TABLE OF CONTENTS

INTRODUCTION ..........................................................................................................................

SECTION I. ONE-YEAR REPORT FOR SAMS AND OARE ON STS-62/USMP-2 ................

1.1

1.2

1.3

1.4

1.5

xvii

1-1

1-3 Introduction ............................................................................................................. 1-3

Accelerometer Systems .............................................................. 1-3

Columbia Low-Gravity Environment--STS-62 ..................................................... 1-4

1.3.1 Crew Sleep ..................................................................................................... 1-4

1.3.2 Crew Exercise ................................................................................................ 1-4

1.3.3 Flash Evaporator System ............................................................................... 1-4

1.3.4 Circulation Pumps .......................................................................................... 1-4

1.3.5 Thrusters ......................................................................................................... 1-5

1.3.6 Space Shuttle Attitude .................................................................................... 1-5

Summary ................................................................................................................. 1-6

Publications .............................................................................................................

SECTION II.

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

ISOTHERMAL DENDRITIC GROWTH EXPERIMENT ....................................

2-15

2-17

Introduction ............................................................................................................. 2-18

Background on Dendritic Growth _._._.;_.. The Isothermal Dendritic Growth L_,,v_,--,,_,-,._.,-,_, iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii 2-20

2.3.1 IDGE Growth Chamber ................................................................................. 2-21

2-22

2.3.2 Flight Experiment Procedure ......................................................................... 2-22

Results .....................................................................................................................

2.4.1 Dendritic Growth Velocities .......................................................................... 2-23

2-24

2.4.2 Dendritic Tip Radii ........................................................................................ 2-24 2.4.3 Growth P6clet Number ................................................................................... 2-24

2.4.4 Kinetic Scaling Parameter .............................................................................. 2-25

Teleoperations ............................................................................................. iiiiiiiiiiii 2-25

2.5.1 IDGE Operating Plans ....................................................................... 2-26

2.5.2 Flight Operations ............................................................................................ 2-27

Summary and Conclusions ...................................................................................... 2-28

Work in Progress ..................................................................................................... 2-29

References ............................................................................................................... 2-30 Publications and Presentations ................................................................................

SECTION III.

3.1

3.2

3.3

3.4

IN SITU MONITORING OF CRYSTAL GROWTH USING MEPHISTO ......... 3-45

3-47

Overview ..................................................................................... 3-47

Background .............................................................................................................

USMP-2 Mission Description ................................................................................. 3-48

3.3.1 Experimental Facility and Techniques ........................................................ 3-48 3-48

3.3.2 Flight Summary ........................................................................................... 3-49 Microstructural and Data Analysis ......................................................................... 3-49

3.4.1 Microstructural Analysis ............................................................................. 3-49

3.4.2 Kinetics Data Analysis ................................................................................

.°.

111
3.5

3.6

SECTION IV.

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

SECTION V.

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

3.4.3 Microstructural Analysis ............................................................................. 3-49

3.4.4 Kinetics Data Analysis ................................................................................ 3-52

Summary ................................................................................................................. 3-56

References ............................................................................................................... 3-57

GROWTH OF SOLID SOLUTION SINGLE CRYSTALS ................................. 4-89

Introduction ............................................................................................................. 4-92

Study Rationale ....................................................................................................... 4-93

Driving Force for Convection ................................................................................. 4-93

Previous Results for Compositional Redistribution ................................................ 4-94

Problems Associated With Growth Interface Shape Control .................................. 4-94

Advantages of Microgravity Growth ...................................................................... 4-95

Ground-Based Results ............................................................................................. 4-95

Flight Results .......................................................................................................... 4-96

Flight Timeline ........................................................................................................ 4-96

Residual Acceleration Results ................................................................................ 4-97

Radiography ............................................................................................................ 4-97

Photography ............................................................................................................ 4-98

X-Ray Diffraction ................................................................................................... 4-98

Scanning Electron Microscopy (SEM) ................................................................... 4-100

Energy Dispersion Spectrometry (EDS) ................................................................. 4-100

Cutting Strategy ...................................................................................................... 4-101

Optical Microscopy ................................................................................................. 4-102

Wavelength Dispersive Spectrometry ..................................................................... 4-102

Synchrotron Radiation Studies ................................................................................ 4-103

Summary and Preliminary Conclusions .................................................................. 4-103

References ............................................................................................................... 4-105

HIGHLIGHTS OF THE ZENO RESULTS FROM THE USMP-2 MISSION ...... 5-135

Introduction ............................................................................................................. 5-137

Timeline .................................................................................................................. 5-138

Instrument Performance .......................................................................................... 5-138

5.3.1 Thermostat and Thermometry ..................................................................... 5-138

5.3.2 The Light-Scattering Spectrometer ............................................................. 5-139

Determining Tc On Orbit ........................................................................................ 5-140

Turbidity Measurements and Related Effects ......................................................... 5-141

Critical-Point Fluctuation Decay Rates ................................................................... 5-143

Conclusion .............................................................................................................. 5-145

Selected Bibliography ............................................................................................. 5-146

iv
Figure

STS-62

Figure 1-1.

Figure 1-2.

Figure 1-3.

Figure 1-4.

Figure 1-5.

Figure 1-6.

Figure 1-7.

Figure 1-8.

Figure 1-9.

Figure 1- l 0.

Figure 1-11.

Figure 1-12.

Figure 2-1.

Figure 2-2.

Figure 2-3.

Figure 2-4.

Figure 2-5.

Figure 2-6.

LIST OF ILLUSTRATIONS

Title

Onboard Photo: Shuttle Cargo Bay .......................................................................

USMP-2 Payload ...................................................................................................

Orbital acceleration research experiment data from STS-62 .................................

Crew sleep period ..................................................................................................

Crew active time with exercise period ...................................................................

Time domain vector magnitude of SAMS data for Unit F, Head B,

MET 012/00:45:00--012/00:47:10 .........................................................................

Power spectral density vector magnitude of SAMS data for Unit F,

Head B, MET 012/00:45:00-012/00:47:10 ...........................................................

Time domain vector magnitude of SAMS data for Unit F, Head B,

MET 008/14:19:30--008/14:21:40. Circulation pump activation ..........................

Time domain vector magnitude of SAMS data for Unit F, Head B,

MET 009/17:09:00-009/17:11:10. OMS ...............................................................

Time domain vector magnitude of SAMS data for Unit F,

Head B, MET 010/17:12:00-O10/17:14:10. Primary reaction

control system activity ...........................................................................................

Power spectral density vector magnitude of SAMS data for Unit F,

Head B, MET 010/17:12:00-O10/17:14:10. Primary reaction

control system activity ...........................................................................................

Space shuttle attitudes for USMP-2 on STS-62 ....................................................

Attitude transition during USMP-2 operations (-ZLV/+YVV

to -XLV/-ZVV) ....................................................................................................

Attitude transition during USMP-2 operations (-XLV/-ZVV

to -XLV/+ZVV) ....................................................................................................

IDGE flight growth chamber .................................................................................

IDGE temperature controlled tank and optic system .............................................

Examples of 35-mm photographic images and low-resolution,

binary, digitized electronic images ........................................................................

Dendritic growth tip velocity as a function of supercooling ..................................

Dentritic growth tip velocity as a function of supercooling

at the lower supercoolings .....................................................................................

Dentritic growth tip radius as a function of supercooling ......................................

Page

ii

xix

1-7

1-8

1-8

1-9

1-9

1-10

1-10

1-11

1-11

1-12

1-13

1-13

2-33

2-34

2-35

2-36

2-37

2-38

V
Figure 2-7.

Figure 2-8.

Figure 2-9.

Figure 2-10.

Figure 2-11.

Figure 2-12.

Figure 3-1.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-3.

Figure 3-3.

Figure 3-3.

Figure 3-4.

Dentritic growth tip Prclet numbers as a function of supercooling ........................ 2-39

Dentritic growth tip Prclet numbers as a function of supercooling

in the diffusion-limited supercooling range ............................................................

Dentritic growth scaling parameter as a function of supercooling .........................

The first telescience measured velocity calculated from the SSTV data ................

Dendrite tip displacement versus time data from the SSTV data ...........................

Extension of the velocity data range that was obtained through

the use of telescience .............................................................................................

(a) MEPHISTO core assembly ..............................................................................

(b) Arrangement of the experimental samples in MEPHISTO. Shows

three rod-shaped samples processed in parallel, the location of

thermocouples, and the two solid/liquid (s/l) interfaces from which

a differential Seebeck electro motive force (emf) is measured. Also

shown is the overall temperature distribution in the apparatus

during processing (schematic) ...............................................................................

Initial microstructure showing a faceted cellular/dendritic morphology.

Samples grown via directional solidification at a rate of 0.2 cm/s. Optical

micrograph, differential interference contrast (DIC) illumination ........................

(a) Summary of sample sections preserved during final solidification.

Also shows the orientation of microsections of the samples which are

taken in a direction pointing to the diffuser center and thus thermally

equivalent ...............................................................................................................

(b) Microstructural evolution starting with the initial microstructure

at growth velocities V1 and V2. (c) Continued growth at V2 and V3 ..................

(d) Microstructural evolution during continued growth at V3 and

after change in the growth rate to V4. (e) Continued growth at V4 ......................

(f) Microstructural evolution during continued growth at V4 and

growth rate changes to V5, V 1, and V3. Note the cellular breakdown

during growth at V5. (g) Continued growth at V3, followed by a

growth rate change to V6. Note the development of a cellular

structure at V6 ........................................................................................................

(a) Microstructural appearance of the interface between the Earth-grown

(faceted cellular/dendritic) and microgravity-processed (plane front)

portions of the Prltier sample, moving furnace side. The composite

micrographs were obtained under a polarized light/sensitive tint

illumination and show the emergence of a few dominant orientations

during directional solidification in microgravity ...................................................

2 -40

2-41

2-42

2-43

2-44

3-62

3-62

3-63

3-64

3-65

3-66

3-67

3-68

vi
Figure3-4.

Figure3-4.

Figure3-5.

Figure3-6.

Figure3-7.

Figure3-8.

Figure3-9.

Figure3-10.

Figure3-11.

Figure3-12.

Figure3-13.

Figure3-14.

(b)Microstructuralappearanceof theinterfacebetweentheEarth-grown

(facetedcellular/dendritic)andmicrogravity-solidified(planefront)

portionsof theResistance/Quenchingsample,fixed furnaceside.

Themicrostructuredevelopedduringfinal cool down.Polarized

light/sensitivetint illumination ..............................................................................

(c) Interfacebetweentheinitial microstructureandmicrogravity

solidificationatagrowthvelocity of 1.85gm/s.Micrographrepresents

thebottomhalfof thesamplecrosssection.Notetheemergenceof two

dominantorientationsfrom theinitial structure....................................................

Planefront solidificationatvelocitiesrangingfrom 1.85to 6.71txrn/s................

(a)Schematicof theshapeof aninterfaceatthetransitionbetween

V5 andV1.(b)Low magnificationmicrographof theinterface.The

interfaceis slightlycurvedattheedgesdueto thesurfacetension

forcesatthesolid/liquid/crucibletriple junction ...................................................

Detailof interfacebetweenV5 andV1.Notethechangesin grain

boundaryanglesupondecreasingthegrowthrate.Optical

Micrograph,DIC illumination ............................................................................

SecondaryIon MassSpectroscopic(SIMS) analysisof theV5/V1

interfaceillustratingthesharpchangein averageSncontentat

theinterface ...........................................................................................................

(a)Schematicrepresentationof thefeaturesof aselectivebreakdown

at agrowthvelocityof 26.6gm/s.(b)Microstructuralappearanceof

theselectivebreakdown.........................................................................................

Selectiveplanarto cellulartransitionat agrowthvelocity of 26.88_trn/s............

(a)Schematicillustrationshowingthesalientfeaturesof acellular

transitionat agrowthvelocityof 40/tm/s. (b) Low magnification

micrographof V6 breakdownregion.(c)Highermagnificationview

of cellulargrowthatV6 .........................................................................................

(a)Schematicof cellulargrowth,transversesection.Showstwo

dominantorientations.(b) Opticalmicrograph(B.F.) illustrating

thecell boundaries.(c) Opticalmicrograph(P.L.S.T.)showing

twodominantgrains.(d) Opticalmicrograph(D.F.) showing

cell boundaries.......................................................................................................

(a)Typicalresistancetrackingof interfacepositionandmeasurement

of furnacetranslation(by linearpotentiometer)asa functionof processing

time.TypicalSeebeckmeasurementcycle(solidification-stabilizationremelting-stabilization).(b)Thehysterisisbetweenresistancechange

duringsolidification(bottomcurve)andthatduringsymmetrical

remelting(topcurve) .............................................................................................

FFTeditingof resistancemeasurementduringa typicalSeebeckcycle.

Notetheremovalof superimposednoisesothatfine slopechanges

duringtheresistancetrackingcanbedetected......................................................

3-69

3-70

3-70

3-71

3-72

3-72

3-73

3-74

3-75

3-76

3-77

3-78

vii
Figure3-15.

Figure3-16.

Figure3-17.

Figure3-18.

Figure3-19.

Figure3-20.

Figure3-20.

Figure3-20.

Figure3-20.

Figure3-20.

Figure4-1.

Figure4-2.

Figure4-3.

Figure4-4.

Figure4-5.

Subtractionof resistancechangeduringsolidificationfrom thatduring

symmetricalremelting.FFT editeddata.Theexistenceof anegative

plateauindicatesinterfacialretardationduringsolidification ...............................

Resistancechangeduringthefinal quench,showsasharpincreasein

theresistanceat thetimeof quench.Thesubsequentdecayis dueto the

slowermeltingof theportionof thesamplepushedinto thehotzoneduring

thequench.Also shownarefurnacepositionandoutputsof quenching

samplethermocouples...........................................................................................

Outputof shortandlongthermocouplesin thequenchingsampleduring

a typicalSeebeckmeasurement.Also shownis thecorresponding

furnacemovement.................................................................................................

Evolutionof differentialSeebeckemf dueto microstructure/thermal

gradientinteraction.(a)Initial anddirectionalgrowthmicrostructure

at two interfaces.(b)Thermalgradientzoneduringdirectional

solidification.(c) Evolutionof theSeebeckemfduringtranslation

of a thermalgradient..............................................................................................

Theprocessingsequenceof microstructural/Seebeckemf evolution

describedin figures20(a)-(e).Showstwo meltingcyclesandtwo

solidificationcyclesseparatedby stabilizationperiods.Eachstepin

theprocessis identifiedby aletter(athroughe),correspondingto

figures20(a)-(e) ...................................................................................................

(a)Initial interfacebetweentheEarth-grownfacetedcellular/dendritic

microstructureandliquid alloy.Theinterfaceis slightly curvedatthe

solid/liquid/crucibletriplejunction.ThebaselineSeebeckemfis steady

with ameanvalueof ~70_tV.................................................................................

(b) Directional2-mmmeltingof theinitial interfaceandthecorresponding

Seebecksignal .......................................................................................................

(c) Directional2-mmsolidificationresultingin plane-frontgrowthwith

columnargrainsandthecorrespondingevolutionof theSeebecksignal..............

(d) Directional7-mmmeltingof theplane-front(2mm) andcellular/dendritic

(last5mm) regionandthecorrespondingevolutionof theSeebecksignal..........

(e)Directional7-mmsolidificationresultingin plane-frontgrowthwith

columnargrainstructure.Alsoshownis thedisappearanceof unfavorable

orientationsasgrowthproceedsandthecorrespondingevolutionof the

Seebecksignal .......................................................................................................

Superconductingmagnetfurnace ..........................................................................

Estimatedcompositionalprofilebasedondiffusioncontrolledsolidification ......

Z-componentOAREdataduringattitudechange.................................................

OAREdataatinstrumentfor -ZLV/+YVV attitude.............................................

OAREdatatransformedto AADSF locationfor-ZLV/+YVV attitude...............

3-78

3-79

3-80

3-81

3-82

3-83

3-84

3-85

3-86

3-87

4-110

4-111

4-111

4-112

4-112

VIII
Figure 4-6.

Figure 4-7.

Figure 4-8.

Figure 4-9.

Figure 4-10.

Figure 4-11.

Figure 4-12.

Figure 4-13.

Figure 4-14.

Figure 4-15.

Figure 4-16.

Figure 4-17.

Figure 4-18.

Figure 4-19.

Figure 4-20.

Figure 4-21.

Figure 4-22.

Figure 4-23.

Figure 4-24.

Figure 4-25.

Figure 4-26.

Directions of residual acceleration vectors with respect to AADSF

and orbiter ..............................................................................................................

Resolved OARE data for AADSF flight sample ...................................................

Radiographs of flight and ground-truth cartridges showing the last to

freeze material .......................................................................................................

Appearance of ground-truth and flight cartridges after processing .......................

AADSF flight sample after removal from ampoule----J7 position .........................

(a) Typical Laue back reflection pattern of surface of flight sample.

(b). Broad beam back reflection Laue pattern--same region as figure 11 a ..........

Back scattered electron image (SEM) from flight sample--note the

channeling pattern ..................................................................................................

(a) Thermal etch pits on surface of flight sample---close to [111].

(b). Thermal etch pits on high index surface .........................................................

Surface of flight sample showing interface breakdown during

orbiter maneuver ....................................................................................................

Surface composition of ground-truth sample along J7+90 ° orientation ................

Surface composition of flight sample along J7 orientation ...................................

Surface composition of flight sample along J7-90 ° orientation ............................

Surface composition of flight sample along J7+90 ° orientation ............................

Surface composition of flight sample along J7+ 180 ° orientation ..........................

Surface composition of flight sample close to J7-90 ° orientation ........................

Two-dimensional analysis of a slab of the ground-truth sample cut

through the rate change region--EDS results .......................................................

Microstructure of the slab shown in figure 21--ground-truth sample

etched in Polisar II .................................................................................................

Flight sample cut to show major grain in [111] direction---etched

in Polisar II ............................................................................................................

Ground-truth sample cut at right angles to growth axis---etched in Polisar II ......

Composition across diameter of ground-truth sample--EDS results ....................

Composition across diameter of flight sample with superimposed

diffusion profile .....................................................................................................

4-113

4-114

4-115

4-116

4-117

4-118

4-119

4-120

4-121

4-122

4-122

4-123

4-123

4-124

4-124

4-125

4-126

4-127

4-128

4-129

4-129

ix
Figure 4-27.

Figure 4-28.

Figure 4-29.

Figure 5-1.

Figure 5-2.

Figure 5-3.

Figure 5-4.

Figure 5-5.

Figure 5-6.

Figure 5-7.

Figure 5-8.

Figure 5-9.

Figure 5-10.

Figure 5-11.

Three-dimensional representation of composition of flight

sample--WDS results ............................................................................................

Composition map for flight sample in +YVV/-ZLV attitude (62 mm) ................

Composition map for flight sample in -XLV/-ZVV attitude (106 ram) ...............

A schematic of the optical layout of the instrument, showing its main

components: Beamsplitter (B 1), mirrors (M1, M2, and M3), apertures

(A1-A4), photodiodes (PD1 and PD2), and photomultiplier tubes (PMT1

and PMT2). The thermostat is the cylindrical object in the center ........................

The timeline of the experiment, given as the temperature of the

sample (measured in distance from Tc) versus MET in days ................................

The stability of the sample temperature at 1 mK, over a four-hour

period while collecting correlograms. The rms of the variations is

1.7/tK. (Time is in hhmm format.) ........................................................................

The temperature of the sample cell during a slow temperature step of -2

mK, shown as distance from To The time constant for thermal relaxation

of the sample cell is about 30 minutes.

(Time is in hhmm format.) .....................................................................................

The temperature of the sample cell during a fast temperature step of -200

BK, shown as distance from Tc. (Time is in hhmm format.) .................................

A correlogram example showing simple exponential decay on a

logarithmic time scale. Taken at T-Tc = 100 mK at a laser power

of 17 _W, this is a forward-scattering correlogram ...............................................

A section of the zig-zag Tc search, showing the pattern of sample

temperature changes: Cooler by 20 _K, then warmer by 10 #K. The

data are smoothed over a 300-second interval. The turbidity during

this same time is shown in figure 8. (Time is in hhmm format.) ...........................

A section of the zig-zag Tc search, showing the response of the sample

turbidity to the zig-zag temperature steps shown in figure 7 for the same

time period. The data are smoothed over a 300-second interval. (Time is

in hhmm format.) ....................................................................................................

The turbidity signal during the entire 64-hour zig-zag Tc search. (Time

is in day: hhmm format.) ........................................................................................

The temperature of the high-precision control shell of the thermostat,

during the first scan for To The temperature of this shell, shown

making -50/_K steps, determines the sample temperature,

shown in figure 11. (Time is in hhmm format.) .....................................................

The sample temperature during the first scan for To in response to the

control-shell steps shown in figure 10. The result is a uniform sample

cooling of 500 _K/hour. (Time is in hhmm format.) .............................................

4-130

4-131

4-131

5-151

5-151

5-152

5-152

5-153

5-153

5-154

5-154

5-155

5-155

5-156

X
Figure 5-12.

Figure 5-13.

Figure 5-14.

Figure 5-15.

Figure 5-16.

Figure 5-17.

Figure 5-18.

Figure 5-19.

Figure 5-20.

Figure 5-21.

Figure 5-22.

The turbidity signal during the first scan for Tc. The small feature just

past 0600 hours is where the sample crosses Tc; the large peak is spinodal

decomposition. (Time is in hhmm format.) ............................................................

Forward-scattering photon count rate during the first scan for Tc, showing

from the break in slope that the sample crosses Tc just past 0600 hours.

(Time is in hhmm format.) .....................................................................................

The turbidity signal during a 500 _K/hour scan for Tc done in 1-g during

preflight testing. Compare this scan with the/_-g scan shown in figure 12 ..........

The turbidity signal, in preflight testing, during a warming sequence

between Tc+l K to Tc+5 K, made in steps of 0.1 K. The sinusoidal

shape is due to cell expansion; the small spikes result from changes

in fluid density .......................................................................................................

Data taken on the engineering model of the instrument show, in the lower

curve, the response of the turbidity signal to a change in temperature from

Tc + 0.6 K to Tc + 0.7 K. The upper curve is the change due solely to cell

expansion. The vertical units are the phase along the interference curve

for the cell ..............................................................................................................

The difference between the two curves in figure 16, showing the response

of the turbidity signal due solely to changes in the fluid density. Compare

with figure 18. (These data are from the engineering model of the

instrument.) ............................................................................................................

The time derivative of the temperature change which produced the

turbidity signal shown in figure 16; this curve has the same shape as

the curve in figure 17. (These data are from the engineering model

of the instrument.) ..................................................................................................

The turbidity response to a temperature change of -70 mK, starting

at Tc+O. 1 K, showing cell interference, adiabatic response, and an

as-yet unidentified feature .....................................................................................

Dual correlogram taken at Tc + 560 mK with 17/zW of laser power; the

integration time was six minutes. Forward scattering (upper curve) had

an average count rate of 710 counts/second; backscattering (lower curve)

had 435 counts/second ...........................................................................................

Dual correlogram taken at Tc+l mK with 17/_W of laser power; the

integration time was ten minutes. Forward scattering (upper curve) had

an average count rate of 446,000 counts/second; backscattering (lower

curve) had 17,800 counts/second ...........................................................................

Dual correlogram taken at Tc÷l mK with 1.7 ,uW of laser power; the

integration time was fifteen minutes. Forward scattering (upper curve)

had an average count rate of 27,100 counts/second; backscattering

(lower curve) had 2,200 counts/second .................................................................

5-156

5-157

5-157

5-158

5-158

5-159

5-159

5-160

5-160

5-161

5-161

xi
Figure5-23.

Figure5-24.

Figure5-25.

Dualcorrelogramtakenat Tc+lO0 _K with 1.7 _W of laser power;

the integration time was fifteen minutes. Forward scattering (upper

curve) had an average count rate of 45,000 counts/second;

backscattering (lower curve) had 3,000 counts/second .........................................

Backscattering decay rates as a function of temperature; the vertical scale

is linear, the horizontal scale logarithmic. The crosses are 1-g data from

preflight testing; the diamonds are the mission data. The curve is our best

theoretical estimate. The error bars show the rms of the data ...............................

Forward-scattering decay rates as a function of temperature; both scales are

logarith-mic. The crosses are 1-g data from preflight testing; the diamonds

are the mission data. The curve is our best theoretical estimate. The error

bars, generally smaller than the symbols, show the rms of the data ......................

5-162

5-162

5-163

xii
LIST OF TABLES

Table Title Page

Table 3-1.

Table 3-2.

Table 4-1.

Table 4-2.

Table 4-3.

Summary of Growth Conditions and Shuttle Activities During

Final Solidification Steps ..................................................................................... 3-60

Summary of Morphological Changes in USMP-2 Samples ................................ 3-61

OARE Data Transformed to AADSF Coordinates .............................................. 4-132

Quality of Surface on Flight Sample .................................................................... 4-132

Orientation of Flight Sample--Transformed Laue Patterns ................................ 4-133

XII"1""
USMP-2 Acronyms

AADSF

BCC

CENG

CNES

DSO

EAC

EDS

EMF

ETR

FES

JSC

ICP

IDGE

MEPHISTO

MET

MPESS

MSFC

NIST

NSLS

NRA

OARE

OAST

OMS

PIMS

POCC

RCS

RPI

SAMS

SCN

SCU

SEM

SIMS

S/L

SSTV

STS

SUNY

USMP

WDS

XLV, YVV, ZLV, ZVV

Zeno

Advanced Automatic Directional Solidification Furnace

Body Centered Cubic

Centre d'Etudes Nuclaires National de Grenoble

Centre National d'Etudes Spatiales

Detailed Supplementary Objective

Experiment Apparatus Container

Energy Dispersion Spectrometry

Electro Motive Force

Experiment Tape Recorders

Flash Evaporator System

Johnson Space Center

Inductively Coupled Plasma

Isothermal Dendritic Growth Experiment

Material por l'Etude des Phenomenes Intrressants la Solidification sur

Terre et en Orbite

Mission Elapsed Time

Mission Peculiar Experiment Support Structure

Marshall Space Flight Center

National Institute of Standards and Technology

National Synchroton Light Source

NASA Research Announcement

Orbital Acceleration Research Experiment

Office of Aeronautics and Space Technology

Orbital Maneuvering System

Principal Investigator Microgravity Sciences

Payload Operations Control Center

Reaction Control System

Rensselaer Polytechnic Institute

Space Acceleration Measurement System

Succinonitrile

System Control Unit

Scanning Electron Microscope

Secondary Ion Mass Spectroscopy

Solid/Liquid

Slow Scan Television

Space Transportation System

State University of New York

United States Microgravity Payload

Wavelength Dispersion Spectrometry

Orbiter Attitudes (plus or minus)

Light-Scattering Spectrometer Experiment

xiv
Orbiter

Mission Number

Dates of Flight

Crew Size

Number of Shifts

Accelerometers

Co-Manifested Payload

USMP-2 Mission Information

Columbia

STS-62

March 4-18, 1994

Five

One

OARE--Measures low-level accelerations in the frequency range below 1

Hertz down to essentially steady state and is mounted near the center of

gravity of the Orbiter.

SAMS--Measures low-level accelerations from about 0.01 Hertz up to

100 Hertz and is mounted in or near the science experiment equipment.

Office of Aeronautics and Space Technology' s experiment OAST-2

XV
xvi
l'Etude des Phenomenes Intrressants la Solidification sur Terre et en Orbite (MEPHISTO), and the

Critical Fluid Light Scattering Experiment (Zeno). The mission was highly successful and resulted in

full operation of all four apparatus.

Experience and Lessons

USMP-2 required the second largest command load (through March 1994) directed through

Marshall Space Flight Center's Payload Operations Control Center (POCC). The system control unit

(SCU) routed approximately 10,000 commands to USMP-2 and relayed some 27 gigabits of experiment

data to the ground for real-time and postflight analyses. All data were also recorded on the flight data

recorders aboard USMP-2 with the experiment tape recorders (ETR) serving as backup. The ETR failed

an hour before the end of USMP-2 microgravity operations, but flight recorders and downlink captured

all the remaining data. Overall, the USMP carrier performed exceptionally well during the mission.

Telescience. IDGE and Zeno provided the most extensive demonstrations of telescience. IDGE

investigators controlled conditions as they watched crystal growth through the two TV cameras, then

commanded the film cameras to take high-resolution pictures at the appropriate times. While

photography was suspended for 3 days to conserve film, the investigators collected valuable data via

slow-scan TV. Zeno demonstrated telescience as the principal investigation team controlled temperature

ranges, manually stepping changes in extremely small increments and observing the xenon film's

responses. This added flexibility was critical since some of the experiment time constants proved to be

longer than expected in space. The AADSF team created a demarcation point on the sample to establish

the position and shape of the solid-liquid growth interface before the orbiter attitude changed 150 hours

into the microgravity portion of the flight. MEPHISTO utilization of telescience was greatly enhanced

by near real-time Seeback and resistivity data which provided information on the samples' responses to

gravity perturbations and on the interfacial morphological stability. This information was utilized to set

the fixed and moving furnace temperature, solidification rates, and conditions for Prltier markings.

USMP-2's success sets a new standard in the U.S. Microgravity Sciences and Applications

program. The anticipated scientific results are expected to expand our understanding of fundamental

aspects of materials behavior with and without gravity as a factor. Already, the extensive operational

experience has expanded our confidence and capabilities in manned and telescience operations in space.

XVIll
INTRODUCTION

Peter A. Curreri

USMP Mission Scientist

George C. Marshall Space Flight Center, Huntsville, Alabama 35812

The second United States Microgravity Payload (USMP-2), flown in March 1994, carried four

major microgravity experiments plus a sophisticated accelerometer system. The USMP program is

designed to accommodate experiments requiring extensive resources short of a full Spacelab mission.

The four USMP-2 experiments dealt with understanding fundamental aspects of materials behavior,

three with the formation of crystals from melts, and one with the critical point of a noble gas. This

successful, scientifically rich mission also demonstrated telescience operations.

USMP Overview

The USMP program grew out of early automated materials payloads carded by the space shuttle

during 1983-1986. Of special value was the Materials Science Laboratory flown on STS-24 (Space

Transportation System) in 1986. USMP supports up to 2,268 kg of experiment apparatus mounted on the

Mission-Peculiar Experiment Support Structure (MPESS), an aluminum truss structure which spans the

shuttle payload bay and occupies less than 1 meter of length. For USMP, two MPESS units are

connected to provide mounting spaces (up to 86x 102 cm) and utilities for up to six experiments. Payload

services include electrical power (450 W @ 28 V), active and passive cooling, and command and data

handling. The latter is routine through the shuttle orbiter and provides the potential for remote operations

or telescience.

Candidate flight experiments are selected by NASA from among the broad range of proposals

submitted in response to NASA Research Announcements (NRAs) issued for various microgravity

disciplines. Experiments are assigned to Spacelab, USMP, or other carriers based on the best match

between investigators' needs and facility services. USMP experiments generally have needs between the

resources of the middeck and Spacelab, and which can also be packaged for exposure to the space

environment. At least two more USMP missions are planned through the late 1990s.

The USMP-2 mission included the Space Acceleration Measurement System (SAMS) which was

comprised of five sets of triaxial sensor heads, each with three orthogonal accelerometers, located at

several positions on the payload. Gravity's effects are not totally eliminated in space; some acceleration

occurs at any position removed from the spacecraft center of mass. Additional micro-accelerations are

caused by "normal" spacecraft activities: Crew movement, antenna pointing, shuttle maneuvers,

atmospheric drag, etc. Some accelerations are intermittent while others are quasi-steady or periodic and

can have subtle, unwanted effects on experiments. SAMS was designed to measure and record the

microgravity environment during USMP-2 operations and provide high-resolution, time-tagged data

which can be correlated with events aboard the shuttle and within experiment events and features on

specimens. Data are stored on optical disk and transmitted to the ground.

USMP-2 Experiments

Four experiments flown on USMP-2 represent a range of investigations from crystal formation to

fundamental physics. Their common need was extended operations in an extremely quiet microgravity

environment. This was provided during the STS-62 mission, March 4-18, 1994, aboard Space Shuttle

Columbia. Of the 14 days in orbit, USMP-2, the prime payload, was allocated 9.25 days of microgravity

starting shortly after orbital insertion. USMP-2 experiments were the Advanced Automated Directional

Solidification Furnace (AADSF), the Isothermal Dendrite Growth Experiment (IDGE), the Material por

xvii
1.3.1. Crew Sleep

The five-member crew of STS-62 worked on a single-shift schedule. The vibration environment

is reduced during crew sleep periods compared to periods of crew activity because some equipment is

turned off and crew push-off forces are minimal or nonexistent. Figure 1 shows tARE data for the

extent of the STS-62 mission. Crew sleep times appear as periodic, relatively quiet regions of the plot,

especially evident in the first half of the mission. Other characteristics of this plot will be described later.

The spectrogram in figure 2 shows the SAMS data from a 2-hour period (mission elapsed time

MET 012/06:00 to 08:00) in one of the crew sleep periods. It can be seen in this data that the

environment is very quiet across the frequency range of interest (25 Hz).

1.3.2. Crew Exercise_

The spectrogram in figure 3 shows the SAMS data from a 2-hour period (MET 012/00:00 to

02:00) during a typical crew active time. This data includes nominal operations (from MET 012/00:50 to

02:00) and the beginning of a sequence of crew exercise activities (from 00:10 to 00:50).

