Crystallization mechanism of barium titanate in amorphous titania gel pellets by acid–base chemical densification near room temperature

To achieve carbon neutrality by 2050, novel ceramic processes which can fabricate it at lower temperatures compared with conventional sintering process are required. Bulk barium titanate (BaTiO₃) is directly synthesized from hydrous titania gel (TiO₂·nH₂O) pellets near room temperature via the acid–base chemical densification (ABCD) process, which affords perovskite oxides via neutralization reactions. TiO₂·nH₂O used as precursor material has amorphous structure and includes a trace amount of water in its structure. Scanning electron microscopy images show that the mechanism of BaTiO₃ crystallization in TiO₂·nH₂O pellet is dependent on the reaction conditions in the barium hydroxide solution, including the heat-treating temperature and time. The bulk BaTiO₃ are obtained from TiO₂·nH₂O pellets by treating below 100 °C for a long time more than 25 h; however, heat-treating at higher temperatures caused TiO₂·nH₂O pellets to disintegrate. It is consolidated that dominant reaction, whether the diffusion reaction or a dissolution–precipitation reaction, in this process depends on the reaction temperature and time. Then, at higher temperatures than 120 °C, a part of TiO₂·nH₂O pellets dissolve to barium hydroxide solution as a dissolution–precipitation reaction. In contrast, at below 120 °C, BaTiO₃ bulk is obtained by diffusion reaction of Ba ion into TiO₂·nH₂O pellet.