Agglomerate-free BaTiO 3 particles by salt-assisted spray pyrolysis
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Sotiris E. Pratsinis Department of Mechanical and Process Engineering, Swiss Federal Institute of Technology (ETH), ETH Zurich, Zurich, CH-8092, Switzerland
Kikuo Okuyamaa) Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Optimum conditions for the synthesis of nonagglomerated BaTiO3 particles by salt-assisted spray pyrolysis (SASP) were investigated. The effect of particle residence time in the reactor and salt concentration on the crystallinity and surface morphology of BaTiO3 was examined by x-ray diffraction and scanning electron microscopy. Mixtures of a metal chloride or nitrate salt, dissolved in aqueous precursor solutions, were sprayed by an ultrasonic atomizer into a five-zone hot-wall reactor. By increasing the salt concentration or the particle residence time in the hot zone, the primary particle size was increased, and its surface texture was improved compared to BaTiO3 particles prepared by conventional spray pyrolysis. The SASP-prepared BaTiO3 crystal was transformed from cubic to tetragonal by simply increasing the salt concentration at constant temperature and residence time. Further thermal treatments such as calcination or annealing are not necessary to obtain nonagglomerated tetragonal BaTiO3 (200–500 nm) particles with a narrow size distribution. Increasing the carrier gas flow rate and decreasing the residence time in the hot zone resulted in cubic BaTiO3 particles about 20 nm in diameter.
I. INTRODUCTION
Barium titanate has many applications, most notably for piezoactuators and multilayer ceramic capacitors, as the result of its excellent dielectric, ferroelectric, and piezoelectric properties. Tetragonal barium titanate is used in electronic devices for its ferroelectric properties, and its cubic phase has a high dielectric constant, thus making it suitable for use in capacitors.1 The dielectric constant and crystal structure are dependent on the grain size of the particles. Arlt et al. reported that the maximum dielectric constant for pure BaTiO3 ceramics is realized at a grain size of 0.8 m.2 Sakabe et al. prepared tetragonal BaTiO3 particles as small as 70 nm by hydrolysis of Ba(OH)2 and Ti-alkoxide.3 Kajiyoshi et al.4 and Dutta and Gregg5 reported that the critical particle size of the tetragonal structure is in excess of 100 nm. Cho et al. reported that BaTiO3 particles prepared from organic precursors are in the tetragonal phase when they are larger than 20 nm.6
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 13 Mar 2015
Several routes for the synthesis of BaTiO3 particles, including coprecipitation,7 sol-gel, hydrothermal synthesis,1 and mechanochemical8 techniques have been reported. Hydrothermal and solid-state processes are used commercially for the preparation of BaTiO3 powder because of their low cost and ease of production. In solidstate synthesis, BaCO3, TiO2 and other addit
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