Papers by Author: Katsuhiro Sagara

Abstract: The reaction behavior of CuAlO₂ during the annealing at high temperatures was investigated. The relationship between thermoelectric properties including electrical resistivity, excess oxygen doping and decomposition of CuAlO₂ was discussed. The evolution of CuAlO₂ compact during the annealing mainly included excess oxygen doping, decomposition of CuAlO₂ into CuAl₂O₄ and CuO and complete decomposition of CuAlO₂ as following pattern : CuAlO₂ → CuAlO_2+x → CuAlO₂+CuO+ CuAl₂O₄ → CuAl₂O₄+CuO. Electrical resistivity of the compacts was decreased with excess oxygen doping, but increased when CuO and CuAl₂O₄ were formed. Thermoelectric performance of the compacts was improved due to the excess oxygen doping.

Abstract: Analysis model of finite element method with a random distribution for thermoelectric composites was built. Thermoelectric properties including electrical resistivity, Seebeck coefficient and thermal conductivity of M/TiO_2–x (M = Cu, Ni, 304 stainless steel (304SS)) thermoelectric composites were investigated by the proposed model. Cu/TiO_2–x composite showed a large decrease in electrical resistivity while 304SS/TiO_2–x composite thermal conductivity was slightly increased. Calculated dimensionless figure-of-merit, ZT of Ni/TiO_2–x composite was higher than those of TiO_2–x and the other composites in a wide range of metal volume fractions because Ni has large absolute values of Seebeck coefficient, power factor and dimensionless figure-of-merit compared to the other two metals. It was found that power factor and dimensionless figure-of-merit of thermoelectric composites depended on the balance among electrical resistivity, thermal conductivity and Seebeck coefficient. The results revealed that it is important for M/TiO_2–x composites to choose suitable addition metal with high power factor and dimensionless figure-of-merit.

Abstract: In the present study, the TiO₂-Ti compacts with Magneli phases Ti_nO_2n-1 were fabricated using the mixed powder of TiO₂ powder and addition Ti powder by spark plasma sintering (SPS). The composition and the crystal types of Magneli phases Ti_nO_2n-1 were examined. The results showed that various Magneli phases Ti_nO_2n-1 (single or multi Magneli phases) with the composition of Ti_1+yO_2-x were obtained. The Magneli phases Ti_nO_2n-1 were formed in the transformation from the mother phase rutile TiO₂ to TiO with increase in Ti addition fraction. In addition, the thermoelectric properties of the sintered compacts were also measured. The electrical resistivity of the sintered compacts decreased with increase in Ti addition fraction. The thermoelectric performance of the sintered compacts was improved by the formation of Magneli phases Ti_nO_2n-1 with the composition of Ti_1+yO_2-x.