Heat-conducting properties of thermobarically-sintered detonation nanodiamond

The research was conducted to study the thermal conductivity of detonation nanodiamonds-based composites. Composite nanodiamond materials were obtained in the course of thermobaric sintering at the press-free high-pressure apparatus (BARS) under 5 GPa and at temperatures within the range of 1100 -1500 degrees C. It was ascertained that unlike diamond monocrystals with their thermal conductivity reaching up to 2100 W / (mK), the thermal conductivity of a nanodiamond composite is considerably lower and does not go beyond 18 W / (mK). Specifically, the temperature dependence of the thermal conductivity coefficient of a nanodiamond composite is anomalous as compared to a similar dependence in diamond monocrystals. The thermal conductivity coefficient in diamond monocrystals grows in compliance with the rising temperature, whereas it shows practically no changes in a nanodiamond composite in the temperature range of 50 - 300 degrees C. Such a temperature dependence of the thermal-conductivity coefficient is apparently related to the features of the phonon spectrum of diamond monocrystals. This feature is stipulated by the dependence of the phonon spectrum of nanocrystals on their size, represented by a set of phonon modes in the range of the wave vector 0 < q<1/L, i.e., the size of a diamond nanocrystal of 4.5 nm is alleged to limit the excitation of harmonics during nanodiamond composite heating, as opposed to macroscopic crystals that demonstrate the excitation of higher-frequency phonon modes during temperature growing.