Effect of boron-doping on structure and some properties of carbon-carbon composite

Abstract

Boron-doped carbon-carbon composites with boron concentration around 11–15 mass % were prepared from a carbon fibre felt with dispersed boron carbide powder by infiltration of pyrolytic carbon. The composite was heat treated at several different temperatures from 2000–2800 °C. The highest bending strength was obtained for the composite at a heat treatment temperature (HTT) of 2200 °C. Carbon fibre began to be destroyed after heat treatment at 2400 °C and the structure of the composite was drastically changed above 2600 °C where the anisotropy of the composite originally existing in the thermal expansion coefficient and the thermal conductivity has been faded away. X-ray diffraction measurement indicated that graphitization of the composite was enhanced by boron doping. At HTTs above 2400 °C, the composite became graphitic, the crystallite sizes of which were more than 100 nm in L_c (004) and L_a (110). It was shown that boron was uniformly distributed in the composite at an HTT of 2400 °C and also that heat treatment at higher temperatures, such as 2600 °C, incurred condensation of boron. Air-oxidation loss at 800 °C appeared to be the lowest for the composite with an HTT of 2400 °C and the rate of oxidation loss was 22 times lower than that of the non-boron-doped composite.