Effect of Partial Equilibrium and Redox Reactions in a Zirconia Solid Electrolyte on the Electromotive Force in a Zirconia Oxygen Sensor

A zirconia oxygen sensor is an electrochemical device for rapidly measuring oxygen concentrations in molten metals at high temperatures. It is desirable to make the sensors capable of continuous, long-time measurements. Here, the effect on the electromotive force of a partial equilibrium state on the surface of a zirconia solid electrolyte in a zirconia oxygen sensor was investigated to reveal the factors preventing long-time measurements. The reference electrode was a mixture of W and WO2. Continuous measurements were performed in molten iron containing carbon for deoxidization. The electromotive force between the positive reference electrode and the negative sample electrode of the sensor decreased from a positive value to zero over time. This was because a metallic tungsten layer formed on the surface of the zirconia electrolyte by the reduction of the tungsten oxide via oxygen diffusion through the zirconia tube. In addition, the equilibrium in the reference electrode was disturbed during the measurements. A direct electrical current was applied to the sensor to examine the relationship between the surface and measured values. The electromotive force during the current was maintained at a higher value than that without the current because of induced re-oxidation at the zirconia surface. However, a long-time application could cause over-oxidation and dissolution of the reference electrode surface.