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Materials Science 101 It's back to school for astronaut
Sept. 15, 1999: You've climbed to the top of the pack, you're a PhD in astrophysics or a Top Gun pilot, you've just been selected to be an astronaut. So what's your first task? Go back to school to become a generalist after studying to be a specialist.
Right: Dr. Sharon Cobb of NASA's Marshall Space Flight Center examines a model of a crystal lattice. One of the key points of her lecture is that processing materials in the microgravity of space reduces defects like the spot, at the center of the model, where an extra row of atoms has wedged into the lattice. Links to 600x425-pixel, 96KB JPG. Or click here for 3024x2252-pixel, 1.2MB JPG. Credit: NASA/Marshall
Because astronauts are called upon to do a little of everything on the job, their training encompasses far more than how to operate the Space Shuttle or - with the new candidates in training - the International Space Station (ISS).
Among the many fields astronauts must know is materials science in microgravity, one of the principal missions for ISS. Materials science research aboard the ISS is sponsored by the Microgravity Research Program at NASA's Marshall Space Flight Center in Huntsville, Ala. Approximately 60 percent of the planned science experiment time aboard ISS is devoted to the microgravity sciences and commercial microgravity investigations.
"This type of training is challenging because the astronauts have a range of backgrounds from mathematics to materials science," said Dr. Sharon Cobb, the project scientist for the Materials Sciences Research Facility here at NASA/Marshall. Cobb recently gave an introductory class, including hands-on labs, to the most recently selected astronaut candidate class at Johnson Space Center.
"I gave an overview - with a number of essential details - of what materials science is and why we want to do this research in microgravity. Later the astronauts will get more detailed training on specific experiments as the hardware is developed for flight."
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From this came the realization that no one fully understood what would happen if gravity's effects were removed from materials that were liquefied, mixed, and resolidified. Early flight experiments were conducted aboard the last Apollo missions. The discipline grew and became a major aspect of Skylab's experiments in 1973-74, and a centerpiece of Space Shuttle and Spacelab missions during 1983-98.
|Animated images show the growth of dendrites, tree-like structures, as a liquid cools in space (at a rate much faster than actually occurred). While the materials are transparent organic compounds - succinonitrile (left) and pivalic acid (right) - their behavior is a close mimic for what happens at a smaller scale inside opaque molten metals. Credit: Rensselaer Polytechnic Institute|
Cobb reminded the astronaut candidates that advances in materials science in 1-g make their missions possible, from the new lithium-aluminum alloy that lightens the Shuttle External Tank so more payload mass can be carried to orbit, to the urethane-coated nylon pressure bladder that will keep them in a safe atmosphere during space walks.
"Manufacturing is 17 percent - $1.2 trillion - of the U.S. gross domestic product," Cobb explained. "That means that even modest improvements in materials and their production can have great economic impact.
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For example, the 1998 Metalcasting Industry Technology Roadmap lists "lack of knowledge of process-microstructure-chemistry-property interactions [as one of the ] major technology barriers in materials."
"To make substantial advances," Cobb continued, "materials processing must transition from a historically trial-and-error art to become a predictable, controllable technology in the future."
The epitome of the older method is the story of Thomas Edison perfecting (not inventing) the light bulb by trying everything as a long-life filament, and then testing virtually every type of bamboo after he happened upon that. The properties of materials, especially under various conditions, were just becoming known to scientists then.
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The NASA Microgravity Research includes
Life and Microgravity programmatic information from NASA headquarters.
To read about recent microgravity science missions, visit the web sites for Microgravity Sciences Laboratory-1 - (July 1997), the U.S. Microgravity Sciences Payload -4, and the press kits for other recent microgravity missions.
Scientists have reached the point where a material's interactions with its container may alter sophisticated measurements of a property, or mask a fundamental phenomenon. For example, unavoidable convection - where warm, light fluids rise and cold, dense fluids sink - disturbs the formation of an alloy or electronics crystal and causes defects.
Gravity is an inescapable factor in the equations because it's always there - unless you go to orbit.
Unless you go into orbit. You're still a captive of Earth's gravity - that's what holds you and the Moon in orbit - but you're falling continually so the effect is indistinguishable from gravity being turned off. Tiny residual accelerations remain, so scientists refer to microgravity, not zero-G.
The net result is that a new range of possibilities now opens for materials science.
"The goal of materials processing in space is to develop a better understanding of the relationship between processing, structure, and properties so that we can reliably predict the conditions required on Earth to achieve desired materials properties," Cobb said.
