Thermomechanical Characterization of Fresnoite Glass-Ceramics

Glass-ceramics containing highly surface and bulk preferentially oriented fresnoite Sr2TiSi2O8 crystals have been synthesised by a simple isothermal heat treatment of suitable glasses compositions in the Sr-Ti-Si-K-B-O system. For all tested compositions, crystallisation starts from the free surfaces of the specimens and propagates to bulk with time. If most of these compositions lead to (00l) preferential orientation at the specimens' surfaces, bulk crystal texture is very composition dependent. Effects of variation in K2O content on the crystallisation have been studied. It is shown that low K2O content is required to keep the (00l) orientation from the surface into the bulk. This result seems to be explained by the viscosities of the initial and residual glasses at the temperature of crystallisation: a low viscosity leads to a fine and homogeneous microstructure with small and strongly (00l) bulk oriented crystals.