interfacial free energies, γ. Along the grain boundary mirror plane, 

equilibrium is maintained if 

cos2  MM MS

(5) 

with subscripts MM for metastable/metastable and MS for metastable/stable. The activation 

barrier at the critical radius, ΔG*, is evaluated based on equating surface and volume terms, A

and V respectively, at the critical cluster radius r* where dG/dn goes to zero such that 

G

r

A MSMS A MM MM  GV V 2*   

*

(6) 

m

V

T

TH

G





with ΔH the enthalpy of fusion, ΔT the undercooling set by the melting points of the metastable 

and stable phases, and Tm the melting point of the stable phase. The area and volume terms are 

readily evaluated from equations for spherical sections. If we assume typical values for 

thermophysical properties for steel alloys[9] to be γMM = 0.468 J/m2, γML = 0.212 J/m2, 

γSL = 0.302 J/m2, ΔGV = 2.56 x107

 J/mol, DGB = 1 x 10-10 m/s2, DS = 1 x 10-14 m/s2, 

DL = 3 x 10-9 m/s2, and a grain boundary thickness of t = 5 Å, then the angle is 2θ = 41.2°, the 

activation barrier is ΔG*/kT = 102 for a nucleus containing 15000 atoms with a delay time of 

τ = 5.9 x10-8 sec. 

Note that the value for the interfacial free energy between stable and metastable phases is highly 

speculative given the inability to experimentally define this value. Thus a value of 

γMS = 0.250 J/m2 was selected as representative. Throughout the remainder of this paper, the 

errors associated with this selection are investigated. 

195
When a dual cap intersects the liquid, the common circular basal surface is truncated. This is 

shown in Figure 1 where the overall shape of the transformed volume is seen in (a) and each of 

the three sub-volumes which comprise the shape are visually blown apart in (b). Note that top 

and bottom represent the converted section of the dual cap within the metastable solid while the 

pointed ellipsoid extends out into the liquid. 

Figure 1. Geometry of a truncated dual cap. (a) overall volume (b) exploded view to show 

upper and lower truncated cap sections and the ellipsoid protruding out into the 

liquid (c) truncated cap for typical properties and γMS = 0.25 J/m2 (d) truncated 

cap for typical properties and γMS = 0.37 J/m2

In Figure 1 the influence of the value of the interfacial free energy between metastable and stable 

phases is illustrated where in (c) the interfacial free energy is set at γMS = 0.25 J/m2 while in 

(d) γMS = 0.37 J/m2. Of particular interest is the strong influence of the value of the γMS

parameter on the ellipsoid dimensions. As seen in Figure 1(c), at low values the interfacial 

properties dominate and the point where metastable solid/metastable solid/liquid are in 

equilibrium is pronounced. In Figure 1(d) the volume properties dominate and the point is less 

pronounced. This point is of critical concern because at this location surface frustration, a local 

instability caused by anisotropy of surface tension, causes modeling of the geometry to break 

down. Isotropic interfacial energies require that the boundary between metastable solid and 

liquid have the same wetting angle which means that the “ellipsoid” should, in fact, be a 

spherical cap section with a-axis and b-axis the same. This is certainly not true in (c) but the 

(a)

(c)

(b)

(d)

196
geometry comes closer to this shape in (d). Surface frustration is due not only to the elliptical 

cross-section of this face but also due to the definition of surface angles. 

Along the solid/solid interface (left side of the shape) equation 5 is used to define the interior 

angle 2θ relative to the horizontal mirror plane as before. At the point where the metastable 

solid-solid boundary intersects the liquid-stable solid boundary (the point in the ellipsoid) the 

interfacial free energy balance results in the definition of two equations in two unknowns along 

horizontal and vertical axes 

LM MS S LS L

 cos  cos 

(7) 

MS S LS L

sin  sin 

(8) 

These define the angle the truncated section makes to the vertical, θS, and the angle the exterior 

surface of the ellipsoid makes to the vertical, θL. Note that ideally the angle should be identical 

along the a-axis and b-axis of the ellipsoid but surface frustration makes this not possible due to 

the required existence of the pointed intersection. To estimate the surface area and volume it 

was assumed that the shape could be represented by rotating the destroyed metastable solidliquid interface about the solid-solid line resulting in a shape similar to half a football. 

