bluuebunny/crossref_metadata_2025 huggingface dataset:
The Influence of Natural Convection on Heating Characteristics of Space Furnaces
Authors: Lenski H. , Piller J.
Published: March 2011

During the Dl-Spacelab mission, several samples of semiconductor and aluminium alloy have been processed in a single ellipsoid mirror furnace (ELLI) and a gradient furnace (GFQ), respectively. Both furnaces operate under inert gas atmosphere at reduced pressure (100 to 200 mbar). In weightlessness, the heat transfer characteristic changed due to absence of convection. For the ELLI, power consumption in space decreased by 15 to 21 %, while the thermal inertia of the system increased by a factor of 2. In contrast to that, the gradient furnace required higher heater temperatures in space, while the influence on the sample thermal profile was rather low.
Importance of Meteoritic Materials in Assessing the Influence of Microgravity on Solidification
Authors: Budka P. Z. , Milillo F. F.
Published: March 2011

A new hypothesis is presented which proposes that major microstructural features of many meteorites are products of non-equilibrium solidification under microgravity conditions. These materials exhibit microstructural features both similar to and different from materials produced on Earth. Evidence is given to show that the differences can be attributed to microgravity solidification while the similarities can be attributed to nonequilibrium solidification. This hypothesis leads to a new interpretation of the formation mechanism for the Widmanstatten structure commonly observed in meteoritic nickel-iron alloys. Several meteorite synthesis experiments are suggested to test the hypothesis. The hypothesis implies that the study of meteoritic materials can lead to a deeper understanding of microgravity solidification; conversely, the principles of microgravity solidification can lead to insights into meteorite formation.
Theoretical Studies of Gravitational Effects in Chemical Vapor Deposition
Authors: Stinespring Charter D. , Kolb Charles E. , Annen Kurt D.
Published: March 2011

Chemical Vapor Deposition (CVD) processes are generally carried out under large temperature gradients. These gradients, temperature dependent fluid properties, and the earth's gravitational field give rise to buoyancy-driven free convective fluid flow which augments heat and mass transport in the CVD reactor. Under certain conditions, this free convective flow may alter the gas phase chemistry associated with the deposition process. In order to understand these free convective effects and their implications for the deposition process, a computational model describing the combined effects of fluid mechanics and chemistry has been developed. This model uses a coupled chemical equilibrium/mass transport code in conjunction with a 2-D elliptic fluid dynamics code to describe gas phase species profiles and deposition rates. This paper briefly describes the development of the model, its use, and the results of typical calculations.
Convection and Diffusion Effects During Physical Vapor Transport of Hg2Cl2
Authors: Singh N. B. , Hopkins R. H. , Mazelsky R. , Glicksman M. E.
Published: March 2011

We have carried out extensive experimentation on the physical vapor transport growth of mercurous chloride, which is an important material for opto-electronic devices. Because of the extraordinary combination of properties found in Hg2Cl2, including transmittance from 0.36 to 20 μm, anomalously slow soung velocity, high birefringence, and large acousto-optic diffraction efficiency, we are exploring the conditions for growth of large crystals of device quality. Our experiments have led so far to measured Hg2C12 growth rates that were orders of magnitude smaller than those expected by theories based on laminar, unidirectional flow between source and sink. Slight disturbances of the solid-vapor interface give rise to instabilities which lead to interfacial convection. This kind of convection should enhance the growth rate due to the acceleration of the materials exchange by hydrodynamic coupling, which is contrary to our findings. Interfacial convection is often connected with buoyancy-induced instabilities which dominate other instabilities under 1-g conditions. Despite attempts to reduce the Rayleigh numbers in our system, buoyancy-induced instabilities might be the cause of discrepancy between the measured and calculated growth rates. Observations of these phenomena under a microgravity environment might permit a better assessment of the transport mechanisms in physical vapor phase crystal growth of Hg2C12.
Thermogravimetric Measurements in an Electroynamic Balance
Authors: Spjut R. E. , Elliott J. F. , Bolsaitis P.
Published: March 2011

An Electrodynamic Thermogravimetric Analysis (EDTGA) Instrument has been developed for the study of kinetics of gas-solid reactions and phase transformations of levitated particles at high temperature. The levitated particles may be heated with a focused laser beam and their temperature and weight monitored during the process. The particles that can be studied are, approximately, 10 to 150 microns in diameter and it is aimed to produce temperature pulses comparable to those encountered in plasma torches and reactors. The instrumentation implemented in the system permits the measurement and or control of relative temperature, weight change, and energy flux to the particle with a time resolution of less than one millisecond. Selected results obtained from pulse heating aluminum oxide and silicon dioxide are presented.
Heavy Metal Halide Glasses
Authors: Drexhage Martin G.
Published: March 2011

Optical fibers with attenuations much lower than those attainable in present-day silicate waveguides are possible in theory if mid-infrared operating wavelengths are used. The loss mechanisms, materials, and problems inherent in obtaining such fibers are discussed. Among several candidates, heavy metal fluoride glasses are suggested as being must suited to low and moderate loss fiber applications in the 2-5 micrometer spectral region.
Ultraviolet Curable Silicone Optical Fiber Coatings
Authors: Dehli Gail J. , Lee Chi-Long , Lutz Michael A. , Suzuki Toshio
Published: March 2011

