Continuous Production of BaTiO 3 Nanoparticles by Hydrothermal Synthesis

Continuous production of BaTiO 3 fine particles was performed by hydrothermal synthesis using a flow reaction system. The effect of reaction temperature on particle size and its distribution was investigated. Aqueous 0.1 M TiO 2 sols and 0.12 M Ba(OH) 2 solutions were used as starting materials. The reaction temperature was in the range 300−420 °C, and the reaction pressure was constant at 30 MPa. The reaction time was varied from 0.1 to 40 s with a flow rate of 15 or 75 g/min. Characterizations of products were performed by X-ray diffractometry, scanning electron microscopy equipped with energy dispersion X-ray spectrometry, and transmission electron microscopy. The conversion of TiO 2 into BaTiO 3 was also evaluated. At 300 °C, the Ti conversion increased with the reaction time and achieved a 95% yield at 38 s. With the increase in temperature, both the Ti conversion and the crystallinity of BaTiO 3 became high. With an increase in temperature from 300 to 380 °C, the average particle size of BaTiO 3 decreased from 46 to 36 nm and these particle size distributions became narrow. At 400 and 420 °C, the reaction was complete within several seconds and highly tetragonal crystalline BaTiO 3 particles around 50 nm in diameter were produced, whereas the particle size distribution was wide ranging from 10 to 150 nm. However, the finer BaTiO 3 particles at a size of 32 nm and a narrow size distribution could be produced by increasing the flow rate to 75 g/min even at 400 °C. We discuss the mechanism of particle formation and growth and the feasibility of continuous production of BaTiO 3 particles.