The effect of viscosity on nanocrystallization of strontium fluoride from a silicate glass

This paper describes the importance of the viscosity development in the glass-ceramics process with respect to the mechanism of nano crystallization in oxyfluoride glasses. In the glass system under investigation nano crystalline SrF₂ is formed from a silicate glass Na₂O/K₂O/SrF₂/Al₂O₃/SiO₂. The occurring crystallization mechanism is clarified by the variation of the SrF₂ concentration and the resulting structures. As expected, the mechanism shows similarities to other recently described oxyfluoride systems. The feature of the present system is that during cooling of the melt a liquid/liquid phase separation occurs which is not the case in similar systems containing BaF₂. A droplet phase enriched in strontium and fluoride is formed which is embedded in a silicate rich matrix phase. During thermal annealing, nano crystalline cubic SrF₂ is precipitated inside the droplet phase. As a result, the viscosity of the residual glassy phase increases during the course of the crystallization. The crystal sizes do not exceed 8 nm, neither if the annealing time nor if the temperature is increased.