Effect of Oxygen on Pyrolysis of SiC Fiber Synthesized from Polycarbosilane

Abstract

The fibers containing 5% O (fiber-A) and 17% O (fiber-B) were prepared from polycarbosilane by the oxidation at 453K and 503K in O₂ stream and by the heat treatment at 1473K in Ar stream, and the effect of oxygen on the mechanism of the pyrolysis of SiC fibers was studied. The rate of pyrolysis of the SiC fiber was measured with a thermo-balance in an atmosphere of Ar from 1673K to 1973K. The pyrolysis products were examined by X-ray diffraction, SEM and TEM observations. The core of the original fiber-A crystallized slightly, though its surface was amorphous. The pyrolysis proceeded rapidly in the early stage of reaction, but it was remarkably sluggish in the late stage. The SiC crystallite in the fiber coarsened only at the surface. The surface layer with high concentrations of oxygen decomposed rapidly, but the core with low concentrations of oxygen was difficult to decompose. The fiber-B, which was amorphous in the original state, decomposed to β-SiC rapidly. SiC crystallite grew and coarsened throughout the whole region of the fiber. The rate of pyrolysis was described by an Avrami-Erofeev equation. The rate-determining step changed from the crystal growth controlled by diffusion to that controlled by chemical reaction with rising pyrolysic temperature.