Influence d'inclusions de PbTe ou de ZnO sur les propriétés thermoélectriques de matériaux skutterudites

This work was carried out as part of a study on nanostructured bulk thermoelectric materials, compounds whose properties can potentially be improved with many interfaces. It is related to the synthesis and characterization of skutterudites (CoSb3 or In0.4Co4Sb12) containing nanoinclusions of PbTe or ZnO. Techniques of powder metallurgy and laser fragmentation in liquid medium were combined to X-rays diffraction analyses, SEM and TEM observations to develop the experimental procedure for the preparation of materials. The microstructures of the composites show that the dispersion step is the most difficult part of the preparation as it leads to agglomerates located at the grain boundaries. The porosity is also relatively large when the quantity of inclusions is high especially with ZnO. The electrical and thermal properties (thermoelectric power, electrical resistivity, Hall effect, thermal conductivity) have been measured over a wide temperature range (2-800 K) and correlated with the microstructures. The results analysis showed that PbTe tends to degrade the electrical properties of the two matrixes because of a reaction between the nanoparticles and the skutterudites. Contrarily, the addition of ZnO seems more promising although it is difficult to conclude definitively on its effectiveness given the complexity of the microstructures (defects, pores, grain boundaries). A theoretical model developed to better understand the impact of the particle sizes on the thermal properties showed that it is more interesting to work with partially filled skutterudites instead of pure CoSb3 since the inclusions mostly affect the scattering of long wavelength phonons. The trends that emerge from this study are rather in the sense of a deterioration of the thermoelectric properties with this type of nanostructure when the quantities of nanoparticles exceed a few percentage. The use of smaller quantities and a better controlled porosity should be consideredCe travail a été réalisé dans le cadre d'une étude sur les matériaux massifs thermoélectriques nanostructurés, composés dont les propriétés peuvent potentiellement être améliorées grâce aux nombreuses interfaces. Il concerne la réalisation et la caractérisation de composites à matrices skutterudites (CoSb3 ou In0,4Co4Sb12) contenant différents taux d'inclusions nanométriques de PbTe ou de ZnO. Des techniques de métallurgie des poudres et de fracturation laser en milieu liquide ont été conjuguées à divers moyens de caractérisation (RX, MEB, MET) pour mettre au point le protocole d'élaboration des matériaux. L'observation des microstructures des composites a révélé que l'étape de dispersion des particules dans le matériau est la plus délicate de la préparation. Celles-ci forment des amas localisés aux joints de grains. La porosité est également relativement importante lorsque le taux d'inclusions est élevé en particulier avec ZnO. Les propriétés électriques (pouvoir thermoélectrique, résistivité électrique, effet Hall) et thermiques (conductivité thermique) ont été mesurées sur une large gamme de température (2-800 K) puis corrélées aux microstructures. L'analyse des résultats a permis de montrer que le PbTe tend à dégrader les propriétés électriques des deux matrices du fait notamment d'une réaction des nanoparticules avec les skutterudites. Par contre, l'ajout de ZnO semble plus prometteur même s'il est difficile de conclure définitivement quant à son rôle réel compte tenu de la complexité des microstructures (défauts, pores, joints de grains). Un modèle théorique développé afin de mieux comprendre l'impact des tailles de particules sur les propriétés thermiques a mis en évidence qu'il est plus intéressant de travailler avec des skutterudites partiellement remplies plutôt qu'avec CoSb3 puisque les inclusions affectent majoritairement les phonons les moins énergétiques. Les tendances qui se dégagent de cette étude vont plutôt dans le sens d'une détérioration des propriétés avec ce type de nanostructuration lorsque les taux de nanoparticules excèdent quelques pourcents mais l'utilisation de quantités plus faibles et une porosité mieux maîtrisée pourrait avoir un effet positi

Influence of PbTe or ZnO inclusions on the thermoelectric properties of skutterudites materials

