Inhibition of catalytic oxidation of carbon/carbon composite materials

An investigation coupling experimental efforts with computational chemistry analysis was conducted to study the inhibition effects of phosphorous or boron on the oxidation of carbon/carbon composite materials catalyzed by potassium or calcium acetate (KAC or CaAC). Commercial aircraft brakes were used, which are exposed during use to K- or Ca-containing runway deicing agents. The reactivity of inhibitor-doped carbon materials was determined by temperature programmed oxidation (TPO) and isothermal oxidation in 1 atm O₂. The structure and surface chemistry of inhibitor-doped samples were characterized, and the inhibition mechanisms were explored with the help of ab initio molecular orbital calculations.; The catalytic effects of KAC or CaAC were found to be dependent on catalyst loading, pretreatment procedure, temperature and O₂ partial pressure. Experimental observations showed that K is a more effective catalyst for carbon composite oxidation than Ca as expected from prior studies of catalyzed carbon gasification. This was attributed to its ability to form and maintain good interfacial contact with carbon, as well as to its insensitivity to carbon structure because of its excellent wetting ability and mobility. The experimental results suggested that the interfacial catalyst/carbon contact is the critical factor determining the catalytic effectiveness.; Thermally deposited phosphorus, upon heat treatment of P-containing compounds such as CH₃OP(OH)₂ and POCl₃ at around 600°C in the presence of inert gas, exhibited a good inhibition effect in the oxidation of C/C composites used in aircraft brake systems. These P compounds were also effective inhibitors for Ca- or K-catalyzed oxidation. The P loading up to a certain amount (ca. 4.0 wt%) was found to suppress Ca-catalyzed oxidation completely. It also improved the resistance of carbon to K-catalyzed oxidation, but the effect was much less significant than in the case of Ca-catalyzed reaction. The characterization of P-doped carbon samples by XPS, XRD, SEM and TPD and the theoretical explorations by ab initio molecular orbital calculations showed that P doping had no effect on the carbon structure and that oxygen-containing P groups preferentially block the active sites on the carbon surface, thus being responsible for the inhibition effect.; Boron doping of the composites at 2500°C was found to have a strong inhibition effect in Ca-catalyzed carbon oxidation and a weak effect in K-catalyzed oxidation. Boron as a dopant was confirmed, by XRD, to enhance the graphitization of the composites. The XPS results supported that the chemical state of doped boron is substitutional, that it is oxidized during carbon oxidation, and that it remains on the carbon surface as boron oxide. The substitutional boron and its oxide appear to have a strong effect on the interfacial contact between the carbon substrate and the catalysts. Accordingly, the catalytic effect of Ca can be almost completely suppressed. (Abstract shortened by UMI.)...