Continuous Production of BaTiO3 Nanoparticles by Hydrothermal Synthesis

Continuous production of BaTiO3 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 TiO2 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 TiO2 into BaTiO3 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 BaTiO3 became high. With an increase in temperature from 300 to 380 °C, the average particle size of BaTiO3 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 BaTiO3 particles around 50 nm in diameter were produced, whereas the particle size distribution was wide ranging from 10 to 150 nm. However, the finer BaTiO3 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 BaTiO3 particles.