Effets de valence dans les composés base manganèse isotypes de HfFe6Ge6 ou ses variantes : Matériaux magnétoréfrigérants

The first part of this work concerns the study of the magnetic and magnetoresistance properties of RMn6X6-xX x (X, X = Sn, Ge, Ga, In) compounds known as 166 with non trivalent R element [R= tetravalent (Hf, Zr), divalent (Mg, Yb) and monovalent (Li) element]. In this part of the study, our result confirm at the same time the dependence of the magnetic properties with the valence electron concentration (VEC) and the fact that the parameters such as the interatomic Mn-Mn distances or the electronic configuration of the layer of valence of R element take part, with a less degree, to define the magnetic behaviour of these phases. Finally, the last of this work concerns the identification of the magnetocaloric properties of these 166 compounds and that of some binary of formulas Mny(Sn, Ga)z. In the case of 166 compounds, the magnetocaloric effect is insufficient to consider use these materials in the technological applications. On the other hand in the case of Mn3Sn2, the presence of two successive ferromagnetic transitions involves a magnetocaloric response to two peaks. The form and extended in temperature of Mn3Sn2 is interesting for the thermodynamic effectiveness, which classifies this material among the promising candidates for future magnetic refrigeration applications.La première partie de ce travail porte sur l'étude des propriétés magnétiques et magnétorésistives des phases RMn6X6-xX x (X, X = Sn, Ge, Ga, In) dites 166 à élément R non trivalent [R= métal tétravalent (Hf, Zr), divalent (Mg, Yb) et monovalent (Li)]. Dans cette partie de l'étude, nos résultats confirment à la fois la dépendance des propriétés magnétiques vis-à-vis de la Concentration en Electron de Valence (CEV) et le fait que les paramètres tels que les distances interatomiques Mn-Mn ou la configuration électronique de la couche de valence de R participent, à un degré moindre, à définir le comportement magnétique de ces phases. Enfin, la dernière partie de ce travail porte sur l'identification des propriétés magnétocaloriques de ces phases 166 et de celle de quelques binaires de formule Mny(Sn, Ga)z. Dans le cas des phases 166 l'ampleur de l'effet magnétocalorique est insuffisante pour envisager l'utilisation de ces matériaux dans les applications technologiques. En revanche dans le cas de Mn3Sn2, la présence de deux transitions ferromagnétiques successives entraîne une réponse magnétocalorique à deux pics. La forme et étendue en température de Mn3Sn2 sont intéressantes pour l'efficacité thermodynamique, ce qui classe ce matériau parmi les candidats prometteurs pour des futurs applications de la réfrigération magnétique

Effets de valence dans les composés base manganèse isotypes de HfFe6Ge6 ou ses variantes (Matériaux magnétoréfrigérants)

La première partie de ce travail porte sur l étude des propriétés magnétiques et magnétorésistives des phases RMn6X6-xX x (X, X = Sn, Ge, Ga, In) dites 166 à élément R non trivalent [R= métal tétravalent (Hf, Zr), divalent (Mg, Yb) et monovalent (Li)]. Dans cette partie de l étude, nos résultats confirment à la fois la dépendance des propriétés magnétiques vis-à-vis de la Concentration en Electron de Valence (CEV) et le fait que les paramètres tels que les distances interatomiques Mn-Mn ou la configuration électronique de la couche de valence de R participent, à un degré moindre, à définir le comportement magnétique de ces phases. Enfin, la dernière partie de ce travail porte sur l identification des propriétés magnétocaloriques de ces phases 166 et de celle de quelques binaires de formule Mny(Sn, Ga)z. Dans le cas des phases 166 l ampleur de l effet magnétocalorique est insuffisante pour envisager l utilisation de ces matériaux dans les applications technologiques. En revanche dans le cas de Mn3Sn2, la présence de deux transitions ferromagnétiques successives entraîne une réponse magnétocalorique à deux pics. La forme et étendue en température de Mn3Sn2 sont intéressantes pour l efficacité thermodynamique, ce qui classe ce matériau parmi les candidats prometteurs pour des futurs applications de la réfrigération magnétique.The first part of this work concerns the study of the magnetic and magnetoresistance properties of RMn6X6-xX x (X, X = Sn, Ge, Ga, In) compounds known as 166 with non trivalent R element [R= tetravalent (Hf, Zr), divalent (Mg, Yb) and monovalent (Li) element]. In this part of the study, our result confirm at the same time the dependence of the magnetic properties with the valence electron concentration (VEC) and the fact that the parameters such as the interatomic Mn-Mn distances or the electronic configuration of the layer of valence of R element take part, with a less degree, to define the magnetic behaviour of these phases. Finally, the last of this work concerns the identification of the magnetocaloric properties of these 166 compounds and that of some binary of formulas Mny(Sn, Ga)z. In the case of 166 compounds, the magnetocaloric effect is insufficient to consider use these materials in the technological applications. On the other hand in the case of Mn3Sn2, the presence of two successive ferromagnetic transitions involves a magnetocaloric response to two peaks. The form and extended in temperature of Mn3Sn2 is interesting for the thermodynamic effectiveness, which classifies this material among the promising candidates for future magnetic refrigeration applications.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Thermal stability of Mg2Si0.55Sn0.45 for thermoelectric applications

