Simulation of microwave sintering of ceramic bodies with complex geometry

Microwave sintering, an emerging technology in which the energy is applied directly to the material, enabling rapid sintering, shows potential for the synthesis of advanced ceramic materials with superior properties. The process is complex, combining the propagation and absorption of electromagnetic waves in the ceramic material, heat transport within the geometric body, and densification. The densification changes both macroscopic shape and microstructural morphology. A dynamic balance between the rate of electromagnetic energy absorbed within the bulk of the sample and the rate of energy loss from its surface generally results in temperature gradients. These temperature gradients may be especially important during the microwave sintering of bodies with a complex geometry, because neither the diffusion distance nor the electromagnetic penetration depth scale with sample dimensions. The gradients generated in a ZnO green body of a complex geometry were studied theoretically using various microwave-sintering approaches, and it was found that (1) dual-frequency (2.45 and 30 GHz) microwave processing leads to a decrease in the duration of the temperature gradients, and (2) an increase in the heating rate from 5 °C/min to 1400 °C/min at 2.45 GHz decreases the total required microwave energy by a factor of 55, while at the same time the internal temperature gradients are maintained over a substantially shorter time.