Alternating-current electrical properties of graphite, carbon-black and carbon-fiber polymeric composites

Electrical conductivity measurements of graphite, carbon-black and carbon-fiber polymeric composites reported over a broad frequency range covering from d.c. to 10⁹ Hz are comparatively discussed. The d.c. electrical conductivity data from carbon-black and graphite composites exhibit a conducting additive concentration dependence which can be explained on the basis of percolation theory. In both systems, tunneling conduction among particles appears as the predominant mechanism in the concentration range investigated. A frequency-dependent conductivity is observed which is stronger the lower the additive concentration. A modification of the percolation theory which includes the contribution of finite-size clusters is invoked to explain the frequency dependence of the conductivity. In carbon-fiber composites, the high fiber orientation gives rise to materials with higher electrical conductivity levels than those found for particulate composites. The high anisotropic conductivity additionally exhibits an almost absence of frequency dependence. This is explained by assuming the occurrence of a highly interconnected fiber network with almost an absence of electrical barriers.