Technology coupling fast ultraviolet (UV) cure response with elastomeric properties in silicone formulations has been developed. Rapid photocure was obtained through addition of mercaptan to vinyl on silicon via a free radical chain reaction resulting in a monosulfide link. One optical fiber coating based on the technology, OPTIGARD™ X3-6662, is a onepart formulation which can be cured very rapidly (≤0.1 sec) upon UV exposure (≥25 mJ/cm2) to provide a low refractive index (RI=1.42), elastomeric film exhibiting good low temperature properties. A high refractive index counterpart (RI=1.51), OPTIGARD™ X3-6663, was obtained through incorporation of a controlled amount of phenyl groups into the polymer backbone. The technology, an IR-100 Award winner in 1985, is being used by the fiber optic industry to satisfy their need for coatings which will cure fast, perform over a wide temperature range, and be moisture insensitive.
Some Transition Ions in Heavy Metal Fluoride Glasses
Authors: Bogomolova L. D. , Caccavale F. , Krasil'nikova N. A.
Published: March 2011

The heavy metal fluoride glasses (HMFG) have attracted considerable attention in recent years because of their unusual physical and chemical properties which allow potential use in numerous electrical and optical applications. The HMFG are transparent over a wide range of the optical spectrum (from 220 nm to 8,μ) and have sufficient chemical stability to make them a suitable optical material middle IR fibers optics. Their high anionic conductivity permits their application as solid electrolytes. They are also of interest scientifically since their structure is different from that of other inorganic glasses. The high coordination number of the network formers ions is the principal structural peculiarity of the HMFG.
Materials For UV and Far Ir Transmission
Authors: Fleming James W.
Published: March 2011

Motivated by the benefits of optical data transmission, materials for the electromagnetic spectral window from approximately 0.6 to 4.0 microns are being explored by many investigators. I will review materials for the spectral regions surrounding this window. In these spectra regions a primary interest is optical power transmission. Optical power transmission in the UVand far IR has significant application for materials and device fabrication and characterization. Sources such as eximer and CO2 lasers provide potential sources of very high power.

Based on bonding and atomic mass considerations, materials for transmission in these windows can be approximated. Methods for taking advantage of the properties of materials such as reflectivity have the potential for interesting lightguiding structures such as hollow core lightguides. Materials currently contemplated for use in these wavelength regions and properties of potential materials will be discussed.
Effects Of Sintering Atmosphere On Defects In SiO2:GeO2 Vad Fiber
Authors: Kashiwazaki A. , Muta K. , Kohketsu M. , Kawazoe H.
Published: March 2011

Effects of the sintering atmosphere for 90SiO2 :10GeO2 preform-rods produced by VAD process were examined. The sintering atmosphere was changed from reducing to oxidizing, and the type and relative concentration of defects were estimated by photoluminescence, UV absorption and ESR absorption. Reducing condition accelerated the formation of defects, because these centers were considered to be the intermediate products of the thermal decomposition reaction of GeO2 which gives rise to reduction of Ge(IV) to Ge(II).
Coating Adhesion Effects on Fiber Strength and Fatigue Properties
Authors: Skutnik Bolesh J. , Munsey Barbara D. , Brucker Carol T.
Published: March 2011

Organic/polymeric materials remain the most prevalent protective coatings for optical fibers. Many studies have chronicled significant differences inn the levels of protection against moisture and other adverse environments by such coatings. Recently an attempt was made to identify those coating properties which correlate with these protective characteristics. Water absorption and adhesion between the coating and fused silica were proposed to correlate the best.

In this study a series of coatings have been prepared with varying levels of adhesion to fused silica substrates, without significant differences in water absorption or in water permeation properties. The tensile strength, static fatigue and dynamic fatigue properties of the coated fibers were measured and correlated with the lap shear strengths of correspondingly coated slides. As the adhesion of the coating/silica glass interface increases, the corresponding fiber's strength and fatigue properties improve also.

The strong positive correlation between the coating's adhesion and the fiber's strength/fatigue properties will be reported for a number of coating/fiber combinations. Effects on lifetime predictability will also be addressed.
Photo-Luminescence of SiO2:GeO2 Vad Fibers Heat-Treated Under Reducing Conditions
Authors: Kohketsu M. , Kawazoe H. , Kashiwazaki A. , Muta K.
Published: March 2011

Photo-luminescence(PL) in VAD silica preforms sintered under reducing or oxidizing condition was observed. Two types of diamagnetic luminescence centers and the Si-E′ center were found in pure silica glass. The intensity of photoluminescence was strongly dependent on the sintering atmosphere. This observation suggests that reduced Si species, Si(III) and Si(II), are responsible for the luminescence.
Magnetic Properties of Pbl-xGdxTe
Authors: Gorska M. , Anderson J. R. , Golacki Z.
Published: March 2011

The magnetization and magnetic susceptibility of Bridgman-grown Pb1-xGdxTe have been measured over a temperature range from 2 to 300 K and in magnetic fields from 0.01 to 50 κOe. The x-values of the crystals ranged from 0.03 to 0.07. The magnetic susceptibility followed a Curie-Weiss behavior, χ = C/(T + θ), with positive θ implying an antiferromagnetic exchange interaction between Gd ions. The magnetic field dependence of the magnetization was fitted to a modified Brillouin function with parameter values that agreed fairly well with those from Curie-Weiss plots. The magnitude of θ was comparable to the value found for Pb1-xMnxTe for similar x values; but since the ion spin is bigger for Gd this suggests that the exchange interaction in Gd-doped PbTe is roughly half the value in Mn-doped PbTe.
NMR and EPR Study of Sn0.98 Gd0. 02 Te at Low Temperatures.
Authors: Han B. S. , Symko O. G. , Zheng D. J.
Published: March 2011

