This experiment was one of two investigations housed within the G-479 Get Away Special canister during STS-031. The other experiment of the two is also applicable to this database (see Rey, STS-031 "Primary Mirror Production Using Vapor Deposition").
This crystal production experiment, which was designed, built, and tested by scientists and engineers at the National Research Council of Canada, was integrated into the G-479 GAS canister with the Telesat Canada Primary Mirror Production Experiment when it was determined that the mirror experiment configuration (1) left unused space in the canister and (2) had excess battery capacity to power a second experiment.
The experiment proposal was accepted by Telesat Canada in June 1984 (the canister was scheduled to fly just 4 months later on the shuttle in October 1984). Several constraints (dictated by the primary mirror production experiment already within the gas can) had to be addressed prior to experiment assembly: volume, weight, operating temperature, power (65 W), etc. Nevertheless, 2 months later, the completed experiment assembly was integrated into the G-479 canister ready for the October launch. Eventually, the canister was removed from the originally scheduled flight in order to clear the cargo bay for the rescue of two satellites. The canister was finally flown (no upgrades allowed) in November 1985.
The experimental concept was based on (1) the Principal Investigator's scientific experience with cadmium crystals, (2) Walter's earlier Skylab InSb quasi-containerless crystal growth experiments (see Walter, SL-3 and SL-4) and (3) Witt's ASTP Ga-doped Ge experiment (see Witt, ASTP).
During the earlier Skylab experiment, a containerless section of an InSb cylindrical sample was melted in a gradient furnace and resolidified. Although it was expected that the solidified free section of the crystal would be spherical, the crystals grew into a longer tear-drop shape (containing facets around their circumference) and unexpectedly made contact with the container wall. During the earlier ASTP experiment, "...it was found that gallium-doped germanium crystals regrown in a closely fitting container (in the bridgman method) did not contact the container as they would have on earth, but developed 'ribs' which contacted only about 1% of the container surface. Thus a form of containerless growth was achieved without special measures." (8, p. 55)
Based on the three factors above, the specific objective of the experiment was to determine if polycrystalline cadmium rods would behave similarly to the crystals flown during these earlier experiments.
Prior to the shuttle flight 10 cadmium specimens were prepared. "The metal was purified, mainly for the removal of oxides, by the casting in a vacuum. The castings were used as billets for the hot extrusion of 2 mm diameter rods in a press. The rods, cut to length became the specimens [each specimen 44 mm long]. No special precautions were taken to clean the surface of the rods before sealing into a container [with an argon atmosphere], an omission which had notable consequences."
"The containers were designed so that the cadmium rod would not be supported everywhere, but would be held at one end and either stand free for the remainder of its length or be in some form of contact with the inside of the container. The latter portion of the container would be heated until the metal melted and then cooled to permit resolidification." (8, pp. 55-56)
The specimen containers were made from two standard sizes of Pyrex tubing - 6 mm o.d. and 10 mm o.d. - blown to the the shape desired. Some of the containers had circular cross-sections (such that no section of the sample's free section would touch the container). The other containers had circular cross-sections with either three or four equally spaced indentations made during container fabrication by pressing the softened Pyrex inwards with a chuck along the length of the tubing in three or four equally spaced positions around the tube circumference (such that the sample would touch the tube at three or four longitudinal locations, see Reference (8), Figure 1.) The specimens were anchored at one end by melting the end and shrinking the tubing over the form.
Bundles of containers "...were assembled by wrapping them with Nicrome ribbon, which served both to secure and heat them. Asbestos tape was wrapped around the outside and secured with cement. Finally aluminum foil was wrapped around the outside and secured with wire. Since only free-standing [or semi-free standing] section of the specimen was melted, resolidification could start from the unmelted part." (8, p. 56)
During the shuttle mission, power was supplied to the experiment for 10 min, 40 seconds. Since time to fuse blowout on Earth was about 8 minutes, it was thought that the experiment might not have proceeded as planned "...since the survival of the fuse showed that the controlling thermocouple had not reached its cutoff temperature." (8, p. 57)
Post-flight examination of the flight specimens indicated that (1) all of the eight specimens in the 6 mm o.d. containers had melted to some degree (although not to the degree desired) and (2) the two specimens in the 8 mm o.d. containers had not melted. Reportedly, the samples did not melt as expected because of an inadequate estimate of convective heat transfer in space.
The general appearance of the flight specimens was much the same as before the flight. "Although unexpected[,] the near retention of the original shape proved due to the presence of an oxide coating, left from the [ground-based, preflight] extrusion process, which acted as a container in microgravity...." (8, p. 55) Soft X-ray wavelength dispersive analysis showed that the oxide layer was approximately 100-170...angstroms thick-substantially more than a normal layer on earth." (8, p. 55)
"The melted lengths, determined with a measuring microscope, ranged from 5 to 28.5 mm [the expected meltback was 25 mm]. There appears to be no firm correlation between the number of indentations and melted length although containers with indentations in general had longer melted lengths than those without. Grain size was in general larger with increased melted length, but the maximum diameter shows no relation. Only the solid-melt interface changed in a regular way with irregular interfaces for short melted lengths and precise circular melts for longer lengths." (8, p. 58) All melted specimens exhibited a slight increase in diameter.
"The grain structure of the specimens was revealed by etching. X-ray photographs were taken with Laue back reflection to show the crystal structure. The grains were large in some specimens, smaller in others, but consistently larger than grains in the unmelted metal." (8, p. 55)
It was concluded that:
"Although prepared in haste, the experiment showed several phenomena not to be seen on earth. The external shape of the specimens showed that the metal remained clear of the container walls and that an oxide layer remaining from extrusion of the rods acted as a slightly elastic container. The surface of the melted length did not retain dye marks, although traces remained on some specimens. The grain size of the specimens varied greatly, but was always larger than that of the starting material. The crystallinity of the specimens ranged from highly perfect to greatly imperfect. Nucleation occurred both at grains at the solid-melt interface and at the tips of some specimens." (8, p. 61)
A detailed description of several of the samples is provided in Reference (8).
Cadmium was selected for this experiment because (1) the Principal Investigator was familiar with the material, (2) it has a low melting point (321 ¡C), (3) it can be contained in and does not wet Pyrex (Pyrex is easy to mold and work with), and (4) the crystal structure is hexagonal close packed (in contrast to the cubic structure of InSb and Ge).
(2) Mirror Experiment Wins Telesat Space Competition. Telesat Canada News Release, Personnel and Public Affairs Division, January 18, 1984. (Preflight)
(3) Covault, C.: Astronauts Deploy Commercial Payloads, Ready Structures for Space Assembly. AW&ST, December 2, 1985. (post-flight)
(4) Hoffer, D.: Towards a Better Mirror. In Goddard Space Flight Center's 1986 Get Away Special Experimenter's Symposium, October 7-8, 1986, pp. 49-57, NASA CP-2438. (post-flight)
(5) Get Away Special... the first ten years. Published by Goddard Space Flight Center, Special Payloads Division, The NASA GAS Team, 1989, p. 32. (post-flight; very brief description)
(6) Ridenoure, R.: GAS Mission Summary and Technical Reference Data Base. Ecliptic Astronautics Co., Technical Report #EAC-TR-RWR 87-11, October 2, 1987. (Get Away Special canister mission history)
(7) Personal communication with Principal Investigator F. Lipsett, September 1993.
(8) Lipsett, F. and Sewell, P. B.: Resolidification of Cadmium in Space and the Role of an Oxide Coating as a Container. Appl. Microgravity Technology, 1, Vol. 2, 1988, pp. 55-61. (post-flight)