Surface Crystallization Phenomena in MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Glass

Abstract

The surface crystallization of glass with a composition MgO(16.7), Al2O3(22.3), SiO2(61) (wt%) was investigated, using XRD, DTA and SEM. Aggregated particles of β-quartz precipitated from fire-polished surface of the glass. The aggregation of non-crystalline spherical particles of about 500-1000A was observed in the region cotiguous to the surface layer containing β-quartz. However, the number of spherical particles decreased with depth from the crystallized surface by the logarithmic function, with substantially the same particle size regardless of the depth. The precipitation of β-quartz, petallite and α-cordierite was observed for the sample with mechanically polished surface. The surface crystallization in this case developed into the glass with the dendrite structure. In the glass having a fire-polished surface, the amounts of β-quartz and petallite passed through maxima during isothermal treatment at 950°C, whereas the amount of α-cordierite increased monotonously with time. This suggests that α-cordierite was formed not only by the direct precipitation from the glass, but also by the transformation of β-quartz and petallite.