Self-organized nanocrystallinity in MgO–Al2O3–SiO2 glasses with ZrO2 as nucleating agent

From glasses with the composition 21.2 MgO·21.2 Al2O3·51.9 SiO2·5.7 ZrO2 (in mol%) glass–ceramics were prepared by thermal annealing at temperatures in the range from 950 to 1200 °C. The prepared glass–ceramics were characterised by dilatometry, X-ray diffraction and transmission electron microscopy. From the XRD-patterns, Rietveld refinement was done in order to study the transformation from β- to α-quartz-solid-solution. After annealing at temperatures ≤ 1000 °C β-quartz-solid-solutions were detected after cooling. The β-quartz-solid-solution is stabilized by MgO and Al2O3. With increasing annealing temperature and time, crystalline MgAl2O4 was formed and the quartz-solid-solution was depleted in magnesia and alumina. During cooling, the solid solution is then successively transformed into the α-quartz modification. This also explains the increasing thermal expansion coefficient measured by dilatometry. Furthermore, with increasing the annealing time, the transformation temperature of the residual glassy phase increases to a value (968 °C) slightly above the annealing temperature of 950 °C. Then the crystals are embedded in a highly viscous layer which hinders further crystal growth.