Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

Abstract

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.