Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Recently, we found a reliable high-ZT thermoelectric oxide, Ba_1/3CoO₂. The crystal structure and electrical resistivity of the Ba_1/3CoO₂ epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ~1.2 mW m⁻¹ K⁻² and the thermal conductivity gradually decreased to ~1.9 W m⁻¹ K⁻¹ with increasing temperature up to 600 °C. Consequently, the ZT reached ~0.55 at 600 °C in air, the highest among oxides and comparable to that of p-type PbTe and p-type SiGe.

  Mg₂Sn has attracted attention as one of the efficient thermoelectric (TE) materials operating at 400 K~800 K. To improve its TE performance, Mg₂Sn single crystals (SCs) containing lattice defects were prepared. The lattice defects in Ga-doped and Li-doped Mg₂Sn SCs were investigated by single-crystal X-ray diffraction measurements and transmission electron microscope observations. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the Ga-doped and Li-doped Mg₂Sn SCs were also measured. The lattice defects less affected the hole carrier transport, whereas they scattered phonons effectively. The dimensionless figure-of-merit zT of a Li-doped Mg₂Sn SC was higher than that of a Li-doped Mg₂Sn polycrystal, demonstrating that the potential of SCs with lattice defects can be a good candidate for TE applications.

  Tin-based Zintl compounds with a crystal structure containing guest atoms of Na in tunnel space were synthesized, and their thermal and electrical properties were characterized in order to develop innovative thermoelectric materials. Experiments and calculations revealed that the compounds with rattling Na atoms in the tunnel space exhibit extremely low thermal conductivity and high thermoelectric properties and that the lattice thermal conductivity decreases with decreasing Na–Na interatomic distance. This clarified the mechanism by which the closeness of the rattling Na atoms increases the anharmonicity of phonons responsible for heat conduction, resulting in lower lattice thermal conductivity.

  Borides are prospective candidates for high temperature thermoelectric applications owing to their attractive properties such as high melting point, excellent hardness, and so on. Especially, the rare earth boron cluster compounds, RAlB₁₄ and RB₂₂C₂N, were found to exhibit good thermoelectric properties at high temperature range and n-type semiconducting behaviors, and thus they are expected to be applicable as n-type elements in a new-type boride-based thermoelectric generator for high temperature applications. In the conventional processes for synthesis of these materials, however, complicated processes involving time-consuming and multiple re-sintering steps are required. This article reviews new synthesis processes for polycrystalline Y_xAl_yB₁₄ and YB₂₂C₂N bulk samples via spark plasma sintering utilizing liquid phase-assisted sintering and gas/solid reaction technology. These newly developed synthesis routes could facilitate the rapid and cost-effective preparation of complex rare earth boron cluster compounds.

  This report addresses the multi-element strategy and nanostructuring for enhancing thermoelectric figure of merit zT of materials through reduction in lattice thermal conductivity. Double and triple half-Heusler alloys developed following a valence balance rule reduce lattice thermal conductivity. Huge compositional space in double and triple half-Heusler alloys is good place to explore high-zT thermoelectric and advanced functional materials. The nanoprecipitates embedded in lead telluride PbTe effectively scatter the heat-carrying phonons, reducing the lattice thermal conductivity. The large band offsets for minority carrier between PbTe and nanostructures result in the blocking of minority carrier transmission, boosting thermoelectric power factor. As a result, thermoelectric figure of merit zT of materials was dramatically enhanced by nanostructuring. The nanostructured PbTe-based thermoelectrics have begun to bridge from materials development to module construction. High-efficiency power generation was demonstrated in nanostructured PbTe-based thermoelectric modules.

  In this study, p-type Ba₈Cu₆Ge₄₀ and n-type Ba₈Cu₅Ga_xGe_41−x−yP_y (x=0, 1, 2, 3; y=0, 1) were synthesized by arc melting and spark plasma sintering processes, and their thermoelectric properties were investigated. p-type Ba₈Cu₆Ge₄₀ shows a relatively high Seebeck coefficient due to the many-valley effect in the valence band, and its thermoelectric figure of merit ZT is about 0.78 (about 720 K), which is higher than that of n-type Ba₈Cu₅Ga_xGe_41−x−yP_y. On the other hand, n-type Ba₈Cu₅Ga_xGe_41−x−yP_y has a lower electron concentration than the comparative material Ba₈Cu₅Ge₄₁ due to simultaneous Ga−P substitution, and ZT at x=2 is about 0.60 (about 670 K), which is higher than the previously reported value.

  Multinary materials have attracted considerable attention for energy applications due to their tunable physical properties. Thermoelectrics are an important class of technology that harvest electric power directly from heat sources. When designing both high performance nad environmentally friendly thermoelectric materials, the pseudo-cubic structure has great potential to maximize the dimensionless figure of merit. The thermoelectric multinary single crystal with a pseudo-cubic structure paves a new path toward manipulating valley degeneracy and anisotropy with low thermal conductivity caused by short-range lattice distortion. Here, I describe solution growth of multinary thermoelectric material Cu₂ZnSnS₄ bulk single crystal and its fundamental properties. The progresses in multinary materials will promote the use of thermoelectric technology to play a positive role in future energy solutions.