The catalytic effect of boric acid on polyacrylonitrile-based carbon fibers and the thermal conductivity of carbon/carbon composites produced from them

The catalytic effect of boric acid on the graphitization and surface structure of polyacrylonitrile-based carbon fibers was investigated by dipping fibers in boric acid before heating at 2500 °C. The thermal conductivity of carbon/carbon composites produced from the modified carbon performs by chemical vapor infiltration was also studied. The results show that the treatment by boric acid has a catalytic-graphitization effect on the fibers that increases the crystallinity and changes the surface state of carbon fibers during high temperature treatment. The modified carbon fibers induce the deposition of pyrocarbon with high crystallinity and an obvious transition interface between the carbon fiber and pyrocarbon during chemical vapor infiltration. By changing the microscopic structure of the carbon fibers, the interface bonding between fibers and pyrocarbon is improved and the microstructure of pyrocarbon is regulated. The thermal conductivity of the carbon/carbon composites is therefore improved, especially that in the direction perpendicular to fiber axis.