Abstract
The temperature dependent conductivity of yttria stabilized zirconia (YSZ) exhibits a bending in Arrhenius’ plots which is frequently discussed in terms of free and associated oxygen vacancies. However, the very high doping concentration in YSZ leads to such a strong defect interaction that the concept of free vacancies becomes highly questionable. Therefore, the temperature dependent conductivity of YSZ is reconsidered. The conductivity of YSZ with different doping concentration was measured in a broad temperature range. The data are analyzed in terms of two different barrier heights that have to be passed along an average path of an oxygen vacancy in YSZ (two barrier model). For 8–10 mol% yttria, the two barriers are in the range of 0.6 eV and 1.1–1.2 eV, respectively. The conductivity and thus the barrier heights also depend on the cooling rate after a high temperature pre-treatment. This indicates that different frozen-in distributions of dopants affect the vacancy motion by different energy landscapes. Temporarily existing defect configurations, possibly with a strong effect of repulsive oxygen vacancy interaction, are suggested as the reason of high barriers. Future dynamic ab-initio calculations may reveal whether this modified model of the YSZ conductivity is mechanistically meaningful.
Highlights
Numerous theoretical studies were performed in order to understand these experimental observations and to get a deeper insight into defect thermodynamics and kinetics of doped zirconia and of the closely related ceria-based ion conductors, see e.g. Refs. 10–29
The vacancy migration barrier strongly depends on the two cations of the tetrahedral edge, which is crossed by an oxide ion during a jump to a nearest neighbor site of the anion sub-lattice:[13,14,15,20,23,26] Refs., 13,14,26 report barriers between 0.3 and 0.67 eV for a Zr-Zr edge and 0.85–1.29 eV for a Y-Zr edge
Only few numerical simulation studies dealt with the temperature dependent activation energy of yttria stabilized zirconia (YSZ), e.g.,13,14,26,27,37,38 An activation energy lowering at high temperatures was reported in Refs
Summary
Numerous theoretical studies were performed in order to understand these experimental observations and to get a deeper insight into defect thermodynamics and kinetics of doped zirconia and of the closely related ceria-based ion conductors, see e.g. Refs. 10–29. For YSZ the measured activation energies were used to determine migration barriers of free vacancies and defect association enthalpies. It is the scope of this paper to reconsider measured temperature dependent conductivities of YSZ by employing this different view on vacancy motion in terms of a modified model for data analysis.
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