Microscopic model of intergrain boundary junction

The statistical mechanics differential-difference equations for the ions concentration distribution that account for the diffusion, electrical conductivity and Poisson contributions are derived. They represent differential-difference analogues of the phenomenological continuity equations for the ions. Spatial inhomogeneities (grain interiors, grain boundaries, intergrain regions, etc.) can easily be taken into account by proper adjusting the system material parameters (diffusion coefficients or particle transition rates, electric conductivities, thermodynamic factors or chemical capacitances). The solution of the equations allows investigating impedance spectra of inhomogeneous systems, e. g. electro-conducting ceramics. The results can be used for interpretation of experimental impedance spectra and evaluation of the medium transport characteristics. A simple example of the intergrain boundary junction is considered

NMR Investigations in Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 Ceramics. Part I: Structural Aspect

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NMR Investigations in Li_1.3Al_0.3Ti_1.7(PO₄)₃ Ceramics. Part I: Structural Aspect

Because of its high Li⁺ conductivity, the family Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃ has already been widely studied; previous structural characterizations reported that aluminum occupied two types of sites in the NASICON framework: the first one in octahedral coordination corresponding to Ti/Al substitution; the second one in tetrahedral coordination corresponding to P/Al substitution. In this work we show that it is possible to synthesize samples presenting only the Ti/Al substitution in the octahedral site, which is more consistent with the formulation. The static local properties of our samples were characterized by multinuclear nuclear magnetic resonance (NMR) and X-ray diffraction. The MAS NMR aluminum spectrum is characterized by a strong parameter of asymmetry (η_Q = 0.9), indicating that aluminum ions are situated in sites which lost their axial symmetry. This loss of symmetry is accompanied by an increase of the number of chemical sites of the phosphorus, among which some are characterized by broad lines. The strong asymmetry quadrupolar parameter, together with the strong broadening of the ³¹P lines assigned to phosphorus with three Ti⁴⁺ and one Al³⁺ are marks of the M₂(IV)­PO₄ skeleton’s distortion. The multinuclear NMR experiment also allowed us to analyze the abnormal behavior of the lithium quadrupolar parameter ν_Q in relation with the flexibility of the M₂(IV)­PO₄ skeleton characteristic of the NASICON family

Changes in properties of scandia-stabilised ceria-doped zirconia ceramics caused by silver migration in the electric field

Silver is one of the most promising cathode materials for low temperature (300e500 C) solid oxide fuel cells. The most important disadvantage of silver is its migration in the electric field. For better understanding of this phenomenon, an in situ observation of the migration mechanism was undertaken with the use of high-temperature microscopes. Scandia stabilised ceria doped zirconia CeScSZ electrolyte prepared from commercial powder was examined before and after silver migration experiments using scanning electron microscope. X-ray diffraction, broadband electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The silver electrodes for solid oxide fuel cells were prepared using magnetron sputtering. The described cells under polarisation were examined using a high-temperature low energy electron microscope. Reference cells and post-mortem cells were observed using a scanning electron microscope equipped with high temperature stage. Under polarisation, silver moved inside the electrolyte and along the surface towards the region between electrodes. The structures thus formed were similar to those previously described in the literature; however, direct observation of the deposit growth was unsuccessful. In situ scanning electron microscopy observations of the silver electrode at 650 C revealed neither melting of the smallest silver particles nor movement of silver structures. Silver migration through the electrolyte caused a reduction in grain interior conductivity of the electrolyte, whereas its grain boundary conductivity remained unaffected