Glass-Ceramics of the Lithium Aluminosilicate System Nucle-ated by TiO2. The Role of Redox Conditions of Glass Melting in Phase Transformations and Properties

TiO2 is an effective nucleating agent to obtain glass-ceramics of the lithium aluminosilicate sys-tem. Reducing conditions of glass melting, which allow to get ions of variable valence in lower oxidation state, can influence the ability of titania to provide proper phase assemblage, structure and properties of lithium aluminosilicate glass-ceramics. The model glass nucleated by TiO2 was melted with and without addition of As2O3. Using heat-treatments from 680° to 1300 °C, XRD, SEM and DSC data, Raman and absorption spectroscopy, transparent glass-ceramics based on nanocrystals of β-quartz and/or γ-Al2O3 with spinel structure and opaque glass-ceramics based on nanocrystals of β-spodumene were obtained and characterized. Three-phase immiscibility develops during secondary heat-treatments. Al2TiO5 crystallizes from aluminotitanate amor-phous regions simultaneously with appearance of β-quartz solid solutions, while traces of ana-tase and then rutile appear at elevated temperatures. Phase assemblage and sequence of phase transformations are independent of the redox conditions of glass melting, while the rate of these transformations is significantly higher in glass melted without addition of As2O3. Absorption in the visible and near-IR spectral ranges in glass melted without addition of As2O3 and corre-sponding glass-ceramics originates from octahedrally coordinated Ti3+ ions and Ti3+-Ti4+ pairs in glass and nanocrystals of γ-Al2O3, Al2TiO5 and β-quartz. Transparent glass-ceramics with thermal expansion coefficient of ~0.3 × 10-6 K-1 were obtained.