Effect of Äo2 on Sic Volatilization Rate

Introduction: Our previous work on the volatilization rate of SiC in reducing gases [1, 2] was restricted to IW-2.8 and IW-6, oxygen fugacities (ÄO2Õs) 2.8 and 6 log units below the iron-wustite (IW) buffer. These data allowed us to estimate lifetimes of interstellar SiC grains (several months at 1200¡C) in a gas of solar composition (~IW-6). There is, however, evidence that ÄO2Õs for many components in chondrites differed significantly (±4 log units) from IW-6 (e.g., [3]). In this work, we explore the possibility that nebular lifetimes of interstellar SiC grains were significantly different from those we calculated previously because of exposure to vapor at different ÄO2Õs. Experimental: Experimental materials (CVD b-SiC and single crystal a-SiC) and procedures were as in [1, 2]. Experiments were conducted at 1300¡C in a vertical Deltech furnace using H2-CO2 gas mixtures. The log ÄO2 was varied from IW-10 to IW-3 as measured by an SIRO2 oxygen sensor after taking mixed ionicelectronic conductivity of the solid electrolyte [4] into account. Results: Fig. 1 shows the dependence of volatilization rate of single crystal a-SiC on ÄO2 at 1300¡C. We used a single SiC wafer for all experiments shown in Fig. 1 and run durations < 2hrs; the experiments also alternated between reducing and oxidizing gases. This procedure was chosen to prevent the formation of a thick silica layer at high ÄO2Õs, and disintegration of the wafer surfaces at low ÄO2Õs due t o pit formation. The latter process led to destruction of the surface of CVD b-SiC, which caused non-reproducible results when this type of material was used in the experiments. Weight loss was a linear function of time for all experiments with single crystal a-SiC. The figure clearly indicates that the volatilization rate of SiC is approximately constant within the range of log ÄO2 from ~IW-6 to IW-3, in accord with our previous experiments at IW-6 and IW-2.8. Under more reducing conditions (log ÄO2 from ~IW-6 to IW-10), the SiC volatilization rate gradually increases as the reacting gas becomes more reducing and, at log ÄO2~ IW-10, the volatilization rate is ~10 times higher than at IW-6.