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

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 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 singlephase 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.