K. Majima

Thermoelectric properties of the skutterudite Co1−xFexSb3 system

We have examined the phase equilibrium and thermoelectric properties of Co1−xFexSb3 ternary system up to high iron context x=0.40. Traces of Sb were observed in the hot-pressed samples with x⩾0.06, and FeSb2 (Fe0.73Co0.27Sb2) compound with marcasite structure was also observed in the samples with x⩾0.25 by x-ray diffraction. The lattice parameter of Co1−xFexSb3 is slightly larger than that of the binary compound CoSb3. The Seebeck coefficient and the electrical resistivity are generally reduced by the substitution for Co by Fe. The thermal conductivity is also reduced by the substitution especially at high iron content region. These behaviors of the thermoelectric properties in the samples with low iron content are ascribed to the substituted Fe, while those in the samples with high iron content are ascribed to the precipitated FeSb2 (Fe0.73Co0.27Sb2) compound. For x⩽0.04, the figure of merit for Co1−xFexSb3 decreases with increasing x. However, above x=0.06 the figure of merit increases with x and as a r...

Thermoelectric properties of (Zn1−yMgy)1−xAlxO ceramics prepared by the polymerized complex method

(Zn1−yMgy)1−xAlxO powders were synthesized by the polymerized complex method and then consolidated by spark plasma sintering apparatus. The microscopic structure and thermoelectric properties were examined comparing with the experimental results of the samples prepared by the conventional solid-state reaction method. A small amount of ZnAl2O4 spinel phase as the second phase was observed in the sintered samples with x⩾0.02 by x-ray diffraction and a scanning electron microscope. The grain size of the samples prepared by the polymerized complex method is much smaller than that of the samples prepared by the conventional solid-state reaction method. The absolute values of the Seebeck coefficient and electrical resistivity decrease with increasing x up to about x=0.01, but above x=0.01 they are almost independent of x. This result indicates that the solubility limit of Al in Zn1−xAlxO is about x=0.01, which is also confirmed by 27Al nuclear magnetic resonance spectroscopy. At a fixed composition of x, the ab...

Thermoelectric properties of CoSb3 with dispersed FeSb2 particles

We have prepared a sintered CoSb3–FeSb2 composite where the FeSb2 particles are dispersed in the CoSb3 matrix by mechanical grinding (MG) and hot pressing, and investigated the Seebeck coefficient, electrical resistivity, and thermal conductivity in order to estimate the corresponding figure of merit. The thermal conductivity of the composite is lower than that of CoSb3. The electrical resistivity of the composite increases with increasing MG time, but it is lower than that of CoSb3 at high temperature. The decrease in the thermal conductivity and the lesser increase in the electrical resistivity are ascribed to the enhancement of phonon scattering caused by the dispersion of FeSb2 particles in the CoSb3 matrix and the low electrical resistivity of FeSb2, respectively. As a result, the composite whose molar ratio of CoSb3 to FeSb2 is 0.7:0.3 and the MG time is 25 h has a maximum figure of merit value of 6.1×10−4 K−1 at 756 K. This value is much larger than the maximum value of CoSb3 at 482 K of 3.2×10−4 K−1.

Effect of LiF addition on the preparation of transparent Y2O3 by the vacuum hot pressing method

Abstract In this experiment, the effect of LiF as sintering additive and the method of loading in the preparation of transparent Y 2 O 3 were investigated, together with its spectroscopic properties. Transparent Y 2 O 3 was fabricated by vacuum hot pressing of the loading stepwise in graphite dies under the following conditions: temperature, 1573 K; maximum pressure, 44 MPa; and LiF content, 1.0 mass%. It has a transmittance of 78% at 644 nm and of greater than 80% from 860 nm to 6 μm wavelength.

Effect of substitution of La by alkaline earth metal on the thermoelectric properties and the phase stability of γ-La3S4

The Seebeck coefficient, electrical resistivity, and thermal conductivity of La3−xAxS4 with the Th3P4-type structure (the γ-phase) for A=Ca, Ba, and x=0–0.8 have been measured as a function of alkaline earth metal content and temperature to determine the optimum dopant concentration and doping agent by calculating the corresponding figure of merit (Z). With an increase of x, the Seebeck coefficient and electrical resistivity increase, while the thermal conductivity decreases. The decrease of thermal conductivity is ascribed to the decrease of the electron concentration and the increase of the phonon scattering accompanying the substitution for La by an alkaline earth metal. As a result, La2.2Ca0.8S4 has a maximum value of Z of 2.3×10−4 °C−1 at 700 °C. We also have examined the long term stability of γ-La3−xAxS4. The substitution for La by alkaline earth metal of La3S4 stabilized the Th3P4-type structure at high temperature.

