Texture formation of pyrolytic carbon in chemical vapor deposition and infiltration

Carbon textures, as a function of methane pressure, obtained at 1100 degrees C by chemical vapor infiltration of a carbon fiber felt with a fiber volume fraction of 7% and a capillary diameter of 1.1 mm, at different surface area/volume ratios of the substrate were characterized using the optical extinction and orientation angle. Significant texture changes with increasing methane pressure were attributed to the nucleation-growth mechanism of carbon formation with growth dominating at lower pressures, and the increasing influence of the nucleation mechanism of carbon formation at higher pressures. Carbon textures resulting from the growth mechanism perfectly agree with the particle filler model. It is postulated that high textured carbon is formed from a gas phase with an optimum ratio of aromatic hydrocarbons (molecular particles) to small linear hydrocarbons (molecular filler), whereas medium and low textured carbon are deposited with an excess of either aromatic or small linear hydrocarbons. Most significant differences in texture formation between chemical vapor infiltration and deposition result from the much lower hydrogen partial pressures in infiltration because hydrogen diffuses to the surface of the porous substrate.