Anisotropic Thermal Conductivity of Pyrolytic Graphite

The thermal conductivity K of a bulk sample of pyrolytic graphite has been measured from 3 to 300^ both perpendicular and parallel to the c axis. Over the whole temperature range below 100^ 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^, and K_ decreases monotonically to 1. 410^-4 W/cm deg at 3^. The measured anisotropy K_{}{K₈₈} decreases monotonically from 47 at 300^ to 2. 5 at 3^. A theoretical calculation of the anisotropy from the elastic constants yields a value of 2. 27 for T1^K. A suggestion is offered to explain the rapid rise in the anisotropy with increasing temperature. Previous measurements of K_{}{K₈₈} in natural and commercial graphite samples are much smaller.