Impact of Spark Plasma Sintering Conditions on Ionic Conductivity in La1.95Sr0.05Zr2O7-δ Electrolyte Material for Intermediate Temperature SOFCs

International audienceSolid Oxide Fuel Cells (SOFC) have attracted much attention as potential energy source. Their high operating temperatures (800°C-1000°C) can lead to thermal, mechanical and chemical problems such as densification of electrodes or formation of an insulating layer at the electrode/electrolyte interface by interdiffusion [1-2]. To overcome these drawbacks, the Proton Ceramic Fuel Cell (PCFC) technology was developed. This technology, where the electrolyte is an H+ ion conductor in the form of ceramic oxide material, exhibits the intrinsic benefits of proton conduction in Polymer Exchange Membrane Fuel Cells (PEMFC) and the advantages of the SOFC technologies. Since the discovery of high temperature protonic conductivity in cerates [3-4], many investigations about pyrochlore-type proton conductors are performed [5-6]. These systems are characterized by mixed valence oxides (often rare earth) and anion vacancies as primary lattice defects. Under wet atmosphere, the proton conduction occurs via the hydration of oxygen vacancies after the material is exposed to a vapour-containing atmosphere according to the following equation 1 (inserted as part of the image file).     The conventional route for the preparation of lanthanum zirconate pyrochlore (LSZO) via solid-state reactions requires multiple milling and high temperature calcination steps. Also, this method leads generally to an heterogeneity of the final product, whereas wet chemical route, which consists of mixing precursors in a solution, could improve compositional homogeneity and stoichiometry. In this work, we have synthesized nano-sized La1,95Sr0,05Zr2O7-d using an oxalic co-precipitation method. As impedance spectroscopy measurements require high densification, only spark plasma sintering (SPS) gives dense materials. Other sintering processes such as hot isostatic pressing induce a segregation of strontium at the surface of the pellet[7],and thereby decrease proton conductivities. LSZO powders were densified using SPS apparatus under different sintering conditions: holding time, temperature and pressure. To maintain the same compacity for different grain sizes, starting powder materials were calcined at different temperatures in order to increase of the particle size. Thus several pellets with either different relative densities or grain sizes were obtained. The grain size increases with increasing of the sintering temperature. The proton conductivity behavior of those pellets was investigated by AC impedance spectroscopy under dry and wet atmospheres. The data were measured in the frequency range 0.1Hz – 6 MHz (Materials mates M2-7260 impedance analyzer) at intermediate temperatures 400-600°C. In order to verify the dependence of total resistance and capacitance, a DC-bias (UDC from 0 to 1V) was applied. The Nyquist diagrams were modeled by equivalent circuits based on resistors and constant phase elements (CPEs).  The ionic conductivities are clearly dependent on grain sizes (see Figure 1). In order to elucidate this dependence, it will be necessary to assess a porosity correction equation. The activation energies, calculated using the Arrhenius equation, increase with increasing grain sizes. The proton conductivities are higher in wet atmosphere than dry atmosphere. For example, the ionic conductivities of 120 nm-LSZO are 2.45 × 10-5 S.cm-1 and 3.30 × 10-5 S.cm-1 under dry and wet atmosphere (5% H2O) at 600°C, respectively. Figure 1 - Nyquist plots of impedance spectra for LSZO with different particle sizes at 600°C References[1]      S.C. Singhal, Solid State Ion. 135 (2000) 305. [2]      C. Xia, W. Rauch, F. Chen, M. Liu, Solid State Ion. 149 (2002) 11. [3]      F. Chen, O.T. Sørensen, G. Meng, D. Peng, J. Mater. Chem. 7 (1997) 481. [4]      H. Iwahara, H. Uchida, K. Ono, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. [5]      K.E.J. Eurenius, E. Ahlberg, C.S. Knee, Dalton Trans. 40 (2011) 3946. [6]      T. Shimura, M. Komori, H. Iwahara, Solid State Ion. 86–88, Part 1 (1996) 685. [7]      D. Huo, D. Gosset, G. Baldinozzi, D. Siméone, H. Khodja, B. Villeroy, S. Surblé, Solid State Ion. (submitted)

