Theory of the effect of crystallite boundaries on the principal thermal conductivities of highly oriented graphite—The effect of the elastic constant C 44

Abstract

The theory of the principal conductivities of highly oriented graphite as limited by crystal boundaries is re-examined using the recently measured value of the elastic constant C 44. The principal conductivities are calculated using the relaxation time approximation and assuming the presence of crystal boundaries parallel and perpendicular to the hexagonal axis defining scattering lengths L a and L c respectively. The lattice dynamics are those given by the semi-continuum model of Komatsu. It is shown that the introduction of the experimental value of C 44 in the lattice dynamics leads to a satisfactory explanation of the “anomalous” temperature dependence of the basal conductivity. Parallel to the hexagonal axis it is shown that, as surmised previously, the out-of-plane acoustic mode contributes the major part of the conduction for reasonable ratios of L a L c . Experimental data on very perfect pyrolytic graphite is compared with the theory and a new comparison made of crystallite dimensions L a derived from electrical and thermal property measurements.