Working principle of zirconia oxygen sensing

Thread Content

Zirconia tubes are stable zirconia ceramic sintered bodies formed by sintering zirconia material mixed with a certain amount of yttrium oxide or calcium oxide at high temperatures. Due to the oxygen ion vacancies in its cubic lattice, it is an excellent oxygen ion conductor at high temperatures. Due to this property, at certain high temperatures, when the oxygen content on either side of the zirconium tube differs, it functions as a typical oxygen concentration cell. In such a cell, air serves as the reference gas, while the flue gas is located at the inner and outer electrodes respectively. In a practical oxygen probe, air flows through the outer electrode while flue gas flows through the inner electrode. When the oxygen content in the flue gas, P, is less than that in the air, P0 (20.6% O2), oxygen molecules in the air take away 4 electrons from the outer electrode to form 2 oxygen ions; the electrode reaction that occurs is as follows: O(P0) + 4e- → 2O2-. These oxygen ions migrate rapidly toward the flue gas side within the zirconia tube, where the opposite electrode reaction takes place: 2O2- → O(P0) + 4e-. The difference in oxygen concentration causes oxygen ions to migrate from the air side to the flue gas side, and the resulting electric potential causes them to migrate back from the flue gas side to the air side. When these two types of migration reach equilibrium, an electric potential signal E, related to the oxygen concentration difference, is generated between the two electrodes. This potential signal conforms to the Nernst equation: E = (RT/4F) ln(P0/P) (1). In this equation, R and F are respectively the gas constant and Faraday’s constant; T is the absolute temperature of the zirconia tube in Kelvin, P0 is the oxygen content in air (20.6% O2), and P is the oxygen content in the flue gas. As can be seen from equation (1), under certain high-temperature conditions (typically 600°C), a specific oxygen content in the flue gas corresponds to a particular potential output. Under ideal conditions, this potential value corresponds to the oxygen content in that high-temperature range. Under ideal conditions, when the concentration of the smoke gas under test is the same as that of the reference gas, the output potential E is 0 mV. However, in practical applications, both the actual conditions of the zirconium tube and the on-site circumstances are not ideal. Therefore, the actual zirconium tube deviates from this value. In fact, the potential output by the zirconium tube at a certain oxygen content is the sum of the theoretical value and the background potential. The potential output by the zirconium tube under conditions without concentration differences is referred to as the background potential or zero potential. The value of this potential varies at different temperatures, and it also changes as the zirconium tube is used for an extended period of time. Therefore, if this situation is not addressed, it will severely affect the accuracy of the entire oxygen meter as well as the lifespan of the probe.