Magnetic resonance behavior of Sn0.98 Gd0. 02 Te has been investigated from 2.5 K down to 0.015 K using a SQUID magnetometer and a 3He-4He dilution refrigerator. Detection of magnetic resonance with a SQUID allows the detection of NMRan d EPR of some of the constituents of the sample. This method is particularly useful for broad lines. We have investigated the NMRfor 125Te, 119 Sn, and 117Sn, and the EPR for the Gd ion in a field of 265 Oe. The contribution of Gd impurities is maily important in the Knight shift of the Sn isotopes; this shift is 7%. The EPR shows 2 broad lines with structure; one line we attribute to the paramagnetic ions and the second one we attribute to Gd ions which have formed clusters.
EPR of Mn2+ and Gd3+ Ions in PbTe and SnTe Semiconductors
Authors: Bartkowski Marian , Northcott D. J. , Reddoch A. H. , Williams D. F. , Hedgcock F. T. , Korczak Z.
Published: March 2011

The EPR spectra of manganese and gadolinium doped PbTe and SnTe semiconductors were measured from 4.2K up to 400K for dopant concentrations ranging from 50 to 50000 ppm. Resolved fine, hyperfine and superhyperfine structure as well as forbidden and cluster lines were measured. The combination of Lorentzian absorption and dispersion derivatives fits the recorded spectra well. Nonlinear broadening of linewidths with temperature is characteristic in these semiconductors. A manganese-induced broad maximum in linewidth versus temperature was found in PbTe around 180K for dopant concentrations in excess of 1.5at%.
Very High Mobility in Semimagnetic Semiconductors with Rare Earth
Authors: M Averous , B.A. Lombos , A. Bruno , J.P. Lascaray , C. Fau , M.F. Lawrence
Published: March 2011

A semimagnetic semiconductor with rare earth like magnetic ion is studied: Pb1-xGdxTe. Very high Hall mobilities were found up to 107 cm2/V.s. The magnetization is well fitted at 4.2K by a Curie-Weisslaw, with a Gaj's parameter of 1K. The value of JNN' the exchange in a Gd pair deduced from the susceptibility measurements is small, as expected, due to the specific character of Gadolinium (S state and 4f shell): JNN/k = - O.3°K for x = 0.025.
Magnetic Properties of Gd-Substituted Yttrium Nitride
Authors: van Dover R. B. , Schneemeyer L. F. , Gyorgy E. M.
Published: March 2011

Gd2Y1-xN is an instantiation of the broad series of diluted magnetic materials composed of lanthanide nitrides in a Sc, Y or La nitride host. These lanthanoid nitrides are interstitial compounds which all form in the Bi structure and with similar lattice constants, so solid solutions are readily obtained. We have prepared single-crystal thin films by epitaxial growth on sapphire substrates, and obtained material with moderate carrier densities (n Hall∼10 21 cm-3) and mobilities (μHall∼10 cm2V-1sec-1). Below the ordering temperature Tc,∼15 K the GdxY1-x,N films (0.25<x<0.40) exhibit a ferromagnetic-like M-H loop. The saturation magnetization measured at 15 kOe and 4.2 K is anomalously low, representing an effective moment of only 4.2 μB per Gd compared to the expected value of at least 7.2 μB. The piecewise-linear M-H curve is inconsistent with conventional models for a simple anisotropic ferromagnet. These features illustrate some of the unusual properties we are beginning to identify in the behavior of these new materials.
II-VI Compounds with Fe - New Family of Semimagnetic Semiconductors
Authors: Mycielski Andrzej
Published: March 2011

Several experimental methods: absorption, photoemission and transport measurements were used to determine the energy position of substitutional Fe2+ (3d6) donor state in the band structure of the semimagnetic semiconductor Hg1-v-xCdvFexSe for 0≤v≤0.7 and v+x=l, and x≤0.15. For v≤0.40, Fe2+(3d6) level is a resonant donor located in the conduction band. For v=O (HgSe) we obtain 230 meV for the position of Fe2+(3d6) level with respect to the bottom of the conduction band which coincides with the position of the Fermi level for electron concentration N ≅5x1018 cm-3. Surprisingly, the mobility of free electrons (T∼4.2K) is abnormally high and the Dingle temperature measured in quantum magnetoresistivity oscillations (SdH effect) and magnetooptical measurements is abnormally low. Because of the Coulomb interaction between the ionized donors, at low T, there will appear some correlation of their positions. This may lead to a kind of “liquefying” of the system of ions and to its “crystallisation” (i.e. formation of a superlattice or hyperlattice of ionized donors) at even lower T. The space-ordering of ionized donors influences dramatically the free-carrier scattering and correspondingly explains the high mobility and low Dingle temperature. Finally, we shall also present some magnetic properties of these new semimagnetic materials.
Electronic Transport Properties of Hg1-xFexSe
Authors: Pool F. , Kossut J. , Debska U. , Reifenberger R. , Furdyna J. K.
Published: March 2011

