Thermal expansivity and conductivity of pure and silicon-alloyed pyrocarbons

Measurements were made of the thermal expansivity (22°–1000°C) and the thermal conductivity (22°–800°C) of isotropic pyrocarbons deposited in fluidized beds and containing up to 34 wt% silicon in the form of β-silicon carbide particles. The thermal expansivity of the pure carbons was proportional to their density, while that of the silicon-alloyed carbons decreased with increasing silicon content, falling from 6·3 × 10^?°C^?1 for material containing 4 wt% silicon to 4·6 × 10^?6°C^?1 for material with 34 wt% silicon. The thermal conductivity of both pure and silicon-alloyed pyrocarbons increased with increasing temperature up to about 500°C. The room-temperature thermal conductivity of the pure carbons increased with increasing apparent crystallite height (L_c), and the conductivity of the silicon-alloyed carbons was significantly lower than that of pure pyrocarbon with the same L_c. The results suggest that silicon entering substitutionally into the carbon lattice may reduce the conductivity.