Theory of the effect of crystallite boundaries on the principal thermal conductivities of highly oriented graphite

Abstract

A theory of the principal conductivities of highly oriented graphite as limited by crystal boundaries parallel and perpendicular to the hexagonal axis is derived. The lattice dynamics of the graphite crystal are assumed to be those of the semi-continuum model due to Komatsu. In the direction parallel to the layer planes it is found that each of the three acoustic vibrational modes contributes roughly the same amount to the conductivity. The crystallite boundaries parallel to the hexagonal axis defining a scattering length L α parallel to the basal planes essentially control the conductivity in this direction. In the direction parallel to the hexagonal axis a major contribution to the conductivity comes from the acoustic mode in which the atomic vibrations are polarized parallel to the hexagonal axis. The principal conductivity is affected by both types of crystal boundary, the boundaries parallel to the hexagonal axis being more important at high temperatures while those parallel to the basal planes are most important at low temperatures. The theoretical results are briefly compared with the available experimental data on the principal conductivities of highly oriented pyrolytic graphite.