Crystallization, and microstructure of Bi ₂ O ₃ -B ₂ O ₃ -ZnO glass-ceramics containing nanocrystalline phase

The crystal structure of Bi 2 ZnB 2 O 7 exhibits a non-centrosymmetric structure, enabling it to exhibit nonlinear optical properties. Crystallization heat treatment of Bi 2 O 3 -ZnO-B 2 O 3 glass results in the formation of nanometer-sized Bi 2 ZnB 2 O 7 crystals within the glass. However, crystallization typically initiates at the glass surface, leading to microstructural and property inhomogeneity. In this study, nucleating agents were added to a 33.3Bi 2 O 3 -33.3ZnO-33.3B 2 O 3 (mol%) glass composition, and thermal behavior was analyzed using differential scanning calorimetry (DSC). A decrease in crystallization temperature was interpreted as an indication of nucleation enhancement. Compositions exhibiting such effects were subsequently subjected to crystallization heat treatment. The results show that the addition of 1 mol% TiO 2 and 3 mol% ZrO 2 effectively lowered the crystallization temperature and increased the degree of crystallinity compared to the baseline glass, indicating that TiO 2 + ZrO 2 is a promising nucleating agent system. The primary crystalline phase was Bi 2 ZnB 2 O 7 , with possible secondary phases including BiB 3 O 6 , ZnO, TiO 2 , and ZrO 2 . While increasing the crystallization temperature and time enhanced crystallinity, the phase assemblage remained unchanged. In bulk glass samples, Bi 2 ZnB 2 O 7 crystals formed on the surface with a pronounced preferred orientation, while the interior remained amorphous. In contrast, for samples fabricated from glass powders, simultaneous densification and crystallization occurred during sintering–nucleation–crystallization treatment. Although the crystalline phases remained similar to those in bulk-derived glass–ceramics, the preferred orientation of Bi 2 ZnB 2 O 7 disappeared. Crystals were observed to nucleate on the surface of individual glass particles and grow inward, resulting in uniform internal crystallization. The average Bi 2 ZnB 2 O 7 partical size was approximately 20 nm. Residual porosity was observed in the sintered glass–ceramics, ranging from 1.35% to 9.1%. Reducing the initial glass powder particle size and applying pressure during sintering were effective strategies for lowering porosity.