Yoshikazu Okino

The effect of MgO and rare-earth oxide on formation behavior of core-shell structure in BaTiO3

The formation mechanism of the core-shell structure of a BaTiO3(BT)–MgO–Ho2O3-based system was studied. Mg reacted with BT at low temperatures and showed poor diffusivity compared with Ho. The core-shell structure was considered to be formed by the suppression of the diffusion of Ho into the core region by Mg. Also, replacement modes of Mg and Ho in perovskite were investigated. Lattice parameters were determined at temperatures higher than the Curie temperature in order to avoid crystal structure change. It was confirmed that Mg dissolved in Ti site, and Ho dissolved in both Ba and Ti sites. This indicates that Mg ions act as acceptors and Ho ions act as both donors and acceptors in the shell phase.

Dielectric Properties of Rare-Earth-Oxide-Doped BaTiO3 Ceramics Fired in Reducing Atmosphere

In order to gain an understanding of highly reliable electrical characteristics for the Ho-doped multilayer ceramic capacitors with Ni electrodes, dielectric properties of various rare-earth-oxide-doped BaTiO3 ceramics were studied. The smaller ionic radius rare-earth-oxide (Dy, Ho, Er)-doped samples showed lower resistivity in reducing atmosphere, but higher resistivity in oxidizing atmosphere at the cooling stage, compared with the larger-ion (La, Sm, Gd)-doped samples. Multilayer ceramic capacitors with Ni electrodes using the smaller-ion-doped materials showed smaller aging rate and longer lifetime. We developed Ni-electrode MLCs with X7R specification as 1 µ F in the 2125 type.

INFLUENCE OF THE MICROSTRUCTURE EVOLUTION ON ELECTRICAL PROPERTIES OF MULTILAYER CAPACITOR WITH NI ELECTRODE

The influence of the microstructure evolution on electrical properties as a parameter of the firing temperature was studied for materials in the BaTiO3(BT)–MgO–Ln2O3 (Ln=Ho and Dy) system. The sintering behavior and the formation of the core-shell structure were dependent on the kind of doped rare earth elements. The stability of the core-shell structure and electrical properties of Dy doped specimens against the firing temperature were much lower than those of the Ho doped specimens. Especially, the Dy doped disk specimens fired at more than 1320°C, in which the core-shell grains were destroyed as judged by the differential scanning calorimetry (DSC) measurement and the transmission electron microscopy (TEM) observation, exhibited characteristic electrical properties. The electrical properties of the Ho doped multilayer capacitor (MLC) specimen were superior to those of the Dy doped one. It was found that the microstructure had a definite influence on the electrical properties, such as the temperature dependence of the dielectric constant and the capacitance aging behavior under an unloaded field.