For release: June 19, 2000
Contacts:
Valerie Sheares, Materials Chemistry, (515) 294-2474
Susan Dieterle, Public Affairs, (515) 294-1405
AMES, Iowa -- Valerie Sheares mixed her love of polymers with a curiosity about mysterious materials known as quasicrystals and made a breakthrough that poses tantalizing new research and development possibilities.
Sheares, a researcher at the U.S. Department of Energy's Ames Laboratory, combined the best qualities of quasicrystals and polymers in a composite that outperforms similar materials in wear-resistance tests. In addition to the improved performance of the polymers, the composite offers a more versatile way of using quasicrystalline powders, which could make the materials more appealing to industry.
"It's a unique material," said Sheares, who has applied for a patent on the quasicrystal-filled polymers. "It's very hard, it's not abrasive and it has low thermal conductivity -- that's pretty neat."
Quasicrystals, typically aluminum-rich alloys of specific compositions, were discovered in the 1980s. They possess an unusual combination of properties: they are hard, highly resistant to wear and corrosion, and don't conduct heat as well as most metals. These properties make them ideal as coatings on automotive and mechanical parts and cookware, but scientists are still trying to understand why quasicrystals have these characteristics. Manufacturers have expressed interest, but the difficulty of processing quasicrystal materials has been a stumbling block to their widespread use thus far.
Polymers, long chains of flexible molecules used in making plastics, are easier to process, said Sheares, who is also an assistant professor of chemistry specializing in polymer research at Iowa State University.
Colleagues at Ames Lab -- a national leader in quasicrystal research -- described the difficulties of processing quasicrystals, and the dilemma gave Sheares an idea. She wanted to find out if the techniques for adding other inorganic fillers to high-performance polymers would work with quasicrystal powders. She wasn't sure what would happen because polymers don't always mix well with other materials.
"Polymers don't like other materials; they don't even like different polymers," she said. "And quasicrystals are the same way. We didn't know if we could disperse the quasicrystals in the polymers, but it worked. The quasicrystals dispersed quite readily. That surprised us."
To determine whether the wear-resistant properties of quasicrystals had transferred to the composite, half-dollar-sized disks of the material were placed on the turntable of a wear-testing device similar to a record player. A small stainless-steel ball was placed in the device's arm, roughly where a record needle would go. A weight of 1-2 pounds was attached to the middle of the arm to hold the ball in contact with the composite material as the disk spun at a rate of 125 rpm.
Afterward, the disks and the steel balls were examined to determine how much of each surface had worn away. Results indicated that the quasicrystal-filled polymers were between five and 10 times better in resisting wear than any other polymer or polymer composite that was tested.
Even more significant was the near-perfect condition of the steel ball. "As hard as quasicrystals are, you have to wonder what happens to the other surface scraping against it," Sheares said. "Quasicrystals outperform every other hard filler in that the steel ball remains basically unchanged. When we tested silicon-carbide fillers, the surface of the ball was completely eroded away because silicon carbide is hard and abrasive. Quasicrystals are hard and nonabrasive. Those two things don't usually go together."
Her tests indicate that quasicrystals significantly improve the wear resistance of a variety of high-performance polymers. "We've gone far enough with the wear tests to say that quasicrystals are a unique filler," she said.
And since polymer-processing techniques are already well known, Sheares said the composite should be fairly easy to produce once industry begins the large-scale manufacture of quasicrystal powders used in the material. "It's reasonable to think that the composite could be industrially useful because once you get beyond the scale-up for quasicrystal production, everything else is pretty simple," she said.
Her next goal is to make small parts out of the composite material and see what happens when two surfaces of the composite rub against each other over significant periods of time. Another quasicrystal property -- low thermal conductivity -- may come into play in those tests. "When things rub together over a long time, the friction creates heat," Sheares said. "We're hoping that far less heat will be generated with the quasicrystal composites because they don't heat up as much as other metallic fillers."
Sheares said more research is needed to understand the composite. "Every time you do something new, you open up a new box of questions. We're entering a new area with the composite," she said.
She's counting on colleagues at Ames Lab to help her address those questions. "Quasicrystals aren't my area of expertise; polymers are," she said. "My group counts on the quasicrystal experts for a basic, fundamental understanding of why quasicrystals do what they do."
Pat Thiel, director of Ames Lab's Materials Chemistry Program and a quasicrystal researcher, said Sheares' work "is an important stimulation for further basic research, particularly because some of the properties she's exploiting in the composite are fundamental to the quasicrystal. The low thermal conductivity and the low coefficient of friction are still rather mysterious on a fundamental level."
Sheares' research is funded by the Department of Energy, the nation's leading science and technology agency. Ames Laboratory is a DOE research facility operated by ISU. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
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Last revision: 6/19/00 sd
