Energetics and structural changes associated with phase separation and crystallization in lithium silicate glasses

Abstract

Abstract The liberation of the enthalpy of vitrification of Li 2 Si 2 O 5 glass as a function of annealing time at 500 and 520°C has been studied by high temperature drop solution calorimetry to relate the energetics of crystallization and concomitant structural changes in the glass. Enthalpy is liberated such that partially crystallized glasses behave like mechanical mixtures of glass and crystal. 29 Si MAS-NMR spectroscopy corroborates this observation. There is good agreement between four independent determinations of the degree of crystallization as a function of time, for glasses annealed at 500 and 520°C, namely by density measurements, X-ray intensity measurements, optical microscopy and calorimetry. Preliminary calorimetric and spectroscopic results on phase separation and crystallization in Li 2 Si 4 O 9 glass show that the enthalpy difference between optically clear and phase-separated glass is almost zero, the phase-separated glass being 2.3±2.6 kJ/mol more stable in energy. This lack of a difference is consistent with macroscopic phase separation representing coarsening of the size of heterogeneous local environments already present in the clear glass.