R. A. Mendybaev

Volatilization kinetics of silicon carbide in reducing gases: an experimental study with applications to the survival of presolar grains in the solar nebula

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO_2 (cristobalite or glass) were determined in H_2-CO_2, CO-CO_2, and H_2-CO-CO_2 gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wustite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin (<1 μm thick), continuous layer of cristobalite. SiC exposed to IW-6.0 lacks such a layer (or its thickness is <0.01 μm), although some SiO_2 was found within pits and along incised grain boundaries. In H_2-CO_2 gas mixtures above ∼IW-3, the similarity of the SiC volatilization rate and of its dependence on temperature and fO_2 to that for SiO_2 suggests that SiC volatilization is controlled by volatilization of a SiO_2 layer that forms on the surface of the SiC. With decreasing log fO_2 from ∼IW-3 to ∼IW-6, the SiC volatilization rate is constant at constant temperature, whereas that for SiO_2 increases. The independence of the SiC volatilization rate from the gas composition under these conditions suggests that the rate-controlling step is a solid-solid reaction at the internal SiC/SiO_2 interface. For gas compositions more reducing than ∼IW-6, the SiC volatilization rate increases with decreasing fO_2, with both bare SiC surfaces and perhaps silica residing in pits and along incised grain boundaries contributing to the overall reaction rate. If the volatilization mechanism and reaction rate in the solar nebula were the same as in our H_2-CO_2 experiments at IW-6.0, then estimated lifetimes of 1-μm-diameter presolar SiC grains range from several thousand years at ∼900°C, to ∼1 yr at 1100°C, ∼1 d at 1300°C, and ∼1 h at 1400°C. The corresponding lifetimes for 10-μm SiC grains would be an order of magnitude longer. If the supply of oxidants to surfaces of presolar SiC grains were rate limiting—for example, at T > 1100°C for P^(tot)= 10^(−6) atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.

Measurement of oxygen fugacities under reducing conditions: non-Nernstian behavior of Y2O3-doped zirconia oxygen sensors

A calibration procedure is presented for the use of a Y_2O_3-stabilized zirconia (YSZ) oxygen sensor in 1 atm gas-mixing furnaces in the temperature range 1200–1500°C and 0–8 orders of magnitude below the iron-wustite (IW) buffer. Corrections to the Nernst equation were obtained by measuring apparent oxygen fugacities of gases in equilibrium with graphite (equilibrated with pure CO vapor), Cr + Cr_2O_3, and Ta + Ta_2O_5. Under reducing conditions, fO_2s calculated using the ideal form of the Nernst equation are erroneously high, by <0.1 log units at IW but by nearly three log units for Ta-Ta_2O_5 at 1000°C. The deviations between measured emfs and those calculated assuming Nernstian behavior of the electrolyte in the oxygen sensor reflect mixed ionic-electronic conduction. Measured emfs under reducing conditions are readily corrected for this effect via experimentally determined values of P_θ, the oxygen fugacity at which electronic conduction constitutes half of the total conductivity. For the oxygen sensors used in this study, log P_θ(± 0.20,lσ)3.70(±0.72)-32.95 ± 1.15 X10^3 T(K). Even under conditions more reducing than a gas of solar composition (f_(O2) = 10^(−18) at 1200°C), YSZ oxygen sensors can be used to determine absolute values of the oxygen fugacity to within ±0.2 log units.

The measurement of oxygen fugacities in flowing gas mixtures at temperatures below 1200°C

We measured oxygen fugacities in H_2-CO_2 and CO-CO_2 gas mixtures in the temperature interval 700–1350°C using an yttria-stabilized zirconia (YSZ) oxygen sensor. At high temperatures in excess of 1200°C, measured emfs are consistent with expectations based on the gas composition. At lower temperatures in H_2-CO_2 gas mixtures, the oxygen fugacity (f_(O2)) obtained assuming Nernstian behavior of the oxygen sensor is as much as two log units more reducing (∼900°C) to one log unit more oxidizing (∼700°C) than expected by assuming equilibrium speciation. The deviations in H_2-CO_2 gas mixtures arise from two sources: (1) poor contact between the electrode and the zirconia electrolyte, leading to apparent f_(O2) values that are higher than expected and (2) disequilibrium in the vapor, leading to lower than expected f_(O2) values in the temperature range ∼700–1200°C (for experiments near the iron-wustite (IW) buffer) and higher than expected f_(O2) at lower temperatures. The first problem can be alleviated by spring-loading and lightly sintering a Pt mesh internal electrode against the electrolyte and the second by forcing the entire gas stream to equilibrate by passing it through a Pt catalyst. With these measures, experiments employing H_2-CO_2 gas mixtures can be conducted routinely in the temperature range 700–1200°C and the f_(O2) determined with an accuracy comparable to that obtained at higher temperatures (2σ < ±0.1 log units). Above ∼770°C, apparent oxygen fugacities measured using an oxygen sensor in CO-CO_2 gas mixtures near IW are consistent with equilibration in the vapor regardless of whether or not a Pt catalyst is present. At lower temperatures, however, the measured values are more oxidizing than the expected equilibrium values. Under more reducing conditions, the deviations begin to occur at even higher temperatures, ∼930°C for IW-3. The anomalously high f_(O2) values are probably related to the condensation of graphite, which removes C from the gas and generates a lower temperature limit for practical gas mixing experiments using CO-CO_2 gas mixtures.