High Temperature X-Ray-Diffraction of Lithiumdisilicate-Glass-Ceramic Material

Objective: The purpose of this study was to investigate the dissolution equilibrium of lithiumdisilicate crystal phase in residual glassy matrix as a function of temperature. Dissolution of crystal phase at the temperature where the glass-ceramic is hot-pressed would significantly influence the viscosity of the material. The viscosity would be lowered by reduction of crystalline phase and by enrichment of the residual glassy phase with flux Li₂O. Method: Lithiumdisilicate glass-ceramic ingot e.max Press MO was cut to a disk with a diameter of 12mm and a thickness of 0.5mm. X-Ray diffraction was carried out in a high temperature chamber of a Siemens D5005 diffractometer. Measurements were made every 20°C beginning from 880°C with a cooling rate of 0.84°C/min to 700°C and then with a heating rate of 0.88°C/min to 920°C. The sum of the net height of peaks [110], [130], [040], [111] of a-Li₂Si₂O₅ (JCPDS 40-0376) was taken as a measure for relative crystal content. Estimated error is +/-10°C and +/-100cps. Result: Peak intensity is higher with lower temperature and is lower with higher temperature (Fig.1). There is only a small difference between cooling curve and heating curve, indicating that equilibrium is achieved. Dotted line is extrapolated to the temperature of total dissolution at 960°C, which was confirmed by differential thermo analysis. Conclusion: The peak intensity at 920°C is only 65% of maximum intensity. Partial dissolution of lithiumdisilicate crystal phase is responsible for the strong reduction of the viscosity at hot-pressing temperature of the glass-ceramic e.max Press MO.