Anisotropic Thermal Conductivity of Pyrolytic Graphite

Abstract

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