The evidence of phase separation droplets in the crystallization process of a Li2O-Al2O3-SiO2 glass with TiO2 as nucleating agent: An X-ray diffraction and (S)TEM-study supported by EDX-analysis

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

Effect of the concentrations of nucleating agents ZrO2 and TiO2 on the crystallization of Li2O–Al2O3–SiO2 glass: an X-ray diffraction and TEM investigation

In order to study the effect of ZrO2 and TiO2 on the nucleation mechanism in a lithium aluminosilicate glass, a composition similar to that of the commercially available ROBAX™ glass (Schott AG) was modified. Glasses with different concentrations of ZrO2 and TiO2 were melted and studied using differential scanning calorimetry measurement. The glasses are thermally treated for different periods of time and at various temperatures and then investigated by X-ray diffraction and electron microscopy. Both scanning electron microscopy and transmission electron microscopy were applied to study size and shape of the formed crystal phases and possibly to obtain lattice plane distances. Increasing the concentration of the nucleation agents led to the spontaneous formation of Zr1−xTi1+xO4 nano crystals during cooling. In any case, the nuclei contain much more titania than zirconia, possess an elongated shape, and are embedded in a shell enriched in alumina

A modified B2O3 containing Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent : Crystallization and microstructure studied by XRD and (S)TEM-EDX

Glass-ceramics based on lithium-alumo-silicate glasses are commercially important for a wide range of applications, due to their special properties, like a vanishing thermal expansion. In order to tailor these properties, the composition of the glass and the temperature/time schedule are crucial factors. For the industrial production of most lithium-alumo-silicate glasses, high melting temperatures are required due to the high viscosities of the respective melt compositions. In this study, a simplified lithium-alumo-silicate glass composition with ZrO2 as nucleating agent, on the basis of the commercially available Robax® composition, is studied. Adding boron oxide leads to lower viscosities of the glass melt and notably lower melting temperatures may be supplied. The resulting glass is investigated using X-ray diffraction and transmission electron microscopy. During the crystallization process, phases such as ZrO2 and β-quartz types are formed. The microstructure of the glass ceramics is notably coarser than that of glass-ceramics which are obtained from lithium-alumo-silicate glasses of standard compositions. EDX-analyses indicate a considerable enrichment of chemical elements in comparatively small areas of the microstructure. Especially boron oxide is found to be enriched in the residual glass of the investigated glass-ceramics

The formation of nanocrystalline ZrO2 nuclei in a Li2O-Al2O3-SiO2 glass - a combined XANES and TEM study

The high economic importance of glass ceramics based on Li2O/Al2O3/SiO2 (LAS) is mainly due to their low coefficients of thermal expansion (CTE), which make these materials suitable candidates for a number of applications. The exact mechanism of the crystallization processes in LAS glasses is still not fully understood. The present work focuses on the formation and development of nanocrystalline ZrO2 within an LAS base composition which contains only ZrO2 as nucleating agent. Using a combination of transmission electron microscopy and X-ray absorption spectroscopy techniques, the temporal evolution of the ZrO2 nanocrystal formation is described. It is found that the formation of ZrO2 is initiated by liquid-liquid phase separation droplets with high Zr content, which eventually evolve into the nanocrystalline ZrO2 precipitations. This process is accompanied by a gradual change of the coordination of the tetravalent Zr ions from sixfold in the glass to eightfold in the crystals. The diameters of the ZrO2 crystals stay well below 4 nm, even at late stages. The degree of crystallization at each step of the crystallization process is deduced, and from that, the Avrami coefficient n is determined to be n ≈ 1, which describes a barrier-limited crystal growth process