Chemical vapor infiltration of carbon fiber felt:: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 T and methane pressures from 5 to 30 kPa to confirm the inside-outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside-outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 T and up to 13.5 kPa at 1095 degreesC, leading to bulk densities above 1.9 g/cm(3). A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 degreesC. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures. (C) 2002 Elsevier Science Ltd. All rights reserved.