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Determination of thermoelectric properties is rather a challenging task, especially during a phase transition, where a drastic change in magnitude of thermoelectric properties is generally observed. In this study, we performed a precise measurement of the Seebeck coefficient on Cu2Se, which was recently reported to possess a colossal Seebeck coefficient with metallic electrical conductivity during a structural phase transition. We confirmed that the colossal Seebeck effect was stable over a long period of time, as high as −0.9 mV under an applied temperature gradient of 1 K. This fact clearly proved that the colossal Seebeck effect observable in metallic Cu2Se is potentially usable for a variety of thermoelectric applications. more...

Materials best-suited for direct application exhibit a high thermoelectric figure of merit (ZT) close to unity, from room temperature to ∼ 400 K. In this work, we investigated the thermoelectric behavior of Ag2Se, and a sulfur substituted Ag2Se system, i.e. Ag2S0.2Se0.8 and Ag2S0.4Se0.6 , in the temperature range of 300 K to 500 K. With strong anharmonic lattice vibration and semiconducting electronic structure, these materials showed thermal conductivity less than 1 W m−1 K−1, electrical resistivity ∼ 1 mΩ cm, together with moderate Seebeck coefficient values of ∼ − 140 μV K−1 in the low-temperature phase. We were able to achieve ZT equal to unity in a wide temperature range of 350–400 K, with a maximum value of ZT = 1.08 at 350 K for the Ag2S0.4Se0.6 material. more...

In this study, the effect of the grain boundary density on the transport properties of the Re-substituted higher manganese silicide Mn30.4Re6Si63.6 has been investigated. The efficiency of electric... more...

We investigated the $5d$ element substitution effect on the thermal conductivity in $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattice thin films epitaxially grown on a MgO (100) substrate. We found unique crossover behavior in the period dependences of thermal conductivity for Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$ and $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices. The $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices with periods more than 40 nm appeared to have much lower thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattices due to the substitution of V by Ta. Unexpectedly, at a shorter periodic length, less than 20 nm, $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices appeared to have higher thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattices despite such a heavy element substitution by Ta. This surprising experimental fact was well accounted for with theoretical calculations, which predicted the dominant contribution of phonons having a shorter mean free path in the Ta substituted samples. more...

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m−1 K−2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K−1 in a narrow temperature range, 340 K T σ possessing larger magnitude exceeding 600 S cm−1 leads to a colossal value of S2σ = 2.3 W m–1 K–2. The small thermal conductivity less than 2 W m−1 K−1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase. more...

The thermal transport properties of $M/\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}/M$ ($M=\mathrm{Mo}$, W) multilayer thin films, which were epitaxially grown on the [100] MgO single-crystal substrates by means of a radio-frequency magnetron sputtering system, were systematically investigated using a picosecond pulsed laser heating time-domain thermoreflectance method. Not only the thermal diffusivity but also the boundary thermal resistance was strongly affected by both the Ta substitution for V in $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$ and the elements in the metallic layer of top and bottom sides. From the first-principles density-functional theory calculations, we found that such changes in heat transport properties were directly related to the intrinsic phonon dispersion. In the $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$ layer thickness region of less than 14 nm, the boundary thermal resistances of all thin-film series abruptly decreased with decreasing the thickness, which was due to the coherently ordered interfacial structure. more...

We investigated the thermoelectric properties and electronic band structure calculation of Sn1−xAgxTe and Sn1.03−xAgxTe (x = 1, 3, 5, 7 mol%) compounds. From the formation energy calculation, the Sn vacancy by Ag doping gives rise to the increase of carrier density as well as the decrease of lattice parameters. There are several possibilities of polar point defects and various neutral Ag complex defects (linear, orthogonal, square type defects etc.) which compensate the increase of charge carriers. The carrier scattering by Ag-related complex defects reduces Hall mobility, resulting in the decrease of electrical conductivity in spite of the increase of Hall carrier density by Ag doping. We do not observe any resonant level formation from the electronic band structure calculation. The change of the scattering exponent by an Ag complex defect (AgSn–AgTe–AgSn) may be associated with the increase of the Seebeck coefficient. Even if the defect scattering reduces electrical conductivity, the increase of the Seebeck coefficient and reduction of lattice thermal conductivity by Ag complex defects result in the enhancement of ZT value 0.62 at 800 K for the Sn-excess Sn0.96Ag0.07Te compound. It indicates that the Sn vacancy and Ag complex defects are associated with the enhancement of thermoelectric figure-of-merit in SnTe based compounds. more...