Thermodilatometric characterization for devitrification of a micaceous dental glass-ceramic

The devitrification of an uncerammed micaceous glass-ceramic used for denta l applications was studied by thermodilatometry and compared with the kinet ics of mica crystallization studied by differential thermal analysis. The t hermodilatometry plots and their derivative plots revealed thermally impede d processes, namely, structural relaxation, glass softening and nucleation, and crystallization, and they were characterized by glass-transition tempe rature, glass-softening temperatures, and crystallization temperature. The heating-rate dependence for these characteristic temperatures was used to d etermine the activation energy for structural relaxation of 338 kJ . mol(-1 ), the activation energy for viscous flow of 276 kJ . mol(-1), and the appa rent 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 ac tivation energies suggested the mechanisms for different thermal processes involved analogous molecular motions, Furthermore, the activation energy fo r crystallization of the micaceous phase of the glass-ceramic could be esti mated from thermodilatometry plots, because it was comparable in magnitude to that obtained from the widely used nonisothermal differential thermal an alysis method. Finally, the dilatometry and scanning electron microscopy st udies strengthened the earlier opinion that the devitrification of the base glass is a single-step process, that is, without occurrence of an intermed iate phase before the formation of the final crystalline phase.