Thermal properties of bulk polycrystalline CVD diamond

Abstract

The thermal properties of bulk polycrystalline CVD diamond are presented and correlated with measured optical properties. A technique for measuring thermal diffusivity is described, and measurements of thermal conductivity as a function of temperature are reported. UV, visible, Fourier transform IR (FTIR) and Raman spectroscopies were used to assess the quality of bulk polycrystalline diamond plates produced by chemical vapour deposition (CVD), which is compared with that of type IIa natural diamond. The refractive index at a wavelength of 10.0 μm was measured as 2.375 ± −0.014, which is in good agreement with the value of 2.3756 accepted for type IIa natural diamond. For colourless, transparent CVD diamond plates, the absorption coefficient in the 8–14 μm wavelength region is between 0.1 and 0.3 cm −1. The data presented show that the thermal and optical properties of CVD diamond with thermal conductivities between 4 and 16.5 W cm K −1 are dictated by the quantity of impurity phases. However, other mechanisms dominate in the very highest quality polycrystalline CVD diamond plates. In these, values of thermal conductivity (up to 21 W cm −1 K −1 at 300 K) and optical transmission (71.3% in the IR and 68% in the visible) approaching those of natural type IIa diamond are achieved.