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)

Near transferable phenomenological n -body potentials for noble metals

International audienceWe present a semi-empirical model of cohesion in noble metals with suitable parameters reproducing a selected set of experimental properties of perfect and defective lattices in noble metals. It consists of two short-range, n-body terms accounting respectively for attractive and repulsive interactions, the former deriving from the second moment approximation of the tight-binding scheme and the latter from the gas approximation of the kinetic energy of electrons. The stability of the face centred cubic versus the hexagonal compact stacking is obtained via a long-range, pairwise function of customary use with ionic pseudo-potentials. Lattice dynamics, molecular statics, molecular dynamics and nudged elastic band calculations show that, unlike previous potentials, this cohesion model reproduces and predicts quite accurately thermodynamic properties in noble metals. In particular, computed surface energies, largely underestimated by existing empirical cohesion models, compare favourably with measured values, whereas predicted unstable stacking-fault energy profiles fit almost perfectly ab initio evaluations from the literature. All together the results suggest that this semi-empirical model is nearly transferable

Grain size-dependent electrical properties of La 1.95 Sr 0.05 Zr 2 O 7-δ as potential Proton Ceramic Fuel Cell electrolyte

International audienceThe effect of grain size on the electrical properties of Sr-doped La 2 Zr 2 O 7-δ ceramics densified by spark plasma sintering was studied by impedance spectroscopy. Structural and microstructural analyses were performed using X-ray and scanning electron microscopy techniques. Sets of samples with different grain sizes and relative densities have been obtained by playing on pressure or temperature during sintering. Depending on the grain size, different equivalent circuits are proposed and discussed. We show that varying the sintering conditions produces significant changes in the bulk and grain boundary conductivities, with a monotonic increase of the total conductivity with the increase of grain size. The Brick Layer Model (BLM) is used to explain the size and temperature dependencies of the total ionic conductivity of doped La 2-x Sr x Zr 2 O 7-δ (x = 0.05)

Understanding Local Structure versus Long-Range Structure: The Case of UO 2

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Influence of sintering methods on microstructure and ionic conductivity of La1.95Sr0.05Zr2O6.975 synthesized by co-precipitation

International audienceWe present here the synthesis of Sr-doped lanthanumzirconate pyrochlores La1.95Sr0.05Zr2O6.975with controlled grain sizes and we compare different methods of sintering. Pyrochlores are systems where it is hard to achieve high density without a significant coarsening of the sample microstructure. It is shown that the use of Spark Plasma Sintering (SPS) can produce high density ceramic samples while maintaining a nanometric grain size, a key factor for enhancing electrical and mechanical properties. Furthermore, ionic conductivity is found higher in samples sintered using SPS than in those produced by hot pressing

Nano-Structured Materials under Irradiation: Oxide Dispersion-Strengthened Steels

International audienceOxide dispersion-strengthened materials are reinforced by a (Y, Ti, O) nano-oxide dispersion and thus can be considered as nanostructured materials. In this alloy, most of the nanoprecipitates are (Y, Ti, O) nano-oxides exhibiting a Y$_{2}$Ti$_{2}$O$_{7}$ pyrochlore-like structure. However, the lattice structure of the smallest oxides is difficult to determine, but it is likely to be close to the atomic structure of the host matrix. Designed to serve in extreme environments—i.e., a nuclear power plant—the challenge for ODS steels is to preserve the nano-oxide dispersion under irradiation in order to maintain the excellent creep properties of the alloy in the reactor. Under irradiation, the nano-oxides exhibit different behaviour as a function of the temperature. At low temperature, the nano-oxides tend to dissolve owing to the frequent ballistic ejection of the solute atoms. At medium temperature, the thermal diffusion balances the ballistic dissolution, and the nano-oxides display an apparent stability. At high temperature, the nano-oxides start to coarsen, resulting in an increase in their size and a decrease in their number density. If the small nano-oxides coarsen through a radiation-enhanced Ostwald ripening mechanism, some large oxides disappear to the benefit of the small ones through a radiation-induced inverse Ostwald ripening. In conclusion, it is suggested that, under irradiation, the nano-oxide dispersion prevails over dislocations, grain boundaries and free surfaces to remove the point defects created by irradiation