IONIC-CONDUCTIVITY OF ZRO2-12 MOL PERCENT-Y2O3 SINGLE-CRYSTALS

Fast ionic conductors are important to study because of their use in the construction of technologically useful devices such as electrochemical cells, oxygen monitors, and the high-temperature fuel cell. Oxygen-ion conductors form a major subgroup of these materials, and, in particular, stabilized zirconia is one of the more important solid electrolytes. However, the ionic conductivity of this material is still only rather poorly understood. The aim of the present work is to describe, by means of a method of local fits (LF's) to Arrhenius's law, the experimental values of the ionic conductivity of ZrO2-12 MOI % Y2O3 single crystals in the temperature range from 200-degrees-C to 1600-degrees-C. This method yields two sets of data: the preexponential factor, A(LF)i, and the activation enthalpy, DELTAH(LF)i. The In A(LF)i versus DELTAS(T)/k plot [where DELTAS(T) is the entropy change in the process] is a very good test of the accuracy of the LF method. The DELTAH(LF)i values are fitted by a least-squares procedure to an empirical temperature-dependence function with four adjustable parameters. In order to interpret these results and to understand the physical meaning of the fitted parameters, a microscopic model is proposed that allows us to deduce a theoretical function of temperature for the activation enthalpy similar to the empirical function. Then, from this function, we determine the association (0.57 eV) and migration (0.73 eV) enthalpies for oxygen vacancies, and analyze the temperature variation of the free energy (DELTAG) and entropy (DELTAS), as well as the degree of dissociation of the vacancies in the conduction process for this material. A noteworthy result is that, for the range of temperature studied here, the extrinsic dissociated regime (where it is assumed that all oxygen vacancies are free) is never reached. Finally, taking into account the contribution of the jumps up to the second-next-nearest anionic neighbors, we obtain the value of 1.31 X 10(13) Hz for the attempt frequency of the oxygen vacancies.