Tetrahedrites: Prospective Novel Thermoelectric Materials

Since their discovery in 1845, tetrahedrites, a class of minerals composed of relatively earth‐abundant and nontoxic elements, have been extensively studied in mineralogy and geology. Despite a large body of publications on this subject, their transport properties had not been explored in detail. The discovery of their interesting high‐temperature thermoelectric properties and peculiar thermal transport has led to numerous experimental and theoretical studies over the last 4 years with the aim of better understanding the relationships between the crystal, electronic, and thermal properties. Tetrahedrites provide a remarkable example of anharmonic system giving rise to a temperature dependence of the lattice thermal conductivity that mirrors that of amorphous compounds. Here, we review the progress of research on the transport properties of tetrahedrites, highlighting the main experimental and theoretical results that have been obtained so far and the important issues and questions that remain to be investigated

Reduced phase space of heat-carrying acoustic phonons in single-crystalline InTe

Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity κph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of κph(T) in single-crystalline InTe along the c axis show that κph exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of In+ cations in tunnels formed by edge-sharing (In3+Te42−)− tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the In+-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low κph value of 1.2Wm−1K−1 at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the In+ sit

Synthèse et caractérisation microstructurale de poudres nanométriques à base de Bi2Te3 et Sb2Te3 (contribution à l'état de l'art des nanocomposites thermoélectriques)

L étude de matériaux thermoélectriques nanocomposites et nanostructurés est en plein essor suite à l intérêt de multiplier le nombre d interfaces et de diminuer la taille des objets pour améliorer les performances. Nous avons mis au point une nouvelle méthode de préparation de nanopoudres thermoélectriques de type n (Bi0.95Sb0.05)2(Te0.95Se0.05)3 et de type p (Bi0.2Sb0.8)2Te3, à partir de la fracturation laser en milieu aqueux de poudres de taille micrométrique. La cellule de préparation développée permet d obtenir par jour environ 200 mg de poudres nanométriques cristallisés présentant la structure cristallographique des poudres initiales et dont la taille moyenne est comprise entre 7 et 12 nm. Les mécanismes mis en jeu dans l obtention des nanoparticules ont été abordés. Ils dépendent fortement de la densité d énergie du faisceau laser. Les nanopoudres ont ensuite été mélangées mécaniquement aux poudres micrométriques de même nature et ont été compactées à froid. Les propriétés thermoélectriques (résistivité électrique, pouvoir thermoélectrique, conductivité thermique) des nanocomposites ont été évaluées à température ambiante. Les premiers résultats montrent que même si le pouvoir thermoélectrique est maintenu dans les matériaux nanostructurés et nanocomposites et que la conductivité thermique totale peut, de manière tout à fait exceptionnelle, être diminuée d un facteur deux, la résistivité électrique obtenue est jusqu alors trop élevée pour conduire à de bonnes performances en terme de facteur de mérite adimensionnel, par rapport à un matériau massif conventionnel de même compositionThe study of thermoelectric nanostructured and nanocomposite materials is expanding because of the interest to multiply the number of interfaces and to decrease the size of the objects in order to improve the thermoelectric performance. We developed a new method to prepare thermoelectric n type (Bi0.95Sb0.05)2(Te0.95Se0.05)3 and p type (Bi0.2Sb0.8)2Te3 nanopowders, from the laser fracture in a liquid medium of powders of micrometric size. The developed cell preparation makes it possible to obtain per day approximately 200 mg of crystallized nanometric powders having the crystallographic structure of the initial powders and whose mean size lies between 7 and 12 nm. The mechanisms concerned in obtaining the nanoparticules were approached. They strongly depend on the density of energy of the laser beam. The nanopowders then were mechanically mixed with the micrometric powders of comparable nature and were cold pressed. The thermoelectric properties (electrical resistivity, thermoelectric power, thermal conductivity) of the nanocomposites were evaluated at room temperature. The first results show that even if the thermoelectric power is maintained in nanostructured and nanocomposite materials and that the total thermal conductivity can, in a completely exceptional way, being decreased by a factor two, the electrical resistivity obtained is hitherto too high to lead to high values of the dimensionless thermoelectric figure of merit, with regard to conventional bulk materials of same compositionNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Ingénierie des matériaux et de microgénérateurs thermoélectriques planaires

NANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Optical reflectivity as a simple diagnostic method for testing structural quality of icosahedral quasicrystals

International audienceOptical reflectivity as a simple diagnostic method for testing structural quality of icosahedral quasicrystals 2 The optical reflectivity of Al-based and Ti-based quasicrystalline and approximant samples were investigated versus the quality of their structural morphology using optical reflectometry, X-ray diffraction and transmission electron microscopy. The different structural morphologies were obtained using three different preparation processes : sintering, pulsed laser deposition and reactive cathodic magnetron sputtering. The work demonstrates that the canonical behaviour of icosahedral state in specular reflectivity is extremely sensitive to different and very fine aspects of the microstructure : sizes of grains smaller than 50 nm, slight local diffuse disorder and shifts away from the icosahedral crystallographic structure (approximants). The work explains why the optical properties of the same kind of quasicrystals found in literature sometimes reveal a different behaviour from one author to another. The study then confirms the work of some authors and definitely shows that the canonical behaviour of icosahedral state in specular reflectivity over the 30000-50000 cm-1 domain is characterized by a decreasing function made of steps. It also shows that this behaviour can be interpreted thanks to the cluster hierarchy of the model of Janot

Réalisation et étude des propriétés thermoélectriques de couches minces et nanofils de types Bi2-XSbxTe3 et Bi2Te3-xSex

De récentes études montrent que les films minces présentent des performances thermoélectriques nettement plus importantes (jusqu'à un facteur 3) que celles obtenues dans les matériaux massifs. Nous avons choisi de développer des couches minces thermoélectriques Bi0,5Sb1,5Te3 de type p et Bi2Te2,7Se0,3 de type n présentant les performances thermoélectriques les plus intéressantes à des températures proches de l'ambiante. La technique de dépôt utilisé est la PVD magnétron. L'optimisation des conditions de dépôt (pression Ar, puissance plasma, distance cible-substrat et temps de dépôt) ainsi que du traitement thermique de recuit a permis obtenir des figures de mérite ZT les plus élevées possibles. De plus, les phénomènes physiques mis en jeu dans les films minces étant différents de ceux des massifs, il a été nécessaire des les étudier pour améliorer les performances thermoélectriques des couches minces. De petits dispositifs thermoélectriques en couche minces ont pu être réalisé et caractérisé. En parallèle nous avons exploré la possibilité de faire croître des filaments thermoélectriques de compositions semblables aux couches et de dimensions manométriques au sein d'une matrice d'alumine nanoporeuse. En effet la réduction des dimensions géométriques permet d'augmenter les performances thermoélectriques des matériaux. Nous avons pu réaliser les premiers fils n et p ainsi que les premières caractérisations thermoélectriques.Recent studies showed that thermoeletrical performances are larger for thin films (factor 3) than for bulk materials. We chose to develop p-type Bi0,5Sb1,5Te3 and n-type Bi2Te2,7Se0,3 thermoelectrical thin films in order to work at room temperature. Thin films have been realized using magnetron sputtering process. The optimisation of the deposition conditions (Ar pressure, power plasma, target-substrate distance, deposition time) as well as the annealing treatments has helped to obtain figure of merits as high as possible. On the other hand, the physical mechanisms in thin films are different than those of bulk materials so it was necessary to study such mechanisms for a better understanding. In parallel, we explore the possibility to grow thermoelectrical wires with nanometric dimensions inside of a nanoporous alumina matrix. It has been recently shown theoretically and experimentally that thermoelectrical properties are strongly improved when the nanometric dimensions are obtained.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Synthèse, caractérisation physico-chimique et propriétés de transport de composés de type Mo3Sb7

