Hiroshi Kishi

The effect of rare-earth (La, Sm, Dy, Ho and Er) and Mg on the microstructure in BaTiO3

Abstract The effect of rare-earth elements, La, Sm, Dy, Ho and Er, on the microstructure in BaTiO3(BT)–MgO–rare-earth oxide based system was studied. Larger amount of MgO was required to suppress the grain growth for the larger ionic radius rare-earth ion (La, Sm)-doped samples than for the smaller ion (Dy, Ho, Er)-doped samples. Also, substitution modes of rare-earth elements and Mg in BT lattice were investigated. The solubility of rare-earth ions in BT lattice and the substitution ratio of rare-earth ions into Ba-site decreased as ionic radius decreases. This suggests that the formation of core-shell structures in BT–MgO–rare-earth oxide based system is dependent on the substitution ratio of rare-earth ions into Ba-site.

X7R Multilayer Ceramic Capacitors with Nickel Electrodes

Electrical properties and microstructures of a holmium-doped (Ba1.01Mg0.01)O1.02(Ti0.98Zr0.02)O2 system were studied. Additions of Ho2O3 had little effect in preventing the dielectrics from reducing at high temperature, but the resistivity at low temperature increased with increasing amount of Ho2O3 when treated in oxidizing atmosphere at the cooling stage. From transmission electron micrograph (TEM) observation, it was noted that the microstructure exhibited a grain core-grain shell structure. Newly developed X7R multilayer ceramic capacitors with Ni electrodes revealed highly reliable electrical properties.

Miniature Cantilever-Type Ultrasonic Motor Using Pb-Free Multilayer Piezoelectric Ceramics

A Miniature cantilever-type ultrasonic motor was fabricated using multilayer piezoelectric ceramics (MLPC) made of (Sr,Ca)2NaNb5O15 (SCNN), a Pb-free piezoelectric material, and the electrical driving properties were investigated. The displacement of SCNN-MLPC was 177 nm at 100 V, which was 10-fold the voltage of MLPC made of Pb(Zr,Ti)O3–Pb(Ni,Nb)O3–Pb(Zn,Nb)O3 (PZT). The ultrasonic motor using SCNN-MLPC started to rotate at 3.8 Vp–p, and showed the following characteristics: a revolution speed of Ω0=517 rpm, a torque of T0=1.4 µN m, and an efficiency of η=7% at 13.4 Vp–p. The power consumption, in this case, was 0.3 mW, which is not so high. On the other hand, an ultrasonic motor of the same form made of PZT-MLPC showed Ω0=610 rpm, T0=1.6 µN m, and η=9% at 1.9 Vp–p, and the same power consumption as the SCNN motor. That is, it was found that the SCNN motor was not so different from the PZT motor in terms of their characteristics; however, the SCNN motor needed 10 times as high a voltage as the PZT motor in electrical driving. In conclusion, it can be said that we succeeded in realizing a miniature cantilever-type ultrasonic motor using Pb-free MLPC.

Dielectric Properties of BaTiO3-Based Ceramics under High Electric Field.

The dielectric properties under a high electric field (ac-field) of BaTiO3-based ceramics with core grains, shell grains and core-shell grains were compared with those of multilayer ceramic capacitors (MLCCs) with these three kinds of grains. The MLCCs with the X7R specification had a core-shell structure, and the relative dielectric permittivity (er) of the dielectric layers in the MLCCs increased with increasing ac-field. Similar behavior was observed in the MLCCs consisting of only cores, indicating that the core predominantly determined the dielectric properties of MLCCs under high ac-fields. The dielectric properties of MLCCs and ceramic plates consisting of only shell grains showed that the shell was the relaxor ferroelectrics. A slight change in the shell composition yielded a large shift of the peak temperature of er. The shell improved the temperature stability of er at low temperatures under low ac-fields. In a ceramic plate with relatively large BaTiO3 grains (approximately 3 µm), the maximum er was observed at a moderate ac-field, which was explained from the electric displacement vs electric field hysteresis curves of ferroelectric BaTiO3. The MLCCs and ceramics plates with fine BaTiO3 grains (0.4 to 0.5 µm) showed similar dielectric behavior to the MLCC with the core-shell structure. The size effect of BaTiO3 played an important role in determining the temperature stability of er. For future MLCCs with very thin dielectric layers, a microstructure with fine BaTiO3 grains and grain boundary layers of the shell was proposed.

Occupational sites and dielectric properties of rare-earth and Mn substituted BaTiO3

Abstract The effect of re-oxidation treatment on the solubility in dopants and the dielectric properties of La–Mn- and Ho–Mn-substituted BaTiO 3 ( AB O 3 ) was investigated. The lattice parameter and electrical resistivity results indicated that La ions occupied A -sites and Ho ions occupied both A - and B -sites. In the case of Ho–Mn-substituted samples, the decrease in lattice parameter and a shift of the Curie point ( T c ) to higher temperatures by re-oxidation treatment were observed in the range in which Ho ions predominantly occupy B -sites (act as acceptors). On the other hand, La–Mn-substituted samples showed little change. Electron spin resonance measurements revealed that these changes due to re-oxidation were based on the decrease of the ionic size of Mn due to the oxidation of Mn 2+ to Mn 3+ or Mn 4+ . The occupational sites of Ho ions strongly affected the valence state of Mn and the dielectric properties of BaTiO 3 .