The electrical resistivity, electron concentration, and mobility of Hg1-xFexSe are reported for 4.2K < T < 300K and for 0.0001 < x < 0.12. The data are interpreted within an electronic band structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground state energy coincides with the conduction band continuum. The electron concentration data enable determination of the value of the donor energy as a function of the temperature and the crystal composition. The low temperature electron mobility for ∼ 0.0003 ≤ x ≤ 0.01 is considerably higher than expected and indicates a reduction of the charged impurity scattering effects at low temperatures.
Synchrotron Radiation Studies of Ternary Semimagnetic Semiconductors
Authors: Franciosi A.
Published: March 2011

A number of spectroscopic techniques that exploit synchrotron radiation are providing new insight into the properties of ternary semimagnetic semiconducting alloys. Photon energy-dependent photoemission methods are being used to characterize the electronic structure and the elemental and orbital contributions to the valence states. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) is providing a detailed understanding of the atomic structure and of the local phenomena that make the virtual crystal approximation inadequate. The new experimental information is stimulating novel theoretical work on the electronic structure, lattice stability and thermodynamics of these materials. We will review the results of a number of recent experimental studies and emphasize systematic trends and promising new areas of research.
Magnetoresistance and Hall Effect Near the Metal-Insulator Transition of Cd1-xMnxSe
Authors: Shapira Y.
Published: March 2011

A large magnetoresistance was observed in n-type Cdl-xMnxSe (x ≤ 0.10) at low temperatures. The concomitant changes in the Half coefficient as a function of H were also studied. These magnetotransport effects are caused by the s-d interaction. Some specific magnetoresistance mechanisms are discussed. The most likely dominant mechanisms are related to the giant spin splitting of the conduction band.
Strain Modification of Alloy Fluctuations in CdTe/(Cd,MnTe) Superlattice Systems
Authors: Jackson S. A. , McIntyre C. R.
Published: March 2011

We have calculated the effect of strains on the valence band offset and bandgap in the CdTe/(Cd,Mn)Te superlattice system for various growth directions and alloy compositions. We find that strain can modify the effect of alloy fluctuations on band offset and bandgap. The consequences of this for the formation of interface states will be discussed.
EXAFS Determination of Bond Lengths in Zn1−x MnxSe
Authors: Bunker B. A. , Pong W.-F. , Debska U. , Yoder-Short D. R. , Furdyna J. K.
Published: March 2011

EXAFS has been used to determine bond lengths in the diluted magnetic semiconductor Zn1−xMnxSe. These alloys change from a cubic (zincblende) crystal structure to a hexagonal (wurtzite) structure as a function of composition, with the transition at x ≈ 0.3. Although x-ray diffraction measurements show the lattice parameter to change by approximately 0.2Å through the series and the crystal structure changes from cubic to hexagonal, the nearestneighbor bond distances show no change within the experimental uncertainty of 0.0Å.
Universality of the Spin-Glass Transition in the Cd1−xMnXTe System
Authors: Datta T. , Amirzadeh J. , Barrientos A. , Jones E. R. , Schetzina J. F.
Published: March 2011

Universality of the spin-glass (SG) transition in bulk Cd1−xMnxTe was investigated by determining the scaling behavior of the spin-glass order parameter q as a function of the reduced temperature t = (Tg-T)/Tg, where Tg is the transition temperature. q(T) was determined from the SQUID magnetometric data both above and below the transition. It can be shown that, q(T) = 1 + T[│θ│ − C/χ(T)]−1. C and θ, the Curie and Curie-Weiss parameters, were obtained from a non-linear regression of the dc low field susceptibility χ(T) above Tg. q(T) thus obtained exhibits the cannonical order parameter criteria, viz. q(T>Tg) = 0, q(T→Tg)= 0, and q(T→Tg)→0, and q(T→0)→1. In range of Mn concentration studied (0.3 ≦ x ≦ 0.55), Tg ranged between 13 K and 23.5 K. Thus q as a function of absolute temperature behaves differently for different x. However, universality is clearly evidenced when the dependence on t is determined. q(t) exhibits a universal scaling law. We observe q ˜ tβ with β ≅ 0.95. Overall good agreement was noted with the Sherrington-Kirkpatrick (SK) infinite range SG model. Observed value of β is in excellent agreement with the model prediction β −1. But in the Cd1−xMnxTe system we find θ<0, indicating a net antiferromagnetic interaction and │θ│/Tg > 1 as opposed to the SK model. We believe this to be additional evidence that the spin interactions are not distributed as a Gaussian function.
Antiferromagnetic Exchange Constants Between Mn2+ Ions in II-VI Semimagnetic Semiconductors
Authors: Aggarwal R. L.
Published: March 2011

Mn2+ ion spins in II–VI compound semimagnetic semiconductors (SMS) such as Cd1−xMn Te interact with one another through an antiferromagnetic exchange which is responsible for many interesting properties of Mn-alloyed II–VI SMS. In particular, this interaction leads to unique magnetization (M) behavior of SMS as a function of applied magnetic field (B). The nearest-neighbor (NN) Mn2+ −MN2+ interaction has been shown to yield a series of five steps in M vs. B curves above about 10 T, providing a direct determination of the NN exchange constant JNN. A detailed analysis of the magnetization steps has yielded values for the next NN exchange constant JNNN. The magnetization steps also yield information on the distribution of Mn2+ ions. Inelastic neutron scattering and Raman scattering provide alternative methods for the direct determination of the exchange constants.
Transient Spectroscopy and Related Optical Studies in Diluted Magnetic Semiconductor Superlattices
Authors: Nurmikko A. V. , Kolodziejski L. A. , Gunshor R. L.
Published: March 2011

