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International audience; Mo-based cluster compounds form a large class of materials with interesting thermoelectric properties primarily due to their very low lattice thermal conductivity. Here, we investigate the influence of the insertion of Cu on the crystal structures and thermoelectric properties of the ternaries In2Mo15Se19 and K2Mo15Se19. Inserting Cu+ cations into the intercluster voids of the rhombohedral lattice enables adjusting the hole concentration, resulting in higher thermopower values with respect to the Cu-free compounds. While Cu+ cations occupy a single crystallographic site in Cu3In2Mo15Se19 as in the Ag-containing analogues, a distinctive crystallographic feature of Cu3K2Mo15Se19 is the different distribution of these cations that reside on split positions. The high degree of structural disorder of these compounds gives rise to very low lattice thermal conductivity kL, which directly depends on the total concentration of inserted cations. The sensitivity of the electronic and thermal properties to the Cu content is evidenced by a Cu-rich sample with nominal composition Cu4In2Mo15Se19 that exhibits enhanced power factor and reduced kL, yielding a peak ZT value of similar to 0.42 at 1145 K. The good thermoelectric performance and high melting point of these compounds make them potential candidates for thermoelectric applications above 1000 K.

Chalcogenide semiconductors continue to be of prime interest for designing novel efficient materials for energy-conversion applications. Among them, the narrow-band-gap p-type semiconductor InTe exhibits high thermoelectric performance that mostly stems from its poor ability to transport heat. Here, we show that its thermoelectric figure of merit can be further enhanced by finely tuning the hole concentration through Pb doping. X-ray diffraction and scanning electron microscopy performed on the polycrystalline series In1−xPbxTe confirm that Pb substitutes for In with an estimated solubility limit below 1%. Pb acts as a donor-like impurity, driving the degenerate behavior of pristine InTe towards semiconducting-like properties. The enhanced disorder caused by this substitution increases point-defect scattering of phonons, leading to reduced lattice thermal conductivity. The resulting increase in the power factor and decrease in the phonon transport to values as low as 0.22 W m−1 K−1 above 700 K yields a peak ZT value of 1.05 at 780 K for x = 0.1% along the direction parallel to the pressing direction. According to a single parabolic band model, this sample exhibits a near-optimal thermoelectric performance.

Chalcogenide semiconductors and semimetals continue to be of prime interest for thermoelectric applications in power generation. As another representative of this broad class of materials, tetragonal InTe has recently emerged as a promising candidate due to its very limited ability to transport heat leading to high thermoelectric performance near 800 K in polycrystalline samples. However, little is known on the basic physical mechanisms governing its electronic and thermal properties, an in-depth study of which requires the growth of single crystals. Here, we report a detailed investigation of the transport properties of InTe single crystals grown by the Bridgman–Stockbarger technique over a wide range of temperatures (5–800 K). Except for the Hall coefficient that remains nearly isotropic below 300 K, all the transport coefficients show a significant anisotropy between the c and [110] direction of the crystal structure. In contrast to electronic band structure calculations suggesting a semimetallic ground state, the high-temperature dependence of the thermopower α(T) indicates that InTe is a semiconductor with a band gap estimated to be 0.26 eV from the Goldsmid–Sharp relation. Despite the absence of grain boundary scattering, an extremely low lattice thermal conductivity κph of 0.32 W m−1 K−1 at ∼780 K is achieved along the [110] direction. Remarkably, this value is equivalent to the glassy limit κglass based on phonon-mediated heat transport suggesting that, at high temperatures, the thermal transport in InTe has reached its minimum value. The combination of extremely low κph values with a relatively high power factor yields a maximum dimensionless thermoelectric figure of merit ZT of 0.61 at 780 K along the [110] direction. The present study provides a solid basis for future doping strategies of InTe for high-temperature thermoelectric applications.

