Thermoelectric properties of Ca0.9Yb0.1MnO3-x prepared by spark plasma sintering in air atmosphere

In this paper we report the densification of Ca0.9Yb0.1MnO3-x ceramics by spark plasma sintering (SPS) in different atmospheres and using different tools. The thermoelectric properties of these ceramics are reported and compared to those of ceramics obtained by conventional sintering. This study clearly demonstrated the possibility for processing oxide materials by SPS under air atmosphere and overcomes the problem of reduction due to the low oxygen partial pressure (graphitic environment, dynamic vacuum or neutral atmosphere) commonly encountered

Induced Ferromagnetism and Colossal Magnetoresistance by Ir-Doping in Pr1-xCaxMnO3

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_{1-x}Ca_xMnO_3 (0.4 < x < 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180K and CMR ratio in 7T as large as 10^4. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr, ...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state.Comment: 9 pages, 5 figures, accepted for publication in Eur. Phys. J.

Etude cristallochimique et caracterisation de nouveaux oxydes supraconducteurs a haute temperature critique, a base de bismuth

SIGLEINIST T 75157 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Synthèse et utilisation de matériaux hybrides lamellaires pour l'élaboration et la caractérisation de nanocomposites polymères

L objectif de la thèse est d élaborer des matériaux hybrides organiques-inorganiques lamellaires originaux et de les disperser dans une matrice polymère afin d obtenir des nanocomposites. La mise en œuvre des nanocomposites s est faite à chaque fois dans deux types de matrices, le polyéthylène (PE) et le polyamide-12 (PA-12). Deux types d hybrides lamellaires y ont été dispersés. Le premier est l oxyde lamellaire KTiNbO5, modifié chimiquement par intercalation d alkylamines de différentes longueurs de chaîne carbonée pour améliorer sa compatibilité avec les différentes matrices polymères. La dispersion de ces matériaux hybrides a permis d améliorer plusieurs propriétés des matrices hôtes comme la stabilité thermique, la résistance mécanique et la résistance au feu. La deuxième famille de matériaux élaborés est celle des phosphonates de cuivre lamellaires porteurs de chaînes alkyles de différentes longueurs. L objectif de cette étude est de préparer un matériau hybride avec un métal de transition en vue de son incorporation dans le polymère. Une étude structurale approfondie de la série homologue a mis en évidence deux types de composés : hydratés pour les chaînes alkyles courtes (nombre de carbone =10. Ces deux types structuraux ont montré des comportements magnétiques différents. Enfin, l incorporation de ces charges dans les matrices polyéthylène et polyamide-12 a révélé que la nature covalente de la liaison chimique entre la partie organique et le feuillet inorganique ne permet pas d obtenir un état d exfoliation suffisant pour bénéficier de l effet nano sur les propriétés physiques.The aim of this work is to elaborate original lamellar hybrid materials and disperse them in a polymer matrix to obtain nanocomposites. They have been prepared using two different matrixes, one apolar (polyethylene) and one polar (polyamide-12). The two lamellar hybrid systems have been dispersed in these matrixes and studied as part of the hybrid nature. The first one, the lamellar oxide KTiNbO5, has been chemically modified with alkylamines having different carbon chain lengths in order to be compatible with the different polymer matrixes. The dispersion of such materials has allowed improving several properties like thermal stability, mechanical resistance and fire resistance. The second family studied is the copper alkylphosphonate one. They have been prepared with different alkyl chain lengths. The aim of this study is to prepare a hybrid material with a transition metal and to disperse it in the polymer matrixes. A deep structural study of the homologue series has revealed two types of compounds: hydrated for the short alkyl chains (number of carbon atoms=10. These two structural types have shown different magnetic behaviours. Finally, the incorporation of these fillers in polyethylene and polyamide-12 has revealed that the chemical nature of the bond which links the organic part to the inorganic one of the hybrid does not allow obtaining an efficient exfoliation mechanism.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Thermoelectric Ca0.9Yb0.1MnO3−δ grain growth controlled by spark plasma sintering

International audiencen-Type Ca0.9Yb0.1MnO3−δ thermoelectric (TE) powders were prepared by solid state synthesis (SSS) and co-precipitation method (Cop). The bulk TE materials were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) respectively. The shrinkage behavior, as well as the sample densification strongly depends on the starting particle size. Consequently, the bulk samples from normal powder (SSS) and nano-powder (Cop) were prepared with similar density by using different sintering temperatures, of 1400 °C and 1200 °C, then 1200 and 950 °C for CS and SPS respectively. Such a decrease (up to 200 °C) of the sintering temperature is a consequent progress in terms of engineering for applications. Another advantage of the co-precipitation process compared to the conventional solid state synthesis is that, due to the small particle sizes and the decreased sintering temperature, grain growth was limited and TE properties were enhanced. The interest of the SPS process was also evidenced and we are presenting here the structural and microstructural investigations. In addition, the thermoelectric properties of samples prepared with two different processes were studied with the figure of merit of 0.18 at 750 °C

Thermoelectric Ca0.9Yb0.1MnO3−δ grain growth controlled by spark plasma sintering

International audiencen-Type Ca0.9Yb0.1MnO3−δ thermoelectric (TE) powders were prepared by solid state synthesis (SSS) and co-precipitation method (Cop). The bulk TE materials were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) respectively. The shrinkage behavior, as well as the sample densification strongly depends on the starting particle size. Consequently, the bulk samples from normal powder (SSS) and nano-powder (Cop) were prepared with similar density by using different sintering temperatures, of 1400 °C and 1200 °C, then 1200 and 950 °C for CS and SPS respectively. Such a decrease (up to 200 °C) of the sintering temperature is a consequent progress in terms of engineering for applications. Another advantage of the co-precipitation process compared to the conventional solid state synthesis is that, due to the small particle sizes and the decreased sintering temperature, grain growth was limited and TE properties were enhanced. The interest of the SPS process was also evidenced and we are presenting here the structural and microstructural investigations. In addition, the thermoelectric properties of samples prepared with two different processes were studied with the figure of merit of 0.18 at 750 °C

Control of the Shrinkage during Hybrid Microwave Sintering of Nano-Sized ZnO Based Powder for Varistors

International audienc

Control of the Shrinkage during Hybrid Microwave Sintering of Nano-Sized ZnO Based Powder for Varistors

International audienc

Decrypting the TEM Images for Deciphering the Microstructural Code of Complex Oxides

International audienceThe knowledge of the structure of the real solids is required for achieving the desired architectures in the research of new materials and/or optimizing the relationships between structure and properties. Understanding complex oxides needs accurate characterization at different length scales and the combined application of all solid‐state techniques. Deciphering the relationships between all this information provides codes that allow the identification of the different structural levels, their roles and the way they interact. These step‐by‐step routes are illustrated through two basic mechanisms of solid‐state chemistry: to determine the building units of one complex oxide in order to predict the existence of other arrangements on the one hand and to correlate complex ordering phenomena, such as those involving charges, orbitals and spins of manganese atoms in perovskite‐type manganites on the other hand