The Advanced Automated Directional Solidification Furnace (AADSF) is a five zone tubular furnace designed for Bridgman-Stockbarger, other techniques of crystal growth involving multiple temperature zones such as vapor transport experiments and other materials science experiments. The five zones are primarily designed to produce uniform hot and cold temperature regions separated by an adiabatic region constructed of a heat extraction plate and an insert to reduce radiation from the hot to the cold zone. The hot and cold zone temperatures are designed to reach 1600 C and 1100 C, respectively. AADSF operates on a Multi-Purpose Experiment Support Structure (MPESS) within the cargo bay of the Space Shuttle on the United States Microgravity Payload (USMP) missions. Two successful flights, both employing the directional solidification or Bridgman Stockbarger technique for crystal growth have been made, and crystals of HgCdTe and PbSnTe grown in microgravity have been produced on USMP-2 and USMP-3, respectively. The addition of a Sample Exchange Mechanism (SEM) will enable three different samples to be processed on future flights including the USMP-4 mission

Cole, J. M.

Gillies, D. C.

Jeter, L. B.

Lehoczky, S. L.

Reeves, F. A.

Sledd, J. D.

English

NASA Technical Reports Server

NASA-TM-112678
PROCEED NGS REPR N--
SPIE--The International Society for Optical Engineering
Reprinted from
C ,"
ZJ
Space Processing of Materials
4-5 August 1996
Denver, Colorado
Volume 2809
©1996 by the Society of Photo-Optical Instrumentation Engineers
Box 10, Bellingham, Washington 98227 USA. Telephone 360/676-3290.
https://ntrs.nasa.gov/search.jsp?R=19970021160 2020-06-16T02:29:39+00:00Z
The Advanced Automated Directional Solidification Furnace
D. C. Gillies, F. A. Reeves, L. B. Jeter, J. D. Siedd, J. M. Coie
and S. L. Lehoczky
NASA Marshall Space Flight Center, Huntsville, AL 35812
ABSTRACT
The Advanced Automated Directional Sohdification Furnace (AADSF) is a five zone tubular
furnace designed for Bridgman-Stockbarger, other techniques of crystal growth involving
multiple temperature zones such as vapor transport experiments and other materials science
experiments. The five zones are primarily designed to produce uvMorm hot and cold temperature
regions separated by an adiabatic region constructed of a heat extraction plate and an insert to
reduce radiation from the hot to the cold zone. The hot and cold zone temperatures are designed
to reach 1600°C and i ]00°C respectively. AADSF operates on a Multi-Purpose Experimem
Support Structure (MPESS) within the cargo bay of the Space Shuttle on the United States
Microgravity Payload (USMP) missions. Two successful flights, both employing the directionai
solidification or Bridgman Stockbarger technique for crystal growth have been made, and
crystals of HgCdTe and PbSnTe grown in microgravity have been produced on USMP-2 and
USMP-3 respectively. The addition of a Sample Exchange Mechanism (SEM) will enable three
different samples to be processed on future flights including the USMP-4 mission.
2. INTRODUCTION
One of the major apphcatiom of microgravity is in materials, in particular the processing of
electronic materials, and metals and alloys by directional solidification, and also by chemical
vapor transport. There are many reasons for such studies in low earth orbit; these include the
need to reduce the fluid flow caused by gravity driven convection, the possibility of reducing the
solutal Rayleigh numbers by several orders of magnitude, and the ability to study the effects of
small accelerations on fluid flow and solidification. In addition, microgravity will reduce a
material's tendency to deform under its own weight immediately after solidification when it is
weakest. The possibility of containerless processing, including float zone techniques will also
aid in the production of more pure material, with less defects induced by contact with the
container walls. NASA has built and flown on the shuttle two versatile, complex furnace
