Precipitation of nanometer-sized SnO 2 crystals and Sn depth profile in heat-treated float glass

Abstract

The effect of oxygen diffusion from the atmosphere on tin depth profile in the bottom face of a soda-lime–silica float glass at temperatures above T g was investigated. The heat treatment was performed in 18O 2/N 2 and argon (Ar) atmospheres. The significant diffusion of tin to the surface was observed for the glass heat-treated in 18O 2/N 2 atmosphere, resulting in the formation of a tin-enriched layer near the surface region. It was found that the tin was supplied from the region shallower than the ‘hump’ which is commonly observed in the tin profile of a commercial soda-lime–silica float glass. No significant change in the tin depth profile was observed for the glass heat-treated in Ar atmosphere. These results indicate that 18O diffusion into the glass, which causes the change in chemical state of tin from Sn 2+ to Sn 4+, induces the significant diffusion of tin. Furthermore, the precipitation of crystalline SnO 2 particles with a diameter of ∼1 nm was clearly recognized in the tin-enriched layer. This fact indicates that a phase separation was induced by the oxygen diffusion into the glass. Consequently, Sn 2+ may be supplied to the surface in order to compensate for the marked decrease in Sn 2+ concentration in the glass system. The significant diffusion of tin to the surface was suppressed by increasing the iron content in the glass. This suppression was ascribed to the increase in Sn 4+ concentration as a result of the redox reaction between tin and iron because the diffusion coefficient of Sn 4+ is much smaller than that of Sn 2+.