Figure 4 shows an example of the vibration environment as recorded by SAMS while a crew

member was exercising on the bicycle ergometer. In this figure note the increased acceleration level at

MET 012/00:46:00 indicating an increase in crew exertion level. In figure 5 the exercise activity is

identified by the excitation of 1.2 and 2.4 Hz frequencies. These frequencies correspond to the pedaling

and body motion frequencies of the crew member.

1.3.3. Flash Evaporator System_

Some heat rejection is provided on-orbit by the flash evaporator system (FES) of the space

shuttle. FES activity was noted during STS-62 and the effect on the microgravity environment was

similar to previous missions.

1.3.4. Circulation Pumps

The electric motor-driven pumps of the three independent space shuttle hydraulic subsystems

have an impact on the low-gravity environment. The pumps must be activated periodically on-orbit.

Figure 6 shows SAMS data collected when circulation pump 2 was activated during STS-62. Note the

relatively large magnitude, short duration transient at MET 008/14:19:55, and the two lower magnitude

transients that occur about 75 and 85 seconds later. This data signature appears to be characteristic of the

initiation of circulation pump activity.

1.3.5. Thrusters

The space shuttle orbital maneuvering system (OMS) and reaction control system are used to

provide thrust on-orbit. OMS thrust is used to modify an orbit for payload rendezvous or deploy, or to

change orbit. During Office of Aeronautics and Space Technology-2 (OAST-2) operations on STS-62,

three OMS burns (designated OMS-3, OMS-4, and OMS-5) occurred. Figure 7 shows SAMS data

collected during the OMS-4 burn. Note the 3x10 -2 g shift of the acceleration mean level during the

burn, followed by a return to the original mean upon burn completion. This signature is typical of OMS

engine bums.

1-4
SECTION I. ONE-YEAR REPORT FOR SAMS AND OARE ON STS-62/USMP-2

Richard DeLombard

PIMS Project Manager

NASA Lewis Research Center, Cleveland, Ohio 44135

1.1. INTRODUCTION

The second United States Microgravity Payload (USMP-2) flew on the Space Shuttle Columbia

on mission STS-62 (Space Transportation System) in March 199& The USMP-2 part of STS-62 was

dedicated to low-gravity experiments. To support these experiments, two accelerometer systems were

flown in the Orbital Acceleration Research Experiment (OARE) and the Space Acceleration

Measurement System (SAMS).

The Principal Investigator Microgravity Services (PIMS) project supports principal investigators

of microgravity experiments in the evaluation of the effects of varying acceleration levels on their

experiments. A summary report was prepared by PIMS to furnish interested experiment investigators

with a guide to evaluate the acceleration environment during STS-62 and as a means of identifying areas

which require further study. The summary report provides an overview of the STS-62 mission, describes

the accelerometer systems flown on STS-62, discusses some specific analysis of the accelerometer data

in relation to the various activities which occurred during the mission, and presents plots resulting from

this analysis as a snapshot of the environment during the mission.

Numerous activities occurred during the STS-62 mission that are of interest to the low-gravity

community. Specific activities of interest during STS-62 were crew exercise, experiment latch

operations, closed circuit camera motion, radiator latch operations, flash evaporator system and water

dump operations, circulation pump activity, Ku band antenna activity, orbital maneuvering system and

primary reaction control system firings, and attitude changes. The low-gravity environment related to

these activities is discussed in the summary report.

1.2. ACCELEROMETER SYSTEMS

Two accelerometer systems, OARE and SAMS, measured the low-gravity environment of the

Space Shuttle Columbia during the STS-62 mission. The OARE instrument measures low-level

accelerations in the frequency range below 1 Hz down to essentially steady-state. It is mounted near the

center of gravity of the space shuttle vehicle. The SAMS units have the capability of measuring lowlevel accelerations from about 0.01 Hz up to 100 Hz. The SAMS sensors are mounted in or near the

science experiment equipment.

1.3. COLUMBIA LOW-GRAVITY ENVIRONMENT - STS-62

The acceleration environment measured by an accelerometer system on the space shuttle is

contributed to by numerous sources. All ongoing operations of crew life support systems and activities

and operations of the space shuttle, crew, carrier, and experiments tend to have vibratory and/or

oscillatory components that contribute to the "background" acceleration environment. The following

sections describe several significant areas of the microgravity environment.

1-3
1-2
SECTION I.

ONE-YEAR REPORT FOR SAMS AND OARE

ON STS-62/USMP-2

i!1m

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Figure 8 shows SAMS data collected during Detailed Supplementary Objective-324 (DSO-324),

a NASA Johnson Space Center (JSC) DSO aimed at evaluating the payload on-orbit, low-frequency

environment. For this DSO, the primary reaction control system engines were fired numerous times over

a period of 6 minutes. Note that the time domain magnitudes are on the order of 10-2 g and that the

power spectral density representation in figure 9 shows an increase in energy level across the entire

frequency spectrum.

1=3.6. Space Shuttle Attitude

For a typical microgravity science mission, the attitude is maintained to optimize certain

secondary parameters, such as the net quasi-steady acceleration, duration and frequency of thruster

firings, and attitude changes during the course of the mission.

The STS-62 mission had several primary attitudes defined for the two primary payloads, USMP2 and OAST-2. The attitude flown for the USMP-2 payload during the majority of the first 7 days of the

mission was the -ZLV/+YVV, figure 10. This attitude was chosen to minimize attitude changes and the

amount of thruster firings. Two additional attitudes,-XLV/-ZVV and-XLV/+ZVV (figure 10), were

flown at the end of the USMP-2 timeline before the OAST-2 attitudes. During the OAST-2 operations

from MET 009/16:45 through MET Day 012, the attitude of the space shuttle was changed frequently in

support of various experiments.

The transitions from the -ZLV/+YVV to the -XLV/-ZVV and then to the -XLV/+ZVV attitude

can be seen at MET hours 163 and 192 in the tARE data shown in figure 1. Two 20-minute windows

which encompass these attitude changes are shown in figures 11 and 12. The large excursions in the

acceleration levels at this time are due to the space shuttle thrusters used to change the attitude.

The quasi-steady acceleration vector changes direction as a result of these transitions as can be

seen by the x, y, and z levels before and after the attitude transition. This illustrates why some

experiments require a certain attitude during their critical operations so that a particular quasi-steady

acceleration environment is maintained.

1A. SUMMARY

There were two primary payloads for the STS-62 mission: USMP-2 and OAST-2. Two SAMS

units were on board STS-62 to support the USMP-2 experiments. Five SAMS triaxial sensor heads were

mounted at various locations on the carrier among the four science experiments. The tARE instrument

was mounted in the space shuttle cargo bay to support the USMP-2 experiments with low-frequency

acceleration measurements.

A mission summary report was prepared to describe the microgravity environment of the

mission.

1-5
,

.

°

1.5. PUBLICATIONS

DeLombard, R., M.J.B. Rogers, K. Destro-Sidik, and W.O. Wagar, "Real-Time Acceleration Data

Analysis on STS-62 (USMP-2)," AIAA95-0612, January 1995.

Matisak, B., L. French, M.J.B. Rogers, K. Destro-Sidik, R. DeLombard, and W.O. Wagar, "Results

of the Quasi-Steady Acceleration Environment From the STS-62 and STS-65 Missions," AIAA95-

0691, January 1995.

Rogers, M.J.B., and R. DeLombard, "Summary Report of Mission Acceleration Measurements for

STS-62," NASA TM 106773.

1-6
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012/06:00 012/07:00

Mission Elapsed Time (Day/HH:MM)

Figure 1-2. Crew sleep period.

012/08:00

10-3

10-4

10-5

10-6

10-7

13b

v

tO

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<

25.0

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Figure 1-3. Crew active time with exercise period

10-3

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Figure 1-4.

12.0

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Time domain vector magnitude of SAMS data for Unit F, Head B, MET 012/00:45:00-

012/00:47:10.

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Figure 1-5. Power spectral density vector magnitude of SAMS data for Unit F, Head B, MET

012/00:45:00-012/00:47:10.

1-9
70.0

I

0')

,r

z

0

m

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UNIT F, HEAD B

Figure 1-6.

0.0

0 20 40 60 80 100 120

TIME [sec]

Time domain vector magnitude of SAMS data for Unit F, Head B, MET 008/14:19:30-

008/14:21:40. Circulation pump activation.

1000.0 T T T T T 1"'----

MET 009/17:09:00

UNIT F, HEAD B

i

'_ 00.0

600.0

400.0

¢_ 200.0

,<

0.0

0 20 40 60 80

TIME [sec]

100 120

Figure 1-7. Time domain vector magnitude of SAMS data for Unit F, Head B, MET 009/17:09:00-

009/17:11:10. OMS.

1-10
I I I I I I

_,_ 500.0

400.0

300.0

,,=,

200.0

,,_ 100.0

0.0

0 20 40 60 80 100 120

TiME [sec]

Figure 1-8. Time domain vector magnitude of SAMS data for Unit F, Head B, MET 010/17:12:00-

010/17:14:10. Primary reaction control system activity.

1000.0

100.0

0.01

Figure 1-9.

I I I

!

MET 010/17:12:00

UNIT F, HEAD B

I I I I

0 5 10 15 20 25

FREQUENCY [Hz]

Power spectral density vector magnitude of SAMS data for Unit F, Head B, MET

010/17:12:00-010/17:14:10. Primary reaction control system activity.

1-11
IJ-t

W

LH

W

LV

Orbiter-ZLV / +YVV Attitude Orbiter -XLV / +ZVV Attitude Orbiter -XLV / -ZVV Attitude

Figure 1-10. Space shuttle attitudes for USMP-2 on STS-62.

1-12
_o

I-!1'

,ll -1

-2

-ZLV / +YVV -XLV ! -ZVV

Mission Elapsed TFme (Imurl)

Figure 1-11. Attitude transition during USMP-2 operations (-ZLV/+YVV to -XLV/-ZVV).

orbiter IM<hr Axes Attitude transition

-, at. d l! , . !

. .r, A!v I . J_

i i

i

..zvv i;_'_..¢ ' ' -XLV / +ZVV

-2 i i i | i .... | | | I • I • • • • = • • • = •

Mleelon EIIpeed Time (houee)

Figure 1-12. Attitude transition during USMP-2 operations (-XLV/-ZVV to -XLV/+ZVV).

1-13
1-14
SECTION II.

ISOTHERMAL DENDRITIC GROWTH EXPERIMENT
Acknowledgments

The requirements for designing, building, testing, and operating a complex, highly interactive

scientific experiment aboard the space shuttle are beyond the skills and resources of any one individual or

organization. Diverse teams of dedicated professionals are required. Over the last 10 years the following

individuals contributed significantly to the design, construction and operation of this experiment: L.T.

Bushnell, R.C. Hahn, B.A. Herbach, P.A. Kobryn, J.C. LaCombe, T.A. Lagrasso, E.R. Rubinstein,

N.B. Singh, M.E. Selleck, S.H. Tirmizi, and A. Velosa, at RPI, Troy, NY; and E. Winsa's team: R.

Abramczyk, C. Blachly-Olah, D. Bums, A. Fedak, R. Grant, A. Kawecki, F. Kohl, G. Kraft, M. Lehto,

L. Levinson, D. Malarik, J. McDade, D. Miller, A. Patel, D. Priebe, J. Robertson, A. Sobota, D.

Vachon, G. Weeks, and R. Whittlesey, at or associated with NASA's Lewis Research Center, Cleveland,

OH.

Special thanks are due for the continued understanding, interest, and financial support provided by

the NASA Life and Microgravity Sciences and Application Division, (Code U), Washington, DC, under

Contract NAS3-25368, with liaison provided by the Microgravity Experiments Branch, Space

Experiments Division, Lewis Research Center, Cleveland, OH.

The sucessful operation of the experiment would not have been possible without the efforts of the

mission scientist and the mission management team. Thanks also to those who oversaw POCC training

and simulations, and the POCC cadre who coordinated and implemented the project's operations. Finally,

we also extend our thanks to the crew of STS-62 and the support crews at the NASA research centers

involved with the space shuttle's activities.

2-16
SECTION II. ISOTHERMAL DENDRITIC GROWTH EXPERIMENT

M.E. Glicksman and M.B. Koss

Materials Science and Engineering Department

Rensselaer Polytechnic Institute, Troy, New York 12180-3590

E. A. Winsa

Space Experiments Division, NASA Lewis Research Center

Cleveland, Ohio 44135

2.1. INTRODUCTION

A vast amount of the products and devices that we use every day---everything from aluminum foil

and soda cans, to cars and jet engines--are made from metals and alloys. Early in the creation of these

products, the metals are in a liquid, or molten state, that freezes to form a solid, similar to the way water

freezes to form ice. If you were to look at some just frozen, or freshly solidified metalfic alloy with a

strong magnifying glass you would see that its surface is not uniform, but is made up of tiny individual

crystalline grains. Moreover, if you were able to look even more carefully at the individual grains through

a powerful microscope, you would see that each grain is made up from what looks like tiny metallic pine

trees crowding and growing into each other. These metallic tree-like crystals are called dendrites. This

description is appropriate because metallic dendrites actually grow analogously in the manner trees grow,

with a main branch or trunk, from which grow side branches, from which grow smal/er side branches,

eventually filling all space.

Dendritic growth is a common mode of crystal growth encountered when metals and alloys solidify

under low thermal gradients, as occurs in most casting and welding processes. Furthermore, in

engineering materials, the details of the dendritic morphology is directly related to various material

responses and properties, such as hot cracking, corrosion resistance, toughness, and yield strength.

Although, the effects of the initial dendritic microstructure can be modified by subsequent heat treatments,

the final material properties are generally dependent on the details of the original dendritic microstructure.

Thus, the understanding and control of dendritic growth in solidification processing is crucial in order to

achieve desired physical properties in products created by casting. According to J. S. Langer, writing on

dendritic growth in Physics Today:

"Metallurgists have long sought to predict and control alloy microstructures. The

development of automated, cost effective manufacturing techniques ultimately depends on

the precision with which we can solve this problem in non-equilibrium pattern formation.

In principle, we would like to incorportate fundamental understanding of microstructures

into computer codes that simultaneously help us design materials with made to order

properties and optimise their manufacturability and performance. ''1

Of more generic interest, perhaps, dendritic growth is also an archetypical problem in morphogenesis, where a complex pattern evolves from simple starting conditions. Thus, the physical

understanding and mathematical description of how dendritic patterns evolve during the solidification

process are of interest to scientists and engineers.

2-17
In the caseof cast alloys that solidify dendritically, the long-term goal is the development of

computational methods to elucidate both the microstructure and chemical microsegragafion, and the

macroscopic end product made up by the agglomeration of millions of dendritic grains. Then, metallurgical

engineers can understand and control the metal formation process, and thus be able to make better, less

expensive, and more reliable metal products. This requires detailing physical processes over a scale change

of at least two orders in magnitude. The mesoscopic dendritic grains, which are strongly influenced by the

growth pattern of individual dendrites, are considered the starting point to the problem of micro to macro

scale modeling. A full understanding of the behavior of a single, isolated dendrite represents an important

step in achieving better understanding and control of the final properties of dendritically solidified

materials.

To be sure, remarkable progress has been made recently by numerical simulations of dendritic

patterns employing phase field models, and in simulating the grain structure of castings. These types of

numerical studies contribute greatly to our understanding of dendritic growth processes and their effect on

cast properties. The immediate goal of IDGE's research in dendritic growth is to provide a complete

physical and mathematical description of how a single, isolated, dendrite grows.

Over the years, scientists and engineers have learned a great deal about how dendritic crystals

grow. Now, the growth of dendrites is known to be controlled by the transport of heat, and/or solute,

from the moving solid-liquid interface into the supercooled melt. Under terrestrial conditions, the gradient

of the melt density near the solid-liquid interface, caused by the rejection of solutes or latent heat, is acted

upon by gravity, resulting in a buoyancy-induced convective flow affecting all the other energy and

species transport processes occuring in the liquid phase. The gravity-induced convection severely

complicates how dendrites grow, making the study of certain aspects of dendritic growth intractable.

The Isothermal Dendritic Growth Experiment (IDGE), which flew aboard the Space Shuttle

Columbia (Space Transportation System-62 (STS-62)) in March 1994, was designed, built, and operated

to grow dendrites, and photograph them as they grow from the molten state over a range of supercooling,

while orbiting the Earth. On orbit, the acceleration field that promotes convection is reduced about one

million times from that on Earth. The main scientific objective was to produce data sets of dendritic tip

velocities and radii, on orbit, that will become the scientific benchmark for critically testing

phenomenological and theoretical models.

2.2. BACKGROUND ON DENDRITIC GROWTH

A number of theories of dendritic crystal growth to predict the dendrite' s tip velocity, V, and radius

of curvature, R, as a function of the supercooling, AT, have been developed over the last 40 years (see the

recent review paper). 2 In 1946, a mathematical solution to the dendritic heat diffusion problem was first

analyzed by Ivantsov, 3 who, based on thermal transport mechanisms, physical assumptions, and

mathematical approximations, modeled the dendrite as a paraboloidal body of revolution, growing at a

constant velocity, V. The resultant solution of the partial differential heat diffusion equation can be

expressed exactly in paraboloidal coordinates moving at a constant velocity, V, with the dendrite tip as the

representative equation,

AO = Pe(eP_)EI(Pe) ---Iv(Pe).

In this formulation, Iv(Pe) is identified as the Ivantsov function, where

(1)

2-18
AT

(2)

is the nondimensional scaled supercooling, Pe is the growth Ptclet number,

VR

Pe = _ _ Iv-l(AO), (3)

and EI(Pe) is the first exponential integral function. In these equations, AT is the applied supercooling,

AHf is the molar latent heat, Cp is the constant pressure molar specific heat, V is the dendrite tip velocity,

R isthe dendrite tip paraboloidal radius of curvature, a is the thermal diffusivity of the melt phase, and

Iv-l(AO) is a formal representation of the inverse of the Ivantsov function (which cannot be expressed in

terms of known functions).

This diffusion solution is, however, incomplete, insofar as it specifies only the tip growth Ptclet

number, VR/2et, as a function of the supercooling, and not the explicit dendritic dynamic operating state,

V, and R. The Ptclet number obtained from the Ivantsov solution for each specified supercooling yields,

instead, an infinite manifold of V, and R, ordered pairs (V, R), that satisfy the diffusion solution at that

particular value of AT. Specifying the driving force alone through Ivantsov's solution does not uniquely

describe the operating state.

The early experimental data on the interface velocity of pure transition metals such as Ni and Co

provided more of a qualitative test of theory, inasmuch as there were no corresponding morphology

measurements, the velocity measurements themselves had a large experimental scatter (20-50 percent),

and the thermophysical properties needed for checking theory were either unavailable or inaccurate.

In the early 1970's, Glicksman and co-workers showed that succinonitrile (SCN), a body centered

cubic (BCC) organic plastic crystal, could be used as a model metal analog system for studying dendritic

growth. 4 This was possible because SCN solidifies like the cubic metals, i.e., with an atomiciaUy "rough"

solid-liquid interface, yet retains advantages because SCN displays convenient properties for solidification

experiments, such as a low melting temperature (58.08 °C), optical transparency, and the potential for

accurate characterization of its thermophysical properties. The use of SCN greatly facilitated dendritic

growth studies over the past 20 years, where now dendritic tip velocities and radii could both be accurately

measured and used as a critical test of theory.

The earlier studies convincingly showed that the then current theories of dendritic growth, i.e.,

combining the Ivantsov solution with the "maximum velocity" hypothesis, was incorrect. Furthermore,

one found that unique, steady, operating states (Vex_, Rexp) are observed at any specified supercooling,

AT. Considerable theoretical efforts within the physics commumty have been directed to answering the

question as to whether, and under what conditions, a second independent equation or length scale exists,

which, when combined with the Ivantsov diffusion solution, mathematically selects the unique (i.e.,

observed) dynamic operating state 2 5 which nature seems to have no trouble selecting.

This effort to find a second lengthscale has in large part been shaped by the observations that for

several materials undergoing unconstrained dendritic growth, VR 2, is constant, or very weakly varying, as

a function of the supercooling. Two of the candidates for the additional crystal growth physics that provide

the second crucial equation are the effects from the solid-melt interfacial energy, the influence of

temperature fluctuations in the melt, or both. Furthermore, the scaling equation derived from these

additional physical considerations may be based on several different dynamical considerations, such as

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morphologicalstability,microscopicsolvability, shapeanisotropy,noiseamplification,trappedwaves,

etc.Althoughtheunderlyingmechanisms,or eventhephysics,for these"theoriesof the secondlength

scale"mightin factbequitedifferent,theirresultsareoftenencapsulatedwith ascalingconstant,

o* = 2ad°

VR 2 . (4)

Here, o* is usually referred to as the stability, selection, or scaling constant, and d o is the capillary

length scale, a material parameter defined as,

(5)

where _s is the molar volume, ASf is the molar entropy of fusion, and _, is the solid-liquid interracial

energy. Solving equations (3) and (4) for V and R yields,

V = 2a°*Iv-2(AT)

d ' (6)

0

and

R-- do

o, Iv_I(AT), (7)

where now, V and R are unique functions of the supercooling.

An experimental requirement to test dendritic growth theories fully is then to measure

simultaneously the dendrite tip velocity and radius of curvature, and then test the validity of equations (6),

(7), and (1). Early SCN dendritic growth data showed that heat transfer due to natural convection might be

modifying the diffusional heat transfer, i.e., the shape of the dendritic growth chamber, and the direction

of [100] growth of the primary dendrite with respect to gravity, affected the measurements. It became

necessary to characterize dendritic velocity and radius measurements not only as a function of the applied

supercooling, but also parameterized by the growth orientation with respect to gravity.

2.3. THE ISOTHERMAL DENDRITIC GROWTH EXPERIMENT (IDGE)

Unfortunately, more detailed subsequent work showed that gravity-induced convection actually

dominates dendritic growth, particulary in the lower supercooling range typical of industrial castings. 6 7

Convection confounds any straightforward analysis based on diffusive heat transfer. There have been

some attempts to estimate the effects of natural or forced convection on dendritic growth,8 but these

calculations are themselves coupled to yet unproven elements of basic dendritic growth theory, and,

consequently, cannot provide an independent test of the theory. In the higher supercooling range, where

convective influences diminish in comparision to thermal conduction, the morphological scale of dendrites

becomes too small to be resolved optically at the high growth speeds encountered.

The experimental situation prior to the microgravity experiment reported here, was that there

appeared to be too narrow a range of supercoolings in any crystal-melt system studied terrestrially that

remains both free of convection effects, and also permits an accurate determination of the dendrite tip

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radius of curvature.Thus, a low-gravity dendritic growth experimentwas proposed to measure

definitively the kinetics and morphologyof dendriticgrowth, and to measurethe effectsof natural

convectionondendriticgrowth.Suchatestrequiresthata single,clearlydefinedexperimentalparameter,

namely,thesupercooling,fully determinetheresultingsteady-statedendriticgrowth velocity, V, and tip

size, R. NASA's Microgravity Science and Applications Division supported the development of a critical

microgravity space flight experiment to evaluate dendritic growth theories. 8-10 Designated the IDGE, the

first flight, manifested on the second United States Microgravity Payload (USMP-2) mission, was

launched and operated on the STS-62 on March 4-18, 1994. The IDGE enabled the acquisition of accurate

dendritic growth speed and morphology data under diffusion-controlled conditions, while greatly reducing

buoyancy-induced convective heat transport through the reduction of gravity. 11 As shown, the

microgravity environment broadens this range of supercoolings and permits for the first time a critical

assessment of both the heat transport theory and the interfacial kinetics behavior.

2.3.1. IDGE Growth Chamber

The techniques for accomplishing simultaneous kinetic and morphological measurements for SCN

dendrites are well established. 4 6 However, the constraints of an autonomous, or tele-operational,

microgravity experiment require extensive modifications of established ground-based laboratory

techniques.9 10 This is most evident in the design and construction of the flight growth chamber (figure

1), which is physically and metaphorically central to the operation of the IDGE apparatus.

In order to withstand the 5 go launch acceleration, and the other rigors of space flight, the usual,

delicate, all glass laboratory growth chamber was replaced with a more rugged combination of glass and

metal. The flight chamber provides an unobstructed view of the growing dendrites through four windows,

set perpendicularly, which allow for stereographic correction of the measurements. This type of chamber

requires that the dendrites grow within the field of view of the windows. This requirement is accomplished

by providing a thin capillary injector tube, called the stinger. Dendritic growth is initiated by energizing

thermoelectric coolers attached to one end of the stinger. The other end of the stinger, where the dendrite

emerges in the center of the chamber, is notched, much like the tip of a fountain pen. This helps isolate the

leading dendrite and reduces the time needed until the leading dendrite is isothermal and has achieved

steady state.

Two stable thermistors (the second thermistor is redundant) monitor the temperature inside the

chamber. The phase change volume expansion and the thermal expansion of the melt are compensated by

stainless-steel bellows which prevents either stray vapor cavities from forming or overpressuring the

growth chamber. The growth chamber is sealed under vacuum, and the external pressure which is

transmitted through the bellows from the thermostatic bath is sufficient to prevent any free surface

formation due to cavitation. The attachment of all components, joints, and seals used in the growth

chamber that come in contact with SCN employs autogenous electron beam welds. This joining method

preserves the purity of the SCN from contamination by avoiding the introduction of any solder, fluxes,

elastomers, or other alloying components that could contaminate the SCN.12

The growth chamber is mounted inside a temperature-controlled tank (figure 2) or thermostat inside

an experiment apparatus container (EAC). Also inside the EAC is the modified shadowgraphic optic and

photographic system, the dendrite growth detection system utilizing optic ram chips, and the control

electronics. The inner pressurized vessel of the EAC is controlled in an atmosphere of moist nitrogen at 15

lb/in 2 and 25 °C. The temperature control tank is filled with a mixture of ethylene glycol and water in a

proportion such that its index of refraction matches that of molten SCN. Index matching enhances the

capability of the optic system. Two heaters (one redundant) and a rotating propeller permit temperature

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control to the desiredlevel (within 2 mK of the set point) both temporally and spatially. All control,

including data acquisition is performed by on-board computers.

2.3.2. Flight Experiment Procedure

The IDGE flight experiment was divided into 60 operational cycles. The first cycle, the warm-up

cycle, consists of bringing the system up to the operational temperature where all system components am

checked and initialized. Also, the purity of the SCN test specimen was confm'ned by examining the

flatness of the melting plateau. The second cycle measured the liquidus temperature of the SCN as a

calibration point from which all subsequent supercoolings are measured. The remaining cycles were

dendritic growth cycles where dendrites are grown and photographed at varying supercoolings.

For each dendritic growth cycle, the thermostatic bath is heated to over 3 K above the SCN melting

temperature. The bath is maintained at this elevated temperature until the SCN inside the chamber reaches,

and becomes stable, at this temperature. This insures that all the SCN in the chamber is melted and that no

stray crystals are present. The bath temperature is then reduced to the temperature that will produce the

desired supercooling in the SCN. After the molten SCN has cooled and stabilized at the prescribed

supercooling, thermoelectric coolers initiate solidification at the capped end of the stinger. The solidifying

SCN grows down the inside wall of the stinger until it emerges as a dendritic crystal from the stinger in the

center of the growth chamber, which is still molten and spatially uniform in temperature at the prescribed

supercooling.

For each dendritic growth cycle, the experiment obtained a time series of stereo-pairs of

35-mm photographic images, low-resolution, binary, digitized electronic images (figure 3), and various

data streams characterizing the dendrite's growth conditions during the cycle. For each image pair, either

35-mm film negative or digitized image, we measured the position of the dendrite tip, and the associated

time that the image was recorded. We determined the dendritic velocity by calculating the slope of the

displacement-time curve from the steady-state regime of the growth cycle. The velocity is then adjusted by

appropriate stereographic and magnification factors. This procedure yields a set of dendrite velocity

measurements accurate to about - 2 percent.

The overall experimental task of measuring the size and shape of steady-state dendritic crystal's

tips is more complex. The complexity is evidenced by noting that the typical experimental uncertainty

encountered in the best terrestrial laboratory-based tip radii measurements is typically _10 percent. Some

of the details of our tip shape analysis methods are described elsewhere, 13 although additional important

changes for improving radii measurements are currently under development in our laboratory. In

summary, the grey-scale film image is digitized to locate the solid-liquid interface, which is reduced to a

list of ordered coordinate pairs that define the dendrite profile or edge. The data describing the dendrite

profile is regressed to an equation for the model of a dendrite, from which it is straightforward to calculate

the radius of curvature at the tip. The tip radii data from the IDGE are usually accurate to better than __.5

percent for supercoolings less then 1.0 K.

2.4. RESULTS

The IDGE apparatus, from the multilayer thermal insulation blanket to the succinonitrile dendritic

growth chamber, worked flawlessly during the USMP-2 mission, and the 9 days of dedicated microgravity time was employed to the fullest. At mission completion, the IDGE had obtained scientific data

comprising approximately 450 photo negatives, over 1000 slow scan TV (SSTV) images, and numerical

data characterizing the temperature environment of the dendritic growth cycles, spanning the supercooling

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rangefrom circa 0.05 to 2.0 K--a dynamic data range of almost40, greatly exceedingpreflight

expectations.

To date,we havecompletedthe reconstructionof theexperimentmonitoringprintouts for each

dendriticgrowthcycle,andcalculatedthesteady-statetemperatures,pressures,andcorrectedsupercooling

for eachof 59 cycles.We havecompleteddetailedcalculationsof dendriticvelocitiesfrom the SSTV

imagesand from the 35-mm photo negatives.We have completedcalculationsof the stereographic

corrections,dendriteprojectionview orientation,dendriteunit vectors,andthestarttime, endtime, and

durationof steady-stategrowth time, for all applicablegrowth cycles.Finally, we havecompletedthe

selection,imagecapture,imageprocessing,andradiuscalculationof thebestfocused,steady-statephoto

negatives,for eachgrowth cycle.A complete"canonical"datasetis beingpreparedat this time (March

1995).

2.4.1. Dendritic Growth Velocities

The two ground-based dendritic growth velocity data sets used for comparision with the

microgravity data (Glicksman/Huang and IDGE terrestrial) are in close agreement over the entire

supercooling range (figure 4). Thus, negligible differences exist between these two data sets arising from

the different chamber size and shape, the different SCN specimens employed, and the different eulerian

angles (between 0 ° and 45 °) of the [100] growth direction with respect to gravity. However, there are

dramatic differences in the observed growth rates, particularly at the lower supercoolings, between

terrestrial and microgravity conditions. At about 1.7 K supercooling there still appears to be a slight

reduction in the growth velocity under microgravity conditions as compared to terrestrial conditions. This

reduction in velocity becomes more evident at 1.3 K supercooling, where the velocity measured in

microgravity is approximately 15 percent lower than the terrestrial data, increasing to almost 200 percent

reduction at small supercoolings. These particular results show that there are substantial convective effects

at 1 go, even at the highest supercoolings for which the solidification microstructures can be resolved

during in situ observations.

The slope of the microgravity data changes from being steeper than the terrestrial data at the higher

supercoolings, to being almost parallel to it at the lower supercoolings (less then ca. 0.4 K (figure 5)).

This reveals that even in microgravity, substantial convective effects reappear at the lower supercoolings.

The scatter in the microgravity data at the lower supercoolings may be ascribed (although not yet proven)

to variations in the magnitude of the acceleration environment, or to variations of the direction of growth of

the measured dendrite with respect to the direction of the local net microgravity acceleration, or perhaps to

some combination of the two effects.

The discussion of the radii results (figure 6) parallels that already given on the velocity data, except

that here the radius is not as strong a function of supercooling as the velocity, so that the ratios between the

terrestrial and microgravity results are smaller, although still evident. An experimentally determined

microgravity scaling constant, o* = 2ado/(VR2), calculated from the microgravity measurements of V and

R, at high supercoolings where convection affects are assumed minimal, yields o* = 0.0194. Curiously,

this value of or* is almost exactly the same as the terrestrially measured value of o*, and this surprising

agreement will be discussed in more detail later.

Theoretical velocity calculations using the microgravity measured o* and Ivantsov's transport

theory do not describe the microgravity velocity data over any supercooling range (figure 4). The

theoretical predictions are, however, parallel to and above the microgravity data at the higher

supercoolings. If instead of directly calculating o'*, we treat o* as an adjustable scaling parameter, which

2-23
wedesignate,av, we are able to fit the observed microgravity velocity data at supercoolings of 0.47 K and

above with crv = 0.0164. The deviations between the microgravity data at the lower supercoolings and the

fit at the larger supercoolings using crv provide quantitative evidence that the quasi-static residual

microgravity level of approximately 0.7 l_go has a significant, and eventually dominant, effect on dendritic

growth. Thus, at sufficiently low supercoolings, even in microgravity, convection effects substantially

alter the growth velocity from what would be expected by heat diffusion alone (figure 5). The excellent

agreement of velocity calculations with a v with the microgravity data over the range of supercoolings from

0.47 to 1.7 K show that this is the supercooling range over which one must test dendritic growth theories

which employ heat conduction as the rate limiting transport process.

2A.2. Dendritic Tip Radii

Theoretical radii calculations also using the microgravity measured value of o* and Ivantsov's

transport theory do not describe the microgravity radius data over any supercooling range either (figure 6).

