Aug. 6, 1998 COLUMBUS, Ohio -- Researchers at Ohio State University have developed a new technique for creating near net-shaped ceramic parts for high-tech devices like fuel cells, medical implants, cellular phones, gas or temperature sensors, and even automobile engines.
Manufacturers of expensive, complex-shaped ceramic components must fire freshly-molded parts at high temperatures in order to obtain a ceramic body that is free of pores. This pore-filling process, called sintering, shrinks the parts. Sometimes the ceramic part shrinks nonuniformly, which causes it to deform and develop cracks.
“Most people won’t notice if cups and saucers vary a few millimeters from their ideal proportions, but the ceramics used in a number of advanced devices won’t work well unless they conform to exactly the right size and shape,” said Ken Sandhage, associate professor of materials science and engineering.
Sandhage and his graduate students have come up with a solution, which they described in a recent issue of the Journal of Materials Research. Instead of starting the manufacturing process with just ceramic powder, they start with a precursor that consists of a mixture of ceramic and metal powders.
Upon firing in air or oxygen, the metal powders oxidize and become ceramics. Two types of metals are used: one type expands upon oxidation, whereas the other type contracts. By mixing the proper amounts of both types of metals, the researchers produce a final all-ceramic part that retains it’s original shape and dimensions.
This avoids the distortion and cracking of conventional ceramics processing, so manufacturers who use this process would produce fewer defective parts, generate less waste, and save money.
Most metals expand upon oxidation, so a ceramic mixture containing only these metals would swell and crack during firing. Alkaline earth metals, however -- such as magnesium, calcium, strontium, and barium -- are unique because they shrink when oxidized.
“We figured that if we started with a precursor that contained some alkaline earth metals, which shrink, and some non-alkaline earth metals, which expand, then we could tailor the composition so that the net volume change when we fired the mixture would be zero,” said Sandhage.
The researchers mix alkaline earth metal, non-alkaline metal, and ceramic powders in a device that resembles a paint mixer at a hardware store. The material sloshes around at high speeds, and emerges as a thoroughly-mixed, malleable powder that is easy to compact and form into complicated shapes.
“It’s like putting M&M’s® into ice cream, where the M&M’s® are the ceramic powder and the soft ice cream is the metal,” said Sandhage.
The compacted and shaped parts remain the same size and shape after firing. Because the material contains malleable metal and not just brittle ceramic, the researchers can shape it with metallurgical techniques like rolling, forging, extrusion, and machining. In laboratory experiments, the researchers were able to roll a sheet of such a precursor that measured only 20 micrometers thick -- about five times thinner than a human hair.
The metals in the precursor provide other benefits. Because they bind the ceramic powder together, they eliminate the need for traditional organic binding materials such as carbon. Manufacturers normally burn these organic materials from the final ceramic, which releases hydrocarbons into the atmosphere and leaves pores in the material.
Also, the lack of organic material in the precursor means that the process can produce certain ceramic compounds at lower temperatures, which saves energy. The researchers have produced ceramic compounds at temperatures as low as 300ºC, well below half the temperature normally required.
One of Sandhage’s graduate students developed a way to speed up the method by dispersing sources of oxygen within the shaped precursor so that the metal would oxidize faster. This reaction works at higher temperatures, on the order of 900ºC, but is capable of producing larger ceramic parts in a relatively short time -- a few hours.
Sandhage and his students have also used such processing to fabricate shaped ceramic composites -- another mixture of ceramic and metal. For these materials, some of the metal in the precursor is left unoxidized. For some applications, such residual metal can enhance the electrical conductivity, thermal conductivity, or toughness of the part.
The idea of oxidizing precursors containing alkaline earth metals to produce near net-shaped ceramic parts is a simple one, and Sandhage said he’s surprised that nobody thought of it before.
“For years, some of my colleagues have studied high-temperature oxidation of metals for the purpose of stopping it, because it was considered to be a corrosion process,” he said. “We’re doing the opposite -- intentionally oxidizing metals to make ceramics.” The researchers have received several patents on this process and filed several more patent applications.
Manufacturers may use this process to produce radomes -- covers that protect sensitive electronic devices while allowing certain wavelengths of radiation to pass through. Such a cover would protect the sensors located in missile or aircraft noses. The process has also produced ceramic microwave resonators -- devices that amplify signals at particular frequencies, such as those in cellular phones.
Other ceramics that have been produced by this process conduct hydrogen ions in fuel cells or act as permanent magnets for speakers, motors, and portable electronic appliances. One calcium-based ceramic that Sandhage and his students have produced is the major component of teeth and bones, and so is suitable for medical implants.
In sodium vapor lamps -- the lights often found along highways -- transparent ceramic acts as a containment vessel. Ceramic components for automobile engines are lighter and more wear-resistant than their metal counterparts, and can perform better at high temperatures. Sandhage said these are both possible applications for ceramics made with this new process.
Sandhage and his students are currently working to develop ceramic gas sensors and heat-resistant components for rocket nozzles.
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