Textures of pyrolytic carbon formed in the chemical vapor infiltration of capillaries

Capillaries, 1.1 mm in diameter and 17.0 or 32.5 mm in length, were
infiltrated at a temperature of 1100 degreesC and methane pressures
from 5 to 30 kPa. Layer thickness and carbon texture were determined at
cross-sections of 2, 16 and 32 mm from the open end of the capillaries
using polarized light microscopy. Average deposition rates, determined
from layer thickness and infiltration time, as a function of methane
pressure indicate a rate increase up to a saturation adsorption at
pressures between 10 and 15 kPa (range 1) and a strong rate increase
above these pressures (range 2). This result implies carbon formations
based on the growth mechanism in range I and the nucleation mechanism
in range 2. The carbon texture shows a maximum in range I and a minimum
in the transition from range 1 to range 2 followed by a clear increase
in range 2. The maximum in range I corresponds to the particle-filler
model describing formation of various textures of carbon by the ratio
of aromatic species to C, species. Increasing texture degrees in range
2 suggest that the nucleation mechanism may lead to high textured
carbon provided that the residence time for intramolecular
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rearrangments of polycyclic aromatic hydrocarbons is sufficient.