Simulation of the kinetics of pyrolytic carbon deposition in C/C composites

A multistep heterogeneous reaction kinetic model for pyrocarbon deposition is proposed to investigate the sharp and clear transition between the high-texture(HT) and medium-texture(MT) pyrocarbons in C/C composites synthesized by chemical vapor infiltration(CVI). The model is based on the Langmuir-Hinshelwood mechanism and a particle-filler conceptual model, which models both the pyrocarbon deposition and the texture formation. The model assumes that adsorption, desorption and dehydrogenation reactions are involved. Unimolecular dehydrogenation reactions of either light linear hydrocarbons as the source of fillers(F) or light aromatic species as the source of particles(P) result in the formation of MT pyrocarbon, while a bimolecular dehydrogenation reaction between P and F species leads to the formation of HT pyrocarbon. The relationship between the types of pyrocarbons and gas-phase compositions is simulated under steady state. It is found that MT and HT pyrocarbon formation are two dominant stable processes with a hysteresis interzone that is affected by gas composition, initial linear hydrocarbon concentration and deposition temperature. Simulated results account for the sharp and clear transition between MT and HT pyrocarbon, and agree well with most pyrocarbon evolution studies under various conditions during CVI with only a few exceptions that may be caused by simplification in constructing the model.