Phase formation during crystallization of a Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent - An X-ray diffraction and (S)TEM-study

Glass ceramics based on the system Li2O/Al2O3/SiO2 (LAS) often show a coefficient of thermal expansion close to zero. Although these glass-ceramics are of high economic importance, the fundamentals of the crystallization process are still not fully understood. In this paper, the effect of ZrO2 addition as a sole nucleation agent on the crystallization of the LAS glass is described predominantly using transmission electron microscopy and X-ray diffraction. The composition of the studied green glass was close to that of the commercially available Robax (TM) glass (Schott AG), which, however, contained both, ZrO2 and TiO2 as nucleating agents. It was found that during thermal treatment, in a first step, already at temperatures around 10-20 K below the glass transition temperature, T-g, ZrO2 nanocrystals with sizes in the range from 5 to 15 nm were precipitated. The next crystalline phase that forms during the crystallization process was LAS with a structure similar to the hexagonal high temperature phase of quartz. These crystals were much larger than the ZrO2 crystals. If thermal treatment was carried out at higher temperatures, a dense network of LAS crystals was formed. Differently shaped crystals in samples with different thermal history were visualized, and an enrichment of Ba and Sb in the residual glass phase in the late stages of thermal treatment was found. Also, an enrichment of aluminum around the ZrO2 crystals was evident, which is a hint at a preceding droplet phase separation from which the ZrO2 crystals were precipitated. The crystallization is notably different from that of mixed ZrO2/TiO2 nucleating agents used in commercial lithium alumosilicate glass ceramics.