Reza Abbaschian is going home with what scientists often hope to find in an experiment: the unexpected. He's also going with almost three times as much data as he had planned on collecting during the two-week mission of the U.S. Microgravity Payload (USMP-4).
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.
MEPHISTO is a dual furnace carrying a total of three samples of bismuth
alloyed with a trace of tin. The alloy has potential uses in the electronics
industry. One furnace is fixed in position while the other one moves down
the length of the samples in three cartridges. It uses three samples to
accommodate three different methods of measuring how the samples melt and
then freeze.
One measurement method is the Seebeck effect where an electrical is produced solid/liquid interfaces are grown. Another sample's electrical resistance is a measure of how the crystal is growing.
The ideal is to have the atoms line up in sheets, like ball bearings filling the bottom of a box. After each layer is complete, the next begins. What often happens is that the crystals will switch growth methods and spawn cellular structures and then dendrites (treelike structures). Because cellular and dendritic growth produce more surface area than planar growth - just as a mountain has more surface area than a plain - the rougher surface will accommodate more atoms.
Abbaschian said that MEPHISTO was scheduled to run 13 Seebeck cycles during the mission. It's coming home with 35, close to triple the planned number.
"We just worked hard and used every opportunity to get an additional cycle," said Abbaschian of the University of Florida in Gainesville. "Since we did not have any problems with our measurements, we were more efficient and productive."
Helping make that happen was the furnace itself which worked perfectly and provided stable temperatures so the investigators did not have to wait for it to warm or cool.
"Some of these Seebeck cycles during melting were not as high as we expected based on what we know from experiments on the ground," he said.
The voltage peaks in the cycles for the melting and freezing cycles were not as widely separated as expected.
"The peak at melting should have happened at a much higher temperature," he continued. "That's a finding we had not expected. We've seen that in every cycle."
Equally important, the 35 different Seebeck cycles have produced accurate, reproducible readings. This is crucial to any scientific endeavor.
Preliminary analyses indicate that the Seebeck signals vary with the speed of the freeze front - the furnace speeds ranged from 3.3 to 140 mm/h (about 1/8 to 5.5 inches per hour) - and the length of the solidified section, and with the internal structure.
From the voltage measurements, Abasschian and his team have calculated a new value for the diffusion of tin through bismuth. This is important in fine-tuning manufacturing processes on Earth.
More understanding will emerge from the months of data analyses that will follow the mission. With the flight nearing its end, MEPHISTO is running the last experiment cycle. A different technique, the Peltier effect, is used during the final freezing. A 15-amp electrical current flows through the sample every a second every few minutes. This freezes the interface for study after the mission. By late morning, 125 mm of the 150 mm (5 of 6 inches) length had been processed.
The third sample will be quickly solidified - or quenched - at the end for comparison with the Seebeck and Peltier samples after the mission.
"The solid-liquid interface is at most two or three atomic layers thick," Abbaschian said. Understanding what happens in that solid-liquid interface will help in designing advanced metallic alloys and electronic devices in the next century.