Temperature dependence of electrical properties in conductive polymer composites

Conductive graphite and carbon black filled composites have been prepared using polypropylene, poly(buthylene-terephtalate) and polycarbonate as matrix, and the effect of temperature on conductivity has been studied. Conductivity of graphite filled composites changed significantly as temperature and filler content increased while the conductivity of carbon black filled composites was insensitive to temperature change. To understand the differences in the conductivity change of composites containing different amount of fillers, apparent thermal expansion of composites was measured and it was found that graphite reduced the thermal expansion of composites more than carbon black. The contradiction was solved by analyzing the current-voltage characteristic of composites. It revealed that the conduction mechanism of graphite filled composites was dominated by ohmic conduction, however, in carbon black filled composites the characteristic charge transport process was tunneling and hopping conduction. Thermal activation of tunneling and hopping conduction compensated for the heat expansion of carbon black filled composites.