Papers by Author: Nobuyuki Wada

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Authors: Atsushi Honda, Shin'ichi Higai, Takafumi Okamoto, Noriyuki Inoue, Yasuhiro Motoyoshi, Nobuyuki Wada, Hiroshi Takagi
Abstract: We performed first-principles calculations to examine the interaction among rare-earth (RE), alkaline-earth (AE) elements and oxygen vacancy (VO) in BaTiO3, in order to clarify the combined effects of VO trapping by two different elements. It was found that there is a synergistic effect of VO trapping by RE at Ba site and AE at Ti site, so that the co-doping is effective to improve insulating reliability of BaTiO3. We also verified that the local atomic structures around dopants obtained by calculations well agree with that obtained experimentally by extended X-ray absorption fine structure (EXAFS) analyses. The present work is the first one to clarify the structural environment around dopants including VO by both theoretical and experimental approaches.
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Authors: Takashi Oyama, Nobuyuki Wada, Hiroshi Takagi
Abstract: The role of grain boundaries (GBs) in the diffusion of oxygen vacancies (VO••s) in barium titanate (BaTiO3) and its mechanism were investigated using atomistic simulation techniques. It was found that GBs trapped VO••s at specific sites in the course of the diffusion, and the excess energy reflecting structural distortion of the GB was closely related to the availability of the trapping. GBs therefore act as a resistance of the diffusion of VO••s, suggesting that electrical degradation of multilayer ceramic capacitors (MLCCs), which is derived from vacancy diffusion, enables to be additionally improved by controlling GB structures in BaTiO3-based dielectrics.
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Authors: Makito Nakano, Akira Saito, Nobuyuki Wada
Abstract: The electrical degradation mechanisms of BaTiO3-based ceramics were investigated by measuring the dependence of leakage current on high electric fields. Before the degradation, the leakage current predominately obeyed Ohm’s law and Poole-Frenkel relation. As the degradation progressed, the Poole-Frenkel emission current increased. Moreover, the total current at the high electric fields also comprised Schottky emissions between cathodes and dielectric layers.
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Authors: Makito Nakano, Akira Saito, Nobuyuki Wada
Abstract: The factors causing a decrease in the resistivity of multilayer ceramic capacitors (MLCCs) with a decrease in the thickness of dielectric layers were examined by carrying out measurements and finite element method (FEM) simulations. The obtained electric field vs. current density plots (E–J characteristics) indicated that the local concentration of electric field increased with the decrease in the thickness of the dielectric layers. The investigation of the local concentration of electric field at the grain boundaries using an FEM showed that the decrease in resistivity was caused by the roughness of the interface between dielectric layers and inner electrodes and the presence of large grains in the dielectric layers.
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Authors: Naoki Iwaji, Chiharu Sakaki, Nobuyuki Wada, Hiroshi Takagi, Shigeo Mori
Abstract: We investigated domain structures in Pb(Zr,Ti)O3 (PZT) ceramics whose composition lies on the morphotropic phase boundary (MPB) using transmission electron microscopy (TEM) and evaluated the piezoelectric properties of PZT. We found that monoclinic nanosized domains (nanodomains), which form in tetragonal domains, strongly correlated with the piezoelectric properties of PZT. The degree of formation of nanodomains depends on the grain composition. Thus, controlling the homogeneity of grain composition in the ceramics is crucial for optimizing the piezoelectric properties of PZT.
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Authors: Takashi Oyama, Nobuyuki Wada, Yukio Sakabe
Abstract: Atomistic mechanisms that restrain diffusion of oxygen vacancies in BaTiO3 doped with rare earth ions as donors were analyzed using molecular dynamics simulation. It was confirmed that formation of cation vacancies and reduction of lattice volume are sources of resistance for the diffusion. The cation vacancies trap the oxygen vacancies at the nearby O2- sites by an attractive force associated with Coulombic interaction. In contrast, the rare earth ions repel the oxygen vacancies, which migrate via O2- sites, and accelerate the diffusion. This is one of the factors that determine the restraint behavior of the diffusion, which depends on the type of rare earth ion in BaTiO3-based materials.
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