The successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack-free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced "hot spots" in the parts, which led to differential sintering and subsequent cracking. To produce crack-free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 137.5" to 1200°C by microwave processing and that the resulting grain size was finer.[

Normal grain growth in dense, fine-grained, aluminum oxide-0.1 wt% MgO was studied under both conventional furnace and 28-GHz microwave furnace annealing conditions. The microstructural changes that occurred were the same for both sets of samples; soap bubble microstructures were observed and the aspect ratios and shape factors did not change during the anneals. The kinetics of grain growth were greatly increased by the 28-GHz microwave anneals; e.g., the grain growth rate at 1500°C in the microwave furnace was the same as the rate at 1700°C in the conventional furnace. Also, the activation energy for grain growth was reduced by the microwave anneal from 590 kJ/mol (conventional) to 480 kJ/mol (microwave). Finally, these results demonstrate that a "microwave effect" can exist in a dense ceramic body and that no free pore-solid interfaces are necessary. [

Processing oxide-based ceramics using microwave ,leating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion.