The effects of Cr and Mo contents on the properties of hot isostatic pressed (HIPed) materials with the compositions of Ni-X%Cr-Y%Mo-1.5%B (mass%) ((X,Y)=(10,36),(22,26),(34,16)) were investigated. Raw powders for these P/M materials were produced by gas atomization, followed by sieving less than 500 μm. The consolidation by HIP was carried out at 1423 K for 5 h under 147 MPa. Crystal structures and microstructures of these materials were investigated by X-ray diffraction analysis and scanning electron microscopy, respectively. Mechanical properties of Rockwell hardness and bending strength, corrosion resistance by immersion tests into various acid solutions and wear resistance by Ohgoshi-type abrasion test were also evaluated.
Tetragonal M₃B₂ and M₅B₃ type complex borides were observed in the consolidated P/M materials. These complex borides with the size of approx. less than 5 μm were finely dispersed in Ni-based matrix. They showed Rockwell hardness from 40 to 50 HRC, which value increased with increasing Mo/Cr ratio, and bending strength comparable to that of P/M material with the composition of Co-1.6%C-30%Cr-8%W (ST-12). In addition, the present Ni-22%Cr-26%Mo-1.5%B P/M material with the most excellent mechanical properties revealed higher corrosion resistance than Ni-16%Cr-16%Mo-3.5%W-6.5%Fe (C-276) alloy and wear resistance comparable to ST-12.

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Phase relationships of Mn-substituted Ru₂Si₃ with the chimney-ladder structure have been investigated by using scanning and transmission electron microscopy. A series of chimney-ladder phases Ru_1-xMn_xSi_y are formed over a wide compositional range between Ru₂Si₃ and Mn₄Si₇. The c-axis dimension of the Si subcell increases with the Mn content accompanied with the increase in the Si/M ratio (y=Si/(Ru+Mn)). A large extent of compositional variation is observed in the directionally solidified chimney-ladder compounds. The interface between adjacent chimney-ladder phases with different Mn contents is considered to have no influence on the electrical properties but have some influence on the lattice thermal conductivity. The dimensionless figure-of-merit increases with the increase in the density of the interfaces.

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Polycrystalline homogeneous samples of Mn(Al_xSi_1-x)_2+δ solid solutions were synthesized by heating the compacts of Mn, NaAlSi and Si powder mixtures with molar ratios Mn:Al:Si=0.95:y:2.05-y (y=0.80, 0.85, 0.90) at 1173 K for 12 h, followed by rinsing the samples with alcohol and water. The chemical analysis of the samples revealed that Mn(Al_xSi_1-x)_2+δ solid solutions were formed with composition ranges of 0.37≤x≤0.40 and δ=0.04-0.08, and approximately 0.4 wt.% of Na was contaminated. The bulk sample of MnAl_0.80Si_1.25, which was prepared from the starting mixtures with y=0.85 and sintered at 1153-1178 K and 40 MPa for 10 min by the pulsed electric current sintering method, showed the highest thermoelectric figure of merit at 622 K (ZT=0.15).

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We investigated the thermoelectric properties of Ba₈Al₁₆Si₃₀ clathrate samples prepared by combining arc melting and spark plasma sintering methods under two different arc melting conditions. The x-ray diffraction, scanning electron microscopy, and energy dispersive analysis of x-ray studies indicate that Al composition is close to 15, which is higher than that of a single crystal reported, for both arc melting conditions. The carrier concentration of samples decreases to about 1×10²¹ cm^-3, which is lower than reported values so far in Ba₈Al₁₆Si₃₀ system. The Seebeck coefficient and the electrical conductivity alter slightly with the arc melting condition due to the slight difference in the carrier concentration. The lattice thermal conductivity (RT) is as low as about 1.3 Wm^-1K^-1, which is comparable to Bi₂Te₃-based bulk alloy, independently of the arc melting condition. The thermoelectric figure of merit ZT is about 0.4 at 900 K. An estimation indicates that ZT reaches 0.8 (1200 K) at the optimum carrier concentration of about 3×10²⁰ cm^-3 in Ba₈Al₁₆Si₃₀ system.

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In this review, I discuss the effect of spin and orbital degrees of freedom on thermopower in the transition metal oxides. The thermopower is nothing but the entropy flow along with the electric current. In the strongly correlated electron systems, the Coulomb interaction causes the enhancement of the entropy flow by spin and orbital degrees of freedom. I propose the new path to thermoelectrics on the strongly correlated electron systems.

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The effect of the planar defects on the electrical transport properties was investigated in the non-stoichiometric titanium oxide; TiO_2-x. TiO_2-x with rutile structure is known to have a semiconductor property and planar periodic structure with lattice defects of oxygen vacancies. TiO_2-x with point defects and planar defects are synthesized by the reduction of TiO₂ and the oxidation of TiO. The defects structure was estimated from the powder XRD patterns. The thermal conductivities were measured using laser flash technique. The electrical properties were measured using the van der Pauw method. The TiO_2-x with planar defects showed lower thermal conductivity than those with point defects. In contrast, the TiO_2-x with planar defects showed slightly higher carrier nobilities than those with point defects.

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The composite of (Bi_0.3Sb_0.7)₂Te₃ and ion liquid, BMImTFSI, was synthesized by the immersion of the ion liquid to the pores of the porous sintered (Bi_0.3Sb_0.7)₂Te₃, and its microscopic structure and thermoelectric properties were investigated. The Seebeck coefficient of the composite is smaller than that of the porous sintered (Bi_0.3Sb_0.7)₂Te₃, while the electrical resistivity of the composite is a little larger. The thermal diffusivity of the composite is smaller than that of the porous sintered (Bi_0.3Sb_0.7)₂Te₃, which seems to indicate that the interface between the ion liquid and the crystal grain of (Bi_0.3Sb_0.7)₂Te₃ can be effective for the enhancement of the phonon scattering. As a result, the dimensionless figure of merit ZT of the composite shows the smaller value than the porous sintered (Bi_0.3Sb_0.7)₂Te₃.

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In the present study, the effect of Cu doping into the Ga site on the thermoelectric properties of AgGaTe₂ with chalcopyrite structure was investigated. The samples of AgGa_1-xCu_xTe₂ (x=0, 0.5, and 0.1) were prepared by a solid-state reaction followed by hot pressing. The electrical resistivity, Seebeck coefficient and thermal conductivity were measured from room temperature to about 680 K. The Cu doping increased the carrier concentration, leading to enhancement of the dimensionless figure of merit ZT of AgGaTe₂. The maximum ZT value was 0.45 at 680 K obtained in AgGa_0.9Cu_0.1Te₂, which was approximately two times higher than that of stoichiometric AgGaTe₂.

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