Yusuke Moriyoshi

Control of particle size and phase formation of TiO2 nanoparticles synthesized in RF induction plasma

TiO2 nanoparticles have been synthesized in this work via Ar/O2 RF thermal plasma oxidation of atomized liquid precursors containing titanium tetrabutoxide and diethanolamine. Quench gases (Ar or He), either injected from the shoulder of the reactor (transverse injection) or injected counter to the plasma plume from the bottom of the reactor (counter-flow injection), are used to affect the quench rate and therefore the particle size and phase constituent of the resultant powders. The experimental results show that counter-flow injection is more effective in reducing the particle size, while He is more effective than Ar. As a result, well-dispersed TiO2 nanopowders with controllable phase structure (up to ~90% of anatase) and average particle size (down to 20 nm) are obtained. The experimental results are well supported by numerical analysis on the effects of the quench gas on flow pattern and temperature field of the thermal plasma as well as trajectory and temperature history of the particles.

Boron nitride nanotubes, webs, and coexisting amorphous phase formed by the plasma jet method

Boron nitride (BN) nanotubes were prepared by a high-temperature plasma jet method. While the BN nanotubes were found in quantity on the surface of a BN sintered disk, amorphous BN webs were found dominant in the water-cooled collector. These structures were studied on the basis of transmission electron microscopy observations. One of them was composed of 21 layers grown coaxially with a regular spacing of 0.34 nm and their tip ends were terminated with parallel bases of the coaxial cylindrical shapes. The diameter of the innermost tube was 0.6 nm and this dimension is in fairly good agreement with the diameters of the smallest BN fullerenes.

Preparation of lead magnesium niobate by a coprecipitation method

The ferroelectric complex perovskite lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), was prepared by a coprecipitation method. As the niobium component, niobium oxalate was used. Among the various precipitants, tetramethylammonium hydroxide was effective for the formation of single-phase PMN without pyrochlore phase. The dielectric constant and the dissipation factor of PMN changed either the sintering temperature or the grain size. The maximum dielectric constant at the Curie point was obtained by sintering at 1220 °C in air for 2 h.

Influence of atomic hydrogen on the growth reactions of amorphous boron films in a low-pressure B2H6+He+H2 plasma

The activation energy (E) of the growth reactions of boron films in plasma‐assisted chemical vapor deposition from B2H6+He+H2 at a pressure of 300 Pa was measured at a range of substrate temperature from 750 to 1200 K. It was found that the E decreases continuously from 1.83 to 0.54 kcal/mol and then increases up to 4.97 kcal/mol with the increase of partial pressure of atomic hydrogen ([H]). The continuous change of E is supposed to be due to the transition of the dominant rate determining growth reaction, which is accompanied with the change of the composition of adsorbed layer on a film surface. The variation of E as a function of [H] can be explained with a kinetic model proposed here, in which the E of the growth reaction through BH3 is assigned to 1.83 kcal/mol, that through BH2 to 0.54 kcal/mol, and E of migration of BH2 on the film surface covered with hydrogen is related to be 4.97 kcal/mol.

Ion Exchange Behavior of Natural Zeolites in Distilled Water, Hydrochloric Acid, and Ammonium Chloride Solution

This study presents basic research on the use of natural zeolites for the removal of ammonium ions from aqueous solutions. Three natural zeolites from Shimane, Japan, used in this study are shown to contain varying amounts of clinoptilolite and mordenite. The chemical behavior of these natural zeolites was investigated in distilled water, hydrochloric acid, and ammonium chloride solution. The amount of ammonium ions adsorbed by these zeolites was measured using an electrode specific to ammonium ion. The results were compared with published adsorption values for other zeolites. Hydrogen ions displaced the exchangeable cations (Na+, K+, Ca2+, and Mg2+) on the natural zeolites in distilled water and hydrochloric acid. With the decrease in equilibrium pH, the amounts of dissolved silica and alumina increased. This behavior indicates that the structures of the zeolites are damaged under this condition. Ammonium ions were preferentially adsorbed on the natural zeolites in ammonium chloride solution. The amount of ammonium ions adsorbed quantitatively corresponded to the theoretically expected amount at high ammonium concentrations. This result indicates that ammonium adsorption occurs by ion exchange of ammonium ions with Na+, K+, Mg2+, and Ca2+ in cation exchange sites on zeolites. The order of selectivity of ammonium ions for other cations on the zeolites was Na+≫K+>Ca2+>Mg2+. Ammonium adsorption was the highest in the zeolite containing the highest proportion of clinoptilolite of the three natural zeolites. The maximum adsorption was 1.28 mmol/g.

