A Forming Evolution

June 1, 2010
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A new forming process using a modest electric field could provide significant energy and cost savings for ceramic manufacturers.

Researchers from North Carolina State University have developed a new way to shape ceramics using a modest electric field, making the process more energy efficient. The new forming process could result in significant cost savings for ceramic manufacturing over traditional manufacturing methods.

At issue are defects that are found in crystalline materials such as ceramics. "One of these defects is called a grain boundary, which is where crystals with atoms aligned in different directions meet in the material," says Hans Conrad, Ph.D., emeritus professor of materials science and engineering at NC State and co-author of the study. These boundaries have electrical charges.

"We found that if we apply an electric field to a material, it interacts with the charges at the grain boundaries and makes it easier for the crystals to slide against each other along these boundaries. This makes it much easier to deform the material." In other words, the material becomes super-plastic, which enables the ceramic to be shaped into a desirable form using a small amount of force.

"We've found that you can bring the level of force needed to deform the ceramic material down to essentially zero, if a modest field is applied," Conrad says. "We're talking between 25 and 200 volts per centimeter, so the electricity from a conventional wall socket would be adequate for some applications."

Practical Applications

Researchers expect this new forming process to enable manufacturers that make anything out of ceramics to do so using less energy. "It will make manufacturing processes more cost-effective and decrease related pollution," Conrad says. "And these findings also hold promise for use in the development of new ceramic body armor." Conrad is planning to do additional work using this approach to fabricate ceramic body armor with better properties at a lower cost.

The research, "Influence of an Applied DC Electric Field on the Plastic Deformation Kinetics of Oxide Ceramics," was published in the journal Philosophical Magazine and was funded by the U.S. Army Research Office. The study was co-authored by Conrad and Di Yang, Ph.D., a senior research associate at NC State.

For additional information, visit www.ncsu.edu.

Sidebar: Study Abstract

A modest DC electric field markedly reduced the tensile flow stress at high temperatures in three polycrystalline oxides: MgO, Al2O3 and yttria-stabilized tetragonal ZrO2 (Y-TZP). The reduction in flow stress in Y-TZP consisted of three components: (i) that due to Joule heating; (ii) a rapid, reversible component obtained in on-off and electric field step tests; and (iii) the cumulative effect of the field on microstructure. Only that due to Joule heating and the rapid reversible component occurred in MgO and Al2O3.

It is concluded that the rapid, reversible component results from a reduction in the electrochemical potential for the formation of vacancies corresponding to the diffusion of the rate-controlling ion in the space charge at the grain boundary. The calculated magnitude of the space charge zone width and its temperature and solute composition dependence are in accord with theory and experiment. The cumulative effect of the field on the microstructure is attributed mainly to the retardation of grain growth by the field. The retardation could be due to one or more of the following effects of the field on the space charge zone: (i) an increase in the segregated solute ions; (ii) a decrease in grain boundary energy; and (iii) a decrease in solute ion mobility.

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