Again, however, a one-parameter fit to the radii data over the supercooling range from 0.47 to 1.57 K

yields the adjustable scaling parameter crn = 0.0179, which describes the microgravity radii data at higher

supercooling. Thus there appears to be no single scaling parameter, whether calculated directly from the

velocity and radius data combined or inferred as an adjustable parameter from the velocity or radius data

separately that when coupled to the Ivantsov heat transfer results, properly describes both the velocity and

radius data. This suggests strongly that the Ivantsov function combined with a unique scaling constant is

not an accurate description of the dendritic growth process.

2.4.3. Growth P_clet Number

The failure of Ivantsov' s diffusion solution, when combined with a unique scaling rule to describe

the diffusion-limited growth regime, is apparent from the differences found between the experimentally

determined growth P6clet number data calculated from the velocity and tip radius data sets from both the

microgravity and terrestrial experiments, and the theoretical Prclet numbers (figures 7 and 8). This theory

has no adjustable parameters. The microgravity measured Prclet number data set is clearly separated from

the terrestrial data. Most importantly, even in the diffusion-limited regime, the microgravity data are

systematically lower than that predicted for paraboloidal dendrites as the Ivantsov theory curve.

Before the IDGE, it was not possible to separately test the Ivantsov solution and the scaling rule

hypothesis. To our knowledge, this is the first clear evidence gathered that shows that Ivantsov's

formulation for paraboloidal dendritic growth does not accurately describe dendritic growth in SCN. The

approximate agreement achieved between the transport theory and the microgravity data indicates that

dendritic growth is indeed most likely governed by the diffusion of latent heat from the crystal-melt

interface, but the detailed Ivantsov formulation to describe that diffusion process is in need of some

modification.

2.4.4. Kinetic Scaling Parameter

Nonetheless, it remains instructive to examine the scaling law data (figure 9). The scatter, both

within and among the three data sets, is significant. There appears to be a trend where o* increases

inversely with supercooling. If o* has a dependence on supercooling, that dependence is rather weak

compared to the scatter observed in the new dendritic growth data, especially in the diffusion-limited

regime established by the microgravity velocity data. Thus, assuming o* remains constant over this limited

supercooling range would only introduce a slight error in any quantity that depends on o*. Most striking is

the observation that although the terrestrial and microgravity velocity and radii data are markedly different,

2-24
the terrestrialand microgravityo* values are virtually indistinguishable. This surprising finding is in

agreement with our earlier observation that the average of o* from the diffusion-limited microgravity data

is almost exactly the same as the average value of o* derived from terrestrial measurements.

2.5. TELEOPERATIONS

2.5.1. IDGE Operating Plans

As a default, the IDGE had an autonomous preprogrammed experimental protocol. The IDGE

autonomous flight protocol is divided into 38 cycles. The initial warm-up cycle consists of heating the

system up to the operational temperature, where all system components are checked and initialized. A slow

melting plateau is performed as a check of specimen purity. Cycle 01 provides a freezing plateau, which

yields the precise liquidus temperature of the SCN (_0.001 K) in the chamber, and serves as the

calibration point from which all subsequent supercoolings are measured. Cycles 02-11 provides dendritic

growth data acquisition, where dendrites were grown and photographed at each of 10 programmed

supercoolings taken in logarithmic steps between 0.1 and 1.0 K. Cycles 12-31 repeat the cycle 02-11

sequence twice more, so that each prescribed supercooling in the nominal range is performed three times.

An additional seven cycles are scheduled for repeating some of the growths at the larger

supercoolings, where, because of the difficulties in measuring small tips (< 25 btm), redundant data would

be beneficial. These cycles also provide the option to extend the supercooling range of the experiment

above and below the nominal preprogrammed range. The protocol specifies that a sequence of six

photographs are exposed for each camera view during each growth cycle. The photo intervals for each

growth cycle were determined from theory based on the assumptions of diffusion-limited heat transfer and

a paraboloidal tip shape coupled to the simple scaling rule that VR 2 = const., where V and R are the speed

and radius during steady-state growth.

The IDGE operating plans allowed the initial 12 cycles to run autonomously. Then, based on the

data analyses performed during those initial 12 cycles, minimal parameter changes were made to the

experiment protocol for the next 10 cycles. For example, when necessary, we changed the photo times and

nothing else. Finally, after these 22 cycles were completed, we used the full telescience capabilities of the

IDGE to maximize the quantity and quality of the data return. Specifically, we were able to improve the

data return by modifying the photo timing intervals, the number of photographs per growth cycle, and the

selection of supercoolings to be examined. The telescience operations must manage all these changes to

produce the optimal data return under the constraint of two limiting resources: (1) Time, limited to

approximately 9 days of microgravity with priority commanding when the USMP-2 payload was

designated as the primary payload of STS-62; and (2) film, limited to approximately 225 usable 35-mm

photo frames available on each camera. The IDGE operations are terminated when either of these two

limiting resources are exhausted. Therefore, the optimal strategy was to use all the film before the

microgravity time was completed, and to complete the microgravity time without any unexposed frames

remaining in the cameras.

The IDGE science team could, by obtaining telescience data, and performing the analysis outlined

above, monitor the progress of the experiment, and, where needed, revise the protocol parameters in

accordance with the replanning operations. Once a day, at the completion of the first shift consol duties,

the science team then integrated all the data generated and analyzed at the Payload Operations Control

Center (POCC) and transmitted from Rensselaer Polytechnic Institute (RPI), and replanned the mission for

the next 30 hours.

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2.5.2. Flight Operations

On March 4, 1994, 8:53 AM EST, the Space Shuttle Columbia (STS-62), lifted off from the

Kennedy Space Center, Cape Canaveral Florida, into a low Earth orbit of approximately 163 nautical

miles. On board the Columbia, as part of the USMP-2 was the IDGE. The IDGE powered up four hours

after launch, ran through the calibrations cycles, and then commenced the dendritic growth cycles. Cycle

03, at a supercooling of approximately 0.17 K, afforded the first opportunity to get critical feedback from

the experiment. The initial velocity analysis, completed around day 01, hour 08 Mission Elapsed Time

(MET), showed a significant departure from theory.

The velocity calculated from the SSTV in microgravity was about 3.6 _tm seconds, as compared to

9.8 _tm seconds at terrestrial gravity, and 2.8 _tm seconds as predicted by theory (figure 10). Based on the

preprogrammed photo intervals for this growth cycle, the disparity between the predicted velocity and the

observed velocity, meant that three of the six programmed photographs were taken when the dendrite was

beyond the camera's field of view. Later, a more detailed analysis revealed that steady-state growth ceased

after only 5.5 mm of growth (figure 11), rather than after 10 mm of growth experienced under terrestrial

conditions. This unexpected occurrence, combined with the error in the estimated velocity, resulted in only

one photograph being exposed during the steady-state regime. Thus, insufficient film data were available

to calculate the steady-state velocity from this growth cycle.

A similar analysis conducted for all analyzable data obtained through growth cycle 11, for

supercoolings between 0.1 and 1.0 K, revealed four significant trends that were used in subsequent

mission replanning: (1) The measured velocity in microgravity is different from the theoretical estimates;

(2) steady-state dendrite growth ceases in microgravity over a total growth distance smaller than under

terrestrial conditions; (3) the total steady-state growth distance is proportional to the supercooling; and (4)

the growth transient prior to achieving steady-state growth is smaller in microgravity than in terrestrial

gravity, and that displacement decreases as the supercooling is increased.

Many of the experimental parameters for the subsequent growth cycles were adjusted using the

empirical "rules" developed from the analysis of the initial preprogrammed growth cycles. During cycles

12-21, we revised the photo times only. These changes provided two advantages: (1) Accurate velocity

and radius data from the film data could be obtained because we were now using precise photo timing

intervals; and (2) with these revised photo timing intervals, each dendritic growth cycle took significantly

less time than had been allocated in the preprogrammed _ght protocol. Therefore, besides improving the

quality and quantity of the data we were now obtaining, one of the IDGE's limiting resources, time in low

Earth orbit, was effectively increased. However, while making these changes to the photo timing intervals,

we discovered that the time to enter, uplink, and verify a command sequence that implemented the

parameter change, took four times as long as what we experienced while training for the IDGE mission.

Slow commanding increased the time and effort expanded by IDGE team members who planned and

executed the commanding sequences during the flight.

The telescience analysis performed during cycles 12-21 convinced the team to "stretch" the IDGE

film resources to match the increased on-orbit time by readjusting the experimental parameters to expose

only four photos per growth cycle through cycle 30, and then to expose only two photos per cycle from

cycle 31 on. These changes saved two photos per growth through cycle 30, and four per growth from

cycle 31-37. At two photos per cycle, there was enough film for more than 60 growth cycles. Also, the

telescience results known at that point in the mission suggested that the best strategy for the remainder of

the experiment was to extend the range of supercooling both above and below that selected originally.

Also, a different selection of supercoolings was sampled within the specified range rather than at the 10

preprogrammed supercoolings from which data had already been adequately acquired.

2-26
TheIDGEexperimentunfoldedaccordingto therevisedplansuntil cycle33, when it was realized

thatthe operatingsoftwarewould not functionproperlyaftercycle37--the last preprogrammedgrowth

cycle.The experimentwasrepeatedlyinterruptedby executinganuplinked"pause"command,while the

teamverifiedthatthiswasin facttrue.Theexperimentwas supendeduntil a solutionto this problemwas

determinedandimplemented.Thesolutionchosenwasto setthephototimesandthenumberof photosso

thattherequireddatacouldbeobtained,andthenacommandwas sentto rebootthecomputerbeforethe

startof the next growth cycle. Since, the cycle index recorded by the software only updated at the end of a

growth cycle, the cycle counter never incremented. When the computer finally rebooted, the software read

the current disk memory, and acted as if the current growth cycle still needed to be performed. However,

during the flu'st part of the growth cycle, a set of commands was sent to the IDGE to change the

supercooling and the photo times. Then, when the "new" required data had been collected, we again sent

the reboot command. In this manner, the IDGE continued to operate sucessfully well beyond 37 growth

cycles without advancing the cycle index and reaching the shut-down condition which would have

occurred after the cycle count exceeded 37.

The limitation to this scheme was that it was not possible to perform dendritic growths at

supercoolings above 0.2 K, because the photo intervals were too short to insure that the computer reboot

command could be entered, uplinked, and executed in time before the cycle count would increment. We

were, nonetheless, able to obtain critical microgravity growth data at three supercoolings above 0.2 K, by

using three of the remaining five cycles in the uninterrupted, and incrementing mode.

At completion of the USMP-2 flight, where 38 experimental cycles were originally planned, 60

cycles had actually been completed. Fifty-seven of the 60 were dendritic growth cycles, from which 56

SSTV velocities were calculated. Of those, photographic sequences were attempted for 52, of which 40

useful velocity measurements were made, and 12 failed. Of the 12 where velocity measurements were not

possible, 3 had incorrect preprogrammed photo intervals based on theory, so that only 1 photo was

exposed during steady-state growth. Nine of the last 24 cycles (all with only 2 photos exposed) also failed

to yield a steady-state velocity measurement. Either the In'st photo was taken before steady-state began, or

the second photo was taken after steady-state ended. In both cases, it was known that these measurements

should not be included in any final data set, and two photo cycles could be attempted, because the SSTV

telescience data located the steady-state regime. The reliable and accurate "canonical" data obtained in

microgravity describe dendritic growth over a supercooling range from about 0.06 to 1.9 K. Without

teleoperations, that range would have been from 0.22 to 1.3 K supercooling, as shown in the inner

rectangle of figure 12.

2.6. SUMMARY AND CONCLUSIONS

We measured the dendritic growth velocities and tip radii of curvature of succinonitrile in

microgravity using the IDGE instrument flown on the USMP-2 platform in the payload bay of the Space

Shuttle Columbia (STS-62). The on-orbit microgravity data, when compared to terrestrial dendritic growth

data, demonstrate that (1) convective effects under terrestrial conditions cause growth speed increases of a

factor of 2 at lower supercoolings (AT < 0.5 K), and remain significant even up to values as high as AT --

1.7 K supercooling; (2) in the supercooling range from 0.47 to 1.7 K, the data remain virtually free of

convective effects, and may be used reliably for examining diffusion-limited dendritic growth theories; (3)

a diffusion solution to the dendrite problem, combined with a unique (measured) scaling constant, o*,

does not yield individual growth velocity and radius predictions consistent with the observed dendritic

growth velocities and radii as a function of supercooling; and (4) the failure of this conventional

formulation is currently attributed to departures from the Ivantsov diffusion solution, which is formulated

for paraboloidal dendrites. Ivantsov's theory describes the overall dependence of Prclet number on

2-27
supercooling,butpredictsavaluehigher(5-15 percent)thanthedataweobservedin thediffusion-limited

regime.

Weoffer here two speculations that might account for the descrepancy between the IDGE data and

the Ivantsov solution. The first is that the diffusion field of the Ivantsov formulation is based on a dendrite

tip which is a paraboloid of revolution, which is only strictly true at the tip itself. In addition, it has also

been assumed that all the available supercooling is consumed by the heat conduction field. If a small

portion of this temperature drop is used, say, in the "creation" of interfacial energy as the dendrite

propagates, then the value of the supercooling used to plot the data in figures 7 and 8 would be too high,

and to correct for this, the data would need to be shifted slightly to the left. Although neither of these

speculations have yet to be examined in quantitative detail, both seem reasonable hypotheses, consistent

with the current microgravity data.

When modeling dendritic growth terrestrially, the effect of convection on dendritic growth must

also be included. With a working model of diffusion-limited dendritic growth, one can use the data

provided here at the lower supercoolings to compare quantitatively diffusion-limited theory with dendritic

growth at two well-characterized different gravity levels (0.7x10 -6 and 1 go). This feature must also be

solved before computer codes will simulate dendritic growth terrestrially.

Based in part on the description of the experiment operations narrated in this report, an

experiment's operations are as important as the experiment's apparatus, and should be treated accordingly

with respect to both effort and expenditures. Furthermore, telescience--the remote observation, and

operation of an experiment---can greatly enhance experiment operations, and remains an important tool for

future microgravity spaceflight experiments

2.7. WORK IN PROGRESS

The IDGE team has completed the bulk of the data measurements and analysis for a complete and

detailed archival journal publication on the IDGE flight on USMP-2. We have begun preparing this paper,

which will include the canonical microgravity data from the IDGE flight on USMP-2, and plan to have it

submitted to Phys. Rev. E., by the end of September 1995.

In addition we are pursuing improved image processing methods to examine and extract radii data

at the highest supercooling (above 1.0 K) performed on the flight. We did not expect to be able to measure

dendritic radii at these elevated supercoolings, however, advances in available image processing

techniques suggest that we should attempt this. We are also preparing to perform measurements on the

sidebranching that occurs in dendritic growth. These issues, and others, are interesting and important, and

will be pursued after the submittal of the archival publication on the 1DGE that describes the data and

analysis of this report.

2-28
°

.

°

°

,

°

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°

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10.

11.

12.

13.

2.8. REFERENCES

Langer, J.S., "Issues and Opportunities in Materials Research," Physics Today, 24, October

1992.

Glicksman, M.E., and S.P. Marsh, "The Dendrite," Handbook of Crystal Growth, edited by

D.J.T. Hurle, Elsevier Science Publishers B.V., Amsterdam, 1993, vol. lb, p. 1077.

Ivantsov, G.P., "Temperature Field Around Spherical, Cylindrical, and Needle-Shaped Crystals

Which Grow in Supercooled Melts," Dokl. Akad. Nauk, USSR, 58, 56, 1947.

Glicksman, M.E., R.J. Schaefer, and J.D. Ayers, "Dendritic Growth--A Test of Theory," Met.

Trans. A, 7A, 1747, 1976.

Brener, E.A., and V.I. Mel'nikov, "2-Dimensional Dendritic Growth at Arbitrary P_clet Number,"

Adv. Phys. 40, 53, 1991.

Huang, S C., and M.E. Glicksman, "Fundamentals of Dendritic Solidification: I. Steady State Tip

Growth," Acta Metall., 29, 701, 1981.

Glicksman, M.E., and S.C. Huang, "Convective Heat Transfer During Dendritic Growth,"

Convective Transport and Instability Phenomena, ed. Zierep and Ortel, Karlsruhe, 557, 1982.

Ananth, R., and W.N. Gill, "Serf-Consistent Theory of Dendritic Growth with Convection," J.

Crystal Growth, 91, 587, 1988; and "Dendrite Growth with Thermal Convection," J. Crystal

Growth, 108, 173, 1991.

Glicksman, M.E., E.A. Winsa, R.C. Hahn, T.A. LoGrasso, S.H. Tirmizi, and M.E. Selleck,

"Isothermal Dendritic Growth--A Proposed Microgravity Experiment," Met. Trans. A, 19A,

1945, 1988.

Glicksman, M.E., R.C. Hahn, M.B. Koss, S.H. Tirmizi, A. Velosa, and E.A. Winsa,

"Isothermal Dendritic Growth Experiment: Science, Engineering, and Hardware Development for

USMP Space Flights," Advances in Space Research, 11, 53, 1991.

Glicksman, M.E., M.B. Koss, and E.A. Winsa, "Dendritic Growth Velocities in Microgravity,"

Phys. Rev. Lett., 73, 573, 1994.

Rubinstein, E.R., S.H. Tirmizi, and M.E. Glicksman, "Long Term Purity Assessment of SCN,"

J. Crystal Growth, 106, 89, 1990.

Glicksman, M.E., M.B. Koss, V.E. Fradkov, M.E. Rettenmayer, and S.S. Mani, "Quantification

of Crystal Morphology," J. Crystal Growth, 137, 1, 1994.

2-29
°

.

.

,

,

.

.

.

°

.

2.9. PUBLICATIONS AND PRESENTATIONS

Peer Reviewed Journal Articles

Glicksman, M.E., M.B. Koss, and E.A. Winsa, "Dendritic Growth Velocities in Microgravity,"

Phys. Rev. Lett., 73(4), 573, 1994.

Review Articles

Smith, R.N., M.E. Glicksman, and M.B. Koss, "Effects of Buoyancy on the Growth of Dendritic

Crystals," Annual Review of Heat Transfer, vol. VII, in press, 1995.

Proceedings and Other Articles

Glicksman, M.E., M.B. Koss, L.T. Bushnell, J.C. LaCombe, P. Kobryn, and E.A. Winsa, 'q'he

Tele-Operational Capabilities of the IDGE," 6th International Symposium on Experimental

Methods for Microgravity Science, edited by R.A. Schiffman and J.B. Andrews, The Minerals,

Metals & Materials Society, p. 141, 1994.

Winsa, E.A., D.C. Malarik, M.E. Glickmsan, M.B. Koss, L.T. Bushnell, and J.C. LaCombe,

"Operation of the Isothermal Dendritic Growth Experiment on USMP-2," Reprint AIAA 95-0610,

33rd Aerospace and Sciences meeting and exhibit, Reno NV, January 9-12, 1995.

Glicksman, M.E., M.B. Koss, L.T. Bushnell, J.C. LaCombe, and E.A. Winsa, "Space Flight

Data From the Isothermal Dendritic Growth Experiment," Adv. Space Res., 16(7), 181, 1995.

Glicksman, M.E., M.B. Koss, L.T. Bushnell, J.C. LaCombe, and E.A. Winsa, "Dendritic

Growth of Succinontrile in Terrestrial and Microgravity Conditions as a Test of Theory," ISIJ

International, 35(6), 1216, 1995.

Smith, R.N., M.E. Glicksman, M.B. Koss, L.T. Bushnell, and J.C. LaCombe, "Experimental

Study of Dendrite Growth in an Undercooled Melt Under Microgravity Conditions," Heat Transfer

Session at the 1994 ASME Winter Annual Meeting in Chicago, in press, 1995.

Glicksman, M.E., M.B. Koss, L.T. Bushnell, J.C.

Growth in Terrestrial and Microgravity Conditions,"

Meeting, Symposium P, Boston, MA, in press, 1995.

LaCombe, and E.A. Winsa, "Dendritic

Material Research Society (MRS) Fall

Glicksman, M.E., M.B. Koss, L.T. Bushnell, and J.C. LaCombe, "The Isothermal Dendritic

Growth Experiment: Implications for Theory," Modeling of Casting, Welding, and Advanced

Solidification Processes VII, The Minerals, Metals & Materials Society, in press, 1995.

Glicksman, M.E., M.B. Koss, L.T. Bushnell, J.C. LaCombe, and E.A. Winsa, "Evaluation of

Teleoperating the IDGE on STS-62," 7th International Symposium on Experimental Methods for

Microgravity Science, edited by R. A. Schiffman, The Minerals, Metals & Materials Society, in

press, 1995.

2-30
°

.

.

°

.

.

o

°

,

10.

11.

12.

13.

14

15.

Presentations

Glicksman, M.E., "Dendritic Growth in Microgravity: IDGE Flight 1," Aachen Foundry Institute,

Aachen, Germany, March 1994.

Glicksman, M.E., "Dendritic Growth in Microgravity: IDGE Flight 1," DLR, Space Simulation

Institute, Koln-Portz, Germany, March 1994.

Glicksman, M.E., "The Isothermal Dendritic Growth Experiment: First Results from USMP-2,"

Pittsburgh Chapter of the ACCG, Pittsburgh, PA, May 1994.

Glicksman, M.E., "The Isothermal Dendritic Growth Experiment: First Results from USMP-2,"

Microgravity Materials Science Conference, Von Braun Civic Center, Huntsville, AL, May 1994.

LaCombe, J.C., "Dendritic Solidification in a Microgravity Environment," Joint Meeting of the

Eastern New York Chapter of ASM International and the Hudson-Mohawk Section of TMS,

Latham, NY, May 1994.

Koss, M.B., "Preliminary Results from the Isothermal Dendritic Growth Experiment," at the

Space Experiments Division Awards Ceremony, NASA Lewis Research Center, Cleveland, OH,

July 1994.

Glicksman, M.E., "Space Flight Data From the Isothermal Dendritic Growth Experiment," at 26th

COSPAR, Hamburg, Germany, July 1994.

Glicksman, M.E., "Space Flight Data From the Isothermal Dendritic Growth Experiment,"

Giesserei Institut, RWTH Aachen, Aachen, Germany, July 1994.

Glicksman, M.E., "The Isothermal Dendritic Growth Experiment: Implications for Theory,"

NIST, Gaitherburg, MD, August 1994.

Glicksman, M.E., "Space Flight Data From the Isothermal Dendritic Growth Experiment: What

Have We Learned?," Materials Engineering Department Seminar, Rennselaer Polytechnic Institute,

Troy, NY, September 1994.

Bushnell, L.T., "Effects of Convection on Dendritic Growth in Microgravity," ACCG-East/9,

Atlantic City, N J, October 1994.

LaCombe, J.C., "Three Dimensional Characteristics of Dendrites," ACCG-East/9, Atlantic City,

N J, October 1994.

Smith, R.N., "Experimental Study of Dendrite Growth in an Undercooled Melt Under

Microgravity Conditions," ASME Winter Annual Meetings, Chicago IL, November 1994.

Glicksman, M.E., "Convection in Microgravity: First Flight of the Isothermal Dendritic Growth

Experiment," AIChE meeting in San Francisco, CA, November 1994.

Glicksman, M.E., "Isothermal Dendritic Growth Experiment USMP-2,3,4," Raumsimulation

Institut, DLR, Koln, Germany, November 1994.

2-31
16.

17.

18.

19.

Glicksman, M.E., "Dendritic Growth in Terrestrial and Microgravity Conditions," Material

Research Society (MRS) Fall Meeting, Boston, MA, November 1994.

Winsa, E.A., "Operation of the Isothermal Dendritic Growth Experiment on USMP-2," Paper 95-

0610, 33rd Aerospace and Sciences Meeting and Exhibit, Reno NV, January 1995.

Koss, M.B., "Evaluation of Teleoperating the IDGE on STS-62," 7th International Symposium on

Experimental Methods for Microgravity Science, TMS Annual Meeting and Exhibit, Las Vegas,

NV, February 1995.

Glicksman, M.E., "The Isothermal Dendritic Growth Experiment: Anniversary Review of Space

Flight Result," Burgess Memorial Lecture, Washington, DC Chapter, ASM, March 1995.

2-32
MOUNTING FLANGE

FILL PORT

COOLER UNIT

POTTING COMPOUND

TOP SHELL

WINDOWS

STINGER

CAP

;TOR PROBE

BELLOWS

BOTTOM SHELL

)R CONNECTIONS

Figure 2-1. IDGE flight growth chamber.

2-33
Uniform Temperature

Bath

Camera

Growth

Chamber I

_L_

Xenon

Flash

Slow-Scan

Television

Camera

LED

Dendrite

Figure 2-2. IDGE temperature controlled tank and optic system.

2-34
I

i _ _ _ _ii _i '

E

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°_

b

o

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,...q

r_

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2-35
1000

O Huang & Glicksman

[] IDGE (terrestrial)

• IDGE (microgravity)

Diffusion Theory (_*=0.0194)

- - - Diffusion Theory (%=0.0164)

100

.i

0

Figure 2-4.

0.1 1

Supercooling (K)

Dendritic growth tip velocity as a function of supercooling.

2-36
O Huang & Glicksman

[] IDGE (terrestrial)

• IDGE (microgravity)

.... Diffusion Theory (%=0.0164)

tool

°

°

10

Figure 2-5.

E

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,

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d

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Supercooling (K)

Dentritic growth tip velocity as a function of supercooling at the lower supercoolings.

2-37
E

=L

v

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rr"

100

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i

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Diffusion Theory (_*=0.0194)

..... Diffusion Theory (0R=0.0179)

I I I ! I I I I

0.1

I I I I I |

Supercooling (K)

Figure 2-6. Dentritic growth tip radius as a function of supercooling.

2-38
m

O

_)

13..

0.01

0.001 B

m

, , , ' I , . i , , , , I

O Huang & Glicksman

[] IDGE (terrestrial)

• IDGE (microgravity)

...... Diffusion Theory (paraboloidal tip): P = Iv-I(AT)

O I1"

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Figure 2-7. Dentritic growth tip P6clet numbers as a function of supercooling.

2-39
11

10

o 9

O

O

_- 8

X

7

S

E

Z 4

m

o 3

tl)

2

0

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o

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, . I , , . i J i I * * I I , , . I , , , . I , , , , I ,

0.5 0.6 0.7 0.8 0.9 1.0

!

m

Supercooling (K)

Figure 2-8. Dentritic growth tip P6clet numbers as a function of supercooling in the diffusion-limited

supercooling range.

2-40
04

rv"

O

04

V

t_

0.04

0.03

0.02

0.01

e ! | m | | I I • i i I

o Huang & Glicksman

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O

i •

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Figure 2-9. Dentfitic growth scaling parameter as a function of supercooling.

2-41
' ' I ' ' ' ' ' : ' ' I

10 2

CCJ

E

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0

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Figure 2- l 0.

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• IDGE Terrestrial(SSTV)

..... Diffusion Theory (o*=0,0195)

IDGE Microgravity (SSTV)

t4

fl

g .°°

es

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a13

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°

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The first telescience measured velocity calculated from the SSTV data.

2-42
10000

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2000

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I I I I I I I I I | ! I I I I I I I I I I I I Ill I I I I I I I I ] I I I I I I I I I I'l 1/I I I I I I I I I I I I I I I I l I . I " IlI" I I

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Microgravity (best fit)

...... Zero gravity (predicted)

J Photograph Exposed

0 600 1200 1800 2400 3000 3600

°

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4200

Time (s)

Figure 2-11. Dendrite tip displacement versus time data from the SSTV data•

2-43
=I

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| | I I Illl

1

Supercooling (K)

Figure 2-12. Extension of the velocity data range that was obtained through the use of telescience.

2-44
SECTION III.

IN SITU MONITORING OF CRYSTAL GROWTH

USING MEPHISTO
Acknowledgments

We would like to extend our special appreciation (in alphabetical order) to B. Angelier, M.

Bodaert, P. Bioulez, J. Comera, P. Contamin, C. Fenwick, D. Gadonniex, G. Hieu, F. Herbillon, R.

Krishnasamy, J.E. Mazille, B. Merkel, R. Panchapakesan, J.P. Praizey, A. Rouzaud, and G. Santoro for

their efforts. We would also like to gratefully acknowledge the financial support from NASA (grant NAG3-1096) for this program.

3-46
SECTION III. IN SITU MONITORING OF CRYSTAL GROWTH

USING MEPHISTO

Mission STS 62 - Program USMP-2

Experiment Analysis

Reza Abbaschian

Professor and Chairman, Department of Materials Science and Engineering

University of Florida, Gainesville, Florida 32611

3.1. OVERVIEW

This report summarizes the results of the In situ Monitoring of Crystal Growth Using MEPHISTO

(Material pot l'Etude des Phenomenes lnt_ressants de let Solidification sur Terre et en Orbite) experiment

on USMP-2 after 8 months of data and microstructural analysis. It consists of an analysis of the

microstructural evolution of microgravity processed samples and of the telemetry data. The flight

experiment was accomplished with the collaboration of the University of Florida, the Centre National

d'Etudes Spatiales (CNES), the Centre d'Etudes Nucleaires de Grenoble (CENG), the National Institute

of Standards and Technology (NIST), and NASA (P.I.: R. Abbaschian, Co-P.I.'s: A.B. Gokhale, J.J.

Favier, and S.R. Coriell). The MEPHISTO Project Manager was G. Cambon (CNES) and the NASA

Project Scientist and Project Manager were H.C. de Groh III and R. DeWitt (NASA-Lewis Research

Center (LeRC)), respectively.

The research program has yielded a wealth of scientific data. A total of more than 45 cm of

directionally solidified alloy were recovered. Metallurgical analysis of the samples has shown that the

interfacial kinetics play a key role in controlling the morphological stability of faceted alloys. We believe

this to be the first unambiguous demonstration of such an effect during directional solidification under

diffusion-dominated conditions. In addition, analysis of the differential Seebeck thermoelectric signals for

very slow growth velocities was carried out. The analysis indicates that it is possible to calculate the kinetic

undercooling from the Seebeck signal decay at the end of solidification. Such information is not accessible

from ground-based studies due to effects associated with convective destabilization.

3.2. BACKGROUND

The In situ Monitoring of Crystal Growth experiment, flown aboard USMP-2 using the

MEPHISTO flight hardware, was a fundamental investigation of the directional solidification of a faceted

metallic alloy. The experiment was carried out on-board Columbia (Mission STS-62, Program USMP-2,

March 4-18, 1994). The hardware used was MEPHISTO Flight Model 2, which was developed by

CNES and flown for the first time on this mission. The hardware is similar to MEPHISTO-I which was

flown aboard USMP-1 for Sn-Bi alloys by J.J. Favier and his colleagues.

This research program was initiated by the University of Florida (NASA grant NAG-3-1096) in

January 1990. As such, the ground-based experiments as well as sample preparation and integration for

the microgravity study were carried out in the relatively short period of approximately four years after the

start of the program. This was accomplished by the establishment of a collaborative effort involving the

CENG, CNES, and NIST, with program scientist and management provided by LeRC.

3-47
The experimentswere performedto gain a detailedunderstandingof two fundamentaland

interrelatedaspectsof theliquidto solidtransformationin facetedalloys:(a) Morphologicalstabilityof the

solid/liquid (s/l) interfacesand the resulting macro- and microsegregationpatterns,and (b) atomic

attachmentkinetics at the freezinginterface,deducedvia measurementof the growth rate-interface

supercoolingrelationship(s).Thealloy systemusedwasBi dopedwith 0.1at.%Sn.

3.3. USMP-2 MISSION DESCRIPTION

3.3.1. Experimental Facility and Techniques

The experiments used the MEPHISTO directional solidification facility which processes three

samples in parallel, each approximately 900 mm long and 6 mm in diameter. One sample was used for

interfacial temperature measurements (termed the Seebeck sample), another for resistance and thermal

gradient measurements (termed the resistance sample), and the third for demarcation of the interface shape

during directional solidification (termed the P61tier sample). A 2-cm length of the resistance sample was

also quenched at the end of the microgravity experiment.

The core structure of the MEPHISTO facility is shown schematically in figure 1(a), while the

arrangement of the experimental samples along with important dimensions, positions of the two s/1

interfaces, locations of thermocouples and the overall temperature profile are shown schematically in figure

1(b). The general experimental procedure consisted of melting the middle -500-mm length of the samples

using two furnaces, thus creating two s/1 interfaces. One of the furnaces was kept stationary and the other

moved back and forth at specified rates in order to melt or solidify at that side of the samples. In addition,

the sample ends were maintained at a controlled and constant temperature in order to facilitate the

measurement of a differential voltage from the two interfaces and the resistance change during

solidification and melting.

3.3.2. Flight Summary

The experiment was monitored and controlled using real-time telemetry. After a nominal hardware

initialization sequence, the condition of the samples was checked by monitoring a variety of sensors

(Seebeck electro motive force (emO, resistance, P61tier circuit continuity, thermocouple outputs,

microfurnace heating rates, heat sink behavior, etc.). It was found that all three samples were nominal,

without any evidence of loss of integrity. However, during the In'st 31 hours of the mission, three main

anomalies were encountered: (1) An apparent indication of a "grounding" of the P61tier sample, followed

by an indication of a loss of the sample continuity; (2) anomaly occurred in the temperature output of the

thermocouples in the P61tier sample; and (3) anomaly manifested itself as a saturation of the Seebeck signal

after the completion of two melting/solidification cycles. Following this, the Seebeck signals saturated at

either the positive or negative voltage limits of the amplifier, with intermittent, highly amplified signals

which could not be correlated with any microstructure-related phenomena. These anomalies have been

described previously in detail in our postlaunch, 6-month report.

