The evidence of phase separation droplets in the crystallization process of a Li2O-Al2O3-SiO2 glass with TiO2 as nucleating agent

Lithiumalumosilicate glasses are of great importance for industrial applications, because they enable the preparation of glass ceramics with coefficients of thermal expansion close to zero. While detailed studies of the effect of ZrO2 on nucleation and crystal growth have already been performed in recent years, the effect of TiO2 was up to now not reported in detail. It is shown for the first time, that liquid/liquid phase separation is the initial step of nucleation in lithiumalumosilicate glasses containing TiO2. During temperature treatment above the glass transition temperature, at 740 °C for 0.25-24 h, in the formed droplets, TiO2 nanocrystals precipitate. The formed phases were investigated by XRD- and TEM and STEM-EDX to illustrate the phase developments as well as the resulting microstructures and the local enrichments of the respective components as a function of time. Longer crystallization times resulted in the formation of crystalline lithiumalumosilicate (LAS) with a high-quartz structure. These crystals are notably larger than the TiO2 crystals and are growing with increasing treatment time and temperature. The evidence of anatase formation was obtained from high resolution TEM from the lattice spacings because it cannot be distinguished from the high quartz structure using XRD.