Recent successes in molecular beam (MBE) epitaxial growth of diluted magnetic semiconductor (DMS) artificial microstructures are now generating structures in which optically excited lower dimensional electronic and magnetic phenomena can be investigated. We consider an example which illustrates the present early state of affairs: transient magnetic polarons in connection with localized quasi 2-dimensional (2D) electronic excitations. We also comment on recent measurements and antiferromagnetic ordering in ultrathin layers in magnetic semiconductors.
Liquid Materials and Flow Processes in Reduced Gravity
Authors: Scriven L. E.
Published: March 2011

Fluid states are central to melt and vapor processing. Fluids flow. As they do they convect heat, solutes, and inclusions. Liquid/gas interfaces have tension. Their importance rises when scale is small or effective gravity low.

The forces that move fluids and mold fluid interfaces are recounted here. Fundamentals of viscous flow, convective transport, and capillary phenomena are summarized with special attention to the roles of gravity in liquid processing. Research needs are highlighted.
An Experimental Study of Heat Induced Surface-Tension Driven Flow
Authors: Kamotani Y. , Ostrach S. , Lowry S.
Published: March 2011

Velocity and temperature measurements are taken of heat induced surface-tension driven flows in both silicone oil and Fluorinert FC-43. Each fluid is contained in an open rectangular box and heated from above by a single strand of electrically heated nichrome wire suspended slightly above the fluid's surface. Velocity measurements are obtained for both liquids with a laser anemometer system. For silicone oil the general flow pattern is recorded using time-lapse photography. The surface temperature measurements are taken by a radiation thermometer and the bulk temperature distributions are measured by thermocouples. The velocity and temperature measurements are compared with numerical solutions obtained for the present configuration.
Overview of Containerless Processing Technologies
Authors: Barmatz M.
Published: March 2011

In the near future, a large segment of the scientific community will have an opportunity to perform materials processing experiments on the Space Shuttle in the reduced gravity environment of space. Many of these experiments will require containerless processing techniques that provide manipulation and control of weightless (molten) materials without physical contact with container walls or other holding devices. A variety of containerless processing technologies are now being developed for space and ground-based materials processing facilities. The utilization of air jets or high intensity acoustic, electromagnetic or electrostatic fields can produce forces that support and manipulate materials. Most of the present containerless research is directed toward the development of high temperature systems capable of melting and resolidifying materials. This paper will review the materials processing capabilities and level of developmental progress of each technique. An introduction to available NASA test facilities will also be given.
Glass Processing in a Microgravity Environment
Authors: Uhlmann Donald R.
Published: March 2011

The scientific and technological aspects of processing glasses in a microgravity environment offer considerable challenge and potential. Activities directed to obtaining information relevant to processing materials on earth and eventually processing materials in space for use in space will be considered. The features on the space environment—reduced gravitational forces, vacuum of large capacity, unique radiation conditions, and unlimited dimensions of space—will be discussed, with emphasis directed to the first. The implications of reduced gravitational forces for containerless processing, freedom from deformation, the study and control of weak forces, and the absence of convection and density segregation will be explored. On-going programs concerned with glass processing in space will be reviewed, and additional areas which merit investigation will be considered.
Bubble Behavior in Molten Glass in a Temperature Gradient
Authors: Meyyappan M. , Subramanian R. S. , Wilcox W. R. , Smith H.
Published: March 2011

Gas bubble motion in a temperature gradient was observed in a sodium borate melt in a reduced gravity rocket experiment under the NASA SPAR program. Large bubbles tended to move faster than smaller ones, as predicted by theory. When the bubbles contacted a heated platinum strip, motion virtually ceased because the melt only imperfectly wets platinum. In some cases bubble diameter increased noticeably with time.
Gels and Gel-Derived Glasses in the Na2o-B2o3-Sio2 System
Authors: Mukherjee Shyama P.
Published: March 2011

The containerless melting of high-purity multicomponent homogeneous gels and gel-monoliths offers a unique approach to making ultrapure multicomponent optical glasses in the reduced gravity environment of space. Procedures for preparing and characterizing gels and gel-derived glasses in the Na 2o-B2o3-SiO2 system are described. Preparation is based on tLe polymerization reactions of alkoxysilane with trimethyl borate or boric acid and a suitable sodium compound. The chemistry of the gelling process is discussed in terms of process parameters and the gel compositions. The physicochemical nature of gels prepared by three different procedures were found to be significantly different. Infrared absorption spectra indicate finite differences in the molecular structures of the different gels. The melting of the gel powders and the transformation of porous gel-monoliths to transparent “glass” without melting are described.
Glass Shell Manufacturing in Space
Authors: Nolen Robert L. , Downs Raymond L. , Ebner Matthias A.
Published: March 2011

Highly-uniform, hollow glass spheres, which are used for inertial-confinement fusion targets, are formed from metalorganic gel powder feedstock in a drop-tower furnace. The modelling of this gel-to-sphere transformation has consisted of three phases: gel thermochemistry, furnace-to-gel heat transfer, and gravity-driven degradation of the concentricity of the molten shell.

