Microwave heating mechanisms in high-purity alumina ceramics

Experiments were conducted to demonstrate mechanisms involved in microwave heating of alumina ceramics, in a single mode cavity. The microwave generator operated at 2.45 GHz producing a maximum power of 6kW. 99.9% pure alumina rods were heated in a single mode TE 107 waveguide applicator. It has been shown in the past that impurities are concentrated at grain boundaries and can lead to conducting phases with increasing temperature. It is also known that rapid increase in dielectric loss factor with temperature is mainly due to the formation of glassy and conductive phases at grain boundaries. Consequently, it has been inferred that increasing microwave energy loss with temperature is due to conduction through connecting conductive paths and microarcing in gaps between nearby conductive streams. Arguments are also presented to clear up misunderstandings about the reduction in activation energy with microwave sintering in comparison to conventional sintering, due to the so called microwave effect.