Ce travail a été réalisé dans le cadre d’une étude sur les matériaux massifs thermoélectriques nanostructurés, composés dont les propriétés peuvent potentiellement être améliorées grâce aux nombreuses interfaces. Il concerne la réalisation et la caractérisation de composites à matrices skutterudites (CoSb3 ou In0,4Co4Sb12) contenant différents taux d’inclusions nanométriques de PbTe ou de ZnO. Des techniques de métallurgie des poudres et de fracturation laser en milieu liquide ont été conjuguées à divers moyens de caractérisation (RX, MEB, MET) pour mettre au point le protocole d’élaboration des matériaux. L’observation des microstructures des composites a révélé que l’étape de dispersion des particules dans le matériau est la plus délicate de la préparation. Celles-ci forment des amas localisés aux joints de grains. La porosité est également relativement importante lorsque le taux d’inclusions est élevé en particulier avec ZnO. Les propriétés électriques (pouvoir thermoélectrique, résistivité électrique, effet Hall) et thermiques (conductivité thermique) ont été mesurées sur une large gamme de température (2-800 K) puis corrélées aux microstructures. L’analyse des résultats a permis de montrer que le PbTe tend à dégrader les propriétés électriques des deux matrices du fait notamment d’une réaction des nanoparticules avec les skutterudites. Par contre, l’ajout de ZnO semble plus prometteur même s’il est difficile de conclure définitivement quant à son rôle réel compte tenu de la complexité des microstructures (défauts, pores, joints de grains). Un modèle théorique développé afin de mieux comprendre l’impact des tailles de particules sur les propriétés thermiques a mis en évidence qu’il est plus intéressant de travailler avec des skutterudites partiellement remplies plutôt qu’avec CoSb3 puisque les inclusions affectent majoritairement les phonons les moins énergétiques. Les tendances qui se dégagent de cette étude vont plutôt dans le sens d’une détérioration des propriétés avec ce type de nanostructuration lorsque les taux de nanoparticules excèdent quelques pourcents mais l’utilisation de quantités plus faibles et une porosité mieux maîtrisée pourrait avoir un effet positifThis work was carried out as part of a study on nanostructured bulk thermoelectric materials, compounds whose properties can potentially be improved with many interfaces. It is related to the synthesis and characterization of skutterudites (CoSb3 or In0.4Co4Sb12) containing nanoinclusions of PbTe or ZnO. Techniques of powder metallurgy and laser fragmentation in liquid medium were combined to X-rays diffraction analyses, SEM and TEM observations to develop the experimental procedure for the preparation of materials. The microstructures of the composites show that the dispersion step is the most difficult part of the preparation as it leads to agglomerates located at the grain boundaries. The porosity is also relatively large when the quantity of inclusions is high especially with ZnO. The electrical and thermal properties (thermoelectric power, electrical resistivity, Hall effect, thermal conductivity) have been measured over a wide temperature range (2-800 K) and correlated with the microstructures. The results analysis showed that PbTe tends to degrade the electrical properties of the two matrixes because of a reaction between the nanoparticles and the skutterudites. Contrarily, the addition of ZnO seems more promising although it is difficult to conclude definitively on its effectiveness given the complexity of the microstructures (defects, pores, grain boundaries). A theoretical model developed to better understand the impact of the particle sizes on the thermal properties showed that it is more interesting to work with partially filled skutterudites instead of pure CoSb3 since the inclusions mostly affect the scattering of long wavelength phonons. The trends that emerge from this study are rather in the sense of a deterioration of the thermoelectric properties with this type of nanostructure when the quantities of nanoparticles exceed a few percentage. The use of smaller quantities and a better controlled porosity should be considere

Wet air oxidation for the treatment of solid wastes generated on autarkic sites

International audienceIndustrial effluents have variable compositions depending on the industry: refineries, pharmaceuticals, distilleries, food processing, paper mill… The most widespread treatment of effluents containing organic pollutants is the biological way, but microorganisms are unsuitable in the case of refractory or toxic products. Legislations regulate more and more severely the management of these wastes and favor the development of alternative processes allowing to treat effectively particular pollutions, as in pre-treatment before a biological process or for a complete degradation of organic matters in carbon dioxide and in water. Other technologies are under research and development, as for instance Wet Air Oxidation (WAO), especially for wastewaters that contains high chemical oxygen demand

Wet air oxidation for the treatment of solid wastes generated on autarkic sites

International audienceIndustrial effluents have variable compositions depending on the industry: refineries, pharmaceuticals, distilleries, food processing, paper mill… The most widespread treatment of effluents containing organic pollutants is the biological way, but microorganisms are unsuitable in the case of refractory or toxic products. Legislations regulate more and more severely the management of these wastes and favor the development of alternative processes allowing to treat effectively particular pollutions, as in pre-treatment before a biological process or for a complete degradation of organic matters in carbon dioxide and in water. Other technologies are under research and development, as for instance Wet Air Oxidation (WAO), especially for wastewaters that contains high chemical oxygen demand

Wet air oxidation for the treatment of solid wastes generated on autarkic sites

International audienceIndustrial effluents have variable compositions depending on the industry: refineries, pharmaceuticals, distilleries, food processing, paper mill… The most widespread treatment of effluents containing organic pollutants is the biological way, but microorganisms are unsuitable in the case of refractory or toxic products. Legislations regulate more and more severely the management of these wastes and favor the development of alternative processes allowing to treat effectively particular pollutions, as in pre-treatment before a biological process or for a complete degradation of organic matters in carbon dioxide and in water. Other technologies are under research and development, as for instance Wet Air Oxidation (WAO), especially for wastewaters that contains high chemical oxygen demand

