Addition of GeO2to Reduce the Viscosity of Parent Glasses for Low-Expansion, Transparent Glass?Ceramics Containing High-Quartz Solid Solutions

Abstract

Parent glasses for fabricating glass–ceramics with nanometer-sized crystals usually have high viscosities, resulting in high processing temperatures. In this study, GeO2 was added to a transparent, near-zero thermal-expansion Li2O–Al2O3–SiO2 glass–ceramic to reduce the viscosity of the parent glass. The effects of this compositional modification on the viscosity and crystal-nucleation rate of the parent glasses, and on the crystal size, thermal expansion, and optical transparency of the resulting glass–ceramics were investigated. It was found that addition of GeO2 was useful in reducing the glass viscosity. Owing to the reduced nucleating rate with the increase in the GeO2 content, the nucleating times required for reaching the smallest crystal size, the lowest coefficient of thermal expansion, and the highest transparency were all increased. With increasing GeO2 content, the lowest coefficient of thermal expansion that can be reached for glass–ceramics increased (0.14–2.9 × 10−6 K−1). The highest transparency of the GeO2-containing glass–ceramics is almost as good as that of the GeO2-free glass–ceramic and is almost independent of GeO2 content when the crystal size is smaller than about 65 nm.