No abstract

( Zn 1−y Mg y ) 1−x Al x O 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 Al27 nuclear magnetic resonance spectroscopy. At a fixed composition of x, the absolute values of the Seebeck coefficient and electrical resistivity for the samples prepared by the polymerized complex method are smaller than those for the samples prepared by the solid-state reaction method, which indicates that the doping of the carrier into the material can be more easily realized in the samples prepared by the polymerized complex method. The thermal conductivity decreases with increasing x, but the further suppression of the thermal conductivity was attained by the additional substitution on the Zn site by Mg. The Seebeck coefficient of (Zn1−yMgy)1−xAlxO is almost independent of Mg content y, but the electrical resistivity increases with increasing y. As a result, (Zn0.90Mg0.10)0.9975Al0.0025O shows a maximum dimensionless figure of merit of 0.10 at 1073 K.

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 result, Co0.75Fe0.25Sb3 has a maximum value of figure of merit of 4.8×10−4 K−1 at 773 K. This value is much larger than the maximum value for CoSb3 3.2×10−4 K−1 at 482 K.

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 nondoped sample in spite of the larger amount of the metallic ε phase. The figure of merit was significantly enhanced by P doping as compared with that of the nondoped β-FeSi2, and a high thermoelectric performance, almost the same as that of the conventional Fe0.98Co0.02Si2, was obtained by P doping into β-FeSi2.

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.