Passive Oxidation of an Effluent System: The Case of Polymer‐Derived SiCO

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.

Boria Effects on the High Temperature Oxidation of Silicon Carbide

“…Although a dark appearance to the glass layers, indicative of carbon retention, was not mentioned in Schlichting's study, retained carbon oxidation has since been shown to occur in bulk sol-gel derived and crosslinked polysiloxane-siliconoxycarbide (SiOC) specimens at elevated temperature in air 77 .…”

“…A free carbon phase was confirmed by Raman analysis. The issue of carbon retention is important to address since its oxidation is expected in the high-temperature oxidizing environment 77,80,82 that glass coatings will be exposed to during SiC oxidation testing.…”

Si and SiC Nanocrystals by Pyrolysis of Sol–Gel‐Derived Precursors

“…According to this route, the preceramic polymer is shaped, cross-linked, and pyrolized in controlled atmosphere (inert or reactive) at temperature exceeding 800 C and converted into self-similar ceramic devices and components [4]. PDCs have shown extraordinary properties, especially at ultrahigh temperature such as oxidation and creep resistance [5,6], chemical durability [7], electrical conductivity (from insulating up to semiconducting behavior) [8], and photo-and electroluminescence [9]. Such unusual properties originate from the atypical nanostructure of these ceramics in which various nanocrystalline phases grow in situ into an amorphous C-containing matrix [10,11].…”

Influencia del tamaño del material híbrido en las características de los oxicarburos de silicio obtenidos

“…Esta incorporación de carbono en la red vítrea supone un fortalecimiento de dicha red, lo que produce tanto una mejora de sus propiedades con respecto a la sílice vítrea como la aparición de nuevas propiedades que anteriormente no se habían apreciado en este tipo de materiales (2). Estos materiales presentan unas buenas propiedades mecánicas (3,4,5,6) térmicas (resistencia a la fluencia a elevadas temperaturas) (7,8), resistencia frente a la oxidación (9,10,11,12) estabilidad química a elevadas temperatura o frente al ataque de ácidos y bases (13,14). Recientemente, se han encontrado otras características tales como que presentan un comportamiento viscoelástico a elevada temperatura (15), así como propiedades piezoeléctricas (16).…”