Chemistry and kinetics of heterogeneous reaction mechanism for chemical vapor infiltration of pyrolytic carbon from propane

A heterogeneous reaction mechanism based on elementary reaction steps is proposed for the prediction of pyrolytic carbon deposition in chemical vapor infiltration, involving 72 surface species and 277 surface elementary reactions. A "prototype" gas-phase reaction is chosen to match with the corresponding surface reaction, and correction methods for kinetic data are also proposed for three kinds of surface reactions (i.e., surface/gas reactions, unimolecular surface reactions, surface/surface reactions) in the present work. Simulation results of deposition kinetics and gas-phase compositions of propane pyrolysis are validated by previously published experimental results of Marquaire's group, with a surface to volume ratio from 5 to 175 cm^-1 and temperature from 1173 to 1323 K. The reaction pathways of the heterogeneous surface reactions and homogeneous gas-phase reactions are studied on the basis of the reaction flow rate analysis. An acceptable agreement indicates that the present heterogeneous reaction mechanism is reasonable, and acetylene, ethylene, and benzene are recognized as the main precursors of the pyrolytic carbon deposition at the present processing conditions.