Chemistry and kinetics of chemical vapor infiltration of pyrocarbon - VII:infiltration of capillaries of equal size

Capillaries, 1 mm in diameter and 17 mm in depth, were infiltrated at a tem perature of 1100 degrees C and pressures of 20 kPa (pure methane) and 30 kP a (methane-hydrogen mixture (7.1)). The residence time of the gas in the de position reactor was 0.5 s, at the lower pressure of 20 kPa, residence time s of 1 and 2 s were studied additonally. The thickness of the deposited car bon layers was determined using a X-ray microscope, i.e., without cutting t he samples. In the experiments with 0.5 and 1 s residence times an increase of the deposition rate from the mouth to the depth of the pores was obtain ed. The gradient of the deposition rate decreases with increasing residence time of the gas and becomes negative at 2 s. Hydrogen, either formed by de composition of methane or added to the feed gas, leads to an increase in th e gradient. The homogeneous -heterogeneous model of carbon deposition from methane is used to interpret the results with special consideration of depo sition chemistry and diffusivity of the relevant hydrocarbon species. (C) 2 000 Elsevier Science Ltd. All rights reserved.