HEAT-CONDUCTING PROPERTIES OF SINGLE-CRYSTAL AND THERMOBARIC SINTERED DIAMOND MATERIALS

10.25712/ASTU.1811-1416.2022.03.001

Abstract

A study was made of the thermal conductivity of metal-diamond composites based on diamond powders with a particle size of 30-300 µm. Composite metal-diamond materials were obtained in the course of thermobaric sintering on a non-press high-pressure apparatus BARS at a temperature of 1300 °C and a pressure of 5 GPa. The developed technique for conducting experiments made it possible to obtain samples with a volume of more than 250 mm³. It has been established that, unlike diamond single crystals, whose thermal conductivity can reach 2100 W/mK, the thermal conductivity of a metal-diamond composite is lower and can reach 490 W/mK. Characteristically, the temperature dependence of the thermal conductivity coefficient of diamond single crystals is monotonically increasing, which corresponds to the phonon mechanism of heat transfer. The thermal conductivity of metal-diamond composites is an effective value and is a combination of the low thermal conductivity of the binder metal and the high thermal conductivity of micron-sized diamond particles, taking into account weight parameters. Obviously, the thermal conductivity depends on the presence of impurity atoms, especially nitrogen, in the lattice of diamond single crystals, on which phonons are scattered. However, the interfaces, on which phonon scattering also occurs, have a significant effect on the thermal conductivity. With the presence of a carbide-forming element in the initial charge, the thermal conductivity of the composite increases due to the formation of a diamond skeleton and good wettability of iron carbide by copper. If in single crystals of diamond the coefficient of thermal conductivity increases with an increase in temperature, then in a metal-diamond composite it decreases in the range of 50-300 °C. This dependence of the thermal conductivity on temperature is obviously related to the competing contribution of the phonon and electronic mechanisms of thermal conductivity. The phonon mechanism leads to an increase in the thermal conductivity of the diamond crystal; on the contrary, the electronic mechanism of heat transfer reduces the thermal conductivity with increasing temperature due to an increase in the resistance of the copper bond.