Harunobu Sano

Effects of Rare-Earth Oxides on the Reliability of X7R Dielectrics

The effects of rare-earth oxides, e.g., La, Nd, Sm, Dy and Yb, on the reliability of multilayer capacitors (MLCs) with X7R dielectrics and Ni electrodes were investigated. Microstructures of the dielectrics were analyzed by transmission electron microscopy (TEM) and electron probe microanalysis (EPMA) in order to characterize the rare-earth ions. Incorporation of rare-earth ions to BaTiO3 ceramics depended on their ionic radius, resulting in different microstructures and electric performances of dielectrics. Dy ions provided BaTiO3 ceramics with ideal X7R characteristics and high reliability. The mechanism governing leakage current was discussed in terms of the voltage dependence of leakage current. Electric properties and related reliability of the capacitors were attributed to solubility, distribution of rare-earth oxides and their occupation site in BaTiO3.

BATIO3-BASED NON-REDUCIBLE LOW-LOSS DIELECTRIC CERAMICS

Dielectrics composed of BaTiO3-rare earth oxide-MgO have been studied to design a non-reducible low-loss dielectric material. Curie temperatures of dielectrics composed of BaTiO3–Gd2O3–MgO were lowered by Gd2O3, which provided the dielectrics with low power loss. It was found that the addition of BaZrO3 to BaTiO3–Gd2O3–MgO decreased the capacitance change with temperature. Gd2O3 diffused easily into BaTiO3, but, it could not easily diffuse into BaTiO3 to which BaZrO3 had been added. It was considered that BaZrO3 prevented the Gd2O3 diffusion into BaTiO3, resulting the remains of ferroelectricity and reduced capacitance change. Thus non-reducible and low power loss X7R dielectric ceramics were developed.

Relationship between Dielectric Properties and Microphonics of Mulutilayer Ceramic Capacitors

As a result of investigating the relationship between the dielectric properties of BaTiO3-based dielectrics and the microphonics of multilayer ceramic capacitors (MLCCs), it was confirmed that microphonics, electric-induced strain, and polarization have very high correlation, and piezostriction becomes dominant in electric-induced strain when high DC bias is applied to BaTiO3-based dielectrics in spite of their very low spontaneous polarizations. Dielectrics with a large paraelectric phase area in which rare-earth elements have diffused into BaTiO3 are useful for applications of MLCCs, especially under high DC bias.

Effect of Microstructure on Reliability of Ca(TiZr)O3-Based Multilayer Ceramic Capacitors

We examined the reliability of Ca(TiZr)O3 (CTZ)-based Ni-electrode multilayer ceramic capacitors (MLCs) prepared by two different processes with particular interest in the microstructure. One process was to calcine the mixture of CaCO3 and TiO2 to prepare CaTiO3 (CT) powder and the mixture of CaCO3 and ZrO2 to prepare CaZrO3 (CZ) powder, and then mix these calcined powders and sinter them to synthesize the CTZ-based ceramics. The other was to calcine the mixture of CaCO3, TiO2 and ZrO2 powders together to prepare CTZ powder and then sinter them. These two processes of CTZ ceramic preparation resulted in a different crystallinity and distribution of the elements. We found that these factors influenced the reliability of CTZ-based MLCs.

The Effect of Rare Earth (Ln = Gd, Dy, Y and Yb) Doping on the Microstructure and Reliability in BaTiO3-Based Monolithic Ceramic Capacitors (MLCs)

The effect of rare earth (Ln = Gd, Dy, Y and Yb) and Mg ions on the microstructure and reliability of BaTiO3-based monolithic ceramic capacitors (MLCs) with Ni electrodes was investigated. The X-ray diffraction results about the lattice volume of sintered specimens suggested that Gd and Dy ions predominatly substituted into the Ba-site, Yb ions gave exclusive substitution at the Ti-site, while Y ions occupied either the Ba- and Ti-site. The reliability of ceramic capacitors was increasing with increasing the ionic radius of the rare earths in this study. The nonlinearity coefficient() etsimated from the leakage currents and the lifetime measured from the highly accelerated lifetime testing (HALT) showed a negative correlation which was observed only from Dy and Y ions doped specimens. The quantity of Ln and Mg in the grains tended to increase with increasing the Ln ionic radius. In order to improve the reliability and the insulation property of BaTiO3 based MLCs with Ni electrodes, it is important that acceptor ions at the Ti-site compensate donor ions which are rare earth ions at the Ba-site, so the overall quantity of the dopants required for the charge compensation with acceptor and donor ions increases accordingly.

Effects of Grain Boundary and Segregated-Phases on Reliability of Ba(Ti,Zr)O3-Based Ni Electrode MLCs

The reliability in the highly accelerated life test (HALT) for multilayer ceramic capacitors (MLCs) composed of the Ba(Ti,Zr)O3-Gd-Mg-Mn-Si system was markedly improved by controlling the amount of SiO2 and the ratio of Ba/(Ti+Zr). We observed the following characteristics in the microstructures when the reliability was improved. 1) The segregated-phases, which consisted of Mg oxides and Si oxides, were dispersed well in the dielectric layers and their size was considerably small. 2) The corrosion resistance of grain boundaries was improved. 3) The electric potential as a contrast in the focused ion beam scanning ion microscopy was homogeneous in both the grains and the grain boundaries. We can design these microstructures by controlling the additives to generate BaMg2Si2O7 and BaSi2O5, which have low melting points.

Dielectric Properties in the Insoluble Compositions of BaTiO3-CaTiO3 Ceramics

A conventional solid phase reaction method was followed to prepare (1−x)BaTiO3-xCaTiO3 based ceramics with content fractions ranging from x = 0.00 to 1.00. Phase segregation was observed when x exceeded 0.10. DC breakdown voltage (DC-BDV) abruptly changed at x = 0.30 and 0.85. Furthermore, the DC-BDV values of insoluble compositions (x = 0.30−0.85) were much higher than those of solubility limit (x = 0.20 and 0.90). Results obtained by investigation of the amount of absorbed current and the dependence of the rate of voltage increase for DC-BDV and AC-BDV confirmed the formation of space charge in insoluble compositions. This suggests that the dependence of the DC-BDV value for (1−x)BaTiO3-xCaTiO3 based ceramics was affected by the formation of space charge.