A model based on the aspect of the distribution of the length of conduction paths accessible for electric charge flow reproduces the universal power-law dispersive ac conductivity observed in polymer networks and, generally, in disordered matter.Power exponents larger than unity observed in some cases are physically acceptable within this model. A saturation high frequency region is also predicted, in agreement with experimental results. There does not exist a 'universal fractional power law' (and is useless searching for a unique common critical exponent), but a qualitative universal behavior of the ac condductivity in disordered media.

The a.c. conductivity σac of polyaniline prepared with a.c. plasma polymerization was studied recently at low angular frequencies (ω⩽1 MHz) (Mathai J et al 2002 J. Phys. D: Appl. Phys. 35 240). The data were analysed considering a power dependence of the a.c. conductivity on frequency σac∝ωn, n⩽1. It was found that the index n varies between 0.4 and 1. In this work, we show that these values conflict with the bounds of the n parameter predictions of the Austin and Mott model, which assumes a single mode of charge hopping in amorphous media. We suggest that multiple a.c. conductivity mechanisms of the Austin and Mott type may contribute to the measured a.c. conductivity. A numerical calculation for a couple of mechanisms showed that the resulting a.c. conductivity curve exhibits effective n values which can explain the wide distribution of the indexes evaluated in polyaniline.

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The complex electric modulus and the ac conductivity of carbon nanoonion/polyaniline composites were studied from 1 mHz to 1 MHz at isothermal

We investigate the dielectric behavior of sandstone, which consists of a porous matrix with a small amount of inherent humidity, by the thermal-stimulated depolarization current technique. Nine different relaxation mechanisms are detected by the thermal sampling scheme, and are characterized. The activation energy distribution and the pre-exponential factor are obtained by analyzing the signals under the constraint of a normal distribution in the activation energy. The drying of the specimen at elevated temperature under dynamic vacuum affects some of the relaxation mechanisms. The model of freely rotating dipoles may not account for all the drying-sensitive mechanisms. It is probable that water molecules are organized in a way that provide either conductive layers over the surface of the grains or for conductive inclusions inside the bulk. Longdistance charge-transport mechanisms are also affected by the removal of the humidity.