Volume crystallization of β-cristobalite from CaO–Al2O3–SiO2 glasses—Influence of different nucleation strategies

This study examines whether different strategies for increasing crystal number density in CaO–Al 2 O 3 –SiO 2 glasses influence the crystallization mechanism of cristobalite and the stabilization of β -cristobalite during cooling to room temperature. Four bulk glasses were investigated: two compositions in the mullite primary phase field and two compositions containing the classical nucleating agents ZrO 2 or TiO 2 . Phase formation after heat treatment was analyzed by X-ray diffraction and dilatometry, while the resulting microstructures and local chemical distributions were studied by electron microscopy including energy-dispersive X-ray spectroscopic analyses. The mullite-containing glasses crystallize predominantly in the volume and, together with mullite, form β -cristobalite, whereas the ZrO 2 - and TiO 2 -containing glasses show surface crystallization under the conditions studied. For all compositions, heat treatment at 1,250 °C leads to the precipitation of β -cristobalite which does not transform into α -phase during cooling, while higher crystallization temperatures promote partial transformation to α -cristobalite. Among the investigated compositions, the ZrO 2 -containing glass shows the smallest increase in the cristobalite-related dilatometric volume jump after high-temperature treatment. The results show that mullite can promote volume crystallization, whereas ZrO 2 and TiO 2 do not act as nucleating agents in these glasses. The thermal stability of β -cristobalite is controlled by both composition and microstructure.