The liberation of the enthalpy of vitrification of Li2Si2O5 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. 29Si 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 Li2Si4O9 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Materials Chemistry