Over 55 melting and solidification cycles with different thermal and velocity conditions were

performed. The total experimentation time was 326 hours 14 minutes. After the mission, over 45 cm of

directionally solidified alloy was recovered in good condition. As discussed below, the metallurgical

analyses of these samples have started to provide valuable information on the morphological stability of

faceted alloys.

3-48
Also, despite the Seebeck anomalies, the first 30 hours of Seebeck signal nevertheless provided

valuable data on the interfacial undercooling of the faceted alloy during plane-front solidification at a low

growth velocity. As presented later, the measurement of such low levels of kinetic undercooling are not

feasible on Earth.

The final solidification sequence for the samples is given in table 1, summarizing the lengths of the

regions which were preserved after controlled directional solidification at the specific growth velocities

listed. Also included are the shuttle attitude and possible sources of g-perturbations.

3.4. MICROSTRUCTURAL AND DATA ANALYSIS

The following main themes were identified for the postflight analysis:

3.4.1. Microstructural Analysis

• Plane front growth

• Plane front to cellular transition

- Orientation-dependence of transition thresholds

- Direction of cell growth with respect to the growth direction

- Cell spacing as a function of growth velocity

• Compositional changes.

3.4.2. Kinetics Data Analysis

• Resistance measurements

• Temperature gradient in the liquid and solid (GI, Gs)

• Seebeck thermoelectric measurements.

3.4.3. Microstructural Analysis

Overview of Microstructural Evolution--The initial (Earth-grown) microstructure of the samples is shown

in figure 2. The samples were produced from a homogeneous master alloy via directional solidification at a

growth velocity of 0.2 cm/s. The high growth velocity was used to avoid macrosegregation along the

length of the grown sample. This was further verified by inductively coupled plasma (ICP) analysis of the

sample ends after the flight. The optical micrograph (differential interference contrast (DIC) illumination)

in figure 2 shows relatively uniform cross sections with a faceted cellular/dendritic morphology; the

growth is radially inward.

An overview of the microstructural evolution of the samples grown in space is obtained from the

low magnification (xl0) composite of a microsection of the quenched sample, figures 3(a)-(g), while the

microstructural changes observed are summarized in table 2. Figure 3(a) shows schematically a summary

3-49
of processing lengths and velocities as well as the microstructure in each section. Also shown is the

location and orientation of the microsection with respect to the furnace graphite diffuser center. Note that

the microsection orientations of all the samples are such that they are thermally equivalent. The

micrographs in figures 3(b)-(g) show the successive development of the microstructure as a function of

the distance solidified and the growth velocity. As detailed later, for solidification at velocities ranging

from V1 to V3 (see table 2 for velocity values), the growth occurs in a planar mode, while a distinct

cellular morphology is seen at the V5 and V6 velocities. The planar to cellular transitions at V5 and V6

reveal many important aspects of the solidification of faceted materials in microgravity as discussed later.

The microstructural evolution at the intermediate V4 velocity appears to be selectively cellular, with the

transition to cellular mode detected in one of the grains, but not the others, after approximately 14 mm of

directional solidification. The other grains continued to grow in a plane-front mode until the end of the

solidification cycle (30 mm). This behavior is currently being investigated further. The P61tier and Seebeck

samples generally had microstructures similar to those shown in figure 3, except that no cellular growth

was observed at V4 in these samples, as discussed in more detail in the following sections.

Plane Front Solidification--The comparative development of a plane-front microstructure is illustrated in

figures 4(a)-(b) which show the transition from a faceted cellular/dendritic structure of the Earth-grown

portion of the samples to a plane-front morphology at the moving furnace interface (P61tier sample) and at

the fixed furnace interface (during final cool down, quenching sample), respectively. At both interfaces,

the initial cellular to plane-front transition interface was sharply delineated, as seen clearly by the low

magnification micrograph in figure 4(c). The optical micrograph shows that only a few dominant

orientations emerge from the initial (rapidly cast) microstructure, which was found to be a common feature

of all three samples.

The microstructural appearance of continued growth in the plane-front mode (V1, V2, and V3) is

shown in figure 5. Only a few orientations again dominate the growth front and the microstructure is

characterized by a complete absence of microsegregation patterns (Sn-rich areas), indicating plane-front

solidification. Our recent preliminary bulk orientation analysis via combined electron

channeling/backscattered electron imaging (measurements at 600 points) indicates that the dominant

growth direction was the trigonal <111> axis, with the grains rotated with respect to each other by a few

degrees around this axis.

S/L Interface Shape--In all cases where an interface was revealed, for example during cellular to planefront transition upon decreasing the growth rate, it was found that the interface was nearly flat, with a

slight curvature (concave toward solid) near the s/l/crucible triple junction. The general shape of the

interface is illustrated in figures 6(a)-(b) and 7. Figure 6(a) shows a schematic of the interface, while

figure 6(b) shows a micrograph of the interface between steps llM and llV (table 1), where the growth

rate was decreased from 26.9 to 1.85 /_m/s. Upon closer examination, the boundary across each grain

appears to be fairly flat, with the small angles between them giving the appearance of an overall slight

curvature of the interface, as shown by the higher magnification micrograph of the interface center, figure

7. It can be seen from this differential interference contrast illumination micrograph that a distinct feature of

the microstructural morphology at these interfaces is the sharp changes in the angles of columnar grain

boundaries upon reducing the growth rate. The result is the disappearance of some of the orientations

because of the net effect of the anisotropy of thermal conductivities and interfacial kinetics. It was found

also that the interface was associated with a sharp compositional change, detected via secondary ion mass

spectroscopy (SIMS) analysis, figure 8.

We note that in the absence of a Pdltier interface demarcation capability, the above analysis

provides only marginal information on the shape of the interface during growth at various velocities.

3-50
Notwithstanding, the results indicated that, in spite of the sharp temperature gradients imposed across the

interface, the interface for each grain remains faceted.

In order to overcome the inability to pass Ptltier pulses through the sample, attempts were made to

demarcate the growing interfaces by closing the MEPHISTO isolation valve at predetermined times toward

the end of the mission, thus providing short-term g-perturbations. The compositional analysis of these

regions has so far not provided conclusive evidence of solute banding. More detailed analysis using SIMS

is currently underway.

Transition to a Cellular Growth Mode--Microstructural examination of the microgravity-processed

sections indicated that those regions of the samples grown at the V5 and V6 velocities exhibited a

morphological transition to a cellular growth mode. One of the most interesting aspects of the transition at

the V5 (26.9 /_m/s) velocity was the existence of a distinct bias toward preferential breakdown of some

grains. The preferential breakdown is shown schematically in figure 9(a) and via a low magnification

micrograph in figure 9(b). Referring to these figures, it can be seen that one grain continues to grow in the

plane-front mode for a significant distance while the other orientation is growing in a cellular mode. The

distance of separation between the morphological instability of the two grains at this velocity is

approximately 1.2 cm. The microstructural appearance of a preferential transition at a higher magnification

is shown in figure 10. Figures 9 and 10 additionally show that the orientation which exhibits the cellular

transition early is also the one that is tilted more with respect to the heat flow direction (6.5°; in

comparison, the tilting of cells in the other orientation was weak and close to zero). These observations

clearly suggest an influence of the anisotropy in interface properties on the morphological stability of this

alloy.

Cellular transition at a higher growth velocity (V6:40/trn/s) exhibited features which were similar

to that at V5, except that the differential instability of the neighboring grains was not as pronounced,

resulting in a significantly narrower planar to cellular transition zone, as shown in figures 1 l(a)-(c).

Figure 1 l(a) shows schematically the main features of cellular breakdown at this velocity, while the

microstructural appearance of the cellular breakdown for the entire cross section of the sample is shown in

figure 1 l(b) and at a higher magnification in figure 1 l(c). In this region also, two dominant grain

orientations were present, albeit with additional subgrain boundaries. The cell spacing was significantly

finer (0.179-mm average), with a cell tilt of 4.5 ° with respect to the heat flow direction for one grain. For

the other grain, the growth orientation was even closer to the dominant growth direction. The crosssectional view of cell geometry on a transverse section is shown in figures 12(a)-(d). Figure 12(a) shows

schematically the appearance of cell boundaries in the two dominant orientations detected in this region,

while 12(b) shows an optical micrograph in the bright field mode. The dominant orientations are clearly

revealed under a sensitive-tint polarized light illumination, figure 12(c), while the cell boundaries are

delineated by a dark-field illumination, figure 12(d). It can be seen that the microstructure consists of long

plate-like cells, with only a marginal difference in the cell spacings of the two orientations. A measurement

of the extinction angles under polarized light indicated that the two orientations were axially rotated by an

angle not greater than 15 ° from each other. It may thus be concluded that while the two regions are

separated by a large-angle boundary, both orientations are close to the dominant growth direction.

The microstructural analysis presented above indicates that selected orientations are morphologically more stable than others, resulting in a staggered planar to cellular transition. In general,

anisotropic interfacial properties are expected to play a role in the morphological stability of planar

interfaces, as well as the evolution of cellular and dendritic structures; this has been predicted theoretically

by Coriell and Sekerka [1976], Coriell et al. [1994] by extending the linear stability analysis of Mullins

and Sekerka [1964], and by Young et al. [1987] in the weakly nonlinear regime. These treatments indicate

3-51
thatsuchanisotropiestendto stabilizethegrowthof aplanarinterface.Experimentalobservationsreported

by Tiller and Rutter [1956] for lead-tinalloys and by Trivedi [1990] and Trivedi et al. [1991] for

transparentorganicsystemshavebeenfoundto begenerallyconsistentwith thetheoreticalpredictions.

The most important aspect of the present observations is the extremely long extension of the planefront stability in one grain with respect to the neighboring grain(s). We believe that it is the anisotropy in

interfacial kinetics, rather than in the surface energy or thermal conductivity, which is exerting such a

strong influence on the morphological stability.

In addition, as shown above in figures 11 (a)-(c), it was found that the extent of staggered cellular

transition diminished at a higher growth velocity (V6, 40 /_m/s). This observation is contrary to the

theoretical predictions, which indicate that the stabilizing influence of interface kinetics should become

more pronounced with increasing growth velocity. It is our belief that when the growth rate increases, the

faceted interface(s) go through a kinetic roughening transition, become nonfaceted, and reduce the

stabilizing effect of interfacial kinetics. Additional considerations include factors such as dislocation

assisted versus dislocation free growth.

3.4.4. Kinetics Data Analysis

Resistance Measurements--Internal tracking of the interface movement was carried out by measuring the

resistance change across the sample as the moving interface translated at preset velocities. The resistance

change as a function of the processing time during a typical Seebeck measurement cycle* is shown in

figure 13(a), along with the furnace (external) movement, which was tracked through a linear

potentiometer. It can be seen that while the correlation between the external measurement of interface

position and the internal measurement is good, there appears to be a slight lag in the resistance change at

the beginning of solidification. The lag can be seen clearly when the melting and solidification parts of the

cycle are plotted as a function of the distance, as shown in figure 13(b). It should be noted that the signals

contain a superimposed (analog-to-digital conversion related) "noise" which can be removed via a full

spectrum Fourier analysis, as illustrated in figure 14. Note that the slope changes in the resistance can now

be seen without ambiguity. The difference between the resistance change during solidification and reverse

(symmetrical) melting (AR = dR Is-dR Ira ) is shown in figure 15 using the filtered data. The figure shows

that during solidification, the AR increases continuously until a steady plateau is reached after about 15

mm of solidification. After the external furnace translation is stopped, the difference diminishes back to

nearly zero. Such behavior seems to be consistent with the retardation of the interfacial temperature from

solute buildup and/or kinetics sources. However, since the corresponding Seebeck data are not available, a

more definite conclusion cannot be reached at this time.

It may be recalled that the sample which was used to measure the resistance change during

processing was also used to quench a 2-cm section at the end of the microgravity experimentation period.

The quenching behavior was monitored through the resistance change, as illustrated by the plot in figure

16, showing the sharp increase in the resistance at the instant of quench. Also shown on this figure is the

furnace position and the output from two thermocouples in the resistance sample, which were located

approximately 40 and 90 mm from the interface. The subsequent decrease in the resistance after quenching

indicates a melting of that 2-cm section of the sample which was pushed from the cold zone into the hot

zone at the fixed interface. Note that the time constant for melting is appreciably larger in this case.

*The processing sequence during Seebeck measurements typically consisted of solidification/stabilization (i.e., no furnace

movement)/reverse melting/stabilization.

3-52
Thermal Gradient Measurements--The temperature gradients were measured continuously during

solidification and melting by using thermocouples in the quenching sample, designated as T1 and T3. A

typical thermal measurement is shown in figure 17. Also shown in the figure are the corresponding

furnace position and the melting temperature. For these thermocouples, the preflight predicted positions

after volume compensation were 105 and 61 mm. The positions deduced from the thermal data were

100.3__.1.4 mm and 61.9___1.6 mm, respectively.

The data were linearized in the vicinity of the melting temperature (~ 271.4 °C) separately for the

solid and the liquid. The thermal gradients are represented as:

dT dT dt

dX- dt dX

1 dT

V dt"

(1)

The gradients were found to be: (G L, as)solidification = 216 and 219 °C/cm and (G L, as)melting = 213 and

219 °C/cm.

Seebeck Measurements--The interfacial undercooling was measured using the Seebeck technique in

which the differential thermo-emf is measured from two s/1 interfaces, 8-10 one stationary and the other

moving. The stationary interface acts as a reference junction, against which the temperature changes of the

moving interface can be measured from a differential signal at the sample ends. For isotropic materials or

single crystals, the differential voltage is directly proportional to the difference in the temperatures of the

moving and stationary interfaces, while for polycrystals and particularly for anisotropic materials,

additional terms are introduced from effects associated with thermo-emfs of grain boundaries and

orientations in the gradient zone. Thus,

/1

Es = ,Ts/,Ari+fVT(x) , "o,/o,+,dx

i=1

(2)

where r/s_ is the difference in the Seebeck coefficients of the solid and liquid, ATi is the interfacial

undercooling, VT(x) is the temperature change as a function of distance in the solid, and rio,/o,, 's are the

crystallographic orientation-dependent Seebeck coefficients.* The summation is implied over _+ range of

orientations in the microstructure. Therefore, because of the interaction between microstructural features

(e.g., grain boundaries) and the temperature gradients generated during processing, the contribution to the

overall Seebeck emf can vary if the microstructure undergoes changes during interface migration. This

situation is shown schematically in figures 18(a) to 18(c), illustrating the passage of a temperature gradient

converting an equiaxed cellular structure to a columnar one during plane-front directional solidification. As

the gradient zone passes over the microstructure, there is a corresponding change in the differential

Seebeck emf, associated with the changes in the growth morphology, grain shapes, and orientations, as

shown schematically in figure 18(c). Obviously, when the microstructure forming at the interface is the

same as that leaving the gradient zone, the change in the Seebeck signal due to structural contributions

must diminish.

t In order to eliminate such structural effects and determine the interfacial contributions, Favier and Rouzaud used a technique

of subtracting the Seebeck signals during melting from those during solidification for Sn-Bi alloys processed on USMP-I.

For anisotropic Bi-Sn alloys, however, we found the technique to be inaccurate.

3-53
Theinterfacialtemperaturedepressionis madeup of contributions from three sources: Chemical

(ATc), kinetic (ATk), and capillarity (curvature-related, ATr). For plane-front solidification, the capillarity

contribution can be ignored, while, as discussed below, the chemical contribution can be calculated

provided the convecto-diffusive state of the liquid can be quantified. The kinetic undercooling in the case

of dislocation assisted growth is generally related to the growth velocity by

V=aAT; (3)

where V is the growth velocity and a and n are constants. In order to compare with microgravity

experiments, the kinetic law for pure Bi single crystals was previously determined based on 72

measurements at growth velocities ranging from 2x 10 -4 to 9.5x 10 -3 cm/s. The resulting values for a and n

were 0.0274 cm/s(K) n and 1.65, respectively, 11 which agreed fairly well with those reported in the

literature. 12-14

Seebeck Signals During Plane Front Solidification--Two melting/solidification cycles were performed at

the beginning of microgravity processing prior to the Seebeck anomalies. The melting/solidification

sequence of these cycles is shown schematically in figure 19, while the corresponding microstructural

evolution is shown in figures 20(a)-(e), along with the Seebeck signals obtained during each processing

step. The microstructural evolution sketched in figure 20 was based on the microstructures obtained during

final solidification of the sample under nearly identical growth conditions, as shown earlier in figures 4(a)-

(c).

As noted earlier and shown in figure 20(a), the interface that formed after thermal stabilization of

the furnace is nearly flat with very slight curvature at the triple-junction between the solid, liquid, and the

crucible (fused quartz) wall. The corresponding Seebeck emf during this stationary state is constant with a

mean value of-70/_V, representing the baseline for the measurements. During melting at a velocity of

26.7×10 --4 cm/s, figure 20(b), the Seebeck emf is seen to decrease to 55 _V at a rate of -7.5 ktV/mm.

Upon stopping the furnace translation, the signal goes through a transient and increases by 7.5 /_V to a

stable level of 62.5 ktV. This transient, lasting approximately 200 seconds after the furnace had stopped, is

related to local adjustments in the thermal fields as well as stabilization of the microstructure near the

interface. Thus, for the initial faceted dendritic microstructure, the net Seebeck change during melting is

3.75/_V/mm.

During the reverse 2-mm solidification at a velocity of 1.85x10 -4 cm/s at the same location, figure

20(c), the Seebeck signal decreases from 62.5 to 22.5 /_V, and upon stopping the furnace translation, the

signal increases through a transient by 10.5 ktV. The time interval for the transient in this case is, however,

much longer than that after melting (-900 seconds versus -200 s), indicating an additional slower

decaying component. The transient in the Seebeck signal at the end of solidification is related primarily to

the solutal decay (diffusional decay of the solute-rich boundary layer and concurrent decay of the chemical

undercooling), with some early contributions from thermal and kinetics adjustments when the interface

comes to rest. As discussed later, the decay can be used to determine the kinetic undercooling at the

interface. It should be noted that even though the same 2 mm which was melted was solidified again, the

Seebeck signals behave differently. This is due to the fact that two important changes take place during this

plane-front solidification: (1) A solute-rich boundary layer begins to build up at the interface, and (2) the

new structure formed comprises predominantly of columnar grains aligned parallel to the heat flow

direction (figures 4(a)-(c)) and therefore is appreciably different from the microstructure which was

melted.

3-54
Thereversemeltingof 7 mm ata velocityof 26.7x10-4cm/s,figure20(d),showsmanyinteresting

featureswhichgive quantitativeinformationonthemicrostructuralevolution.First,for theinitial 2-mmof

melting,thesectionthathadbeensolidifiedin space(columnargrains)is meltedback. In this case,the

Seebecksignaltrackstheinitial solidification,exceptin reverseorder(figure 20(c)). After meltingof the

space-grownsection,thereis aninversionin theSeebecksignal,asit beginsto trackthemelting of the

ground-grownportionof thesample,with thesameslopeasthefirst 2-mmmelting(figure 20(b))of the

ground-grownsample.It shouldnotedthatthefirst 2-mmof solidcontainedaconcentrationof Sn ranging

fromkC0(0.0029at.%)to 0.0048at.%,whereasthelattersectioncontainsCo(0.1 at.%). In addition,the

first sectioncontainedonly a few directionallygrown grains, while the latter section is faceted

cellular/dendritic.Thebehaviorof thesignalthusclearlysuggestsaverysensitiveresponseof theSeebeck

emftothemicrostructureaswell astheinterfacetemperature.Upon stoppingthefurnacetranslation(after

7mm),thesignalundergoesatransient,andstabilizesbackto alevelobservedafterthefirst melting(62.5

#V), indicating the establishment of an interface very similar to the one encountered after

2-mm melting.

The directional solidification following this melting is shown in figure 20(e). The evolution of the

Seebeck emf is identical to the first 2-mm solidification (figure 20(c)), indicating nearly identical structural

development. However, the signal shows an inflection point after approximately 4.6-mm solidification and

continuously decreases in slope thereafter, resulting in a lower rate of change at the end of the

solidification cycle (7 mm). The behavior of the signal for this measurement as well as the first

solidification shown in figure 20(c) can be described empirically by an error function of the form

Es=a+1 b[1+erf(t-c)]]' (4)

where a, b, c, and d are constants and t is the time in seconds.

Based on our earlier discussion (figure 18), the decrease in slope indicates a change in the

contribution from microstructure/thermal field interaction related effects. It is clear that, as the oriented

plane-front microstructure develops, as shown in figure 4(a), the Seebeck emf from gradient affected zone

decreases accordingly. Upon stopping the furnace translation (after a solidification of 7-mm), the signal is

seen to decay in a characteristic manner, until a stable thermosolutal and interface profile is attained.

The temperature depression due to compositional changes for the purely diffusive state can be

calculated using the classical Tiller et al. [1953] approach. The governing equation for the plane-front

solidification in initial transient (for the present velocity, a steady state would be achieved at a distance >

15 cm) is

(5)

The chemical contribution is then given by

ATc =ml(C 1-CO) (6)

where C1 is the composition of the liquid at the interface, Co is initial alloy composition (0.1 at.%), k is

the equilibrium partition coefficient (0.029), V is the growth velocity (1.85x10 -4 cm]s), D_ is the liquid

3-55
diffusivity (5.315x10-5cm2/sfor Snin Bi for anaveragetemperaturein thegradientzoneof 435.5°C),16

x is the distance solidified, and ml is the liquidus slope (-2.32 K/at.%).

Based on equations (5) and (6), the temperature depression due to compositional changes after 2-

mm and 7-mm solidification is 0.155 and 0.533 K, respectively, from which we can deduce the magnitude

of nonsolutal contributions, such as the kinetic undercooling. The resulting values of 2.75 and 3.5 _V

corresponded to a kinetic undercooling of 0.055 and 0.07 K after 2-mm and 7-mm solidification,

respectively.

The above determined interface kinetics supercoolings compare well with those calculated based on

the kinetic undercooling for pure Bi single crystals (0.048 K), using equation (3) and the values of

constants given earlier. However, it should be emphasized that the constants in equation (3) were derived

for pure Bi single crystals, which can be appreciably different for the rate equation for alloys, as discussed

by Abbaschian and Mehrabian [1978]. Unfortunately, because of the Seebeck signal anomalies, additional

data could not be gathered during the rest of the mission to determine the effect of compositional changes

on interfacial kinetics.

We note that, because of thermosolutal convection, accurate measurements of the kinetic

undercooling in alloys are not feasible on Earth, particularly when the growth velocities are on the order of

a few Itm/s. As shown above, the kinetic measurements can however be performed in a high-quality

microgravity environment.

3.5. SUMMARY

We report two major findings based on the first 8 months of data and microstructural analysis:

(1) The morphological stability of various crystallographic orientations is significantly affected by

the anisotropy of interface kinetics and surface energies. The effect was clearly demonstrated via postflight

microstructural analysis. The analysis showed that the growth of one orientation occurred in a plane-front

mode while the neighboring orientations exhibited a transition to a cellular mode. The most interesting

aspect of such a staggered breakdown was the relatively large extent to which the plane-front growth

continued in one orientation. It is clear that the anisotropy in interfacial properties of the faceted alloy in

general, and the interface kinetics in particular, play a major role in stabilizing plane-front growth. The

very significant interplay between the crystallography-dependent thermophysical properties (e.g., interface

kinetics coefficients, surface energy, and thermal conductivity) of the faceted alloy and effects associated

with any possible solute diffusion parallel to the interface (resulting from a staggered morphological

transition) thus warrant further experimental and theoretical investigation.

(2) The differential Seebeck signals allowed the monitoring of interfacial phenomena. While these

signals are extremely sensitive to the interaction between thermal gradients and local microstructural

details, the magnitude of kinetic undercooling could nevertheless be calculated from the decay in the

Seebeck signal at the end of solidification. Using this method, the kinetic undercooling was measured even

at very slow growth rates. We note that such measurements are not possible on Earth because of

thermosolutal convective effects.

3-56
3.6. REFERENCES

1. Coriell, S.R., and R.F. Sekerka, "The Effect of Anisotropy of Surface Tension and Interface

Kinetics on Morphological Stability," J. Crystal Growth, 34(2), 157, 1976.

2. Coriell, S.R., B.T. Murray, and A.A. Chernov, "Kinetic Self-Stabilization of a Stepped Interface:

Binary Alloy Solidification," J. Crystal Growth, 141,219, 1994.

3. Mullins, W.W., and R.F. Sekerka, "Stability of a Planar Interface During Solidification of a Dilute

Binary Alloy," J. Appl. Phys., 35, 444, 1964.

4. Young, G.W., S.H. Davis, and K. Brattkus, "Anisotropic Interface Kinetics and Tilted Cells in

Unidirectional Solidification," J. Crystal Growth, 83, 560, 1987.

5. Tiller, W.A., and J.W. Rutter, "The Effect of Growth Conditions Upon the Solidification of a

Binary Alloy," Can. J. Phys., 34, 96, 1956.

6. Trivedi, R., "Effects of Anisotropy Properties on Interface Pattern Formation," Appl. Mech. Rev.,

43(5), Part 2, $79, 1990.

7. Trivedi, R., V. Seetharaman, and M.A. Eshelman, "The Effects of Interface Kinetics Anisotropy on

the Growth Direction of Cellular Microstructures," Met. Trans. A, 22A, 585, 1991.

,

,

10.

11.

12.

13.

14.

Favier, J.J., "Etude des Cinetiques Cristallisation par Application de l'Effet Termoelectrique Analyse

de la Temperature d' une Interface de Solidification," Ph.D. Dissertation, Grenoble, 1977.

Alvarez, J., S.D. Peteves, and G.J. Abbaschian, "Thermoelectric Effects Across Solid-Liquid

Gallium Interface," Thin Solid Films and Interfaces, edited by J.E.E. Baglin et al., MRS Symp.

Proc. 25, 345, 1984.

Peteves, S.D., and G.J. Abbaschian, "Growth Kinetics of Faceted Solid-Liquid Interfaces and

Kinetic Roughening," J. Crystal Growth, 79, 775, 1986.

Abbaschian, R., A.B. Gokhale, J.J. Favier, G. Cambon, S.R. Coriell, H.C. de Groh III, and R.L.

DeWitt, "A Study of Directional Solidification of Faceted Bi-Sn Alloys in Microgravity," AIAA 95-

0608, Washington, DC, 1995.

Kobayashi, K.F., M. Kumikawa, and P.H. Shingu, "Growth Rate of Crystals of Pure Metals

Nucleated From the Undercooled Melt," J. Crystal Growth, 67, 85, 1984.

Alfintsev, G.A., and D.E. Ovsienko, "Study of the Growth Mechanism of Certain Metal Crystals

From the Melt," in Growth and Imperfections in Metallic Crystals, edited by D.E. Ovsienko,

Consultants Bureau, NY, p. 32, 1968.

Sixou, B., A. Rouzaud, and J.J. Favier, "Growth Kinetics in Ultra-Pure Bismuth Using a

Thermoelectric Method for Interface Temperature Measurements," J. Crystal Growth, 137, 605,

1994.

3-57
15.

16.

17.

°

.

Tiller, W.A., K.A. Jackson, J.W. Rutter, and B. Chalmers, "The Redistribution of Solute Atoms

During Solidification of Metals," Acta. Met., 1,428, 1953.

Niwa, K., M. Shimoji, S. Kado, Y. Watanabe, and T. Yokokawa, "Studies on Diffusion in Molten

Metals," J. of Metals, 91, 96, 1957.

Abbaschian, G.J., and R. Mehrabian, "Effect of Aluminum on the Solidification Kinetics and

Morphology of Gallium," J. Crystal Growth, 43, 433, 1978.

Publications/Presentations from the Current Research Program

Gokhale, A.B., R. Krishnasamy, and R. Abbaschian, "In situ Monitoring of Solid-Liquid Interfaces

During Crystal Growth," MRS Spring Meeting, San Francisco, CA, April 28, 1992.

de Groh, H.C. III, "A Numerical and Experimental Study of Natural Convection and Interface

Shape in Crystal Growth," First International Conference on Transport Phenomena in Processing,

1993.

. Gadonniex, D., A.B. Gokhale, and R. Abbaschian, "Morphological Stability of Faceted

Solid/Liquid Interfaces in Dilute Bi-Sn Alloys," AIAA 94-0791, 32nd Aerospace Sciences Meeting

& Exhibit, Reno, NV, January 10-13, 1994.

°

de Groh, H.C. III, and E.S. Nelson, "On Residual Acceleration During Space Experiments," in

Heat Transfer in Microgravity Systems, edited by S.S. Sadhal and A. Gopinath, HTD-290, pp. 23-

33, ASME, 1994.

5. Abbaschian, R., NASA Research Summaries: Microgravity Materials Science Conference,

Huntsville, AL, May 24-25, 1994.

6. Gokhale, A.B., "USMP-2 Preliminary Results," NASA-Lewis Research Center, Cleveland, OH,

July 5, 1994.

.

.

Abbaschian, R., A.B. Gokhale, J.J. Favier, S.R. Coriell, H.C. de Groh III, and R. DeWitt,

"Directional Solidification of Faceted Bi-Sn Alloys in Microgravity," AIAA 95-0608, 33rd

Aerospace Sciences Meeting and Exhibit, Reno, NV, January 9-12, 1995.

Yao, M., R. Raman, and H.C. de Groh III, "Numerical Modeling of Bridgman Growth in Space

with MEPHISTO," International Conference on Computational Engineering Science, Presented at

the 1995 ICES Meeting, July 30-August 3, 1995.

9. Gadonneix, D., "Morphological Stability of Faceted Bi-Sn Alloys During Directional Solidification,"

Master's Thesis, University of Florida, 1995.

10. Yeoh, G.H., G. de Vahl Davis, E. Leonardi, H.C. de Groh III, and M. Yao, "A Numerical and

Experimental Study of Natural Convection and Interface Shape by Crystal Growth," J. Crystal

Growth, accepted, 1995.

3-58
11. Yao,M., R. Raman,andH.C. deGroh III, "NumericalSimulationof Heatand MassTransport

DuringSpaceCrystalGrowthwith MEPHISTO,"NASA TM, in review, 1995.

3-59
.o

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_{ >: > > >" > > >" _ >"

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3-60
Table3-2. Summaryof Morphological Changes in USMP-2 Samples

A:

B:

BI:

CI:

C2:

C3:

C4:

Sample

Earth V1

Grown 1.85

_m/s

Quench A B

Seebeck A B

P61tier A B

Growth Conditions

V2 V3

3.38 6.71

/_m/s #rn/s

B B

B B

B B

V4 V5 V6

13.34 26.88 40.01

_m/s /_m/s /_m/s

B1 C1 C3

B C2 C4

B D D

Faceted cellular/dendritic with radially inward growth. Microstructure shown in figure 2.

Plane front. Columnar grains. Microstructure shown in figure 5.

Predominantly plane front. Columnar grains. One grain shows preferential cellular breakdown.

Two dominant grains. Transition of one grain to cellular mode after 2.54-mm growth at V5. Cellular

breakdown in the second grain occurs in a stepwise sequence, with the first breakdown ~0.18 mm

after the breakdown in the f'trst grain and the last breakdown 1.7 mm after that in the first grain. The

grain which breaks down first shows a cell tilt of 4 ° with respect to the heat flow direction and an

average cell spacing of 0.356 ram. The second grain shows a cell tilt close to zero and an average cell

spacing of 0.517 mm.

Two dominant grains. One grain breaks down -0.6 mm after growth at V5, the second grain

continues plane-front growth for an appreciable length of time (12.22 mm) before transitioning to a

cellular mode. The cells in the grain which breaks down first are tilted 6.5 ° with respect to the heat

flow direction, with an average cell spacing of 0.265 ram, while the cell tilt in the second grain is

close to zero, with an average cell spacing of 0.276 mm. Microstructure shown in figure 9.

Two dominant grains. The entire cross section shows a transition to a cellular growth mode almost

immediately after the start of growth at this velocity. The cells are tilted -4.5 ° with respect to the heat

flow direction, with an average cell spacing of 0.279 mm.

Two dominant grains. The entire cross section shows a transition to a cellular growth mode -3.2 mm

after the start of growth at this velocity. The cells are tilted -4.5 ° with respect to the heat flow

direction, with an average cell spacing of 0.179 mm. Microstructure shown in figure 11.

D: Under investigation.

3-61
Figure3-1. (a)MEPHISTOcoreassembly.

I

332 --

MOVING FURNACE

CLOSEST APPROACH

, ,; ' i

_ ,,:" , I IIII I I II RE_,_NCE i

tPEL,==--', ' ' ,, II II IIIIII I I

J' ,dlk

[RE:FRECK ,

1 50 100 149 ',,,

MOVING FURNACE PROCESSING

FARTHEST PosmoN ZONE

LIQUID

COLUMN

l ........... i

i

STATIONARY

INTERFACE

THERMOCOUPLES

T = 50°C

J

MELTING 4 P SOLIDIFICATION

i Texp

\

rf

Figure 3-1. (b) Arrangement of the experimental samples in MEPHISTO. Shows three rod-shaped

samples processed in parallel, the location of thermocouples, and the two solid/liquid (s/l)

interfaces from which a differential Seebeck electro motive force (emf) is measured. Also

shown is the overall temperature distribution in the apparatus during processing (schematic).

