67 research outputs found

    NOUVEAUX MATERIAUX COMPOSITES POUR ELECTRODES NÉGATIVES A BASE D'ETAIN

    No full text
    Currently, commercialised batteries work with negative electrodes based on carbon or graphite which show good cycling properties, but have limited mass and volume capacities and and may be unsafe under certain conditions In order to increase the specific energy, several metals forming alloys with lithium were proposed as new generation accumulators due to their high energy density. The aim of the present work is the search and development of new composite materials synthesized by ex situ dispersion of tin in an inactive matrix (CaSiO3). The performances of the composite ‘‘Sn-0,4CaSiO3'' chosen as the reference compound are interesting: high reversible mass capacity of 480 mAh.g-1, low polarization of only 140 mV. However, the first-cycle capacity loss of 146 mAh.g-1 is too important and the cycling stability is insufficient.To understand the reasons responsible for these two phenomena we have undertaken a detailed study of the reaction mechanism governing the first cycle of restructuration. To this end, several experimental techniques were combined. We show that the charge/discharge rate influences the restructuration process. At a C/50 rate, the formation of tin-rich and stable intermediate alloys like LiSn leads to poorer performances than the restructuration process taking place at a C/10 rate. The chemical composition of the matrix influences the electrochemical properties and in particular the first-cycle capacity loss. The use of sodium borosilicate, which is a better conductor than calcium silicate, significantly reduces the irreversible capacity in the first cycle (90 mAh.g-1).Actuellement, les batteries commercialisées fonctionnent avec des électrodes négatives à base de carbone qui présentent une bonne tenue en cyclage mais des capacités massique et volumique limitées et des problèmes de sécurité. Pour améliorer les performances des accumulateurs de nouvelle génération, les métaux purs alliables avec le lithium ont été proposés en raison de leur grande densité d'énergie.L'objectif de cette thèse consiste à élaborer de nouveaux matériaux composites, synthétisés par dispersion ex situ de l'étain dans une matrice inactive (CaSiO3).Les performances du composite de référence sélectionné ‘‘Sn-0,4 CaSiO3'' sont intéressantes : capacité massique réversible de 480 mAh.g-1 et faible polarisation de 140 mV. Cependant, la perte au premier cycle (146 mAh.g-1) est encore trop importante et la tenue en cyclage insuffisante. Pour comprendre les causes de ces deux phénomènes nous avons entrepris l'étude détaillée du mécanisme mis en jeu au cours du premier cycle de restructuration en couplant différentes techniques expérimentales. Les études montrent que le régime influe sur l'étape de restructuration. En régime C/50, la formation d'alliages intermédiaires stables, riches en étain, type LiSn, entraîne une restructuration moins performante que celle réalisée en régime C/10.Nous avons montré que la modification de la matrice de dispersion joue un rôle important sur les paramètres électrochimiques et en particulier sur la perte au premier cycle. Ainsi l'utilisation d'un borosilicate de sodium, plus conducteur, réduit nettement cette perte (90 mAh.g-1)

    Nouveaux matériaux composites pour électrodes négatives à base d'étain

    No full text
    MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

    A Novel Approach to the Sintering Schedule of Ba (Co0.7Zn0.3)1/3Nb2/3O3 Dielectric Ceramics for Microwave Applications

    No full text
    The present research is devoted to the optimization of the sintering schedule of Ba (Co0.7Zn0.3)1/3Nb2/3O3 (BCZN) dielectric ceramics for microwaves applications. A novel approach to the heat treatment of these ceramics based on the rapid-rate sintering (RRS) technique followed by a lower temperature annealing cycle has been developed. The relationships among the heat treatment process optimization, the structural, microstructural characteristics, and the microwave dielectric properties of the BCZN ceramics were investigated using X-ray diffraction, scanning electron microscopy, energy dispersion analysis, and vector network analysis. The RRS-technique shortens substantially the time required for the elaboration of these components in comparison with conventional sintering techniques and prevents simultaneously the formation of secondary phases as Ba5Nb4O15 and Ba8(Co, Zn) 1Nb6O24 on the surface of the ceramics. All of the sintered and annealed ceramics exhibit a high quality factor QF close to 110 000 GHz at 6 GHz. The high dielectric constant εr of ∼34.5 and a temperature coefficient of the resonant frequency τf of ∼0 ppm.C-1 were obtained in all annealed ceramics

