Crystallization sequence of barium osumilite and MAS osumilite glass-ceramics with low thermal expansion

Abstract

Glass powders with compositions of the series BaMg2Al6Si9O30 (barium osumilite)-Mg2Al4Si11O30 were crystallized isothermally and with a constant heating rate schedule. The phase diagram of the series was constructed from equilibrium solid state reaction studies in order to distinguish stable from metastable crystallization products. Depending on the glass composition and the conditions of heat treatment different phases can crystallize. Metastable crystallization products are a high-quartz solid solution (MgAl2SixO4+2x). hexacelsian (BaAl2Si2O8) and MAS osumilite (Mg2Al4Si11O30). As accessories which can also be metastable with respect to the chosen conditions, cordierite, celsian and a spinel-Al2O3 solid solution appear in the products. BaO-rich glasses of the series crystallize easily to give a monophase barium osumilite glass-ceramic. Glass-ceramics which mainly consist of MAS osumilite can be prepared by crystallizing glasses of the series with low BaO content. Barium osumilite and especially MAS osumilite have very low thermal expansion coefficients: 2.6 ∙ 10^-6 and 1.7 ∙ 10^-6 K^-1 respectively, in the temperature range 25 to 1000 °C. The expansion behaviour of MAS osumilite is similar to that of the low-expansion phase cordierite.