The truncated dual cap and ellipsoid shapes cannot be characterized readily analytically and a 

graphical approach was used to estimate areas and volumes. Using the predicted characteristic 

lengths and angles, a model was developed in SolidWorks® to evaluate areas and volumes as a 

function of the interfacial free energy γMS. Defining the base of the truncated cap as AMM, the 

front as AML, the surface as AMS, and the exterior of the ellipsoid as ASL, the activation barrier 

can be evaluated from 

G

r

A MSMS A SLSL A MLML A MM MM  GV V 2*   2   

*

 (9) 

Using typical thermophysical properties presented earlier, θ = 41.2°, θS = 81.2°, θL = 54.9°, and 

the ellipsoid has major axis length a = 19.9 x10-9 m and minor axis length b = 3.3 x10-9 m. The 

activation barrier is calculated to be ΔG*/kT = 3315 for a nucleus containing 156000 atoms with a 

delay time of τ = 2.8 x10-9 sec. The results of calculations of this type for all three mechanisms 

as defined over the entire range of possible interfacial free energies are presented in Figure 2 and 

Figure 3. 

Discussion 

Prediction results show that all three mechanisms are possible but only for low values of the 

interfacial free energy. At these energies, the shape of both the dual cap and truncated dual cap 

are thin saucer-shaped volumes due to the small interior angle along the metastable solid grain 

boundary. The truncated portion thus has a highly anisotropic elliptical shape with major axis to 

minor axis ratio varying from 11 to around 6 over the range of interfacial free energies 

investigated. The sensitivity to selection of values for interfacial energies can readily be 

investigated beyond what has been presented here as part of a larger evaluation of error 

associated with use of this type of technique. For example, if the value of the interfacial free 

energy of the metastable solid-solid boundary is reduced from the nominal value of γMM = 0.468 

197
J/m2

to γMM = 0.212 J/m2(the value for γML) then major to minor axis ratio is reduced to around 

1.6 and is insensitive to the selection of γMS. This ratio represents geometries closer to the 

desired isotropic, circular shape. Note that all interfacial free energy values have a high inherent 

uncertainty and thus the results should be viewed as indicating trends and not as numerical 

predictions. Results are ordinal and not cardinal. 

Figure 2. Energy barrier for nucleation of each cluster shape over the range of possible 

interfacial free energy values. 

Figure 2 also clearly shows that the truncated cap geometry does successfully combine elements 

of both the solid-solid dual cap mechanism and the solid-liquid crevice mechanism. Predictions 

for the truncated dual cap tend to lie between the extremes calculated for the other two 

mechanisms however if all were operational this mechanism would dominate since the fastest

possible mechanism is favored and the truncated cap is kinetically fastest over the entire range, 

as seen in Figure 3. However, since the energy barrier is higher for the truncated dual cap than 

for the embedded dual cap it is unlikely to be operative except at interfacial free energies less 

than a value of around γMS = 0.24 J/m2

. 

One additional weakness of the approach used to estimate the surface area and volume for the 

pointed ellipsoid section for this geometry is that in order to obtain a tractable solution for the 

ellipsoid form, the foot-ball shaped rotation element was selected. This technique was used 

because SolidWorks® can readily generate this shape. Unfortunately, this ignores the calculated 

angle θL= 54.9° for γMS = 0.25 J/m2 which should be constant along the three-phase interface. 

198
Figure 3. Kinetics of cluster formation for each shape over the range of possible interfacial 

free energy values. 