Influence of ZnO nano-inclusions on the transport properties of the CoSb3 skutterudite

International audienceAddition of nanoparticles into bulk materials is one of the strategies designed to synthesize thermoelectric materials with superior efficiency. The presence of nanoparticles is expected to act as a barrier of the heat transport due to an increased amount of nanoscopic interfaces and to enhance the electronic properties via quantum confinement effects. Here, this approach was applied to the skutterudite CoSb3 matrix in which different amounts of ZnO (2-12 wt.%) nanoparticles were introduced. The samples were densified by spark plasma sintering and characterized by X-ray diffraction and scanning electron microscopy. The thermoelectric properties were investigated in a broad temperature range (2-800 K) to better assess the influence of the ZnO nanoparticles. The nanostructured materials showed a significant enhancement in the thermopower values combined with a slight decrease in the thermal conductivity. A small amount of ZnO nanoparticles (2 wt.%) has a beneficial influence on the dimensionless thermoelectric figure of merit ZT suggesting that introducing stable oxide nanoparticles may be an interesting way to improve the thermoelectric properties of skutterudite compounds

Thermoelectric properties of In0.2Co4Sb12 skutterudites with embedded PbTe or ZnO nanoparticles

International audienceTransport properties of the skutterudite compound In0.2Co4Sb12 containing ZnO or PbTe nano-sized particles (2-12 wt%) were investigated by means of electrical resistivity, thermopower and thermal conductivity between 5 and 800 K. The composite powders were prepared by freeze-drying the nanoparticles with micron-sized In0.2Co4Sb12 powders. Densification was achieved by spark plasma sintering. All composites were characterized by X-ray powder diffraction and scanning electron microscopy. All the transport coefficients show similar temperature dependences suggesting little influence of the nature, semiconducting or insulating, of the nanoparticles. Both the electrical and the thermal conductivities decrease with increasing the PbTe or ZnO content. The impact of ZnO and PbTe on the thermal conductivity was modelled based on the Debye model taking into account a relaxation time constant reflecting phonon scattering by spherical nanoparticles. A maximum dimensionless figure of merit ZT of 1.05 at 700 K was achieved in a sample containing 2 wt% ZnO, a value quite similar to that of the reference In0.2-Co4Sb12 compound

Thermoelectric properties of In0.2Co4Sb12 skutterudites with embedded PbTe or ZnO nanoparticles

International audienceTransport properties of the skutterudite compound In0.2Co4Sb12 containing ZnO or PbTe nano-sized particles (2-12 wt%) were investigated by means of electrical resistivity, thermopower and thermal conductivity between 5 and 800 K. The composite powders were prepared by freeze-drying the nanoparticles with micron-sized In0.2Co4Sb12 powders. Densification was achieved by spark plasma sintering. All composites were characterized by X-ray powder diffraction and scanning electron microscopy. All the transport coefficients show similar temperature dependences suggesting little influence of the nature, semiconducting or insulating, of the nanoparticles. Both the electrical and the thermal conductivities decrease with increasing the PbTe or ZnO content. The impact of ZnO and PbTe on the thermal conductivity was modelled based on the Debye model taking into account a relaxation time constant reflecting phonon scattering by spherical nanoparticles. A maximum dimensionless figure of merit ZT of 1.05 at 700 K was achieved in a sample containing 2 wt% ZnO, a value quite similar to that of the reference In0.2-Co4Sb12 compound

Crystal structure and high temperature thermoelectric properties of the Mo_3-xRu_xSb₇ compounds

International audienceZintl phases are currently focusing great attention for their thermoelectric potential typified by the discovery of a high ZT value in Yb14MnSb11 based compounds. Here, we report on the crystallographic characterization via neutron and X-ray diffraction experiments and on the thermoelectric properties measured in the 300-1000 K temperature range of Mo3Sb7 and its isostructural compounds Mo3-xRuxSb7. Even though Mo3Sb7 displays rather high ZT values given its metallic character, the partial substitution of Mo by Ru improves substantially its thermoelectric properties resulting in a ZT value of ~ 0.45 at 1000 K for x = 0.8

Crystal structure and high temperature thermoelectric properties of the Mo_3-xRu_xSb₇ compounds

International audienceZintl phases are currently focusing great attention for their thermoelectric potential typified by the discovery of a high ZT value in Yb14MnSb11 based compounds. Here, we report on the crystallographic characterization via neutron and X-ray diffraction experiments and on the thermoelectric properties measured in the 300-1000 K temperature range of Mo3Sb7 and its isostructural compounds Mo3-xRuxSb7. Even though Mo3Sb7 displays rather high ZT values given its metallic character, the partial substitution of Mo by Ru improves substantially its thermoelectric properties resulting in a ZT value of ~ 0.45 at 1000 K for x = 0.8