Anisotropic Thermal Conductivity of Pyrolytic Graphite

The thermal conductivity $K$ of a bulk sample of pyrolytic graphite has been measured from 3 to 300°K both perpendicular and parallel to the $c$ axis. Over the whole temperature range below 100°K the thermal conductivity in both directions appears to be limited by the crystallite size. The measured value of $K_{\perp }$ is 0.72 W/cm deg at 300°K, and $K_{\perp }$ decreases monotonically to 1.4×${10}^{-4\mathrm{}}$ W/cm deg at 3°K. The measured anisotropy $\frac{K_{\perp }}{K_{\mathrm{II}}}$ decreases monotonically from 47 at 300°K to 2.5 at 3°K. A theoretical calculation of the anisotropy from the elastic constants yields a value of 2.27 for $T\le 1°$K. A suggestion is offered to explain the rapid rise in the anisotropy with increasing temperature. Previous measurements of $\frac{K_{\perp }}{K_{\mathrm{II}}}$ in natural and commercial graphite samples are much smaller.