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The three-dimensional carbon fiber fabric was densified by chemical vapor infiltration and precursor impregnation cracking composite process, and the high thermal conductivity 3D C/C composite material with density of 1.95 g/cm3 was obtained. Using SEM, XRD, thermal conductivity test, linear expansion coefficient test and three-point bending experiment to study the effect of different heat treatment temperatures of 2350,2550,2850℃ on the microscopic morphology,structure, thermal conductivity, linear expansion coefficient and bending performance of 3D C/C composites. The results show that with the increase of the heat treatment temperature, the mesophase pitch-based carbon fiber graphite sheet structure becomes more obvious, the order of the pyrolytic carbon sheets evenly wrapped around the carbon fiber increases, and the arrangement between the sheets becomes more dense; the graphitization and thermal conductivity of 3D C/C composite materials improve; in the test temperature range of 250-1400℃, the linear expansion coefficient increases slightly with the increase of the test temperature, and the 3D C/C composite after different heat treatment temperatures. The linear expansion coefficient of the materials is -1×10-6-2×10-6℃-1, showing good "zero expansion". In addition, when the heat treatment temperature increases, the bonding of the carbon fiber and the matrix of the 3D C/C composite materials are weakened, resulting in the decrease in the bending strength and flexural modulus of the materials. A large number of long fibers are pulled out on the bending fracture surface of the materials after the high temperature heat treatment at 2850℃. In general, the fracture after three different heat treatment temperatures shows "pseudoplastic" fracture characteristics.

The carbon fiber preform is the skeleton of the carbon/carbon material. During the densification process,the pores of the preform is filled by the matrix carbon to form a carbon/carbon composite material. In order to explore the relationship between the microstructure of carbon fiber preform and densification, three kinds of three-dimensional woven preforms were used as the research objects. The microstructure of the preform was observed through a metallurgical microscope, and digital models of the theoretical structure of the preforms were established to calculate the volume content of different types of pores in the body, and to study the effect of pore structure on the densification efficiency of chemical vapor deposition. The results show that the pores in the three-dimensional woven preform are periodically and repeatedly arranged. The fiber volume fraction in the preform is related to the XY fiber layer spacing and the Z direction fiber spacing, the smaller the XY fiber layer spacing and the Z direction fiber spacing are, the higher the fiber volume content of the preform is; the preform is mainly composed of the pores in the fiber bundle and the inter-bundle pores surrounded by the X, Y, Z fiber bundles; the pore size and structure of the preform affect the densification efficiency, and the densification rate of the preform with high porosity is higher in the early stage of densification, and the preform with uniform pore structure gains weight quickly in the later stage of densification.

Porous carbon/carbon (C/C) composites prepared by pitch-based carbon fiber and mesophase pitch were densified by chemical vapor infiltration (CVI) and precursor impregnation process (PIP). After different heat temperature treatment (HTT), the unidirectional C/C composites and bidirectional C/C composites were obtained. The microstructure of C/C composites was investigated by SEM, XRD, and the thermal conductivity was analyzed based on the thermal conductivity mechanism. The results show that unidirectional C/C composites and bidirectional C/C composites have dense structures and excellent thermal conductivity along the fiber axis. After 3000 ℃ treatment, the structure of graphite flat layers become more distinct and the graphitization degree is increased by 18.84%, leading to higher thermal conductivity. Furthermore, the thermal conductivity in X direction and Y direction of the bidirectional C/C composites can be calculated from that of unidirectional C/C composites in X direction and Z direction.