Thermodilatometric characterization for devitrification of a micaceous dental glass-ceramic

The devitrification of an uncerammed micaceous glass-ceramic used for dental applications was studied by thermodilatometry and compared with the kinetics of mica crystallization studied by differential thermal analysis. The thermodilatometry plots and their derivative plots revealed thermally impeded processes, namely, structural relaxation, glass softening and nucleation, and crystallization, and they were characterized by glass-transition temperature, glass-softening temperatures, and crystallization temperature. The heating-rate dependence for these characteristic temperatures was used to determine the activation energy for structural relaxation of 338 kJ . mol(-1), the activation energy for viscous flow of 276 kJ . mol(-1), and the apparent activation energy for crystallization of 286 kJ . mol(-1) by one model and 342 kJ . mol(-1) by another model. The similar magnitudes for these activation energies suggested the mechanisms for different thermal processes involved analogous molecular motions, Furthermore, the activation energy for crystallization of the micaceous phase of the glass-ceramic could be estimated from thermodilatometry plots, because it was comparable in magnitude to that obtained from the widely used nonisothermal differential thermal analysis method. Finally, the dilatometry and scanning electron microscopy studies strengthened the earlier opinion that the devitrification of the base glass is a single-step process, that is, without occurrence of an intermediate phase before the formation of the final crystalline phase.