Elektrochemische Untersuchung der chemischen Diffusion in Wüstit mit Hilfe eines oxidischen Festelektrolyten/ Electrochemical Investigation of Chemical Diffusion in Wüstite Using a Solid Oxide Electrolyte

Abstract

Chemical diffusion measurements in wüstite were carried out with the help of the solid state galvanic cell pO₂, Ptl |ZrO2(+Y2O3)| Fe1 - δ(ϰ)O| Pt2, N2 with doped ZrO2 as a solid electrolyte which exhibits practically pure conduction for oxygen ions. Starting from an initial homogeneous stoichiometry as given by δ, the voltage E of the cell, i. e. the chemical potential of oxygen (or δ) in Fe1 - δO at the phase boundary with the electrolyte, was changed in small steps, corresponding to 3,5 - 15% of the whole stoichiometric range. The re-equilibration of the compound was observed by the current as a function of time. Alternatively, a small constant current was applied to the galvanic cell and the voltage between the wüstite and a reference electrode was measured. From the relaxation behaviour the chemical diffusion coefficient can be determined in several ways. The values are consistent with those calculated from tracer diffusion coefficients, the thermodynamic factor, which could be estimated from coulometric titration measurements, and a correlation factor of the order of 1. In contradiction to previous results with thermogravimetric methods the chemical diffusion coefficient increases with growing deviation from the ideal stoichiometry.