Yoshiaki Kinemuchi

Magnetic properties of nanosize NiFe2O4 particles synthesized by pulsed wire discharge

Nanosize particles of nickel ferrite, NiFe2O4, have been successfully synthesized by pulsed wire discharge (PWD). In PWD, a simple circuit consisting of a capacitor and a gap switch drives the discharge. The wires of Ni and Fe were simultaneously discharged in a chamber filled with oxygen. The particles floating in the ambient gas were collected by pumping the gas through a membrane filter, and subjected to further analysis. The specific surface area of the particles were measured by the Brunauer-Emmet-Teller (BET) method. X-Ray diffraction showed the formation of NiFe2O4 and the inclusion of NiO. The NiO inclusion was estimated to be 18 vol.%. Magnetization hysteresis was measured for the particles synthesized at 600 torr. X-Ray diffraction and BET measurements reveal that particle size increases with increase in oxygen pressure. The saturation magnetization was found to be 33 emu/g for the particle with 45 nm in the size.

Enhanced boundary-scattering of electrons and phonons in nanograined zinc oxide

Nanoscale constituents in bulk materials can promote enhanced boundary-scattering in the transport of phonons as well as electrons, which is considered a key design factor for enhancing thermoelectric properties. Here, we demonstrate a method for synthesizing zinc oxide bulk materials from nanoparticles without significant grain growth by means of pressure-induced deformation at 200 °C. This allows us to comprehensively analyze the grain size dependence of thermoelectric properties in the nanoscale range above 30 nm, the size of a nanoparticle. Grain size was found to largely influence thermal conductivity as well as electrical conductivity. The observed thermal conductivity agreed with the Callaway model, indicating that enhanced phonon boundary-scattering was responsible for the variation. On the contrary, Seebeck coefficient was mostly governed by effective mass and carrier concentration, and was independent of the grain size. The dimensionless figure of merit systematically increased with grain size, ...

Synthesis of Nanosize Powders of Aluminum Nitride by Pulsed Wire Discharge

Using a thin wire of aluminum, we have successfully synthesized the aluminum nitride powders in N<inf>2</inf>/NH<inf>3</inf> mixed gas. The powders were characterized by X-ray diffraction and specific surface area measurement. The results of X-ray diffraction indicated that the synthesized powders consists of würtzite aluminum nitride. In the powders synthesized in N<inf>2</inf> gas weak peak intensity of aluminum nitride was observed. However, it is significantly increased using the mixed gas of N<inf>2</inf>/NH<inf>3</inf>. The average particle sizes were 30 ∼ 70 nm.

Enhancement of Nitridation in Synthesis of Aluminum Nitride Nanosize Powders by Pulsed Wire Discharge

High-purity aluminum nitride (AlN) nanosize powders were synthesized by pulsed wire discharge (PWD) of aluminum wires in nitrogen gas mixed with ammonia. The AlN content in the powders, which was determined by X-ray diffraction analysis, was increased by increasing the discharge energy. The highest AlN content of 98 wt% was obtained at the discharge energy of 360 J, and the average particle size was 30 nm. The increase in the AlN content at higher discharge energy can be explained by the energy deposited in the wire during the ohmic heating process. The number of micrometer-sized Al particles, which might be generated from liquid droplets, decreased by increasing the amount of energy deposited in the wire. The production rate of AlN powder was evaluated to be 46 g/kWh, which was 15 times higher than that obtained by the conventional arc discharge method.

Thermoelectric properties of tungsten-substituted Heusler Fe2VAl alloy

A Heusler Fe2V1-xWxAl sintered alloy was synthesized to evaluate the effect of W substitution on thermoelectric properties of the Heusler alloy. The Seebeck coefficient and the electrical conductivity are simultaneously enhanced through electron injection resulting from the W substitution. Comparison with the Si-substituted Fe2VAl alloy reveals that the additional electronic states derived from W 5d orbital in the vicinity of pseudogap are likely to degrade the Seebeck coefficient. Thermal conductivity is effectively reduced by the W substitution because of the large atomic mass and volume of W compared to the constituent elements of Fe2VAl alloy. The appreciable reduction of thermal conductivity, without a serious deterioration in electrical conduction, enhances the thermoelectric figure of merit in the Heusler alloy.

Preparation and Thermoelectric Property of Highly Oriented Al-Doped ZnO Ceramics by a High Magnetic Field

Highly oriented Al-doped ZnO ceramics were prepared by a high magnetic field method and their anisotropic thermoelectric properties were examined. The c-axis oriented specimen along the ab-plane showed a higher electrical conductivity compared to the a-axis and non-oriented specimens as a result of high electron mobility. On the other hand, the differences in the Seebeck coefficients and thermal conductivities between oriented and non-oriented specimens were a small. Consequently, the dimensionless figure of merit of the c-axis oriented specimen increased by about 30% compared to the other specimens.

Grading porous ceramics by centrifugal sintering

Graded porous structures in ceramics have conventionally been synthesized by applying centrifugal acceleration during sintering. A gradual change in porosity is caused by enhanced sintering using centrifugal force, which increases linearly along the radius of rotation. A theoretical understanding of the formation of graded structures has been demonstrated based on the kinetic model of liquid-phase sintering, which is considered to be driven by both capillary and centrifugal forces. The observed structure is in accordance with the theoretical approach which predicts linear shrinkage in the radial direction that is proportional to the power of one fifth of the distance from the surface.

Thermoelectric properties in the series Ti1-xTaxS2

Polycrystalline samples in the series Ti1-xTaxS2 with x varying from 0 to 1 were prepared using solid-liquid-vapor reaction and spark plasma sintering. Rietveld refinements of X-ray diffraction data are consistent with the existence of a full solid solution for x ≤ 0.4. Transport measurements reveal that tantalum can act as electron donor when substituted in the Ti sites. As a consequence, the electrical resistivity and the absolute value of the Seebeck coefficient decrease with Ta content due to an increase in the carrier concentration. The lattice thermal conductivity being reduced due to mass fluctuation effect, the ZT values in Ti0.95Ta0.05S2 is slightly increased as compared to TiS2.

Fabrication of c-axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties

Fabrication of c-axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties

Novel Critical Temperature Resistor of Sintered Ni–Fe–O Nanosized Powders

Nanosized powders of Ni-Fe-O were synthesized by a pulsed wire discharge method and sintered at 600 °C for 1 h in air. Abrupt electrical resistivity changes were observed in the temperature dependence of resistivity for the sintered Ni-Fe-O powders above 203 °C. Similar resistivity curves, which had been observed in V-O samples and had been used for the critical temperature resistors, had never been reported in Ni-Fe-O samples. Possible mechanisms to explain the resistivity change in NiFe 2 O 4 , including order-disorder transition, semiconductor-metal transition, and surface spin pinning, are discussed.