Ammonium Ion Exchange of Synthetic Zeolites: The Effect of Their Open‐Window Sizes, Pore Structures, and Cation Exchange Capacities

Abstract Ammonium ion exchange behavior of synthetic zeolites, i.e., sodalite (SOD), rho zeolite (RHO), Linde type A zeolite (LTA), and faujasite zeolite (FAU), was investigated by changing the initial concentration of ammonium ions and reaction time. Ammonium ion exchange behavior was dependent on the open‐window sizes, the pore structures, and the cation exchange capacities of these zeolites. Regarding sodalite with Na+ ions (Na‐SOD), ammonium ion exchange did not completely occur at the equilibrium state because the open‐window size is smaller than the diameter of ammonium ions. Regarding RHO with Na+ and Cs+ ions (NaCs‐RHO), the larger cations, Cs+ ion, in the cages obstructed the ion exchange at the initial stage. However, the amount of exchanged ammonium ions gradually increased with increasing reaction time, finally achieving equilibrium. Regarding LTA with Na+ ions (Na‐LTA), the amount of exchanged ammonium ions decreased with increasing reaction time, and then reached plateau. It indicated that both the ion exchange on the α‐cages and physical adsorption on the β‐cages occurred at the initial stage, which was followed by the equilibrium state of ion exchange on the α‐cages. Regarding FAU with Na+ ions (Na‐FAU), no dependence on reaction time was observed, because the size of the open‐windows is large enough for ion exchange of ammonium ions. The Na‐FAU, which has the biggest open‐windows among these zeolites, showed the highest exchange capacity for ammonium ions, 3.20 mmol/g.

Growth steps and etch pits appearing on {100} planes of diamonds prepared by combustion-flame deposition method

Abstract The growth steps and etch pits were observed on {100} planes of diamonds prepared by C2H2-O2 combustion-flame deposition method. The direction of the growth steps and the edges of the etch pits were both parallel to 〈110〉. They are in good agreement with the (2X1) surface reconstruction hypothesis, which suggests that the surface dangling bonds under these growing conditions are not terminated with hydrogen.

Transition from thermal‐ to electron‐impact decomposition of diborane in plasma‐enhanced chemical vapor deposition of boron films from B2H6+He

The apparent reaction orders for the growth of boron films in plasma‐enhanced chemical vapor deposition from B2 H6 + He were measured as a function of the plasma power, which ranged from 0.4 to 1.6 kW. The conditions were as follows: substrate temperature, 890 °C; pressure, 200 Pa; and total flowrate of gases, 200 sccm. It was found that the reaction order (n) makes a transition from 1/2 to 1 at 900 W with an increase of the plasma power, and which accompanied a corresponding morphological transition. On the basis of a simple kinetic model, the observed reaction‐order transition is related with a transition from thermal‐ to electron‐impact decomposition of diborane occurring at 900 W: the n of 1/2 observed at the power range below 900 W is attributed to a pyrolysis‐dominant growth mechanism in which the association of two BH3 into B2 H6 cannot be neglected, whereas the n of 1 observed at the power range above 900 W is related to a growth mechanism in which the electron‐impact dissociation of diborane is d...

Deposition from the vapour phase during induction plasma treatment of alumina powders

A study was carried out of the induction plasma melting of alumina powders (particle mean diameter, ¯dp=24.5 μm), (Ar/H2 or Ar/N2 plasma, plate power, 40 kW) under reduced pressure conditions (400 torr). The results reveal that in the process, partial vaporization of the alumina powders takes place in the hot region of the discharge. As the molten particles cool down and solidify, the deposits from the vapour phase was formed with the spheroidized particles. In all treatments with the Ar/H2 and Ar/N2 plasmas, a condensate of ultrafine alumina fume (dp<200 nm) was obtained. The fine particles consisted essentially of metastable γ-, δ- and θ-phases. Needle-like crystals(0.1–0.3 μm diameter, by 5–15 μm long) were observed when operating with an Ar/N2 plasma at powder feed rates exceeding 10 g min−1. Electron diffraction analysis revealed that the needles were whiskers, whose structure was very similar to κ- or χ-aluminas with an hexagonal close-packed oxygen lattice. The change of morphology is related to the degree of supersaturation in the vapour phase.

Novel Long-Term Immobilization Method for Radioactive Iodine-129 Using a Zeolite/Apatite Composite Sintered Body

The amount of radioactive iodine generated from nuclear power plants is expected to increase with the proliferation of nuclear energy production, and long-term immobilization methods for such radioactive elements need to be developed to make nuclear energy sustainable. The standard immobilization method of radioactive elements, vitrification, is not very effective for radioactive iodine-129 because of the low solubility of iodine in silicate melts, its very high volatility at standard vitrification process temperatures, and its instability in the alkaline environment of deep geological layers below 300 m. We have developed a novel three-phase ceramic composite produced by a sintering process. Iodine adsorbed onto Ca-type zeolite A was covered with a hydroxyapatite nanolayer through the exchange reaction of ammonium with calcium. Clusters of iodine of 30 nm within the zeolite structure were found to be thermally stable up to 1253 K because of the partial blockage of the alpha-cage apertures by ammonium ions and the partial change from a crystalline phase to an amorphous phase at 473 K. No gasification of iodine molecules was found to occur during the sintering process. The outer phase was highly crystalline hydroxyfluorapatite in which the hydroxyapatite nanolayer plays an important role for successful sintering. The elution of iodine in low-dioxygen water, similar to that found within the Earths crust, was investigated and was found to occur only in the surface layer of the sintered body.