The Experts below are selected from a list of 15984 Experts worldwide ranked by ideXlab platform

Andreas Tschöpe - One of the best experts on this subject based on the ideXlab platform.

  • Space Charge Layers in Polycrystalline Cerium Oxide
    MRS Proceedings, 2020
    Co-Authors: Andreas Tschöpe
    Abstract:

    ABSTRACTThe effect of space charge layers in polycrystalline Cerium Oxide was analyzed by comparing experimental results of grain size-dependent electrical conductivity with theoretical models. Modeling included the calculation of space charge segregation of acceptor ions and of the effective electrical conductivity of polycrystalline Cerium Oxide in both the macroscopic and mesoscopic range of grain sizes. It is shown that an L-3 power law for the electronic conductivity in the nm-regime is characteristic for the equilibrium space charge model and different from the scaling behavior of alternative models. The origin of space charge potential was investigated by numerical calculation of the electrical potential in a two-phase model. It was found, that a positive excess charge at grain boundaries of Cerium Oxide is caused by an enhanced oxygen deficiency at the grain boundary core. The influence of acceptor ion doping in the dilute limit and of non-equilibrium distribution of acceptor ions on electrical conductivity was also studied.

  • Interface Defect Chemistry and Effective Conductivity in Polycrystalline Cerium Oxide
    Journal of Electroceramics, 2020
    Co-Authors: Andreas Tschöpe
    Abstract:

    Polycrystalline Cerium Oxide exhibits increasing electronic and decreasing ionic conductivity upon reduction of the grain size. In the present study, the origin of this effect was examined. Temperature-programmed reduction (TPR) and oxygen titration measurements on nanocrystalline Cerium Oxide revealed a large excess oxygen deficiency associated with the surface. Using a two-phase model for the combined system of the bulk phase in equilibrium with a surface layer, this enhanced oxygen deficiency could be explained by a reduced binding energy of surface oxygen ions in agreement with results from atomistic computer simulations. The model also revealed that this segregation of oxygen vacancies is the origin of an intrinsic space charge potential. Translating this effect to polycrystalline Cerium Oxide and taking into account the segregation of dopants and the accumulation/depletion of charge carriers, it was possible to model the grain size dependence of electrical conductivity and thermopower of polycrystalline Cerium Oxide. A straightforward 1-dimensional numerical model and a change from Boltzmann to Fermi-Dirac statistics allowed to calculate the conductivity of heavily doped polycrystalline Cerium Oxide for grain sizes in the range of 5–10,000 nm and acceptor concentrations up to 20%. Using this approximation, the effect of grain size on mixed ionic/electronic conductivity and the electrolytic domain boundary was investigated.

  • Grain-size-dependent thermopower of polycrystalline Cerium Oxide
    Solid State Ionics, 2002
    Co-Authors: Andreas Tschöpe, S. Kilassonia, B Zapp, Rainer Birringer
    Abstract:

    The effect of grain boundaries on the defect chemistry of polycrystalline Cerium Oxide was investigated by measuring the thermopower of Cerium Oxide samples with grain sizes in the range of 0.13–11.5 Am. The samples were prepared by sintering pellets from a single batch of nanocrystalline 500 ppm Gd-doped Cerium Oxide powder at different temperatures. A change in the sign of the Seebeck coefficient as function of the grain size was observed, indicating a transition from predominantly ionic conductivity at large grain size to electronic conductivity at small grain size. The experimental results were analyzed using the space charge model for ionic solids, which has already been successfully employed in the analysis of the grain-size-dependent electrical conductivity of Cerium Oxide. The agreement between the experimental data and the space charge model, regarding both electrical conductivity and thermopower, suggested that the essential effect of the grain boundaries in Cerium Oxide is the accumulation/depletion of charge carriers in space charge layers, whereas the effect of microstructure on charge carrier mobilities is negligible. From the analysis of experimental results, a value of DU=0.7 V was obtained for the space charge potential at the grain boundaries in Cerium Oxide. D 2002 Elsevier Science B.V. All rights reserved.