Les préoccupations environnementales actuelles ont conduit à un regain d intérêt pour la conversion d'énergie par effets thermoélectriques au cours de ces 20 dernières années. Le challenge lié à cette technologie consiste à découvrir des matériaux qui possèdent à la fois une faible conductivité thermique, une forte conductivité électrique et un fort pouvoir thermoélectrique. Les travaux présentés dans ce mémoire se sont orientés vers l'étude de phases cristallines complexes à base de Mo3Sb7. Contrôler finement les propriétés électriques et thermiques de ces matériaux par le biais de substitutions appropriées et relier les propriétés physiques aux propriétés structurales et électroniques ont été au coeur de ces travaux de recherche. Des résultats significatifs ont ainsi pu être obtenus tant au niveau de la synthèse et de la caractérisation physico-chimique qu'au niveau des propriétés magnétiques et de transport. En particulier, nous avons pu mettre en évidence les propriétés exotiques du composé Mo3Sb7 dont la compréhension s'est révélée indispensable pour l'étude des propriétés de transport des matériaux substitués ternaires et quaternaires. Les différentes possibilités de substitution ont alors permis d'améliorer de façon substantielle les performances thermoélectriques du composé Mo3Sb7 et ont, de ce fait, conduit à la découverte de nouveaux matériaux surpassant les meilleurs matériaux connus à ce jour (Si-Ge) et utilisés sur la gamme 900 - 1200 K dans des applications en génération d'électricitéDue to current environmental concerns, a resurgence of interest in thermoelectricity have been witnessed by the last 20 years. The challenge raised by this technology lies in identifying materials that display low thermal conductivity as well as both high electrical conductivity and thermopower. The work presented in this manuscript deals with a thorough study on molybdenum-antimony based complex crystalline structure. To finely control the thermal and electrical properties of these compounds through judicious substitutions and to link up physical and structural properties were at the heart of this in-depth study. Not only did we obtain outstanding results regarding the synthesis and both the chemical and structural characterizations but we also discovered intriguing magnetic and transport properties. Particularly, we emphasized the exotic properties exhibited by the binary Mo3Sb7 compound whose a deep understanding were essential to study the transport properties of the ternary and quaternary alloys. The different substitutions we have considered were found to substantially improve the thermoelectric properties of the Mo3Sb7 compound and thus, led to the synthesis of new prospective thermoelectric materials that surpass the best compounds discovered up-to-now (Si-Ge) and used in power generation applications in the 900 1200 K temperature rangeNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Beneficial influence of Ru on the thermoelectric properties of Mo₃Sb₇

International audienceZintl phases have recently been identified as potential thermoelectric materials typified by the discovery of a high thermoelectric figure of merit in the Yb14MnSb11 compound above 1000 K. We report on the synthesis and the thermoelectric properties measurements over a wide temperature range 300–1000 K of other Zintl phases exhibiting a complex crystalline structure, namely, Mo3Sb7 and its related compounds Mo3−xRuxSb7. While the binary compound displays low figure of merit values due to its metallic nature, we show that the partial substitution of Mo by Ru significantly improves its value to reach 0.45 at 1000 K in Mo2.2Ru0.8Sb7

High Temperature Transport Properties of Tetrahedrite Cu12−x M x Sb4−y Te y S13 (M = Zn, Ni) Compounds.

International audienceNatural and synthetic tetrahedrites are currently the focus of attention in thermoelectricity due to their abundance, numerous possibilities of substitution, and their complex crystal structure (I4¯3m) resulting in extremely low lattice thermal conductivity (below 1 W m−1 K−1 above 300 K). Here, we report on the synthesis, structural and chemical characterizations, and high-temperature thermoelectric properties of polycrystalline Cu12−x M x Sb4−y Te y S13 (M = Zn, Ni) tetrahedrites. Upon substituting on both the Cu and Sb sites, we successfully improved the thermoelectric properties with respect to the ternary compound Cu12Sb4S13 with peak ZT values of 0.7 at 700 K