Thermal conductivity of sintered nanodiamonds and microdiamonds

We studied the sintering mechanisms and thermal conductivity of composites based on well purified detonation nanodiamonds (ND) prepared by Dr. Aleksenskii A.E. The thermal and electrical conductivities of composites from natural diamonds of 10–14 μm in size were also examined. Both types of composites were sintered at high pressures (5.0–7.0 GPa) and high temperatures (1200–2300 °C) for 10–25 s. It was found that the thermal conductivity of composites from natural diamonds increased as the sintering temperature approached the diamond–graphite equilibrium in the pressure range of 4.5–6.5 GPa because of the interdiffusion of the diamond particles. Above the phase equilibrium temperature, the thermal conductivity was observed to decrease due to the sample bulk graphitization. The maximum value of this parameter for the ND samples was observed at approximately 1900 °С. Higher temperatures caused sample damage at lowered pressures, which seems to be due to the ND transition to the nondiamond carbon phase possessing a lower density. When we added 5 wt.% of С 60 fullerene to the initial ND, the diamond transition to a nondiamond carbon-like state occurred at a temperature below 1400 °С and the thermal conductivity increased from 50 to 100 W/(m · K). Thermal conductivity was found to be about 50 W/(m · K) for the ND samples and about 500 W/(m · K) for the microdiamonds.