Dielectric Properties and Microstructure of BaTiO 3 Ceramics
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Dielectric Properties and Microstructure of BaTiO3 Ceramics Dae-Chul Park, Jun-ichi Itoh1, Isao Sakaguchi, Naoki Ohashi, Toyohiko Yano2, and Hajime Haneda Advanced Materials Laboratory, National Institute for Materials Science Tsukuba, Ibaraki 305-0044 Japan 1 Mitsui Mining & Smelting Co.,LTD Ageo-shi, Saitama 362-0021 Japan 2 Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology Meguro, Tokyo 152-8550 Japan ABSTRACT Dielectric properties and microstructure were investigated in BaTiO3 ceramics with various additives, Ho2O3, MgO, Ho2O3/MgO, and La2O3. The dielectric constants were increased up to ~4000 and ~3000 at 25oC in the 1 mol% Ho-doped and 0.5 mol% Mg-doped BaTiO3 materials, respectively. The BaTiO3 material codoped with 3 mol% Ho + 1.5 mol% Mg led to increase dielectric constant up to ~6000 at 25oC and the dielectric constant peak around Curie temperature was suppressed at temperature range of from 25oC to 125oC. The size of BaTiO3 grains depended on the content and kind of an additive. Core-shell grains and secondary phase were also dependent on an additive. Core-shell grains were formed completely in Ho-doped BaTiO3 except for 0.5 mol%, but the structure was little observed in Mg- and La-doped BaTiO3 material. Codoped BaTiO3 also formed the core-shell grains.
INTRODUCTION Barium titanate (BaTiO3) ceramic has wide application as the main substance in modern dielectrics technology[1-3]. Since the discovery of the high dielectric of BaTiO3 ceramics, many investigators have tried to modify this compound by different means in order to achieve stable capacitors with satisfactory operational capacity. Two techniques are generally adopted to carry out this modification. The first is by self-modification of BaTiO3, by controlling the particle size and the optimum sintering temperature or trying different pressing techniques. The second procedure of BaTiO3 modification is by adding certain additives of different concentration[4]. Ceramic multilayer capacitor formulations based on BaTiO3 can be chemically or physically modified to exhibit nearly temperature-independent dielectric behavior, over the temperature range –55o to 125oC. The stability can result either from chemical substitution in the ceramic, from a small-grained microstructure, or from the presence of core-shell grains[56]. These three effects were found to be responsible for the flat temperature characteristics of X7R materials[7]. The formation of a core-shell microstructure not only requires liquid phase sintering, solubility of BaTiO3 in the glass, and reprecipitation of BaTiO3 including additives into the perovskite structure, but there also has to be a limited grain-growth process and limited interdiffusion to a homogeneous distribution of dopants[8]. MgO-doped BaTiO3 is important for practical uses in ceramic capacitors. Rare-earth oxides are one of the most important additives to BaTiO3-based dielectrics in multilayer
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