Toru Iwaki

Preparation of high coercivity magnetic FePt nanoparticles by liquid process

FePt nanoparticles have been synthesized using a liquid process, by mixing two precursor liquids: ferric acetyl ferric acetyl acetonate, Fe(acac)3, and platinum acetyl acetonate, Pt(acac)2 in polyol solution of sodium hydroxide at high temperatures. To avoid the agglomeration of the produced particle, aminoethoxy ethanol has been used. The particle size was monodisperse and non- agglomerating. The fraction of Fe in FexPt(100−x) was a linear function of Fe(acac)3 molar fraction. The highest room temperature coercivity (up to 10 kOe) was observed in Fe53Pt47 sample after annealed at 580 °C, which also exhibited the most ordered fct structure.

Studies of the surface of titanium dioxide: II. The effect of reduction by hydrogen on the weight and the magnetic susceptibility

Abstract Simultaneous measurements of weight and magnetic susceptibility of several kinds of titanium dioxide have been carried out after an introduction of hydrogen at temperatures between 400 and 550 °C and at pressures between 0.01 and 0.5 atm. The weight of titanium dioxide decreased gradually, while the magnetic susceptibility increased, when hydrogen was introduced above 400 °C. It was found that the rate of reduction obeys partly the Elovich equation and that the weight decrease per unit weight of the sample tends to increase with an increase in the specific surface area. It could not be revealed definitely whether the crystal structure of titanium dioxide, rutile or anatase, affects the behavior of the reduction. The activation energy increased gradually with the reduced amount. The magnetic susceptibility increased linearly with a decrease in the weight. These results support the previous findings of the hydrogen reduction of titanium dioxide in connection with the anatase-rutile transformation performed by Shannon. On the basis of these results, the reduction mechanism was discussed and the rate-determining step in the initial stage was presumed to be formation of the surface hydroxyl groups followed by rapid removal of water molecules from the surface.

Mesopore-free silica shell with nanometer-scale thickness-controllable on cationic polystyrene core.

The formation of mesopore-free silica shell with homogenous shell thickness, smooth surface, and controllable thickness in the nanometer range (from 4 to 12 nm) on core material was studied. Cationic polystyrene particles with various sizes (ranged from 80 to 300 nm) were used as a model of core material, which could be effective to support the electrostatic attraction between the core material and the negatively charged silica without any additives. Different from other reports, mesopore-free shell was produced due to the absence of additive. Basic amino acid (i.e., lysine) was used as a catalyst for forming the silica, which is harmless and able to control the silica growth and produce shell with smooth surface. Homogenous thin shell (thickness <13 nm) with nanometer-scale-controllable was reported, while in the current reports, the modification of the shell in this thickness range was typically difficult and relating to the formation of incomplete/inhomogeneous silica coating and rough surface. The relationships among the reaction parameters were also investigated in detail along with the theoretical consideration and the proposal of the silica coating formation mechanism. The present mesopore-free silica shell was efficiently used for various applications because of their tendency not to adsorb large molecules, as confirmed by the nitrogen sorption and large molecule adsorption analysis.

Facile synthesis of single-phase spherical α″-Fe16N2/Al2O3 core-shell nanoparticles via a gas-phase method

When nitrogen was inserted into the spherical α-Fe/Al2O3 core shell of 45 nm nanoparticles, the XRD pattern showed a clear change in the crystal modification from a body-centered cubic crystal to that of a single-phase α″-Fe16N2 structure. SEM, TEM, and energy-dispersive X-ray spectroscopy mapping analysis gave the particle size distributions, the shell thickness, and the Fe and Al elements. An examination of the total electron yield (surface sensitive) and fluorescence yield (bulk sensitive) of X-ray absorption fine structure on Fe and N atoms of these core shell nanoparticles confirmed the nitriding of the core iron and showed iron oxide formations on the core surface, indicating stability and resistivity performance. The nitriding process also changed the magnetic properties from paramagnetic to ferromagnetic with a coercivity above 3000 Oe, indicating a promising material for a “rear-earth-free” giant magnet.

Correlation between particle size/domain structure and magnetic properties of highly crystalline Fe 3 O 4 nanoparticles

Highly crystalline single-domain magnetite Fe3O4 nanoparticles (NPs) are important, not only for fundamental understanding of magnetic behaviour, but also for their considerable potential applications in biomedicine and industry. Fe3O4 NPs with sizes of 10–300 nm were systematically investigated to reveal the fundamental relationship between the crystal domain structure and the magnetic properties. The examined Fe3O4 NPs were prepared under well-controlled crystal growth conditions using a large-scale liquid precipitation method. The crystallite size of cube-like NPs estimated from X-ray diffraction pattern increased linearly as the particle size (estimated by transmission electron microscopy) increased from 10 to 64.7 nm, which indicates that the NPs have a single-domain structure. This was further confirmed by the uniform lattice fringes. The critical size of approximately 76 nm was obtained by correlating particle size with both crystallite size and magnetic coercivity; this was reported for the first time in this study. The coercivity of cube-like Fe3O4 NPs increased to a maximum of 190 Oe at the critical size, which suggests strong exchange interactions during spin alignment. Compared with cube-like NPs, sphere-like NPs have lower magnetic coercivity and remanence values, which is caused by the different orientations of their polycrystalline structure.

Preparation and evaluation of magnetic nanocomposite fibers containing α″-Fe₁₆N₂ and α-Fe nanoparticles in polyvinylpyrrolidone via magneto-electrospinning.