Distortion of the density of states induced by specific impurities, a mechanism known as resonant level (RL), is an efficient strategy to enhance the thermoelectric performances of metals and semiconductors. So far, experimental signatures identifying the resonant nature of an impurity have relied on the so-called Ioffe–Pisarenko plot that enables visualizing the induced thermopower enhancement at specific carrier concentrations. However, this method cannot solely discern RL from other possible band-structure-related sources of thermopower enhancement such as band-shape modifications or band convergence. An independent method of resolving this problem is proposed here. A detailed theoretical and experimental analysis of the low-temperature electrical resistivity ρ0 and carrier mobility μ0 of the resonant-level system SnTe doped with In is presented as a function of the impurity concentration x. By comparing to non-resonant cases of SnTe doped with I, Mn, and Ga, we demonstrate that the construction of residual resistivity ρ0(x) and residual mobility μ0(x) plots allows to distinguish between resonant and non-resonant impurities, even when some of them induce similar thermopower enhancements. This methodology is further confirmed by analyses performed for Na- and Tl-doped PbTe, illustrating how the combination of transport measurements at low temperatures can be used to determine the resonant nature of an impurity.

International audience; Rutile is the most common and stable polymorph form of titanium oxide TiO2 at all temperatures. The doping of rutile TiO2 with a small amount of niobium is reknown for being responsible for a large increase of the electrical conductivity by several orders of magnitude, broadening its technological interest towards new emerging fields such as the thermoelectric conversion of waste heat. The electronic conduction has been found to be of a polaronic nature with strongly localized charges around the Ti3+ centers while, on the other side, the relatively high value of the thermal conductivity implies the existence of lattice heat carriers, i.e. phonons, with large mean free paths which makes the nanostructuration relevant for optimizing the thermoelectric efficiency. Here, the use of a high-pressure and high-temperature sintering technique has allowed to vary the grain size in rutile TiO2 pellets from 300 to 170 nm, leading to a significant reduction of the lattice thermal conductivity. The thermoelectric properties (electrical conductivity, Seebeck coefficient and thermal conductivity) of Nb-doped rutile nanostructured ceramics, namely NbxTi1−xO2 with x varying from 1 to 5%, are reported from room temperature to ∼900 K. With the incorporation of Nb, an optimum in the thermoelectric properties together with an anomaly on the tetragonal lattice constant c are observed for a concentration of ∼2.85%, which might be the fingerprint of the formation of short Nb dimers.

Microstructure and physicochemical properties of a thermally sprayed coating depend on the dynamics of the particles interacting with the spray jet. This is especially the case for electrical properties. In this study, different spraying processes were used to spray p-type and n-type half-Heusler powders. Thermoelectric powders, Hf20Zr75Ti05CoSb80Sn20 (p-type) and Hf60Zr40NiSn98Sb02 (n-type), were selected due to their interesting electrical properties. The spray processes were evaluated based on coating composition and mechanical property measurements. The only coatings of practical interest were those that were plasma sprayed and they were examined in detail to assess the effect of process parameters on coating properties.

International audience; We report the syntheses and crystal structures of the cluster compounds Tl2Mo15S19 and Ag3Tl2Mo15S19, two novel members of the series M2Mo15X19 (M = In, Tl, K, Rb, Cs, Ba; X = S, Se). While only single crystals of Tl2Mo15S19 were obtained, Ag3Tl2Mo15S19 powders were synthesized, allowing for a study of its thermoelectric properties. Their structures contain Mo6S8iS6a and Mo9S11iS6a units separated by Tl+ cations. Silver insertion increases the electron transfer towards the Mo clusters leading to modifications of the Mo-Mo distances. Electrical resistivity measurements on Tl2Mo15S19 evidence its metallic character. Inserting Ag drives the transport properties towards a semiconducting state characterized by relatively low electrical resistivity and thermopower values. The large, complex unit cell of these compounds results in very low lattice thermal conductivity values below 1 Wm−1 K−1. Although the Ag insertion improves the thermoelectric performances, the ZT value remains moderate with a maximum value of 0.2 at 800 K.

International audience; Solid-state energy conversion through thermoelectric effects remains the technology of choice for space applications for which, their low energy conversion efficiency is largely outweighed by the reliability and technical requirements of the mission. Radioisotope thermoelectric generators (RTGs) enable the direct conversion of the heat released by nuclear fuel into the electrical power required to energize the scientific instruments. The optimization of the conversion efficiency is intimately connected to the performances of the thermoelectric materials integrated which are governed by the transport properties of these materials. Recent advances in the design of highly efficient thermoelectric materials raise interesting prospects to further enhance the performances of RTGs for future exploratory missions in the Solar system. Here, we briefly review the knowledge acquired over the last years on several families of thermoelectric materials, the performances of which are close or even higher than those conventionally used in RTGs to date. Issues that remain to be solved are further discussed.

