120 research outputs found
Thermoelectric chalcogenides and antimonides for very high temperature applications
No full textCe travail de thèse porte sur l’étude de matériaux thermoélectriques pour des applications à très hautes températures. Plusieurs systèmes physico-chimiques ont été explorés. En premier lieu, des composés chalcogénures de structure type pseudo-hollandite et « hollandite-like » formant des canaux unidimensionnels ont été étudiés avec la découverte d’un nouveau composé de formule Rb0,2Ba0,4Cr5Se8. L’occupation mixte du rubidium et de baryum à l’intérieur des canaux de cette structure a permis d’améliorer les propriétés de transport de ce composé. L’exploration de ce système a également permis de mettre en évidence de l’existence d’une solution solide CsxCr5Te8 (x = 0,73 ; 0,91 ; 0,97), ces trois composés cristallisants tous dans le type structural B « hollandite-like ». Dans un second temps, des chalcogénures de niobium de formule générale Nb3X4 (X = Se, Te) ont été étudiés. La synthèse et la caractérisation des propriétés de transport de ces matériaux ont révélé un comportement métallique. L’amélioration des propriétés thermoélectriques de ces composés par substitution sur le site du chalcogène et par l’insertion d’un élément ternaire n’a pas été significative. Enfin, l’étude d’antimoniures de terres rares de formule Yb4Sb3 ont présenté une figure de mérite zT de 0,6 à 1000 °C. Les substitutions par du lanthane et du bismuth sur les deux sites cristallographiques n’a pas permis d’améliorer les propriétés thermoélectriques de ces composés.This PhD work aimed at the study of different systems of thermoelectric ma-terials for very high temperature applications. First, chalcogenide compounds of pseudo-hollandite and hollandite-like structure forming one-dimensional channels were studied with the discovery of a new compound of formula Rb0.2Ba0.4Cr5Se8. The mixed occupancy of rubidium and barium within the channels has improved the transport properties of this compound. The exploration of this system also revealed the existence of a CsxCr5Te8 solid solution (x = 0.73; 0.91; 0.97), these three com-pounds all crystallising in the "hollandite-like" structural type B. In a second step, niobium chalcogenides of the general formula Nb3X4 (X = Se, Te) were studied. The synthesis and characterisation of the transport properties of these materials revealed a metallic behaviour. The improvement of the thermoelectric properties of these compounds by substitution at the chalcogen site and by the insertion of a ternary element was not significant. Finally, the study of rare earth antimonides of the for-mula Yb4Sb3 resulted in a promising figure of merit zT of 0.6 at 1000 °C. The substitution by lanthanum and bismuth on both crystallographic sites has not improved the thermoelectric properties
Machine-Learning-assisted discovery of new thermoelectric clusters
No full textL'objectif de cette thèse est la conception de nouvelles phases à clusters condensés de molybdène, dérivées des phases de Chevrel, possédant des propriétés thermoélectriques améliorées. L'approche employée combine le calcul théorique (DFT) et le machine learning, une méthode d'intelligence artificielle qui sera utilisée pour prédire les propriétés des nouveaux composés, à plus grande échelle. L'objet du premier chapitre est de faire le lien entre les structures cristalline et électronique de ces phases à clusters condensés, lesquelles possèdent un VEC (concentration d'électrons de valence) « idéal » pour lequel elles deviennent semi-conductrices. Le second chapitre recense les propriétés thermoélectriques rapportées de ces phases. Ces propriétés sont prometteuses en vue d'applications à haute température, notamment du fait de la très faible conductivité thermique de ces phases. Enfin, une méthode de calcul du coefficient de Seebeck par DFT y est détaillée. Le troisième chapitre commence par la création d'une base de données, composée du coefficient de Seebeck calculé de 92 formules hypothétiques semi-conductrices. Enfin, un modèle de machine learning, basé sur un algorithme de régression utilisant des arbres décisionnels, est entraîné pour prédire le coefficient de Seebeck de plus de 900 formules hypothétiques. Celles dont on pense qu'elles présentent des propriétés thermoélectriques améliorées sont proposées comme candidates à la synthèse.The aim of this PhD is the conception of new condensed molybdenum cluster