Phase evolution of solid-state BaTiO 3 powder prepared with the ultrafine BaCO 3 and TiO 2
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Chi-Yuen Huanga) and Che-Yuan Chang Department of Resources Engineering, National Cheng Kung University, Tainan 70101, Taiwan
I-Kuan Cheng, Ching-Li Hu, Chun-Te Lee, and Masayuki Fujimoto MLCC R&D Division, Yageo Corporation, Kaohsiung 81170, Taiwan (Received 3 May 2012; accepted 28 June 2012)
The phase evolution, nucleation, and sintered ceramics of barium titanate (BaTiO3, BT) powder prepared by solid-state synthesis with an ultrafine starting material (27 m2/g of BaCO3 and 190 m2/g of TiO2) were investigated in this study. Surface diffusion between BaCO3 and TiO2 was observed at a relatively low temperature of 400 °C by transmission electron microscopy. Rapid nucleation of the BT and cubic BT phases was observed at 500 °C by x-ray diffraction. The derivative thermogravimetry curve clearly shows a single step of BT formation at 600 °C. In short, pure BT particles with an average particle size of 250 nm and high tetragonality were prepared by solid-state synthesis, which produced X7R ceramics with high dielectric permittivity, high insulation resistance, and a clear core–shell structure.
I. INTRODUCTION
A small, uniform, and well-dispersed barium titanate (BaTiO3, BT) powder is needed for use as the dielectric material in multilayer ceramic capacitors (MLCCs) with ultrathin active layers.1–5 Two main challenges in the current development of MLCCs are the trends for both smaller size (miniaturization) and greater capacitance values. For example, the number of dielectric layers in one capacitor now often exceeds 1000, and a thickness of less than 1 lm for each active layer is required. Solid-state BT has the advantages of good crystallinity, low cost, and high precision of stoichiometric control. The phase evolution of solid-state BT has been studied by many researchers.6–17 Beauger et al.10,11 built a model that explains how barium ions diffuse though the TiO2 surface and control BT growth. Rössel et al.15 used Raman spectral imaging to confirm the mechanism by which the BT layer continues to form and consumes the TiO2 surface. Kobayashi and Lee16,17 reported the presence of a thin layer of BT on a rutile TiO2 surface and confirmed the existence of Ba–Ti–O precursor and nucleation through in situ transmission electron microscopy (TEM) observations. With regard to the mixing mechanism used to prepare the solid-state BT, Hennings et al.1,18,19 reported that BT with a low calcination temperature and a pure a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.255 J. Mater. Res., Vol. 27, No. 19, Oct 14, 2012
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crystal phase (without the secondary phase, Ba2TiO4) can achieve the viscosity stability of an aqueous slurry in the MLCC process. Ando et al.20,21 used high-energy bead milling to prepare finer BT and proposed a two-step (nucleation ! BT particle growth) schematic model of BT formation. Three important factors for solid-state BT synthesis have been proposed: an increase in contact point density between BaCO3 and
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