Computational Study of Thermal Runaway in Microwave Processing of Zirconia

Up to now, reported attempts of multiphysics modeling of high-temperature microwave processing have been limited to fairly idealized models. In this contribution, an original accurate computer model enabling virtual experimentation in microwave processing of zicronia is presented. The model, implementing fully coupled FDTD electromagnetic and thermal algorithms, uses detailed data on temperature-dependent electromagnetic and thermal material parameters of zirconia and alumina-based insulation in the temperature range from 25 to 1100C. Computational results including characteristics of energy coupling and 3D temperature fields are analyzed in the context of conditions for occurrence of thermal runaway. The rate of microwave heating is found to be dependent on the frequency of processing: operation at a resonance closest to 2.45 GHz results in substantial acceleration of heating and generation of a particularly strong hot spot. It is demonstrated that thermal runaway is caused by an increasing loss factor and develops independently of energy coupling.