Effect of Nanostructuring on the Thermoelectric Properties of β-FeSi2

Nanostructured β-FeSi2 and β-Fe0.95Co0.05Si2 specimens with a relative density of up to 95% were synthesized by combining a top-down approach and spark plasma sintering. The thermoelectric properties of a 50 nm crystallite size β-FeSi2 sample were compared to those of an annealed one, and for the former a strong decrease in lattice thermal conductivity and an upshift of the maximum Seebeck’s coefficient were shown, resulting in an improvement of the figure of merit by a factor of 1.7 at 670 K. For β-Fe0.95Co0.05Si2, one observes that the figure of merit is increased by a factor of 1.2 at 723 K between long time annealed and nanostructured samples mainly due to an increase in the phonon scattering and an increase in the point defects. This results in both a decrease in the thermal conductivity to 3.95 W/mK at 330 K and an increase in the power factor to 0.63 mW/mK2 at 723 K

Synthèse par magnésioréduction d'intermétalliques thermoélectriques : mécanismes, caractérisations multi-échelles et propriétés

An alternative synthesis route for thermoelectric intermetallics by magnesioreduction was developed and applied to two classes of materials: CoSb3-based skutterudites and the β-FeSi2 and MnSiγ (γ ≈ 1.74) silicides. These materials present a strong industrial potential but their syntheses by conventional methods are difficult and their lattice thermal conductivities must be reduced. The results of this work show that lower reaction temperatures and durations can be achieved by this route. In addition, it enables the direct synthesis of powders with (i) high purities, (ii) controlled chemical compositions and especially dopants concentration, and (iii) submicronic grain sizes which are suitable for the fabrication of mesostructured materials (grain size between 500 nm et 1 μm) by spark plasma sintering. The structural characteristics were studied by powder X-ray diffraction using advanced models taking into account the composite structure of MnSiγ (3D+1 space group) and the presence of (100)[011]/2 stacking faults in β-FeSi2. The microstructure was characterized by EBSD and TEM in order to evidence relations with the thermoelectric properties and especially the reduction of the lattice thermal conductivities. However, the figure-of-merit ZT has only been improved in the case of β-Co0.07Fe0.93Si2 because the electrical resistivities of the materials are also affected by the mesostructuration. The influence of the reaction parameters as well as the reaction mechanisms were studied in detail in order to apply the magnesioreduction syntheses to a wider range of materials.Une voie de synthèse alternative de matériaux intermétalliques thermoélectriques par magnésioréduction, a été mise au point et appliquée à deux types de matériaux : les skutterudites basées sur le composé CoSb3 et les siliciures β-FeSi2 et MnSiγ (γ ≈ 1.74). Ces matériaux présentent un fort intérêt industriel mais ont des synthèses complexes par les méthodes conventionnelles et nécessitent un abaissement de leurs conductivités thermiques. Les résultats de ce travail montrent que ce type de synthèse permet, tout en diminuant les températures et durées de réaction, d’obtenir directement des poudres (i) de hautes puretés, (ii) de compositions chimiques contrôlées, notamment vis-à-vis de la concentration en dopants, et (iii) de tailles submicroniques adaptées à la fabrication par frittage SPS de matériaux massifs mésostructurés (grains entre 500 nm et 1 μm). Les caractéristiques structurales ont été étudiées par diffraction des rayons X sur poudre en prenant en compte des modèles avancés pour la structure composite (groupe d’espace 3D+1) de MnSiγ et les défauts d’empilement (100)[011]/2 de β-FeSi2. La microstructure a été caractérisée par EBSD et MET afin de mettre en évidence les relations avec les propriétés thermoélectriques, notamment avec la réduction de la conductivité thermique de réseau. Le facteur de mérite thermoélectrique ZT n’est toutefois amélioré que dans le cas de β-Co0,07Fe0,93Si2 car la résistivité électrique est aussi impactée par la mésostructure des matériaux. L’influence des conditions réactionnelles ainsi que les mécanismes de réaction ont été étudiés en détail afin de permettre l’application de cette méthode de synthèse à une large gamme de matériaux

Processing of Thermoelectric Transition Metal Silicides Towards Module Development

International audienceThis chapter presents recent advances on the processing of transition metal silicides for thermoelectricity through various techniques, such as magnesiothermic reduction synthesis, mechanical alloying, melting processes and additive manufacturing. It emphasizes the importance of a better understanding of the crystal structure of these materials and its evolution during operating conditions through some examples of advanced structural characterization studies of transition metal silicides. The chapter discusses several scalable techniques to synthesize silicide materials as well as the importance of carefully investigating their structural properties to obtain equilibrium materials with stable performances. It also presents recent studies on module/leg development of silicide materials which will hopefully pave the way for future applications. Lots of efforts have been devoted to the synthesis and process development of transition metal silicide materials but the realization of functional devices integrating them still remains a challenge

