The near-surface defect structure of yttria-stabilised zirconia determinedby measurement of the differential capacity

The space charge density at the surface of yttria-stabilised zirconia (YSZ) containing 2-10 mol% yttria was determined from differential capacity meas urements of YSZ/gold interfaces at 750-850 K. The oxygen vacancy concentrat ion in the sub-surface layer was determined as a function of temperature an d bias potential by fitting the experimental data at positive bias potentia ls to predictions based on a Boltzmann-type expression. The vacancy concent ration at the interface increases with decreasing bias potential, and reach es a temperature-dependent maximum value at negative bias potentials. The t hermal activation parameter for this process is 0.27-0.46 eV. A thermodynam ic model for the near-surface defect structure of YSZ is proposed, in which oxygen vacancies are assumed to be distributed randomly over a fraction of the total number of oxygen sub-lattice sites. The fraction of sites that i s available for oxygen vacancy distribution increases with yttria content a nd temperature.