Voltage Step Characteristics of Oxygen Concentration Cell Sensors for Nonequilibrium Gas Mixtures

Abstract

Concentration cell zirconia oxygen sensors show steady‐state output characteristics having an abrupt voltage step at critical oxygen combustible gas concentration ratio. The voltage step characteristics were investigated experimentally at various temperatures using simple gas mixtures containing oxygen and combustible gas. Experimental results indicated that there are three temperature regions for the voltage step characteristics: (i) a high temperature region (region I) in which the voltage step occurs near the stoichiometric composition. This results from equilibrium reaction occurring in the gas mixtures; (ii) a middle temperature region (region II) in which the voltage step deviates from the stoichiometric composition of the nonequilibrium gas mixtures. The magnitude of the deviation does not depend on temperature; (iii) a low temperature region (region III) in which the voltage characteristics strongly depend on temperature. In the region II , the magnitude of the deviation in the voltage step characteristics is somewhat influenced by the flow rates of the gas mixtures and the thickness of a porous coating on the sensor electrode for protection. This behavior of the sensor was analyzed by a kinetic model in which the sensor electrode is assumed to have a high catalytic activity for the reaction of the combustible gas and oxygen. The model also considers mass transport through the porous coating and gaseous boundary layers. The sensor characteristics in the region II , in which the sensors are mainly used in practice, can be explained by this model.