Studies on the Solid Solution of Mn in BaTiO3

BaTi1-xMnxO3 solid solutions with 0≤x≤0.04 were synthesized in reducing atmosphere (PO2<10-9 Pa) and in air at 1350°C for 2 h. The effect of Mn ions with different valences on the crystal structure of BaTiO3 was studied by the room-temperature Raman scattering, electron spin resonance (ESR) and X-ray powder diffraction analyses. The crystal structure of the solid solution was analyzed to determine the change of its lattice parameters. It was found that in a reducing atmosphere, Mn2+ ions promoted the tetragonal-to-cubic transformation, as both a contraction of c-axis and an extension of a-axis were observed with an increase in Mn content. In contrast, Mn ions in the air-synthesized samples promoted the tetragonal-to-hexagonal transformation. The formation of the hexagonal phase was accompanied by a strong extension of the c-axis of the tetragonal phase from 4.032 A to 4.042 A. This was attributed to Jahn-Teller distortions caused by Mn3+ ions.

Dielectric properties of BaTiO 3 –BaZrO 3 ceramics under a high electric field

Multilayered ceramic capacitors (MLCCs) with BaTiO 3 –BaZrO 3 (BTZ) dielectric layers were fabricated, and the dielectric permittivity of the BTZ layers with different thicknesses in MLCCs was measured. The dielectric permittivity of the BTZ ceramic disk was also measured under various ac electric fields. The variation in the dielectric behaviors with the thickness of BTZ layers in MLCCs was explained by the ac-field dependence of dielectric permittivity observed in the BTZ ceramic disk. The ac-field dependence of dielectric permittivity of BTZ was markedly observed below the temperature of a broad maximum in the dielectric permittivity versus temperature (є versus T ) curve. It was found that the temperature of the broad maximum shifted to the low-temperature side and the peak shape became asymmetric with increasing ac field. These changes in the dielectric properties under high ac fields were explained by a model of relaxors with the concept of the formation of polar microregions (PMRs) and the freezing of fluctuating ipoles in PMRs.

Effect of Ho/Mg Ratio on Formation of Core-shell Structure in BaTiO3 and on Dielectric Properties of BaTiO3 Ceramics

Ceramics were prepared from hydrothermally-synthesized BaTiO3 (100 mol%), BaSiO3 (1.5 mol%), MnO (0.1 mol%) with Ho2O3 (0.5 mol%) and MgO (0.25–1 mol%). The compositions with Ho/Mg molar ratios of 4:4, 4:2, 4:1 were selected for examination. The ceramic samples were sintered at 1340°C (PO2<10-9 Pa) and, then, annealed at 1000°C (PO2<30 Pa). The distribution of Ho and Mg in the samples was examined by transmission electron microscope (TEM) equipped with an element analyzer (EDS). Measurements of temperature dependence of dielectric permittivity over -55–155°C showed that the sample with a Ho/Mg ratio of 4/4 showed the least change of dielectric permittivity with temperature compared to the sample with a high Ho/Mg ratio of 4/1. The material with a Ho/Mg ratio of 4/1 is characterized by the deepest Ho diffusion into BaTiO3 and the slowest decrease of Ho concentration that is considered to result in a formation of a shell with considerable amounts of low-concentrated Ho-containing solid solutions.

Effect of milling process on core-shell microstructure and electrical properties for BaTiO3-based Ni-MLCC

Abstract The effect of the process parameter in the milling process on the microstructural evolution and the electrical properties of multilayer ceramic capacitor (MLCC) samples were investigated in the BaTiO 3 (BT)–Ho 2 O 3 –MgO system. The microstructure for MLCC samples fired at 1320°C was dependent on the degree of damage for BT given by the milling process, judging from the field emission scanning electron microscopy observation and differential scanning calorimetry measurement (DSC). The mean grain size ( D 50 ) determined from the chemically etched samples decreased as the damage increased. The endothermic peak of DSC profile at around 125°C was broadened and the peak area decreased as the damage increased. Furthermore, the electrical properties were dependent on the degree of damage. The dielectric constant for MLCC samples decreased and the peak of dielectric constant at around room temperature shifted to a higher temperature as the damage increased. It was found that the MLCC sample showed the small leakage current and long mean lifetime as the degree of damage increased.

Evaluation of Residual Strain and Oxygen Vacancy in Multilayer Ceramic Capacitor Using Laser Raman Spectroscopy

Micro-Raman spectroscopy was employed to investigate the residual strain and oxygen vacancies of BaTiO3-based multilayer ceramic capacitors (MLCCs) with a Ni internal electrode. Strain was found to accumulate around the internal Ni electrodes layers than at other parts of the MLCCs. In addition, the number of oxygen vacancies near the internal Ni electrode was larger than that at any other parts of the MLCCs. These observations clearly show that Raman spectroscopy is extremely useful for evaluating the residual strain and oxygen vacancies in MLCC devices.