International audience; Chalcogenide seChalcogenide semiconductors and semimetals are a historical fertile class of materials for discovering novel compounds for energy conversion applications, such as photovoltaic cells or thermoelectric devices. Here, we report on a detailed investigation of the low-temperature transport properties (5 – 300 K) of the n-type series Bi8-xSbxSe7 (0  x  2.4), the thermoelectric performances of which can be optimized around 300 K. The complex layered crystal structure, built up by Bi bilayers and quintuple Bi-Se layers stacked perpendicular to the c axis of the trigonal unit cell, gives rise to very low lattice thermal conductivity κ_L values on the order of 0.6 W m-1 K-1 at 300 K. Upon cooling, κ_L (T) exhibits a crystalline-like behavior with a well-defined dielectric maximum. Despite Sb and Bi are isovalent, both the thermopower α and the electrical resistivity ρ vary upon increasing the Sb content, possibly due to variations in the nature and concentration of native defects. Our results indicate the existence of two substitutional ranges, in which the evolution of the electronic properties with x are not equivalent. Combined with the poor ability of these materials to conduct heat, the high power factors, optimized for x = 1.6, result in a maximum dimensionless thermoelectric figure of merit ZT as high as 0.40 at 300 K, making these compounds competitive among n-type thermoelectric chalcogenides for near-room-temperature thermoelectric applications.

International audience; Because the binary chalcogenide SnTe is an interesting Pb-free alternative to the state-of-the-art thermoelectric material PbTe, significant efforts were devoted to the optimization of its thermoelectric properties over the last few years. Here, we show that saturation-annealing treatments performed at 823, 873 or 973 K under Sn-rich conditions provide a successful strategy to prepare polycrystalline samples with a controlled concentration of Sn vacancies. Both scanning transmission electron microscopy and Mössbauer spectroscopy demonstrate the absence of Sn-rich areas at the grain boundaries in the saturation-annealed samples. Transport property measurements, performed over a wide range of temperatures (5-800 K), show that this technique enables achieving thermoelectric performances at 800 K similar to those obtained using Sn self-compensation. The three saturation annealing temperatures result in comparable transport properties across the entire temperature range due to similar hole concentrations ranging between 1.0 and 1.5 Â 10 20 cm À3 at 300 K. As equally observed in samples prepared by other synthetic routes, the temperature dependence of the Hall mobility evidences that charge transport is strongly affected by point-defect scattering caused by the random distribution of Sn vacancies.

International audience; Highly flexible free-standing polypyrrole films were prepared by the interfacial chemical polymerization of pyrrole (in cyclohexane) using aqueous FeCl 3 as an oxidant and p-toluene sulphonic acid (PTSA) as a dopant. Morphological characterization revealed the granular morphology of the synthesized films and also suggested a reduction in their thickness with the increase in PTSA concentration. The increase of the doping content in these polypyrrole films, also resulted in the enhancement of electrical conductivity from 4.8 S/cm to 162.7 S/cm without much affecting the Seebeck coefficient~4e8 mV/K. The combined results of X-ray photoelectron spectroscopy and Electron spin resonance spectroscopy (ESR) confirms the enhancement of doping content (N þ /N content) i.e. formation of charge carriers such as polarons/ bipolarons with the increase in PTSA concentration. The synergetic combination of high electrical conductivity along with moderate Seebeck coefficient in the doped free-standing PPy films resulted in the highest average power factor of~0.45 mW/mK 2. A wrist-band type thermoelectric power generator was also designed by integrating seven number of free-standing PPy films to manifest their potential for practical thermoelectric applications. The fabricated device exhibited maximum open-circuit voltage and current respectively as 336 mV and~46 nA, for a temperature difference of 80 C. The work reveals new avenues towards the development of wearable energy harvesting devices that can be possibly designed using free-standing PPy films.

