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

Abstract

The crystal structure of Bi₂ZnB₂O₇ exhibits a non-centrosymmetric structure, enabling it to exhibit nonlinear optical properties. Crystallization heat treatment of Bi₂O₃-ZnO-B₂O₃ glass results in the formation of nanometer-sized Bi₂ZnB₂O₇ 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 33.3Bi₂O₃-33.3ZnO-33.3B₂O₃ (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% TiO₂ and 3 mol% ZrO₂ effectively lowered the crystallization temperature and increased the degree of crystallinity compared to the baseline glass, indicating that TiO₂ + ZrO₂ is a promising nucleating agent system. The primary crystalline phase was Bi₂ZnB₂O₇, with possible secondary phases including BiB₃O₆, ZnO, TiO₂, and ZrO₂. While increasing the crystallization temperature and time enhanced crystallinity, the phase assemblage remained unchanged. In bulk glass samples, Bi₂ZnB₂O₇ 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 Bi₂ZnB₂O₇ disappeared. Crystals were observed to nucleate on the surface of individual glass particles and grow inward, resulting in uniform internal crystallization. The average Bi₂ZnB₂O₇ 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.