Formation of nanometer-sized SnO2 colloids and change in Sn-depth concentration profile in float glass induced by oxygen diffusion from atmosphere at temperatures above Tg

The effect of oxygen diffusion from the atmosphere on tin depth profiles in the bottom face of float glass at temperatures above the glass transition temperature (Tg) was investigated by secondary ion mass spectrometry (SIMS) with an oxygen tracer (18O) gas and transmission electron microscopy (TEM). The heat treatment was performed in 18O2/N2 and argon (Ar) atmospheres. A significant movement of tin to the surface was observed for the glass heat-treated in the 18O2/N2 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 `humpʹ which is commonly observed in tin profiles of 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 Sn2+ to Sn4+, induces a significant diffusion of tin. Furthermore, the precipitation of crystalline SnO2 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, Sn2+ may be supplied to the surface in order to compensate for the marked decrease in Sn2+ concentration in the glass system. It was considered that the oxidation of Sn2+ ions into Sn4+ in the tin-rich layers by O2-diffusion from the atmosphere leads to the phase separation, and the resulting decrease in the Sn2+ concentration in the top layers that induces the out-diffusion of Sn2+ ions from the inside.