Iron concentration and the physical processes of dynamic oxidation in an alkaline earth aluminosilicate glass

Rutherford backscattering spectroscopy was used to investigate the persiste nce of cation-diffusion-limited oxidation in three, low-Fe2+-bearing MgO-Al 2O3-SiO2 glasses (base glass compositions along the enstatite-cordierite-li quid cotectic; total Fe levels of 0.04, 0.19, and 0.54 at%). The glasses we re reacted in air at temperatures of 700-850 degrees C (similar to T-g), an d changes in the composition of the near-surface region (less than or equal to 2.5 mu m) of the glass resulting from oxidation were characterized. The reaction morphology produced by oxidation at temperatures above 800 degree s C, for all of the glasses studied regardless of Fe concentration, was con sistent uniquely with an oxidation process dominated by diffusion of Fe2+ c ations to the free surface that was charge compensated by a (counter) flux of electron holes into the material. In the high-Fe material (0.54 at%), th e activation energy for the cation-diffusion-limited reaction was estimated at similar to 475 kJ/mol. Below 800 degrees C, the two glasses with lowest Fe concentration displayed a reaction morphology consistent with oxidation occurring by the motion of an oxygen species. High levels of transition me tal cations are not required to ensure the dominance of cation-diffusion-li mited oxidation reaction in silicate glasses and melts; thus, monitoring in ternal Fe3+:Fe2+ equilibrium, even at trace amounts, seems untenable as an indicator of the diffusion behavior of molecular or ionic oxygen.