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 < 400 K, where a structure phase transition takes place. The metallic behavior of 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.
Ternary compounds with a tetragonal chalcopyrite structure, such as CuGaTe 2 , are promising thermoelectric (TE) materials. It has been demonstrated in various chalcopyrite systems, including compounds with quaternary chalcopyrite-like structures, that the lattice parameter ratio, c/a, being exactly 2.00 to have a pseudocubic structure is key to increase the degeneracy at the valence band edge and ultimately achieve high TE performance. Considering the fact that ZnSnSb 2 with a chalcopyrite structure is reported to have c/a close to 2.00, it is expected to have multiple valence bands leading to a high p-type zT. However, there are no complete investigations on the high temperature TE properties of ZnSnSb 2 mainly because of the difficulty of obtaining a single-phase ZnSnSb 2 . In the present study, pure ZnSnSb 2 samples with no impurities are synthesized successfully using a Sn flux-based method and TE properties are characterized up to 585 K. Transport properties and thermal analysis indicate that the structure of ZnSnSb 2 remains chalcopyrite with no order− disorder transition and clearly show that ZnSnSb 2 can be made to exhibit a high zT in the low-to-mid temperature range through further optimization.
Filled skutterudites are known as excellent thermoelectric (TE) materials. The voids in the structure of skutterudites such as cobalt antimonide (CoSb3) can be filled or partially filled with a variety of different atoms. In the present study, we tried to fill both gallium (Ga) and indium (In) into the voids of CoSb3. The polycrystalline samples with nominal compositions of Ga0.2InxCo4Sb12 (x = 0.15, 0.20, 0.25, and 0.30) were prepared and their TE properties were examined from room temperature to 773 K. Ga and In added to CoSb3 partly filled the voids and most of the residual precipitated as nanoparticles at the grain boundaries. A tiny amount of Ga substituted for Sb at the Sb site of CoSb3, while it was more likely for In to fill the voids than Ga. All the samples exhibited negative values for the Seebeck coefficient. The lattice thermal conductivity was reduced effectively by the co-addition of Ga and In without degrading the power factor. The maximum dimensionless figure of merit (ZT) value of 0.95 at 725 K was obtained for the sample with the nominal composition Ga0.2In0.3Co4Sb12.
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