Ultra-rapid microwave sintering employing thermal instability and resonant absorption

Ultra-rapid microwave sintering of ceramics has been recently demonstrated by the authors. In the experiments with oxide ceramic samples carried out in a 24 GHz gyrotron system for microwave processing of materials, full density was achieved in the sintering processes with a duration of the high-temperature stage of one to several minutes and zero hold at the maximum temperature. The implementation of the ultra-rapid microwave sintering processes was made possible due to fast and efficient control over the temperature of the materials and the supplied microwave power. The absorbed microwave power density was typically in the range of 10–100 W/cm3, which is within the same order of magnitude as the power of Joule heat in the DC electric field–assisted flash sintering processes. At this power level, a thermal instability is triggered by the volumetric heating, which results in a drastic enhancement of mass transport. In addition, possibility of ultra-rapid microwave sintering of powder metals has been demonstrated within a model accounting for the effective electromagnetic properties and resonant absorption effects.