OXIDATION OF SI IN 15 MBAR ISOTOPICALLY LABELED O-2 H2O-GAS MIXTURES AT 750 TO 1000-DEGREES-C/

The oxidation of Si in 15 mbar isotopically labeled O-2/H2O-gas mixtur es at 750 to 1000 degrees C was studied in situ by probing the gas-pha se composition of an enclosed volume with a mass spectrometer. The exp eriments reveal interactions between the gas phase and the oxide which are not accessible from analyses of the solid reaction product. The d issociation rate of O-2 was calculated from oxygen exchange reactions. O-2 reversible arrow O-2 and O-2 reversible arrow SiO2, and compared with the oxidation rate. In the O-2 + H2O-gas mixture, the dissociatio n rate of H2O was calculated from the O-2 reversible arrow H2O oxygen exchange and compared with the oxidation rate. In the oxidation proces s, the dissociation rate of H2O decreased by a factor of more than 20. A general discussion on the dependence of the dissociation rate on th e oxidation rate is made in relation to different types of migrating s pecies in various systems.