Crystal nucleation in P2O5-doped lithium disilicate glasses

The bulk (volume) crystallisation of Li2O . 2SiO(2) (LS2) glasses with addi tions of 1, 2 and 5 mol % P2O5 (G1P, G2P and G5P) was investigated. In the G1P and G2P glasses heated at 454 degrees C metastable alpha'- and beta'-LS 2 crystal phases formed initially and only stable LS2 at later stages. In G 1P heated at 463 degrees C transmission electron microscopy (TEM) revealed particles (greater than or equal to 2 mu m) of stable LS2 but also smaller (< 0.1 mu m) crystalline particles which were unstable under the electron b eam and could not be identified. Crystalline lithium orthophosphate (Li3PO4 ) was detected by XRD and P-31 magic angle spinning nuclear magnetic resona nce (MAS-NMR) in both as-quenched and heat treated G5P but not in G1P and G 2P. In G5P heated at 476 degrees C, metastable alpha'-LS2 phase initially f ormed and at later times stable LS2. In as-quenched G5P, a trace of lithium metasilicate (LS) was observed. Li3PO4 crystals were not detected in G5P b y TEM but a marked increase in nucleation rate due to P2O5 addition was obs erved. Si-29 MAS-NMR revealed amorphous Q(2) (LS) and Q(4) (SiO2) species i n an average Q(3) (LS2) environment in the as-quenched glasses. Increased P 2O5 concentration caused a greater degree of amorphous phase separation. (3 )1P MAS-NMR showed amorphous lithium phosphate units in the as-quenched and nucleated G1P and G2P. The presence of crystalline Li3PO4 phase in glass G 5P suggests that Li3PO4 crystals may act as sites for heterogeneous nucleat ion of stable lithium disilicate. (C) 1999 Kluwer Academic Publishers.