The heat transfer from the furnace to the free-falling gel particle was modelled with forced convection. The gel mass, dimensions, and specific heat as well as furnace temperature profile and furnace gas conductivity, were controlled variables. This model has been experimentally verified. In the third phase, a mathematical model was developed to describe the gravity-driven degradation of concentricity in molten glass shells.
The Role of Fluid Flow Phenomena in the Czochralski Growth of Oxides.
Authors: Miller D.C.
Published: March 2011

In the Czochralski growth of single crystals from large melts, fluid flow phenomena have a major effect on interface shape, growth striations, defect density and the length of crystals which can be grown from a melt of given volume and thermal geometry. Because of the technical difficulties encountered in making direct measurements in molten oxides, simulation experiments have been extensively utilized to gain insight into melt behavior.

Both temperature profile and flow geometry results from simulation experiments are discussed. This data is supported by direct melt observations and results from the characterization of grown crystals. When reviewed together, this information offers new insights into the complex behavior of Czochralski growth processes, including the role of thermal gradients, crystal rotation, and surface tension driven (Marangoni) convection.
Modelling of Tgs Growth in Space
Authors: Liu L. C. , Wilcox W. R. , Kroes R. , Lal R.
Published: March 2011

To maintain a constant growth rate in the absence of convection it is necessary to program down the temperature. Trial and error numerical computations were performed to find the first approximation for the required isothermal dissolution period, linear ramp; and polynomial temperature variation period. The linear ramp rate was limited by the specified maximum temperature gradient. The isothermal dissolution turned out to be unnecessary. The linear ramp had to be stopped before the polynomial period was begun to avoid overshooting the specified maximum growth rate. After a few hours the temperature profile approached steady state behavior.
Growth of Triglycine Sulfate (Tgs) Crystals by Solution Technique
Authors: Lal R. B. , Kroes R. L. , Wilcox W. R.
Published: March 2011

The growth of crystals from solution is greatly influenced by buoyancy driven convection. In a low-g environment, convection is greatly suppressed and diffusion becomes the predominant mechanism for thermal and mass transport. An experiment to grow TGS crystals by solution technique during the orbital Spacelab III mission has been designed. Crystals are grown by a new and unique technique of extracting heat from the crystal through a sting. The cooling at the sting tip is responsible for the desired supersaturation near the growing crystal. Calculations indicate that the cooled sting technique for solution crystal growth is necessary in low-g to maintain a maximum growth rate of 1 mm/day. Results of groundbased work in support of the flight experiment are discussed.
Point Defects in Doped and Undoped Hgl−xCdxTe Alloys
Authors: Vydyanath H. R.
Published: March 2011

EXTENDED ABSTRACT

Using a law of mass action approach in which lattice defects are treated as chemical species, the variation of the defect concentrations as a function of .the physicochemical conditions of preparation has been established via measurements of electrical characteristics which are directly related to the defect concentrations.
Increased Uniformity of Silicon Needed for Critical Devices How Can We Improve the Float-Zone Process?
Authors: Kern Edward L.
Published: March 2011

EXTENDED ABSTRACT
Convective and Interfacial Instabilities During Solidification of Succinonitrile Containing Ethanol
Authors: Schaefer R. J. , Coriell S. R.
Published: March 2011

Although slow convective flow is difficult to detect in solidifying metals, it can readily be observed in transparent materials by observing the motion of small neutrally buoyant particles. An excellent material for such studies is succinonitrile, which solidifies with an unfaceted solid/liquid interface and which has well characterized physical properties. For studies of solute-induced convection, ethanol is a useful addition to succinonitrile because it has a lower density and a somewhat similar molecular structure.

Samples of high purity and ethanol-doped succinonitrile are solidified unidirectionally in a vertical temperature gradient. Latex microspheres, 2μm in diameter, are suspended in the liquid to delineate convective flow. Convective and morphological stability are observed as a function of solute concentration and growth velocity. These measurements are compared to theoretical calculations which predict the transition from stability to instability as a function of solidification conditions. The predicted transitions occur at low concentrations and solidification velocities, so that extreme care is required to eliminate the effects of impurities or thermally-induced convection.
A Dynamic Technique for Measurements of Thermophysical Properties at High Temperatures
Authors: Cezairliyan Ared
Published: March 2011

A technique is described for the dynamic measurements of selected thermophysical properties of electrically conducting substances in the range 1500 K to the melting temperature of the specimen. The technique is based on rapid resistive self-heating of the specimen from room temperature to any desired high temperature in less than one second by the passage of an electrical current pulse through it and on measuring the pertinent quantities, such as current, voltage and temperature, with millisecond resolution. The technique was applied to the measurements of heat capacity, electrical resistivity, hemispherical total emittance, normal spectral emittance, thermal expansion, melting temperature, heat of fusion, and temperature and energy of solid-solid phase transformations. Upper temperature of the measurements has been limited by the melting temperature of the specimen, at which point, the specimen collapses due to the gravitational force. In order to be able to extend the measurements to the liquid phase, performance of the dynamic experiments in a near-zero gravity environment is suggested.
Gravitationally Induced Convection During Directional Solidification of off-Eutectic Mn-Bi Alloys
Authors: Pirich Ron G.
Published: March 2011

