Voltage-assisted 18O tracer incorporation into oxides for obtaining shallow diffusion profiles and for measuring ionic transference numbers: basic considerations

Abstract

In ¹⁸O tracer diffusion experiments slow oxygen exchange kinetics at the surface (i.e. a small oxygen exchange coefficient k*) often drastically decreases the ¹⁸O concentration compared with cases of pure diffusion limitation and this restricts the applicability of such experiments to certain parameter regimes. In particular, measurements on fast ion conductors requiring small diffusion lengths (e.g. thin film investigations) or low temperatures can be rather difficult. As a tool for overcoming some of the restrictions, voltage-assisted ¹⁸O incorporation into an oxide is proposed in this contribution. It is shown that the resulting effective oxygen exchange coefficient k* linearly depends on the current and can thus be tuned to experimental requirements. The role of the electrical field on ion transport in the bulk is quantified and it is discussed how far a simple determination of the diffusion coefficient from the resulting ¹⁸O profiles is still possible. Moreover, it is shown that the same tool of voltage-assisted ¹⁸O incorporation can be used for determining the ionic transference number of mixed conducting oxides.