Nitrogen doped zirconia (N-YSZ) : preparation, characterization and electrode processes

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2006

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DOI:
http://dx.doi.org/10.22029/jlupub-10036

Abstract

Firstly the influence of nitrogen on the defect structure properties of pure ZrO2-delta and yttria stabilized zirconia (YSZ) is studied. On the basis of the defect equilibria and theappropriate Brouwer approximations the Kroeger-Vink diagrams are constructed and the defect structure of these materials is discussed. The thermodynamics of electrochemical cells of the type M/MX/X2(Me), is analyzed and we found an invariant point in the E/ lg a (X2) plot corresponding to the free enthalpy of formation of the compound MX at T = 0 K. The preparation of nitrogen-doped YSZ thin films by pulsed laser deposition (PLD) is systematically studied as a function of the temperature, gas pressure in the deposition chamber, distance between the target and the substrate and the yttria content. The surface morphology, the structure and the nitrogen content of these films are analyzed by AFM, HRSEM, XRD, SIMS, XPS and optical spectroscopy. Both nitrogen-doped (YSZ:N) and nitrogen free YSZ films prepared by PLD shown a flat surface topography and crystalline structure, but the films show a slight distortion of the fluorite-type cubic cell, mostly pronounced for the nitrogen-doped samples suggesting a possible order of the nitrogenions in the films but also can be a result of the nano-sized grain structure. An orientation dependence of the films from the background gas is also reported. The maximal nitrogen content of 14 at. percent was measured for the 7YSZ:N sample. The 7YSZ:N films show a different behaviour than YSZ:N films with lower or higher yttria content. The XPS and optical absorption experiments suggest an other charge or an other position of the nitrogen ions in the 7YSZ:N lattice. The kinetics of the electrochemical reduction of nitrogen on micro-electrodes is studied by potentiodynamic (LSV, CV) and steady state (potentiostatic, galvanostatic) polarization techniques. The appropriate electrode material, electrolyte and temperature are experimentally determined and discussed. The results clearly show a complicated multi-step charge transfer controlled process. Applying the concept of the stoichiometric number we suggest a mechanism for the cathodic electrochemical reduction of nitrogen where as a rate determining step we suggest the intermediate reaction: N2 + e = N22 with a stoichiometric number nu = 3. In addition we confirmed the nitrogen incorporation into YSZ thin films and as well in single crystals by SIMS analysis. Spatially resolved XPS studies during cathodic polarization confirmed for the first time in-situ the electrochemical reactivity of nitrogen. In this work we demonstrate that nitrogen can be incorporated upon cathodic polarization in YSZ even from the air. The experiments in ammonia gas phase show that the nitrogen incorporation proceeds much easier than from N2 gas and thus we suggest that nitrogenincorporation is limited by the surface reaction and not by the diffusion into the bulk. The transport properties of YSZ:N thin films as a function of the nitrogen content show an increase in the activation energies of the total ionic conductivity with increasing nitrogen content. On the basis of the deconvolution of the impedance spectra we conclude that for films with higher nitrogen content the ionic transport in the grain boundaries limits the diffusion.

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