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