systems specifically designed for materials processing. These are the Crystal Growth Furnace
(CGF) l, and the AADSF 2. Of these the CGF is designed to fly within the Spaceiab, and in a man
tended environment; the samples are loaded by the crew members on board the orbiter, but the
furnace is designed to run autonomously once the samples have been inserted. In contrast, the
AADSF or)crates on the MPESS structure within the cargo bay of the orbiter, the samples are
loaded prior to launch and are inaccessible once the mission is underway. As such the AADSF is
subject to the harsh environment of outer space with wide fluctuations of temperature, and has
tmdergone rigorous ground testing to simulate these conditions. Both CGF and AADSF were
built by Teledyne Brown Engineering in Huntsville, AL.
0-8194-2197-9/96/$6.00 SPIE Vol. 2809 /329
3. HARDWARE
AA.DSF consists of three major constituent sections of hardware, namely the Experimental
Apparatus Container (EAC), the Data Acquisition System (DAS) and the Signal Conditioning
and Control System (SCCS). The functions of these three elements are summarized in Table 1.
The EAC is an environmental container designed to isolate the heating element and the
translation mechanisms of the furnace system within a controlled environment. As such it is
vacuum tight, with feedthroughs for required power, coolant loops and monitoring devices.
During use, the EAC normally operates with a positive pressure of argon. The outer jacket of the
ftmaace is 20.3 cm in diameter and incorporates the coolant loops on the outside. The coolant
used during flight is freon, which is provided through the MPESS carrier as part of the USMP
mission. Ground based testing uses water for cooling.
A major component of the EAC is the five zone furnace module itself. The zones consist of
beryilium oxide cores wrapped with platinum-rhodium (60/40) furnace wire. The Lurer diameter
of the furnace is 2.54 cm, but with the high temperature metallic insert the working diameter is
restricted to 1.91 am The gradient region is shown schematically in Figure 1. The hot zone is
25.4 cm long and the cold zone 12.7 crn long. The hot zone is equipped with a hot guard heater
to prevent "rolloff'., of the temperature, while the _'adient region can be adjusted by means of a
booster heater just above the adiabatic zone. Similarly, the cold zone is maintained more
isothermal by means of a short guard heater. Temperature control of the zones is by means of
thermocouples positioned close to the elements themselves. There are two thermocouples per
heater element, thus ensuring continuity of operation in the case of a failure of one of the
thermocouples. Two schematics of the EAC and its components are shown ha Figure 2.
The adiabatic zone consists of a re-configurable assembly with a cold plate or adiabatic ring,
a hot plate, both constructed of Zircar AL-30AA, a heat extraction plate of inconel 718, and a
high temperature insert composed of Haynes alloy 214. The nominal thicknesses of these
components are 4.32mm, 1.60 mm and 7.62 mm for the hot plate, heat extraction plate and cold
plate respectively. The configuration can be modified to include IX, 2X or 3X of each of these
components in any combination. The USMP-2 flight used the IX configuration to obtain a high
gradient. The USMP-3 flight used the 2X configuration.
The translation mechanism consists of a d.c. motor driving a lead screw. Translation rates in
the range of 0.5 to 50 mm per hour are availabie. This wide range of speeds allows for extremely
slow growth for directional solidification, and rapid insertion or withdrawal for other scientific
reasons such as the quenching ha of an interface. Experimenters have regularly used rapid
withdrawal for examining the shape and position of the solidifying interface.
The Sample Exchange Mechanism will be added for the USMP-4 Mission. This facility does
not affect the thermal characteristics, and in fact will be retro-fitted to some of the existing
furnace assembles. Three samples will be accommodated, and will be processed ha sequence.
