Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz

Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two-dimensional (2-D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.