Direct Synthesis of Barium Titanate Nanoparticles Via a Low Pressure Spray Pyrolysis Method
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Yoshitake Terashi and Yu-Cong Wang Kyocera Corporation R&D Center, Kokubu, Kagoshima 899-4312, Japan
Kikuo Okuyamaa) Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi, Hiroshima 739-8527, Japan (Received 16 May 2005; accepted 26 July 2005)
The one-step synthesis of barium titanate (BaTiO3) nanoparticles was studied by employing a low-pressure spray pyrolysis (LPSP) method. The effects of temperature, pressure, and the addition of urea to the precursor were investigated experimentally. The results were compared with the experimental data of the conventional (atmospheric) spray pyrolysis method. It was shown that the BaTiO3 nanoparticles could be synthesized by the low-pressure method, while only spherical hollow particles with smooth surfaces could be produced by the conventional spray method. The addition of urea greatly improved the crystal growth and particle breakup due to extra heat supplied during the combustion reaction coupled with the evolution of gases. The dispersity of nanoparticles increased with the quantity of urea and with a decrease in pressure. The possible mechanism of the formation of BaTiO3 nanoparticles in the LPSP process was also proposed.
I. INTRODUCTION
Since barium titanate (BaTiO3) possesses excellent dielectric,1 ferroelectric,2 and piezoelectric properties,3 it is considered the most important perovskite-type ceramic material. It is widely used for multilayer ceramic capacitors (MLCCs),4 PTC thermistors,5 and a variety of electro-optic devices.6 For achieving good dielectric properties, a material should have a fine particle size, narrow size distribution, non-aggregation, spherical morphology, and uniform composition.7 Moreover, electronic devices such as the MLCCs have been miniaturized recently. High-dielectric constant BaTiO3-based materials with ultrafine grains are essentially required for achieving a thinner dielectric layer. In the past, extensive research has been conducted for synthesizing BaTiO3 particles by employing various methods, such as the solid reaction method,8 spray pyrolysis,9–13 hydrothermal method,14–17 sol-gel method,18 and others.19 BaTiO3 powders have conventionally been prepared by the solid reaction method, which involves
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0359 J. Mater. Res., Vol. 20, No. 10, Oct 2005
the calcination of BaCO3 and TiO2 at high temperatures for enhancing the diffusivity between solid raw precursors. However, only large powders (submicron and micron) with polydispersity and inhomogeneity could be achieved, and some impurities were introduced in the subsequent milling process. Hydrothermal reaction and the sol-gel method are currently the most popular methods for synthesizing BaTiO3 nanoparticles with a good crystallite structure and a pure phase at low temperatures.14–17 However, a serious problem in these wet chemical methods is that the resulting particles may contain hydroxyl groups in the crystal lattice due to the alka
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