The effects of thermal and solute gradient, gravity induced convection during vertical directional solidification, on longitudinal macrosegregation of Bi and Mn rich off-eutectic starting compositions, has been studied as a function of composition, growth velocity and gravity vector orientation. Since the morphology of these alloys is characterized by an aligned, rodlike permanent magnet composite when grown cooperatively, the magnetic properties were used to measure composition segregation and the transition from dendritic to composite growth. Severe macrosegregation was observed in all cases studied and the degree of convection inferred by modeling the observed composition segregation using a stagnant film approach. Morphological stability was found to follow a constitutional supercooling-type law for both Bi and Mn rich compositions.
The Effect of The Natural Convection on The Transition from Columnar to Equiaxed Crystals:
Authors: Fredriksson Hasse
Published: March 2011

During dendritic solidification of metal alloys there normally occur temperature gradients in the liquid ahead of the solidification front. The gradient causes a natural convection ahead of the front. The natural convection causes a heat transfer from the interior of the liquid, which gives the possibility for crystals to grow ahead of the solidification front. The growth of these crystals is determined by the cooling rate and the kinetics of the solidification process. The paper will deal with the theory behind the growth of free crystals ahead of a solidification front. Some experimental results will be given. The effect of gravity on the growth process will. be discussed.
Theory and Applications of Electromagnetic Levitation
Authors: Frost R. T. , Chang C. W.
Published: March 2011

The basic theory of electromagnetic levitation of conducting specimens by alternating fields is reviewed. Emphasis is placed upon simple approximation formulae useful in planning and interpreting laboratory measurements. Recent work involving computer solutions for forces and heating by complex field coil configurations, including those with polyphase excitation, is described. Applications and limitations of the technique both terrestrially and in a microgravity environment are discussed.
Growth and Characterization of High Quality, Low Defect, Subgrain Free Cadmium Telluride by a Modified Horizontal Bridgman Technique
Authors: Allred W. P. , Khan A. A. , Johnson C. J. , Giles N. C. , Schetzina J. F.
Published: March 2011

A low stress modified horizontal Bridgman technique has been developed and used to grow low defect, large area, subgrain free CdTe crystals for use as substrates in the epitaxial growth of HgCdTe and related IR detector materials. CdTe wafers cut from horizontal Bridgman grown boules exhibit, resistivities in the 107ohm-cm range. Etch pit counts are in the 104cm−2 range. Etch pit patterns as well as x-ray topographs indicate the absence of low-angle grain boundaries. Double crystal x-ray rocking curves are single peaked and very narrow with FWHM(333) as low as 9 arc-sec. Rocking curves of FWHM(333) = 9 to 15 arc-sec, measured at several different laboratories, have been obtained for CdTe wafers cut from several boules. This is in contrast to standard vertical Bridgman grown CdTe samples, which generally show broader x-ray rocking curves sometimes with multiple peaks as a result of subrgrain structure. Low temperature (1.6–4.5 K) photoluminescence (PL) measurements on these low defect samples reveal bright edge emission lines which are the main feature of the spectrum. Additional bound exciton lines and other sharp features associated with donor and acceptor impurities are also present. The very weak defect band luminescence (1.40–1.46 eV) provides additional evidence of sample quality.
Growth and Characterization of Ti3PSe4 Single Crystals
Authors: Singh N. B. , Hopkins R. H. , Mazelsky R. , Gottlieb M.
Published: March 2011

Good quality single crystals of Ti3PSe4 were grown from the melt by the Bridgman technique following improvements in the method of purifying the parent components, and optimization of growth parameters. Crack-free crystals 8 cm in length and 17 mm in diameter were produced.

The quality of the crystals was evaluated by optical transmittance and metallographic techniques. In the range 0.7 to 14 μm the optical transmittance shows elimination of absorption bands exhibited in crystals grown without special purification steps. Etchpit studies showed that the crystals were free from inclusions and lamellar twins and that they show a uniform cross sectional etch pit density.
HgCdTe Versus Hgznte: Electronic Properties and Vacancy Formation Energies
Authors: Berding M. A. , Chen A.-B. , Sher A.
Published: March 2011

The alloy variation of the band gap and the electron and hole effective masses have been calculated for HgCdTe and HgZnTe. Band-gap bowing is larger in HgZnTe than in HgCdTe because of the larger bond length mismatch of HgTe and ZnTe; electron and hole effective masses are found to be comparable for the two alloys for a given band gap. We have calculated the electron mobility in both alloys with contributions from phonon, impurity, and alloy scattering. Contributions to the E1 line width due to alloy and impurity scattering in Hg0.7Cd0.3Te have been calculated. Results of calculations of the vacancy formation energies in HgTe, ZnTe, and CdTe are discussed.
Photochemical Processes in Photo-Assisted Epitaxy of CdxHg 1-xTe
Authors: Irvine S J C , Mullin J B , Blackmore G W , Dosser O D , Hill H
Published: March 2011

