On the solid-state formation of BaTiO3 nanocrystals from mechanically activated BaCO3 and TiO2 powders: innovative mechanochemical processing, the mechanism involved, and phase and nanostructure evolutions

Abstract

It still remains a challenge for the scientific community to obtain high quality barium titanate nanocrystals using high-energy ball mills while avoiding unwanted (carbonate) by-products. The current work aims to address this challenge. In order to improve the kinetics of barium titanate formation, the starting materials, BaCO₃ and TiO₂, were mechanically activated with a high-energy ball mill before their mixing and initiating the solid-state reaction. This step induces anatase to rutile polymorphic transformation simultaneous with particle refinement for TiO₂ starting particles while BaCO₃ does not experience any transformation during its refinement. Very fine monosized barium titanate nanocrystals free from secondary phases and by-products are obtained at a lower calcination temperature. The progress of reactions for the formation of barium titanate is monitored by analyzing the X-ray diffraction patterns and DTA results. According to the proposed mechanism of formation, the formation of BaTiO₃ in the initial stage of the interfacial reaction between BaCO₃ and TiO₂ depends on the BaCO₃ decomposition. Mechanical activation of the BaCO₃ accelerates its decomposition and as a result barium titanate is obtained at a lower temperature and shorter time span in contrast to the literature. Moreover, the mechanical activation reduces the size of the starting materials and increases their specific surface area and stored energy. The high-energy sites are potential sites for the nucleation of barium titanate crystallites during calcination and as a consequence a large density of fine crystallites is obtained. In the second stage, the barium titanate formation is controlled by Ba²⁺ diffusion through the formed barium titanate layer, which acts as an inhibiting layer against further Ba²⁺ diffusion. The creation of a high density of lattice defects, dislocations and free surfaces during mechanical activation of the starting materials activates short-circuit diffusion paths and in turn accelerates the formation of pure single-phase barium titanate. In this stage, in contrast to the literature, no secondary phase and by-product was detected. The synthesis of barium titanate through this avenue is attractive for large-scale production and device application and may provide a strategy for the synthesis of other perovskites.