Koji Kajiyoshi

Heteroepitaxial Growth of BaTiO3 Thin Films on SrTiO3 Substrates under Hydrothermal Conditions

Heteroepitaxial growth of BaTiO3 films on SrTiO3 single-crystal substrates was studied in Ba(OH)2 solution under hydrothermal conditions above 450°C. The epitaxial relation, BaTiO3 (hkl)//SrTiO3 (hkl), was confirmed for the films on (100), (110), (111) and (210) faces of the SrTiO3 crystals. Morphology of the grown surfaces faceted by {100} faces was directly affected by the indices of the substrate surface as a result of the epitaxial relation. In particular, monolithic films having a smooth surface were formed on SrTiO3 (100) substrates.

CONTRIBUTION OF ELECTROLYSIS CURRENT TO GROWTH OF SRTIO3 THIN FILM BY THE HYDROTHERMAL-ELECTROCHEMICAL METHOD

The electrolysis treatment of the hydrothermal-electrochemical method has been modified so that it permits SrTiO 3 thin films to be grown on Ti electrodes being oxidized anodically in Sr(OH) 2 solutions far beyond a thickness limit of several tens of nanometers hitherto attained. The relation between the total current passed through the Ti anode and the amount of the resulting SrTiO 3 film was analyzed on the basis of a reaction model that interprets the anodic current to be compensated with electrons generated partly by oxidation of Ti and partly by decomposition of H 2 O. Current efficiency for the film growth was estimated to be in the range from 0.8 to 3% depending on the Ti electrode potential.

Electrical properties of BaTiO3 thin film grown by the hydrothermal-electrochemical method

Crystalline BaTiO3 thin films were prepared directly on Ti substrates in aqueous solutions of Ba(OH)2 at 150 °C by a hydrothermal-electrochemical method and their electrical properties for capacitors were measured. The leakage current-voltage characteristics exhibited rectifications, the degree of which depended on the top electrodes used. Resistivities as high as 1012 Ω cm were obtained in the voltage range up to 2 V for the 0.40-µ m-thick BaTiO3 thin film and its breakdown voltage was higher than 12 V. The grown films were paraelectric with dielectric constants of 340–350 and dielectric losses of 7–10% at 1 kHz, 0.1 V rms, and 25 °C. The capacitance variation with dc bias voltage from -2.5 to +2.5 V was 9% of the zero-bias value, and that with temperature from -60° to 130 °C was 24% of the room-temperature value.

Piezoelectric and dielectric aging of Bi0.5(Na0.82K0.18)0.5TiO3 lead-free ferroelectric thick films

Ferroelectric Bi0.5(Na0.82K0.18)0.5TiO3 (NKBT) thick films have been prepared using screen printing on Pt electroded alumina substrates. The room-temperature dielectric constant of resulting 60 μm thick NKBT film reaches as high as 823 at 1 kHz and the depolarization temperature, Td, locates at 130 °C. The effect of amplitude and frequency of the applied ac field and the aging temperature as well as the Bi excess on the dielectric and piezoelectric aging characteristics are systemically studied. The results show that the dielectric and piezoelectric aging are associated with the reorientation of defect dipoles which stabilized the non-180° domain walls. By this reorientation the domain wall motions are clamped by a time dependent force resulting in the observed decrease in the dielectric and piezoelectric properties during the aging procedural. Moreover, the aging in NKBT thick films exhibits strong frequency dependence, which is caused by that the reorientation and alignment of the defect dipoles with th...

Catalytic wet oxidations of aromatic compounds over supported copper oxides

Catalytic wet oxidations of naphthalene as a model compound of persistent aromatic compounds were carried out with hydrogen peroxide in a closed autoclave lined with Teflon. CuO/Al2O3 and CuO/AC catalyst showed the high activity for the naphthalene oxidation with hydrogen peroxide of 1.0 mol L−1 at 100 °C. Naphthalene, whose initial concentration was 1.0 g L−1, was converted completely and the concentration of water-soluble organic compounds in the resultant aqueous solution was less than 25 ppm-C. In contrast, platinum, and manganese oxide, silver oxide, and ruthenium oxide catalysts consumed hydrogen peroxide preferentially. Iron and nickel oxides catalysts showed lower activity than the copper oxide catalyst. During the reaction, the intermediate organic acids were formed and then were oxidized. Simultaneously, copper species of CuO catalysts were dissolved and then were precipitated. The precipitated copper species on the catalyst support showed the catalytic activity. CuO/Al2O3 catalysts showed high activity for the six successive batch reactions with the treatment of sodium carbonate after the reaction to precipitate copper ions.

