THERMAL TRANSPORT ACROSS BOUNDARIES IN DIAMOND STRUCTURE MATERIALS

Previous measurements of the thermal conductivity kappa of chemical va pour deposition diamond films show values of kappa near, or even excee ding, that of natural diamond in spite of the polycrystalline nature o f the films. These data have led us to consider whether there can be a 'resonant' transfer of energy between identical crystallites separate d by a material with different vibrational properties. We consider her e a model of energy transfer between diamond-structure crystallites (S tillinger-Weber silicon) separated by a barrier region in which the ma ss is altered. We find that, for a pulse of energy deposited in one cr ystallite, there can be an efficient transfer of energy through the ba rrier region and subsequent build-up in a neighbouring crystallite if the vibrational spectrum of the barrier region is harder than that of the crystallites. If the vibrational spectrum of the barrier material is softer, the energy accumulation in the barrier region is at least a s rapid as in the neighbouring crystallite and energy is retained long er in the region where it was deposited. The microscopic reasons behin d this behaviour are discussed, and we conclude that the transmission probability between neighbouring crystallites leads to a more physical interpretation than a resonant transfer between next-neighbour crysta llites.