[en] BiCuSeO based materials have been reported as very promising thermoelectric materials, with thermoelectric figure of merit values that make them among the best p-type Pb-free polycrystalline thermoelectric materials, and they could be suitable for applications in thermal to electrical power conversion modules in a 300–650 °C temperature range. In this paper, we report on the phase stability and thermal stability of these materials under inert and oxidizing atmosphere and we discuss their actual potential for wide scale applications. We show that although they are stable in inert atmosphere up to 650 °C, with very slow volatilization rate, it is not the case under oxidizing atmosphere, with a quick oxidation starting at a moderate temperature. - Graphical abstract: BiCuSeO oxidation under air, starting below 300 °C. - Highlights: • Decomposition and oxidation mechanisms of BiCuSeO were studied. • BiCuSeO does not volatilize in inert atmosphere up to 650 °C. • BiCuSeO starts oxidizing for temperatures as low as 200 °C

[en] We report on the electrical and thermal transport properties of the Sn_1_−_xPb_xBi_2Te_4 series and we discuss the potential of these materials for thermoelectric conversion applications. From the evolution of the XRD patterns, we can confidently conclude that a complete solid solution exists between SnBi_2Te_4 and PbBi_2Te_4, with no miscibility gap. A crossover from p-type conduction in Sn-rich samples to n-type conduction in Pb-rich ones has been observed, with a transition between x=0.3 and 0.4. A concomitant increase of the electrical resistivity and of the Seebeck coefficient has been observed in the solid solution, which leads to almost constant values of the thermoelectric power factor. Moreover, the thermal conductivity is slightly reduced in the solid solution. The best figure of merit ZT values at room temperature have been observed for p-type Sn_0_._8Pb_0_._2Bi_2Te_4 with ZT=0.25 and for n-type Sn_0_._3Pb_0_._7Bi_2Te_4 with ZT=0.15. - Graphical abstract: Seebeck coefficient in (Pb/Sn)Bi_2Te_4 solid solution. - Highlights: • A complete solid solution exists between PbBi_2Te_4 and SnBi_2Te_4_. • A crossover between p-type and n-type is observed for 0.3< Pb fraction <0.4. • The lattice thermal conductivity is hardly reduced in the solid solution

[en] Entropic contributions to the stability of solids are very well understood and the mixing entropy has been used for forming various solids, for instance such as inverse spinels, see Nawrotsky et al., J. Inorg. Nucl. Chem. 29, 2701 (1967) [1]. A particular development was related to high entropy alloys by Yeh et al., Adv. Eng. Mater. 6, 299 (2004) [2] and Cantor et al., Mater. Sci. Eng. A 375-377, 213 (2004) [3] (for recent reviews see Zhang et al., Prog. Mater. Sci. 61, 1 (2014) [4] and Tsai et al., Mater. Res. Lett. 2, 107 (2014) [5]) in which the configurational disorder is responsible for forming simple solid solutions and which are thoroughly studied for various applications especially due to their mechanical properties, e.g. Gludovatz et al., Science 345, 1153 (2014) [6] and Lu et al., Sci. Rep. 4, 6200 (2014) [7], but also electrical properties, Kozelj et al., Phys. Rev. Lett. 113, 107001 (2014) [8], hydrogen storage, Kao et al., Int. J. Hydrogen Energy 35, 9046 (2010) [9], magnetic properties, Zhang et al., Sci. Rep. 3, 1455 (2013) [10]. Many unexplored compositions and properties still remain for this class of materials due to their large phase space. In a recent report it has been shown that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, see Rost et al., Nature Commun. 6, 8485 (2015) [11] these new materials were called ''entropy-stabilized oxides''. In this pioneering report, it was shown that mixing five equimolar binary oxides yielded, after heating at high temperature and quenching, an unexpected rock salt structure compound with statistical distribution of the cations in a face centered cubic lattice. Following this seminal study, we show here that these high entropy oxides (named HEOx hereafter) can be substituted by aliovalent elements with a charge compensation mechanism. This possibility largely increases the potential development of new materials by widening their (already complex) phase space. As a first example, we report here that at least one HEOx composition exhibits colossal dielectric constants, which could make it very promising for applications as large-k dielectric materials. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] Several devices able to measure the Seebeck coefficient have been devised over the years but most of these were designed to perform under vacuum or under inert atmosphere. We describe here a system designed for measuring thermoelectric power, electrical resistivity and differential conductance up to 1200 K, under the controlled partial pressure of oxygen including vacuum or reducing atmosphere for samples that are sensitive to oxygen, which allows the in situ investigation of the electrical properties of oxide materials and their phase diagram. The instrument we present here can also be used to ‘tune’ the properties of a compound through thermal treatment. This system is reliable and has been in use in our laboratory for more than three years. (paper)

[en] Highlights: • A new entropy-stabilized oxide with pyrochlore structure was synthesized. • Entropy stabilization was proved by XRD and EDS with a reversible phase transition. • The low thermal conductivity is potentially interesting for thermal barrier coating. -- Abstract: This article reports on a new entropy-stabilized oxide with the pyrochlore structure: Dy₂(Ti_0.2Zr_0.2Hf_0.2Ge_0.2Sn_0.2)₂O₇. The entropy-stabilization was demonstrated using X-ray diffraction and elemental mapping (SEM/EDS) by the existence of a reversible phase transition between a low-temperature multiphase and a high-temperature single-phase. Moreover, some physical properties were investigated: Dy₂(Ti_0.2Zr_0.2Hf_0.2Ge_0.2Sn_0.2)₂O₇ is paramagnetic (with µ_eff = 10.95 µ_B), it is a dielectric (resistivity of 8.0.10¹⁰ Ω m and a dielectric constant of 78 at room temperature) and it displays an amorphous-like thermal conductivity (1.4 W m⁻¹ K⁻¹, constant from room temperature to 900 °C), interesting for potential applications as thermal barrier coating.

