Metastable phases in lithium disilicate glasses

This article deals with crystal nucleation in glass, with special emphasis on new findings about metastable phase formation in lithium disilicate glass (LS2) and its effects on crystallization of the stable LS2 crystal phase. The early crystallization stages of slightly sub-stoichiometric LS2 glasses were probed with SAXS and selected area diffraction (SAD). The second task was accomplished by carefully measuring the crystal nucleation rate (I), growth rate (U), the time evolution of crystallinity and the apparent induction periods (t′) for crystal nucleation and growth at 500°C (Tg ∼ 450°C). Finally, the independently determined kinetic parameters (I and U) were used in a modified form of the JMAK equation, which account for transient nucleation conditions, to test the nucleation mechanism. Depending on the heat treatment, one or more metastable phases precipitate in the early crystallization stages of LS2 glass, however, they do not play a significant role on the crystallization mechanism of the thermodynamically stable phase.