Sophisticated ceramics
|
|
|
|
|
|
By Ken Howard
A huge steel-making plant dominated the landscape of Anthony Evans's childhood in South Wales, England.
It loomed over his hometown and became part of stories he heard growing up, as many of his parents' friends worked there.
According to Evans, who is director of the Princeton Materials Institute and Gordon Wu Professor of Mechanical and Aerospace Engineering, the steel factory may have been as important as anything else in guiding him along the path to materials design.
This path has lead him to develop sophisticated ceramics and, more recently, novel aluminum-based systems, which often replace steel (though Evans says steel is cheap and will always be in demand, and the South Wales plant is still open).
Evans's goal is to make materials that are both lighter and at the same time stronger and more temperature-resistant. When he is successful, these materials often find their way into the latest power generators, airplanes, high-end automobiles and even tanks. And while getting the right mix of a new composite's raw materials and hitting upon the ideal geometry for the finished product often takes years, he thrives on the challenge.
"To me, the fun part is to be able to spend five years working on a research program and finding you've created a whole new industrial technology with the materials being used to do new things," Evans says.
While the steel factory of his youth may have spurred initial interest in materials, and a PhD in metallurgy from Imperial College in London gave him the skills and training to study the subject, it was his first job that nudged Evans towards what emerged as his specialty. The British Atomic Energy Commission assigned him to study how to boost the efficiency of nuclear fuel by studying one of its components, uranium dioxide, a ceramic material.
"I developed a certain expertise in ceramics," he notes, "and when I came to the States, it was still a virgin field."
Close cousins of clay
Ceramics, close cousins of clay, emerge from a combination of a nonmetallic mineral and high temperature. For example, "Start with sand, get your carbon from coal, and mix it to make silicone carbide," says Evans. "But," he adds, "to refine and get the purity you need, it's a much more complicated story."
Evans has been playing out that story in the United States for the past 25 years, starting in industry at the US National Bureau of Standards and then the Rockwell International Science Center. In 1978 he joined the faculty at the University of California at Berkeley, where he stayed for seven years before moving to U.C. Santa Barbara for 12 years. While at Santa Barbara, he also joined the faculty at Harvard, where he later moved before joining the Princeton faculty in 1998.
The ceramics he works with are particularly useful in high-temperature environments where lighter is better, such as inside a jet turbine. Light material that withstands high temperatures allows the engines to be run more efficiently, saving on fuel costs and reducing ozone-depleting gas emissions. But ceramics tend to be brittle as well as light. Over the years Evans has designed specialty ceramics to be less brittle.
One example of a successful ceramic product is a thin coating, a thermal barrier, applied to metal components in a plane's engines to insulate the metal. The coating, only about 100 microns thick, reduces the temperature of the metal by approximately 200 degrees C, says Evans. He has also created ceramic fibers that he weaves together and layers in particular geometric patterns to make composites used for other critical hot section components.
Partnership with industry
Most of Evans' research is done in partnership with industry, a back and forth dialogue that takes place over the years as a particular material is being formulated. This arrangement came about out of a mutual necessity to share resources.
"It's impossible to reproduce manufacturing processes used in industry at the university level; it's too expensive," explains Evans. "We do measurements and develop the coats and testing. Industry supplies the material. At some point, they stop telling us what they're doing, and we know our job is done. For us, success is passing the technology on."
Examples include ceramic composite components used by GE in their jet engines, Evans says, and cellular A1 used by Rockwell for cooling high power electronics used in motor drives.
Evans and other researchers have brought ceramics to such a high level of development that price often dictates applications rather than technology. "Ceramic components have proven themselves in terms of durability," he says. "At this stage, the implementation of ceramics is virtually all related to cost relative to competing materials. It's out of academics."
Light, cheap, stiff aluminum
While Evans continues to work on refining properties of various ceramics, such as decreasing peeling of a ceramic coating exposed to extreme temperature cycles, he is also turning his interest towards another material: cellular aluminum.
His goal is to create lightweight, inexpensive and super-stiff aluminum alloys for a number of applications, including structural units for planes and tanks. As with ceramics, the ultimate application is usually up to industry researchers, after Evans and colleagues bring the technology far enough along to hand off to the manufacturing partners.
So after years testing the behavior of the materialscompressing, stretching, twistingand sitting in front of a computer writing code to tell the manufacturers what alloys and configurations to use and the specifications for the material, what is Evans's take-home?
"Our products are design codes, research articles and satisfaction," he says with a smile.
top