High temperature thermophysical properties of carbon/carbon composites prepared by chemical liquid-vapor deposition

To reduce the cost and shorten the production time of carbon/carbon (C/C) composites for high speed train brakes, C/C composites were prepared from preforms made of polyacrylonitrile-based preoxidized fibers. They were densified by rapid chemical liquidvapor infiltration followed by cycles of impregnation-carbonization and graphitization at 2300℃. Thermophysical properties of the composites at temperatures from 30 to 800℃ were investigated. Results showed that the thermal diffusivities of the C/C composites decreased nonlinearly with temperature both in parallel (X-Y) and perpendicular (Z) directions to the side of the preforms, while the specific heat increased nonlinearly with temperature. The thermal conductivity was anisotropic. In the X-Y direction it exhibited a maximum value of 186. 7W · m^-1 · K^-1 at 200℃, but in the Z direction it remained almost unchanged with temperature. The maximum thermal conductivity in the X-Y direction was 5. 8 times higher than in the Z direction. It was found by analyzing the Wiedemann-Franz ratio that thermal conduction in the C/C composites might be dominated by phonons at low temperature and by electrons at high temperature.