ZnO precipitation during sintering of Ni-Zn ferrite used for the substrate of thin-film heads

The main purpose of this work was to study the microstructural changes during the sintering process of Ni-Zn ferrite, in order to fabricate a magnetic recording head of high quality.

Structure and magnetic properties of Sm2(Fe,Mn)17Nx coarse powders with high coercivity

The influence of a substitution of Mn for Fe in the Sm2(Fe,Mn)17Nx alloys on the structure and the magnetic properties was studied. Sm2(Fe,Mn)17Nx alloy powders of 32–74 μm in particle size with several values of x were successively synthesized using NH3+H2 gas. It was found that the primary phase of the powders maintained the Th2Zn17-type structure in the wide range for 0 3. The Sm2(Fe0.95Mn0.05)17N5 powder proved to have a coercivity of 0.62 MA/m (7.7 kOe), without a fine grinding. Microstructural analysis by transmission electron microscopy and energy-dispersive x-ray analysis revealed the powder consisted of Sm2(Fe,Mn)17N3 nanocrystalline grains of 10–30 nm in diameter surrounded by Mn-enriched amorphous phase.

Effect of mechanical grinding and mechanical alloying on Sm2Fe17

Abstract Mechanical grinding (MG) in Ar and mechanical alloying (MA) in NH 3 have been used to prepare Sm 2 Fe 17 N x permanent magnets, where three different kinds of alloy powders, i.e. Sm 2 Fe 17 , Sm 2 Fe 17 C and Sm 2 Fe 17 C 2 , were used as the starting materials. The heat-treated (1023 K-1.8 ks in Ar) Sm 2 Fe 17 specimen after the MG exhibited the α-Fe peak in addition to the Sm 2 Fe 17 peak. The similarly heat-treated Sm 3 Fe 17 specimen, however, consisted of the single Sm 2 Fe 17 phase, and it was confirmed that the nitrided (723 K-21.6 ks in N 2 ) specimen after the MG and heat treatment had a coercivity higher than 1.2 MA m −1 . Nitriding has been carried out thoroughly for all the three specimens treated by the MA process in NH 3 .

Phase relation and thermoelectric properties of the ternary lanthanum chalcogenide system La-A-S (A=Ca, Ba)

Abstract The electrical resistivity, Seebeck coefficient and thermal conductivity of La3−xS4, La3−yAyS4 and La2AzS3 (A=Ca, Ba) with the Th3P4-type structure have been measured as a function of the composition and temperature. The carrier concentration of these systems can be controlled by the content of x, y and z. With an increase of x and y, or with a decrease of z, the electrical resistivity and Seebeck coefficient increase, while the thermal conductivity decreases. The increase of the electrical resistivity and Seebeck coefficient is due to the decrease of the carrier concentration. For low carrier concentration, the thermal conductivity is mainly governed by the lattice contribution, while for high carrier concentration the carrier contribution dominates. The disordering of the arrangement of the atoms in the crystal lattice accompanied by the substitution or the insertion may affect the phonon scattering, resulting in decreasing the thermal conductivity. The samples with lower carrier concentration show a large figure of merit. La2.27Ca0.73S4 has a maximum figure of merit of 2.9×10−4 K−1 at about 1000 K.

Thermoelectric properties of β-FeSi2 with B4C and BN dispersion by mechanical alloying

The B4C- and BN-dispersed β-FeSi2 thermoelectric materials were synthesized by mechanical alloying and subsequent hot pressing. The effects of the B4C and BN dispersion on the thermoelectric properties, such as Seebeck coefficient, electrical resistivity and thermal conductivity etc., of the β-FeSi2 were investigated. For the sample with B4C and BN addition, a larger amount of the residual ∈ phase was detected in the X-ray diffraction patterns than the sample without addition. In the case of the BN addition, the Seebeck coefficient was enhanced by BN addition above 700 K, and the electrical resistivity also increased with increasing amount of BN. This is considered to result from doping of a small amount of B into the β phase due to partial decomposition of the BN phase. The fine dispersion of BN particles in the β phase matrix was quite effective for reducing the thermal conductivity as compared to the B4C addition over the entire temperature range. The figure of merit, Z, of the β-FeSi2 was significantly enhanced by BN addition.