Since composite 5# has not only the highest density, but also PyC in it has the largest crystallites and the most RL tissue, the phonon mean free path is dominated by the phonon-phonon interaction. Therefore its thermal diffusivity and TC are very high under the same temperatures. As the test temperature goes up, the phonon vibration frequency will be quickened to make the collision possibility increase, so the mean free path decreases rapidly, which leads to the rapid decrease of its thermal diffusivity. Because its specific heat ascends slowly, the TC is dominated mainly by the phonon mean free path, and TC decreases with increasing temperature. Compared with composite 5#, the PyC in composite 4# is mainly the ISO tissue, with possessing the more defects, so the phonon mean free path is determined by both the phonon-phonon and the phonon-defect interactions. However, its specific heat increases in a little degree with temperature, while the phonon mean free path decreases much, which make its TC first increases slowly and then decreases with temperature. Because the crystal lattice defects in the composites can be activated and will increase under higher temperature, so that the phonon-defect interaction is intensified, which make its TC decrease rapidly again. In consequence, under the same preform conditions, the TC of composite 4# is the lowest among composites 1#, 4#, and 5# at the same temperature. Composite 2# possesses more pores and lower density, and there exists the interface between the PyC and the resin carbon, so the phonon mean free path is mainly dominated by the phonon-defect and the phonon-interface interaction, which leads to its lower TC under the same temperature in the X-Y direction.
In the X-Y direction, a large number of long fibers provide the continuous channels for the phonon transmission. However in the Z direction, there are only the random chopped fibers or small number of long needle-pricked fibers or no fibers for the five C/C composites, so the phonon movement is readily obstructed. In addition, the phonon-interface interaction should not be neglected and the formula can be expressed as
λανLT3 (5)
where L is the crystallite size. PyC grow continuously in the direction of the fiber axis, so L is much larger in the X-Y than the Z direction, which also accounts for the differences in thermal diffusivity and TC in the two directions for five composites.
Three needled carbon felts impregnated with pyrocarbons (PyC) of different microstructures, chopped fibers/ resin carbon + PyC were studied, and carbon cloth/PyC. Their X-Y direction thermal expansion coefficient is negative in the range 0-100C with values ranging from -0.29 to -0.85 *10-6/K. In the range 0-900C, their CTE is also very low, and the CTE vs. T curves have almost the save slope. In the same temperature range composites prepared using chopped fibers show the least CTE value 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/mk). Heat transmission in these composites is by phonon interaction and is related to the preform and PyC structures.