Influence of boron on carbon fiber microstructure, physical properties, and oxidation behavior

An investigation was conducted to determine the influence of substitutional boron on carbon fiber microstructure, physical properties and oxidation behavior in 0.1 MPa UHP O2. Mesophase pitch P55 and PAN T-300 carbon fibers were substitutionally doped with boron at concentration levels between 4 × 10−5 and 0.05 BC atom ratio. Boron was found to enhance graphitization in these fibers at concentrations greater than 2 × 10−4BC. Below this concentration level the fiber microstructure and strength were unaffected. The presence of boron at all concentration levels inhibited carbon fiber oxidation. Oxidation inhibition at high boron concentrations was attributed to changes in the fiber microstructure and specific site blockage by an oxide of boron which developed on the fiber surface during gasification. At relatively low boron concentrations, the decrease in the reactivity of the fiber was related to a change in the fibers' electronic structure which, in turn, influenced the chemistry of the active sites. Boron was also found to significantly influence the fiber structure. Boron modulated the (002) reflection by enhancing the development of two separate microstructures in the same fiber, one more oxidation resistant than the other. The room temperature mechanical properties of these doped fibers were relatively unaffected up to 5 × 10−4BC, but strength and modulus sharply decreased above this dopant level.