Formation of TiO ₂ nanocrystallites in the TiO ₂-ZnO-B ₂O ₃-Al ₂O ₃ glass-ceramics

We have examined the structure and the crystallization behavior of the 13TiO ₂-xZnO-17Al ₂O ₃-(70-x)B ₂O ₃ (TZAB) glass. Coordination number of B ₂O ₃ and Al ₂O ₃ is changed by substitution of B ₂O ₃-ZnO, whereas coordination states of TiO ₂ and ZnO are not affected by the substitution. With increasing amount of ZnO, network forming AlO ₄ increases instead of network modifying AlO ₅ and AlO ₆. In the case of B ₂O ₃, BO ₄ unit is preferentially formed with increasing amount of ZnO. The glass-ceramics show selective crystallization of TiO ₂, and the precipitated phase of TiO ₂ depends on the amount of ZnO. Dendritic-like aggregates of anatase crystallites were precipitated in the glass-ceramic containing lower amount of ZnO, whereas isolated granules of rutile crystallites were precipitated in the glass-ceramic containing higher amount of ZnO. We have demonstrated that the transparency of the glass-ceramic strongly depends on the crystallinity of precipitated crystallites. The crystallization behavior indicates that both main network and the interstitial components were affected by the substitution.