The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO2, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO2 content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO2, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al2O3-SiO2 phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al2TiO5 preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO2 content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.