Cellulose carbon-CVD carbon composites

Abstract

Carbon-carbon composites have been fabricated using a cellulose powder which is molded at low pressures, carbonized at 800°C and infiltrated by a CVD technique at 700°C using propylene as a source of carbon. Infiltration rate at time t is governed by the amount of open porosity existing in the sample at this time. These composites are then treated at temperatures ranging from 1000°C to 2400°C. Flexural strength, elastic modulus, Knoop hardness and electrical conductivity of the composites are progressively changed with increasing apparent density (d), following expressions of the type Y=adb. Coefficient of thermal expansion, wear resistance and oxidation resistance increase with infiltration time, while surface area sharply drops to a low value after initial infiltration. Heat treatment (HT) of the composites considerably changes their properties, but not monotonically with increasing HTT. Rather they show maxima or minima at 1500-2000°C HTT. However, propertydensity relationships of the same type (but with different constants a and b) as for non-heat-treated samples are generally preserved after HT. Comparison of the densest composites with corresponding commercial glassy carbons heat treated to similar temperatures shows that oxidation rate and coefficient of thermal expansion of the composites are lower, flexural strength and electrical conductivity are similar, while Young's modulus, wear resistance and hardness are greater for the glassy carbons.