Abstract
Thermodynamics of oxidation of crystalline silicon carbide (cubic form) by atomic oxygen (O) and ozone (O3) was derived to understand the thermodynamic stability of SiC in the upper atmosphere. Equilibrium constants and equilibrium partial pressures were computed for each of eight possible reactions of SiC with O and O3. Equilibrium activity diagrams were derived, showing the most stable oxidation products of SiC, represented in temperature-oxygen pressure (T-PO/O3) 2D diagrams. Programs were developed in Mathematica. The diagrams provide an understanding of the oxidation routes of SiC under changing levels of O/O3 and temperature, as encountered during reentry of space vehicles. At high levels of the volatiles, CO2, CO, and SiO and temperatures between 1000 and 1500 K, oxidation by atomic oxygen or ozone first produced SiO2 + C followed by SiO2 + CO and finally SiO2 + CO2. When volatiles were at very low pressures, the sequence of oxidation was SiO + CO followed by either SiO2 + CO or SiO + CO2 and finally SiO2 + CO2. Stability of SiC in ozone was much lower than in atomic oxygen. With both oxidants, the oxidation of the Si in SiC occurred prior to the oxidation of C. Implications for mechanisms of thermal protection are discussed.
Highlights
Silicon carbide is an important material for space applications
Mogilevsky and Zangvil [6] modelled the oxidation of SiC-reinforced mullite–zirconia matrix composites and evaluated oxygen permeabilities of the matrix as well as the silica layer
Phase diagrams of Si-C-O system are available in the literature [10, 11] as either triangular diagrams (Si-C-O) or biaxial diagrams (T-SiC/SiO2)
Summary
Silicon carbide is an important material for space applications. It is a useful thermal protection system for very high temperature zones like nose-cones in reentry vehicles. Several experimental studies on the oxidation of SiC with O2 have been reported It was observed [1] that oxidation of SiC between 900–1600◦C resulted in a surface layer of SiO2 that formed a protective film. Tropospheric chemistry would be dominated by oxidation with molecular oxygen whose partial pressures vary from about 0.2 bar to 10−3 bar at the tropopause [12, 13]. The mesosphere is located about 50–85 km above Earth’s surface, and the lower thermosphere is located between 80 and 180 km Within this layer, temperature decreases with increasing altitude. To obtain an understanding of the chemical behaviour of SiC in the upper atmosphere, knowledge of the nature of reactions with atomic oxygen and ozone is important. The studies would be useful for modelling chemical transformations of SiC in the stratosphere and mesosphere-lower thermosphere
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