Papers by Author: Makoto Kuwabara

Abstract: Barium titanate (BT) and strontium titanate (ST) nanocubes which have been synthesized by hydrothermal method with surfactants were assembled in order directly on the substrates by using capillary-force-assisted self-assembly method. The ordered structures, crystallinity and orientation of the nanocubes were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning probe microscopy (SPM) and X-ray diffraction (XRD). The piezoresponse properties of the nanocubes ordering structures characterized by Piezoresponse Force Microscopy (PFM) seemed to depend on the constituents and their interface.

Abstract: Barium titanate (BTO) inverse opal photonic crystals were fabricated by a process of self-assembly of polystyrene opal template in combination with electrophoretic deposition (EPD) of nanoparticles from BTO suspension. In this process, stable monodispersed suspension of BTO nanoparticles was prepared by dispersing BTO gel into a mixed solvent of 2-methoxyethanol and acethylacetone. Then the BTO nanoparticles were infilled into the interstices of the opal template formed by monodisperse polystyrene microspheres by electrophoretic deposition, and then polystyrene template was removed by calcining the specimen at a final temperature of 500oC. SEM images show that the inverse opals possess face-centered cubic (fcc) structure with center to center distant of the air spheres 310 nm. A photonic bandgap in the visible range is observed from reflection spectra of the sample. Such BTO inverse opals as photonic crystals should be useful in device applications.

Abstract: Using the electrophoretic deposition (EPD) method, we prepared barium titanate nanoparticle (BT-NP) thin films by depositing BT-NP monodispersed in ethylene glycol monomethyl ether (EGMME) onto an electrically conductive substrate coated with polyacrylic acid (PAA). Deposition efficiency was increased compared to conventional EPD, and the uniformity of the film thickness was also improved as a result of the BT-NPs cohesion effect of the PAA coating on the substrate in addition to the electrophoretic deposition effect of the electric field. When low-molecular-weight PAA was used, the film was reinforced since PAA, which coats the substrate, diffused close to the surface of the BT-NP film during film formation. A crack-free BT-NP film was produced with a film thickness of several micrometers.

Abstract: We synthesized nano-sized Eu-doped Y2O3 (Y2O3:Eu) phosphor powder by an ion-exchange po-lymer/water/oil (W/O) emulsion method using carboxymethyl cellulose (CMC) as the ion-exchange polymer, in which the CMC-(Y, Eu) gel formed was calcined at 800°C in air to convert into Y2O3:Eu particles. The obtained phosphor powder consists of unagglomerated particles with sizes in the range of 30-80 nm and exhibits strong red emission at 612 nm. This CMC/W/O method for producing nano-sized oxide powder has significant advantages over conventional solid-phase and chemical solution-based methods because this method can provide unagglomerated nanoparticles of oxide phosphor without employing any mechanical pulverizing process for reducing the particles’ size into the nanometer range; mechanical grinding, such as ball milling, of phosphor powders has been well known to cause a significant degradation of their luminescent properties.

Abstract: We investigated epitaxial assembly in aggregates of CaTiO3 nanoparticles, which were synthesized under a hydrothermal condition at 150°C using water/ethanol solvent, by transmission electron microscopy analysis combined with a three-dimensional tomography technique. The obtained results indicate that epitaxial assembly is involved in the crystal growth of CaTiO3 nanoparticles, and that in aggregates of CaTiO3 nanoparticles epitaxial assembly occur on their {101} planes.

Abstract: Nanosized Y2O3:Eu3+ crystalline powders with average particle sizes around 100 nm, measured by field-emission scanning electron microscopy (FE-SEM), have been synthesized by a polymerized precursor method using carboxymethyl cellulose (CMC). The nanocrystalline phosphors exhibit intense photoluminescence (PL) and cathodoluminescence (CL) around 612 nm, being attributed to the 5D0-7F2 transition of Eu3+. Electroluminescence (EL) properties of the nanophosphors in the form of thick films were investigated, and emission of red light with maximum intensity at 612 nm (corresponding to x = 0.57 and y = 0.32 in the CIE chromaticity diagram) was observed under high ac field at 1 kHz. The obtained results suggest a possibility that Y2O3:Eu3+ nanopowders can be used as a red-emitting phosphor for EL devices.

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.

Abstract: We synthesized barium titanate (BaTiO3) nanoparticles by sol-gel process and investigated their crystallization behavior using differential scanning calorimetry and X-ray diffraction. BaTiO3 nanoparticles with various degrees of crystallinity were obtained by adjusting synthesis conditions. Under aging conditions that do not allow dealcholization reaction to complete, many hydroxyl ligands remain in as-synthesized BaTiO3 nanoparticles, resulting in the formation of voids or defects in the nanoparticles after calcination. It is essential to use high concentration alkoxides precursor solutions for producing BaTiO3 nanoparticles with high crystallinity at low temperature.

Abstract: Barium titanate nanoparticle (BT-NP) suspensions were prepared by dispersing sol-gel-derived BT-NPs in ethylene glycol monomethyl ether (EGMME) under sonication. When an electric field was applied, a part of the BT-NPs in a suspension was deposited on the cathode by electrophoretically deposition and most of the rest of the BT-NPs settled down towards the bottom of a container. The concentration of BT-NPs in the lower part of the container (Cbottom) increased with time until the Cbottom reached a specific concentration (threshold concentration) at which sedimentation of BT-NPs start to occur. The threshold concentration of the nanoparticles sedimentation gave a significant influence upon the aspect of a BT-NPs deposit formed on the cathode.