Effect of ZnF2 addition on the crystallization behaviour of a zinc borosilicate glass

- Incorporation of F− into zinc borosilicate glasses caused the SiOSi bond breakage.
- Heat-treatment of the glasses induced the precipitation of willemite (Zn2SiO4).
- Precipitation of willemite was favoured in glasses with the highest ZnF2 addition.
- The mean size of precipitated willemite crystals increased with ZnF2 addition.

A series of zinc borosilicate glasses with a molar composition (65-x) ZnO-x ZnF2-15B2O3-20SiO2, where x ranged from 0 to 5 mol%, were prepared by a melt quenching technique. The effect of zinc fluoride (ZnF2) substitution for ZnO on the structural, thermal and crystallization features of the glasses was investigated. Structural information via vibration modes was provided by FTIR analysis that revealed an increased band definition and intensity for the highest content of ZnF2. The incorporation of F− ions caused the SiOSi bond breakage and appearance of non-bridging oxygens in the network. DTA results indicated that ZnF2 addition decreased slightly the onset and peak crystallization temperatures, lowering the thermal stability of the glasses. XRD analysis of glasses heat-treated at 615 °C revealed the formation of willemite (Zn2SiO4) crystals, which was enhanced in glasses containing the highest ZnF2 addition. Microstructural observations by SEM of heat-treated glasses confirmed that the amount and the mean size of willemite crystals precipitated within the glass matrix increased with the addition of ZnF2, which acted as a crystallization promoting agent.