A numerical simulation of heat transfer during the microwave heating process of magnetite, which is a two-dimensional (2-D) magnetic dielectric, subjected to heat conduction, convection, and radiation was performed. The heat transfer process was modeled using an explicit finitedifference approach, and the temperature profiles for different heating parameters were generated through developing a code in Mathematica 7.0 (Wolfram Research, Inc., Champaign, IL). The temperature in the sample increases rapidly in 1 minute and nonuniform temperature distribution inside the object is observed. An obvious temperature hot spot is formed in the corner of the predicted temperature profile initially, which shifts to the center of the object as heating power increases. Microwave heating at 915 MHz exhibits better heating uniformity than 2450 MHz mainly because of the larger microwave penetration depth. It is also observed that the heating homogeneity in the object can be improved by reducing the dimension of object. The effects of heating time, microwave power, microwave frequency, and object dimension need to be considered to obtain high heating performance and avoid/minimize thermal runaway resulting from temperature nonuniformity in large-scale microwave heating.

The permittivity and permeability measurements of ferric oxide (Fe2O3) were carried out over a broad temperature range from 24°C to above 1 000°C at 915 and 2 450 MHz. The real part and imaginary part of complex relative permittivity (εr' and εr") of ferric oxide slightly increase with temperature below 450°C, above which εr' increases significantly while εr" presents a broad dielectric loss peak between 450 and 1 000°C. Contrary to εr' and εr", the real part and imaginary part of complex relative permeability (μr' and μr") remain relatively invariable (1 and 0, respectively) until 700°C. The μr' values subsequently exhibit a decreasing tendency due to the increased electrical conductivity at higher temperatures while the μr" values stay negligible as temperature increases. The results demonstrate that the dielectric loss is the primary factor contributing to microwave absorption of Fe2O3. The calculation of microwave penetration depth shows that Fe2O3 undergoes a transition from a microwave transparent material to a good microwave absorber with increasing temperature.

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