Toshitaka Ota

Electrical and elastic properties of conductor-polymer composites

Several series of conductor-polymer composites were prepared from metal, graphite and conducting ceramics as filler materials, and epoxy, silicone rubber, polyethylene and polypropylene as polymer matrix. Their percolation curves, pressure dependence of resistivity, and Youngs modulus were examined for applications such as a pressure sensor.

Porous ceramics prepared by mimicking silicified wood

Abstract Porous titania, alumina and zirconia ceramic woods with wood-like microstructures, analogous to that of silicified wood, were prepared from natural woods as templates. The production of these ceramic woods was performed by the following process: (1) infiltration of metal alkoxide into wood specimens, (2) hydrolysis of the alkoxide in the cell structure to form titania, alumina or zirconia gels, (3) firing in air to form titania, alumina or zirconia ceramic woods. The resulting titania, alumina and zirconia ceramic woods were studied by means of an X-ray diffractometer, a mercury porosimeter and a scanning electron microscope. The microstructure of these ceramic woods retained the same structure as that of the raw wood: with the pore sizes corresponding to those of the original wood, and the major pores being unidirectionally connected.

Low thermal expansion and low thermal expansion anisotropy ceramic of Sr0.5Zr2(PO4)3 system

A low thermal expansion ceramic with a very low thermal expansion anisotropy was synthesized from the Sr0.5Zr2(PO4)3 system. The sintering was promoted by addition of MgO, and the solgel technique also improved the sinterability. The thermal expansion of the crystal was lowered by substituting Nb5+ for Zr4+ and 1/2Sr2+ pairs, becoming near-zero for Sr0.25Nb0.5Zr1.5(PO4)3. All dense ceramics in this system had a strength of about 80 MPa, and did not suffer microcracking even in the coarse-grained polycrystalline ceramics, owing to the very low thermal expansion anisotropy of the crystals.

Thermal and mechanical properties of sintered LaPO4-Al2O3 composites

Abstract Mixtures of (1 − x)LaPO 4 and xAl 2 O 3 (x = 0 to 1 mass) were dry-pressed to disks or bars. Relative densities larger than 94% and apparent porosities less the 6% were achieved when the specimens were sintered at 1600°C for 5 hours in air. The sintered ceramics (x = 0 to 0.7) were found to be machinable — they could be drilled easily using conventional metallic WC drills. The thermal and mechanical properties of the sintered (1 − x)LaPO 4 - xAl 2 O 3 composites had the following ranges: 10 × 10 −6 /°C (x = 0) to 9.0 × 10 −6 /°C(x = 1) (linear thermal expansion coefficient at 200°–1000°C); 5.0 (x = 0) to 43 W/(m·K)(x = 1) (thermal conductivity at 25°C); 140±19 (x=0) to 352±23 MPa (x = 1)(bending strength at room temperature); 5.7 (x = 0) to 16.5 GPa (x = 1) (Vickers hardness); and 155 (x = 0) to 400 GPa (x = 1) (Young’s modulus).

Flux growth of perovskite-type La2/3TiO3-x crystals

Crystals of perovskite-type lanthanum titanate La2/3TiO3-x were grown from a flux of the system KF-Na2B4O7. Cubic crystals were obtained above 1000°C and tetragonal crystals were obtained below 950°C. Oxygen deficiency seemed to be not so much a problem as has been reported for powdered sample. Growth conditions, X-ray diffraction analysis, crystal morphology and dielectric properties are described.

Leakage Current Properties of Cation-Substituted BiFeO3Ceramics

Cation-doped BiFeO3 ceramics were fabricated by sintering coprecipitated and calcined powders at 700–900 °C to study the effect of cation doping on the leakage current properties of the sintered samples. Among the dopants examined in this study, Ti4+, Sn4+, or Zr4+ doping was found to effectively reduce the leakage current of the samples. In particular, a marked decrease in the leakage current density was achieved at 10% Ti4+ doping, which also resulted in the structure change from rhombohedral to cubic. The codoping of Ti4+/Zn2+ ions or Ti4+/Ni2+ ions brought about a substantial reduction in the leakage current density of the bulk samples by about four or five orders of magnitude at a small doping amount of 2%. This can be explained by the combined effect of Ti4+ doping, which basically contributes to the decreased number of oxygen vacancies in the sample, and Zn2+ or Ni2+ doping, which might assist the homogeneous substitution of Ti4+ ions for the Fe3+ sites.

