Controllable preparation of CaF2 transparent glass ceramics: Dependence of the light transmittance mechanism on the glass crystallization behaviour

Abstract

Transparent glass ceramics containing CaF2 nanocrystals were synthesized using a conventional melting and heat treatment process. The mechanism associated with the maintenance of the high transparency of the as-prepared glass ceramics was further clarified. The maximum light transmittance of the as-obtained transparent glass ceramics was approximately 85%, with microhardness value that reaches 761.62 ± 7.93 HV. To obtain transparent glass ceramics, in addition to controlling the CaF2 nanocrystal size to be smaller than the wavelength of visible light, the average grain spacing (or concentration) of the CaF2 is also a key factor. CaF2 transparent glass ceramics were prepared following heat-treatment at a temperature between 630 °C and 700 °C for a period of 1–4 h; herein, the CaF2 grain size was approximately 10–25 nm. The transmission electron microscopy (TEM) results showed that the CaF2 grains were evenly distributed within the glass phase and that the average grain spacing was greater than 5.64 nm. The Raman analysis results indicate that at 730 °C, there was a significant increase in the degree of polymerization and an obvious increase in the concentration of CaF2 grains. The average grain spacing was much lower than 5 nm, and therefore, light energy was constantly reflected between the grains and eventually consumed, resulting in the observed opacity.