Nanocrystallization of CaF2 from Na2O/K2O/CaO/CaF2/Al2O3/SiO2 glasses

In glasses with composition (100-x)(9.9Na(2)O(.)8.8K(2)O(.)12.1CaO(.)6.6Al(2)O(3)(.)62SiO(2))(.)xCaF(2) the glass-transition temperature decreases with increasing CaF2 concentration. Samples with x = 12.5 were thermally treated at temperatures in the range from 520 to 560 degrees C for 1-80 h. This leads to crystallization of CaF2. The quantity of crystalline CaF2 increases with increasing crystallization temperature, while the mean crystallite size remains constant. The glass-transformation temperature of partially crystallized samples increases with increasing crystallization temperature and crystallization time and approaches a value equal to the temperature at which the samples were treated. This was explained by formation of a highly viscous layer enriched in SiO2 formed during crystallization, which acts as a diffusion barrier and hinders further crystal growth.