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Optimization of the carrier concentration by doping/alloying is a key prerequisite for improving thermoelectric performance. In reality, it is, however, challenging to identify effective dopants in some specific systems. Herein, we discovered that Y doping significantly reduced the carrier concentration, enhanced the power factor, and, therefore, boosted thermoelectric performance. The high Y-doping efficiency had origin in the fact that the compensating effect due to Ge vacancies was weaker in comparison to other dopants in GeTe because the slightly increased rhombohedral interaxial angle suggested that the Ge-vacancy concentration would not be altered dramatically. In addition, first-principles calculations revealed that Y doping did not induce significant modifications on the band structure, consistent with electrical transport measurement results. Moreover, Bi doping was found to be more powerful than Sb doping to further increase electrical resistivity and to enhance performance in which the highest peak ZT ∼ 1.8 together with a high average ZT ∼ 1.1 from 300 K to 723 K was achieved. Our work not only highlighted the efficacy of Y doping in enhancing the thermoelectric performance of the GeTe system but also developed advanced GeTe-based materials for power generation applications. [ABSTRACT FROM AUTHOR]

p -type TlSbTe 2 and its In- and Yb-doped variants were prepared by hot pressing of the elements in the stoichiometric ratios, and their thermoelectric properties were determined after verifying the purity of the samples by powder X-ray diffraction. Theoretically, TlSbTe 2 is an intrinsic semiconductor possessing a narrow band gap; however, the as-prepared TlSbTe 2 exhibited relatively high electrical conductivity, decreasing with ascending temperature, which provides evidence for the presence of p -type extrinsic charge carriers (holes) along with the positive Seebeck coefficient. Doping In onto the Tl site inserts more electrons, thereby decreasing the hole concentration; as a result, the electrical and thermal conductivity decreased and the Seebeck coefficient increased. The best thermoelectric performance of In-doped TlSbTe 2 was achieved at 625 K, with a figure of merit (zT) of 0.77. The overall best zT was improved to 0.85 at 620 K with the sample of nominal composition Tl 0.98 SbYb 0.02 Te 2. An anisotropy test was implemented by comparing the electrical conductivity and Seebeck coefficient measured on the prismatic bars cut parallel and vertical to the pressing direction. The results indicate basically isotropic behavior in polycrystalline TlSbTe 2 along these two orthogonal directions. • TlSbTe2 is bestowed with an extraordinarily low thermal conductivity. • Our processing conditions caused an increase in the figure-of-merit by 15%. • Doping with Yb caused a further increase by an additional 20%, for a total increase of 39%. • Doping with In led to no significant changes in performance. • Direction-dependent measurements revealed no significant anisotropies in the electrical properties. [ABSTRACT FROM AUTHOR]

A series of samples with the initial formula Mo 3– x Fe x Sb 7 and Mo 3– x Ni x Sb 7 have been prepared by solid state reaction at 873 K, starting from the elements in the stoichiometric ratios. The materials were compressed via hot-pressing, and their thermoelectric properties determined. Single crystal data show that Ni atoms in part occupy the cubic voids of the Mo 3 Sb 7 structure, as well as replace some Mo atoms, in contrast to the Fe analogues, where we found no evidence for Fe in the cubic voids. Supported by electronic structure calculations, replacing Mo with Fe or Ni increases the valence-electron count, thus decreases the numbers of p -type carriers of Mo 3 Sb 7 . On the other hand, filling the holes with Ni atoms appears to have no such effect. As a consequence, the Ni containing samples exhibit higher electrical conductivity than the Fe analogues, and ultimately lower thermoelectric figure-of-merit, as the p -type carrier concentration remains too high. [ABSTRACT FROM AUTHOR]

First, a large sample of Ni0.05Mo3Sb5.4Te1.6 was prepared by heating the elements in the stoichiometric ratio. β-SiC nanoparticles were added in volume fractions, f, of 0.01, 0.02, and 0.034 to three different portions of the material, and a portion without SiC nanoparticles was retained as a reference. All four samples were subjected to consolidation by hot-pressing. Furthermore, Al2O3 nanoparticles were added in volume fractions of f = 0.01, 0.0216, and 0.0325 to three other portions, again retaining a fourth protion as reference. These four samples were consolidated by spark-plasma sintering. The thermoelectric transport properties of these composites were characterized from 325 to 740 K. For the sample with 0.01 volume fraction of SiC, there was an enhancement in figure of merit by 18 % compared with the reference sample, mainly due to an 18 % reduced thermal conductivity. The 9 % reduction in thermal conductivity of the sample with 0.01 volume fraction of Al2O3 was not enough to compensate the loss in the power factor of 18 %, leading to a decrease in the figure of merit. Microstructural information obtained by SEM, TEM, and BET was used to elucidate the phase and transport properties. The spark-plasma-sintered bulk sample has a figure of merit that is 35 % higher than the bulk sample consolidated through hot-pressing. [ABSTRACT FROM AUTHOR]

Abstract: Various materials of the formulas Mo3−x Fe x Sb7 and Mo2.6Fe0.4Sb7− y Te y have been prepared by solid state synthesis at 873K, starting from the elements in the stoichiometric ratios. The Fe content can be as high as x=0.45, while attempts to incorporate 0.5 Fe lead to the formation of FeSb2 as a minor side product. At x=0.4, the Te content can vary within 0≤y≤0.75, while the sample with y=1.0 shows FeSb2 as a minor phase. The materials were densified via hot-pressing, and their thermoelectric properties determined. Increasing both the Fe and the Te content leads to a decrease in the p-type carrier concentration, and thus an increase in Seebeck coefficient and a decrease in both electrical and thermal conductivity. In the end, the best thermoelectric properties were obtained in case of Mo2.6Fe0.4Sb6.25Te0.75, i.e. at the edge of the phase range, culminating in a figure-of-merit of zT=0.34 at 800K. [Copyright &y& Elsevier]