Mesoscopic Fast Ion Conduction in Nanometre-Scale Planar Heterostructures

Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany.
Nature (Impact Factor: 41.46). 12/2000; 408(6815):946-9. DOI: 10.1038/35050047
Source: PubMed

ABSTRACT Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.

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Available from: Noriko Sata, Sep 27, 2015
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    • "The solid curves represent the best fit to Eq. (3). the assumption that the hopping distance is constant. There are different factors that may contribute to the enhanced conductivity of SPS ceramics compared to the CS-900 one, including; (i) the reduced grain size of the SPS ceramics, which facilitates the migration of Li þ ions and leads to improved mobility [16] [17] [18] [19] [20] [21] [22], and (ii) the reduced porosity and/or improving the grain-to-grain bonding in SPS ceramics as observed in SEM micrographs for the CS and SPS ceramics [32]. Scaling of the conductivity spectra can represent a test of the universality of the dynamic response. "
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    ABSTRACT: Nanocrystalline Li5La3Nb2O12 (LLN) lithium ion conductor with garnet-like structure is synthesized by mechanical milling and solid state reaction routes. The nanopowder has an average grain size of 26 nm. The product nanopowder is sintered by conventional (at 900 °C) and spark plasma sintering (SPS) at different temperatures of 800 °C, 850 °C and 900 °C. The conventionally sintered ceramics have coarse grained structure with grain size of 1-2 μm, whereas the SPS ceramics have nano-sized grains with grain size as small as 50-100 nm at low SPS temperatures. The electrical properties have been studied by impedance spectroscopy measurements. The SPS-850 ceramics exhibit ionic conductivity value of 3.7×10−5 S/cm at 27 °C, which is one order of magnitude larger than the conventionally sintered sample. For the first time we explore two regions of the temperature dependent conductivity below and above room temperature with different activation energies, most probably due to two Li+ ions migration pathways. The relaxation properties of the studied materials have been analyzed in the conductivity and electric modulus formalisms. Different factors are suggested to contribute to the enhanced conductivity including the hopping frequency and the concentration of mobile Li+ ions.
    Ceramics International 06/2015; 41(5). DOI:10.1016/j.ceramint.2015.01.077 · 2.61 Impact Factor
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    • "Introducing interfaces is regarded as one effective strategy to dramatically enhance the ionic conductivity in solid electrolytes, leading to the redistribution of ions in the space-charge regions [7] [11]. Sata et al. [12] prepared the defined heterolayered films composed of CaF 2 and BaF 2 by molecular-beam epitaxy. For interfacial spacing greater than 50 nm, the ionic conductivity, which is parallel to the interfaces, increases proportionally with interface density. "
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    ABSTRACT: Sm0.2Ce0.8O1.9 (SDC)/Na2CO3 nanocomposite synthesized by the co-precipitation process has been investigated for the potential electrolyte application in low-temperature solid oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na2CO3 nanocomposite has been studied. The performance of 20 mW cm−2 at 490 °C for fuel cell using Na2CO3 as electrolyte has been obtained and the proton conduction mechanism has been proposed. This communication demonstrates the feasibility of direct utilization of methanol in low-temperature SOFCs with the SDC/Na2CO3 nanocomposite electrolyte. A fairly high peak power density of 512 mW cm−2 at 550 °C for fuel cell fueled by methanol has been achieved. Thermodynamical equilibrium composition for the mixture of steam/methanol has been calculated, and no presence of C is predicted over the entire temperature range. The long-term stability test of open circuit voltage (OCV) indicates the SDC/Na2CO3 nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.
    International Journal of Hydrogen Energy 03/2011; 36(6):3984-3988. DOI:10.1016/j.ijhydene.2010.12.061 · 3.31 Impact Factor
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    • "It is known [5] that thin multiple layer structures, because of their many interfaces and space-charge effects, have higher ionic conductivities parallel to the interfaces than the bulk materials. Since it is essential to grow smooth interfaces at a nanometer thickness level, we have studied surface roughness as well as surface state properties in Ceria films by D.C. magnetron sputtering. "
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    ABSTRACT: Synthesis and Characterization of Pure and Doped Ceria Films by Sol-gel and Sputtering. KURT T. KOCH (University of Missouri, Rolla, MO, 65409) LAXMIKANT SARAF (Environmental and Molecular Science Laboratory (Part of Pacific Northwest National Laboratory), Richland, Washington 99352). Pure and doped Ceria are known for their ability to gain or lose Oxygen, which is of interest to the Solid Oxide Fuel Cell (SOFC) and catalyst community. Current efforts are focused in SOFCs to reduce the operating temperature of the cell while maintaining ionic conduction. Ceria is known for its high ionic conductivity in the intermediate temperature region. (600-800 C) We have prepared pure and doped Ceria films by Sol-gel and magnetron sputtering methods. These films were characterized by X-ray diffraction (XRD), nuclear reaction analysis (NRA), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Oxygen conduction measurements. We have observed greater volume diffusion in nanocrystalline Ceria compared to bulk polycrystalline films as a result of low density. Near surface diffusion properties with increasing temperature indicate a decrease in the volume diffusion as a result of grain growth. However, a linear increase in O2 content at {approx}600nm depth was observed and can be correlated to the redistribution of O2 in the samples. Surface roughness of <111> and <200> oriented Ceria films on Al2O3 and YSZ was observed to be 0.13nm and 0.397nm, respectively. In the case of Ceria grown on YSZ, structural properties from XRD results showed a highly oriented structure with cube on cube growth. XRD results from Ceria grown on Al2O3 showed an oriented state near the surface. structure whose degree of orientation appeared to be partially dependent on substrate temperature. Preliminary XPS results indicate reduction in Ceria from the Ce4+ to Ce3+ state near the surface.
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