The cartridge design will not change, and the samples will be securely held during launch and
landing. Due to the need to withdraw all of the samples above the opening to the furnace, the
new model will be taller than the original AADSF. This means that it can no longer be
accommodated at one side of the orbiter on the MPESS, but has to be positioned on the center
line. This modification, in fact, may help the science_ as will be described below.
330 / SPIE Vol. 2809
The other two main components of the AADSF system, as shown in Table I, are the Data
Acquisition System and the Signal Control and Conditioning System. These are housed outside
the EAC, being bolted to the MPESS as shown in Figure 3.
Other related components of the AADSF include the cabling to provide the system with 28
volt d.c. power from the main bus of the orbiter, and the fact that in the event of a computer
malfunction, the AADSF can be controlled by means of the orbiter on-board computer which can
re-boot the system be means of a program called the Bootstrap and Table Load Software (BTLS).
This would be done by the orbiter crew during the mission.
4. SCIENCE
The first two flights of the AADSF examined the solidification of two solid solution
semiconductor alloys, namely mercury cadmium telluride and lead tin telturide. As shown in
Figure 4, these alloys exhibit different fluid flow characteristics as driven by gravity on earth. Iv..
the case of lead tin telluride, the component rejected at the interface is less dense than the bulk
liquid, and so complete mixing ensues. In the case of mercury cadmium telluride, the rejected
component is denser and fluid flow is restricted to a small region close to the interface. The twc
experiments thus complemem each other very well. In the flight experiments, the intentions of
both investigators was to examine the amount of fluid flow present under different conditions of
small residual accelerations. The experiments are shown in Table 2.
The results from the mercury cadmium teUuride experiment showed that low gravity levels
alone would not result in diffusion controlled growth. Even with static residual acceleration
levels lower than 10tg0, there was considerable fluid flow. Figure 5a shows the effect in the first
half of the mission when the vector had a com_nent which was destabilizing with respect to the
solute. There is three dimensional fluid flow in the direction of the vector manifested by the
pushing of the higher density mercury rich material in the direction of the residual transverse
vector. The degree of inhomogeneity is in fact greater than on earth. On the other hand, as
shown in Figure 5b, when the vector is towards the solid, the flow is reduce, even with a large
(i.55 lag) transverse residual component. This flight gave clear demonstration of the nature of
three dimensional fluid flow in microgravity.
5. MISSION
As mentioned above, the AADSF is attached to the MPESS carrier within the payload bay
of the orbiter. The arrangement, and how the AADSF fits with other payloads is shown in Figure
3. This is the configuration as used for USMP-2 and USMP-3. For USMP-4, the AADSF is
taller, due to the addition of the sample exchange mechanism; this results in the necessity of
moving the instrument to the center line of the orbiter. This latter factor has significant
implications on the science. During shuttle missions, there can be restrictions on the attitude in
which the orbiter can fly'. These restrictions can lead to undesirable residual acceleration vectors
on the experiment itself. In the case of USMP-2, the two different attitudes shown in Figure 4
produced widely differing results. There are two principle origins of the residual acceleration
vectors. The first of these is the drag on the orbiter caused by residual ionized particles. This
varies with the night and day cycles of the orbiter. The second contributor is the gravity gradient
factor which is caused by the fact that he ex_r_dment may be located away from the center of
