One moment you're a liquid, the next you're a solid. In
an instant of time, atoms shift from one to the other and, as they settle
in to the new neighborhood, determine the characteristics of the larger
metal.November 23, 1997
United States Microgravity Payload-4 - Flight
Day 4
One moment you're a liquid, the next you're a solid. In
an instant of time, atoms shift from one to the other and, as they settle
in to the new neighborhood, determine the characteristics of the larger
metal.
It almost seems like the transition from shadow to light.
"It's sharper than that," said Dr. Reza Abbaschian of the University of Florida in Gainesville. "It's at most two or three atomic layers thick." Understanding what happens in that solid-liquid interface will help in designing advanced metallic alloys and electronic devices in the next century.
Abbaschian is principal investigator for MEPHISTO, the device for the study of interesting solidification
phenomena in space and in orbit. MEPHISTO, one of the major experiments
in the U.S. Microgravity Payload-4 (USMP-4) mission,
was developed by the French National Center
for Space Studies (CNES) which has an active program in microgravity
sciences (see sidebar).
Part of the agreement that puts the French device aboard the American Space Shuttle is to have an American as principal investigator on alternate missions. Abbaschian filled that role on USMP-2 (flown in March, 1994) and now on USMP-4. MEPHISTO is a dual furnace carrying a total of three samples of bismuth alloyed with a trace of tin. One furnace is fixed in position while the other one moves down the length of the samples which are held in three cartridges. This way, each sample is solid at the ends and molten in the middle. As the moving furnace and a heat sink (cooling unit) slide across the samples, the material solidifies. |
Microgravity science is a major area of research for France's National Center for Space Studies (CNES). "We have two kinds of research activities," said Dr. Bernard Zappoli, microgravity science program manager at CNES. "The first is connected with materials sciences, and the second is with fluid science and convection." Zappoli spoke here Friday at Spacelab control before returning to France. In addition to the full range of space sciences, CNES has about 60 investigators in France working on microgravity sciences with a 12 million Franc budget. |
This zone, just a few atoms deep, where the liquid becomes solid is what interests Abbaschian.
"The emphasis of USMP-2 and USMP-4 is the on the growth of faceted materials and how they behave," Abbaschian explained. "We are interested in the interface kinetics, how atoms join at the interface."
On both his flights, Abbaschian has used bismuth which crystallizes in layers of atoms spreading across the front of the solid material such as silicon and gallium arsenide. Atoms can attach themselves to the top - they must in order to start a second layer - but find it easier to expand the existing layers.
The result resembles a broad, terraced field.
By contrast, non-faceted materials grow in all directions uniformly. Bismuth can head in that direction, too, which is part of this investigation.
"In all crystal growth processes, you are looking at when does an interface break down and become cellular or dendritic," Abbaschian said. "That's important even beyond which you precipitate material between the arms of the dendrites or cells."
That's what happened on the USMP-2 mission. Each tube carried two grains. After several millimeters of growth, one grain started growing in cells while the other kept growing with facets or terraces. This continued for about 12 mm (almost a half inch) with the faceted sample apparently pushing tin out and feeding it to the cellular grain. Then the second grain became cellular.
This was not expected since all the samples in the furnace experienced otherwise identical conditions. For USMP-4, the samples have been isolated in capsules so none can be influenced by what happens in the other.
Three pairs of samples are used because MEPHISTO was designed to provide three kinds of measurements - Seebeck, Peltier, and resistance - which do not work together.
The Seebeck effect treats the solid-liquid interface as a thermocouple, a part of an electrical circuit to measure temperature. It is quite useful because it does not use a physical thermocouple which would be far larger than the zone of a few atoms that make up the liquid-solid interface.
The Peltier effect is different. By sending an electrical pulse through the sample for a second, the atoms at the interface are frozen. Then, normal growth resumes. This provides a series of curves that, like tree rings, show the growth interface of the specimen at every moment during the experiment. It can also show, through an irregular shape, if the sample was disturbed by the spacecraft maneuvering.
The third specimen is a control, grown without resistance measurement and quenched quickly to freeze the specimen. It is compared to the others after the flight.
"We know the temperature (from Seebeck measurements), the velocity of the interface (from the resistance change), the shape of the sample (from Peltier pulses), and the composition of the interface (from post-flight analyses)," Abbaschian said. "All four have to be known in order to shape the predictions during solidification of metals."
Still, surprises happen, as with the two different crystals on USMP-2.
"The difference between the two could not be explained by existing theory," Abbaschian said. "We are repeating the experiment [with the samples isolated] and looking for more details on what makes one crystal behave differently from the others."
Analysis will take about 6 months after the samples are returned to the University of Florida: "These are huge, long samples," he explained. Each is 80 cm (31.5 in.) long and 6 mm (0.24 in.).
After that, MEPHISTO is not manifested to fly, although Abbaschian would like to continue the investigations.
"Every time we do an experiment under conditions that are different," Abbaschian said, "we come up with new findings."
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