Abstract
The oxidation kinetics of non‐oxides depend on the inward diffusion of oxygen from the environment through the passivating silica overgrowth, the outward diffusion of the effluent species, e.g., CO, produced by the oxidation reaction at the interface, and the chemical driving forces for diffusion. An analysis that combines these factors into a unified theory is presented. The analysis is applied to experiments on the oxidation of polymer‐derived amorphous silicon oxycarbide (SiCO) ceramics containing different amounts of carbon. The comparison between theory and experiment suggests that the activity of the so‐called “free carbon” in SiCO is likely to be less than unity, which explains why the oxidation of SiCO is passive in nature. Further, the analysis provides quantitative answers to the following questions: (a) How is the effective diffusivity for the parabolic rate constant related to the composition of the substrate, the inward diffusivity of oxygen, and the outward diffusivity of CO? (b) How does the rate constant depend on the activity of carbon in the substrate and on the activity of carbon in the environment? (c) How is the pressure of CO generated at the interface related to the carbon activity and the diffusion coefficients? The analysis points towards the need for systematic experiments in controlled O2/CO2 environments for a more complete understanding of the oxidation kinetics of carbon‐based ceramics.
Published Version
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