Nonlinear dielectric properties of (Bi0.5Na0.5)TiO3-based lead-free piezoelectric thick films

The effects of applied dc bias field, temperature, frequency, and doping types on the nonlinear dielectric behaviors of Bi0.5(Na0.82K0.18)0.5TiO3 piezoelectric films with thickness of 40 μm have been investigated. It is demonstrated that the dielectric permittivity versus ac field amplitude departed from linearity at high temperature, which is caused by a decrease of the coercive field with increasing temperature. The frequency dependence of the dielectric permittivity is related to domain wall pinning. The thicker piezoelectric films exhibit higher extrinsic contribution due to high domain wall mobility and less domain wall pinning, resulting from the increased grain size and film thickness.

Fabrication and charaterization of silica nanocoatings on ZnS phosphor particles

With the addition of the cationic surfactant, cetyltrimethylammonium chloride (CTAC), continuous and uniform silica nanocoatings on ZnS phosphors have been successfully obtained. The coatings are proven to cover ZnS phosphors completely by using transmission electron microscopy (TEM), zeta potential and x-ray photoelectron spectroscopy (XPS) analysis. The XPS and Fourier transform infrared red (FT-IR) spectroscopy results provide evidence of the presence of Zn–O–Si bonds between the silica coatings and ZnS phosphors. It is suggested that the bridging effect of CTAC favours the formation of silica coatings, and Zn–O–Si bonds are formed during the annealing procedure.

Preparation of Hydroxyapatite Ceramics by Hydrothermal Hot-Pressing Technique

The hydrothermal hot-pressing (HHP) technique was used to prepare hydroxyapatite (HAp) ceramics from a HAp powder with low crystallinity, and the effects of processing factors such as temperature, loading pressure and reaction time on densification were investigated. The crystallinity of HAp was increased by the HHP treatments. With the increase in reaction temperature, loading pressure and reaction time, the density and Vickers hardness of the compacts increased. The HAp compact with high relative density of 83% and high Vickers hardness of 2.9 GPa was successfully prepared by the HHP treatment at 200°C for 3 h under loading pressure of 60 MPa.

Hydrothermal Synthesis of Na(Nb1-xTax)O3 Powder for Fabrication of Lead-Free Piezoelectric Ceramics

Hydrothermal method was used to synthesize the Na(TaxNb1-x)O3 (x = 0.1-0.5) powders with the addition of isopropanol. The phases of the obtained powders were confirmed by XRD. The morphology and particle size were observed by FE-SEM. The specific surface areas were measured by multipoint BET technique. The presence of water and organic group were determined by (FT-IR) spectroscopy. The results indicate that the pure Na(TaxNb1-x)O3 (x = 0.1-0.5) powders with perovskite structure and fine particles were successfully synthesized by hydrothermal method at 240 °C for 8 hours with the addition of isopropanol.

Conversion of Used Glass Bottles to Porous Materials by Using Hydrothermal Technique

Conversion of used glass bottles to porous materials was investigated for recycling of used glasses. The ordinary method to prepare porous materials from glass needs vesicants such as calcium carbonate and silicon carbide that decompose at high temperatures to produce gas, and the gas is trapped in softened glass to form pores. In this study, used glass bottles were converted to porous plates by using hydrothermal technique. First, a glass powder is hydrothermally treated at low temperatures, so that water diffuses into the glass structure. Water is released as vapor to form pores in the softened glass, when the hydrothermally treated glass powder is heated at high temperatures. Thus, this process gives porous materials with a fine structure including closed pores at lower temperature than the ordinary method. The effects of hydrothermal treatment and calcination conditions on bulk density of the resulted porous materials were investigated with a small scale to determine the conditions to get light porous materials. Finally, porous plates (45x45x3 cm3 in size) with density of 0.45g/cm3 were produced by hydrothermal treatment of the glass powder at 180°C in a large autoclave with inside volume of 2.5 m3, followed by calcination at 800°C in a continuous heater with 18 m in length.