Chemistry and kinetics of chemical vapor deposition of pyrocarbon -: VIII.: Carbon deposition from methane at low pressures

Carbon deposition from methane was studied at methane pressures from 5 to 30 kPa, temperatures from 1050 to 1125 degreesC. total residence times of 1, 2 and 4 s, using substrates with surface area/volume ratios, [A(S)/V-R], of 0.79, 1.6, 3.2 and 7.4 mm(-1). The deposition rates were determined as a function of the substrate length using substrates composed of 10 slices. Surface-related deposition rates are not constant, but decrease with increasing [A(S)/V-R] ratio. Increasing methane pressure leads to various states: of saturation adsorption; they are ascribed to different major. carbon forming species. The rate-limiting step results from dissociation of the carbon-hydrogen surface complex, C-x(H): the corresponding activation energy is in the range of 445+/-10 kJ/mol. The results at low pressures confirm those obtained at 100 kPa using argon as diluent gas. (C) 2001 Elsevier Science Ltd. All rights reserved.