Atsuko Kosuga

Ag9TlTe5: A high-performance thermoelectric bulk material with extremely low thermal conductivity

We studied a high-performance thermoelectric material whose chemical formula is Ag9TlTe5. Ag9TlTe5 is simple and easy to prepare. Its highest dimensionless figure of merit (ZT) value is 1.23, obtained at 700K. The values of individual thermoelectric properties at 700K are 2.63×10−4Ωm for electrical resistivity, 319μVK−1 for Seebeck coefficient, and 0.22Wm−1K−1 for thermal conductivity. Ag9TlTe5 is a unique material combining extremely low thermal conductivity and relatively low electrical resistivity.

Thermoelectric properties of Tl9BiTe6

Abstract Polycrystalline-sintered samples of Tl9BiTe6 have been prepared by hot pressing, and the thermoelectric properties have been measured in the temperature range from room temperature to about 700 K. The electrical resistivity is about 10 times higher than those of state-of-the-art thermoelectric materials, and shows a positive temperature dependence. The Seebeck coefficient is positive in the whole temperature range, showing p-type semiconductor characteristics. The maximum value of the power factor of the sample is about 6.2×10−4 W m−1 K−2 at 590 K. The thermal conductivity is extremely low comparable to those of state-of-the-art thermoelectric materials. The maximum value of the thermoelectric figure of merit ZT is 0.86 at about 590 K.

Thermoelectric properties of TlBiTe2

Abstract Polycrystalline samples of TlBiTe2 have been prepared and the thermoelectric properties, such as the electrical resistivity and Seebeck coefficient, have been measured. The electrical resistivity of TlBiTe2 is slightly higher than that of state-of-the-art thermoelectric materials, and has a positive temperature dependence. The Seebeck coefficient of TlBiTe2 is negative in the whole temperature range, showing n-type semiconductor behavior. The thermal conductivity of TlBiTe2 is relatively low compared with that of state-of-the-art thermoelectric materials. The maximum value of the thermoelectric figure of merit ZT is 0.15 at about 760 K.

High-temperature thermoelectric properties of Ca0.9−xSrxYb0.1MnO3−δ (0≤x≤0.2)

Polycrystalline samples of Ca0.9−xSrxYb0.1MnO3−δ (x=0, 0.025. 0.05, 0.1, and 0.2) were prepared by a conventional solid-state reaction and their thermoelectric properties were evaluated at 303–973 K. Each of the samples consisted of a single phase with an orthorhombic structure. All the samples showed a metallic conductivity and their electrical resistivity was markedly affected by the distortion of the MnO6 octahedron. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers were electrons over the entire temperature range examined. The highest power factor achieved (0.22 mW m−1 K−2 at 773 K) was shown by the sample with x=0.1. The thermal conductivity was affected by both the crystal distortion and the difference in mass between the Ca2+ and Sr2+ ions. The highest dimensionless figure of merit obtained was 0.09 at 973 K for the sample with x=0.1; this is a result of its low electrical resistivity and its moderate Seebeck coefficient and thermal conductivity.

Thermoelectric properties of Tl–X–Te (X=Ge, Sn, and Pb) compounds with low lattice thermal conductivity

We prepared polycrystalline-sintered samples of Tl2GeTe3, Tl4SnTe3, and Tl4PbTe3 and evaluated their thermoelectric properties. Although the electrical properties of these compounds were not optimized, the dimensionless figure of merit ZT was relatively high, i.e., 0.74 at 673K for Tl4SnTe3, 0.71 at 673K for Tl4PbTe3, and 0.29 at 473K for Tl2GeTe3, due to the very low lattice thermal conductivity of the compounds. Low lattice thermal conductivity appears to be closely related to the weak bonding of atoms and complex crystal structures of these compounds.

Thermophysical properties of Tl9BiTe6 and TlBiTe2

Abstract Polycrystalline samples of Tl9BiTe6 and TlBiTe2 were prepared by melting stoichiometric amounts of component elements in sealed quartz crucibles. At the temperatures of 400–600 K, the heat capacity of TlBiTe2 was measured using an enthalpy method by means of a differential scanning calorimeter (DSC). An empirical equation for the heat capacity CP of TlBiTe2 was obtained by fitting the experimental data, CP(J K−1 mol−1)=79.2+8.27×10−2T+1.37×105T−2. Other thermophysical properties of Tl9BiTe6 and TlBiTe2, viz., melting point, thermal expansion coefficient, elastic moduli, Debye temperature, and thermal diffusivity were also measured, and the relationships between these properties were studied. The measured heat capacity of TlBiTe2 was compared with the estimate one obtained from the Debye temperature, thermal expansion coefficient, and compressibility. The thermal conductivity of TlBiTe2 was evaluated from the thermal diffusivity, heat capacity, and density.

