Electroceramics in Japan X

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Authors: Kiyotaka Tanaka, Kenichi Kakimoto, Hitoshi Ohsato
Abstract: Various KNbO3 powders have been derived from sol-gel process by using different preparation and heating conditions. KNbO3 powder derived from ethanol solution and fabricated at 800oC was most suitable as the raw material of KNbO3 bulk ceramics, because of the small grain size of about 250 nm and its homogeneous size distribution.
Authors: Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Satoshi Wada
Abstract: Barium titanate (BaTiO3) nanoparticles with various particle sizes from 20 to 430 nm were prepared using a 2-step thermal decomposition method. Powder dielectric measurement clarified that dielectric constant of BaTiO3 particles with 140 nm exhibited a maximum around 5,000. To explain this high dielectric constant, THz-region dielectric properties of BaTiO3 nanoparticles, especially Slater transverse optic (TO) mode frequency, were estimated using the far infrared (FIR) reflection method. As the result, it was found that the Slater TO mode of BaTiO3 particles with 140 nm exhibited a minimum. Therefore, the high dielectric constant around 5,000 at 140 nm can be originated from the softening of the Slater TO mode.
Authors: Mikiko Koyama, Sachi Murakami, Naoki Iwaji, Hiromichi Takebe, Makoto Kuwabara
Abstract: An investigation has been made on the state of barium titanate (BaTiO3; BTO, hereafter) nanoparticles of 10-15 nm in diameter dispersed in various suspension media, such as 2-methoxyethanol (ethylene glycol monomethyl ether: EGMME) with and without acetylacetone (Acac), ethanol, and isopropanol. The state of the nanoparticles dispersed in these suspension media is deduced from the behavior pattern of their optical transmittance (T), in the wavelength (λ) range of 300-900 nm; the concentration (c) of BTO nanoparticles in the suspensions was varied from 0.01 to 0.25 mol/L. T-c plots obtained for EGMME+Acac and EGMME suspensions at various wavelengths indicate that BTO nanoparticles take two and three different states in the suspensions, respectively, from mono-dispersed to agglomerated states with increasing c.
Authors: Keigo Inaba, Shinya Suzuki, Yuji Noguchi, Masaru Miyayama, Minoru Osada
Abstract: Complex perovskite oxides of Sr0.5(Nb0.5Ta0.5)O3 were synthesized by reassembling (SrNb2O7)2− and (Sr1.5Ta3O10)2− nanosheets followed by topochemical dehydration with heat treatment. Atomic force microscopy (AFM) images showed that the thicknesses of the (SrNb2O7)2− and (Sr1.5Ta3O10)2− nanosheets were approximately 6 nm (7 unit layers) and 4 nm (3 unit layers), respectively. Raman scattering measurement has suggested that nanosheet-processed Sr0.5(Nb0.5Ta0.5)O3 with B-site-cation ordering has a superlattice-like structure composed of alternating layers of Sr0.5NbO3 and Sr0.5TaO3 along the c-axis. Nanosheet-processed Sr0.5(Nb0.5Ta0.5)O3 with B-site-cation ordering exhibited a dielectric permittivity of 42 (1 MHz), which was lower than that of solid-state-reacted samples (65) with disordered B-site cations.
Authors: Kayo Takizawa, Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Yoshihiro Kuroiwa, Satoshi Wada
Abstract: Barium titanate (BaTiO3) fine particles were prepared using the 2-step thermal decomposition method of barium titanyl oxalate. At the 2nd step of this method, the intermediate compound (Ba2Ti2O5·CO3) was decomposed into BaTiO3 and CO2 under various degrees of vacuum pressure. As a result, the particle size of prepared BaTiO3 nanoparticles decreased with decreasing pressure. Moreover, the dielectric constants of these BaTiO3 nanoparticles were measured using the powder dielectric measurement method using slurry. The dielectric constant of BaTiO3 nanoparticles increased with decreasing pressure at the same particle size. It is considered that mesoscopic particle structure controlled by vacuum pressure is important for the dielectric properties of BaTiO3 nanoparticles.
Authors: Yuji Noguchi, Masaru Miyayama
Abstract: Crystal structures of Bi4Ti3O12 ferroelectrics have been investigated by high-temperature neutron powder diffraction study. The ferroelectricity in Bi4Ti3O12 is shown to originate from the in-plane displacement of TiO6 octahedra with respect to heavy Bi ions, and that the off-center Ti displacement in the octahedra plays a minor role. Bond valence analysis shows that the underbonding of Bi with the adjacent oxide ions at the perovskite A site in high-temperature paraelectric state plays an essential role in the ferroelectric phase transition in the Bi4Ti3O12 system.
Authors: Yuuki Kitanaka, Yuji Noguchi, Masaru Miyayama
Abstract: Polarization switching and domain dynamics in unpoled and poled crystals of bismuth titanate by applying electric field along the crystallographic c axis were investigated through polarization measurements and domain observations by optical microscope and piezoelectric force microscope. Poled crystals showed a well-saturated polarization hysteresis with a remanent polarization of 4.4 μC/cm2 and a coercive field of 4.7 kV/cm. Domain observations reveal that lenticular domain acts as an initial nucleus during polarization switching. The sidewise motion of the lenticular-domain walls and resultant single domain state were easily established for the poled crystals, while the lenticular domains observed in unpoled crystals were clamped even though a high electric field was applied to them.
Authors: Tomomitsu Muraishi, Keisuke Yokoh, Hirofumi Kakemoto, Takaaki Tsurumi, Satoshi Wada
Abstract: The phase transition behaviors of the [111]c oriented barium titanate (BaTiO3) single crystals (the subscript c means the cubic notation system) were investigated as functions of temperature, uniaxial stress and electric fields. These results suggested that above Tc, combination between uniaxial stress and electric fields might be effective for a poling treatment of BaTiO3 single crystals. Thus, a new poling method for BaTiO3 single crystals was proposed using control of temperature, uniaxial stress and electric fields in this study.
Authors: Minoru Tamada, Yuji Noguchi, Masaru Miyayama
Abstract: Single crystals of PbTiO3 (PT) were grown by a self flux method, and effects of lattice defects on the leakage current properties were investigated. While PT crystals annealed in air at 700 oC showed a leakage current density of the order of 10-5 A/cm2, annealing under a high oxygen partial pressure of 35 MPa increased leakage current density to 10-4 A/cm2. The increase in leakage current by the oxidation treatment provides direct evidence that electron hole is a detrimental carrier for the leakage current property of PT at room temperature. The vacancies of Pb are suggested to act as an electron acceptor for generating electron holes.
Authors: Masaya Nishida, Hiroaki Takeda, Takashi Nishida, Tadashi Shiosaki
Abstract: Growth of ferroelectric Bi2WO6 (BWO) mono-domain bulk crystals was attempted by the vertical-Bridgman (VB) method below the phase transition (ferro- to paraelectric) temperature of 940oC using Li2B4O7 as a flux. In this method, Pt crucibles with different shapes were used. The crucible with a wedged tip bottom produced BWO crystal with a thickness of over 4 mm along the crystallographic c-axis (perpendicular to the spontaneous polarization axis). Using BWO mono-domain crystals grown by slow cooling technique, on the other hand, their electric properties were characterized. The dielectric constants, ε ij, and electromechanical coupling factor, k33, of the crystals were 70-100 and 36% at room temperature, respectively.

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