Electroceramics in Japan XIII

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Authors: Rintaro Mori, Petr Pulpan, Hiroshi Hayashi, Yoshitaka Nagamori, Yuichi Yamamoto, Tetsuo Uchikoshi, Tohru S. Suzuki, Satoshi Wada
Abstract: Barium titanate (BaTiO3, BT) grain-oriented ceramics along [110] direction were prepared by a templated grain growth (TGG) method. The [110] oriented BT platelike particles (t-BT) were used as template particles. BT grain-oriented ceramics with higher relative density (Drel) over 95 %, high degree of orientation along [110] direction (F110) over 80 % and grain sizes around 30 µm were successfully prepared using only the template particles. The relationship between microstructures and piezoelectric constant (d33) was investigated. The higher F110 was, the higher Drel and the smaller grain size were required for enhancement of the piezoelectric properties.
Authors: Akifumi Morishita, Yuuki Kitanaka, Makoto Izumi, Yuji Noguchi, Masaru Miyayama
Abstract: Single crystals of x (Bi0.5K0.5)TiO3-(1-x)(Bi0.5Na0.5)TiO3 (BKT-BNT) solid solution were grown by a flux method, and their ferroelectric properties were investigated along [100]cubic (the cubic notation). While the flux growth soaked at 1250 °C led to the crystals with a composition of x less than 0.14, the crystals with x of 0.45 were obtained by decreasing soaking temperature to 1150 °C. The crystals of BKT-BNT (x = 0.45) exhibited a relatively large remanent polarization of 39  C/cm2 and a low coercive field of 23 kV/cm.
Authors: Shigehito Shimizu, Petr Pulpan, Nobuhiro Kumada, Daisuke Tanaka, Masahito Furukawa, Yoshihiro Kuroiwa, Tohru S. Suzuki, Tetsuo Uchikoshi, Satoshi Wada
Abstract: Barium titanate (BaTiO3, BT) - potassium niobate (KNbO3, KN) solid solution system (0.5BT-0.5KN) ceramics with various microstructures were prepared by conventional sintering method and two-step sintering method using BT and KN nanoparticles. Their microstructures were investigated using X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM), and it was confirmed that two ferroelectric phases, i.e., BT-rich tetragonal and KN-rich orthorhombic phases, always coexisted for all ceramics, which suggested that 0.5BT-0.5KN ceramics had “pseudo-morphotropic phase boundary (MPB)” structure. Thus, the control of the interface area between two phases was important to enhance piezoelectric property. Finally, their piezoelectric property was measured, and the apparent piezoelectric constant d33* increased with increasing interface area.
Authors: Masahiro Saitoh, Yuji Hiruma, Hajime Nagata, Tadashi Takenaka
Abstract: Grain-oriented (Bi1/2K1/2)TiO3 (BKT) ceramics were prepared by the reactive templated grain growth (RTGG) method, and a hot-pressing (HP) method was modified on the basis of the RTGG method to obtain dense ceramics. The BKT ceramic sintered at 1060°C showed a single-phase perovskite structure. The grain-oriented BKT prepared by the HP method exhibited a relatively high orientation factor F of 0.82 and a high density ratio of 99%. SEM micrographs of the HP-grain-oriented BKT indicated a textured and poreless microstructure. On the basis of the density ratio of 87% for the grain-oriented BKT without HP, the HP method was considered very effective for improving the density ratio even for the RTGG sample. In addition, the resistivity of the HP-grain-oriented BKT was 1.73 x 1013 •cm.
Authors: Keisuke Teranishi, Tomohisa Azuma, Masakazu Hirose
Abstract: The effect of adding Al2O3 to the PZT-PMN solid solution material for use for ceramic resonator was investigated. It has been found that when the amount of Al2O3 added to PZT-PMN is 0.3 wt% or below, it exists inside the crystal lattice as Al3+ ion and increases the stability of oscillation frequency over application of heat (heat resistance). It has been found that when more than 0.3 wt% of Al2O3 is added, the Al3+ ion that remained undoped will exist at grain boundary as secondary phase, and increase the strength.