Design of ITO/transparent resin optically selective transparent composite

Abstract Optically selective transparent composites were made using ITO particle mixed into a polymer resin. To fabricate the composites, ITO particles were chosen as filler, and Urethane resin (transparent resin) was chosen as a matrix. Using fine (30 nm ) and small ITO particle (10 μm ) , we obtained a composite with optically selective transparent properties. The best sample was 0.2 vol % of ITO fine particle (30 nm ) dispersed into the composite, it was not transparent in IR wavelength regions and transmitted more than 80% in visible wavelength region; so an optically selective transparent composite was successfully fabricated.

Homogeneous precipitation of alumina precursors via enzymatic decomposition of urea

Abstract A modified homogeneous precipitation technique involving enzymatic decomposition of urea was proposed and was applied to the synthesis of alumina precursors. Spherical aluminum hydroxide aggregates were prepared at 298 K from aqueous solutions containing aluminum sulfate, urea and urease. In contrast to the conventional technique involving thermally induced decomposition of urea, the resultant aggregates were amorphous with no trace of pseudoboehmite. The apparent rate of nucleation of the aggregates was found to correlate with the concentration of urease. As a result, the mean size of the aggregates could be controlled in a range between 0.3 and 1.9 μm by adjusting the urease concentration. The precursors remained amorphous after calcination at 1073 K for 2 h, then transformed to γ-alumina after being calcined at 1273 K. The transformation to α-alumina was completed after calcination at 1473 K.

Thermo-elastic behaviour of a natural quartzite: itacolumite

Itacolumite is a particular type of sandstone constituted of quartz grains, with an interlocked microstructure and large intergranular decohesions. This article is devoted to the study of the thermo-elastic behaviour of this material during thermal cycles between 20 and 800 °C. This was made by using an ultrasonic pulse-echo measurement technique of Young’s modulus E, coupled to thermal expansion experimentation and to acoustic emission. An unusual evolution of elastic properties versus temperature is found, with strong irreversible effects around the temperature of the α–β transition of quartz. A damage parameter, representing the fractional number of cracks, is calculated with a Kachanov type formulation, by using the experimental data for itacolumite and the values of E, obtained from literature for an hypothetic ideal quartz polycrystal. The evolution of this parameter versus temperature is explained by internal stresses that are developed by both thermal and elastic effects in quartz grains.

Synthesis and thermal reactions of rhabdophane-(Yb or Lu)

Abstract Poorly crystalline rhabdophane-(Yb or Lu) was synthesized by precipitation from mixed aqueous solutions of Ln(NO 3 ) 3 (Ln = Yb or Lu), (NH 4 ) 2 HPO 4 , and citric acid with the mixing mole ratios of (NH 4 ) 2 HPO 4 /Ln(NO 3 ) 3 = 4 and citric acid/Ln(NO 3 ) 3 = 5 to 30, at pH 7 and 90°C for 7 to 30 days. However, unknown XRD peaks and C–O bond FTIR absorption peaks were observed for the precipitates. These peaks disappeared with increasing heating temperature up to 400°C in air, and single phase rhabdophane-(Yb or Lu) was obtained when heated at 400 to 800°C in air. Chemical formulas of the single phase rhabdophane were YbPO 4 ·0.4H 2 O and LuPO 4 ·0.5H 2 O, respectively. The water corresponding to 0.4H 2 O (or 0.5H 2 O) was zeolitic water. The lattice constants are a = 0.676 and c = 0.626 nm for Yb, and a = 0.674 and c = 0.630 nm for Lu. Rhabdophane-(Yb or Lu) changed into the xenotime structure above 860°C, which was stable even at 1800°C in air.