Dielectric approaches for estimating microwave heating of ceramics and polymers assuming that resistive effects are small enough to be neglected have been applied quite often. However, these approaches are not valid for semiconductors. Permittivity and resistivity are very sensitive to chemical composition; therefore it is difficult to extrapolate properties from similar materials as can be done with thermal properties. The objective of this work is to estimate the dielectric and resistive contributions during microwave heating, solving the electromagnetic and heat transfer equations by means of Finite Element Method (FEM) using the software ANSYS TM . The experiments were performed over a parallelepiped made of compacted and sintered powders of the manganite, which is a negative thermal coefficient thermistor, placed inside a waveguide and exposed to 2.45 GHz microwaves at different powers. Temperature was taken with sheathed thermocouples that also served as electrodes for monitoring electric resistance. Matching experimental data and simulation results provided values for the properties within acceptable range. An operative model for simulating heating of a sample placed in a waveguide was built, considering either resistive or dielectric effects, which in turn helps to estimate these properties applying it jointly with the experimental technique shown.

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It is not rare to hear arguments against the use of thermocouples for taking temperatures in processes that are taking place under microwave fields. However, the simplicity of this device makes it attractive to consider its use. One question that arises when thermocouples are employed is whether the electric field perturbs the measurement, and if the thermocouple affects the processing. The process that was chosen for conducting this test was the synthesis of spinel (MgAl2O4) using microwaves as a power supply and hematite (Fe2O3) as an additive for both spinel formation promotion and susceptor. The alumina-based systems are very important to study because this is one of the most common ingredients in refractory materials. There are many discussions about the improvement of the process when microwaves are used, but a kinetic comparison cannot be performed if the temperature is unknown, and that is the reason for emphasizing the measurement techniques. The analysis of the obtained samples was carried out by X-ray diffraction of powders. The results of this work show that there is no difference between the products obtained when the thermocouple was inserted in the system, compared to processing without it; hence the thermocouple is appropriate for this application.

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