THERMAL-CONDUCTIVITY OF GRAPHITE IN THE BASAL-PLANE

Theoretical expressions are derived for the phonon thermal conductivity of single crystals of graphite in the basal plane at room and at elevated temperatures. The phonons are treated by a two-dimensional Debye model in the frequency range from 4 to 46 THz. Mean free paths are calculated for anharmonic three-phonon interactions, scattering by point defects and scattering by grain boundaries. The calculated intrinsic thermal conductivity of 19.1 W/cm/K at 300 K agrees with existing data on highly oriented pyrolytic graphite. The spectral distribution of the intrinsic phonon heat current has a maximum at 4 THz and decreases with increasing frequency, making the thermal conductivity sensitive to grain size. Estimates are given for the reduction in conductivity by vacancies and Frenkel pairs, and for grain boundary and point defects in combination. Observed reductions due to neutron irradiation are discussed in terms of Frenkel pairs or residual vacancies. Thermal conductivity reductions are estimated in terms of observed fragmentation of basal planes as a result of prolonged electron irradiation.