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

Rutherford backscattering spectroscopy was used to investigate the persistence of cation-diffusion-limited oxidation in three, low-Fe2+-bearing MgO-Al2O3-SiO2 glasses (base glass compositions along the enstatite-cordierite-liquid cotectic; total Fe levels of 0.04, 0.19, and 0.54 at%). The glasses were reacted in air at temperatures of 700-850 degrees C (similar to T-g), and 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 degrees C, for all of the glasses studied regardless of Fe concentration, was consistent uniquely with an oxidation process dominated by diffusion of Fe2+ cations 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%), the 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 metal cations are not required to ensure the dominance of cation-diffusion-limited oxidation reaction in silicate glasses and melts; thus, monitoring internal Fe3+:Fe2+ equilibrium, even at trace amounts, seems untenable as an indicator of the diffusion behavior of molecular or ionic oxygen.