  • Grain size-dependent electrical conductivity of polycrystalline Cerium Oxide. I. Experiments
    Solid State Ionics, 2001
    Co-Authors: Andreas Tschöpe, E. Sommer, Rainer Birringer
    Abstract:

    The electrical conductivity of polycrystalline Cerium Oxide was investigated in the nanometer and micrometer size range. Nanocrystalline samples of different grain size were prepared by uniaxial hot-pressing of nanocrystalline powder at various temperatures and pressures. Additional annealing at high temperatures was employed in order to obtain microcrystalline samples. An equivalent-circuit analysis of ac-impedance spectra based on the brick-layer model was performed and the apparent bulk conductivity determined. The effect of a variation in temperature or oxygen partial pressure revealed the rather different nature of the electrical transport properties in the nano- and microcrystalline materials. Nanocrystalline Cerium Oxide exhibited electronic conductivity under conditions at which microcrystalline samples showed impurity-controlled ionic conductivity. The electronic conductivity of nanocrystalline samples was larger than the intrinsic electronic conductivity of pure single crystalline Cerium Oxide and was increasing with decreasing grain size. The experimental results were analyzed according to the defect chemistry of Cerium Oxide and consequences of a space charge effect on the partial electronic and ionic conductivity in polycrystalline Cerium Oxide will be discussed.

  • Oxyreduction studies on nanostructured Cerium Oxide
    Nanostructured Materials, 1997
    Co-Authors: Andreas Tschöpe, Rainer Birringer
    Abstract:

    Abstract The effect of high surface area in porous nanostructured Cerium Oxide on chemical equilibrium with oxygen gas phase was investigated. Nanostructured Cerium Oxide was prepared by chemical precipitation. These materials were compacted and sintered at 600°C to porous nanostructured solids. Sample weight under varying oxygen partial pressure and temperature was measured and compared with microcrystalline CeO 2 reference sample. The change in sample weight relative to the oxidized sample was used to determine the degree of reduction. The interfacial equilibrium of Cerium Oxide surface with oxygen gas phase was analyzed on the basis of the van't Hoff equation. The binding enthalpy of oxygen was determined to −7.5 eV per oxygen atom.

Jiao Xiao-yan - One of the best experts on this subject based on the ideXlab platform.

Galen D Stucky - One of the best experts on this subject based on the ideXlab platform.

  • hollow microporous Cerium Oxide spheres templated by colloidal silica
    Chemistry of Materials, 2009
    Co-Authors: Nicholas C Strandwitz, Galen D Stucky
    Abstract:

    We present a simple, solution-based synthetic route to hollow Cerium Oxide spheres. Thin layers (∼12 nm) of Cerium Oxide are deposited onto ∼200 nm silica colloids using Cerium nitrate and the silica cores are subsequently removed to yield hollow spheres. The spheres are composed of ∼4 nm ceria nanocrystals. Nitrogen adsorption isotherms indicate that the spheres are microporous with pore sizes of approximately 10 A. The spheres are thermally stable to collapse and ripening up to 700 °C and are active for the catalytic combustion of carbons. The hollow ceria spheres developed in this work are attractive as building blocks for multicomponent catalysts.

Yong Xiu Li - One of the best experts on this subject based on the ideXlab platform.

  • Preparation of Super-Fine Cerium Oxide Using Mechanically Chemical Method
    Advanced Materials Research, 2012
    Co-Authors: Zi Ying Gu, Hang Yu Gu, Xin Ke Peng, Yong Xiu Li
    Abstract:

    The Cerium Oxide powder has been modified with mechanically chemical method. The effect of modifier, reaction time, calcine temperature, and amount of additives on the particle size, density, and hardness of super-fine Cerium Oxide was investigated. The complex super-fine Cerium Oxide powder was prepared, whose physical state and feature were characterized. The results show that modifier and powder may interact, which improves the properties of super-fine Cerium Oxide.

Nicholas C Strandwitz - One of the best experts on this subject based on the ideXlab platform.

  • hollow microporous Cerium Oxide spheres templated by colloidal silica
    Chemistry of Materials, 2009
    Co-Authors: Nicholas C Strandwitz, Galen D Stucky
    Abstract:

    We present a simple, solution-based synthetic route to hollow Cerium Oxide spheres. Thin layers (∼12 nm) of Cerium Oxide are deposited onto ∼200 nm silica colloids using Cerium nitrate and the silica cores are subsequently removed to yield hollow spheres. The spheres are composed of ∼4 nm ceria nanocrystals. Nitrogen adsorption isotherms indicate that the spheres are microporous with pore sizes of approximately 10 A. The spheres are thermally stable to collapse and ripening up to 700 °C and are active for the catalytic combustion of carbons. The hollow ceria spheres developed in this work are attractive as building blocks for multicomponent catalysts.