Effect of oxygen partial pressure on oxidation rate of Si-C-O fiber

The oxidation mechanism of Si-C-O fiber (Nicalon) has been investigated by considering the effect of partial pressure and temperature on the oxidation rate. The kinetic data of Nicalon were described by the two-dimensional disc-contracting formula. The rate constants were proportional to the partial pressure of O-2. In earliest stage of oxidation, the rate equation for reaction control was applicable and the activation energy was estimated to be 45 kJ/mol. The initial oxidation of Nicalon is considered to be determined by both the gas-phase diffusion and chemical reaction. When the fiber was completely coated with SiO2 film, the oxidation rate obeyed the equation for diffusion control and the activation energy was 102 kJ/mol. It is considered that the rate-determining step is the diffusion of oxygen molecules through the pores of SiO2 film.