Masahiro Uda

Oxidation and Degradation of Titanium Nitride Ultrafine Powders Exposed to Air

Titanium nitride ultrafine powders were synthesized by an active plasma-metal reaction method. Gas desorption measurements were conducted to estimate the surface chemistry of the powders after exposure to air and storage at room temperature. H2O, H2, CO2, CO, and NH3 gases were mainly evolved. These gases were considered to be formed by the surface reaction of adsorbed gases on surface oxide of the powders and decomposition of hydroxide-like or ammonialike compounds, which might be produced during a slow oxidation treatment and storage.

Hydrogen generation from water using Mg nanopowder produced by arc plasma method

Abstract We report that hydrogen gas can be easily produced from water at room temperature using a Mg nanopowder (30–1000 nm particles, average diameter 265 nm). The Mg nanopowder was produced by dc arc melting of a Mg ingot in a chamber with mixed-gas atmosphere (20% N2–80% Ar) at 0.1 MPa using custom-built nanopowder production equipment. The Mg nanopowder was passivated with a gas mixture of 1% O2 in Ar for 12 h in the final step of the synthesis, after which the nanopowder could be safely handled in ambient air. The nanopowder vigorously reacted with water at room temperature, producing 110 ml of hydrogen gas per 1 g of powder in 600 s. This amount corresponds to 11% of the hydrogen that could be generated by the stoichiometric reaction between Mg and water. Mg(OH)2 flakes formed on the surface of the Mg particles as a result of this reaction. They easily peeled off, and the generation of hydrogen continued until all the Mg was consumed.

Preparation of fine Nb3Al powder by hydriding and dehydriding in an arc-melting chamber

Abstract The disintegration due to hydrogenation of NbAl alloys with eight different compositions has been studied. The hydrogenation is performed directly in an arc-melting chamber without exposing NbAl ingots to air after arc-melting. Because the active surface resulting from arc-melting is preserved, NbAl alloys absorb a large amount of hydrogen and disintegrate rapidly into fine particles, even though the pressure of hydrogen is only at 0.1 MPa. This disintegration is closely related to the arc-melting atmosphere and the composition of the alloys. Structural changes of NbAl alloys before and after hydrogenation and dehydrogenation have been examined by X-ray diffraction, electron microscopy and chemical analysis. A fine powder of single Nb 3 Al phase with an average particle size of less than 14 μm can easily be obtained by hydriding NbAl alloy buttons with aluminum compositions from 22 to 28 at.% in an arc-melting chamber, followed by dehydrogenation at 1073 K for 3.6 ks.

Fabrication and characterization of TiN–Ag nano-dice

TiN-Ag nanocomposite was synthesized by dc arc-plasma method. Microstructures of TiN-Ag nanocomposite were carefully characterized by powder X-ray diffraction method and transmission electron microscopy, and nano-morphologies by three-dimensional electron tomography. It was found that the surface of nanocrystalline TiN matrix was densely covered by finely dispersed Ag nanoparticles, and it was found that they were physically attached but not chemically bonded from their orientation relationships.

Removal of Aerosol by Magnetic Separation

We concluded high gradient magnetic separation experiments of iron (ferromagnetic) nano-particles with a nominal diameter of 50 nm by using a superconducting magnet. In the experiment, we fabricated fine iron particles using a thermal reactive plasma method in argon and hydrogen mixed gas and sent them directly to the magnetic separator. We present the experimental results, functions of the capture ratio on the magnetic flux density, and velocity of flow. We conclude that high gradient magnetic separation can separate nano-particles and is useful for nano-technology

Preparation of fine Nb3Al powder by hydriding and dehydriding of bulk material

Abstract Structural changes of the intermetallic compound Nb3Al before and after hydriding and dehydriding have been investigated by X-ray diffraction, electron microscopy and thermal analysis. A fine powder of this compound with an average particle size less than 14 μm can be easily prepared by hydriding at 773 K for 86.4 × 103 s and dehydriding at 1073 K for 10.8 × 103 s.

Preparation of ultrafine powders of AlN and (AlN + Al) by nitrogen plasma-Al reaction

Ultrafine powders of AlN and (AlN+Al) with particle sizes of less than 0.5μm were produced by arc melting of Al in atmospheres of N2+Ar, N2+H2, N2+NH3 at 0.1MPa pressure. The size, surface area, chemical composition, and crystal structure of obtained powders were determined by electron microscope, BET method, chemical analysis and X-ray diffraction analysis respectively. The following results were obtained.(1) AlN ratio in (AlN+Al) powders obtained in (N2+Ar) atmospheres decreases from 0.3 to zero with decreasing nitrogen partial pressure.(2) AlN ratio in (AlN+Al) powders obtained in (N2+NH3) atmospheres increases from 0.3 to 0.95 with increasing NH3 partial pressure.(3) Projections were formed on molten Al surface by evolved nitrogen gas from the melt and by reaction between nitrogen and molten Al.The reaction between nitrogen plasma and molten Al plays an important role in the formation of ultrafine particles. The driving force for the formation of ultrafine particles can be explained as follows; In arc melting of Al in a nitrogen atmosphere, molten Al simultaneously contacts with two gas phases, one is atomic nitrogen in the arc gas phase and another is the molecular nitrogen in the non-arc gas phase. The extent of dissolution of atomic nitrogen in the molten Al is much larger than that of molecular one. Then, the dissolved nitrogen through the arc gives higher nitrogen content for the non-arc gas phase. That is, super-saturated nitrogen will evolve in non-arc gas phase carrying with Al vapor. A kind of enhanced evaporation of Al is induced. This Al vapor reacts with surrounding gas species near the arc such as N2, N, NH3, NH2, NH in high temperature ranges (above 1600K) to form AlN or (AlN+Al) vapor which condense to ultrafine particles.

Hydrogen Storage Properties of Nb-Zr-Fe Alloys Disintegrated by Hydrogen Gas

The disintegration of 6 different Nb-Zr-Fe alloys and their hydrogen storage properties due to hydrogenation have been investigated. The hydrogenation was performed in a 0.1MPa hydrogen pressure held at room temperature in arc melting chamber without exposure to air atmosphere. The thermal stability of hydrogen dissolved in disintegrated powder was measured in the temperature range from room temperature to 600°C by using the thermal analysis equipment. Hydrogen contents in alloys were evaluated from the mass difference of TG curve and chemical determinations for hydrogen gas analysis. The values of hydrogen content are in accordance with both chemical and mass difference methods. The hydrogen contents of disintegrated alloys synthesized were in the range of 1.2-2.0 wt% which depend on the composition of alloys.