Estimation of microwave penetration distance and complex permittivity of graphite by measurement of permittivity and direct current conductivity of graphite powder mixtures

Microwave penetration distance into powder mixtures of FeO(OH)/graphite (C) and tri-calciumphosphate/C was studied by changing the fraction of graphite powder and the degree of the compression. They are combined into a parameter of carbon volume fraction [Pct. V_c]. Experimentally, it was shown that the penetration distance decreased at high [Pct. V_c]. Measurement of permittivity became impossible at certain [Pct. V_c], which is related with the abrupt increase in DC conductivity (occurrence of percolation). In this study, dependence of DC conductivity of the mixture on [Pct. V_c] was expressed using generalized effective medium approximation. And then, average permittivity of the mixture below percolation threshold was measured and analyzed using a mixing rule based on effective medium approximation. In this procedure, permittivity of carbon was estimated to fit the data of the measured average permittivity. Transition from the dielectric to the conductive nature of the powder mixture influences the penetration distance; however, it was shown that the predicted penetration distances by equations using conductivity (σ) or complex permittivity (ε) of the mixtures are consistent with each other, namely, their extrapolations are continuous across the transition region of the percolation threshold.