Improved thermal conductivity in isotopically enriched chemical vapor deposited diamond

The thermal conductivity κ of an isotopically enriched (0.055% 13C) polycrystalline diamond plate made by chemical vapor deposition (CVD) has been measured for heat flowing in a direction either parallel (κ∥) or perpendicular (κ⊥) to the plane of the plate. The room-temperature conductivities (κ∥=21.8 and κ⊥=26 W cm−1 K−1) are higher than for any CVD diamond previously reported, and the κ⊥ value is higher than the best gem-quality single crystal with the natural abundance (1.1%) of 13C. Analysis of the temperature dependence of κ∥ reveals that the point-defect scattering of phonons is in fact significantly lower than expected for the natural abundance of 13C and that it is responsible for the improved conductivity. The observed anisotropy κ∥/κ⊥=0.84 at room temperature is associated with the anisotropic grain structure.