SPIE Vol. 2809 / 331
gravity of the orbiter. Thus, the experiment may lie within a slightly different orbit with respect
to the center of gravity, and there is an effective acceleration component introduced. The
situation and the calculations can be quite complex, and many safety and operational needs of the
orbiter influence the possible attitudes. The re-positioning of the AADSF to the centerline of the
orbiter makes these calculations easier, and means that it will be easier to obtain desired residual
accelerations.
6. REFERENCES
1. R. Srinivas and D. A. Schaefer, "'Crystal Growth Furnace: An Overview of the System
Configuration and Planned Experiments on the First United States Microgravity Laboratory
Mission," Paper 92-0786, AIAA, 30 thAerospace Sciences Meeting and Exhibit, Reno, NV,
January 1992.
2. J. LeCroy and D. Popok, "Design of a High Thermal Gradient Bridgman Furnace", Paper 9_
0336, AJA_A, 32*dAerospace Sciences Meeting and Exhibit, Reno, NV, 3anuary 1994.
Experimental Apparatus Container CEAC)
Houses the five zone Furnace Module with associated freon cooling loop
Translation System, and associated motors and relays
Muffle Tube or Cartridge containing the experimental Ampoule and its
instrumentation
Sample Exchange Mechanism (SEM), and its motors and relays
Data Acquisition System (DAS)
16-bit microprocessor which controls the command and data interfaces
Provides discrete inputs, analog inputs, serial/digital input/outport ports, and
relay drivers
Receives uplinked commands, processes them and transmits them to the SCCS
Signal Conditioning and Control System (SCCS)
Controls the furnace parameters for the five zones
Can operate autonomously with pre-programmed furnace temperatures and
translations
or can receive uplinked commands from the DAS to modify experimental
parameters during the mission
Transmits data to the DAS for downlink
Table 1. The components of the AADSF System
332 / 5PIE Vol. 2809
USMP-2
GROWTH OF SOLID SOLUTION SINGLE CRYSTALS
Dr. Sandor L. Lehoczky, Marshall Space Flight Center, Huntsville, AL
Principal Investigator
Successfully flown - March 1994 on STS-62
USlVlP-3
COMPOUND SEMICONDUCTOR GROWTH IN A MICROGRAVITY ENVIRONNIENT
Dr. Archibald L. Fripp, Langley Research Center, Hampton, VA
Principal Investigator
Successfully flown - March 1996 on STS-75
USMP-4
FIRST FLIGHT of AADSF with SAMPLE EXCHANGE MECHANISM
THREE EXPERIMENTS - up to 3 different teams
Candidates not chosen at time of going to press
Scheduled for October !997 on STS-87
Table 2. AAJ3SF Experiments
,Insert (Haynes Alloy 214)
I III
_Jccar AL-3OAA) t Adial_lic R_g
(Zkcar AL-3OAA]
He_ler5 (Inccx_HealExtraction718)Plate Heiter s
Guard Main ( Main Guard
Figure 1. Schematic of AADSF High Gradient Region
SPIE Vol. 2809 / 333
Translation
Assembly
Support Arm
Assembly
Ampoule
Muffle
,Tube t
Configuration
(in place) Muffle TubeAssembly
Drive Shaff
Heat
Extraction
Plate
Furnace
Assembly
Experiment
Apparatus
Container
;old Zone
Thermal
Gradient
Region
_ot Zone
Freon Coolant
Coils
Figure 2. Two views of the Expe.,-iment Apparatus Container
Advanced
Automated
,irectional
Solidification
Furnace
Signal
Conditioning
and Control
System
Data
Acquisition
System
Figure 3. Advanced Automated Direction_ Solidification Furnace on Shuttle Carrier
334 /5PIE Vol. 2809
HgCdTe, HgZnTe,
HgZnSe, CdZnTePbSnTe
Liquid-. , _.o',;jj..._,._._. ...._r.,_f, ..._.._."
ReJecled componenl lems I Rejected component mmrl
dense than bulk Ikluld [ dense than bulk liquld
resull$ In thotmosolul_ results in in|et|sce shape
©onve¢lion IhroughosJI blstab|lity in presence ol l'adlal
IhQ Hquld volume lemperature _rldtent
Figure 4. Gravity Effects in the Solidification of Pseudobinary Solid Solution Systems
SOUD TO LIQUID LIQUID TO SOLID
0.28_g 0.501_J
CdTe
0.330
0.817
0.305
0.203
0.280
0.268
0.258
0.243
0.2,10
0.218
0.205
0.192
0.180
0.168
0.1U
J7
t mm
gr
1.55rig
(a) +YVV/-ZLV Attitude Co) -XLV/-ZVV Attitude
Figure 5. USMP-2/AADSF Flight Sample of Hgl-x Cd xTe
SPIE Vol. 2809 / 335


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