METASTABLE PHASE-FORMATION IN THE EARLY-STAGE CRYSTALLIZATION OF LITHIUM DISILICATE GLASS

Early stage crystallisation behaviour of binary lithium disilicate [LS 2] glass containing 33.91 mol% Li2O has been investigated. Two metasta ble crystalline phases, alpha'- and beta'-LS2 precipitated prior to cr ystallisation of stable lithium disilicate in samples heat treated at 454 +/- 1 degrees C for 120-480 h. Metastable alpha' and beta'-LS2 pha ses transformed to stable LS2 after extended holds (similar to 551 h) at 454 degrees C, The broader Si-29 MAS-NMR crystalline peaks in sampl es nucleated for > 228 h at 454 degrees C, than those of samples cryst allised at 600 degrees C, indicate the presence of multiple polymorphs or a disordered structure in the former samples. The streaking of som e of the spots observed in electron diffraction patterns from beta'-LS 2 crystals is consistent with this observation. (C) 1998 Elsevier Scie nce B.V.