Crystallization of ZrO2-nucleated MgO/Al2O3/SiO2 glasses - a TEM study

In a 61SiO 2 ·17.5MgO·17.5Al 2 O 3 ·4ZrO 2 (mol%) glass, the course of crystallization at a heat treatment temperature of 950 °C is analyzed between t = 0 and 100 h using X-ray diffractometry and (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy at multiple stages of heat treatment. It is found that after an initial formation of phase-separation droplets from the vitreous parent glass, high-quartz solid solution crystals are formed, in which ZrO 2 nanocrystals are embedded. The diameters of the latter nanocrystals stay approximately constant at around 3.7 nm for all heat treatment times. In contrast to previous investigations of a SiO 2 -MgO-Al 2 O 3 -ZrO 2 parent glass with slightly altered composition, no transformation from high- to low-quartz solid solution with increasing heat treatment time is observed here. Moreover, no precipitation of secondary phases like spinel or indialite occurs. Hypotheses concerning possible crystallization mechanisms that occur during heat treatment of the parent glass are discussed. Hypotheses concerning possible crystallization mechanisms in nanocrystal-bearing MgO-Al 2 O 3 -SiO 2 -ZrO 2 glass-ceramics, analyzed with XRD and analytical TEM techniques, are discussed.