Effective permittivity and microwave heating characteristics of electric conductor and insulator (dielectrics) mixtures

In microwave heating of powder materials that having low absorbability, a susceptor substance is mixed with it, such as a graphite (C). It was found that the microwave penetration depth changed with C concentration. Real part of the permittivity of the mixture became negative above certain C concentration, which corresponds to the percolation threshold of the conductivity. In order to interpret the measured permittivity values, comparison with the theoretical mixing law was attempted. The measured conductivity was well fitted by the GEM (Generalized Effective Medium approximation) mixing law. GEM equation for symmetric state is applied to compare the experimental data for the permittivity values. In this procedure, assumed graphite permittivity was incorporated into the mixing law. This value was compared with the previously reported values. The predicted values of microwave penetration distance by equations for a dielectric and an electric conductor are consistent with each other except the concentration range of threshold.