3-62
=

,0 =

l E :6. "_._=

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3-69
GROWTH DIRECTION

Figure 3-4. (c) Interface between the initial microstructure and microgravity solidification at a growth

velocity of 1.85/_m/s. Micrograph represents the bottom half of the sample cross section.

Note the emergence of two dominant orientations from the initial structure.

Figure 3-5.

300/t m

Plane front solidification at velocities ranging from 1.85 to 6.71 _um/s.

3-70
(a)

V5 =

V1

2,000/_m

(b)

Figure 3-6. (a) Schematic of the shape of an interface at the transition between V5 and V1. (b) Low

magnification micrograph of the interface. The interface is slightly curved at the edges due to

the surface tension forces at the solid/liquid/crucible triple junction.

3-71
Figure3-7.

V5 =

Vl

Detail of interface between V5 and V 1. Note the changes in grain boundary angles upon

decreasing the growth rate. Optical Micrograph, DIC illumination.

Figure 3-8.

140

120

100

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c

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40 "-

20

: AG-BiSn-31 Avg. (Interface)

V1

0 _, ........ i i .... t ....

0 200 400 600 800 1000

Distance in pm

Secondary Ion Mass Spectroscopic (SIMS) analysis of the V5/V 1 interface illustrating the

sharp change in average Sn content at the interface.

3-72
0.6 mm

(a)

• Tilt

12.22 mm

GROWTH DIRECTION

(b)

Grain 1

Grain 2

Breakdown 2,000/_ m

in Grain 2

Figure 3-9.

Note: the top grain is cellular while the bottom grain continues in a plane-front mode for a

significant distance (12.22 mm) before a transition to the cellular mode.

(a) Schematic representation of the features of a selective breakdown at a growth velocity of

26.6/_rn/s. (b) Microstructural appearance of the selective breakdown.

3-73
GROWTH DIRECTION

Note: the bottom grain is growing in the plane front mode while the top orientation is

growing in a cellular mode with cells tilted with respect to the heat flow direction.

Figure 3-10. Selective planar to cellular transition at a growth velocity of 26.88/_m/s.

3-74
(a)

4.5 ° Tilt

I I

3.2 mm

GROWTH DIRECTION

Breakdown Front

500 _m (b)

Figure 3-11.

(c)

!

(a) Schematic illustration showing the salient features of a cellular transition at a growth

velocity of 40 gm/s. (b) Low magnification micrograph of V6 breakdown region. (c)

Higher magnification view of cellular growth at V6.

3-75
(a) (b)

(c) (d)

Figure 3-12.

m

200/_ m

(a) Schematic of cellular growth, transverse section. Shows two dominant orientations. (b)

Optical micrograph (B.F.) illustrating the cell boundaries. (c) Optical micrograph (P.L.S.T.)

showing two dominant grains. (d) Optical micrograph (D.F.) showing cell boundaries.

3-76
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Furnace Position, mm

Figure 3-13. (a) Typical resistance tracking of interface position and measurement of furnace translation

(by linear potentiometer) as a function of processing time. Typical Seebeck measurement

cycle (solidification-stabilization-remelting-stabilization). (b) The hysterisis between

resistance change during solidification (bottom curve) and that during symmetrical remelting

(top curve).

3-77
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58 I

i

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Resistance Data, FFT Edited, Channel > 100 = 0

Figure 3-14.

55.5

20'00 40'00 60'00

Time, Seconds

BOO0

editing of resistance measurement during a typical Seebeck cycle. Note the removal

of superimposed noise so that fine slope changes during the resistance tracking can be

detected.

E

nAR = (dR s - dRm)

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0 500 1000 1500

Time, Seconds

2000

Figure 3-15. Subtraction of resistance change during solidification from that during symmetrical

remelting. FFT edited data. The existence of a negative plateau indicates interracial retardation during solidification.

3-78
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150

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time of quench. The subsequent decay is due to the slower melting of the portion of the

sample pushed into the hot zone during the quench. Also shown are furnace position and

outputs of quenching sample thermocouples.

3-79
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Figure 3-17. Output of short and long thermocouples in the quenching sample during a typical Seebeck

measurement. Also shown is the corresponding furnace movement.

3-80
(a)

(b)

L

Thermal Gradient Zone

(c)

E

_r

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.c}

co

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Gradient

Distance

Figure 3-18. Evolution of differential Seebeck emf due to microstructure/thermal gradient interaction. (a)

Initial and directional growth microstructure at two interfaces. (b) Thermal gradient zone

during directional solidification. (c) Evolution of the Seebeck emf during translation of a

thermal gradient.

3-81
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Time, minutes

IIIII

Figure 3-19. The processing sequence of microstructural/Seebeck emf evolution described in figures

20(a)-(e). Shows two melting cycles and two solidification cycles separated by stabilization

periods. Each step in the process is identified by a letter (a through e), corresponding to

figures 20 (a)-(e).

3-82
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4000 4200 4400 4600 4800 5000

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Figure 3-20. (a) Initial interface between the Earth-grown faceted cellular/dendritic microstructure and

liquid alloy. The interface is slightly curved at the solid/liquid/crucible triple junction. The

baseline Seebeck emf is steady with a mean value of-70 _V.

3-83
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0 100 200 300 400 500 600 700 800 900 1000

Time, seconds

Figure 3-20. (b) Directional 2-mm melting of the initial interface and the corresponding Seebeck signal.

3-84
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3000 4000

Time, seconds

Figure 3-20. (c) Directional 2-mm solidification resulting in plane-front growth with columnar grains and

the corresponding evolution of the Seebeck signal.

3-85
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150

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143

142

141

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5600 6000 6400

Time, seconds

E

E

0

0

fit.

0

cU_

Figure 3-20. (d) Directional 7-mm melting of the plane-front (2 mm) and cellular/dendritic (last 5 mm)

region and the corresponding evolution of the Seebeck signal.

3-86
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f

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142

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0 2000 4000 6000 8000 10000

Time, seconds

Figure 3-20. (e) Directional 7-mm solidification resulting in plane-front growth with columnar grain

structure. Also shown is the disappearance of unfavorable orientations as growth proceeds

and the corresponding evolution of the Seebeck signal.

3-87
3-88
SECTION IV.

GROWTH OF SOLID SOLUTION SINGLE CRYSTALS
Acknowledgments

Research was carried out in part at the National Synchrotron Light Source (NSLS), Brookhaven

National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials

Sciences and Division of Chemical Sciences. Many persons have contributed to this effort. These include

the AADSF Project Management team led by Fred Reeves, the Engineering team led by James Sledd, the

test team led by Michael Cole, and the Teledyne Brown Engineering team. Bill Aldrich was responsible for

much of the preliminary design of AADSF. Without their dedication and skilfful efforts there would be no

furnace. Strong collaboration from the Crystal Growth and Solidification Branch of Space Sciences

Laboratory upon the experimental effort included the magnetic field growth experiments conducted by Dale

Watring and Ching-Hua Su, growth experiments and characterization by Sharon Cobb, and skilfful glass

blowing by Mark Jones. Much of the preliminary crystal growth on this system was done by L.R.

Holland, J.C. Clayton, and M.C. Davidson at MSFC. Important thermophysical property measurements

have been made by D. Chandra, K. Mazuruk, and R.E. Taylor. Chris Coppens of the Structures and

Design Laboratory designed the ampoule, while Greg Jerman of Materials and Processes Laboratory made

the electron microprobe WDS measurements. Helga Alexander of Mevatec performed much of the optical

and scanning electron microscopy work, while Gene Nelson of A.G. Scientific Glass Company prepared

all the ampoules. Finally, we are indebted to Joel Kearns of NASA Headquarters for his encouragement

and interest during the project.

4-90
GROWTH OF SOLID SOLUTION SINGLE CRYSTALS

Sandor L. Lehoczky, Frank R. Szofran, Donald C. Gillies

Space Sciences Laboratory

NASA/Marshall Space Flight Center, Alabama 35812

Abstract

The solidification of a solid solution semiconductor having a wide separation between liquidus and

solidus has been extensively studied in ground-based, high magnetic field and Spacelab experiments. The

material being studied, mercury cadmium telluride with 80 mole percent of HgTe and 20 mole percent of

CdTe, is extremely difficult to grow by directional solidification on Earth due to high solutal and thermal

density differences which give rise to fluid flow and consequent loss of interface shape and composition.

Diffusion controlled growth is therefore impossible to achieve in conventional directional solidification.

The ground-based experiments consisted of growing crystals in several different configurations of heat

pipe furnaces, NASA's Advanced Automated Directional Solidification Furnace (AADSF), and a similar

furnace incorporated in a superconducting magnet capable of operating up to 5T. The flight experiment

took place during the flight of STS-62 in March 1994, with the AADSF installed on the second United

States Microgravity Payload (USMP-2). The alloy was solidified at 3/4 inch per day over a 9-day period,

and for the first time, a detailed evaluation was performed of residual acceleration effects.

The results of the ground-based experiments were compared with the crystal grown in orbit under

microgravity conditions. On Earth, it has been found that the application of the magnetic field leads to a

significant reduction in fluid flow, with improved homogeneity of composition. The field strength required

to suppress flow increases with diameter of the material and the 8-mm diameter sample used was less than

the upper limit for a 5T magnet.

The resulting material grown during the flight experiment was significantly different. One clear

difference is that there is no hydrostatic force pushing the material against the ampoule wall during the

solidification as is the case on Earth. During the mission the orbiter was maneuvered into several different

attitudes with the result that the residual acceleration vector caused by drag on the orbiter and the distance

of the AADSF from the center of gravity of the orbiter was aligned differently with respect to the growth

axis of the solidifying material for distinct parts of the mission. Significant differences were observed

during three long, but uninterrupted, periods at constant attitude. Compositional variations along the

crystal circumference indicate residual fluid flows for the least favorable vector orientations. Identifiable

regions exist in which a transverse vector has pushed the material against the ampoule wall and allowed it

to readily contract away from the opposite wall. Such surfaces showed etch pits produced by preferential

evaporation at defect sites. X-ray scattering showed that the regions pulled away from the wall tended to

be less strained or of higher quality material than the opposite surface, and considerably better than the

Earth-grown material. Composition determination on the surface of the material demonstrated significant

difference, dependent on the direction of the residual acceleration vector. These are clear indications of

three-dimensional fluid flow. A significant portion of the boule was grown with a component of the vector

aligned in a direction from liquid to solid. Synchrotron x-ray studies of this material showed it to be single

crystal and of much lower defect density than found in ground-based growths.

4.1. INTRODUCTION

4-91
The major objective of this research is to establish the limitations imposed by gravity during

growth on the quality of bulk, solid-solution, semiconducting crystals having a large separation between

their liquidus and solidus temperatures. An important goal is to explore the possible advantages of growth

in the absence of gravity. The alloy system being investigated is Hgl_xCdxTe with x-values appropriate

for infrared detector applications in the 8-14 btm region. Both melt and Te-solvent growth methods as well

as growth in magnetic fields are being considered. The study consists of flight experimentation and

ground-based experimental and theoretical work needed to establish material properties and optimum experimental parameters for the on-going flight experiments, and to assist material evaluation. Hgl_xCdxTe

is representative of several II-VI alloys which have electrical and optical properties that can be

compositionally tuned to meet a wide range of technological applications in the areas of sensors and lasers

with applications to optical computing and communications as well as the national defense. The following

specific objectives are deemed essential for accomplishing the primary objectives of the investigation: By

means of ground laboratory and flight experiments and concurrent theoretical analysis, establish

quantitative correlation between growth parameters (translation rates, thermal fields, alloy compositions,

and sample geometries) and growth interface shapes during the melt growth of Hgl_xCdxTe alloy crystals

by unidirectional solidification; establish correlation between growth interface shapes and microstructural

characteristics; develop theoretical models to delineate the effects of segregation, diffusion, and fluid flow

in two dimensions on sample homogeneity and the solid-liquid interface shape during the melt growth of

Hgi_xCdxTe alloy crystals and thus establish a fundamental understanding of the growth process;

ascertain the relative effectiveness of ground-based stabilizing techniques (e.g., applied magnetic fields)

for suppressing convective flow during melt growth of the crystals; establish ternary phase-equilibrium

parameters for selected regions of the Hg-Cd-Te constitutional phase diagram required to model the Tesolvent growth process; by means of growth experiments and theoretical analysis, establish quantitative

correlation between growth parameters (growth rates, thermal fields, alloy compositions, and sample

geometries), growth interface shapes, and microstructural characteristics for various Te-solvent growth

processes; establish relationships between electrical characteristics and densities of charged and neutral

defects for various growth conditions and thus correlate processing variables with the optical response of

the processed alloy; evaluate the possible benefits of microgravity processing for device applications.

The investigation (Experiment No. MPS77F069) was originally selected based on the response to

NASA OA-77-3 Announcement of Opportunity. The content of the current effort is based on an amended

proposal submitted in response to Announcement of Opportunity OSTA-77-3 (formerly OA-77-3). Some

of the initial work was done at McDonnell Douglas Research Laboratories under contract NAS8-33107.1-7

The investigation is consistent with the committee recommendations resulting from the "Review of the

Microgravity Science and Applications Flight Programs" conducted January-March of 1987. The majority

of the ground-based studies are being performed in the Space Sciences Laboratory at the George C.

Marshall Space Flight Center (MSFC). 8-57 In addition to crystal growth and characterization, this work

has included thermophysical property determination, thermal modeling, phase diagram determination, fluid

flow modeling, transient and diffusion analysis, and electronic property modeling. The flight portion of

the investigation is being conducted using the AADSF developed by the MSFC and manifested for flights

on the USMP series of missions. The In'st flight of the instrument took place in March 1994 on STS-62. 58

4-92
4.2. STUDY RATIONALE

Segregation during solidification, high volatility of one of the components (Hg), and strain fields

associated with large temperature gradients make the preparation of homogeneous, high-quality, bulk

crystals of the alloys an extremely challenging problem. The usual melt or solvent growth methods tend to

yield crystals with significant alloy compositional variations, and second phase inclusions, e.g.,

Te-precipitates which we believe are at least partially caused by gravity-induced, thermosolutally-driven

flows just ahead of the growth interface.

The reduction of the gravity-induced convection in a microgravity environment is expected to be

advantageous for maintaining the crystal-melt interface shape required to minimize the densities of crystal

defects while minimizing compositional variation transverse to the crystal growth direction. It is believed

that CdTe, Hgl_xCdxTe, etc. probably possess extremely small yield strengths near their growth

temperatures. If this is the case, the high dislocation density (-10 5 cm -2) usually seen in these crystals

could be due at least in part, to stresses induced by the sample's own weight, that is, self-induced stresses.

Therefore, a second goal of these experiments is to assess the validity of this hypothesis.

4.3. DRIVING FORCE FOR CONVECTION

As noted earlier, both melt and Se- and Te-solvent growth are subjects of the proposed investigation.

Because of the relative maturity of the ground-based, melt-growth method compared with solvent growth,

the unseeded melt growth of Hgo.sCd0.2Te alloys by Bridgman-Stockbarger-type directional solidification

is the primary method selected for the initial USMP flight experiments. On Earth the Hg-rich component

rejected during solidification is more dense than the original melt and the vertical Bridgman-Stockbarger

growth process would appear to be both gravitationally and thermally stable against convection. However,

this is not generally true. Due to the peculiar relationships between the thermal conductivities of the melt,

solid, and ampoule, it is not practicable to completely avoid radial temperature gradients in the growth

region in these alloys. Because of the high Hg partial vapor pressures involved at the processing

temperatures, the confinement of the alloys requires the use of very thick fused silica ampoules which have

thermal conductivities comparable to those of the alloys. This, when combined with the large (a factor of 4

to 10)decrease in the thermal conductivities of Hg alloys upon freezing, leads to isothermal surfaces near

the melt/solid interface that are bowed into the solid. Although the interface under this condition is neither

an isothermal nor an isocompositional surface, it is bowed in the same direction as the isotherms near it.

We have developed a method that relies on a careful control of radiation heat transfer near the growth

interface to minimize the effect for this type of growth system. 26 Nevertheless, the wide separation

between the liquidus and solidus boundaries of each of the pseudobinary phase diagrams (by causing the

growth interface temperature to undergo large changes during growth and yielding growth rate dependent

thermophysical properties in the melt just ahead of the interface) makes a complete elimination of the radial

temperature gradients in the vicinity of the interface nearly impossible. Thus, in spite of the stabilizing

influence of the solutal density gradients, intense thermally-driven fluid flows will occur in a narrow

region near the interface that will control the extent of radial compositional-segregation in a gravitational

environment. 59

4-93
4.4. PREVIOUS RESULTS FOR COMPOSITIONAL REDISTRIBUTION

Over the past several years, we have performed an extensive evaluation of the effects of growth

parameters on the axial and radial compositional uniformity in directionally solidified HgCdTe alloys. 1 2 5

16 21 26 27 29 37 To obtain a measure of the relative importance of the various heat and mass transfer

processes involved, detailed radial and axial composition mappings have been obtained for many ingots.

For most cases, the compositional variation along the growth axis could be fit well by a set of

one-dimensional diffusion equations, indicating that solute transport to the interface region during growth

tends to be dominated by mass diffusion and that the vertical extent of the convective region above the

crystal growth interface has to be small compared to the effective diffusion lengths. On the other hand, the

extent of the typical measured radial segregation for practical growth rates (>0.1-0.4 #rn/s) was found to

be significantly higher and at low growth rates (<0.1 _m/s) significantly lower than would be expected

from diffusion-limited lateral segregation alone, indicating that the crystal compositional distributions

transverse to the growth axis are primarily determined by the nature of the thermally-driven fluid flows in

the vicinity of the growth interface. This contention is strongly supported by the results of a linear-stability

type analysis which indicates that the results are compatible with a transition from steady to time-dependent

flows as the growth rates and thus the stabilizing composition gradients are reduced. In agreement with the

conclusions suggested by the axial composition distribution data, such an analysis also suggests that the

axial extent of the flow regions should be about a factor of 10 smaller than the estimated dimension of the

effective diffusion lengths involved. Recent detailed numerical analysis for the HgCdTe system by D.H.

Kim and R.A. Brown provides an excellent theoretical description of the overall heat and mass transport

phenomenon and supports the main contentions of the proposition. 59 60 Growth experiments performed in

transverse and axial magnetic fields up to 0.6 and 5T, respectively, suggest strong fluid flow effects on

radial compositional segregation.51 61

4.5. PROBLEMS ASSOCIATED WITH GROWTH INTERFACE SHAPE CONTROL

Frequently, the desirable surface geometry for crystal growth is planar or nearly so. Usually,

however, the optimum interface shapes tend to be those that bow slightly into the melt, because such

interface shapes favor grain selection and the outgrowth of extended line defects. These shapes are

expected to produce ingots with better crystal perfection. While such shapes are difficult to obtain for these

alloys, they may be achievable with judiciously chosen thermal boundary conditions. Under the influence

of the stable flow conditions during fast growth, such interface geometries exacerbate lateral alloy

segregation because of the tendency of the more dense Hg-rich liquid to settle at portions of the surface

having the lowest gravitational potential. Due to the decrease of the alloy solidus temperatures with

increased Hg-concentration, the temperature at this portion of the interface will be lowered, causing an

increase in the interface curvature. Thus, a potential interface instability can result from the "settling" of the

rejected Hg-rich solvent into the lowest-lying regions of the interface. Lateral diffusion and incomplete

convective mixing will have the tendency to drive the interface melt compositions to some equilibrium

value, but most ground-based, melt-growth experiments involving the alloys show large radial

compositional variations that are probably a direct consequence of such an interfacial fluid flow

phenomenon. Although the growth at very slow rates, under the influence of reduced stabilizing

composition gradients and thus nonsteady flow conditions, tends to yield radically more homogeneous

ingots, generally, no ingots could be obtained by the Bridgman-Stockbarger method that were simultaneously radially and axially homogeneous for any substantial length.

4-94
4.6. ADVANTAGES OF MICROGRAVITY GROWTH

In microgravity it is expected that the highly desired, slightly convex growth surfaces will be easier

to maintain because of the reduced tendency for stratification of the denser (Hg-rich) fluid components at

the higher growth rates that are usually needed to obtain solute conserving steady-state growth and thus

axial compositional uniformity. At the same time, the near-elimination of transverse temperature

gradient-driven incomplete mixing effects is expected to provide for a better control of the lateral

compositional distribution in the melts and stresses resulting from hydrostatic pressure will be nearly

eliminated. We thus expect that by growing under the influence of low-gravity conditions, crystals with

significantly improved crystallinity and compositional homogeneity can be prepared as compared to the

best crystals that can be produced on Earth. It is also reasonable to expect that careful characterization of

both the space- and ground-growth materials will lead to better insights into the peculiarities of the various

growth mechanisms, permitting improvements in Earth-based processing of semiconductor alloy systems.

4.7. GROUND-BASED RESULTS

Over the past several years, a detailed evaluation has been performed on the effects of growth

parameters on the axial and radial compositional uniformity, defect density, and optical properties in

directionally solidified Hgl_xCdxTe and other similar compounds and pseudobinary alloys. A series of

Hgl_xCdxTe alloy ingots (0<x<0.6) has been grown from pseudobinary melts by vertical BridgmanStockbarger type heat-pipe furnace assembly using a wide range of growth rates and thermal conditions.

Several of the experiments were performed in transverse and axial magnetic fields of up to 5T. 61 The

effect of an axial field of 5T on the radial segregation is shown in figure 1. On the left is shown the

superconducting magnet; the furnace resides in the central bore. On the right are shown the radial

composition profiles. The two experiments were done in the same furnace with the same temperature

settings and translation rates. 26 27 Precision measurements were performed on the ingots to establish

compositional distributions and defect density distributions.

Correlation between growth rates and thermal conditions and growth interface shapes have been

established for the alloy system. To assist in the interpretation of the results and the selection of optimum

in-flight growth parameters, the pseudobinary phase diagram (0<x<l), 3 liquid and thermal diffusivities

(0<x<0.3), 19 melt viscosity, 55 and the specific volumes as a function of temperature (0<x<0.15) have

been measured. 20 From these measurements and other available data, the heat capacity, enthalpy of

mixing, and the thermal conductivity of pseudobinary melts have been calculated using a regular associated

solution model for the liquid phase.

A one-dimensional diffusion model that treats the variation of the interface temperature, interface

segregation coefficient, and growth velocity has been used to establish effective diffusion constants for the

alloy system. 14 Theoretical models have been developed for the temperature distribution and the axial and

radial compositional redistribution during directional solidification of the alloys. 10 11 59 60 These were

used along with the experimental results to select the parameters for the first flight experiment flown on the

USMP-2 mission.

A microscopic model for the calculation of point-defect energies, charge-carrier concentrations,

Fermi energy, and conduction-electron mobility as functions of x, temperature, and both ionized and

neutral defect densities has been developed. 48 52-54 For selected samples, measurements were performed

of electron concentration and mobility from 10-300 K. The experimental data were in reasonably good

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agreementwith theoryand were successfullyanalyzedto obtaindonor and acceptorconcentrationsfor

variousprocessingconditions.

4.8. FLIGHT RESULTS

A five-zone Bridgman-Stockbarger-type furnace system designated as the AADSF has been

designed and developed for operating in the cargo bay of the Space Transportation System (STS). Over 15

growth experiments were performed in the development and ground-based versions of the AADSF to

establish optimum operating parameters and procedures for the initial flight experiment. Three other crystal

growth experiments were made part of the ground-testing program of the flight unit. The experiment was

successfully flown on the STS-62 mission as part of the USMP-2 mission in March 1994. An

approximately 16-cm long and 0.7-cm diameter Hg0.8Cd0.2Te alloy crystal was grown at a rate of 0.79-

mm/hour over a period of approximately 11 days. Following the growth on the USMP-2 mission, an exact

duplication of the experiment was conducted on the ground to provide a "ground truth" equivalent sample.

Characterization of these two boules is in progress. Certain aspects of the characterization of the surface of

the boules have been substantially completed, and much other work is in progress. The following

paragraphs will describe the status of the characterization as of May 1, 1995.

4.9. FLIGHT TIMELINE

In order to optimize science results for all the experiments on the STS-62 mission, certain

compromises were accepted with regard to the operation of the AADSF. First of all, the furnace was

positioned some 13 feet in front of the center of gravity of the orbiter, 3 feet starboard of the center of

gravity, and almost 4 feet above the center of gravity. This factor resulted in there being little or no

possibility of avoiding a residual gravity vector acting parallel to the growth interface. While the most

favorable attitude for flying the orbiter could be calculated, other mission-related issues affected the final

decision. Such issues included the disposition of other experiments, and their desired residual gravity

vectors, communications with other experiments, and orbiter safety-related issues including navigation and

thermal aspects. Nevertheless, a substantial period of time was negotiated during which there were no

orbiter maneuvers, and during which crystal growth could be initiated and continued well into the

presumed "steady-state" region. The attitude during the AADSF operation of the mission remained faithful

to the pre-mission timeline, and 153.5 contiguous hours within a -ZLV/+YVV of the orbiter attitude were

available. Sample translation occurred during some 147 hours, and actual crystal growth was estimated to

take up some 140 hours of this time frame. The remainder of the crystal growth occurred during

approximately 28 hours at -XLV/-ZVV and 56 hours at -XLV/+ZVV. The final centimeter and a half (as

calculated) grew during a period after the end of the dedicated microgravity part of the mission with a very

noisy gravity environment and several changes of attitude. An estimated compositional profile based on

diffusion controlled growth t4 and a supercooled region of 1.7 cm is shown in figure 2. The graph is

plotted against distance grown on a linear scale, with superimposed mission elapsed time (MET) adjusted

to allow for changes in growth rate during the experiment. A decision was made during the mission to

increase the translation velocity of the sample and hence force breakdown of the interface and thereby

insert a time mark on the crystal. This marking was intended to demonstrate both position and shape of the

interface and also to permit the correlation between crystal growth and mission activities to be determined.

The chosen time for the "interface demarcation" was 4 hours before the first attitude change. The

translation velocity was increased from a nominal value of 0.79 mm/hour to 10 mm/hour for 10 minutes

and then returned to 0.79 mm/hour for the remainder of the mission. Figure 2 also shows the calculated

effect of this change on the composition, again assuming diffusion control. It was anticipated that a 4-hour

time period would be sufficient for the sample to re-establish the same conditions that had prevailed prior

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to thechangebeingmade.Thesametimelinewas usedin the groundtruth, with the furnacehot zone

maintainedontop.

4.10. RESIDUAL ACCELERATION RESULTS

Acceleration data of importance are now available, specifically the Orbital Atmospheric Research

Experiment (OARE) data taken during the mission. 62 OARE gives the quasi-steady component of the

gravity vector at the instrument location which is close to and below the center of gravity of the orbiter.

Figure 3 shows the z-component at the instrument for periods at the end of the AADSF primary attitude

and through the change-over period to the second attitude. These data have been "transformed" to the

center of gravity and then to the AADSF location. Of specific interest here is the degree of noise difference

between the activity period and the crew sleep period. While most of the activity is believed to be in the

high frequency range, the location of the change is significant, and will affect the characterization strategy

as will be reported later. Figures 4 and 5 taken from the report of Rogers and DeLombard 62 show a

representative period of the -ZLV/+YVV attitude. In figure 5, the data have been transformed to the

location of the crystal interface in the AADSF. To simplify some of the analysis, the orbital oscillations

which are clearly seen in the X-Axis data are ignored and approximate steady-state components have been

tabulated in table 1. These data are for the three primary attitudes and were calculated by Kim Destro-Sidik

of Tal-Cut Company. Figure 6 is a schematic representation of the three main attitudes with the position of

the AADSF with its hot and cold zones superimposed. The residual acceleration data from table 1 have

been included.

The next step is to correlate such data with cross-sections of the crystal. Figure 7 shows the

locations of the residual acceleration vectors relative to the crystal cross-section for the main primary

attitudes. Three features are of note here: First, only during the first attitude is the total resolved value less

that 1 _tg. Second, the vector points from the cold to the hot end of the furnace in the In-st and third

attitudes, thus giving rise to an unstable convective situation liable to lead to fluid flow. The -ZVV/-XLV

attitude has a small residual acceleration directed from liquid to solid and is thus stabilizing. Finally, the

transverse vector across the solidifying interface is in all three cases lower than the perpendicular vector.

This is also conducive to fluid flow. These parameters are mainly the result of the furnace being located

some 13 feet aft of the center of gravity of the orbiter with a resulting acceleration component always

driving in that direction, irrespective of drag contributions from the orbiter attitude. The significance of

these residual components and the manner in which they affected the sample features will be discussed

later in the characterization section.

4.11. RADIOGRAPHY

The In'st post-landing activity of characterization was to extract the cartridge from the furnace,

photograph, and inspect for damage. The cartridge was in excellent shape and typical of all successful

prior ground growths. A key issue here was the marking of the cartridge to ensure that the direction

relative to the furnace frame and hence the orbiter frame was well established. This was done by

identifying and then marking on the cartridge with a felt-tip pen the J7 position of the furnace, which

corresponded to the forward direction of the orbiter. On-line radiography at Kennedy Space Center using a

microfocus tube and an image intensifier was used to examine the cartridge. The results were recorded on

videotape. The main result was to confirm that the ampoule had not breached and that a single large boule

of material had been produced. Closer examination of the video revealed that there were positions on the

side of the boule, particularly near the end, where the surface was irregular. The other significant

observation was that, on rotation of the cartridge in the x-ray beam, one side of the material at the very last

to freeze showed a 180 ° contact with the wall, while the opposite face showed a 120 ° contact angle.

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Ground-basedresultsnormallyshowanglesof just over90°. The 120° contactangleis mostpronounced

at andaroundtheJ7-45° position.Theseobservationswerelaterconfn'medwith film at MarshallSpace

Flight Center.A radiographshowingtheflight sampleandthegroundtruth is shown in figure 8. It is

believedthattheasymmetryof theendof thesampleis causedby atransverseresidualaccelerationvector.

This doesnot, however,correlatewith the anticipatedvectorfrom the -XLV/+ZVV attitudeshown in

figure7,butthefinal solidificationtakesplacebeyondthemicrogravitypartof themission,andtheexact

correlationof orbital maneuversto crystal growth position is not exactly known due to the solidus

temperaturechangeassociatedwith thefinal transientregionandthethermalendeffectsof thecartridge.

4.12. PHOTOGRAPHY

During this stage of the characterization, all aspects of the removal of the cartridge from the

furnace, the cutting open of the cartridge, and the removal of the ampoule and the boule itself following the

dissolution of the fused silica in hydrofluoric acid were photographed. The cartridges, after processing,

are shown in figure 9. Due to the attitude changes during the mission and the presumed presence of a

residual acceleration component across the liquid-solid interface, radial asymmetry of the sample was

anticipated and all photography was repeated at several locations around the ampoule and boule. Eight sets

of photographs were taken at positions 45 ° apart with J7 as the reference point. The surface features

changed both along the boule and also as one went around the boule at the same distance from the tip. A

typical example is shown in figure 10. As on most of the orientations, the fu'st 70 mm are smooth, and

beyond 110 mm the surface is often rough. This is the J7 position and is fairly smooth throughout. At

certain orientations there is a roughness associated with the solidifying material not being forced against

the edge of the ampoule. A typical rough region is visible between 3 and 6 cm along the surface at about -

45 ° or just blow the center line of the boule as we observe it. Microscopy later confirmed the presence of

thermal etch pits as one of the causes of such a surface. It should be noted that ground-based growths of

this material invariably result in smooth, shiny surfaces, due to the hydrostatic force pushing the liquid

against the ampoule wall during solidification. An interesting feature which was seen on the boule surface

prior to dissolving away the ampoule was the presence of a thin ring approximately 11 cm from the tip of

the ampoule. The ring passes completely around the boule and its presence was confu_ed after the silica

was removed. This ring is clearly shown in figure 10. Later analysis demonstrated that the position of the

ring coincided with the change of attitude of the orbiter from -ZVV/-XLV to +ZVV/-XLV. The degree of

smoothness at these various orientations is shown in table 2. In all ground-based experiments, the surface

of the boule is smooth, with the exceptions of the occasional place where the material sticks and reacts

with the ampoule wall. The table shows that the region between 9 and 11 cm is smooth completely round

the boule; this is an indication of the presence of a downward force on the solidifying liquid and is similar

to the hydrostatic force due to gravity as present in the ground-based work. This correlates well with the

data shown in figure 7. The correlation is also good in the first part of the material to freeze, where the

region near J7+180 ° has the smoothest surface. The rough surface close to J7 is due to the material being

pulled from the wall by the residual acceleration as shown in figure 7. The nominal attitude for the crystal

region in the range 11-15 cm was subjected to many disturbances. For example, the orbiter robot arm was

in operation at day 8, hour 1 and extra vernier firings were deliberately made from day 8, hour 8 onwards.

These may have contributed to the continuation of the banding starting at 11 cm and shown in figure 14.

4.13. X-RAY DIFFRACTION

Several options were considered prior to initiating surface characterization of the sample. The

simplest techniques for observing grain structure and degree of crystallinity is by sand blasting the surface.