Authors: Keisuke Yamato, Nobuhiro Kumada, Petr Pulpan, Satoshi Ohno, Chikako Moriyoshi, Yoshihiro Kuroiwa, Bong Yeon Lee, Takashi Iijima, Satoshi Wada
Abstract: Barium titanate (BaTiO3, BT) – bismuth titanate magnesium oxide (Bi(Mg0.5Ti0.5)O3, BMT) solid solution system ceramics were prepared by conventional sintering method in pursuit of the enhancement of the BT Curie temperature (TC, 132 °C). Normal ferroelectric polarization vs. electric-field (P-E) hysteresis loops were observed for BT-BMT ceramics with BMT contents below 20 and above 60 molar%. On the other hand, broad P-E double hysteresis loops were observed for BMT contents from 30 to 50 molar%. The origin was investigated using synchrotron XRD measurement and Rietveld analysis. The crystal structure was assigned to ferroelectric phase with domain-pinning by certain defect structures. A modified phase diagram was proposed on the basis of the temperature dependence of the crystal structure.
Authors: Yoichi Kigoshi, Saki Hatta, Takashi Teranishi, Takuya Hoshina, Hiroaki Takeda, Osamu Sakurai, Takaaki Tsurumi
Abstract: Barium titanate (BaTiO3) ceramics with various grain sizes from 0.7 to 13 μm on average were prepared by a conventional sintering method, a two-step sintering method and a rate controlled two-step sintering method. The permittivity of the ceramics was increased with decreasing grain size to 1.1 μm on average. However, the permittivity of the ceramics was decreased when the grain size was below 1 μm. The field emission scanning electron microscope (FE-SEM) observations revealed that the 90º domain width decreased with decreasing the grain size. By ultrawide range dielectric spectra from kHz to THz range of the BaTiO3 ceramics, the domain contribution to the permittivity was investigated. For the BaTiO3 ceramics with grain sizes over 1 μm, the dipole polarizability and the ionic polarizability were enhanced by high domain-wall density. In contrast, for the BaTiO3 ceramics with grain sizes below 1 μm, these polarizabilities were weakened.
Authors: Satoshi Yokomizo, Takuya Hoshina, Hiroaki Takeda, Katsuya Taniguchi, Youichi Mizuno, Hirokazu Chazono, Osamu Sakurai, Takaaki Tsurumi
Abstract: We researched the phenomenon that the permittivity of dielectric layers in multilayer ceramic capacitor (MLCC) increases with the number of dielectric layers. Finite element method (FEM) shows that the internal residual stress in MLCC was generated by the difference of thermal expansion coefficient between internal electrodes and dielectric layers. We developed a electric measurement system with applying external stress for understanding the stress influence on dielectric properties. The compressive stress along electric field increased the polarization. The polar nano regions (PNRs) in shell composition dielectrics were easily influenced by stress. Based on these results, the relationship between the number of dielectric layers and their permittivity in MLCCs was explained.
Authors: Koichiro Morita, Youichi Mizuno, Hirokazu Chazono
Abstract: The lifetime determination model for multilayer ceramic capacitors (MLCCs) is discussed. The accumulation of oxygen vacancies on the cathode/ceramics interface by an electro-migration process is a concept accepted by many researchers. However, the lifetimes and leakage currents measured during a highly accelerated lifetime test (HALT) could not be explained by this concept. To investigate the mechanism, we used a polarity reversal method during the HALT, which provided information on the dominant process for the leakage current. Thermally stimulated current (TSC) measurement provided the relative number of oxygen vacancies both on the cathode/ceramics interfaces and the grain boundaries. Moreover, the microstructure of the MLCC samples was evaluated by both electric property measurements and direct observation. From these results, we concluded that the grain boundaries controlled the leakage current as well as the lifetime.

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