Characterization and control of phase segregation in the fine particles of BaTiO3 and SrTiO3 synthesized by the spray pyrolysis method

Abstract

Fine particles of BaTiO₃ and SrTiO₃ have been synthesized by the spray pyrolysis technique, and the chemical homogeneity of their particles was analyzed by x-ray diffraction (XRD) and analytical electron microscopy (AEM). The stock solutions were prepared by dissolving Ti(OC₃H₇)₄ and Ba(NO₃)₂ or Sr(NO₃)₂ in diluted nitric acid solution. They were atomized into a reaction chamber held at 1000 °C through a two-fluids atomizer with N₂ gas. Mostly hollow spherical particles 3 μm in diameter were obtained, composed of very fine grains of 40 nm. As-prepared powders were crystallized to BaTiO₃ or SrTiO₃ with a small amount of by-product such as Ba₂TiO₄, Sr₂TiO₄, and TiO₂. The AEM study revealed that the bulk composition of each particle was chemically homogeneous, but a local chemical composition segregation was observed within each particle. This chemical inhomogeneity was considered to be caused by the difference in the precipitating speed of each component from the precursor salts; that is, the precipitation of Ti⁴⁺ ion as TiO₂ · _xH₂O was faster than those of Ba²⁺ and Sr²⁺ ions. To control this segregation, (a) replacing a part of the solvent of stock solutions with methanol or ethanol, (b) adding H₂O₂ to the solutions, and (c) increasing the concentration of the solution, are found to be effective. The reasons for these effects are discussed.