Nature of the Electrical Conduction Transients Observed in CeO2 and Ca-Doped CeO2

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Abstract
The electrical conduction transients of CeO2 and ca‐doped CeO2 were investigated by employing a constant‐current dc power source and a six‐probe high‐resistance input‐isolated recording system. In contrast to the observations made by Kevane et al. neither voltage‐time transients nor non‐Ohmic behavior were observed when high‐purity CeO2 (99.9% min.) was investigated in the temperature range 300° to 1000°C. Transients were observed for the high‐purity CeO2 specimen when loading due to the instrumentation occurred or when high current densities due to a small contact area were used at the contacts. A model based on electrolysis was used to explain these results. Voltage transients at constant current were observed on a CeO2 specimen doped with 1 wt% CaO in argon but not in oxygen. These transients increased with increasing temperature in contrast to the observations made by Kevane. At 960°C in argon potential distribution curves at several different times showed that the electrical conductivity increases with time and was larger at the negative end of the sample than at the positive end where the conductivity decreases with time initially. After longer periods of time the conductivity increased over the entire specimen. In addition the specimen exhibited a discolored region; the intensity of the discoloration decreased with increasing distance from the cathode. After reoxidation at 1000°C in air the discolored region disappeared and the weight of the specimen increased. These results were explained in terms of a model that assumed blocking of the oxygen vacancies and oxygen ions at the cathode and anode, respectively. The electrons were then distributed so that electroneutrality was preserved. Thus the electrical conductivity was dependent upon the distribution of the oxygen vacancies and the oxygen ions. After longer times the voltage transients were controlled by electrolysis.

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