Transient modeling of chemical vapor infiltration of methane using multi-step reaction and deposition models

Based on multi-step reaction and deposition models including the hydrogen inhibition model of pyrocarbon growth, transient 2D simulations of chemical vapor infiltration of methane were carried out by a finite element method (FEM) coupling the mass transfer (by convection and diffusion) and the evolutive surface area model with gas-phase and surface chemical reactions. The continuous infiltration, pyrolysis and deposition of methane and its consecutive CxHyCxHy products lead to continuously varying hydrogen concentration inside the carbon felt. The higher diffusibility of hydrogen compared to those of hydrocarbons results in complex distributions of [H2H2]/[CxHyCxHy] ratios inside the carbon felt, significantly affecting the deposition rates of pyrolytic carbon from hydrocarbons. The favorable densification mode (from inside to outside) seems to depend not only on the concentration ratio of [C2HnC2Hn]/[C6HmC6Hm] but also on the concentration ratio of [H2]/[CxHyCxHy]. The effect of various feed compositions of methane and hydrogen on densification of carbon felts was investigated at a temperature of 1368 K. An acceptable agreement was found between the predicted density distribution and experimental data taken from literature.