Time-dependent resistivity in carbon fibre sheets

Abstract

The electrical properties of sheets of short carbon fibres in resin, glass-fibre and wood-pulp materials have been investigated. For carbon fibre in wood-pulp, a conductor-to-insulator transition was observed at 3 wt % (0.6 vol %) carbon fibre above which conductivity varied linearly with weight fraction. This result is interpreted in terms of a percolation threshold in a system of high aspect ratio. The data agree well with previous measurements on carbon-fibre in polymer composites, and satisfactorily with two-dimensional Monte Carlo calculations. At high concentrations of carbon fibre in all materials, the in-plane resistivity was found to be strongly time-dependent, the fractional change being proportional to Int. A theoretical model is presented which assumes a continuous increase in the number of interconnecting pathways as fibres physically move together under electrostatic attractive forces. Thermal activation over a continous spectrum of energy barriers leads to logarithmic time dependence as observed experimentally. Studies of the effect of external compression support the model for the time dependence.