Two kinds of ferromagnetic nanocomposite fiber comprising α″-Fe16N2 and α-Fe nanoparticles (NPs), which have the highest magnetic moments as hard and soft magnetic materials, respectively, embedded in polyvinylpyrrolidone (PVP) have been synthesized via the magneto-electrospinning method. Both α″-Fe16N2 and α-Fe were single-domain core-shell NPs with an average outer diameter of 50 nm and Al2O3 as the shell. Ferrofluid precursors used for the electrospinning were prepared by dispersing these NPs in a PVP-toluene-methanol solution. The results show that applying the magnetic field in the same direction as the electric field resulted in smaller and more uniform fiber diameters. Nanocomposite fibers containing α″-Fe16N2 had smaller diameters than those containing α-Fe NPs. These magnetic-field effects on the fiber formation were explained by referring to the kinetic energy of the moving jet in the electrospinning process. In addition, magnetic hysteresis curves showed an enhancement of the magnetic coercivity (H(c)) and remanence (M(r)) by 22.9% and 22.25%, respectively. These results imply a promising possibility of constructing bulk magnetic materials using α″-Fe16N2 NPs, which furthermore reveals attractive features for many other magnetic applications, such as magnetic sensors.

Selective, high efficiency reduction of CO2 in a non-diaphragm-based electrochemical system at low applied voltage

In a typical electrochemical CO2 reduction system, hydrocarbon products are not selectively generated when a diaphragm is used in the cell. However, without the diaphragm, H2 and CH4 are selectively produced with Faradaic efficiencies as high as 96.7% in methanolic NaOH and KOH electrolytes, respectively. We are the first to successfully achieve the selective production of hydrocarbon and hydrogen fuels from the electrochemical reduction of CO2, which can help to meet the rapidly growing energy demands of modern society.

Low-Energy Bead-Mill Dispersion of Agglomerated Core-Shell α-Fe/Al₂O₃ and α″-Fe₁₆N₂/Al₂O₃ Ferromagnetic Nanoparticles in Toluene.

Magnetic materials such as α″-Fe16N2 and α-Fe, which have the largest magnetic moment as hard and soft magnetic materials, are difficult to produce as single domain magnetic nanoparticles (MNPs) because of quasistable state and high reactivity, respectively. The present work reports dispersion of agglomerated plasma-synthesized core-shell α″-Fe16N2/Al2O3 and α-Fe/Al2O3 in toluene by a new bead-mill with very fine beads to prepare single domain MNPs. As a result, optimization of the experimental conditions (bead size, rotation speed, and dispersion time) enables the break-up of agglomerated particles into primary particles without destroying the particle structure. Slight deviation from the optimum conditions, i.e., lower or higher dispersion energy, gives undispersed or broken particles due to fragile core-shell structure against stress or impact force of beads. The dispersibility of α″-Fe16N2/Al2O3 is more restricted than that of α-Fe/Al2O3, because of the preparation conditions. Especially for α″-Fe16N2/Al2O3, no change on crystallinity (98% α″-Fe16N2) or magnetization saturation after dispersion was observed, showing that this method is appropriate to disperse α″-Fe16N2/Al2O3 MNPs. A different magnetic hysteresis behavior is observed for well-dispersed α″-Fe16N2/Al2O3 MNPs, and the magnetic coercivity of these NPs is constricted when the magnetic field close to zero due to magnetic dipole coupling among dispersed α″-Fe16N2 MNPs.

Studies of the surface of titanium dioxide. IV. The hydrogen-deuterium equilibration reaction

The interaction of hydrogen with the surface of titanium dioxide has been studied in connection with the hydrogen-reduction mechanism of titanium dioxide, by means of such measurements as weight decrease, magnetic susceptibility, hydrogen uptake, and electrical conductance. It was postulated in the previous study that the rate-determining step of the hydrogen-reduction reaction may be the formation of surface hydroxyl groups, followed by the rapid removal of water molecules from the surface. In this study, the interactions between hydrogen and the surface of titanium dioxide were investigated by measuring the hydrogen-deuterium equilibration reaction, H/sub 2/ + D/sub 2/ = 2HD, at temperatures above 200/sup 0/C on both surfaces before and after hydrogen reduction to compare the differences in the reactivities.

Aligned Fe3O4 magnetic nanoparticle films by magneto-electrospray method

This work reports for the first time the preparation and evaluation of aligned Fe3O4 nanoparticle films via a magneto-electrospray method, i.e., electrospray under a magnetic field. The magnetic field was applied to align the magnetic moment of Fe3O4 particles. Well-dispersed Fe3O4 nanoparticles (NPs) with average sizes of 10, 25, and 45 nm were obtained using a bead-mill dispersion. The Fe3O4 nanoparticle slurries were mixed with a polyvinyl alcohol (PVA) solution and then deposited on Si-wafers under a 0.1 T magnetic field. The Fe3O4 crystalline structures were maintained after both dispersion and deposition, as characterized by X-ray diffraction patterns. Hysteresis curves revealed that the magnetic coercivity (Hc) of the well-dispersed nanoparticle slurries decreased owing to magnetic interactions among particles. However, the Hc values of the films were larger than those of the nanoparticle slurries. The values further increased from the application of a magnetic field during film deposition. This enhancement was attributed to alignment of the magnetic moments of the Fe3O4 NPs. These results show that tuning of the magnetic properties of materials, such as Fe3O4 NPs, can be achieved by controlling the alignment of their magnetic moment.