    Flash combustion synthesis of electron doped-CaMnO3 thermoelectric oxides

    No full text
    International audienceNano-crystalline particles of calcium manganite Ca1 − xRxMnO3 (R = Yb, Dy, Sm and Bi, with x = 0.0 and 0.1), having a complex orthorhombic-perovskite structure, have been successfully synthesized by flash combustion method. The powders obtained were thoroughly characterized by scanning electron microscopy and thermal analyses. The average crystallite size is found in the range between 100 and 200 nm. The crystallization was followed by in situ high-temperature X-ray diffraction in air. The XRD analyses show that the cell volume of the orthorhombic-perovskite structure for Ca0.9R0.1MnO3 increased with a rise in the ionic radius of the R3 + cations. The stoichiometry of the samples was confirmed by energy dispersive spectroscopy analysis. The temperature dependence of thermoelectric properties was measured from 75 to 400 °C and the R-dopant effects were investigated. The electrical resistivity is strongly dependent on the nature of the substituent ions. All the samples exhibit negative values of the thermopower, which indicates that the electrons are the predominant charge carriers (n-type conduction). Finally, the highest power factor of 165 μW·m− 1 K− 2 has been obtained at 400 °C for Ca0.9Yb0.1MnO3. In this paper, we suggest to use the fine grains of Ca0.9R0.1MnO3 as promising candidates for thermoelectric applications

    Flash combustion synthesis of electron doped-CaMnO3 thermoelectric oxides

    No full text
    International audienceNano-crystalline particles of calcium manganite Ca1 − xRxMnO3 (R = Yb, Dy, Sm and Bi, with x = 0.0 and 0.1), having a complex orthorhombic-perovskite structure, have been successfully synthesized by flash combustion method. The powders obtained were thoroughly characterized by scanning electron microscopy and thermal analyses. The average crystallite size is found in the range between 100 and 200 nm. The crystallization was followed by in situ high-temperature X-ray diffraction in air. The XRD analyses show that the cell volume of the orthorhombic-perovskite structure for Ca0.9R0.1MnO3 increased with a rise in the ionic radius of the R3 + cations. The stoichiometry of the samples was confirmed by energy dispersive spectroscopy analysis. The temperature dependence of thermoelectric properties was measured from 75 to 400 °C and the R-dopant effects were investigated. The electrical resistivity is strongly dependent on the nature of the substituent ions. All the samples exhibit negative values of the thermopower, which indicates that the electrons are the predominant charge carriers (n-type conduction). Finally, the highest power factor of 165 μW·m− 1 K− 2 has been obtained at 400 °C for Ca0.9Yb0.1MnO3. In this paper, we suggest to use the fine grains of Ca0.9R0.1MnO3 as promising candidates for thermoelectric applications

    Etude de la synthèse de CaZrO3 par flash combustion

    No full text
    International audienceLa méthode d’élaboration de poudre par flash combustion permet l’obtention de poudres de taille nanométrique. Des quatre combustibles testés, ceux semblant mener le plus facilement à la formation de la phase recherchée (CaZrO3) sont la glycine et la ß-alanine. Ils conduisent à des poudres cristallisées après synthèse ainsi qu’à des éramiques relativement denses après frittage non optimisé

    Effect of Post-Annealing on the Microstructure and Microwave Dielectric Properties of Ba(Co0.7Zn0.3)1/3Nb2/3O3 Ceramics

    No full text
    International audienceThe effects of post-annealing on the crystal structure, microstructure, and microwave dielectric properties for Ba(Co0.7Zn0.3)1/3Nb2/3O3 ceramics were investigated. The as-prepared materials were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The microwave dielectric properties are measured at 6 GHz using a network analyzer. Ba5Nb4O15 and/or Ba8(Co,Zn)1Nb6O24 secondary phases were found on the surface according to sintering conditions due to volatilization of some Zn and Co elements. The experimental results show that the beneficial effect of the annealing steps to improved the microwave dielectric properties. Excellent microwave dielectric properties were achieved for the coarse-grained microstructures by a higher sintering temperature and with a shorter holding time followed by annealing steps at lower temperatures with a longer holding time. This improvement can be attributed to 1:2 cation ordering within the crystal, which is taking place during annealing process. The Ba(Co0.7Zn0.3)1/3Nb2/3O3 ceramic could be used successfully for realization of dielectric microwave resonators, since it has a high quality factor Qf value of 123,700 GHz, a high dielectric constant εr value of 34.5 and a temperature coefficient of the resonant frequency τf of 0 ppm/°C

    Céramiques diélectriques pour condensateurs multicouches à armatures métalliques non nobles

    No full text
    We present a pre-study carried out on the mixing and grinding conditions of precursor powder slurries (CaCO3, TiO2 ZrO2) according to the dispersing medium, the pH, the addition of dispersant, in order to prepare a solid solution of CZT. Laboratory and high energy attritor, pulverizer, will be used. The impact of these different processes on the characteristics of the powders (BET surface, particle size) and on chamottage was studied.Ceramic pellets will be prepared and sintered to determine the dielectric properties.Nous présentons une prè-étude menée sur les conditions de mélange et broyage des barbotines des poudres précurseurs (CaCO3, TiO2 ZrO2) en fonction du milieu dispersant, du pH, de l’ajout de dispersant, afin de préparer une solution solide de CZT. Attriteur de laboratoire et de haute énergie, pulvérisette, seront utilisés. L’impact de ces différents procédés sur les caractéristiques des poudres (surface BET, granulométrie) et sur le chamottage a été étudié.Des pastilles céramiques seront préparées et frittées afin de déterminer les propriétés diélectriques
    • …
    corecore