This procedure, however, leads to considerable structural crystallographic damage to the surface, the

removal of uneven amounts of material, and the destruction of surface effects. As has been shown in the

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photographicsection,the surfacestructureof the as-grownboulevariesconsiderablyfrom locationto

location;probablythisis theresultof themicrogravityenvironment.It is importantto retainsuchfeatures

for as long during the characterization phase as possible. A second possible preparation technique is to

etch the surface with a bromine solution. This is much more controlled and, depending on time and

concentration, can remove as little as l_tm/minute. The benefits are that the surface can be cleaned

considerably, but delicate features such as etch pits may be removed. To reveal grain structure requires a

considerable length of time in such an etch, and may be orientation dependent. The crystal structure,

however, is not adversely disturbed by this technique. The third option is to use a strong acid etch such as

Polisar or Straughan' s reagent. This will clearly and rapidly reveal the grain structure and will not damage

the crystallographic structure. The surface features, however, will be removed and the chemical reaction is

dependent on both orientation and defect status. Such an acid etch produces etch pits in certain preferential

directions and an oxidized surface in others. It is clear from the above rationale that grain structure of the

surface cannot readily be revealed nondestructively.

One way out of this dilemma is to evaluate nondestructively the crystallinity and to find out where

grains were and what orientation they were by taking x-ray back reflection Laue photographs. Some 60

patterns were taken on four surfaces at 90 ° to each other and at 0.5-mm intervals. There were significant

differences between the four surfaces examined. The sample was mounted within a machined groove and

attached with carbon paste to a graphite block. This arrangement was found to be suitable for x-ray

diffraction in one direction, and for energy dispersion x-ray spectrometry (EDS) in a scanning electron

microscope (SEM) at 90 °, thus reducing the handling involved. The Laues showed that there was a long,

single crystal stretching along the J7 surface from 35-90 mm. This would indicate that the crystal survived

the interface marking translation rate increase which would have been at about 85 mm. As noted above this

surface was rough, and the solidifying material was probably not being pressed strongly against the wall

of the ampoule during the solidification. In contrast, the crystallographic orientation at J7+180 ° was

changing every cm and the surface was smooth.

In order to get a quick estimate of the quality of the material, the collimator of the Laue camera was

removed for some of the photographs; this produces a beam approximately 3 mm across and while it is by

no means a parallel arrangement, it does simulate a topograph and does give an idea of crystal quality.

Figure 11 shows the Laue patterns at a position 73 mm from the tip on the J7-90 ° surface. This pair were

chosen at random, and it can be seen that the Laue pattern with the collimator in place (figure 11 a) is close

to [111] orientation, and with spots which are sharp, but sprit in places. This would indicate a structure

with low angle boundaries. The accompanying topograph (figure l lb) shows well-defined spots with

structural features within, indicative of a good but not perfect crystalline structure. Reflection topographs

were recently taken at the National Synchroton Light Source (NSLS) facility in Brookhaven, New York,

in collaboration with Dr. Michael Dudley of the State University of New York (SUNY)-Stony Brook. The

preliminary results will be reported in a subsequent paragraph.

These Laue pattems give the orientation of the crystal normal to the surface, but do not directly

reveal the preferred direction of crystal growth. This can be done by transposing the normal plane through

90 ° towards the crystal growth axis. The procedure is as follows: First the normal to the surface is found

by indexing the photograph with conventional techniques. If three known index planes which lie in

different zones can be identified, then the angles made by these planes to the center will enable one to

determine the (hckclc) for the center of the film. By repeating the procedure for an arbitrary position on the

line joining hkl to the direction of crystal growth, a second plane can be found, and hence the zone [uvw],

which includes both the normal to the surface and the direction of crystal growth. The precision of the

initial measurement is estimated to be about 0.5 °, and the transformed orientation precision is

approximately 4 °. One can then determine the direction of crystal growth by determining the plane which

makes an angle of 90 ° with (hckclc), and obeys the Weiss zone law for [uvw]. The resulting transformed

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orientationsareshownin table3.As cuttingof theboulehasproceeded,theorientationof thedirectionsof

crystalgrowth canbemeasureddirectlyfrom the Lauepatterns.A cross-sectionof grainsin the flight

sample(MCT27-Q)takenwith thebackscatteredsignalin the SEMis shownin figure 12, togetherwith

theorientationof twoof thelargestcrystals.

4.14. SCANNING ELECTRON MICROSCOPY (SEM)

The central part of the sample surface has been examined by SEM. Due to the size of the chamber

and the movement of the stage, it is impossible to reach the first and last 3 cm of the sample without

cutting the boule. As mentioned above, the surface features varied considerably and several hundred

photographs have been taken. They included what are believed to be thermal etch pits (figures 13a and

13b) and several featureless areas. The etch pits change shape as one moves around the circumference of

the sample, indicating the effect of orientation. Different grains can be readily recognized by an abrupt

change in shape and orientation of the etch pit triangle. The circumferential mark at 11 cm is clearly visible

as a brightly emitting band some 100 _um wide. Several other light bands were visible, but these were

much narrower and were curved, and beyond the 11 cm mark. This part of the surface is shown in figure

14. These bands are also indications of interface breakdown as will be shown in the EDS section of this

report.

4.15. ENERGY DISPERSION SPECTROMETRY (EDS)

In order to determine the compositional uniformity of the boule, EDS analysis in the SEM was

employed. As a preliminary nondestructive evaluation was needed, the as-grown surface of the boule was

examined first. While the results are not of optimum precision due to the fact that the surface is not fiat,

this is the most suitable chemical anaylsis technique known to us. Wavelength dispersion would give

better results, but to justify its use one would need to examine a polished surface. Standards of elemental

cadmium and mercury telluride were employed. Excitation energy of 10 kV was used with a take off angle

of 35 °, a working distance of 25 mm, and the L lines for Cd and Te and the M line for Hg were analyzed.

Initially, separations of 5 mm were used, with better resolution employed when justified by the data. Two

surfaces of the ground truth have been examined, and five of the flight sample. In addition, the flight

sample was rotated about the growth axis within the SEM to obtain some area, as opposed to linear

measurements.

The results are shown in figures 15-20. Figure 15 shows the ground-truth sample surface

compositional values at the J7+90 ° position. The results are similar to those previously obtained in

ground-based growth of mercury cadmium telluride and reflect primarily diffusion limited axial mass

transport. As this is the surface analysis only and the interior of the sample has a lower cadmium telluride

fraction, the curve is higher than normally seen with averaged density results. Of significance is the effect

of the rate change, which is at 83 mm from the tip of the ampoule. Clearly, the sudden increase in

translation velocity has forced the solidification of mercury-rich material which has not been replaced. The

reversion back to the original velocity has then led to the opposite effect with cadmium-rich material being

solidified. The same effect at the same position was found in the other surface examined. The increase in

translation velocity for a short period has been successful in marking the location of the solidification

front.

Figures 16-19 show the composition on the four "primary" surfaces of the flight sample. Figure

16 is the J7 position and shows the sharpest drop in composition. The behavior of the other three surfaces

is similar, but with some significant differences. In all cases the initial transient region is more shallow

than on the ground truth samples. Thus, the curve for flight grown material would seem to indicate a lower

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effective mass transport coefficient. Preliminary fitting of the data to diffusion controlled growth

conditionsindicatesacoefficientof lxl0-4cm2/s,asopposedto avalueof 5.5x10-5cm2/snormallyfound

ontheground.Anothersignificantdifferenceis thatthefour surfacesaredifferentfrom eachother,thus

supportinganotionthatresidualfluid flow is takingplace.For example,theJ7+90° curvelies abovethat

for J7,theeffectof theratechangeis muchless,andthereversionto theoriginalrateresultsin a higher

CdTepeak.Whilenot showingonall thesurfaceresults,thelight bandat 11cmnotedaboveprovedto be

mercurytelluride,this beinga furtherindicationof interfacebreakdown.The otherlight, narrow bands

wereall HgTerich. EDS,particularlywith unpolishedsurfaces,givesmuchscatterin theresults,but the

readingsbetweentheratechangeregionandthe11-cmmarkaremorewidespreadthannormal, andmore

thanthenormalscatter.This regionevidentlycontainsmorevariationsin compositionthantheremainder

of theboule. Anotherfeatureisthattheshapeof thefinal"transient"is notthatnormallyseenin this alloy

in ground-basedgrowth, the curvebeing the oppositedirectionto that which one would expectfor

diffusion-controlledconditions.Theseregionsarebeingexaminednow that the samplehas beencut.

Figure20 showstwo-dimensionalcompositionalanalysiscloseto theratechange.Theresultsshow the

spreadof theratechange,but arenot of sufficientfidelity to demonstratethe subtletrendsthatmightbe

associatedwith changesof fluid flow.

4.16. CUTTING STRATEGY

The cutting strategy is crucial for optimizing characterization and obtaining maximum science

return. Among the factors to be examined are the following: Compositional variations on a threedimensional scale are required to determine the presence or absence of nonaxial rotational fluid flow

caused by transverse residual acceleration vectors. [111] oriented slices are needed to examine defect

structure by etch pit measurement, double axis diffractometry and for some x-ray synchrotron topography

techniques. Finally, it is desirable to retain areas of material of reasonable homogeneity for the preparation

of electrical test pieces. Such samples are not necessarily compatible with each other and so much

precutting measurements have to be made. In both the ground truth and the flight sample tests, wafers

were cut perpendicular to the growth direction and at a location just prior to the start of the rate change.

These wafers were lapped and chemomechanically polished in 2 percent bromine-ethylene glycol. EDS

readings were taken and later high precision, high spatial resolution wavelength dispersion spectrometry

(WDS) readings also. These latter will be described in a later section of this report. During the preparation

of these sections, careful monitoring of the J7 direction was maintained so that nonsymmetric analysis and

presumed asymmetric fluid flow could be correlated with orbiter attitude and any residual acceleration

vector. The cartridge was initially cut through with a wire saw to mark the fused silica ampoule, and the

ampoule was cut after removal from the cartridge. The first cut was made into the glass at a location where

there was no material, and corresponded to the J7 position of the furnace. The second cut had to penetrate

the material and was made in the surface of the boule through the glass into the last-to-freeze portion. This

position was deemed least destructive from the point of view of science return. A small cut was also made

parallel to the growth direction close to the first-to-freeze end so that both ends would be marked after

initial cuts were made. The surface features had already given some indication of the possibility of

asymmetric fluid flow (see table 2). WDS results, however, showed no obvious asymmetry, and so the

cutting strategy was to rely on the OARE calculated data shown in figure 7. The rough area seen in figure

10 is close to the OARE data. The cut was made at -16 ° with respect to J7 or along the resolved transverse

vector calculated from the OARE data. The ground-truth sample was sectioned down the middle through

the rate change region and EDS measurements made as shown in figure 21. The resolution was not good

enough to determine exactly across the entire sample the position of the rate change, and the readings will

be repeated with WDS measurements.

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4.17. OPTICAL MICROSCOPY

This sectioned sample from the ground-truth growth was etched in Polisar II etch to examine for

granularity and etch pits. The structure is shown in figure 22, and it is of interest that there is no

breakdown of the crystallinity during the rate change. In fact there is no metallographic evidence of the

interface position during the rate change. A comparison of this structure with the compositional variation

shown in figure 21 indicates only roughly the history of the solidification and more solid conclusions will

be drawn when the WDS results are available. This contrasts with the surface analysis (figure 15) on

which the start of the rapid translation had an immediate and obvious effect. Similar cross-sections will be

made in the two other primary attitudes of the orbiter. The material is prepared for structural evaluation by

mounting the boule on a goniometer and then taking orientation photographs with the back reflection x-ray

Laue technique. The feedback from the Laue photograph enables adjustments to be made on the

goniometer so that the desired [111 ] orientation can be obtained. This has been done for one grain on the

flight sample adjacent to the region from which the first wafer was cut. The etched surface is shown in

figure 23. Once quantitative evaluation of the etch pits has been completed, the sample will be transferred

to the double axis diffractometer and x-ray rocking curves and reciprocal lattice point scanning by means

of 0/20 will be done. This same section will then be taken at Brookhaven National Laboratory for x-ray

synchrotron topography. Similar cutting and sectioning will be done with the aim of examining one [111]

oriented grain at each location of the surface. In this way it will be possible to observe any effects caused

by the material being forced against the wall during solidification as compared to the regions of the crystal

which grew in a "semicontainerless" configuration. The same procedure will be used for sections of

material grown in all three major attitudes of the orbiter during growth, material permitting. The interface at

11 cm produced by the second attitude change will be examined in a cross section or slab.

A section cut from the ground-truth sample at a similar stage during the growth is shown in figure

24. While cut perpendicular to the growth axis rather than being oriented to [111], the Polisar etch has

been effective in revealing the structure of one of the larger grains which lies 13 ° from the [ 111 ]. At fh'st

sight the structure and defect densities are not very different from the flight sample, but quantitative etch pit

counting and correlation with compositionally dependent stresses due to differences in lattice constant will

be carefully made to complete the characterization effort. A curiosity is the prominent twin present in this

sample; twins are rare in bulk grown HgCdTe and the origin of this one will be carefully investigated.

4.18. WAVELENGTH DISPERSIVE SPECTROMETRY (WDS)

Results with WDS are more sensitive than those from EDS, and are now being done on polished

samples with both high compositional resolution and high spatial resolution. The fh'st samples studied

were the cross sections prepared just prior to the rate change. This was done for both the ground-truth and

the flight samples. Figure 25 shows a cross section for the ground-truth sample and indicates some

asymmetry, possibly caused by the sample not being perfectly centered, not translating exactly along the

center line of the furnace or, most likely, the furnace axis not being exactly vertical. Figure 26 shows a

similar plot for the flight sample with a superimposed three-dimensional diffusion limited calculation done

by Dr. J.C. Wang of Alabama A&M University. The calculation is for an interface deviation of 2 mm from

being fiat. It can be seen that this diametrical plot, which is in the J7 direction indicates a larger asymmetry

than the ground truth sample. This may be an indication of three-dimensional fluid flow. The threedimensional representation of the composition of the flight sample is shown in figure 27. This position

includes material before, during, and after the rate change, and also beyond the attitude change. The first

rate change has brought about a complete change in the solidification characteristics with an almost

quenched in interface, particularly at the center of the boule. The surface data also show the interface as

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has been shown, and it is reasonable to infer its exact position and shape at the instant of the fh-st rate

change. In figure 27, the red material is the heavier, low-x, HgTe rich material, and it can be seen that

there is a slight asymmetry, denoted by the yellow material dominating the right center of the figure at the

bottom.

Recently, much EDS data have become available on two wafers cut from the two fh'st attitudes.

These data were taken within 100/_m of the edge and proceeded at increasing intervals to the center. In all,

some 280 points were measured. By this means good statistics are obtained, and figures 28 and 29

demonstrate certain effects very clearly. In figure 28, the resolved vector in the plane of the boule (but not

in the plane of the solidifying interface, which is curved), is shown. At 62 mm from the In'st to freeze end

there is a vector from the solid towards the liquid of 0.28 /_g, and this has caused much stirring of the

melt, resulting in considerable asymmetry in the resolved vector on the wafer. In contrast, the wafer cut at

106 mm, in the second attitude and shown in figure 29 has a small vector which points from the solid to

the liquid, and the wafer is more homogeneous. There is still an asymmetry in the direction of the resolved

vector, but it is much smaller than one would anticipate in comparison to the material at 62 mm. The

inference here is that the stabilizing vector from liquid to solid is essential to reduce fluid flow. The data in

figure 28 are worse than normally encountered in ground based growths, while the data of figure 29 are

superior to ground based based growths.

4.19. SYNCHROTRON RADIATION STUDIES

The entire boule surface was examined on beamline X19C at NSLS. This line operates as a white

beam source with no monochromator and at an energy range up to 2.584 GeV. The operating current is

initially at 250 mA and declines over a 20-hour period. It is ideally suited for Laue scattering experiments.

While more information can be obtained from transmission synchrotron studies, the sample preparation

involved is much longer and there is the possibility of losing some of the material; such studies will be left

till later. No sample preparation is required for a study of the surface of the as grown boule. Initially the

four surfaces at 90 ° were examined with an oblique incident beam along the length of the boule. The

images of the ground-truth sample were generally of poor quality, indicative of both small crystal size and

highly strained material. The flight sample, in general, was similar, but there were areas which were of

much higher quality. Good quality reflection topographs were obtained from some of the rough, etch pit

laden material, particularly the region between 38 and 42 mm at J7. Other good reflections were observed

from the region close to 105 mm, which is the part of the crystal grown in the -XLV/-ZVV attitude. The

crystal orientation seems to have been maintained entirely around the boule. It is significant that this is the

region of the crystal grown with a stabilizing acceleration vector pointing from liquid to solid.

4.20. SUMMARY AND PRELIMINARY CONCLUSIONS

1. An approximately 16-cm long Hgo.8Cdo.2Te alloy crystal was successfully grown over a period

of 11 days during Mission STS-62 on USMP-2.

2. A rate change inserted into the growth timeline sequence produced in the crystal an effective time

marker for correlating orbital and residual accelerations to various crystal features and alloy compositional

changes. This allowed a detailed evaluation of the effects of the magnitude and direction of residual

acceleration on crystal homogeneity and perfection to be made for the first time. The rate change did not

cause the propagation of new crystals.

3. Circumferential variation of composition and topographic features around the boule indicated

that residual acceleration vectors were present and having a considerable effect on the growth process.

4-103
4. Preliminaryx-raytopographsof theportiongrownin themostfavorableattitude(-XLV,-ZVV)

indicatethataregionconsistingof a completesinglecrystalof significantlyhigherquality thanusually

grownontheground.

5. TheOAREinstrumentprovedto beanaccuratepredictorof the direction of residual acceleration

vectors, as evidenced by the surface features of the boule.

6. The measured transient surface compositions indicate a lower effective mass transport coefficient than those observed in ground-based studies.

7. Certain attitude maneuvers of the orbiter can dramatically affect the growth stability. This is

illustrated by the roll-around in tail-down attitude which reversed the direction of the residual acceleration

perpendicular to the interface and caused thick compositional striations at 11 cm along the crystal.

8. Compositional maps of wafers taken from materials grown in two different orbiter attitudes with

very different resulting residual accelerations vectors demonstrate conclusively the effect of residual

accelerations on solidification of solid solution alloys with a wide separation between liquid and solid.

9. Further microstructural, optical and electrical characterizations of the crystal promise to provide

a wealth of additional information on the growth in low-Earth orbit of solid solution alloy crystals having a

large separation between their liquidus and solidus.

4-104
°

°

.

°

.

°

,

.

,

10.

11.

4.21. REFERENCES

Lehoczky, S.L., F.R. Szofran, and B.G. Martin, "Advanced Methods for Preparation and

Characterization of Infrared Detector Materials," McDonnell Douglas Corporation Report No. MDC

Q0717, July 5, 1980, (final report for the period December 5, 1978-July 5, 1980, for the George C.

Marshall Space Flight Center, Contract No. NAS8-33107), pp. 1-151 (1980) (also NASA CR

161598, July 5, 1980.

Lehoczky, S.L., and F.R. Szofran, "Advanced Methods for Preparation and Characterization of

Infrared Detector Materials," McDonnell Douglas Report No. MDC Q0744, September 30, 1981

(interim report for the period July 5, 1980-September 30, 1981, for the George C. Marshall Space

Flight Center, Contract No. NAS8-33107), pp. 1-33 (1981) (also NASA CR 161949, September

30, 1981).

Szofran, F.R., and S.L. Lehoczky, "The Pseudobinary HgTe-CdTe Phase Diagram," J. Electronic

Matls., 10, 1131-1150, 1981.

Kelley, J.D., B.G. Martin, F.R. Szofran, and S.L. Lehoczky, "Applications of the Regular-Associated-Solution Model to the Cd-Te and Hg-Te Binary Systems," J. Electrochem. Soc., 129,

2360-2365, 1982

Lehoczky, S.L., and F.R. Szofran, "Directional Solidification and Characterization of Hgl_xCdxTe

Alloys," in Materials Research Society Proceedings, Materials Processing in the Reduced Gravity

Environment of Space, ed., G.E. Rindone (Elsevier: New York), 9, 409420, 1982.

Lehoczky, S.L., C.J. Summers, F.R. Szofran, and B.G. Martin, "Electrical Characterization of

Hgl_xCdxTe Alloys," in Materials Research Society Proceedings, Materials Processing in the

Reduced Gravity Environment of Space, ed., G.E. Rindone (Elsevier: New York), 9, 421-431,

1982.

Szofran, F.R., and S.L. Lehoczky, "Liquidus Temperatures of Hg-Rich Hg-Cd-Te Alloys," J.

Electronic Matls., 12(4), 713-717, 1983.

Holland, L.R., "A Thermal Transmission Function for Fused Silica Ampoules," J. Crystal Growth,

49, 246, 1980.

Holland, L.R., "Radiant Heating of Ampoule Contents," NASA Tech Briefs, 349, Fall/ Winter

1981.

Naumann, R.J., "An Analytical Approach to Thermal Modeling of Bridgman-Type Crystal Growth

I. One-Dimensional Analysis," J. Crystal Growth, 58, 554-568, 1982.

Naumann, R.J., "An Analytical Approach to Thermal Modeling of Bridgman-Type Crystal Growth

II. Two-Dimensional Analysis," J. Crystal Growth, 59, 569-584, 1982.

Clayton, J.C., "Transient and Diffusion Analysis of HgCdTe," NASA CR 162049, 1982.

Cothran, E.K., "An Analytical Approach to Thermal Modeling of Bridgman-Type Crystal Growth:

One Dimensional Analysis, Computer Program Users Manual," NASA TM 82487, May 1982.

4-105
14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

Clayton,J.C., M.C. Davidson,D.C. Gillies, andS.L. Lehoczky,"One DimensionalAnalysisof

Segregationin DirectionallySolidifiedHgCdTe,"J. Crystal Growth, 60, 374, 1982.

Gillies, D.C., "Analysis of Mercury Cadmium Telluride by Energy Dispersive Analysis," J.

Electronic Matls., 11,689, 1982.

Lehoczky, S.L., F.R. Szofran, and R.J. Naumann, "Crystal Growth of Homogeneous Hgl_xCdxTe

Crystals from Pseudobinary Melts," Bull. Am. Phys. Soc., 28, 294, 1982.

Naumann, R.J., and S.L. Lehoczky, "Effect of Variable Thermal Conductivity on Isotherms in

Bridgman Growth," J. Crystal Growth, 61,707-710, 1983.

Holland, L.R., R.P. Harris, and R. Smith, "High Temperature, High Pressure Optical Cells,"

Review of Scientific Instruments, 54, 993, 1983.

Holland, L.R., and R.E. Taylor, "Measured Thermal Diffusivity of Hg l_xCdxTe Solids and Melts,"

J. of Vacuum Science Technology, A1, 1615, 1983.

Chandra, D., and L.R. Holland, "Density of Liquid Hgl_xCdxTe," J. of Vacuum Science and

Technology, A1, 1620, 1983.

Lehoczky, S.L., F.R. Szofran, D. Chandra, and J.C. Wang, "Growth Rate Dependence of the Axial

Compositional Variations in Bridgman-Grown Hgl_xCdxTe Crystals," Fall Meeting of the American

Physical Society, San Francisco, CA, November 20-23, 1983, Bull. APS, 28, 1313, 1983.

Szofran, F.R., and S.L. Lehoczky, "High Temperature Electrical Properties of HgTe," Bull. APS,

28, 1313, 1983.

Lehoczky, S.L., and F.R. Szofran, "Growing Crystals for Infrared Detectors," NASA Tech Briefs,

8, 136, 1983.

Jasinski, T., and R.J. Naumann, "One-Dimensional Modeling of Vertical Bridgman-Type Crystal

Growth," J. Crystal Growth, 66, 469, 1984.

Holland, L.R., "Sealed Silica Pressure Ampules for Crystal Growth," J. Crystal Growth, 66, 501,

1984.

Szofran, F.R., and S.L. Lehoczky, "A Method for Interface Shape Control During Bridgman Type

Crystal Growth of HgCdTe Alloys," J. Crystal Growth, 70(1/2), 349-355, 1984.

Szofran, F.R., D. Chandra, J.C. Wang, E.K. Cothran, and S.L. Lehoczky, "Effect of Growth

Parameters on Compositional Variations in Directionally Solidified HgCdTe Alloys," J. Crystal

Growth, 70(1/2), 343-348, 1984.

Holland, L.R., "VLS Growth to Purify Elements of Groups II and VI," J. Crystal Growth, 70,

1984.

Lehoczky, S.L., and F.R. Szofran, "Further Comments on Segregation During Bridgman Growth

of Hgl_xCdxTe," J. Crystal Growth (Lett.), 69, 201-203, 1984.

4-106
30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

Lehoczky,S.L., and F.R. Szofran,"SolidificationandCrystalGrowth of Solid Solution SemiconductingAlloys," inNASA CP2313,editedby CamilleM. Jernigan,p. 320, 1984,Proceedings

of theSecondSymposiumonSpaceIndustrialization,Huntsville,AL, February13-15, 1984.

Szofran,F.R.,andP.N.Espy,"AutomatedACGalvanomagneticMeasurementSystem,"Review of

Scientific Instruments, 56, 1363, 1985.

Chandra, D., "Anomalous Volume Expansion in Hgl_xCdxTe Melts: An Analysis Employing the

Inhomogeneous Structure Model," Phys. Rev., 331, 7706, 1985.

Lehoczky, S.L., and F.R. Szofran, "HCT Crystal Growth Method," U.S. Patent No. 4,545,848,

October 1985.

Su, C.-H., S.L. Lehoczky, and F.R. Szofran, "A Method to Eliminate Wetting During the

Homogenization of HgCdTe," J. Appl. Phys., 60(10), 3777-3778, 1986.

Su, Ching-Hua, "Heat Capacity, Enthalpy of Mixing and Thermal Conductivity of Hgl_xCdxTe

Pseudobinary Melts," J. Crystal Growth, 78, 51, 1986.

Lehoczky, S.L., and F.R. Szofran, "Growth of Solid Solution Single Crystals," NASA TP 2787,

December 1987.

Szofran, F.R., and S.L. Lehoczky, "Bridgman Growth of Mercury Cadmium Telluride Alloys," in

Processing of Electronic Materials, eds., C.G. Law, Jr. and R. Pollard (American Institute of

Chemical Engineers: New York), pp. 342-348, 1987, Proceedings of First International Conference

on Processing of Electronic Materials, held Santa Barbara, CA, February 23-28, 1986.

Lehoczky, S.L., and F.R. Szofran, "Growth of Solid Solution Single Crystals," in The Nation's

Future Materials Needs, International SAMPE Technical Conference Series, eds., T. Lynch, J.

Persh, T. Wolf, and N. Rupert (SAMPE: Covina, CA), vol. 19, p. 332, 1987. Proceedings 19th International SAMPE Technical Conference, Arlington, VA, October 13-15, 1987.

Lehoczky, S.L., F.R. Szofran, C.-H. Su, S.D. Cobb, and R.N. Andrews, "Crystal Growth of

Solid Solution Systems by Directional Solidification," Proceeding of ASM International '87

Materials Congress, Cincinnati, OH, October 12-15, 1987.

Naumann, R.J., S.L. Lehoczky, and D.O. Frazier, "Growing Single Crystals of Compound

Semiconductors," in NASA Tech Briefs, vol. 11, No. 6, pp. 47-48, June 1987.

Naumann, R.J., D.O. Frazier, S. Lehoczky, M. Vlasse, and B. Facemire, "Crystal Growth in

Liquid-Encapsulated Float Zone," in NASA Tech Briefs, vol. 11, No. 8, pp. 44-45, September

1987.

Su, C.-H., S.L. Lehoczky, and F.R. Szofran, "Growth of HgZnTe Alloy Crystals by Directional

Solidification," J. Crystal Growth, 86, 87-92, 1988.

Szofran, F.R., G.L.E. Perry, and S.L. Lehoczky, "Highly Automated Transmission-Edge

Mapping," J. Crystal Growth, 86, 650_555, 1988.

4-107
44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

Dakhoul,Y.M., R. Farmer,S.L. Lehoczky,and F.R. Szofran,"NumericalSimulationof Heat

TransferDuringtheCrystalGrowthof HgCdTeAlloys," J. Crystal Growth, 86, 49-55, 1988.

Andrews, R.N., F.R. Szofran, and S.L. Lehoczky, "Growth and Characterization of Hgl_xCdxSe

Alloys," J. Crystal Growth, 92(3/4), 445-453, 1988.

Andrews, R.N., F.R. Szofran, and S.L. Lehoczky, "Internal Temperature Gradient of Alloy

Semiconductor Melts From Interrupted Growth Experiments," J. Crystal Growth, 86(1-4), 100-

105, 1988.

Su, C.-H., G.L.E. Perry, F.R. Szofran, and S.L. Lehoczky, "Compositional Redistribution During

Casting of Hg0.8Cd0.2Te Alloys," J. Crystal Growth, 91, 20-26, 1988.

Patterson, J.D., and S.L. Lehoczky, "The Second Born Approximation and the Friedel Sum Rule,"

Physics Lett. A, 137(3), 137-138, 1989.

Lehoczky, S.L., and F.R. Szofran, "Method of Preparing Radially Homogeneous Mercury

Cadmium Telluride Crystals," U.S. Patent No. 4,863,553, September 1989.

Andrews, R.N., S.D. Walck, M.W. Price, F.R. Szofran, C.-H. Su, and S.L. Lehoczky,

"Microhardness Variations in II-VI Semiconducting Compounds as a Function of Composition," J.

Crystal Growth, 99, 717-721, 1990.

Su, C.-H., S.L. Lehoczky, and F.R. Szofran, "Directional Solidification of HgCdTe and HgZnTe

in a Transverse Magnetic Field," J. Crystal Growth, 109, 392-400, 1991.

Abdelhakiem, W., J.D. Patterson, and S.L. Lehoczky, "A Comparison Between Electron Mobility

in N-Type Hgl_xCdxTe and Hgl_xZnxTe," Materials Lett., 11(1,2), 47-51, 1991.

Patterson, J.D., W.A. Gobba, and S.L. Lehoczky, "Electron Mobility in n-Type Hgl-xCdxTe and

Hgl_xZnxTe Alloys," J. Materials Res., 7(8), 2211-2218, 1992.

Gobba, W.A., J.D. Patterson, and S.L. Lehoczky, "A Comparison Between Electron Mobilities in

Hg l-xMnxTe and Hg l-xCdxTe," Infrared Phys., 34(3), 311, 1993.

Mazuruk, K., C.-H. Su, S.L. Lehoczky, and F. Rosenberger, "Novel Oscillating Cup Viscometer -

Application to Molten HgTe and Hg0.8Cd0.2Te," J. Appl. Phys., 77 (10), 5098-5102, 1995.

Palosz, W., F.R. Szofran, and S.L. Lehoczky, "The Effect of Heat Treatment on the Magnitude and

Composition of Residual Gas in Sealed Silica Glass Ampoules," J. Crystal Growth, 142, 215-224,

1994.

Price, M.W., R.N. Andrews, C.-H. Su, S.L. Lehoczky, and F.R. Szofran, "Effect of Transverse

Magnetic Field on the Microstructure of Directionally Solidified CdTe," J. Crystal Growth, 142,

215-224, 1994.

Lehoczky, S.L., D.C. Gillies, F.R. Szofran, F.A. Reeves, J.D. Sledd, T.K. Pendergrass, D.A.

Watring, C.R. Coppens, J.E. LeCroy and D. Popok, "Crystal Growth of HgCdTe in the AADSF on

the USNIP-2 Mission," Paper AIAA 95-0609, 33 rd Aerospace Sciences Meeting and Exhibit, Reno,

Nevada, 1995.

4-108
59. Kim, D.H., andR.A. Brown, "Modelling of the Dynamicsof HgCdTe Growth by the Vertical

Bridgman Method," J. Crystal Growth, 114, 411, 1991.

60. Kim, D.H., and R.A. Brown, "Models for Covection and Segregation in the Growth of HgCdTe by

the Vertical Bridgman Method," J. Crystal Growth, 96, 609, 1989.

61. Watring, D.A., and S.L. Lehoczky, "Magneto Hydrodynamic Damping of Convection During

Vertical Bridgman-Stockbarger Growth of HgCdTe," J. Crystal Growth, in press 1995.

62. Rogers, M.J.B., and R. DeLombard, "Summary Report of Mission Acceleration Measurements for

STS-62," NASA TM 106773, November 1994.

4-109
0

i

ii

0

L_

a..

i

C

0

gl

Im

0

a.

E

0

(..)

ii

"0

0

i0

a.

E

0

0

0

70

.I

0

c

v

r7

o

:s

0

I

0

0

OJ

0 _ 0 _ 0 _ 0 _ 0 i

d d d d d d d o d

elPO uo!},oeJ:lepOlAl

o E

E

c

0

o

d

0

n

0

I

rr

o

e_

I

E

o

r_

4

°,,,_

4-110
I---

tO,I

2

m

o

Mission Elapsed Time (MET)

1/00:03 6/19:45 8/00:30

Attilude +VVVf-ZLV ZVV/ X[ _VVl-XLV

0.6

/./.J_ _ EffectiveDiffusionCoefficient= _ 5E_5 c_l_'s

0.5 TranslationRa_e = 079 rn_thl

COh'lp0_ition x = 0 2

Gr_dlen! _ 700 C = 65 Clcm

$_pe_ooled Length = 1.? c_

0.4

First Interlace

0.3 \ Cut Demmcetion

\

0.2

pe_. ion el

9.1 • _,n

0.0 . . . , . . . , . . . _ . . . i , , , , . • • , . . . , . .

