Evaluating dielectric impedance spectra using effective media theories

The immittance spectra (i.e., impedance and modulus representations) are ca lculated for various effective medium theories, i.e., the Maxwell-Wagner (M W), Hashin-Shtrikman (HS), Bruggeman Asymmetric (BA) and Bruggeman Symmetri c (BS) models, with emphasis on their individual microstructures. In additi on the brick-layer (BL) model is also considered. The BL and MW-HS models y ield similar single impedance arcs for a relatively low volume fraction con ductive matrix (coating on the low conductivity phase). The BA model yields single impedance arcs different from the MW-HS models. The BL and MW-HS mo dels yield virtually identical dual impedance arc behavior for a low volume fraction insulating matrix (coating on the high conductivity phase). At lo w volume fractions of insulating matrix, the low frequency arc due to the i nsulating material for the BA model is much smaller than for the MW-HS mode l. The BS model exhibits single impedance arc behavior when the volume frac tion of conductor is above or near the percolation threshold and dual arc b ehavior somewhat below the percolation threshold. Equivalent circuits for t hese model materials are discussed, and application is made to experimental data for various electroceramic systems.