phases, which derive of Chevrel phases, with enhanced thermoelectric properties. The chosen approach combines theoretical calculations (at a DFT level) and machine learning, an artificial intelligence method that will be used to predict the properties of the new compounds, on a larger scale. The objective of the first chapter is to make the link between the crystalline and electronic structures of these condensed clusters phases, which possess an « ideal » VEC (Valence Electrons Concentration) for which they display semiconducting properties. The second chapter lists the reported thermoelectric properties of these phases. Those properties are promising for high temperature thermoelectric applications, especially because of the very low thermal conductivity of those phases. Finally, a DFT calculation process of the Seebeck coefficient is detailed in chapter 2. The third chapter begins with the creation of a database, composed of the computed Seebeck coefficient of 92 hypothetical, semiconducting formulas. Finally, a machine learning model, based on regression algorithms using decisional trees, is trained to predict the Seebeck coefficient of more than 900 hypothetical formulas. Some of those formulas, that we think may display enhanced thermoelectric properties, are proposed as candidates for synthesis
Thermoelectric properties of n-type cobalt doped chalcopyrite Cu1−xCoxFeS2 and p-type eskebornite CuFeSe2
No full textAbstractChalcopyrite CuFeS2 has been recently suggested as a promising thermoelectric material. In the present paper, the transport properties – electrical resistivity ρ and Seebeck coefficient S – of the n-type CuFeS2 chalcopyrite and the p-type CuFeSe2 eskebornite have been measured. Very different groundstates are evidenced with a semimetallic behavior concomitant to a metal-like S(T) curve for CuFeSe2 whereas CuFeS2 is a semiconductor with much larger |S| values. For that reason, charge creation by Co2+ for Cu+ substitution in CuFeS2 has been performed. The veracity of the Co for Cu substitution for x ≤ 0.10 in Cu1−xCoxFeS2 chalcopyrite has been confirmed by EDS analyses, coupled to electron diffraction, with a transmission electron microscope. Also, this study demonstrates the existence of twinning domains. The compounds corresponding to the best compositions in terms of power factor have been densified by Spark Plasma Sintering for thermal conductivity measurements. A maximum ZT value up to 0.22 at 675 K for Cu0.96Co0.04FeS2 has been obtained
Poétique d’un langage vulnérable : Parle de Noémi Lefebvre
No full textComment cerner la vulnérabilité d’un individu si ce n’est en s’attachant à sa faculté de produire du sens et de se faire entendre ? Noémi Lefebvre illustre dans Parle (2021) les tensions présentes dans les interactions entre un individu (Je) et un groupe (Nous) au sein de notre société de consommation. Penser la poétique d’un langage vulnérable, c’est faire un sort aux déraillements du langage, aux jeux de mots, et aux discours angoissés des locuteur.rice.s, en révélant leur recréation permanente. L’autrice prône alors une éthique du langage qui met en péril la quantification du sujet, portée par une idéologie capitaliste délétère : un lyrisme critique.How can we capture the vulnerability of an individual if not by focusing on their ability to produce meaning and to be heard ? In her work Parle (2021), Noémi Lefebvre highlights the tensions lying in the interactions between an individual (I) and a group (We) within our consumer society. Approaching the poetics of a vulnerable language, it is to put forward the eccentricities of language, the plays of words, and the anguished discourses of its speakers, revealing their constant re-creation. Using a form of critical lyricism, the author then advocates an ethics of language that threatens the quantification of the subject brought up by a deleterious capitalist ideology
Nanostructural and Microstructural Ordering and Thermoelectric Property Tuning in Misfit Layered Sulfide [(LaS) x ] 1.14 NbS 2
