Researchers from North Carolina State University have developed a new way to shape ceramics using a modest electric field, making the process significantly more energy efficient. The process should result in significant cost savings for ceramics manufacturing over traditional manufacturing methods.
Ceramics make up significant components of an array of products, including insulators, spark plugs, fuel cells, body armor, gas turbines, nuclear rods, high temperature ball bearings, high temperature structural materials and heat shields.
At issue are crystalline defects 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 Dr. Hans Conrad, 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 superplastic – so a ceramic can 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.”
These findings mean that manufacturers who make anything out of ceramics will be able 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.
A modest dc electric field markedly reduced the tensile flow stress at high temperatures in three polycrystalline oxides, i.e. MgO, Al2O3 and yttria-stabilized tetragonal ZrO2 (Y-TZP). The reduction in flow stress ΔσE in Y-TZP consisted of three components: (i) ΔσT 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 ΔσT and occurred in MgO and Al2O3. It is concluded that 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; 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.