International audienceUnderstanding the thermal stability of the Mg2(Si,Sn) system is essential to define their safe tempera-tures of service. Despite its good thermoelectric performance, Mg2(Si,Sn) is subject to a phase separationduring thermal cycling due to the miscibility gap, which leads to a degradation of its thermoelectricproperties and affects its performance during device operation. Isothermal annealing at 500 !C and750 !C were performed with different annealing time to investigate thermal stability of Mg2(Si,Sn).During the heat treatment, two phases were formed associated with porosity in the matrix. In addition,thickness of specimen was tracked and a significant expansion was detected. This phenomenon isattributed to the Kirkendall effect. The composition and the structure of the two forming phases wereinvestigated by electron probe microanalysis and X-ray diffraction. Finally, the optimized thermaltreatment allowed to stabilize the Mg2(Si,Sn) without porosity and the presence of two thermody-namically stabilized phase (Mg2Si0.41Sn0.59 and Mg2Si0.58Sn0.42) leading to a better reliability of thesilicide thermoelectric modules

Reliability Investigation of Silicide-Based Thermoelectric Modules

International audienceThe reliability and failure mechanisms of silicide-based thermoelectric modules (n-type Mg2(Si,Sn)/p-type HMS) were investigated thanks to two types of thermal tests with either a fixed or a cycling thermal gradient, under different atmospheres. The hot interfaces of the thermoelectric modules were analyzed by scanning electron microscopy and X-ray diffraction after the reliability tests. The current thermoelectric modules do not exhibit any failure mechanism under ambient air for a hot side temperature of 250 °C for tests conducted either during 500 h at a fixed temperature gradient or after 1000 thermal cycles. However, when the temperature was increased to 350 °C, pesting phenomena were detected at the hot side of the n-type Mg2(Si,Sn) legs caused by the decomposition/oxidation of the material. These phenomena are strongly slowed down for thermoelectric modules tested under a primary vacuum, highlighting the predominant role of oxygen in the degradation mechanism. Interdiffusion phenomena are the most pronounced at the interface of the hot side of the n-type thermoelectric materials. The formation of a MgO layer, which is an electrical and thermal insulator, has decreased the thermoelectric modules’ performances. For thermal cycling tests, cracks are observed on the hot side of the n-type legs. The presence of these cracks drastically increases the thermal and electrical resistances, leading to an overheating of the system and limiting its efficiency and failure by breaking electrical continuity. The interfaces at the hot side temperature of the p-type HMS legs remained intact whatever the test conditions were, indicating a high chemical stability and a good mechanical strength

Manufacturing and performances of silicide-based thermoelectric modules

International audienceSilicide-based materials are among the most promising candidates for a mass manufacturing of thermoelectric devices allowing converting waste heat into electricity in the medium temperature range (250–500 °C), as they are formed from abundant, low cost and non-toxic elements while exhibiting good thermoelectric properties. In order to manage the detrimental mismatch of thermal expansion coefficients between the n and p-type materials constituting the thermoelectric legs, inducing thus thermomechanical stresses, we propose in this paper a new design of modules having a 'half-skeleton' structure. Twenty-two modules consisting of two couples of thermoelectric legs combining n-type magnesium silicide Mg2(Si, Sn) and p-type higher manganese silicide have been fabricated according to this design, the thermoelectric materials being manufactured by kilograms. It is clearly shown that all the interfaces present in the modules are free from cracks, oxygen, and diffusion. The remarkable repeatability of the measured thermoelectric performance attests the robustness of our manufacturing process. An average power output of 0.37 W has been achieved, i.e. a power density of 0.95 W/cm2, for a temperature difference of 400 °C (hot side temperature of 450 °C), placing our modules at the state of the art level while using simple production tools and materials mass production. Comparing this performance to finite elements modelling, the performance could be even enhanced. Long term stability tests at a given temperature and under cycling conditions in different working atmospheres are underway