T. Uchikoshi

A study of the passivating oxide layer on fine nickel particles

Abstract The surface oxide layer of fine nickel particles has been investigated by high- resolution electron microscopic observation,, electron diffraction, X-ray diffraction, X-ray photo- electron spectroscopy and temperature-programmed gas desorption experiment. The thickness of the oxide layer formed on the particles was about 2 nm. The oxide layer was composed of innermost NiO, and outermost Ni(OH) 2 and amorphous nickel carbonate-hydroxide. The oxide layer give a good thermal stability to fine nickel particles.

Synthesis and characterization of Fe and composite Fe–TiN nanoparticles by dc arc-plasma

Abstract Fe and composite Fe–TiN nanoparticles were prepared by an active plasma–metal reaction method. The structure and morphology were examined by X-ray diffraction, transmission electron microscopic observation and Mossbauer experiment. The morphology of Fe is spherical and that of the composite Fe–TiN nanoparticle is dumbbell-like. The surface oxide of the nanoparticles exposed to air was characterized and the reduction characteristics in a hydrogen atmosphere were examined. The thermal stability of the nanocomposite particle is vastly superior to that of the metal particle. The hydrogen sorption–desorption characteristics of the nanoparticles were examined by temperature-programmed desorption measurements. Hydrogen desorption at above 450 K was observed for the composite Fe–TiN nanoparticles. The effect of CO coadsorption on the hydrogen desorption was also examined and a large amount of hydrogen desorption was observed.

Pressure filtration and sintering of fine zirconia powder

Abstract Pressure filtration is applied to the consolidation of 3 mol% yttria-doped zirconia fine powder with average particle size of 60 nm over an applied pressure range of 2.5 to 10.0 MPa. Two kinds of aqueous suspension with different rheological behavior, well-dispersed and insufficiently dispersed, respectively, are prepared by changing the amount of dispersant. The kinetics and mechanics of the consolidation of particles are discussed from the dehydration and consolidation rates of the suspensions based on Darcys law and the Kozeny-Carman equation. The consolidated layer prepared from the well-dispersed suspension shows a good sinterability. Cold isostatic pressing at 400 MPa increases the sintered density of the consolidated layer prepared from the insufficiently dispersed suspension.

Preparation of fine-grained monoclinic zirconia ceramics by colloidal processing

Fine-grained monoclinic zirconia ceramic was made from well-dispersed zirconia sol prepared by the hydrolysis of zirconium chloride oxide octahydrate. Dechlorinated and concentrated zirconia sol was consolidated by pressure filtration. The relative green density of the compact was improved by the following cold isostatic pressing treatment at 400 MPa. The compact was densified by pressureless sintering to >98% of theoretical density in air at 1100 °C, which is lower than that of monoclinic to tetragonal transformation of pure zirconia. The average grain size of the sintered monoclinic zirconia ceramics was 92 nm.

Sintering characteristics of Fe and FeCo alloy ultrafine powders

Sintering characteristics of three kinds of iron and FeCo alloy ultrafine powders (UFPs) in vacuum and a hydrogen atmosphere were examined by continuously measuring their dimensions, and observing their structural changes. The UFPs exposed to air contain mixtures of oxide and hydroxide phases. The oxide phase increased during heating in vacuum. The compacts of the UFPs shrunk slightly at temperatures between 450 and 700 K, where the surface oxides sintered, and then densified rapidly above 700 K. The shrinkage in a stream of hydrogen occurred at much lower temperatures than that in vacuum. The reduction reaction is ratecontrolled by an interface reaction and the reduction rate of the UFPs depends on the oxidation level, where the more highly oxidized UFPs tend to the lower reduction rate. Activation energies of the reduction rate constants of the iron UFPs and FeCo UFPs lie in the range 48–59 kJ mol−1.

