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_⊥ is 0.72 W/cm deg at 300^∘{}K, and K_⊥ decreases monotonically to 1.4×{}10^-4 W/cm deg at 3^∘{}K. The measured anisotropy K_⊥KII 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≤1^∘K. A suggestion is offered to explain the rapid rise in the anisotropy with increasing temperature. Previous measurements of K_⊥KII in natural and commercial graphite samples are much smaller.