During microwave sintering of ceramics, the input microwave power and specimen temperature are limited by the breakdown of the gas in the cavity. The ionization potential of the gas, the temperature, and the specimen shape are all factors that influence the onset of breakdown. Thus, in the sintering of SiC rods in a TE111 cylindrical cavity at 1 atm argon or nitrogen, breakdown occurred at temperatures less than 1300 C and 1700 C, respectively, which are well below the required sintering temperature.
Breakdown can be suppressed by applying either high vacuum or high pressure. Sintering in high vacuum would exacerbate the problems of decomposition and vaporization of ceramics such as Si3N4 and SiC at high temperatures. Pressurization not only suppresses breakdown but inhibits decomposition and vaporization.
The TE111 cylindrical cavity has been adapted for pressurization to 1.38 MPa. The allowable input microwave power and specimen temperature have been markedly increased under high pressure conditions.