Three-dimensional lanthanide-organic frameworks based on di-, tetra-, and hexameric clusters

Three-dimensional lanthanide-organic frameworks formulated as (CH3)2NH2[Ln(pydc)2] · 1/2H2O [Ln3+ ) Eu3+ (1a) or Er3+ (1b); pydc2- corresponds to the diprotonated residue of 2,5-pyridinedicarboxylic acid (H2pydc)], [Er4(OH)4(pydc)4(H2O)3] ·H2O (2), and [PrIII 2PrIV 1.25O(OH)3(pydc)3] (3) have been isolated from typical solvothermal (1a and 1b in N,N-dimethylformamide - DMF) and hydrothermal (2 and 3) syntheses. Materials were characterized in the solid state using single-crystal X-ray diffraction, thermogravimetric analysis, vibrational spectroscopy (FT-IR and FT-Raman), electron microscopy, and CHN elemental analysis. While synthesis in DMF promotes the formation of centrosymmetric dimeric units, which act as building blocks in the construction of anionic ∞ 3{[Ln(pydc)2]-} frameworks having the channels filled by the charge-balancing (CH3)2NH2 + cations generated in situ by the solvolysis of DMF, the use of water as the solvent medium promotes clustering of the lanthanide centers: structures of 2 and 3 contain instead tetrameric [Er4(μ3-OH)4]8+ and hexameric |Pr6(μ3-O)2(μ3-OH)6| clusters which act as the building blocks of the networks, and are bridged by the H2-xpydcx- residues. It is demonstrated that this modular approach is reflected in the topological nature of the materials inducing 4-, 8-, and 14-connected uninodal networks (the nodes being the centers of gravity of the clusters) with topologies identical to those of diamond (family 1), and framework types bct (for 2) and bcu-x (for 3), respectively. The thermogravimetric studies of compound 3 further reveal a significant weight increase between ambient temperature and 450 °C with this being correlated with the uptake of oxygen from the surrounding environment by the praseodymium oxide inorganic core

An EXAFS study of the formation of a nanoporous metal–organic framework: evidence for the retention of secondary building units during synthesis

EXAFS data measured from amorphous intermediates and crystallisation solutions provides the first evidence that trimeric iron oxide secondary building units remain intact during crystallisation of the metal-organic framework MIL-89 from starting materials to products

Synthèse et caractérisations physico-chimiques de matériaux hybrides poreux multifonctionnels

