Chemical vapor deposition of carbon on graphite by methane pyrolysis

Chemical vapor deposition (CVD) of carbon by methane pyrolysis onto a nonporous graphite rod was studied experimentally in a hot-wall, laminar-flow reactor and theoretically by finite-element modeling. Reactor pressure ranged from 10 to 40 torr, control temperature from 1,000 to 1,100 degrees C, and methane was diluted with H-2, Ar, or N-2. The average residence time ranged from 1 to 50 s. A combination of experimental measurement and numerical modeling was used to accurately characterize fluid and thermal fields inside the reactor. Tractable numerical models were developed for the CVD of carbon with gas-phase chemical reaction mechanisms based on accepted kinetic expressions. Simpler mechanisms were reduced from ones based on a more extensive list of intermediate reactions by systematic comparison to experimental data. A simplified but rational model is recommended based on the best fit to experimental data at various flow rates and pressures. The study indicates that axial dispersion plays a significant role and the pressure fall-off effect for rate coefficients of unimolecular decomposition reactions must be taken into account.