FORMING INNOVATION: Charged Sintering
Researchers from North Carolina State University have found that applying a small electric field results in faster formation of ceramic products during manufacture at lower temperatures while enhancing the strength of the ceramic itself. During sintering, ceramic powders are compressed into the desired shape and heated. Under high heat, the atoms of the powder material bond by diffusion; the atoms of different powder grains move around, bonding the fine powder particles together. Sintering eliminates porosity in the ceramic product, which significantly strengthens the material.
"By applying a 60-hertz alternating current (AC) field, we were able to eliminate porosity at 1250øC, as opposed to the 1500øC needed without the electric field," says Hans Conrad, Ph.D., emeritus professor of materials science and engineering at NC State and co-author of the study. In addition, the researchers were able to reduce the grain size of the ceramic by 63%, creating grains with a diameter of 134 nanometers (nm), as opposed to the 360 nm diameter grains produced using conventional sintering methods.
Potential ImpactThe study indicates that ceramic manufacturers can make their products more quickly and cheaply while also improving product quality through the use of an inexpensive electric field. "We found that the use of a small electric field-with a current of only 0.6-0.8 amp/cm2-can result in improved sintering rates with much finer grain size," Conrad says. "You don't use much energy, and you put it right at the atomic site where it is needed, rather than using more energy to create higher temperatures in a kiln, which is less efficient.
"If you want to make a strong ceramic, you want to eliminate porosity and keep the grain size as small as possible. And you want to do it at the lowest cost, which means using the smallest amount of energy and doing it at the lowest temperature and the fastest rate possible. Using an electric field achieves all of these goals."
The research is described in "Enhanced Sintering Rate of Zirconia (3Y-TZP) by Application of a Small AC Electric Field," which will be published in a forthcoming issue of Scripta Materialia. The lead author of the paper is Di Yang, Ph.D., a senior research associate at NC State. This research stemmed from previous work by Yang and Conrad that was funded by the U.S. Army Research Office (see "A Forming Evolution," Ceramic Industry, June 2010, p. 24, or online at www.ceramicindustry.com). Conrad and Yang are currently working to determine the effects of the frequency and strength of the electric field and to investigate other ceramic materials. c
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Above: Grain size of ceramic made from 3Y-TZP powder after conventional sintering at 1500øC (left), compared to the same material after sintering at 1250øC with the application of a 60-hertz AC field (right). (Images courtesy of Di Yang and Hans Conrad, North Carolina State University.)