New Results for Electron Transport, Chemical Diffusion and Stability of Solid Oxygen Ion Conductors
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New Results for Electron Transport, Chemical Diffusion and Stability of Solid Oxygen Ion Conductors H.-D. Wiemhoefer, M. Dogan, S. Luebke, V. Ruehrup, Institute for Inorganic and Analytical Chemistry, University of Muenster, 48149 Muenster, Germany ABSTRACT We describe the measurement of electronic conductivity of solid oxide electrolytes by a modified Hebb-Wagner technique based on the use of blocking microelectrodes. Results are presented for a couple of typical solid oxide electrolyte systems mainly derived from ceria and lanthanum gallate. The examples demonstrate a good resolution of the microelectrode technique in particular within the electrolyte domain, i.e. around the minimum of the electronic conductivity. This made possible the detection of deviations from the predicted oxygen partial pressure dependence of simple defect models for the concentrations of electrons and holes. The observed deviations from these defect models, at least partially, reflect the overemphasized ideality of the usually applied semiconductor model. Whereas the effect of dissolved transition metals with variable valence states such as Fe, and Co on the electronic conduction is well known, it was unexpected to find a strong concentration dependent effect of dopants like Y3+ and Zr4+ in ceria or Mg2+ and Sr2+ in the gallates upon the electronic conductivity within the electrolytic domain. Ions like Y3+ and Zr4+ cause a shift and a partial broadening of electronic states in ceria based materials. Indications have been found for band tailing due to high defect concentrations. In some cases, the dopants cause the appearance of additional localized electron states in the gap which give rise to weak superimposed maxima of the electronic conductivity at a particular oxygen partial pressure within the electrolytic domain. Accordingly, one cannot expect that electronic conductivities of solid electrolytes are insensitive to a changing concentration of stabilizers such as Y, Ca, etc. For instance, even a moderate doping of ceria by zirconia leads to a considerable electronic excess conductivity in the electrolytic domain. INTRODUCTION The onset of electronic conduction above or below certain values of the chemical potentials limits the range of application of solid electrolytes and determines the width of the so-called electrolytic domain. Accordingly, it is of interest to know the material properties and further relevant factors that influence the concentration and mobility of electrons and holes in solid electrolytes. Various experimental techniques have been used in the past in order to analyze the electronic conductivity of solid oxide electrolytes. The main difficulty is that the electronic conductivity is lower than the ionic one by several orders of magnitude. Therefore, blocking or minimizing of the oxygen ion flow is necessary in the presence of a chemical potential gradient of oxygen, if the electronic conductivity is to be measured with high accuracy around the minimum of the electronic conductivity in the electrolytic domai
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