Thermoelectric properties of Mo3Te4

Abstract The thermoelectric properties of Mo3Te4 are evaluated in the temperature range from room temperature to about 1000 K. The elastic constants and Debye temperature of Mo3Te4 are also evaluated from sound velocity measurements made by the ultrasonic pulse-echo method. The relationships between several properties of Mo3Te4 are studied. The electrical resistivity of Mo3Te4 is slightly lower than that of state-of-the-art thermoelectric materials. The absolute values of the Seebeck coefficient are relatively low, increase with increasing temperature, and reach a maximum value of about 25 μV K−1 at 1000 K. The thermal conductivity of Mo3Te4 increases gradually with increasing temperature, showing metallic behavior. The maximum value of the dimensionless figure of merit is 0.015 at about 1000 K.

Thermoelectric properties of Chevrel phase Mo6Te8−xSx

The series of Chevrel phases, Mo6Te8−xSx (x=0, 1, 2), was prepared and the thermoelectric properties such as the electrical resistivity, Seebeck coefficient, and thermal conductivity were measured. Sulfur substitution increases the electrical resistivity and decreases the absolute value of the Seebeck coefficient of Mo6Te8−xSx. The Seebeck coefficients of all samples are positive in the whole temperature range, showing that the majority of charge carriers are holes. The thermal conductivities of Mo6Te8−xSx are lower than those of Mo3Te4, which is caused by the enhancement of phonon scattering due to the substitution of sulfur for tellurium. The thermoelectric figure of merit, ZT is not enhanced by the substitution of sulfur, and the maximum value of ZT is 0.015 at about 1000 K for Mo3Te4.

Physical properties of Mo6-xRuxTe8 and Mo6Te8-xSx

Abstract A series of the Chevrel phases, Mo 6− x Ru x Te 8 and Mo 6 Te 8− x S x ( x =0, 1, 2), has been prepared and the various physical properties, such as the elastic modulus, Debye temperature, and electrical resistivity, have been evaluated. The relationships between several properties of the compounds have also been studied. Young’s modulus and Debye temperature of Mo 6− x Ru x Te 8 and Mo 6 Te 8− x S x increase with increasing x value. The relationship between the Vickers hardness and Young’s modulus shows ceramic characteristics for Mo 6− x Ru x Te 8 , while they show glass-like characteristics for Mo 6 Te 8− x S x . The electrical resistivities of Mo 6− x Ru x Te 8 and Mo 6 Te 8− x S x increase with increasing x value.

Thermoelectric Properties of Polycrystalline Ca0.9Yb0.1MnO3 Prepared from Nanopowder Obtained by Gas-Phase Reaction and Its Application to Thermoelectric Power Devices

Ca0.9Yb0.1MnO3 nanopowder prepared by a gas-phase reaction (GPR) consisted of well-dispersed particles with an average diameter of 47 nm. Sintering of this GPR powder proceeded rapidly and at a lower temperature than that required for a comparable powder prepared by conventional solid-state reaction (SSR). The sintered bulk material from the GPR powder (GPR-bulk) consisted of small grains with an average diameter of 620 nm; this morphology is completely different to that of the SSR-bulk in which larger grains bind together to form a network-like structure. A maximum power factor of 0.19 mWm-1K-2 was obtained for GPR-bulk at 973 K; this value is higher than that of SSR-bulk, mainly as a result of the lower electrical resistivity of GPR-bulk. The thermal conductivity of GPR-bulk is also lower than that of SSR-bulk, possibly because of increased phonon scattering at the grain boundary. The maximum value of the dimensionless figure of merit of 0.13 was obtained for GPR-bulk at 1073 K; this value is about 1.5-fold higher than that for SSR-bulk at 773 K. A unicouple device consisting of a p-type Ca2.7Bi0.3Co4O9 leg and an n-type Ca0.9Yb0.1MnO3 (GPR-bulk) leg was fabricated. Both oxide legs used for the measurement are 3.1–3.5 mm in both width and thickness and ~5 mm in height. The device generated up to 0.14 W of power when the hot- and cold-side temperatures at the ends of the oxide legs were 1095 and 390 K, respectively.