Formation and crystallization of zinc borosilicate glasses: Influence of the ZnO/B₂O₃ ratio

Different glasses based on the ZnO-B₂O₃-SiO₂ system, with a ZnO content ≥ 60 mol% and a fixed SiO₂ content (20 mol%), were synthesised using a melt-quenching method. Glass samples with zinc oxide concentrations as high as 65 mol% were prepared successfully without deteriorating the glass-forming ability. The glass samples were submitted to controlled heat-treatments, and the effect of the ZnO/B₂O₃ molar ratio on the formation of crystalline phases within the glass matrix was investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). DTA results indicated that the glass transition and the onset crystallization temperatures of the studied glasses tended to increase with the increment on the ZnO/B₂O₃ ratio. XRD data showed that the zinc binary crystalline phases, willemite (Zn₂SiO₄), a zinc orthosilicate, and zinc borate (Zn₃B₂O₆) could be present in the crystallized glasses, depending on the heat-treatment conditions. The formation of willemite was promoted by an increasing ZnO/B₂O₃ ratio. Microstructural observations performed by SEM indicated that under controlled experimental parameters (glass composition, heat-treatment temperature and time) the precipitation of nanocrystals within the glass matrix can be achieved, resulting in transparent and translucent willemite glass-ceramics.