Dynamic oxidation of a Fe2+-bearing calcium-magnesium-aluminosilicate glass: the effect of molecular structure on chemical diffusion and reaction morphology

Rutherford backscattering spectroscopy (RBS) and optical microscopy (OM) an d transmission electron microscopy (TEM) have been used to characterize the oxidation process in a homogeneous, well-annealed, ferrous iron-bearing ca lcium-magnesium-aluminosilicate (Fe-CMAS) glass. Suites of specimens were e xposed to oxidizing environments of air (p(O2) = 0.21 atm) or of argon (p(O 2) similar to 10(-6) atm) within a time range 1-200 h and a temperature of similar to 750 degreesC (near the glass transition). Oxidation causes: (1) formation of crystalline Mg/Fe oxides on the free surface of the glass, and (2) an internal region that is depleted of divalent cations. In general, t his morphology is unequivocal evidence of the oxidation being rate-limited and dominated by the chemical diffusion of divalent, network-modifying cati ons out of the glass; the cation flux is charge-compensated by an inward fl ux of electron holes (polarons). Specifics of the reaction morphology vary for the two oxidation atmospheres. In the case of the Ar environment, outfl uxing Mg2+ and Fe2+ form discrete particles of crystalline (Mg,Fe)(3)O-4 on the free surface of the glass; in air, a continuous surface film forms tha t contains cubic gamma -Fe2O3. In both cases, MI-scale ferrites precipitate at an internal reaction front; the air-oxidized case sees the presence of a second front that changes the ferrite precipitates to gamma -Fe2O3. The a ir-oxidized case also sees substantial outfluxing of Ca2+. Consistent with rate-limitation by chemical diffusion, parabolic reaction kinetics characte rize the oxidation reaction. The different reaction morphologies seen for t he different oxidizing environments demonstrate directly the applicability of the Modified Random Network (MRN) model to the structure of the oxidized (residual) glass. Removal of Mg2+ and Fe2+ in the case of Ar oxidation cre ates internal ferrite precipitates and a residual glass that retains interc onnected channels for the network-modifying cations, hence the formation of discrete crystalline oxide precipitates on the surface. In air, the intern al transformation of the ferrite to gamma -Fe2O3, by requiring the outward flux of Ca2+, sees the collapse of the interconnected channels in the remna nt glass, and so a continuous oxide film forms on the free surface. The thr eshold network-modifier-oxide content for the existence of the interconnect ed channels of modifiers in the CMAS residual glass is thus estimated as si milar to 10 vol.%. (C) 2000 Elsevier Science B.V. AII rights reserved.