July 6, 1997 10:30 a.m. CDT
Any cook who has watched a cake fall in the oven is familiar with the problem faced by scientists running some materials science experiments. After a sample has been prepared, you don't want the thing to remix or even collapse, leaving you with something that cannot be studied.
Two of the science teams working with the Large Isothermal Furnace (LIF) aboard the Microgravity Sciences Laboratory mission (MSL-1) solved the problem by building "a high temperature Swiss watch" to separate discrete segments of their molten alloy samples before they resolidify. (In Spacelab Mission Operations Control at NASA's Marshall Space Flight Center, below, the LIF team monitors the progress of an experiment.)
LIF, developed by Japan's National Space Development Agency (NASDA) is on its third experiment campaign in space. The first two were Spacelab J and the second International Microgravity Laboratory (IML-2) missions.
LIF is designed to heat an entire sample cartridge evenly, like an oven, to temperatures as high as 1,600 degrees Celsius (2,912 degrees Fahrenheit). Of the six sets of samples carried by LIF, five deal with basic research in how materials diffuse into each other; one studies how alloys are formed by metal powders that are sintered together.
The diffusion studies are fundamental science, explained Dr. Osamu Odawara of Tokyo Institute of Technology.
"Industrial engineers say that an approximate coefficient is OK for practical applications," Odawara said, "so this is for fundamental research. The coefficient is important for a good understanding of materials science." In time, though, fundamental research finds its way into commercial processes.
Diffusion is the process by which atoms and molecules elbow their way past each other and spread out. The fresh smell of a cake in the oven is caused partly by diffusion as hot molecules push across the room. The same process happens in many chemical reactions, including those that make metal alloys and electronic materials. The rate at which atoms and molecules move by these random collisions is called the diffusion coefficient.
Measuring it can be difficult because Earth's gravity causes fluid flows or currents. It's as if a fan was stirring the air in the kitchen, so you can't tell how fast the smell would have moved across the room without the stirring. In the near weightless environment of space, that problem is eliminated.
However, the uncontrolled cooling of the sample you are trying to measure can mess up the experiment, explained Alfonso Velosa of Nyma Corporation in Brook Park, Ohio. Velosa is the project scientist for experiments by Dr. David Matthieson, of Case Western University, on diffusion processes in molten semiconductors - traces of gallium and antimony in samples of germanium. A measurement of the coefficient means knowing how much of the trace elements diffused, and how far, over a period of time.
The challenge is that specimens as hot as those in LIF will not resolidify right away. In effect, they can't be flash frozen. As they cool, more diffusion can take place and that may skew the results when the specimens are analyzed on the ground. The samples may also shrink, so the diffusion length you measure on the ground is different from the diffusion length that was traveled in space.
The next best thing is to slice the samples up and isolate the parts. Matthieson and Dr. Shinichi Yoda of NASDA both chose shear cells, a widely used technique for this purpose.
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Velosa described the shear cells as being like a stack of Lifesavers candy with extra holes. A shaft runs down the center hole, and smaller holes are drilled in the sides to hold the samples. A small tab stands out from each ring to match a gap in the next ring so the rotation is precise. Once in orbit, a specimen is inserted in the LIF, heated, and the rings are rotated to align the chambers and form a continuous specimen and let diffusion take place. Before the furnace is cooled, the rings are rotated to offset the chambers again, effectively freezing the sample diffusion profile in place.
"We call it the high-temperature Swiss watch," Velosa said, because of the precision required. The 21 ``Lifesavers'' are made of graphite, a form of carbon that can withstand high temperatures without deforming, so it will not open gaps that would let the molten sample leak from one cell to another. It is also difficult to machine to the tight tolerances needed for the LIF samples. It took several weeks of machining to make each of the shear cell sets now aboard MSL-1.
"The shear cell is very expensive to manufacture for getting only one data point," Odawara said. "But it's the only way, especially if the samples shrink or contract. Then you cannot measure the coefficient. The shear cell is a very good, efficient technique."
The other LIF sample cartridges measure the diffusion coefficient by different means. Dr. Misako Uchida of Ishiawajima-Harima Heavy Industries will measure how two slightly different samples of lead-tin-telluride diffuse into each other through a capillary, a long, thin tube, 2 mm (1/12 inch) wide and 80 mm (3 inches) long.
Dr. Toshio Itami of Hokkaido University also uses the capillary method, but using a trace of a heavy isotope of tin diffusing into a larger quantity of tin through a 2 mm by 60 mm capillary.
While those scientists must wait for their specimens to return to Earth, Dr. Tsutomu of Tohoku University already has some results. His cartridges are equipped with electrodes so he can measure how the specimen changes the voltage of an electrical current. This tells precisely how silver ions are diffusing through a melt of two salts, lithium chloride and potassium chloride.
"The important thing about this experiment is that on the ground it's difficult to get the precise features because the ions can move even without the current," Odawara said. "So, in microgravity we are experimenting to get precise measurements even with low currents."
Finally, one LIF experiment does not involve diffusion but sintering, the process by which powdered materials are heated to form a new alloy. Sintering often uses metals like tungsten, nickel, and iron that would settle and separate if melted together. Sintering compresses and heats the powders to form a new, tough alloy without fully melting the whole sample.
Samples sintered on the ground (shown at left) slump compared to samples sintered in space (center). Holes, which are spherical in ground specimens (right), assume distorted shapes that are being studied to understand their meaning for improved processing. |
Still, some segregation takes place, and sintered materials often shrink
or slump, so the product must be machined before it can be used. Dr. Randall
German of Pennsylvania State University has found "interesting and
complicated particle reactions," Velosa said, including unusual bubble
formations in the sample.
For the next 11 days, you can follow along and learn about the science being performed on the mission through activities on this WWW site, as well as the "Liftoff" Mission Home Page, and the Shuttle Web Site. Check out our daily image and video highlights on the "Science In Action" page!!
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Headlinesreturn to Space Sciences Laboratory Home
Author: Dave
Dooling
Curator: Bryan Walls
NASA Official: John M. Horack
