15 July 1998: Small electrodes built into the side of a sample tube may offer a peek inside the opaque world of molten metals. A successful series of experiments aboard the Space Shuttle has a team looking for a new shape. And magnets might be employed to help stop molten metals in their tracks in future space experiments.
These were among a number of "works in progress" described at the third biennial Microgravity Materials Science Conference now entering its final day at the Von Braun Center in Huntsville, Ala. More than 300 scientists, engineers, and managers have gathered to discuss each other's progress in work ranging from basic ground tests to flight experiments aboard the Space Shuttle.
Processing materials in the microgravity environment of space is crucial to understanding and improving a number of manufacturing processes on Earth where the effects of gravity can mask subtle phenomena that affect the properties of a product. In space - aboard the Space Shuttle and, soon, the International Space Station - gravity's effects can be reduced or eliminated so that fundamental properties can be studied.
Dr. Timothy Anderson of the University of Florida offered "an interesting solution to a problem," the measurement of fluid flows inside molten metals. While many transparent models have been employed to simulate what happens when metals are molten inside a container, some research questions require understanding the material at hand.
"It's a very difficult problem," Anderson said, noting that most methods, even those that use the actual metals, have various limitations
His team at Gainesville has come up with a technique that lets them trace the flow of metals as they occur and without inserting probes that would themselves disrupt an experiment.
Electrodes made of yttria-stabilized zirconia were embedded in the walls of an alumina tube containing tin with trace quantities of oxygen. Tin was selected because its physical properties are well known from decades of experiments, including several in space. Measuring the electrical current and voltage from one electrode through the molten tin and then back to the other electrodes provided a very sensitive measure of the tin's flow, even at speeds as low as 0.0001 cm/s (less than 1/7th of an inch per hour). The technique was also able to distinguish when samples were circulating like a donut - rising at the center and sinking along the outside wall - or overturning from end to the other, or forming stacks of overturning cells.
"We think it's flight adaptable and suitable for high temperatures," Anderson said. Because the sensors are built into the container walls, they do not interfere with the samples being processed. And while convection dominated the samples studied on Earth, the method is sensitive enough to be used in space samples dominated by diffusion, that is, by the thermal motion of the atoms and molecules in the sample. Diffusion-controlled growth, completely free of flows caused by convection, is the ideal for a number of crystals.
I can see clearly
Anderson's method does not eliminate the need for various types of models that help scientists understand what happens inside metals. A highly successful model was the Isothermal Dendritic Growth Experiment (IDGE) flown on three of the U.S. Microgravity Payloads (USMP). The most recent was USMP-4 in late 1997.
Unlike other experiments which brought back valuable crystals for study, IDGE brought back thousands of photographs of crystals that were grown, melted, and regrown over and over again. The purpose was to study what happens at the tip of a dendrite, a branched crystal that grows, treelike, from a seed spreading into the liquid.
"Dendritic structures are ubiquitous in solids and are important to understanding the final microproperties of materials," said Dr. Matthew Koss of Rennselaer Polytechnic Institute in Troy, N.Y. To make his point, Koss compared a microscope image of a dendrite inside an advanced alloy with a dendrite grown by IDGE.
As the tip of the dendrite grows, molecules or atoms in the liquid attach themselves to the solid surface, releasing part of their energy to heat the remaining liquid. That heat must diffuse away so that more molecules or atoms can join the solid and make the crystal larger. At the same time, branches start to form some distance back from the tip, increasing the surface area even more.
The first equation to describe this was developed 50 years ago and still serves as a good starting point for studies, Koss said. But experimental observations on Earth are limited by gravity's effects. The heating effect described above causes convection that quickly disrupts the fluid flows. (In making metals, it is generally believed that the dendrites are so small, and form so quickly, that gravity has little if any effect. Still, the shape of the dendrite tip remains important.)
Thus, IDGE was developed to study dendrite growth in space. On the USMP-2 and -3 missions in March 1994 and February 1996, IDGE used succinonitrile (SCN), an organic fluid that is clear both as a crystal and a liquid, and is a good model for iron-based metals. On the USMP-3 mission in late 1997, it used pivalic acid (PVA), another clear organic, that is a good model for non-iron metals. It also behaves slightly differently at the solid-liquid interface, so it provided a good check on the results using SCN.
"Our preliminary results [with PVA] confirm the SCN results," Koss said.
But one of the results that has come from the IDGE missions is that the tip of the dendrite is not a simple paraboloid, a simple mathematical curve generated by rotating a parabola.
"What you see depends very much on the rotation angle you are looking at," Koss said. One of the graduate students working with IDGE is studying the different curves that the dendrite tips take and should publish his results late this year.
At the same time, images from the third IDGE run are being studied to understand the effects of chamber size, side branches, and undercooling.
"After all these years, we're still a work in progress," Koss said.
Stopping gravity ... sort of
After all these years, materials scientists are still coping with gravity's effects, even in space where most of those effects are nearly canceled. What passes for weightlessness actually involves tiny gravitational effects. At any distance from a spacecraft's exact center of mass, an object will actually be trying to follow a slightly different orbit. This will cause it to experience accelerations ranging from 1/100,000 to even 1/1,000th the force of Earth's gravity. That's insignificant if you are trying to anchor a camera or clipboard. It's quite a push if you are processing a small, sensitive sample. This has led many scientists to ask that the Space Shuttle be pointed in certain directions during their experiments to make sure the residual gravity force causes the least disruption.
Still it would be nice to get rid of it altogether.
Magnetic fields are emerging as a contender for this role. Several papers were presented on current or planned activities using magnetic fields to dampen fluid flow in molten samples. While we generally associate magnetism with iron, most materials have magnetic moments to some degree.
Dr. Archibald Fripp of NASA's Langley Research Center described his team's work using magnetic fields up to 5 Tesla - about 5,000 times stronger than the Earth's magnetic field.
Fripp has a special interest since several experiments that he ran on the USMP series with NASA/Marshall's Advanced Automated Directional Solidification Furnace were altered by residual microgravity effects.
"The 5 Tesla fields provided some stability in the sample flow" in experiments to provide a force that would counteract the convective flows that normally occur in materials, Fripp said.
Lest anyone think that magnetic fields might substitute for microgravity in orbit, it did not quite work that way.
"Even with the 5 Tesla field on the ground, the predictions showed that the convection is greatly reduced, but not completely stopped," Fripp said. "A few kilogauss [a few Tesla] in microgravity will eliminate convection, but even 50 kilogauss [50 Tesla, 50,000 times Earth's field] on the ground won't stop it."
This was soon seconded by Dr. Arne Croll of the Kristallograpisches Institut der Universitat, Frieburg, Germany, and Dr. Martin Volz of NASA/Marshall's Space Sciences Laboratory.
Croll experimented float zone processing of silicon boules just 8 to 14 mm (1/3 to 1/2 in.) in diameter and 40 mm (1.6 in.) long in a furnace with magnetic fields ranging from 0.5 to 5.0 Tesla. Even when the magnetic field eliminated striations across the width of the boule, Croll said they often found strains from complex vertical flows along the outer surface of the sample.
They ran into a new effect, thermoelectromagnetic convection (TEMC) that left "rather strange striation patterns" in the silicon: since the magnetic field could dampen flow, it could also cause flows.
With a different furnace at NASA/Marshall, Volz described similar work with 5 Tesla fields applied to a furnace growing germanium-silicon crystals.
"At present growing conditions," he said, "a 5-Tesla field is insufficient to achieve diffusion-controlled growth."