Effect of Ba(OH)2 solution concentration on Ba ion diffusion in acid-base chemical densification process for obtaining BaTiO3 bulk ceramics

Near-room-temperature ceramic manufacturing are attracting attention as a way to reduce energy consumption and CO₂ emissions. In this study, we have fabricated barium titanate (BaTiO₃) bulk ceramics at near room temperature via acid-base chemical densification (ABCD) process, which utilizes the crystallization reaction that occurs between amorphous titania gel (TiO₂·nH₂O) and barium hydroxides (Ba(OH)₂) aqueous solutions. The low temperature fabrication of bulk ceramics by ABCD process is based on the diffusion of cation ions into precursor pellets consisting of TiO₂·nH₂O and the crystallization of BaTiO₃. In this study, focusing on concentration of Ba(OH)₂ in aqueous solution, Ba diffusion behavior was evaluated in ABCD process. To prevent collapse due to the dissolution-precipitation reaction, the reaction temperature was set to 80 °C which is optimal temperature. When BaTiO₃ prepared with high Ba(OH)₂ concentration solution, such as 7.0 M, Ba ion is difficult to diffuse into a interior of the pellet sample because the dense crystallized BaTiO₃ layer was formed immediately on the pellet surface which disturbs the penetration of Ba(OH)₂ solution. In contrast, thick crystallized BaTiO₃ layers were observed in the sample prepared in 0.4 or 0.7 M Ba(OH)₂ solution. It was found that Ba ions diffusion was facilitated by reducing the Ba concentration in ABCD process because the crystallization rate can be suppressed using low concentration solution. However, when the Ba(OH)₂ concentration was further reduced to 0.07 M, the Ba ions diffusing into the titania gel was low, so no BaTiO₃ was formed. It was found that the optimal concentration of Ba(OH)₂ can enhance the Ba diffusion in the pellets while maintaining crystallization into BaTiO₃. In addition, by using a low-concentration barium solution, the dissolution-precipitation reaction can be suppressed even at high temperatures of 120 °C, so it was possible to create uniform BaTiO₃ using ABCD process in 25 h. By reducing the barium concentration, it was possible to reduce the reaction time to 1/4 of the previous condition.