Ultra-rapid microwave sintering

The results of microwave sintering of powder materials in the regimes with high heating rates and zero hold time at maximum temperature are reported. Microwave processing of compacted samples based on Al2O3, Y2O3, MgAl2O4, and Yb:(LaY)(2)O-3 ceramics was carried out using a 6 kW/24 GHz gyrotron system. The volumetric absorption of intense microwave radiation resulted in a very rapid densification with the duration of the high-temperature stage of sintering on the order of one to several minutes. In the rapid microwave heating regimes the effective high-frequency conductivity of the materials increased sharply at a certain threshold temperature as a result of the overheating instability, also known as thermal runaway. This suggests that the ultra-rapid sintering occurs via grain-boundary softening and formation of transient liquid phases. The indications of the presence of such phases were observed in the microstructure of the sintered samples. The absorbed microwave power density required for the transition to the ultra-rapid sintering is on the order of 10 ... 100 W/cm(3) for a broad class of the materials. The obtained results suggest that ultra-rapid microwave sintering proceeds via essentially the same mechanism as the so-called flash sintering that occurs in the presence of dc or low-frequency ac electric field.