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November 27, 1997
United States Microgravity Payload-4 - Flight
Day 8
It may seem odd, but in space, "Which way is up?" is an important question for making new materials.
"With our experiment on USMP-2, we discovered the importance of the orientation of the residual acceleration" in growing crystals in space, said Dr. Sandor Lehoczky of NASA's Marshall Space Flight Center. "That is a significant accomplishment."
The residual acceleration defines "up." While the crew cannot sense it, atoms and molecules in materials experiments can be rearranged by it, depending on its direction.
Lehoczky is one of two principal investigators for experiments in the Advanced Automated Directional Solidification Furnace (AADSF) aboard the fourth U.S. Microgravity Payload (USMP-4) mission.
He is investigating alloys of mercury-cadmium-telluride (HgCdTe) made from solutions of cadmium telluride and mercury telluride. "Merc-cad-telluride" is valuable as an infrared detector, but the composition of the material has to be uniform in order to get enough useful material.
The AADSF is a moving furnace that slides along the length of a thin rod of material. Rather than heating and cooling the entire sample at once (as in an oven), the sample is melted from one end and then frozen in a similar manner. A solidification front travels the length of the sample to make crystals grow in one direction rather than at random.
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Even within that thin layer where the sample refreezes, gravity's effects can influence the makeup of the final product.
With AADSF on USMP-2 in 1993, Lehoczky's samples showed a slight variation in composition in the direction of the residual g-vector.
"The best orientation is if the residual g-vector [the remaining acceleration] is aligned with the sample axis, pointing into the solid from the liquid," he said. This puts the hot (lighter) melt on the "top" with respect to the residual g-vector.
A variation on USMP-3 from previous flights is that the attitude control is to be more closely controlled. Lehoczky wants the residual g-vector, that inescapable remnant of gravity's effects, pointed straight down the center of his sample, or as near as the shuttle can reasonably point - for about 70 hours.
Merc-cad-telluride is not something you can process in a hurry. Lehoczky's sample will need 72 hours with the shuttle pointed in one attitude while the furnace crawls at 0.2 microns per second. At the end of the experiment the furnace will move more quickly to provide a clear mark showing the shape of the liquid-solid interface.
Even with the attitude variations, the USMP-2
samples were better than what has been grown on the ground (left).
"We demonstrated that under favorable Shuttle orientations, one can grow crystals with improved composition and structural perfection as compared to those grown on the ground under otherwise identical conditions.
"So, in this second mission," Lehoczky continued, "we have tried to select a more optimum orientation to further demonstrate the correlations between residual g-directions and compositional uniformity."
In addition, he is trying a different material composition (slightly less cadmium telluride in this sample than on USMP-2) in one of his two samples. The optical and electronic properties vary with the composition, meaning that the materials can be blended in different proportions to tune a detector to a specific wavelength of light.
"On the last flight, we selected a material with an electron band energy gap of 10 microns wavelength, which corresponds to an atmospheric 'window' that admits radiation at 10 microns," he explained. In other words, infrared energy is largely absorbed by the atmosphere, except in a few narrow "windows" such as 10 microns.
That means that satellite instruments, for example, could "see" the Earth at 10 microns wavelength. Closer to home, devices could detect hot spots on the body. These would indicate the presence of cancer tumors where the metabolism is elevated.
With his second sample, though, he is looking for a different effect.
The second sample has a slightly different composition - the cadmium telluride ratio is lower - in an attempt to grow the high-electron-mobility crystals at the "band crossover point."
"There is a composition at which point the material goes from semiconductor to semimetallic behavior," he said. "At that composition, there is a large change in the electrical properties and the material demonstrates unique characteristics."
Chief among those is that the apparent electron mass goes almost to zero. This means that electron mobility in the alloy goes up.
Lehoczky said that while silicon has an electron mobility in the thousands, merc-cad-telluride - at the ideal composition - would have a mobility of about a million..
"One of of our objectives on this flight is to get benchmarks," Lehoczky said, near-perfect materials that can be used as comparison points to judge materials made on Earth.
Check
out the daily
Mission Status Reports prepared by Marshall's Public Affairs Office.
Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson
