On the origin of slow processes of charge transport in porous media

Abstract

Dielectric relaxation spectra in porous media reveal the importance of slow processes of charge transport with polarisation currents down at least to mHz. The spatial scale of the relaxation is frequency-dependent, potentially allowing information about the structure of the medium to be obtained from the AC conductivity. Many mechanisms of relaxation have been proposed, but we review only two in any depth. These two approaches relate long relaxation times on the one hand to diffusion limited relaxation, on the other, to geometrical limitations on conduction pathways. For equidimensional non-fractal particles, the first (second) generates a quadratic (linear) dependence of relaxation time on particle (or pore) size. In each a fractal model of the pore space generates a power-law AC conductivity with the power of the frequency a function of the fractal dimensionality, but the powers have different relationships with system parameters. Comparison with data does not support either interpretation generally, though some data sets agree with each. Since surface conduction yields a quadratic dependence for particles elongated in one dimension, but more complex dependences for fractal interfaces, it seems easier and more productive to start from the perspective that surface conduction processes are the basis for the relaxation.