Electrical conductivity of graphite fiber-epoxy resin composites

The electrical conductivity of graphite fiber-epoxy resin composites was measured for samples containing 64 or 65 vol.% of carbon fibers. Measurements made at angles of 0°, 30°, 45°, 60° and 90° with respect to the nominal fiber axis verify that these systems can be described by a conductivity tensor with principal axes along and normal to the fiber axis. The longitudinal conductivity σ1 is calculated from the simple law of mixtures. Since the epoxy matrix is non-conducting, the transverse conductivity σt is theoretically zero for fibers which are all separated. We assumed that the measured conductivity results from contact between neighboring fibers due to an irregular arrangement and we based our theoretical treatment on the random resistor network model of Kirkpatrick. The possible conductance values are determined by the number of fiber contacts in a given sample thickness with the fiber contacts distributed according to a binomial probability law. The model thus is capable of accounting for the observed thickness dependence of σt. A detailed comparison of the predictions of this theory with experiment is presented.