Thermodynamics and Oxidation Behaviour of Crystalline Silicon Carbide (3C) with Atomic Oxygen and Ozone

Abstract

Thermodynamics of oxidation of crystalline silicon carbide (cubic form) by atomic oxygen (O) and ozone (O 3 ) 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 O 3 . Equilibrium activity diagrams were derived, showing the most stable oxidation products of SiC, represented in temperature-oxygen pressure <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mo stretchy="false">(</mml:mo><mml:mi>T</mml:mi><mml:mtext>-</mml:mtext><mml:msub><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mtext>O</mml:mtext><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mtext>O</mml:mtext></mml:mrow><mml:mrow><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub><mml:mo stretchy="false">)</mml:mo></mml:math> 2D diagrams. Programs were developed in Mathematica . The diagrams provide an understanding of the oxidation routes of SiC under changing levels of O/O 3 and temperature, as encountered during reentry of space vehicles. At high levels of the volatiles, CO 2 , CO, and SiO and temperatures between 1000 and 1500 K, oxidation by atomic oxygen or ozone first produced SiO 2 + C followed by SiO 2 + CO and finally SiO 2 + CO 2 . When volatiles were at very low pressures, the sequence of oxidation was SiO + CO followed by either SiO 2 + CO or SiO + CO 2 and finally SiO 2 + CO 2 . 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.