X-ray absorption study on the origin of the deviation from Vegard's law for U(Al1−xGex)3 solid solution

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Magnesioreduction synthesis of silicides: the structure-properties relationship

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Mécanismes de réaction et propriétés thermoélectriques de In0:22Co4Sb12 préparé par magnésiothermie

International audienceThe magnesioreduction synthesis of In 0.22 Co 4 Sb 12 with high In-rattler concentration from Sb 2 O 4 and In-doped Co 3 O 4 precursors is reported. This process directly yields a submicronic powder in a single step of 96 h at 810 K. The reaction mechanism has been investigated by stopping the reaction every 12 h and quantifying the existing phases by X-ray diffraction and Rietveld refinements. The precursors are first reduced in CoO and Sb 2 O 3 lower oxides, then form CoSb 2 O 6 and CoSb 2 O 4 intermediates which are finally reduced in In x Co 4 Sb 12. A powder with 350 nm average size and mostly composed of In-filled skutterudite phase with composition close to In 0.17 Co 4 Sb 12 is obtained. Upon spark plasma sintering, small residual amount of InSb reacts with the skutterudite matrix to form a single-phase densified pellet with composition close to In 0.22 Co 4 Sb 12. The resulting densified material with 1.8 µm average grain size shows a figureof-merit ZT max of 0.95 at 750 K

Crystal structure and high temperature X-ray diffraction study of thermoelectric chimney-ladder FeGeγ (γ ≈ 1.52)

International audienceThe crystal structure of Nowotny chimney-ladder phase FeGeγ with γ ≈ 1.52 was studied by X-ray diffractometry from 300 K to 850 K. The diffraction patterns were fitted by Rietveld refinement considering an incommensurate composite crystal structure. The refined crystal structure of FeGeγ is described in details and compared to MnSiγ, a thermoelectric material with γ ≈ 1.74. The lattice parameters a, cFe and cGe were found to increase with the temperature following a polynomial law, while the modulation vector component γ remained constant up to the peritectoid decomposition into FeGe and FeGe2. Within the temperature range considered, the linear and volumetric thermal expansion parameters increased from about 3 × 10−6 K−1 to 10 × 10−6 K−1 and from 9.6 × 10−6 K−1 to 31.5 × 10−6 K−1, respectively

Crystal structure and high temperature X-ray diffraction study of thermoelectric chimney-ladder FeGeγ (γ ≈ 1.52)

International audienceThe crystal structure of Nowotny chimney-ladder phase FeGeγ with γ ≈ 1.52 was studied by X-ray diffractometry from 300 K to 850 K. The diffraction patterns were fitted by Rietveld refinement considering an incommensurate composite crystal structure. The refined crystal structure of FeGeγ is described in details and compared to MnSiγ, a thermoelectric material with γ ≈ 1.74. The lattice parameters a, cFe and cGe were found to increase with the temperature following a polynomial law, while the modulation vector component γ remained constant up to the peritectoid decomposition into FeGe and FeGe2. Within the temperature range considered, the linear and volumetric thermal expansion parameters increased from about 3 × 10−6 K−1 to 10 × 10−6 K−1 and from 9.6 × 10−6 K−1 to 31.5 × 10−6 K−1, respectively

Influence of Stoichiometry and Aging at Operating Temperature on Thermoelectric Higher Manganese Silicides

International audienceThermoelectric higher manganese silicides, MnSix, were synthesized by magnesioreduction followed by spark plasma sintering with different nominal compositions (x = 1.65, 1.74, and 1.80) and various postsynthesis annealing durations (0, 48, 96, and 336 h). The composite Nowotny chimney-ladder crystal structures of the resulting samples were investigated by synchrotron X-ray powder diffraction. The modulation vector component gamma, generally considered corresponding to the stoichiometry (x) of the material, was accurately determined by Rietveld refinement using a (3 + 1)D superspace approach. Regardless of the initial nominal composition, all the samples have a similar gamma similar to 1.736 after 48 h of annealing at 900 K. This result suggests that MnSix, at a temperature of 900 K, is better described as a defined compound with x close to 1.736, rather than intermediate solid-solution phases with 1.725 < x < 1.75 as predicted by the commonly accepted phase diagram. At the fixed nominal composition MnSi1.74, gamma increases significantly from 1.7313(2) to 1.7411(1) after 336 h of annealing, indicating that the thermal history influences the Si stoichiometry. The evolution of gamma with time is concomitant with a power factor drop (-19%), attributed to a decrease in charge carrier concentration. The drop of the power factor, partially compensated by a decrease in thermal conductivity, results in a -12% reduction of the maximum figure-of-merit ZT, after prolonged annealing under realistic application conditions

Innovative synthesis of mesostructured CoSb 3 -based skutterudites by magnesioreduction

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