Crystallisation kinetics and structural stability of transparent CaF2 glass ceramics: Dependence of light transmittance on the amount of CaF2 added

Abstract

The effect of CaF2 addition (15.4, 20.4, 27.4 and 35.4 wt%) on the crystallisation ability and preparation of transparent glass ceramics (TGCs) containing CaF2 nanocrystals was studied. It was found that samples with 15.4, 20.4, and 35.4 wt% CaF2 are crystallised with multiple crystal phases, making the TGCs opaque. Only when the amount of CaF2 added is increased to 27.4 wt%, TGCs with only CaF2 nanocrystals can be successfully prepared by heat treatment at 630 and 730 °C for 2 h. The crystallisation activation energy of the sample with 27.4 wt% CaF2 is 247.07 kJ mol−1, and the crystallisation energy barrier to be overcome corresponds to the formation of CaF2 nanocrystals rather than the formation of aluminosilicates, whose size and distribution easily make the glass opaque. Similarly, the FT-IR and XPS results indicate that as the amount of CaF2 added is increased to 27.4 wt%, the polymerisation degree of the sample is more suitable for the diffusion of calcium and fluorine atoms to form only CaF2 nanocrystals; therein, the corresponding mole fractions of the bridging oxygen (Oo), non-bridging oxygen (O−), and free oxygen (O2−) are approximately about 0.12, 0.77 and 0.11, respectively.