Measurement of thermal conductivity of polycrystalline CVD diamond by laser-induced transient grating technique

The tangential thermal diffusivity $D_\parallel$ and thermal conductivity $k_\parallel$ of diamond plates grown from the gaseous phase by chemical vapour deposition (CVD diamond) are determined by the transient grating technique in the temperature range 25 – 200$^\circ$C. Samples of insulating and boron-doped polycrystalline diamond of thickness about 0.3 mm and diameter 63 mm were synthesised in a microwave discharge in mixtures of methane and hydrogen. In view of the intense light scattering by the samples, a photosensitive grating recording technique was developed and used for measurements. It was found that the value $k_\parallel$ amounts to 18 – 20 W cm $^{-1}$ K $^{-1}$ at room temperature, approaching the thermal conductivity of the highest purity single crystals of diamond. A comparison of the value of $k_\parallel$ with the normal thermal conductivity $k_\perp$ determined by the flash method reveals a thermal conductivity anisotropy of about 10% – 20% associated with the texture of the diamond film, the normal component of thermal conductivity being larger than the tangential component. Boron-doped diamond displays a dependence of the transient grating kinetics on the excitation wavelength. The obtained results indicate that CVD diamond is a promising material for preparing efficient heat sinks, especially of large size, used in microelectronic devices and laser engineering.