Effect of Silica Doping on the Electrical Conductivity of 3 mol % Yttria-Stabilized Tetragonal Zirconia Prepared by Colloidal Processing

Silica-doped (SiO2 = 0 - 1.0 wt%) 3Y-TZP (3 mol % yttria-doped tetragonal zirconia polycrystal) ceramics are prepared from hetero-coagulated aqueous suspension by colloidal processing. Consolidation of the suspension was carried out by pressure filtration at 10 MPa followed by cold isostatic pressing (CIP) at 400 MPa. Consolidated compacts are densified to a relative density over 99% by sintering at 1573 K for 2 h. The formation of glass pockets at grain boundary multiple junctions was observed by SEM for ≥0.5 wt % silica-doped samples. Electrical conductivity measurements were performed to evaluate the modification of grain-boundaries by silica. The apparent grain boundary conductivity decreased with an increase in silica content and became nearly constant above 0.3 wt % of silica, while the bulk conductivity was constant with silica content.

Morphology and hydrogen desorption characteristic of Ni-TiN nanocomposite particle prepared by RF plasma

Abstract Ni-TiN nanocomposite particles are prepared by RF plasma. Dumbbell-like and dice-like morphologies are seen. Hydrogen sorption-desorption characteristics of the nanocomposite particles were examined by temperature-programmed desorption measurements. Hydrogen desorption at 500–700 K is observed, which implies that a new adsorption state of hydrogen exists. Hydrogen desorption characteristics are not so influenced by coadsorption of CO. The results are compared with the previously reported ones that were produced by a DC plasma reaction method.

Characterization of degraded surfaces of Al and AlN ultrafine powders

Abstract Aluminum (Al) and aluminum nitride (AlN) ultrafine powders (UFPs) were synthesized by an active plasma-metal reaction method. Temperature-programmed gas Desorption (TPD) measurements were conducted for the Al and/or AlN UFPs after exposure to air. H 2 O gas for AlN UFP, and H 2 O and H 2 gases for (Al + AlN) UFPs were mainly detected. The surface chemistry of the powders after exposure to air is estimated in comparison of the desorption characteristics with infrared spectrometry spectra. The evolved H 2 around 420 °C from the (Al + AlN) UFPs is due to the reaction of H 2 O with inner Al. The evolved H 2 O is considered to be formed by the reaction of adsorbed H 2 O on surface oxide of the powders and decomposition of hydroxide-like compounds.

Adsorption of hydrogen on Ni and NiTiO2 composite ultrafine particles

Abstract The adsorption of hydrogen on Ni and NiTiO2 composite ultrafine particles (UFPs) was studied by temperature-programmed desorption (TPD) experiments. The desorption characteristics of hydrogen on Ni and NiTiO2 composite UFPs were similar to those of a Ni single crystal or polycrystal. The amount of adsorption of hydrogen on NiTiO2 composite UFPs was dependent upon the temperature of exposure, and the phenomenon attributed to the spillover of hydrogen from Ni onto TiO2. The CO hydrogenation reaction on NiTiO2 composite UFPs proceeded above 373 K. TPD spectra showed new desorption peaks after the catalytic reaction above 373 K and suggested the formation of new adsorption states.

Preparation of Barium Titanate Grain-Oriented Ceramics by Electrophoresis Deposition Method under High Magnetic Field Using Single-Domain Nanoparticles

Barium titanate (BaTiO3, BT) grain-oriented ceramics were prepared by electrophoresis deposition (EPD) method under high magnetic field (HM-EPD) of 12 T. For this objective, the BT single-domain nanoparticles with high c/a ratio of 1.008 and size of 103 nm were prepared by two-step thermal decomposition method. Using the BT nanoparticle slurry, BT nanoparticle accumulations were prepared by EPD or slipcasting (SC) methods with/without high magnetic field. After binder burnout, these accumulations were sintered at 1350 °C and it was revealed that only the BT ceramics prepared by the HM-EPD method was assigned to grain-oriented ceramics with weak preferential crystallographic orientation along [11 direction.