Microstructural evolution and oxidation resistance of polyacrylonitrile-based carbon fibers doped with boron by the decomposition of B4C

Boron (B) was doped into polyacrylonitrile-based carbon fibers (CFs) in various contents by exposing the CFs in a vapor of B by the decomposition of B4C. The structural evolution of B-doped CFs on different B concentrations and heat-treatment temperatures was studied using scanning electron microscope, transmission electron microscope, X-ray diffraction and Raman spectroscopy. It was found that introducing B changed the morphology of the CFs. At high temperatures, the strong erosion of B vapor, not only changed the primary structure of CFs, but also produced more flaws. Structural analysis indicates that B is mostly substitutionally bonded within the graphene sheet regardless of the experimental conditions. As increasing from 2200 to 2600 °C in the graphite crucible containing 10% B4C, the concentration of substitutional B increased progressively from 0.81 to 2.57 at.%. Thermogravimetric analysis revealed that B-doping greatly improved oxidation resistance of the CFs with the oxidation onset temperature increasing from 640 °C (0 at.% B doped, 2.1% weight loss) to about 800 °C (3.65 at.% B doped, 0.3% weight loss). The correlation between structural evolution and oxidation resistance of B-doped CFs provides a new route for fabricating the CFs with oxidation resistance up to 800 °C.