0 20 40 60 80 100 120 140

10109:00

60

Distance from firstto freeze, mm

Figure 4-2. Estimated compositional profile based on diffusion controlled solidification.

2.0

1.5

1.0

05

0.0

-0.5

-I 0

-I .5

-2.0

6.0

: +YVV/-ZL'_ -ZVV/-XLV

................ ................ _]P,ST _,TTr___,E..........

CH_t.JGE

...................................... ................ i................... t..................

........ ...... ......

,.........r....

6.2 64 6.6 6.8 7.0

MET

Figure 4-3. Z-component OARE data during attitude change.

4-111
i

l

0.!

0.0

-0.1

-0.2

-0.3

-0.4

0,!

0.0

-0.1

-0.2

-0.3

-0.4

X axis

Y axls

--___1 z i *

Z axis

"O,S r 1 , , ,

"'-_.o .o _.o .:o .o _o

Minhm Elape4d Time Ihr]

Figure 4-4. OARE data at instrument for -ZLV/+YVV attitude.

Figure 4-5.

X axhl

0.8

0.7

0.6

0.5

0.4

0.3

0.1

-0.1

-0.2

-0.3

•0.4 , I , ,

Z axis

83 84 85 86 87

ma_n_Ehnp_dTimeI_l

0.6

0.5

0.4

0.3

0.2

0.1

82

OARE data transformed to AADSF location for -ZLV/+YVV attitude.

4-112
0.53

z

0"15 T 0.28 __

z

0.28

Orbiter -ZLV / +YVV Attitude

LH

_ _ o

VV V-V "L oo..'s

0.43

_ LV Ls3

1.42

Orbiter -XLV / +ZVV Ammde Orbiter -XLV / -ZVV Attitude

Figure 4-6. Directions of residual acceleration vectors with respect to AADSF and orbiter.

4-113
MCT 27Q Looking Towards First to Freeze End

Orbiter Transverse Residual Resolved Axial/Transverse Total

Athtude Residual Acceleration Axial Acceleration Transverse Ratio Resolved

-ZLV/+YVV

O-9cm

-XLV/-ZVV

9-11 cm

-×LV/_ZVV

11 - 15cm

17

J7

ttot

0.28

Cold

_o_

0.50

Cold

Hot

0.43

Cold

-0.5]

+0.32

-0.31

0.62

1.63

7.51

Figure 4-7. Resolved OARE data for AADSF flight sample.

4-114
Figure 4-8. Radiographs of flight and ground-truth cartridges showing the last to freeze material.

4-115
Figure4-9. Appearanceof ground-truthandflight cartridgesafterprocessing.

4-116
°_..q

0

r--

0

o

o

E

L

t_

E

_0

<

<

d

4

_0

°_.._

4-117
(a)

(b) "

Figure 4-11. (a) Typical Laue back reflection pattern of surface of flight sample. (b) Broad beam back

reflection Laue pattern same region as figure 11 a.

4-118
o_1 o11

t 111

J7

Position

o11

Figure 4-12. Back scattered electron image (SEM) from flight sample--note the channeling pattern.

4-119
(a)

(b)

Figure 4-13. (a) Thermal etch pits on surface of flight sample---close to [111]. (b) Thermal etch pits on

high index surface.

4-120
"0

im

L_

mm

IE_

E_ 0

C

_0i_

C

ID0

i._.

0

>>

_j -J X X! >> > N N , +

O'J E

E

"E

"0C

E0

00

O

E©

°_,_0=0=

°,,,_ 00

rj

r._

4-121
O6

0.5

_> 0.4

I--

C

0

_- 0.3

u

0

•_ 0_

01

0.0

• !

• i

............. i ......, ..... ', ...... i ........... !................

• ]

i • e _,eo i

• *o

...... ..........i.................................T ...............

i i • i

............. .. .................... _ ......................... _ ................... _.......................... ..........

i i i • i z

i !dl i • •

• . i .. i... ,... , .... i . L ..... i...

0 20 40 60 80 100 120 140 160

Distance from firstto freeze, mm

Figure 4-15. Surface composition of ground-truth sample along J7+90 ° orientation.

0.6

0.5

© 0_4

k--

L)

=...

o

0.3

0

0.2;

0.1

0.0

Surface Composition, MCT-27Q, J7 Position

Flight Sample, AADSF, USMP-2

'''''''''''!'''''''I'*'''''''''

.......................................................... i ............._......................................... i ....................... -.................

i

• i :

.... 4,8 ÷ ........ i i .......! .....! ...........

o e •

aP'

• ". .,,,.

..................................._ ................. " I_' "t--w .... _ ......... _........

I

........................ _.................................. i "............. i..................... _ ............. ............... o

, , , I , . . n . , . J . . i , , , I , , , i , , , i . , .

0 20 40 60 80 100 120 140 160

Distance from firstto freeze, turn

Figure 4-16. Surface composition of flight sample along J? orientation.

4-122
0.5 .................. i .................... + .................. i -

04 ........................ ' ........... i _ _ ..... _........... i -

i • i i i i

o 3 ..... . .......... : ........ i.................. 7 T ........ !...............

o.2 ....................... i.............. _....................... _......................... _.. ........... -...........

oi

0.0 , , , J , . . _ . , . t . , , , , . , _ , , , ] . , _ . • .

0 20 40 60 80 100 120 140 160

Distance from firstI0freeze,mm

Figure 4-17. Surface composition of flight sample along J7-90 ° orientation.

0

0.6 ...... I ' ' ' I ...............

o.5 ............................................... i........................ i.............................................................................................................

0.4 ............................._.............

• i i

0_3........................................................:, ........... ..................................

O'l

02 ........ i .................... i ..... , '-,.4,...............

o

i • i •

• i

0.1 ................ ....................... ........................ _............................ .......................... +....................... +................

0.0 . , . i . , . I , , , _ , , , _ , , , L , , , i , , , i , , ,

0 20 40 60 80 100 120 140 t60

Distance from first Io freeze, mm

Figure 4-18. Surface composition of flight sample along J7+90 ° orientation.

4-123
II

c

c,

o

0.6 ...... I ............ I ' ' ' I ' ' '

o5 ....................... i........................................ i.....................................

i, i

• i

0.4 .......................

•

......._

i

....... +_.............................................................................. i

• +- [

o.3 ............................................ i............................................. "- i..............................................................................................

i "o° ! J i

0_2

++ -:.

++ •

I1.1

.............. i _" .............. _......................... ?..........• ........._ ......... _ .................

, , . 1 . . . I , . * I . • . I . . . I i = + I , , , I , , ,

20 40 60 BO 100 120 140

0.0

6O

Distance from first to freeze, mm

Figure 4-19. Surface composition of flight sample along J7+180 ° orientation.

g.

E

v,-

I

e-

<

60

50

40

30

20

10

0

-10

-20

o 000 - 179

t80-189

[] 190-199

• 200-209

• 210 & above

• • [] • • • • • • • •

• • _ D _ O C, D • • •

• • [] • • • C, • • • •

• • [] • • • • • • • •

• • O D [] • • • • • •

• • • • • • • • • • •

• • _ _ • C, • • • • •

• • O [] • 0 • • • • •

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• • • • • • • • • • •

• _. c_ [] • • o • • • •

r I i i i I i , i I i i i I = I i I i i i I i

84 B6 BB 90 92 94

Distance rrom Firs! to Freeze, mm

Figure 4-20. Surface composition of flight sample close to J7-90 ° orientation.

4-124
8

7--

6--

5_--

4--

3--

2--

1--

0

USMP-2 Ground Truth - Rate Change Region

................................. /__:'<.. _- -..!........................

............. ; __ : ...........< ...... :_- ..... ..........

__........ "__ i----_--J:--; --" W ...._ '.... _ ..........

-_--_-, <3 -- J _3 _ :Y-- _ ...........i,......

S ::: %

' I '

' I '

0 2 4 6 8 10

Figure 4-21. Two-dimensional analysis of a slab of the ground-truth sample cut through the rate change

region--EDS results.

4-125
C

.9

(1)

rr"C

t"

r¢"

t--- 2

¢-

I---I

Q. E

CO

"13

t---

--"1 2

(5

(5

EEI EEI

1...-4 0

-62

¢',1O

..Q

r,,¢3 0

¢3

e,i

¢.q 4

4-126
I

O

ID

°,,,_

O

o

_D

r_

e,-;

t"q

4-127
°

00£

r_E

.6£

,4

4-

4-128
0.30

0.25

c 0.20

0

o 0.15

O

"_ 0.10

005

0.00

-,d

.... I: .... I .... I .... I .... I .... I .... ! ....

....... i........................................

i

.... I-.,o1,,.,I,,.,1..,.I .... I..lll.,_ _

-3 -2 -1 0 1 2 3 4

y-coordinate, rnm

Figure 4-25. Composition across diameter of ground-truth sample--WDS results.

co

"6

kl.

O

0.35

0 30

025

0 20

0.15

0.10

0.05

0.00

....,....,....,....,....,....,....,....1

, _ i i i i i i

• i i i i i i .........i...................

• i i i i

• i i ; [

,is, _ ! i i _ i

.............. ,-i ....................... i........................................... i........................ i....................... i....................... i............. * ....

........................i ................. i

I I Microprobe data

.... - -- -Diffusion limited .................. ..........

theory hyJ Wang

". • • • i • . • , I .... I • , , , I , , , ,

-3 -2 -1 0

Parameters used for

theoretical calculation:

D = 5 5e-05 cm'/s

= 4

co = 2

V = 22 rniororls/s

d=5

i .... I .... i,,.

1 2 3 4

x-coordinate, rrlrn

Figure 4-26. Composition across diameter of fright sample with superimposed diffusion profile.

4-129
86.0

(mm)

85.0

84.0

83.0

82.0

81.0

80.0

79.0

r gr = 0.55 #g

J7-16

78.0

77.0

76.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 (mm)

Figure 4-27. Three-dimensional representation of composition of flight sample--

WDS results through rate change

CdTe

0.264

0.255

0.246

0.237

0.227

0.218

0.209

0.200

0.191

0.182

0.173

0.163

0.154

0.145

0.136

4-130
J7

gr

0.55/_g

Figure 4-28.

CdTe

0.3300

0.3157

0.3050

0.2925

0.2800

0.2675

0.2550

0.2425

0.2300

0.2175

0.2050

0.1925

0.1800

0.1675

0.1550

Composition map for flight sample in +YVV/-ZLV attitude (62 mm).

J7

1 mm

Figure 4-29.

gr

1.55,u g

CdTe

0.3300

0.3175

0.3050

0.2925

0.2800

0.2675

O.2550

0.2425

0.2300

0.2175

0.2050

0.1925

0.1800

0.1675

0.1550

Composition map for flight sample in -XLV/-ZVV attitude (106 mm).

4-131
Table4-1. OAREDataTransformedtoAADSF Coordinates.

AverageAcceleration(micro-g)

A'IqTI'UDE X-BODY Y-BODY Z-BODY

+YVV/-ZLV +0.53 -0.15 +0.28

-ZVV/XLV -1.53 -0.28 +0.50

+ZZV/-XLV -1.42 -0.28 -0.43

Table 4-2. Quality of Surface on Flight Sample.

Surface Position Relative to J7

DISTANCE (CM)

1-9

J7 J7+45 ° J7+90 ° J7+135 ° J7+180 ° J7 - 135 ° J7 - 90 ° J7 - 45 °

M M M S S S M R

9-11 S S S S S S S S

11-15 S M M R R R S S

S--Smooth surface, M--Medium smooth surface, R--Rough surface

4-132
Table4-3. Orientationof Flight Sample--TransformedLauePattems.

LAUES--AADSF--USMP-2

MCT27Q---FLIGHTSAMPLE

I.D.

27 -7

27-13

27 -6

27-5

27 -4

27-1

27-2

27-3

27-8

27-9

27-10

27-11

27-12

27-29

27-28

Position (mm

from first to

freeze)

30

35

40

50

60

70

80

90

100

110

120

130

140

30

Surface

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7+90

J7-180

J7-180

0.29

0.03

0.03

0.03

0.03

0.04

h k 1

(direction of crystal growth)

0.68

3.45

3.45

3.45

3.45

3.45

3.45

3.45

3.45

2.68

40

J7-180 1.57

1

4.66

4.66

4.66

4.66

4.66

4.66

4.66

4.66

3.18

3.18

3.18

3.18

3.50

1.78

27-27 50 2.98

27-26 60 J7-180 1 3.46 3.93

27-25 70 J7-180 1 1.57 2.98

27-24 80 J7-180 1 2.68 3.50

27-23 90 J7-180 0 0.56 1

27-22 100 J7-180 0 0.56 1

27-21 110 J7-180 0.05 0.30 1

J7 -90

J7-90

J7-90

J7-90

J7-90

32

42

52

55

62

65

0.04

0.43

0.31

0.19

0.05

0.14

0.34

0.32

0.12

0.12

72

73

75

82

0.55

0.73

27-31

27-32

27-33

27 -47

27-34

27 -45

27-35

27-53

27 -43

0.33

1.32

0.55

5.57

0.59

0.46

0.46

90

90

27-36

27-51

27-37

27-38

27-39

27 -40

27-41

27-42

J7-90

J7-90

J7-90

J7-90

J7-90

J7-90

J7-90

1

1.05

1

1

2.81

71.13

7.32

1.01

1

1

1

100 J7-90 O.97 1.44

110 J7-90 1 1.56 1.75

120 J7-90 0.46 1 3.80

130 J7-90 1 7.65 36.5

140 J7-90 1 7.65 36.5

4-133
Table4-3. Orientationof Flight Sample--Transformed Laue Patterns Continued

LAUES--AADSF--USMP-2

MCT27Q--FLIGHT SAMPLE

I.D.

27-104

27-102

27-100

27-98

27-96

27-94

Position (mm

from first to

freeze)

30

40

50

55

60

65

27-92 70

27-88 75

27.86 80

27-84 85

27-82 87

27-8O 89

27-78 92

27-76 94

27-74 96

27-72 98

27-71 100

27-68 110

27-65

27-63

27-61

120

130

140

Surface

J7

J7

J7

J7

J7

J7

J7

J7

J7

h k 1

(direction of crystal growth)

0.15

0.23

0.08

0.10

0.41

0.29

0.60

0.59

0.21

1.46

0.31

1

1

1

1.37

8.41

1.40

0.64 1

J7 0.64 1 1.14

J7 0.64 1 1.14

J7 0.64

J7 1

J7 0.58

J7 0.58

J7 0.17

J7 0.17

J7 0.45

J7

1.93

1.114

1 1.14

1.61 1.62

O.98 1

0.98

1 1.03

1 1.03

1 2.98

1.34

J7

J7

multigrains

multigrains

4.22

4-134
SECTION V.

HIGHLIGHTS OF THE ZENO RESULTS

FROM THE USMP-2 MISSION
Acknowledgments

We are grateful for the work of many colleagues who contributed so much to the success of this

mission. At the University of Maryland: John Borden, David Torrealba, Dan Swann, Mark Sienkiewicz,

Paul Mahata, Loft Mora, Matt Bieneman, Meade Larson, John Pickett, Kai-Chang Zhang, Ed Cole, and

Jan Sengers. At Lewis Research Center: Richard Lauver, Andrew Peddie, Greg Zimmerli, Geoff Elliott,

and Klaus Gumpto. At Ball Aerospace: Richard Reinker, Mike Dials, Robert Stack, and Robert Woodruff.

At the National Institute of Standards and Technology: Svetlana Avramov, Nile Oldham, M.R. Moldover,

and Robert Berg.

5-136
SECTION V. HIGHLIGHTS OF THE ZENO RESULTS

FROM THE USMP-2 MISSION

Robert W. Gammon, J. N. Shaumeyer, Matthew E. Briggs, Hac_ne Boukari,

and David A. Gent

Institute for Physical Science and Technology,

University of Maryland, College Park, Maryland 20740

R. Allen Wilkinson

NASA Lewis Research Center, Cleveland, Ohio 44135

5.1. INTRODUCTION

When a pure fluid is near its liquid-vapor critical point the otherwise small, statistical fluctuations

in its density become very large, as large as the wavelength of light when the system is still only 10 mK

from the critical temperature, Tc. These large fluctuations scatter light very strongly and the previously

clear fluid turns milky white, a phenomenon known as critical opalescence.

Indeed, many different systems have critical points, and large fluctuations of some thermodynamic

parameter are a universal feature. Because critical fluctuations become macroscopic in size and involve

enormous numbers of molecules, many features of critical-point behavior are controlled by the statistical

behavior of the fluctuations, so that many types of systems exhibit the same behavior near the critical

point.

Light scattering from critical fluctuations in a fluid is a simple and accurate technique to measure

one fundamental property of the fluctuations: Their decay rates (the inverse of their lifetimes). However,

near the critical point, the fluid becomes highly compressible, so that just the weight of the fluid itself

causes severe density gradients in the sample, distorting these measurements in a terrestrial laboratory.

The goal of the Zeno experiment was to build a precision, light-scattering spectrometer to measure

the fundamental fluctuation decay rates in a sample of xenon, which would operate in the low-gravity

environment of the space shuttle, thus removing the gravity-induced distortion of the measurements. The

instrument performed beautifully on its first mission, and we were able to make our projected

measurements with 1 percent precision to within 100 _K of the critical temperature (16.7 °C).

The fight-scattering spectrometer is shown schematically in figure 1. Light from the low-power,

helium-neon laser was directed into the sample along one of two beam paths determined by the

beamsplitter B 1; the path was chosen by shutters (not shown). Light scattered by the sample (located in the

cylindrical thermostat at the center of the figure) was then collected by the apertured photomulfiplier tubes

(PMT1 and PMT2), the primary signal which was processed to obtain the decay rate information. In

addition, photodiodes (PD1 and PD2) placed behind partially reflecting mirrors (M2 and M3) measured the

intensity of the light entering and exiting the sample. These two signals were then rafioed electronically to

give us information about the turbidity of the fluid, itself an important measurement and our primary

method of locating the critical temperature of the sample.

5-137
Thecollectionanglesfor thescatteredlight (determined by apertures A1, A2, A3, and A4) were

precisely aligned so that the two scattering angles were supplements. This accomplished two things: It

simplified data analysis, and it allowed us to measure decay rates at the same pair of widely-spaced angles

regardless of the beam path through the sample.

The instrument was housed in two separate containers attached to the USMP carrier. The optics

module contained the light-scattering spectrometer, built on a stiff, optical bench, in addition to

temperature-sensitive signal amplifiers and ratio transformers for the thermostat's temperature control and

measurement system. All additional electronics, including the instrument's computer system, were housed

in the Electronics Module.

5.2. TIMELINE

The instrument operated continuously for 13 days and 17 hours, probably a record. Figure 2

shows the timeline of the experiment as the temperature of the sample (in distance from Tc) versus mission

elapsed time (MET). The principle events are delineated by times where the sample was warmed to Tc+ 1 K

in order to bring it back to a state of uniform density, and times where the sample was closer than 1 /2K to

Tc

During MET day 0 the instrument was primarily in a warm-up and initialization phase. Then, just

prior to the start of MET Day 1 the sample was warmed to 1 K, and the first scan to locate Tc began. The

scan was followed by two warming periods. The slow cooling that spans MET days 2 and 3 represents a

zig-zag search for Tc which extended into MET day 4, followed by another warming procedure.

The first period of data collection began midway through MET day 4. Decay rate measurements

were collected at a geometrically spaced set of temperatures progressively nearer Tc, the In'st one starting at

100 mK. The last measurements of this set, during MET day 7, were at temperatures within 100 pK of

Tc, and actually contained some measurements at temperatures just below Tc.

MET Day 8 began with another warming cycle, followed by another period of data collection at

various temperatures between 560 mK and 30/2K above Tc. Between MET days 9 and 12 we took some

additional measurements at temperatures further from Tc, as well as repeating selected measurements from

earlier in the mission. Near the beginning of MET day 11 we began a series of 100 /_K cooling steps

which would again take the sample below Tc, recording decay rate measurements during the process with

the hope of getting measurements which would show the effects of crossing the critical point into the twophase region at temperatures at which it is impossible to make measurements on Earth.

5.3. INSTRUMENT PERFORMANCE

5.3.1. Thermostat and Thermometry

The thermostat was a small device, 23-cm long and 8 cm in diameter, which provided a stable,

controlled thermal environment for the xenon sample cell. It was made of three coaxial, cylindrical shells,

each with a heater and feedback temperature controller. In addition, each shell and the sample cell had

separate temperature-monitoring devices. Thermometry was provided by thermistors inside the thermostat

and sample-cell walls, wired to standard resistors inside the thermostat to form one arm of an alternating

current resistance bridge. The other part of the bridge was made up of ratio transformers. In all there were

5-138
three control bridges and one, multiplexedmeasurementbridge. There were three 6-decaderatio

transformersand one 8-decadetransformer,all calibratedthrougha collaborationwith workers at the

NationalInstituteof StandardsandTechnology[seebibliography:dissertationof S. Avramov;papersof

S.AvramovandN. Oldham].

In flight, thethermostatrequiredastablethermalenvironmento doitsjob: Thetemperaturecontrol

of theopticsmodulebyradiativeheatleakandcontrolheaterswasexcellent.Becauseof thelengthof the

mission andthe long periodsof constantshuttleattitude,we were able to maximizethe temperature

control.Thus,thethermostatwasableto achievearoutinecontrollevelof betterthan__.3_K rms noisefor

periodsof 3 hoursor longer.Figure3 showsoneperiodof 4 hourswhenthe sample-celltemperaturewas

controlledto 10/_K peak-to-peak(with a 5-secondiLrneconstantin themeasurement)and 1.7 /_K rms.

This level of control easily met the requirements for the Zeno experiment.

To change the temperature of the sample, the set points of all three control shells of the thermostat

were moved. Figure 4 shows an example of a simple temperature change (cooling) of 2 mK; the

temperature of the sample cell follows with a time constant of about 30 minutes.

For smaller temperature changes, we were able to make "fast steps" where the set points of the

control shells were temporarily moved well beyond the target temperature and then returned just as the

sample cell reached the desired temperature. Figure 5 illustrates the fast change, which is finished in a

matter of minutes.

5.3.2. The Light-Scattering Spectrometer

Scattered light was detected by a pair of ruggidized, EMI 9863 photomultiplier tubes. These tubes

had better than 5 percent quantum efficiency at 633 nm, total afterpulsing of less than 0.06 percent, and

dark counts of less than 1 s-1 at the operating temperature of the optics module of 9 °C. The background

photon count rates were low enough that we could easily see the 20 counts/second from increased

radiation when the shuttle passed through the South Atlantic Anomaly.

The photomultiplier tubes were operated in photon-counting mode and the two signals were passed

to the dual inputs of an ALV-5000 digital correlator, adapted specifically for this instrument. With this

correlator we were able to make simultaneous measurements of forward-scattered and backscattered light.

The correlator is a special-purpose, digital signal processor, which produces an accurate autocorrelation

function of the stream of photocounts from the photomultipliers. This correlator is designed to cover a

broad range in delay times by calculating the autocorrelation function on a geometrically expanding time

scale so it is natural to plot the resulting correlograms with a logarithmic time scale. Figure 6 shows a

sample correlogram; note that this shape represents an exponential decay of the autocorrelation function.

The experiment was performed with very low light levels. The windows of the sample cell absorb

small amounts of the incident light at their surface (measured to be about 2 ppm). Because the thermal

expansion coefficient of the sample diverges at the critical point, the increase in temperature of the window

surface due to absorbed fight can lead to unwelcome perturbations in the density of the fluid, and

measurements would no longer represent the behavior of the fluid at the temperature we measured for the

bulk [see bibliography: dissertation of M.E. Briggs].

To avoid these problems, we reduced the power of the laser fight by placing filters in the beam

paths, selecting two different power levels that allowed us to cover two temperature ranges with density

perturbations of less than 1 percent. A power level of 17/_W was used between 1 K and 1 mK from Tc,

and the lower level of 1.7 /_W was used at temperatures closer than 1 mK, with overlapping

5-139
measurements made with both powers near 1 mK. With this combination of incident light powers we

were able to cover a wide range of temperature with enough signal to reach our precision requirement of 1

percent in the decay-rate measurements within the length of the mission.

5.4. DETERMINING Tc ON ORBIT

Since all of the measurements in the Zeno experiment are made relative to the critical temperature of

the xenon sample, it was important to understanding the results that we locate Tc precisely using our

experimental thermometry. The possibility that the thermometry would be sensitive to the harsh mechanical

environment of a shuttle launch meant that locating Tc on orbit was a significant part of our mission

activities.

In general, locating Tc involved moving the sample to a region very near the critical temperature

and then adjusting its temperature in a controlled way and watching for signs that the critical point has been

crossed. The signal which we monitored during the searches measured the turbidity of the sample, given

by the logarithm of the ratio of the intensity of the light transmitted by the sample to the intensity of the

incident beam. We attempted two techniques with different expected levels of precision; one worked, and

one did not.

The zig-zag technique, which we used to try to increase our level of precision for the value of Tc

and did not work successfully, is a technique that we had previously used to locate the critical-mixing

temperature of density-matched mixture of two fluids. In a density-matched critical mixture gravity has

little effect and the technique gave clear indications, with 10 #K precision, whenever Tc was crossed.

However, we were not able to try the technique with a pure-fluid system on Earth because the dynamics of

density stratification were too rapid. Our single zig-zag search on orbit was the first time that this technique

had been tried in a pure fluid.

The zig-zag search started only 200/2K above the critical temperature. Figure 7 shows the manner

in which the temperature of the sample changed, altemately cooling by 20 _uK and then warming by 10

/_K; Figure 8 shows the response of the turbidity signal during the same portion of the search. Although it

is clear that the sample is responding to the changes in temperature, we never detected a clear hysterisis in

the turbidity which we expected to indicate that we had crossed Tc. We believe now that several

unexpected effects in the turbidity signal (see the next section) combined to obscure the transition. The

entire period of the zig-zag search is shown in figure 9. At some point just past MET day 3, hour 12, the

sample first crossed Tc and began the early stages of spinodal decomposition, indicated by the increasing

amplitude of the variations in the signal. However, we will not be able to extract more precise information

until we understand all of the effects in the turbidity response better.

Our second technique for locating Tc, which was successful, is a simple scanning procedure. The

set point of the thermostat's control shells was changed in small steps of 50 /_K every 6 minutes

(figure 10), which caused the temperature of the sample to change at a uniform rate of 500 /tK/hour

(figure 11).

During the scan we monitored two signals as the sample crossed T¢: The turbidity of the sample,

and the photon count rates of the light scattered by the sample. Figure 12 shows the turbidity signal

during the first of the three scans done during the mission. Between hour 4 and hour 6 the signal

increased steadily as the critical temperature was approached. At a few minutes past hour 6 there was a

small break in the increase, followed by a swift rise, then decline, in the signal. The break marked the

sample's crossing T¢; the large peak was caused by the sudden increase in density fluctuations as the fluid

5-140
began spinodal decomposition, the initial stages of phase separation in which the fluctuations first become

unstable and begin to grow in size. This increase passed as the fluid began to resolve itself into distinct gas

and liquid phases, and the turbidity signal dropped.

That this rather unassuming feature in the turbidity signal marked the phase boundary was

confu-med by the intensity of the forward-scattered light, shown over the same time period in figure 13.

The light scattered at this angle increased as the critical point was approached, and then dropped as

fluctuations at this wavelength lost stability and the fluid began phase separation. The position of the peak

in the scattering-intensity curve matched that of the feature in the turbidity curve.

For comparison, figure 14 shows a scan done on the instrument during preflight testing on the

ground, at the same rate as the one in figure 12. There are two notable differences in the curves, both

related to the effects of gravity. In the second hour of the terrestrial scan the turbidity appeared to be

decreasing rather than increasing steadily. This was caused by the slowly forming density gradient in the

highly compressible sample: As the gradient became more severe, the amount of fluid near enough to its

critical density to show increasing turbidity decreased, and the net effect was a decrease in the turbidity

signal. In addition, the small feature immediately preceding the phase-decomposition peak that marked the

phase transition is not even visible. At this level of resolution, gravity had broadened the critical point to

such an extent that it effectively disappears from the turbidity response.

During the mission the collection of three scans and the zig-zag Tc determinations seemed to show

a drift in the critical temperature of 800/_K, substantially larger than we had seen during all testing with

this instrument and with the functionally identical engineering model. However, we now believe that this

may have been caused by small density rarifications in the beam path that resulted from repeated crossings

into the two-phase region that were not completely removed by the warming procedures which we used to

rehomogenize the sample. Subsequent ground scans with the instrument, along with the first on-orbit scan

and previous ground measurements now tell us unambiguously that the critical temperature of the sample

had indeed been stable during the mission to within the precision of our measurement of - 50 /_K. This

gives an upper boundary of 120/_K/year on the stability of this sample, a drift rate spectacularly better

than any previously experienced.

5.5. TURBIDITY MEASUREMENTS AND RELATED EFFECTS

Turbidity is the natural logarithm of the sample transmission, per unit length, and is a measure of

the overall cloudiness of the sample. We measured the intensity of the light leaving the sample and ratioed

that with the intensity of the light entering the sample; special electronics then calculated the logarithm of

this ratio, giving us a measurement proportional to the turbidity of the xenon sample.

The turbidity of a fluid increases as the fluid is taken nearer to its critical point; measuring the

turbidity gives information about the average size (the correlation length) of critical fluctuations. The

turbidity measurements which we made during the mission have proved to be the biggest challenge to our

understanding of the flight data. During the mission the turbidity signal showed small, long-time drifts at

constant temperatures, sometimes to smaller values, sometimes to larger values. We believe that this was

related to the small density inhomogeneities which we conjecture caused the apparent drifts in Tc.

However, the analysis of these data are far from complete at this point.

Extracting values which actually represent the turbidity of the sample is complicated by two known

effects which make difficult a direct, simple turbidity measurement. We have modeled these effects

extensively and we now have a better understanding of their contribution. The first has to do with the

5-141
optical properties of the sample cell itself; the second is a manifestation of the thermal response of a fluid

near its critical point.

The interior of the sample cell created a 100-pm thick disk of xenon, the portion of the sample

through which the laser beam passed, by bounding the region with two polished, parallel, fused-quartz

windows. Reflections of the beam from the window surfaces can interfere, changing the intensity of the

transmitted beam depending on the spacing of the windows. When the temperature of the sample cell was

changed, thermal expansion or contraction of its components altered the window spacing and produced a

measurable change in the turbidity signal, independent of the turbidity of the xenon itself.

Figure 15 shows the turbidity signal responding to 100 mK steps in the temperature of the sample

cell, which warmed the cell from 1 to 5 K above the critical temperature of the xenon; this curve is from

preflight testing of the instrument. At these temperatures the actual turbidity of the fluid is negligible and

we are viewing the interferometric behavior of the sample cell only.

The large, sinusoidal variations in the curve were the result of the interference effect, showing that

the separation of the windows changed by about 0.75 orders of interference per Kelvin, at approximately

400 orders of interference. This corresponded to a change in the window separation of 0.28 /urn/K, a

rather small effect. Nevertheless, it is clear from the magnitude of the change in figure 15 that in order to

make good use of our turbidity measurements it is vital that we know where on this interference curve the

measurements were made.

We were able to fit curves like this one with ample precision to make the necessary correction to

the turbidity measurements. Important questions about how this characteristic of the instrument changes

with time are now settled as well. We have found, from analysis of testing both before and after the

mission, that the amplitude of this curve was stable and predictable. However, the phase of the

interference showed a drift with time which suggests that the sample cell exhibits a small mechanical creep

of its longitudinal spacing of about 10 nm/month. With this information we can be confident that we have

placed the flight measurements at the appropriate place on the interference curve.

There is a second feature evident in figure 15 that is related to the fluid behavior itself. With each

temperature step the turbidity signal, rather than simply moving to a new value appropriate to that

temperature, overshot and then returned to the expected value; all of these temperature steps were of the

fast-change variety. Although the fluid at these temperatures showed no critical enhancement to its

turbidity, it was still near enough to the critical point that it was highly compressible. We have shown in

previous work [see bibliography: papers of H. Boukari, J.N. Shaumeyer, M.E. Bridges, and R.W.

Gammon; H. Bourkari, M.E. Briggs, J.N. Shaumeyer, and R.W. Gammon; dissertation of H. Boukari]

that highly-compressible critical fluids can make very fast changes in their density via an adiabatic heattransfer mechanism; this phenomenon is a manifestation of the adiabatic mechanism.

As the temperature change began, heat entered the outer walls of the sample cell and quickly caused

a large thermal expansion of a thin layer (order 10 tma) of fluid near the wall. As a result, because of high

compressibility, the bulk of the fluid was subject to adiabatic compression, which caused the bulk to heat

rapidly. Significantly, the density of the bulk had changed, changing its index of refraction, hence the

optical path length of the laser beam passing through that part of the sample. Thus, during the temperature

change itself, the fluid in the optical path of the cell executed density changes which affected the

interference characteristics of the cell. Note that this transient response depends on the temperature of the

fluid and decreased as the sample was moved further from Tc.

5-142
We have been modeling this phenomenon extensively, as well as performing controlled

experiments with our engineering model of the instrument. These analyses have now converged in a

satisfying way.

