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.