International audience; Narrow-band-gap IV-VI semiconductors represent a historically important class of thermoelectric materials. As one of the representative compound of this class, SnTe has been reinvestigated over the last years demonstrating its potential as a high-temperature p-type thermoelectric material. Here, we present a detailed study of the influence of very low doping levels of In, from 0.05% up to 2%, on the high-temperature transport properties of the selfcompensated Sn1.03Te compound. Our results evidence a strong impact of In on the transport properties, consistent with the presence of an In-induced resonant level (RL) in the valence bands of Sn1.03Te. This peculiar behavior is confirmed by electronic band structure calculations performed using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) revealing a narrow and sharp peak in the density of states (DOS) induced by the hybridization of the In s-states with the electronic states of Sn1.03Te. This distortion in the DOS results in a spectacular increase in both the thermopower and electrical resistivity at 300 K. Although the influence of the RL is somewhat lessened at higher temperatures, a significant enhancement in theZTvalues is nevertheless achieved with a peak ZTof 0.75 at 800 K which represents an increase of 35% over the values measured in Sn1.03Te. Of relevance for practical applications, the weak dependence of the RL on temperature leads to enhanced averageZTvalue.

Besides the intrinsic low heat transfer capability of material, the thermal insulation property of thermal barrier coatings (TBCs) also relies on their microstructures. For better understanding the relationship between process parameters, porous architecture, and thermal properties of coatings, YSZ coatings were firstly manufactured by suspension plasma spray (SPS). Afterwards their total porosities were characterized by using the technique of X-ray transmission, the nano/submicro pores in those coatings were detected with Ultra-Small Angle X-ray Scattering (USAXS), and the thermal properties of coatings were measured using the laser flash method. The results indicated that: i) the porous architecture of SPS coatings can be tailored by adapting process parameters. ii) increasing total porosity is an effective means for reducing the heat transfer capability of SPS coatings. iii) nano/submicro pores have higher influence sensitivity on thermal properties than that of larger pores. Increasing the content of nano-submicro pores is therefore more effective for improving the thermal insulation property of SPS coatings. The potential of SPS technique for fabricating TBCs was discussed as well.

Vehicle collision leading to life threatening accidents is a common problem which is incrementing noticeably. This necessitated the need for Driver Assistance Systems (DAS) which helps drivers sense nearby obstacles and drive safely. However, it’s inefficiency in unfavorable weather conditions, overcrowded roads, and low signal penetration rates in India posed many challenges during it’s implementation. In this paper, we present a portable Driver Assistance System that uses augmented reality for it’s working. The headset model comprises of five systems working in conjugation in order to assist the driver. The pedestrian detection module, along with the driver alert system serves to assist the driver in focusing his attention to obstacles in his line of sight. Whereas, the speech recognition, gesture recognition and GPS navigation modules together prevent the driver from getting distracted while driving. In the process of serving these two root causes of accidents, a cost effective, portable and holistic driver assistance system has been developed.

Semi-crystalline polypyrrole (PPy) nanotubes were synthesized through a chemical polymerization route using methyl orange-ferric chloride (MO-FeCl3) as a template for growth. The thermoelectric properties of these PPy nanotubes have been studied in the temperature range 300–380 K after treatment with various dopants such as hydrochloric acid (HCl), p-toluene-sulphonic acid monohydrate (ToS), and tetrabutyl ammonium hexaflurophosphate (PF6). It has been observed that these dopants affect the electrical and thermal transport properties of PPy nanotubes in different ways. The temperature dependence of electrical resistivity suggests that pure PPy and ToS-doped PPy nanotubes exhibit a critical regime of metal-to-insulator transition, and doping with HCl drives them into the metallic regime. In contrast, PF6 doping of PPy nanotubes carries them into the insulating regime and these samples exhibited the highest figure of merit of ~3.4 × 10‒3 at 380 K, which was 240% higher than the value obtained in the case of pristine PPy nanotubes. Strongly repressed thermal conductivity along with moderately high Seebeck coefficient and electrical conductivity have been found to be responsible for the high figure of merit observed in PF6-doped PPy nanotubes. The suppression of thermal conductivity in PF6-doped PPy nanotubes is attributed to the scattering of the spectrum of phonons via hierarchical length-scale defect structures present in the sample.