Dielectric Relaxation in Dielectric Mixtures
Doctoral thesis, 2001
In this thesis dielectric properties of both ideal composite structures and polymeric composite materials were investigated. Numerical calculations were performed to simulate frequency response of interfacial polarization using the finite element method in two dimensions. Fictitious binary mixtures were considered in the simulations to investigate influences of electrical properties of the constituents, of their concentrations as well as distributions of phases. In addition, effects of randomness in the mixture structure were included. The dielectric properties of regular and disordered structures were compared. A new technique based on the Monte Carlo method and a functional approach using an iterational complex nonlinear least squares algorithm were developed to analyze dielectric responses. It was shown that a single Debye-like dielectric relaxation was not relevant for dense and disordered mixtures. The relaxation in such systems was asymmetric, and depending on the ratios of the complex dielectric permittivities of the constituents even fractional power-law dispersions (low frequency dispersion) could be obtained. It was concluded that by just considering interfacial polarization, it was possible to simulate different dielectric related phenomena in materials. Dielectric spectroscopy measurements performed on polymeric composite materials have indicated that the electrical properties change significantly with filler concentration. Samples with high concentrations of small filler particles have had low frequency dispersions in their dielectric spectra, whereas, for samples with low concentrations of filler particles or with large filler particles having well-defined shapes and shape distribution, the dielectric spectra have shown clear relaxation polarization and ohmic losses.