Large diameter pitch-based graphite fiber reinforced unidirectional carbon/carbon composites with high thermal conductivity densified by chemical vapor infiltration

Carbon/carbon (C/C) composites of high thermal conductivity have been prepared by chemical vapor infiltration (CVI) processing from novel graphite fibers of large diameter (∼50 μm). The fibers bundles were used to fabricate two types of unidirectional cylindrical C/C composites by a CVI densifying process. One type was initially bonded with phenolic resin impregnation, cured, carbonized and graphitized before subsequent CVI densification (ICCG + CVI). In the second process the bundles were densified by CVI processing only. Both types of composite were further densified by secondary CVI treatment. The microstructure of the C/C composites and the relative contributions of fiber and matrix were investigated. The CVI matrix is highly crystalline and oriented around the fibers and, after secondary CVI treatment and graphitization, densities of the C/C composites prepared by the ICCG + CVI and CVI processes are 1.65 and 1.72 g/cm3, respectively. The axial thermal conductivities of the two C/C composites are as high as 569 and 675 W/m K, respectively. Simple mixture rules have enabled the thermal conductivity of the CVI matrix itself to be estimated to be of the order of 1200 W/m K, consistent with the highly crystalline nature of this phase and significantly higher than that of the highly oriented large diameter fibers themselves.

The unidirectional cylindrical C/C composites fabricated by densifying the graphite fiber bundles with CVI process have excellent thermal properties and high anisotropy ratio for the thermal conductivity along the longitudinal plane and transverse section. The structure and content of the graphite fiber and the matrix carbon of the C/C composites which varied with the manufacturing processes are the keys to control the thermal properties of the C/C composites.231