Figure 16 demonstrates the adiabatic effect, using data from the engineering model. The sample

cell was subjected to a fast change of 100 mK, far enough from Tc that the actual turbidity of the xenon is

negligible. Recorded in the lower curve is the response in the measured turbidity signal showing the

overshoot and then return to its final, equilibrium value. It is significant that the final value is reached at

precisely the same time as the temperature change is completed.

The upper curve in figure 16 shows data from our calibration of the apparent change in turbidity,

represented as a change in phase along the interference curve, due solely to the interference effects of cell

expansion. The difference between these two curves, shown in figure 17, shows the effect due solely to a

change in the fluid density from adiabatic heating effects. Since the adiabatic effects came into play only in

response to a changing temperature, we can verify this relationship by comparing the adiabatic response in

figure 17 to figure 18, which is the derivative of the sample-cell temperature measured during the

temperature change: They have exactly the same shape. Note that the temperature is measured at the cell

wall, approximately 1.5 cm from the region of fluid probed by the laser, confirming the long-range action

of the adiabatic effect.

All of the mysteries of the flight turbidity data have not been settled. Figure 19 shows the transient

response of the turbidity signal when the sample-cell temperature was changed by 70 mK (starting from

Tc+O. 1 K). The adiabatic contribution that we have measured on Earth is evident here in the initial sudden

decrease in the signal as the temperature change began. However, this was soon overwhelmed by a much

larger change in the opposite direction, followed by a slower settling to the final value. We know that this

response is apparent because gravity has been removed: We had never seen it in data taken during ground

testing. We believe that it is related to later stages in an adiabatic response that is quenched by convection

in 1 g, but we have not yet completed the modeling of the behavior.

5.6. CRITICAL-POINT FLUCTUATION DECAY RATES

During the mission we collected 32 sets of correlograms at 23 different temperature set-points,

covering a temperature range from 560 mK to 10 /_K above the critical temperature, as well as three

temperatures below critical. In all, there are 500 correlograms for each of the two scattering angles (in the

fluid) of 10.4478 ° (forward scattering) and 169.5636 ° (backscattering); forward-scattering and

backscattering correlograms were collected simultaneously. The data were taken with two different

incident-beam intensities: 17 and 1.7/_W.

The next several figures show representative correlograms, illustrating the range of temperatures

and incident-beam intensities. Figure 20 shows a pair of correlograms taken 560 mK from Tc, the

furthest from the critical temperature at which we made measurements. As in all the correlograms, the

upper curve with the slower decay is the forward-scattering correlogram. At this temperature the intensity

of the scattered light was at its lowest, although we were using the higher power beam path. The decays of

the two curves, particularly the backscattering curve, are barely discernible, although the measurements are

statistically meaningful.

Figure 21 shows a pair of correlograms taken 1 mK above the critical temperature, using the

higher-intensity beam path. At this temperature the critical fluctuations were well developed, the intensity

of the scattered light was relatively high in both directions, and the correlograms appeared virtually

noiseless. However, a new feature is evident here in the backscattering correlogram: It shows two decays.

5-143
The seconddecay,which has the sametime scaleas the forward scatteringcorrelogram,is actually

forwardscatteredlight.

Theforward-scatteringcrosssectionhadbecomelargeenough,100timesthe backscatteringcross

section,thatthesmallamountof light reflected in the backward direction at the exit window of the sample

cell contributed a forward-scattering correlation to this signal. It was for this reason that the two scattering

angles were chosen to be precise supplements: Results from the analysis of the forward-scattering

correlograms could be used to remove this contribution from the backscattering correlograms.

For comparison, figure 22 shows a pair of correlograms at the same temperature as in figure 21,

but at the lower beam-intensity of 1.7/2W. The backscattering correlogram shows an increased amount of

noise (due to the statistical process of the light detection), but the decay rate obtained from it is still

precisely determined. Finally, figure23 shows correlograms collected at Tc+100 /tK. These

measurements are well inside a region in which, on Earth, the observation of fluctuation correlations is

completely distorted by gravity.

The data sets were collected for a total amount of time which we had predicted would result in

fluctuation decay-rate measurements with a precision of 1 percent or better. We achieved this precision

with all of the measurements except for the backscattering measurement at the warmest temperature, where

the extremely low intensity of the scattered light precluded it. Each data set was made up of 10-15 separate

correlograms so that we could examine our results statistically.

The decay rate values were extracted from the correlograms by nonlinear, least-squares fitting. The

forward-scattering correlograms were fit to a simple decaying exponential; for the backscattering

correlograms the forward-scattering contribution was first removed where necessary, and then a simple

decaying exponential was used.

The results of the fluctuation decay-rate measurements are shown in figure 24, the backscattering

decay rates, and figure 25, the forward-scattering decay rates. In both figures we have included data from

ground testing on the instrument (in both cases the upper curve of symbols), as well as a line representing

the best available theoretical predictions.

Quite evident in these graphs is the effect of removing gravity and the distortions it creates in the

density of the sample, already apparent as far as 30 mK from the critical temperature. The limiting values

of these curves is a significant measurement, and it is clear from these graphs that it is not possible to make

accurate measurements of the limits on Earth, whereas we have been able to penetrate well into the limiting

region without the distortions due to gravity. As measurements were made closer yet to Tc, the density

gradient in the Earth-bound sample as probed by the 100-/_m diameter of our focused laser beam led to

errors of 100 percent, doubling the apparent decay rate in the terrestrial measurements compared with the

measurements in _g.

We believe that the limiting values of the fluctuation decay rates from our data set will be the key

contributions from this flight of the Zeno instrument. It is true that some questions remain about the

homogeneity and precise density of the xenon at each temperature, questions raised by the drifts in the

turbidity signal. Nevertheless, we have made measurements so close to the critical temperature that we

have been able to make accurate measurements of the limiting values of the decay rate, accurate regardless

of the possibly small variations in density. These limits are beyond the predictive capabilities of current

theories and represent a challenge and target for future calculations of transport properties in near-critical

systems.

5-144
To conclude, we present our results for the limits. From the backscattering, the limiting decay rate

is F = (51.2---0.4)x103 rad/s at a scattering angle in the fluid of 169.5636 °. From the forward scattering,

the limiting decay rate is F = 36.8___0.3 rad/second at a scattering angle inside the fluid of 10.4478".

5.7. CONCLUSION

The Zeno instrument and the special flight opportunity of the USMP-2 mission, with its

exceptional commitment of mission time to making fundamental scientific measurements, have given us an

excellent data set to contribute to measurements on liquid-vapor critical points. The data are beautiful and

precise. The outstanding performance of the Zeno instrument during the mission gave a fine demonstration

of the possibility of making high-precision, materials measurements in low-gravity, as well as the power

of a flexible, ground-commanded experiment.

5-145
°

.

.

.

,

.

.

.

°

10.

11.

12.

5.8. SELECTED BIBLIOGRAPHY

Zeno Publications

Kopelman, R.B., "Light Scattering Measurements of Critical Fluctuations in an Optically Thin

Binary Liquid Sample," Ph.D. Dissertation, University of Maryland, College Park, MD, 1983

(University Microfilms, Ann Arbor, #AAD84-05662).

Moldover, M.R., and R.W. Gammon, "Capillary Rise, Wetting Layers, and Critical Phenomena

in Confined Geometry," J. Chem. Phys., 80, 528-535, 1984.

Kopelman, R.K., R.W. Gammon, and M.R. Moldover, "Turbidity Very Near the Critical Point of

Methanol-Cyclohexane Mixtures," Phys. Rev., A29, 2084, 1984.

Takagi, Y., and R.W. Gammon, "Brillouin-Scattering in Thin Samples: Observation of

Backscattering Components by 90 ° Scattering," J. Appl. Phys., 61, 2030-2034, 1987.

Gammon, R.W., "Critical Fluid Light Scattering," in The Nation's Future Material Needs, T.

Lynch, J. Persh, T. Wolf, and N. Rupert, eds., Soc. Adv. Mater. & Process Eng., Covina, CA,

1987, Proceedings of the 19th International Technical Conference of the Society for the

Advancement of Material and Process Engineering (SAMPE), Oct. 13-15, 1987.

Gammon, R.W., and J.N. Shaumeyer, "Science Requirements Document for Zeno," NASA,

Washington, DC, 1988.

Shaumeyer, J.N., and R.W. Gammon, "Optimizing Fitting Statistics in Photon Correlation

Spectroscopy," in OSA Proceedings on Photon Correlation Techniques and Applications, Volume

1, J.B. Abbiss and A.E. Smart, eds., Optical Society of America, Washington, DC, pp. 14-17,

1988.

Gammon, R.W., "Description of the Photon-Correlation Instrument for Zeno," NASA Laser Light

Scattering Advanced Technology Development Workshop, NASA Lewis Research Center,

September 1988, NASA Conference Publication 10033, 1989.

Boukari, H., J.N. Shaumeyer, M.E. Briggs, and R.W. Gammon, "Critical Speeding Up in Pure

Fluids," Phys. Rev., A41, 2260-2263, 1990.

Boukari, H., M.E. Briggs, J.N. Shaumeyer, and R.W. Gammon, "Critical Speeding Up

Observed," Phys. Rev. Lett., 65, 2654-2657, 1990.

Gammon, R.W., "Investigations of Fundamental Phenomena in Microgravity," Proceedings

IKI/AIAA Microgravity Symposium, American Institute of Aeronautics and Astronautics, pp.

379-381, 1991.

Shaumeyer, J.N., R.W. Gammon, and J.V. Sengers, "Photon-Correlation Spectroscopy," in

Measurement of the Transport Properties of Fluids, Experimental Thermodynamics, Volume III,

W.A. Wakeham, A. Nagashima, and J.V. Sengers, eds., Blackwell Scientific, Oxford, pp. 197-

213, 1991.

5-146
13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

.

.

,

Boukari, H., "Adiabatic Effects in a Fluid Near its Liquid-Gas Critical Point: Theory and

Experiment," Ph.D. Dissertation, University of Maryland, College Park, MD, 1992.

Zhang, K.C., M.E. Briggs, and R.W. Gammon, "The Susceptibility Critical Exponent for a

Nonaqueous Ionic Binary Mixture Near a Consolute Point," J. Chem. Phys., 97, 8692-8697,

1992.

Briggs, M.E., "Photothermal Deflection in a Supercritical Fluid," Ph.D. Dissertation, University

of Maryland, College Park, MD, 1993.

Briggs, M.E., R.W. Gammon, and J.N. Shaumeyer, "Measurement of the Temperature

Coefficient of Ratio Transformers," Rev. Sci. Instrum., 64, 756-759, 1993.

Avramov, S., and N.M. Oldham, "Automatic Calibration of Inductive Voltage Dividers for the

NASA Zeno Experiment," Rev. Sci. Instrum., 64, 2676-2678, 1993.

Avramov, S., and N.M. Oldham, "Inductive Voltage Divider Calibration for a NASA Flight

Experiment," Proceedings of National Conference of Standards Laboratories Workshop and

Symposium, Albuquerque, NM, July 1993, pp. 225-232, July 1993.

Shaumeyer, J.N., M.E. Briggs, and R.W. Gammon, "Statistical Fitting Accuracy in Photon

Correlation Spectroscopy," Appl. Optics, 32, 3871-3879 (1993).

Avramov, S.,"Voltage Ratio Measurements Using Inductive Voltage Dividers," Ph.D.

Dissertation, University of Maryland, College Park, MD, 1994.

Palik, E.D., H. Boukari, and R.W. Gammon, "Line-Shape Studies for Single- and Triple-Pass

Fabry-Perot Interferometer Systems," Appl. Optics, 34, 58-68, 1995.

Boukari, H., E.D. Palik, and R.W. Gammon, "Closed-Form Expressions to Fit Data Obtained

with a Multipass Fabry-Perot Interferometer System," Appl. Optics, 34, 69-86, 1995.

Boukari, H., R.L. Pego, and R.W. Gammon, "Calculation of the Dynamics of Gravity-Induced

Density Profiles Near a Liquid-Vapor Critical Point," Phys. Rev. E, in press 1995.

NASA Reports

Gammon, R.W., and M.R. Moldover, "Light Scattering Tests of Fundamental Theories of

Transport Properties," in Microgravity Science and Applications Program Tasks, NASA TM

89607, NASA Office of Space Science and Applications, Washington, DC, p. 240, February

1985.

Gammon, R.W., and M.R. Moldover, "Light Scattering Tests of Fundamental Theories of

Transport Properties," in Microgravity Science and Applications Program Tasks, NASA TM

87568, NASA Office of Space Science and Applications, Washington, DC, p. 106, May 1986.

Gammon, R.W., "Critical Fluid Light Scattering," in Microgravity Science and Applications Flight

Programs, January--March 1987, Selected Papers, NASA TM 4069, Vol. 2, NASA Office of

Space Science and Applications, Washington, DC, p. 805, 1988.

5-147
. Gammon, R.W., J.N. Shaumeyer, and M.R. Moldover, "Zeno: Critical Fluid Light Scattering," in

Microgravity Science and Applications Program Tasks: 1987 Revision, NASA TM 4068, NASA

Office of Space Science and Applications, Washington, DC, p. 189, September 1988.

, Gammon, R.W., J.N. Shaumeyer, and M.R. Moldover, "Zeno: Critical Fluid Light Scattering," in

Microgravity Science and Applications Program Tasks: 1988 Revision, NASA TM 4097, NASA

Office of Space Science and Applications, Washington, DC, p. 245, January 1989.

, Gammon, R.W., and J.N. Shaumeyer, "Critical Fluid Light Scattering (Zeno)," in Microgravity

Science and Applications Program Tasks: 1989 Revision, NASA TM 4191, NASA Office of

Space Science and Applications, Washington, DC, p. 201, May 1990.

. Gammon, R.W., and J.N. Shaumeyer, "Critical Fluid Light Scattering (Zeno)," in Microgravity

Science and Applications Program Tasks: 1990 Revision, NASA TM 4284, NASA Office of

Space Science and Applications, Washington, DC, p. 261, May 1991.

, Gammon, R.W., and J.N. Shaumeyer, "Critical Fluid Light Scattering (Zeno)," in Microgravity

Science and Applications Program Tasks: 1991 Revision, NASA TM 4349, NASA Office of

Space Science and Applications, Washington, DC, p. 261, February 1992.

, Gammon, R.W., "Critical Fluid Light Scattering (Zeno)," in Microgravity Science and

Applications Program Tasks and Bibliography for FY 1992, NASA TM 4469, NASA Office of

Space Science and Applications, Washington, DC, p. 11-69, March 1993.

Critical Phenomena

Static Critical Phenomena

. Einstein, A., Ann. Phys. (Leipzig), 33, 1275 (1910); English translation: "Theory of Opalescence

of Homogeneous Liquids and Mixtures of Liquids in the Vicinity of the Critical State," in Colloid

Chemistry, J. Alexander, ed., Rheinhold, New York, vol. I, p. 323, 1913.

2. Stanley, H.E., Introduction to Phase Transitions and Critical Phenomena, Oxford, New York,

1971.

3. Wilson, K.G., and J. Kogut, "The Renormalization Group and the e Expansion," Phys. Rep.,

12C, 77, 1974.

Dynamic Critical Phenomena

1. Swinney, H.L., and D.L. Henry, "Dynamics of Fluids Near the Critical Point: Decay Rate of

Order-Parameter Fluctuations," Phys. Rev., A8, 2586, 1973.

2. Kawasaki, K., in Phase Transitions and Critical Phenomena, C. Domb and M.S. Green, eds.,

Academic, New York, vol. 5a, p. 165, 1975.

. Siggia, E.D., B.I. Halperin, and P.C. Hohenberg, "Renormalization-Group Treatment of the

Critical Dynamics of the Binary-Fluid and Gas-Liquid Transitions," Phys. Rev., B13, 2110.

1976.

5-148
,

.

.

.

.

.

.

.

.

.

.

.

.

.

Giittinger, H., and D.S. Cannell, "Correlation Range and Rayleigh Linewidth of Xenon Near the

Critical Point," Phys. Rev., A22, 285, 1980.

Sengers, J.V. "Universality of Critical Phenomena in Classical Fluids," in Phase Transitions:

Cargese 1980, M. Levy, J.-C. LeGuillou, and J. Zinn-Justin, eds., Plenum Publishing Corp.,

New York, 1982.

Bhattacharjee, J.K, and R.A. Ferrell, "Critical Viscosity Exponent for a Classical Fluid," Phys.

Rev., A28, 2363. 1983.

Burstyn, H.C., J.V. Sengers, J.K. Bhattacharjee, and R.A. Ferrell, "Dynamic Scaling Function

for Critical Fluctuations in Classical Fluids," Phys. Rev., A28, 1567, 1983.

Berg, R.F., and M.R. Moldover, "Critical Exponent for the Viscosity of Carbon-Dioxide and

Xenon," J. Chem. Phys., 93, 1926, 1990.

Hao, H., "Aspects of Dynamic Critical Phenomena in a Single-Component Fluid," Ph.D.

Dissertation, University of Maryland, College Park, Maryland, 1991.

Critical-Fluid Light Scattering

Puglielli, V.G., and N.C. Ford, Jr., "Turbidity Measurements in SF6 Near its Critical Point,"

Phys. Rev. Lett., 25, 143, 1970.

Degiorgio, V., and J.B. Lastovka, "Intensity-Correlation Spectroscopy," Phys. Rev., A4,

2033, 1971.

Lunacek, J.H., and D.S. Cannell, "Long-Range Correlation Length and Isothermal

Compressibility of Carbon Dioxide Near the Critical Point," Phys. Rev. Lett., 27, 841, 1971.

Ford, Jr., N.C., "Light Scattering Apparatus," in Dynamical Light Scattering, Application of

Photon Correlation Spectroscopy, R. Pecora, ed., p. 7, Plenum, New York, 1985.

Low-Gravity Critical Phenomena

Hohenberg, P.C., and M. Barmatz, "Gravity Effects Near the Gas-Liquid Critical Point," Phys.

Rev., A6, 289, 1977.

Moldover, M.R., R.J. Hocken, R.W. Gammon, and J.W. Sengers, Overviews and Justifications

for Low Gravity Experiments on Phase Transitions and Critical Phenomena in Fluids, National

Bureau of Standards Technical Note 925, U.S. Government Printing Office, Washington, DC,

1976.

Moldover, M.R., J.V. Sengers, R.W. Gammon, and R.J. Hocken, "Gravity Effects in Fluids

Near the Gas-Liquid Critical Point," Rev. Mod. Phys., 51, 79, 1979.

Beysens, D., "Critical Phenomena," in Materials Sciences in Space, A Contribution to the

Scientific Basis of Space Processing, B. Feuerbacher, H. Hamacher, and R.J. Naumann, eds., p.

91, Springer, Berlin, 1986.

5-149
,

.

.

l°

,

.

o

,

,

,

.

Beysens, D., J. Straub, and D.J. Turner, "Phase Transitions and Near-Critical Phenomena," in

Fluid Sciences and Materials Science in Space, A European Perspective, H.U. Walter, ed., p. 221,

Springer, Berlin, 1987.

Beysens, D., "Recent Developments in Critical and Near-Critical Point Phenomena," in

Proceedings VIIth European Symposium on Materials and Fluid Sciences in Micro-gravity,

ESA SP-295, 1990.

Guenoun, P., B. Khalil, D. Beysens, Y. Garrabos, F. Kammoun, B. Le Neindre, and B.

Zappoli, "Thermal Cycle Around the Critical-Point of Carbon-Dioxide Under Reduced Gravity,"

Phys. Rev., E47, 1531, 1993.

Adiabatic Effects Near the Critical Point

Dahl, D., and M.R. Moldover, "Thermal Relaxation Near the Critical Point," Phys. Rev., A6,

1915, 1972.

Onuki, A., H. Hong, and R.A. Ferrell, "Fast Equilibration in a Single-Component Fluid Near the

Liquid-Vapor Critical Point," Phys. Rev., A41, 2256, 1990.

Onuki, A., and R.A. Ferrell, "Adiabatic Heating Effect Near the Gas-Liquid Critical Point,"

Physica, A164, 245, 1990.

Behringer, R.P., A. Onuki, and H. Meyer, "Thermal Equilibration of Fluids Near the LiquidVapor Critical-Point-3He and 4He Mixtures," J. Low Temp. Phys., 81, 71, 1990.

Zappoli, B., D. Baily, Y. Garrabos, B. Le Neindre, P. Guenoun, and D. Beysens, "Anomalous

Heat Transport by the Piston Effect in Supercritical Fluids Under Zero Gravity," Phys.

Rev., A41, 2264, 1991.

Ferrell, R.A., and H. Hao, "Adiabatic Temperature Changes in a One-Component Fluid Near the

Liquid-Vapor Critical Point," Physica, A197, 23, 1993.

Berg, R.F., "Thermal Equilibration Near the Critical Point: Effects Due to Three Dimensions and

Gravity," Phys. Rev., E48, 1799, 1993.

Zhong, F., and H. Meyer, "Numerical Calculation of Density Equilibration Above the 3He LiquidVapor Critical Point," in Proceedings of NASA/JPL Microgravity Low Tempera-ture Workshop,

Washington, DC (1994), U. Israelson and D. Strayer, eds., NASA Document JPL D-11775, p.

73, NASA, Washington DC, 1994.

5-150
Figure5-1.

LASER

/_

M2

L2 OPO!

I

PD2_,

IM4 _:'"

PMT t

A schematic of the optical layout of the instrument, showing its main components:

Beamsplitter (B1), mirrors (M1, M2, and M3), apertures (A1-A4), photodiodes (PD1

and PD2), and photomultiplier tubes (PMT1 and PMT2). The thermostat is the cylindrical

object in the center.

le+00

le-Ol

le-02

1t_.-03

I e-04

le-05

le-06

1 2 3 4

i I

\L

i i

5 6 7 8 9

MET (days)

I I I I

I

10 11 12 13

Figure 5-2. The timeline of the experiment, given as the temperature of the sample (measured in distance

from Tc) versus MET in days.

5-151
I

2

0

-4

I I

1100 1200 1300 1400 1500

time on MET day 5 (hours)

Figure 5-3. The stability of the sample temperature at 1 mK, over a four-hour period while collecting

correlograms. The rms of the variations is 1.7/2K. (Time is in hhmm format.)

Figure 5-4.

3.0

2.5

"-_ 2.0

1.5

1.0

0430

I I I

I I I

0500 0530 0600

time on MET day 5 (hours)

0630

The temperature of the sample cell during a slow temperature step of-2 mK, shown as

distance from Tc. The time constant for thermal relaxation of the sample cell is about 30

minutes. (Time is in hhmm format.)

5-152
300,

--- 250 -

;-4

©

L

k9

200 -

u

15o -

100 -

2100

I I

I I

2130 2200 2230

time on MET day 5 (hours)

Figure 5-5. The temperature of the sample cell during a fast temperature step of -200 _tK, shown as

distance from To (Time is in hhmm format.)

2.0

1.8

"_ 1.6

1.4

N

1 9 O "_

1.0

' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '

<>

,I _ i il ' . .I , , ,I . . .I . . .I . . .

le-06 le-05 0.0001 0.001 0.01 0.1

delay time (s)

Figure 5-6. A correlogram example showing simple exponential decay on a logarithmic time scale.

Taken at T-Tc = 100 mK at a laser power of 17 gW, this is a forward-scattering

correlogram.

5-153
Figure5-7.

20

O

10

E

0

L)

,- -10

¢9

-20

-30

0800

I I I

I I I

0900 1000 1100 1200

time on MET day 3 (hours)

A section of the zig-zag Tc search, showing the pattern of sample temperature changes:

Cooler by 20 /_K, then warmer by 10 oK. The data are smoothed over a 300-second

interval. The turbidity during this same time is shown in figure 8. (Time is in hhmm

format.)

Figure 5-8.

0.5

..... 0.25

= 0

-0.25

I I i

_0.51 I I I I

0800 0900 1000 ] 100 1200

time on Mt']T day 3 (hours)

A section of the zig-zag Tc search, showing the response of the sample turbidity to the zigzag temperature steps shown in figure 7 for the same time period. The data are smoothed

over a 300-second interval. (Time is in hhmm format.)

5-154
]0 i , i i i

Figure 5-9.

"u

a_

2

0

m

I I I I I

2:0000 2:1200 3:0000 3:1200 4:0000 4:1200

time (MET day:hours)

The turbidity signal during the entire 64-hour zig-zag Tc search. (Time is in day: hhmm

format.)

Figure 5-10.

_" ]50 _ i

 oov

f -

-d

I

0545 0600 0615

time o_l MET day 1 (hours)

The temperature of the high-precision control shell of the thermostat, during the first scan

for Tc. The temperature of this shell, shown making -50 #K steps, determines the sample

temperature, shown in figure 11. (Time is in hhmm format.)

5-155
Figure5-11.

150

o 100

._

"-_ 50

_o

.-_ 0

_ -50

-_ -100

-150

0545

I

I

0600 0615

time oil MET day 1 (hours)

The sample temperature during the first scan for To in response to the control-shell steps

shown in figure 10. The result is a uniform sample cooling of 500 _K/hour. (Time is in

hhmm format.)

Figure 5-12.

12

10

LE 6

4

= 2

:...s

0

_9

! I

, l i

0,t00 0500 0600 0700

time on ME]' day 1 (hours)

The turbidity signal during the first scan for To The small feature just past 0600 hours is

where the sample crosses Tc; the large peak is spinodal decomposition. (Time is in hhmm

format.)

5-156
40000

"_ :_0000 _

0400 0500 0600 0700

time on MET day 3 (hours)

Figure 5-13. Forward-scattering photon count rate during the first scan for To showing from the break

in slope that the sample crosses Tc just past 0600 hours. (Time is in hhmm format.)

Figure 5-14.

I I I

6

2

.= 0

-2

-6 I I I

0 1 '2 3 ,1

elapsed time (hours)

The turbidity signal during a 500/aK/hour scan for Tc done in 1-g during preflight testing.

Compare this scan with the/_-g scan shown in figure 12.

5-157
Figure 5-15.

¢,

_at3

cO

¢/}

6

4

2

0

-2

-4

-6

-8

0

I I I I I

I I I

l.....

I I I I I

6 8 10 12 14

elapsed time (hours)

I I I I

'2 4 16 18 20

The turbidity signal, in preflight testing, during a warming sequence between Tc+ l K to

T¢+5 K, made in steps of 0.1 K. The sinusoidal shape is due to cell expansion; the small

spikes result from changes in fluid density.

Figure 5-16.

0.4

0.3

0.2

._

0.1

0.0

-0.1

-X",,-. I I I I

N

%\_\\

\\\\\XN\ _

0 200 400 600 800 1000

time (s)

Data taken on the engineering model of the instrument show, in the lower curve, the

response of the turbidity signal to a change in temperature from Tc+0.6 K to Tc+0.7 K.

The upper curve is the change due solely to cell expansion. The vertical units are the phase

along the interference curve for the cell.

5-158
Figure 5-17.

c_

e_

0

-0.1

-0.2

-0.3

I I I I

I I I I

0 200 4 O0 600 800 1000

time (s)

The difference between the two curves in figure 16, showing the response of the turbidity

signal due solely to changes in the fluid density. Compare with figure 18. (These data are

from the engineering model of the instrument.)

Figure 5-18.

o -100

"_ -200

i

-300

I I I I

I I I I

0 200 400 600 800 1000

time (s)

The time derivative of the temperature change which produced the turbidity signal shown

in figure 16; this curve has the same shape as the curve in figure 17. (These data are from

the engineering model of the instrument.)

5-159
k3

I I I

I I I

1930 2000 2030 2100

time on MET day 4 (hours)

Figure 5-19. The turbidity response to a temperature change of-70 mK, starting at Tc+0.1 K, showing

cell interference, adiabatic response, and an as-yet unidentified feature.

2.0

1.8

",.__ 1.6

E

1.4

N

¢I

E

_ 1.2 C

1.0

' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '

o ,0 °

- d...+o o

I iI I

le-06

+ + _v

+ _ '_,, -_

%_.++ +%L........... I

+ ++ _q-+

, ,I , 4-_1 , , ,I . , ,I , , ,I , , ,_

le-05 0.0001 0.001 0.01 0.1 l

delay time (s)

Figure 5-20. Dual correlogram taken at Tc + 560 mK with 17/_W of laser power; the integration time

was 6 minutes. Forward scattering (upper curve) had an average count rate of 710

counts/second; backscattering (lower curve) had 435 counts/second.

5-160
2.0

v 1.8

_ 1.6

-_ 1.4

N

_ 1.2

1.0

' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '1 ' ' '

_O<_Z ii "'" " '

-'H':-

, ,I , , ,I , , ,I , , i] i i ,I , ' li i i i

1e-06 le-05 0.0001 O.OOl 0.01 O.l

delay time (s)

Figure 5-21. Dual correlogram taken at Tc+l mK with 17/_W of laser power; the integration time was

10 minutes. Forward scattering (upper curve) had an average count rate of 446,000

counts/second; backscattering (lower curve) had 17,800 counts/second.

2.0

].8

"-& 1.6

1.,t

_- 1.2 0

1.0 B

, .i , , ,i i

le-06 le-05

i II i i ,I , , ,I , , ,I ,

0.0001 0.001 O.Ol 0.1

delay time (s)

| , .[ | . |

Figure 5-22. Dual correlogram taken at Tc+ 1 mK with 1.7/_W of laser power; the integration time was

15 minutes. Forward scattering (upper curve) had an average count rate of 27,100

counts/second; backscattering (lower curve) had 2,200 counts/second.

5-161
Figure 5-23.

¢d

o_

t_

t'q

0_

©

2.0

1.8

1.6

1.4

1.2

1.0

' '1 ' ' '1 ' ' '1 ' ' 'i .........

- .

• __

, ,I , , ,I , , ,I , , ,I , , ,I , , ,I , I I

le-06 le-05 0.0001 0.001 0.01 0.1

delay time (s)

Dual correlogram taken at Zc+lO0/_K with 1.7 _W of laser power; the integration time

was 15 minutes. Forward scattering (upper curve) had an average count rate of 45,000

counts/second; backscattering (lower curve) had 3,000 counts/second.

Figure 5-24.

,%-

0.;

0_

q,

¢.,

60000

55000

' ' ' ' ''"1 ' ' ' ' ''"1 ' ' ' ' ''"1

5oooo

45000 ........ i ........ i ........ i

1(,-05 0.0001 0.001 0.01

T-Tc (K)

u I -! l llll

t

i | | i|1,

0.

Backscattering decay rates as a function of temperature; the vertical scale is linear, the

horizontal scale logarithmic. The crosses are 1-g data from preflight testing; the diamonds

are the mission data. The curve is our best theoretical estimate. The error bars show the

rms of the data.

5-162
1000 ........ I ........ I ........ ) ........

I00

+ ++ + =k +

o o_ o _ P---°#'

10 ........ I ........ I ........ I .......

le-05 0.0001 0.001 0.01 0.

T-To (K)

Figure 5-25. Forward-scattering decay rates as a function of temperature; both scales are logarithmic.

The crosses are 1-g data from preflight testing; the diamonds are the mission data. The

curve is our best theoretical estimate. The error bars, generally smaller than the symbols,

show the rms of the data.

5-163
Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188

Puldicreportingburdenforthiseofledonof informationis utimated to _erage I hourperre=ponse,includingthe timefor rev_ instructions,torching existingdatatmume=,

garneringand rr=intainingthe data needed,and¢o_ and re_ the ¢olection_ ifdom_ation.Sendcommenlsrngardingthisburdenestimaleor anyothe¢a=pectofthis¢ofleclion

of irdomWk_ includingsugg_(:ms forreducingthisbulden,to Wmhinglon_u,=rte*'s _ Directoratekx Irdom'mtionO_ andReports,12t5 Jeifer_onDavisHigtht_j,

Suite 1204,Arlington,Va 222024302, andto the Officeof Managementand Budget,Papenm_ ReduclionProject(0704-0188),Washington,DC 2050_

1. AGENCY USE ONLY (Leave Blank) 12 REPORTDATE April 1996 13. REPORT TYPE Technical AND DATES COVERED Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Second United States Microgravity Payload: One Year Report

6. AUTHOR(S)

Peter A. Curreri and Dannah E. McCauley.* Editors

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATON

REPORT NUMBERS

M-806

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-4737

11. SUPPLEMENTARY NOTES

Prepared by Space Sciences Laboratory,

*University of Alabama in Huntsville

Science and Engineering Directorate

12a DISTRIBUTION_VAILABILITYSTATEMENT

Unclassified-Unlimited

Subject Category 39

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The second United States Microgravity Payload (USMP-2), flown in March 1994, carried four

major microgravity experiments plus a sophisticated accelerometer system. The USMP

program is designed to accommodate experiments requiring extensive resources short of a

full Spacelab mission. The four USMP-2 experiments dealt with understanding fundamental

aspects of materials behavior, three with the formation of crystals from melts and one

with the critical point of a noble gas. This successful, scientifically rich mission also

demonstrated telescience operations.

14. SUBJECT TERMS

microgravity, USMP, crystal growth, critical point, metal alloys,

Seebeck, Peltier, telescience

17. SECURITY CLASSIFICATION

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassi fled

19. SECURITY CLASSIFICATION

OFABSTRACT

Unclassified

15. NUMBER OF PAGES

185

16. PRICE CODE

A09

20. LIMITATION OF ABSTRACT

Unl imi ted

NSN 7540 - 01 - 280 - 5500 StandardForm298 (Rev2_9)

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