Mechanisms for the low temperature photo-dissociation of alkyl precursors for the epitaxial growth of CdxHg1-xTe (CMT) are discussed. The roles of vapour and surface nucleation are considered in the light of the free radical model which also can provide methods for controlling the vapour phase photochemistry. Higher quality CMT has been grown at 250 °C by using dimethyl mercury as a source of free methyl radicals. Problems encountered in reducing growth temperature for multilayer epitaxy are considered for CdTe and HgTe and conditions established for epitaxial growth at 200°C.The roles of alkyl concentration, substrate temperature, UV intensity and free radical concentration are explored. Results on the first reported growth of CdTe deposition using a cw UV laser source are compared with the arc lamp grown layers.
Characterization of Infrared Materials by X-Ray Diffraction Techniques
Authors: Takei W. J. , Doyle N. J.
Published: March 2011

X-ray diffraction techniques have proved invaluable in the characterization of infrared materials, particularly those prepared by thin film deposition techniques such as molecular beam epitaxy, MBE. The techniques are sufficiently sensitive and rapid to provide the information feedback required for efficient optimization of the growth process. They are nondestructive and permit the correlation with results on the same sample obtained by other characterization techniques such as those being described at this Symposium. Depending on the development status of the growth technology, the information to be acquired includes presence of twinning, quality, and type of epitaxial orientation, strains, and compositional variations. A critical issue in the application of these materials in detector arrays is the question of uniformity control, both laterally and in depth. The techniques to be described include not only modern x-ray topographic and multiple crystal diffractometric techniques but particularly for the early stages of growth process development, classical photographic ones such as the oscillation and Weissenberg methods. Examples of these various aspects are presented with emphasis placed on the characterization involved in MBE growth of HgCdTe films.
Atomic Structure of HgTe and CdTe Epitaxial Layers Grown by MBE on GaAs Substrates
Authors: Ponce F. A. , Anderson G. B. , Ballingall J. M.
Published: March 2011

Using high resolution transmission electron microscopy (HRTEM) it is possible to directly image the projected structure of semiconductors with point resolutions at the atomic level. This technique has been applied to the study of interface and defect structures associated with molecular beam epitaxial (MBE) growth of HgCdTe layers on GaAs substrates. The structure of the CdTe/GaAs interfaces is described for (100) and (111) epitaxy. From the atomic structure, a model for the early stages of epitaxial growth is presented. The structure of HgTe-CdTe superlattices is discussed from the HRTEM point of view.
Dynamical X-ray Rocking curve Simulations of InGaAsP/InP Double Heterostruciures using Abeles' Matrix Method
Authors: Macrander A. T. , Glasgow B. M. , Minami E. R. , Karlicek R. F. , Mitcham D. L. , Riggs V. G. , Berreman D. W. , Johnston W. D.
Published: March 2011

Simulated rocking curves for a light-emitting diode structure are presented. Results for a structure containing uniform layers are compared to rocking curve data for a wafer grown by vapor phase epitaxy (VPE), and we conclude from the comparison that the VPE wafer closely approached the hypothetical ideal assumed in the simulations. Simulations illustrating difficulties in analyses and the effects of a graded active layer are also presented.
Deep Level Defects in CdTe
Authors: Leigh V. B. , Kremer R. E.
Published: March 2011

We have used two complementary techniques to study deep trapping levels in CdTe single crystals. Samples of undoped, semi-insulating material and p-type material doped with phosphorus or cesium have been examined using transient spectroscopic techniques. Both capacitance transients (DLTS) and photocurrent transients (PITS) have been measured. The DLTS measurements showed several trapping levels in all of the specimens, while the PITS data usually revealed only a single level.
Development of HgCdTe for LWIR Imagers
Authors: Schmit Joseph L.
Published: March 2011

This paper provides a historical perspective on the emergency of HgCdTe as the material of choice for long wavelength infrared (LWIR) imagers. The need for devices which see room temperature objects through the atmospheric window actually drove the development of this material. The lack of elemental or compound semiconductors having the desired wavelength response forced the choice of the alloy semiconductor, HgCdTe. The development of this material in several countries and companies beginning in the late 1950's is traced. The crystal growth methods used to grow HgCdTe have included melt growth techniques such as Bridgman, zone-melting, quench-anneal and slushgrowth. The solution growth techniques include growth from HgTe-rich, Te-rich and Hg-rich solutions. Vapor phase growth has included evaporation, sputtering, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). No perfect method has yet been developed, but several have provided material for the large area arrays needed for modern imagers.
Low Temperature Photoluminescence Study of Doped CdTe and CdMnte Films Grown by Photoassisted Molecular Beam Epitaxy
Authors: Giles N. C. , Bicknell R. N. , Schetzina J. F.
Published: March 2011

N-type and p-type (100) CdTe films have been grown on (100) CdTe substrates by photoassisted molecular beam epitaxy, using indium and antimony as n-type and p-type dopants, respectively. The application of this growth technique to substitutionally dope another II-VI material is demonstrated by the successful n-type doping of (100) CdMnTe films with indium. Modulationdoped superlattices consisting of barrier layers of CdMnTe:In alternating with CdTe have also been grown. The point defect nature of these in situ doped films and multilayers is studied with low temperature (1.6–5 K) photoluminescence and excitation photoluminescence measurements. The introduction of the dopant atoms using this new growth technique produces immediate changes in the photoluminescence spectra of the epilayers. Photoluminescence studies of the superlattices show the effects of quantum well confinement and band filling due to free carriers.