INDUCTION TIME ANALYSIS OF NUCLEATION AND CRYSTAL-GROWTH IN DISILICATE AND METASILICATE GLASSES

The nucleation and crystallisation kinetics in three glasses with stoichiometric composition Li2O.2SiO2, Na2O.2CaO.3SiO2 and 2Na2O.CaO.3SiO2 are investigated by determining the nucleation and crystal growth rates and the corresponding induction times. An analysis of the induction times for crystal growth and nucleation indicates that in the two soda-lime metasilicate glasses nucleation and crystallisation of the crystals occur subsequently, i.e., no time lag exists between the two experimentally determined induction times. By contrast with this absence of a time lag, a considerable time lag appears in the case of lithium disilicate, indicating a metastable phase formation and probably compositional changes during crystallisation in this early stage. Transmission electron microscopy shows that a metastable phase (denoted by transient-phase) occurs in the early stage which is totally dissimilar to lithium disilicate and which acts as a precursor to the equilibrium crystallisation of lithium disilicate. This two-stage process leads to the conclusion that the theory of homogeneous nucleation cannot be applied to the Li-disilicate melt.