Effect of Oxygen Partial Pressure on Oxidation Rate of Si-C-O Fiber

Abstract

Amorphous Si-C-O fiber (Nicalon) was oxidized under oxygen partial pressures from 0.25 to 1 at temperatures from 1073 to 1773K. The oxidation mechanism of Nicalon was proposed by carrying out the kinetic analysis of TG data as follows:(1) In the earliest stage of oxidation, its rate obeyed both the linear rate equation for gas-phase diffusion control and the two-dimensional disc-contracting rate equation for reaction control. Both the rate constants, kg and kr, were proportional to the partial pressure of oxygen gas, pO2. The apparent activation energies for gas-phase diffusion and for chemical reaction were estimated to be 50 and 45kJ/mol, respectively. The initial rate seems to be determined by both the gas-phase diffusion and chemical reaction.(2) The oxidation rate at later stage was described by the two-dimensional disc-contracting rate equation for diffusion control. The proportionality between the rate constant, kd, and pO2 was found to hold. The apparent activation energy of 102kJ/mol was obtained.(3) It is thought that oxygen molecules diffuse through the pores of SiO2 film to oxidize Nicalon at the interface bett ween SiO2 film and unoxidized core.