Les solides poreux sont connus comme étant une classe de matériaux stratégiques pour leurs potentielles applications dans des domaines tels que la catalyse, l adsorption et / ou le stockage de gaz, les propriétés optiques. Le but de ce travail était de synthétiser de nouveaux matériaux poreux multifonctionnels. Le chapitre bibliographique retrace l histoire des solides poreux depuis les zéolithes jusqu à nos jours. Une description structurale brève de quelques solides hybrides à base de carboxylates est développée. Les chapitres suivants mentionnent deux types de solides illustrant parfaitement le concept de chimie d échelle et montrent aussi l apport de la simulation numérique lors de la résolution structurale. Les matériaux de type MIL88 sont des solides flexibles isoréticulaires. Ces matériaux respirent, par adsorption de liquides, avec une amplitude unique en son genre dans le domaine des solides poreux : entre 87% et 230%. Le phénomène est par ailleurs sélectif et réversible. Les solides hybrides à pores géants MIL100 et MIL101, dont les structures ont été déterminées à partir des données de diffractions des rayons X par les poudres combinées à la simulation numérique, ce qui est une première dans l histoire de la résolution structurale. Ces solides sont uniques avec une hiérarchie de pores: des cages microporeuses et deux cages mésoporeuses parfaitement cristallisées. Leurs volumes de maille sont sans précédant (380000 Å3 et 700000 Å3) et des surfaces spécifiques records sont mesurées: 3100 m2.g-1 et 5900 m2.g-1. Les deux derniers chapitres rapportent d une part la synthèse de nouveaux carboxylates de terres rares et d autre part une étude spectroscopique des cations Eu3+ dans le but d établir une corrélation entre la structure cristalline et les propriétés de luminescence.Synthesis and physico-chem Porous solids are known as being a strategie mate rials class for their potential applications in fields such a~ catalysis, gas separation, storage or optical properties. This work has aimed to synthesize new multifonctionnal porous materials.The first chapter recalls the history of porous solids from zeolithes to today. A short structural description of several hybrid solid with carboxylates has been developed. The following chapters mention two solids, iIIustrating clearly the concept of scale chemistry and they show the contribution of simulation in resolution of structures from X-ray powder data. Porous solids, ca lied MIL88, are flexible and isoreticular. The Amplitude of breathing is unique in the field of porous materials: between 87% and 230%. This amplitude is selective and reversible.Structural determination of hybrid solids with large pores size (MIL 100 and MIL 101) using X-ray powder data is a turning point i our history. These solids are unique with hierarchy of extra-large pore sizes: microporous cage and two mesoporous cages perfectly crystallized. They have a giant cells volumes (380000 A3 et 700000 A3) and a record surface area : 3100 m2.g-1 and 5900 m2.g-1. The two final sections bring up on the one hand, the synthesis of new carboxylate of rare earths and on the other hand a spectroscopie study of the Eu3+ cations in attempt of proposing a correlation between crystalline structure and optical properties.ical characterization of multifonctionnal porous materialsVERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

Structural changes upon dehydration of Pr(III)(H2O){C6H3 (CO2)3} or MIL-81: A new three-dimensional praseodymium 1,2,4-benzenetricarboxylate with a one dimensional inorganic sub-network

75 FIELD Section Title:Crystallography and Liquid Crystals Institut Lavoisier, UMR CNRS 8180,Universite de Versailles St-Quentin en Yvelines,Versailles,Fr. FIELD URL: written in English.A new three-dimensional lanthanide(III) tricarboxylate, MIL-81 or Pr(III)(H2O)(C6H3-(CO2)3) has been obtained under hydrothermal conditions. Its three-dimensional structure, which has been detd. using X-ray powder diffraction data, is built-up from edge-sharing chains of nine coordinated praseodymium(III) capped square antiprisms linked through 1,2,4-Benzenetricarboxylate (1,2,4-BTC) moieties. Its thermal behavior has been investigated using TGA and X-ray thermo-diffractometry and reveals that dehydration is followed by an irreversible structural change giving the solid MIL-81ht or Pr(III)(C6H3 -(CO2)3) with both a change in the environment of the rare earth and in the connection mode of the carboxylate. Crystal data for MIL-81: monoclinic space group P21 with a = 10.272 (1) .ANG., b = 7.057 (1) .ANG., c = 6.232 (1) .ANG., b = 93.668 (4) Deg and Z = 2. Crystal data for MIL-81ht: triclinic space group P-1 with a = 9.864 (1) .ANG., b = 7.054 (1) .ANG., c = 5.784 (1) .ANG., a = 90.862 (9) Deg, b = 92.439 (6) Deg, g = 91.594 (8) Deg and Z = 2

Impact of disorder on ionic charge in spinel compounds

International audienceIn order to obtain a correlation between the ionic charge and the local environment, the evolution of valence charges of cations in different 2-3 spinel compounds was investigated as a function of the temperature. The evolution of the structural parameters in normal (MgAl(2)O(4)), mixed (MgGa(2)O(4)) and inverse (MgIn(2)O(4)) spinels as a function of the temperature was extracted from X-ray diffraction patterns collected during different thermal annealings. The evolution of these structural parameters as a function of the disorder is analyzed within the bond valence shell model: large variations of the cation valence are observed in these three spinel compounds. From this analysis, a strong correlation between the change of the cation valence and the local disorder is pointed out. Including this dependence in the microscopic models may provide a better agreement between experimental observations and simulations. (C) 2008 Elsevier B.V. All rights reserved