CHEMICAL DIFFUSION AND CRYSTALLINE NUCLEATION DURING OXIDATION OF FERROUS IRON-BEARING MAGNESIUM ALUMINOSILICATE GLASS

Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) have been used to evaluate the mechanism and kinetics of oxidation of a Fe2+-doped MgOAl2O3SiO2 glass (with nominal composition along the enstatite-cordierite-liquid divariant) which was heat treated in air under the time and temperature ranges 10-150 h and 700-800°C, respectively. The results clearly demonstrate that oxidation occurs by a cation diffusion process: specifically, the divalent cations diffuse from the interior of the glass to the free surface where they subsequently react with environmental oxygen to form a two-phase, MgO(Mg, Fe)3O4 crystalline layer which covers the (divalent cation-depleted) glass. Oxidation of some Fe2+ within the glass occurs via the inward flux of electron holes (a counterflux to the divalent cation diffusion required to maintain charge neutrality of the glass); this internal oxidation results in the fine-scale (∼ 1-5 nm), homogeneous nucleation of crystalline (Mg, Fe)3O4 within the divalent cation-depleted layer of the glass. Chemical diffusion of an oxygen species is thus demonstrated to be a slower, parallel kinetic process which is not required for oxidation to occur in this material. A first-order analysis of oxidation kinetics in the glass is presented. © 1990.