Thermophysical properties of carbon/carbon composites and physical mechanism of thermal expansion and thermal conductivity

Abstract Five different carbon/carbon composites (C/C) have been prepared and their thermophysical properties studied. These were three needled carbon felts impregnated with pyrocarbons (PyC) of different microstructures, chopped fibers/resin carbon + PyC, and carbon cloth/PyC. The results show that the X – Y direction thermal expansion coefficient (CTE) is negative in the range 0–100 °C with values ranging from −0.29 to −0.85 × 10 −6 /K. In the range 0–900 °C, their CTE is also very low, and the CTE vs. T curves have almost the same slope. In the same temperature range composites prepared using chopped fibers show the smallest CTE values and those using the felts show the highest. The microstructure of the PyC has no obvious effect on the CTE for composites with the same preform architecture. Their expansion is mainly caused by atomic vibration, pore shrinkage and volatilization of water. However, the PyC structure has a large effect on thermal conductivity (TC) with rough laminar PyC giving the highest value and isotropic PyC giving the lowest. All five composites have a high TC, and values in the X – Y direction (25.6–174 W/m K) are much larger than in the Z direction (3.5–50 W/m K). Heat transmission in these composites is by phonon interaction and is related to the preform and PyC structures.