COMPARATIVE-STUDY OF MICROWAVE SINTERING OF ZINC-OXIDE AT 2.45, 30, AND 83 GHZ

Temperature gradients that develop in ceramic materials during microwa ve heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identic al samples of ZnO ponder compacts mere microwave heated at three disti nct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, t he approximately uniform ''volumetric'' heating tends to raise the tem perature of the sample as a whole, but the interior becomes hotter tha n the exterior because of heat loss from the surface. At 30 and 83 GHz , this interior to exterior temperature difference was found to be rev ersed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated w ith reduced skin depth. a model for solving Maxwell's equations was in corporated into a newly developed two-dimensional (2 -D) heat transpor t 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.