November 28, 1997
United States Microgravity Payload-4 - Flight
Day 9
When
Archie Fripp had a chance to use a special furnace flying on the Space Shuttle
in 1985, he didn't have time to design a plunger to center his specimen.
It turned out great - and then less than wonderful on a reflight in 1996.
Now he's trying to figure out why.
"The primary objective is to figure out what happened on USMP-3," said Fripp, the principal investigator for two of the samples in the Advanced Automated Directional Solidification Furnace (AADSF; left) aboard the U.S. Microgravity Payload (USMP-4) now on Space Shuttle Columbia.
Fripp, a materials scientist from NASA's Langley Research Center in Hampton, Va., first flew a specimen in the Automated Directional Solidification Furnace as a part of the Materials Experiment Assembly (MEA) in 1985.
His sample of choice is lead tin telluride, a promising material which can make excellent infrared lasers or infrared detectors. Their wavelength (i.e., which infrared "color") they sense or emit is set by the ratios of the three metals.
In his 1985 experiment, Fripp got one sample. However, the Shuttle's attitude was not continuously controlled (as it is on this mission), nor were detailed accelerometer measurements made. To expand our understanding of how these materials are formed (and what we might expect them to do), Fripp flew a second set of samples in the AADSF on USMP-3 in 1996.
This time, he got a surprise.
Even before they
opened the ampoules, computer tomography (CT) scans (right)
showed that each specimen had stuck to one end of the quartz ampoule and
had "neck" deformities and internal voids. Oddly, the top and
bottom of each specimen appeared to be in good shape.
"Our rough guess is that on MEA, we had about a half centimeter [0.2 in.] gap above the melted liquid, so the sample was pushed down by the vapor pressure generated when it was heated," he explained. Fripp's experiment was scheduled earlier than expected, so he did not have time to design plunger to position the sample. The results indicated that the plunger was not needed.
On USMP-3, they left only about 1 mm (0.04 in.) of head space to maximize the size of the crystal that could be studied after the mission. The orientation of the sample with respect to the residual gravity vector was expected to be the critical component that had to be controlled for a homogenous sample. Because plungers were not though to be needed, the ampoule was designed to hold three samples, each solidifying in a different direction (hot on top, hot on bottom, and hot on side).
This time, Fripp's team got samples with necking and voids.
Fripp thinks part of the answer to the necking and voids may be that lead-tin-telluride expands when it melts and shrinks when it solidifies. The sample could easily have expanded to touch both ends of the glass ampoule when it was 1 mm away, but not when it was 5 mm away, he said.
 |
"And then when it started to freeze and contract, it held on to the end of the ampoule," he continued. "When it's contracting and freezing, it had to make up this volume somewhere. So you get voids, pits, and channels.
"That helps explain some of these pores inside. It doesn't explain this big neck down in the region close to the bottom. That has me much more perplexed."
Fripp actually has six samples inside two experiment cartridges. Each sample is different from the other, or will be processed slightly differently than the others, in an effort to cover all the bases.
Where MEA was a relatively simple facility - it borrowed heavily from proven designs used on suborbital rockets - AADSF offers extensive measurements, including temperature, sample speed, and the ability to start and stop the furnace as needed.
That will allow him to vary how each sample is processed in addition to changing the samples themselves, including: a lower temperature gradient to reduce convection, a faster growth rate (2 cm [0.8 in.] per hour) to keep the boundary layer thin, the "keep chugging" sample which does not pause after recalescence (a special nucleation effect), a sample that will melt back onto a seed to avoid recalescence, a slow grow rate (0.5 cm/hr), and a sample with a lower tellurium content to reduce vapor pressure.
Recalescence is an effect that happens when a sample is cooled below its normal freezing point and then freezes. The atoms and molecules don't just stop moving: they have to give up their energy of motion. It winds up spreading through the rest of the sample as a rise in temperature and causing a change in composition of the liquid near the newly formed seed.
"Whatever redistribution that is caused in the liquid, we wanted to give it time to rediffuse into the liquid," he explained. "Now why that should cause any kind of a problem, I don't know."
"If we grow them on Earth with the high degree of mixing caused by convection," Fripp explained, "you don't get a uniform composition." On his UMSP-3 samples, Fripp observed a large degree of variation in composition along the length of the samples.
This may have been caused by Maragoni convection which happens when changes in surface tension (like a pinprick of soap on water) cause the surface layer to shrink or expand.
"We always expect
to have some amount of convection (left)
in any kind of microgravity," Fripp said. "The question is, How
fast is it going to convect? Is is slow compared to the speed of the solid/liquid
interface?" If so, then the rate may be acceptable.
The problem that Fripp and his colleagues are trying to solve is how to control conditions to get the right ratio. The enemy is convection.
One of the questions scientists are often asked during space missions is, "What would meet your minimum science objectives?" In Fripp's case, the answer might be just one of the six specimens. The problem is, he doesn't know which of the six, so he needs for all to be processed.
The ironic thing about Fripp's quest is that he could have retired from NASA a few years ago and would have felt that he knew a lot about making materials in space.
"If I had retired when I was first eligible three years ago," he said, "I would have gone out thinking I had learned a lot about processing materials in space, just based on that MEA sample. Now, I'm not so sure."
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