[en] We present a fast Hall effect measurement system that can be used at high temperature. The use of a homogeneous high field permanent magnet in a Halbach configuration allows fast measurements in various DC and AC current fields with step and continuous measurement modes. The results are presented of measurements on platinum film at room temperature and Ge and BiCuSeO between 300 K and 650 K. (paper)

[en] The discovery of a 26 K superconductivity in the tetragonal iron-based pnictide LaFeAsO_1-xF_x (x = 0.05-0.12) has stimulated a tremendous search for superconductivity in a wide class of materials with similar structure. Beside superconductivity, promising thermoelectric properties have been discovered in this family in the cryogenic temperature range and in the p-type BiCuSeO system around 400-600 C. In this paper, we report on the effect of various substitutions on the low temperature properties of this system. Whatever the substitution, all samples revealed p-type, and no superconductivity was observed. Moreover, although some of these compounds exhibit very promising thermoelectric properties above room temperature, this is not the case in the low temperature range. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] The iron-oxypnictide compounds, recently reported as a new class of superconductors when appropriately doped, exhibit large Seebeck coefficients, of the order of -100 μV/K, while keeping good electrical conductivity. Their power factor shows a peak at low temperatures, suggesting possible applications of these materials in thermoelectric cooling modules in the liquid nitrogen temperature range. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Research highlights: Much attention has been paid on the layered rare-earth iron oxypnictides LnFePnO (Ln = La, Pr, Ce, Sm; and Pn = P and As) since superconductivity has been discovered at T_c = 26 K in LaFeAsO_0.89F_0.11. After that, Dragoe and his co-workers found that iron-oxypnictides exhibit large Seebeck coefficients, and suggested that iron-oxypnictides are promising low temperature thermoelectric materials . In this paper, a series of LaFeAsO_1-xF_x (x = 0-0.225) oxyarsenides have been synthesized by a solid-state reaction method in order to optimize electrical transport properties through appropriate F doping. Results show that both electrical resistivity and Seebeck coefficient of undoped LaFeAsO show an anomaly at about 150 K, which is related to a structural phase and/or spin-density-wave (SDW) transition. The enhancement of Seebeck coefficient and electrical properties result in power factor show the maximum value of 1.2 mW/mK² at 80 K for LaFeAsO_0.85F_0.15 sample, this value is the same order to that of the best low temperature thermoelectric Bi₈₈Sb₁₂ compounds. The present results indicate that LaFeAsO_1-xF_x iron-oxypnictides are promising for thermoelectric cooling applications in the liquid nitrogen temperature range. - Abstract: A series of LaFeAsO_1-xF_x (x = 0-0.225) oxyarsenides have been synthesized by a solid-state reaction method in order to optimize electrical transport properties through appropriate F doping. Both electrical resistivity and Seebeck coefficient of undoped LaFeAsO show an anomaly at about 150 K, which is related to a structural phase and/or spin-density-wave (SDW) transition. Seebeck coefficient seems to be determined by two competitive factors: it is enhanced by suppressing the structural phase and/or SDW transition, and reduced by increasing carrier concentration. Seebeck coefficient is significantly enhanced just after suppressing the anomaly, and the maximum Seebeck coefficient reached -142 μV/K for the sample with F doping x = 0.075 from -58 μV/K for undoped LaFeAsO, and then decreased with further increasing carrier concentration through F doping. Meanwhile, the electrical resistivity is decreased with increasing F doping, resulting in a maximum power factor value of 1.2 mW/mK² at 80 K for polycrystalline LaFeAsO_0.85F_0.15 sample_, this value is the same order as that of the best low temperature thermoelectric Bi₈₈Sb₁₂ compounds, and could be significantly higher in single crystals.

[en] Herein, the synthesis of high-entropy wolframite oxide (CoCuNiFeZn)${}_{1-<!--\; ???\; -->x}$Ga${}_x$WO${}_4$ through standard solid-state route followed by spark plasma sintering and their structural, microstructural, and thermoelectric (TE) properties are investigated. X-ray diffraction pattern followed by patterns matching refinement shows a monoclinic structure with the volume of the unit cell decreasing with increasing Ga content. The optical bandgap for these oxides shows a cocktail effect in high-entropy configuration. The Seebeck coefficient indicates electrons as dominating charge carriers with a nondegenerate behavior. The electrical resistivity decreases with increasing temperature depicting a semiconducting nature. Thermal conductivity in high-entropy samples (κ ≈ 2.1 W m${}^{-<!--\; ???\; -->1}$ K${}^{-<!--\; ???\; -->1}$ @ 300 K) is significantly lower as compared to MgWO${}_4$ (κ ≈ 11.5 W m${}^{-<!--\; ???\; -->1}$ K${}^{-<!--\; ???\; -->1}$ @ 300 K), which can be explained by the strong phonon scattering due to large lattice disorder in high-entropy configuration. The TE figure of merit zT increases with Ga doping via modifying all three TE parameters positively. (© 2023 The Authors. physica status solidi (RRL) Rapid Research Letters published by Wiley‐VCH GmbH)