Effects of boron and phosphorus on the reactivity and crystallinity of diverse carbon materials.

摘要

Boron and phosphorus were doped into carbon materials to clarify their effects on carbon crystallinity and oxidation and improve oxidation resistance and high-cost time-consuming processing in the preparation of CFRC composites.; Saran char and SP-1 graphite were doped with B to clarify the intrinsic effect of substitutional B on carbon reactivity. The retentivity and preferential distribution of B were studied. The intrinsic effect of substitutional B in carbon oxidation was confirmed to be a catalytic one. For representative model carbon clusters in a theoretical analysis, the differences in bond angle and bond length, as well as in atomic charges, were evaluated and compared.; To suppress the intrinsic catalytic effect of substitutional B at edge carbon atoms and thus maximize the oxidation resistance, P was doped with B into an activated carbon cloth and a carbon felt. The hypothesis was that both B and P can provide complementary oxidation protection: P can block active site by the formation of C-O-P (or C-P-O) bonds at graphene edges, while substitutional B can alter the chemical reactivity of the residual free active sites by reducing their electron density and thus reduce carbon's affinity for oxygen.; The dopant retentivity and carbon crystallinity depended on carbon surface chemistry. Oxidation inhibition increased with heat treatment and in the presence of dopants; the oxidation inhibition mechanism of P-doped samples appears to be active sites blockage. A subsidiary theoretical analysis of the proposed structures of the active sites blocked by phosphorus suggested that the C-O-P bond is more likely to survive at reaction conditions than the C-P-O bond. The results of B-doped samples are consistent with the previous conclusion.; Substitutional B was also incorporated into CFRC composites to study the same issues. Preferential B distribution was observed due to different processing conditions and structures of the components. High-temperature heat treatment showed a compensation effect; increasing crystallinity and losing B concentration. Boron can be applied to achieve high efficiency of densification cycles for the preparation of the composites. Due to its ability to increase the degree of graphitization, fewer numbers of cycles will be needed.