No full textInternational audienceThe potential to control the structural ordering at the nanoscale in misfit layered systems can open new paths for achieving high thermoelectric performance in these systems. In the present work, we demonstrate that compositional changes can provide nanoscale tuning in misfit layered (La x S x ) 1.14 NbS 2 (x = 0.90, 0.95, 1.00, 1.05, 1.10), leading to improved thermoelectric properties (investigated over the temperature range of 300 to 950 K). The samples were prepared by CS 2 sulfurization and consolidated using pressure-assisted sintering. It was revealed through transmission electron microscopy analysis that nonstoichiometry promotes long-range ordering of the layers and results in elongated lamella formations. The La deficient x = 0.95 sample was found to contain large strain induced stacking disorder, owing to which the long lamellas rolled up to form tubular structures, whereas the La rich x = 1.05 sample had a much ordered structure and only slight curling of lamella edges was observed. Improved structural ordering and textured grain growth in La rich sample resulted in a ∼30% improved power factor (∼460 μW K -2 m -1 at 950 K) and ZT (∼0.2 at 950 K) in the in-plane direction, compared to the x = 1.00 sample. The origin of very low thermal conductivity (1.1-2.5 W K -1 m -1 at 950 K) in our samples has been identified and discussed in detail. © 2015 American Chemical Society
Substitution of indium for chromium in TlIn5−xCrxSe8: crystal structure of TlIn4.811(5)Cr0.189(5)Se8
No full textThe new thallium penta(indium/chromium) octaselenide, TlIn4.811(5)Cr0.189(5)Se8, has been synthesized by solid-state reaction. It crystallizes isotypically with TlIn5Se8 in the space group C2/m. Although the two Tl positions are disordered and only partially occupied, no Tl deficiency was observed. The insertion of chromium in the structure has been confirmed by EDS analysis. Chromium substitutes indium exclusively at one of three In sites, viz. at one of the positions with site symmetry 2/m (Wyckoff position 2a). In the crystal structure, edge-sharing InSe6 octahedra, and (In,Cr)Se6 octahedra and InSe4 tetrahedra make up two types of columns that are linked into a framework in which two different types of channels parallel to [010] are present. The Tl atoms are located in the larger of the channels, whereas the other, smaller channel remains unoccupied
Is LiI a Potential Dopant Candidate to Enhance the Thermoelectric Performance in Sb-Free GeTe Systems? A Prelusive Study
No full textAs a workable substitute for toxic PbTe-based thermoelectrics, GeTe-based materials are emanating as reliable alternatives. To assess the suitability of LiI as a dopant in thermoelectric GeTe, a prelusive study of thermoelectric properties of GeTe1−xLiIx (x = 0–0.02) alloys processed by Spark Plasma Sintering (SPS) are presented in this short communication. A maximum thermoelectric figure of merit, zT ~ 1.2, was attained at 773 K for 2 mol% LiI-doped GeTe composition, thanks to the combined benefits of a noted reduction in the thermal conductivity and a marginally improved power factor. The scattering of heat carrying phonons due to the presumable formation of Li-induced “pseudo-vacancies” and nano-precipitates contributed to the conspicuous suppression of lattice thermal conductivity, and consequently boosted the zT of the Sb-free (GeTe)0.98(LiI)0.02 sample when compared to that of pristine GeTe and Sb-rich (GeTe)x(LiSbTe2)2 compounds that were reported earlier
Suppression of superconductivity and resistivity anomaly in Rh17S15 by cobalt substitution
No full textInternational audienceThe chalcogenide superconductor Rh17S15 is known for having an upper critical field of nearly twice the Pauli limit and an unusual temperature dependence of the resistivity. When doped with small amounts of cobalt, superconductivity in Rh17-xCoxS15 (0 < x < 3) is systematically suppressed. We explore the evolution of the electrical transport properties from 2-300 K as a function of x. We identify three temperature regimes which are differently affected by doping. The disappearance of an electron-like contribution to the transport at low temperature is correlated with the suppression of superconductivity
Nickel bismuth boride, Ni23-xBixB6[x = 2.44&#8197;(1)]
No full textThe &#964;-boride Ni23-xBixB6 [x = 2.44&#8197;(1)] adopts a ternary variant of the cubic Cr23C6-type structure, with Ni8 cubes and Ni12 cuboctahedra arranged in a NaCl-type pattern. Two of the four independent metal sites (8c, overline{4}3m symmetry; 4a, moverline{3}m symmetry) are occupied by a mixture of Ni and Bi atoms in a 0.106&#8197;(6):0.894&#8197;(6) and a 0.350&#8197;(7):0.650&#8197;(7) ratio, respectively
Microstructure and Thermoelectric Properties of Dense YB22C2N Samples Fabricated Through Spark Plasma Sintering
No full text- …