- NASA Quench Module Insert is a furnace capable of reaching 1400oC (iron melts at 1535oC), with a cold end to establish a controlled temperature gradient. This insert will also allow rapid freezing of samples up to 8 mm (1/3 inch) in diameter. This quenching will enable the history of the solidification of complex alloys to be maintained for subsequent examination. The information gained will be applied to foundry practices in industry.
- NASA Diffusion Module Insert is a furnace capable of reaching 1600oC, and able to maintain a constant temperature along a 100 mm (4 inch) length. Controlled gradients can also be obtained. The furnace will be used to study the speed and mechanisms by which electrically active elements can be distributed (diffused) through a molten element such as a semiconductor. These data are important to the electronics industry and real values cannot be obtained on the ground because of the influence of gravity driven convection.
- ESA Low Gradient Furnace Module Insert is a furnace for crystal growth capable of reaching 1600oC. Samples can be translated at slow and precise rates within a temperature controlled environment. Magnetic field capabilities, both static and rotating are available to influence the liquid flow and improve the properties of the crystalline product.
- ESA Solidification Quench Furnace Module Insert is a furnace designed primarily for metallurgical experiments capable of reaching 1600oC, and including a quench capability. While initially designed to be used for ESA experiments, these latter two insert modules may be made available for NASA experimenters.
- NASA Advanced Pattern Formation and Coarsening Research Module will be an on- orbit replacement for an experiment module sponsored under NASA's Space Product Development program. It consists of a low temperature facility with a precisely controlled bath for in situ observation of the solidification and growth of transparent model materials that simulate the behavior of metals and alloys
Microgravity processing benefits
At right are two different materials that are expected to benefit from enhanced processing in microgravity. Non-linear optical polymers (top two pictures) form more uniform fibers when processed in microgravity (noted as Âľg) than in 1-g on Earth. Nearly defect-free optical fibers of ZBLAN, a heavy metal glass, were produced in Âľg instead of the polycrystalline product that normally results on Earth. The polymers are a candidate for advanced optical computing. ZBLAN promises much higher data throughput than conventional silicon-based fibers. A sampling of other beneficiaries includes:
- Isothermal Dendritic Growth Experiments (IDGE) aboard three U.S. Microgravity Payload missions have led scientists to revise their models of how metallic crystals grow based on photographs and videos of crystals growing in transparent model materials.
- An electromagnetic levitation furnace flown on two Spacelab missions has allowed scientists to measure, in detail, the physical properties of several metals without being contaminated by contact with the experiment apparatus, as would happen on Earth.
- Space experiments on one type of electronic material -cadmium-zinc-tellurium alloy semiconductors - led to a 200-fold reduction in imperfections and a potential increase in the number of circuits that can be produced on one wafer.
- Improved understanding how mixtures coarsen &SHY; how small particles grow into larger particles - can help in developing improved turbine blades, among other applications.
- Reduced fluid flow in mercury-cadmium-telluride single crystals could yield larger infrared detectors with improved responses.
The experiments will be conducted in several racks, each 1 meter wide (almost 40 inches) that will be installed aboard ISS over the next few years. Cobb is the science community's point of contact for the Materials Sciences Research Facility which comprises three Materials Science Research Racks. (ISS will also host facilities for research in fluids, combustion, biotechnology, and other areas.)
The MSSR-1 will carry a total of five experiment furnaces from NASA and the European Space Agency. Candidates for MSSR-2 and -3 include furnaces that would take half the rack.
"I often get asked why we need so many different furnaces," Cobb said. "Well, most of us have several ovens at home - a toaster oven, a microwave oven, a regular convection oven, and then four hot eyes on the top of the oven."
In the same manner, it's almost impossible to design one furnace that would satisfy every experiment, so several furnaces are designed with special capabilities. Some are isothermal, meaning that the whole sample is heated and cooled evenly. Some use gradient heating where the hot zone travels down the length of a sample, followed by a cool-down zone. Others provide for rapid quench, like dipping a hot horseshoe in a bucket of water, to freeze in the history of the liquid for subsequent examination on the ground.
Cobb gave the astronaut candidates a little practice in these areas with some basic experiments that parallel what is done in classrooms and in space. In one demonstration, the candidates grew crystals of succinonitrile, a chemical that forms dendrites, tree-like crystals, similar to what happens inside metals.
Already, similar experiments aboard the Space Shuttle have caused scientists to rewrite some basic assumptions about what happens in that magical instant as a metal turns from liquid to solid. More advances are expected as the ISS is completed and becomes an orbiting materials laboratory.
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