14 - Barium titanate nanoparticles synthesized under sub and supercritical water conditions

Abstract

This chapter describes the selective production of tetragonal BaTiO3 nanoparticles by hydrothermal synthesis in supercritical water in a flow reactor. Barium titanate (BaTiO3) is one of the candidates of a high dielectric insulator, and the dielectric constant of cubic BaTiO3 is 100 times larger than that of SiO2 or V205. When BaTiO3 is used as an insulating surface layer material of a capacitor, the thickness of the layer can be several 10 nm, which makes it an extremely fine capacitor. An experiment for producing tetragonal BaTiO3 nanoparticles by supercritical hydrothermal synthesis is presented where titanium oxide sol is used as a source of titanium, barium hydroxide is used as a source of barium, and sodium hydroxide is used as a pH controller. To determine the appropriate solution condition for producing BaTiO3, several preparatory experiments at 400°C, 30 MPa are conducted. The effects of the reaction temperature on the crystal phase after changing it from 300 to 420°C at a constant pressure of 30 MPa are investigated and then the effects of pressure on the crystal phase at 400°C are evaluated. The polymorphism of BaTiO3 depends not only on reaction temperature but also on reaction pressure. The residual OH ions in the crystal structure resulted in the formation of cubic BT during hydrothermal synthesis. As the diffusivity of supercritical water is higher than that of subcritical water, dehydration proceeds rapidly, and the residual OH ions in the lattice are subsequently reduced, which finally helps in obtaining tetragonal BT under supercritical conditions.