CONDUCTION MECHANISMS IN SOME GRAPHITE-POLYMER COMPOSITES - EFFECTS OF TEMPERATURE AND HYDROSTATIC-PRESSURE

This article is devoted to the conduction mechanisms involved in some highly electrically anisotropic resin-graphite particle composites. Th ese materials are known to show a percolation phenomenon as the filler content is varied; they are epoxy or polyurethane based, the conducti ng particles are oriented single-crystal platelets, and samples are in the form of thick films. Because of their strong anisotropy, two type s of measurements were made, i.e., parallel to and perpendicular to th e plane of the films. Study of the resistivity variations of samples c ontaining various concentrations in conducting particles was carried o ut first as a function of temperature from 4.2 to 300 K at ambient pre ssure, and second as a function of hydrostatic pressure up to 1.2 GPa, at room temperature. As the temperature is varied, the changes in res istivity of all the samples studied (i.e., above percolation threshold ) are weak; analysis leads to the conclusion that thermally activated tunneling plays a dominant role above but close to the percolation thr eshold phi(c). As a function of pressure, more samples were studied: w hen the filler content is above phi(c), resistivity changes are quanti tatively in agreement with what is expected from both percolation theo ry and tunneling; below threshold, the observed behavior is partially attributable to an ionic conduction mechanism throughout the polymer. (C) 1998 American Institute of Physics. [S0021-8979(98)05602-3].