Undoped polyaniline in the high voltage domain: nonlinear behavior and aging effects

Nonlinearity of current-voltage characteristics of polyaniline emeraldine base (PANI-EB) have been studied in detail. PANI-EB is insulating under low electric field and becomes more conductive under high electric field. Tests performed under impulse breakdown (1.2/50) show that for an electric field lower than 300 kV.mm^-1 the breakdown probability of PANI-EB/ethylene vinyl acetate (EVA) blends is lower than that for carbon black/EVA ones. It seems therefore possible that PANI-EB can be used as a stress-grading material for high voltage applications. In view of extremely promising results, it is worthwhile to perform basic studies concerning the origin of the above mentioned nonlinearity as well as its changes induced by aging. We show that nonlinear behavior can be explained in terms of space charge limited current combined with an electric field dependent mobility. The field dependence of the mobility (In(μ)αE^1/2), deduced from time of flight experiments, is similar to the Poole-Frenkel effect and related to energetic disorder in the polymeric material. This model is very close to that developed by Bassler for conjugated polymers. The observed increase in nonlinearity behavior of PANI-EB with aging can also be explained in the framework of our model