Shigeru Katsuyama

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...

Synthesis of NaxCo2O4 thermoelectric oxides by the polymerized complex method

Abstract The thermoelectric oxide Na x Co 2 O 4 was synthesized by the polymerized complex method, which provided a flaky powder. The sintered sample showed a high density, a fine microstructure and improved thermoelectric performance compared to a sample prepared by the conventional solid state reaction method.

Effect of NiSb on the thermoelectric properties of skutterudite CoSb3

The phase equilibrium and thermoelectric properties of Co1−xNixSb3 ternary system were examined up to high nickel content x=0.40. NiSb compound with nickel arsenide structure was observed as the impurity phase in the samples with x>0.10 by x-ray diffraction. The sign of the Seebeck coefficient of Co1−xNixSb3 is negative, and the absolute value of the Seebeck coefficient and electrical resistivity generally decreases with increasing x. The thermal conductivity is reduced in the samples with x⩽0.06, but above x=0.06 it increases with increasing x. These behaviors of the thermoelectric properties in the samples with low nickel content are ascribed to the substituted Ni, while those in the samples with high nickel content are ascribed to the precipitated NiSb compound. In order to more reduce the thermal conductivity of the material, we have prepared a sintered CoSb3–NiSb composite where the NiSb particles are dispersed in the CoSb3 matrix by mechanical grinding and hot pressing. The thermal conductivity of t...

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.

The first magnesium–chromium hydride synthesized by the gigapascal high-pressure technique

Abstract The occurrence of new magnesium-based ternary hydrides of chromium was investigated by using the gigapascal high-pressure thermal technique. Powders of MgH 2 and Cr in the molar relation of 3:1 were compressed at a pressure up to 8 GPa by using a multi-anvil machine, and then heated to 873 K. The resultant samples were studied with X-ray powder diffraction and thermal desorption spectroscopy. A new phase, Mg 3 CrH x , was discovered under hydrogen pressure at 6–8 GPa. The hydrogen desorption was observed at around 598 K for the samples containing this new phase.

Thermoelectric properties of (Na1-yMy)xCo2O4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01∼0.35)

Abstract Polycrystalline samples of (Na 1− y M y ) x Co 2 O 4 (M=K, Sr, Y, Nd, Sm and Yb; y =0.01∼0.35) were prepared by a solid state reaction method. In this study, in order to improve the thermoelectric properties of Na x Co 2 O 4 , the effects of partial substitution of other metals for Na on the thermoelectric properties of Na x Co 2 O 4 from room temperature to 1073 K were investigated. For M=Sr, the thermoelectric power and the electrical resistivity increased, and the electronic and lattice contribution to the thermal conductivity decreased compared to the non-substituted sample. These effects suggest that the carrier density was reduced by the substitution of Sr for Na. As a result, the figure of merit of the sample for M=Sr was improved. On the other hand, for other samples in spite of the increase in the electrical resistivity, the thermoelectric power decreased. These results are anomalous effects, which cannot be described merely by a change of the carrier density. For all samples, except for M=Y, the lattice contribution to the thermal conductivity decreased and for all samples, except for M=K, the electronic contribution slightly decreased.

Effects of P doping on the thermoelectric properties of β-FeSi2

The effects of P substitution for Si as an n-type dopant on the thermoelectric properties of hot-pressed β-FeSi2 were investigated. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the FeSi2−xPx were measured from room temperature to 1100 K, and then the power factor and figure of merit were evaluated. The Seebeck coefficient of the hot-pressed FeSi2−xPx was negative, indicating that P atoms were definitely substituted for Si atoms as an n-type dopant in the β phase. The samples with x=0.02 and 0.04 had a Seebeck coefficient greater than that of the conventional hot-pressed Fe0.98Co0.02Si2 below 800 K. The electrical resistivity was significantly reduced by P doping, especially in the lower temperature range, and slightly decreased with increasing P content. The log ρ−1/T plots of the P-doped samples exhibited a specific behavior below 480 K, which was not observed in the case of the nondoped sample. The thermal conductivity of the P-doped sample was smaller than that of the no...

Effects of Ti, Nb and Zr doping on thermoelectric performance of β-FeSi2

Abstract The effects of Ti, Nb and Zr doping on the thermoelectric performance of a hot-pressed β-FeSi2 were investigated. Ti doping did not have a significant influence on the thermoelectric properties. Nb doping decreased the electrical resistivity over the entire temperature range. The thermoelectric power of the Nb-doped samples showed lower values below 800 K and higher values above this temperature than those of the non-doped sample. The figure of merit of the Nb-doped samples showed higher values in a high temperature range as compared to the non-doped sample. In the case of the Zr-doped samples, the thermoelectric power was markedly deteriorated below 900 K. In contrast, in the higher temperature range, the thermoelectric power was enhanced by Zr-doping. The electrical resistivity was significantly decreased by Zr doping. The hot-pressed Fe0.94Zr0.06Si2 showed around 20 μΩm over the entire temperature range. These values of electrical resistivity are extremely low as compared to the β-FeSi2 doped with other elements, particularly in a low temperature range. The figure of merit for the Zr-doped samples was significantly enhanced in a high temperature range. The maximum figure of merit, 0.67×10−5/K, was obtained at 1064 K for Fe0.94Zr0.06Si2, which was ten times larger than that of the non-doped sample at the same temperature. Thus, it is evident that Zr doping is quite effective for enhancement of the performance of β-FeSi2 in a high temperature range.

Thermoelectric properties of Fe0.98Co0.02Si2 with ZrO2 and rare-earth oxide dispersion by mechanical alloying

The n-type Co-doped β-FeSi2 (Fe0.98Co0.02Si2) with dispersion of several oxides, such as ZrO2 or several rare-earth oxides (Y2O3, Nd2O3, Sm2O3 and Gd2O3), was synthesized by mechanical alloying and subsequent hot pressing. The effects of these oxide dispersions on the thermoelectric properties of Fe0.98Co0.02Si2 were investigated. ZrO2 was decomposed in the β phase, and the ZrSi and e-FeSi phases, which are metallic phases, were formed in the samples with ZrO2 addition. The Seebeck coefficient and the electrical resistivity were significantly decreased with increasing amount of ZrO2, indicating that a part of the Zr atoms was substituted for Fe atoms in the β phase. In the case of the samples with rare-earth oxide addition, a decomposition of a large amount of these added oxides did not occur. However, the rare-earth oxide addition caused a slight increase in the amount of the e phase. The Seebeck coefficient was significantly enhanced by the rare-earth oxide addition especially in the low temperature range. These facts indicated that a small amount of rare-earth oxides was decomposed in the β phase, and rare-earth elements were substituted for Fe atoms as a p-type dopant, resulting in the decrease in the carrier concentration. The rare-earth oxide addition was also effective in reducing the thermal conductivity.