Note that the rotated shape has a continuously changing surface angle. The extremes occur at the 

point along the major axis where the angle is θL

min

= 18.9°, and at the top along the minor axis 

where the angle is θL

max= 90°. The effect of this change in angle is a continuously changing 

curvature of the surface which, as previously noted, results in surface frustration. This warping 

of the ideal geometry also results in a slight overestimation of the surface area and volume –

estimated to be on the order of 37% and 58% respectively for these conditions – with the error 

decreasing because the difference between major and minor axis decreases significantly with 

increasing γMS. One interesting result of this surface frustration is that the decreased angle at the 

point strains the equilibrium at the three-phase junction and creates an additional resistance to 

cluster formation. This effect is commonly observed by children blowing up a balloon – if a 

strain is induced along one axis by pinching, the increase in anisotropic surface tension only 

allows expansion of the balloon along orthogonal axes. Thus for our application there is a strain 

term which must be added to the free energy balance and growth along the major axis is 

hindered. The polarity of this term suggests that the resulting activation energy will be greater 

than anticipated. These observations further strengthen the conclusion that clusters will form 

along the grain boundary within the metastable solid and not at the intersection of the metastable 

boundary and the undercooled liquid. 

These theories must be validated over a wide range of convection conditions as is only possible 

through electromagnetic levitation experimentation in microgravity [10].

199
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Summary 

Evaluation of the predicted delay times for all mechanisms shows that the incubation time to 

grow a critical cluster is several magnitudes faster than observed. This supports the theory that, 

following formation of the metastable array, the controlling kinetic mechanism is formation of a 

site where nucleation may occur rather than growth of a critical cluster. Evaluation of the energy 

barrier indicates that over a select and limited range of interfacial free energy values, all of the 

mechanisms – spherical cap, crevice nucleation, and truncated cap – are possible. However, 

since the barrier for formation of a truncated cap is greater than that for a dual cap for all 

interfacial energies evaluated in this study, cluster formation is more likely along a grain 

boundary within the solid rather than at the intersection of the boundary and the liquid. 

Acknowledgements 

This project is sponsored by NASA under grants NNX08AL21G and NNX10AV27G. The 

authors would like to thank the members of the ThermoLAB International Topical Team 

sponsored by ESA under AO-2009-1020 for valued collaboration. 

References 

1. T. Koseki and M. C. Flemings, “Solidification of undercooled Fe-Cr-Ni alloys: Part I. 

Thermal behavior” Metall. Mater. Trans. A26[11] (1995) 2991-2999. 

2. J. H. Hollomon, D. Turnbull, “Nucleation”, Prog. Met. Phys. 4, (1953) 333-338. 

3. D. Turnbull, J.C. Fisher, “Rate of nucleation in condensed systems”, J. Chem. Phys. 17

(1949) 71-73. 

4. R. Becker, “Nuclear formation in the separation of metallic mixed crystals” Anal. Phys.

32[1-2] (1938) 128-140. 

5. A. Kantrowitz, “Nucleation in very rapid vapor expansions”, J. Chem. Phys. 19, (1951)

1097-1100.

6. J. Feder, K. C. Russell, J. Lothe, G. M. Pound, “Homogeneous nucleation of droplets in 

vapours” Adv. Phys. 15 (1966) 111-178.

7. K. C. Russell, “Linked flux analysis of nucleation in condensed phases” Acta Metall. 16, 

(1968) 761-769. 

8. A.B. Hanlon, D.M. Matson, and R.W. Hyers, “Internal Convective Effects on the 

Lifetime of the Metastable Phase Undercooled Fe-Cr-Ni Alloys,” Philosophical 

Magazine Letters, vol. 86, no. 3, (2005), 165-174. 

9. D.M. Matson, “Ch 10 – Nucleation within the Mushy-zone”, in Solidification of

containerless Undercooled Melts, D. M. Herlach and D. M. Matson eds., Wiley-VCH, 

(2012) in press. 

10. D.M. Matson, R. W. Hyers, T. Volkmann, and H.-J. Fecht, "Phase selection in the mushy 

zone: LODESTARS and ELFSTONE Programs", Physics IOP, Proceedings of ISPS 

2011 Conference, July 14, 2011, Bad-Godesberg DE, (2011) in press. 

200
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Materials Research in Microgravity 2012

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M–1342

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materials science, microgravity, modeling, thermophysical properties, combustion, sintering,

phase transformations, solidification

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UU 212

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Reducing gravitational effects such as thermal and solutal buoyancy enables investigation of a large range 

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