Heterogeneous crystal nucleation, viscosity and liquidus temperature in the system lithium metasilicate-lithium disilicate

The heterogeneous crystal nucleation of five glasses along the compositional join between lithium disilicate and lithium metasilicate was studied by differential scanning calorimetry (DSC) and high-temperature optical microscopy (HTOM). In addition to DSC and HTOM, liquidus temperature and viscosity were measured. DSC and HTOM consisted of 147-378 repetitive cooling runs for each melt and method. DSC and HTOM showed that the crystal nucleation rate was highest for a glass of similar to 63 mol% silica and that the first crystallite consisted of intergrown lithium metasilicate and quartz. In the case of DSC, primary crystallization of lithium disilicate was additionally observed, which was treated as an oxygen-related artifact. The viscosity at the maximum nucleation rate was almost constant for each melt, while the liquidus temperature resulting from the linear extrapolation of the DSC-endotherm for a heating rate of 0 K min-1 was 20-30 K higher than that calculated from thermodynamic properties.