Characterization and control of phase segregation in the fine particles of BaTiO 3 and SrTiO 3 synthesized by the spray
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T. Yano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan (Received 31 January 1991; accepted 21 March 1991)
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(OC 3 H 7 ) 4 and Ba(NO 3 ) 2 or Sr(NO 3 ) 2 in diluted nitric acid solution. They were atomized into a reaction chamber held at 1000 °C through a two-fluids atomizer with N 2 gas. Mostly hollow spherical particles 3 /um 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 Ba 2 Ti04, Sr 2 Ti0 4 , and TiO 2 . 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 • xH 2 O 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 H 2 O 2 to the solutions, and (c) increasing the concentration of the solution, are found to be effective. The reasons for these effects are discussed. I. INTRODUCTION Spray pyrolysis is widely known as powder preparation method suitable for continuous synthesis of fine, homogeneous, and multi-component powders in a single step.1 Synthesis is performed by atomizing a solution of precursor salts into a reaction chamber held at elevated temperatures. Therefore the droplets are rapidly dried, and to some extent a reaction occurs, forming a complex compound. Chemical homogeneity of each particle is easily achieved by this method. Syntheses of fine powders of PLZT,2 diopside,3 and BaTiO3 4 by spray pyrolysis have been previously reported. In these papers, the spray-pyrolyzed powders sometimes involved a small amount of second phase. Although these by-products can be completely eliminated by calcining at relatively low temperatures, the necessity of calcination obviates one of the major advantages of spray pyrolysis, i.e., being a one-step process to synthesize fine powders from solutions. The reasons for second phases in the spray-pyrolyzed particles were mentioned before4 and are considered to result from chemical composition segregation during precipitation from the solutions. Some synthetic parameters such as reaction temperature, atmosphere, and 1750
J. Mater. Res., Vol. 6, No. 8, Aug 1991
processing time4 pertain to this phenomenon. The segregation will more or less increase and even precipitate in a very short time. Little is known, however, about synthesizing chemically homogeneous multi-component particles from precursor s
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