The effect of defects on the basal plane thermal conductivity of a graphite crystal

Abstract

The theory of the principal conductivities of a graphite crystal is considered in the relaxation time approximation using theoretical treatments of the lattice vibration spectrum. The results are applied to the calculation of the effect of grain boundaries on the basal plane conductivity of a graphite crystal. The effect of point defects and isotopes is treated using the harmonic mean free path for phonon scattering. In each case an approximation to the graphite lattice in which the layer planes are uncoupled is assumed. It is shown that the effect of point defects and isotopes on the “out-of-plane” vibrations of the graphite lattice is similar to that on the “in-plane” vibrations. The theoretical results are applied to experimental measurements of the conductivity of pyrolytic graphite of varying perfection and also to measurements of the change of thermal conductivity of graphite under fast neutron irradiation. The theory of point defect scattering is found to give good agreement with the changes in conductivity introduced by fast neutron irradiation and also with the temperature dependence of the thermal conductivity of irradiated graphite. The well known relationship between stored energy and fractional change in thermal resistance is explained. The theory yields values of mean free path L a for grain boundary scattering in reasonable agreement with those obtained from electrical resistance measurements.