Research On Electrode Performance Of Zirconia Oxygen Sensors

Solid electrolyte oxygen sensors made of zirconia can monitor and control various combustion ambiences and metallurgical reaction, so they have been widely applied to the fields such as environment protection, material science, chemical engineering, metallurgy, energy source and space navigation. Based on a review of its current condition and development home and abroad, the thesis systematically investigates the key technology such as preparation and test of the electrodes of zirconia sensor.The slurry of noble metal electrode and slurry of Pt/YSZ cermet electrode are fabricated, then the slurries are brushed and sintered to obtain various electrode system. The microstructure of surface and interface of these electrode systems and its influencing factors are studied by scanning electronic microscopy, the results show that the sintering temperature and aging time have remarkable effect on the microstructure. These results are explained by length of tripe phase boundary (TPB) of electrode, and then the appropriate processing parameters of electrode are analyzed and optimized. The analysis of interface indicates that the active reaction centers (TPB) of porous metal electrode only lie in the connecting interface between metal particles and YSZ particles; While in Pt/YSZ cermet composite electrode, its active reaction centers expand into the whole three-dimensional region. And the YSZ electrolyte phase in the cermet composite electrode improves combination between electrode and electrolyte, and effectively depressed growth of metal particles. The relation between the sintering temperature, Pt and YSZ volume ratio of the prepared Pt/YSZ cermet electrode and its microstructure is discussed, and the optimal sintering temperature range and volume ratio range is found to manufacture the cermet composite electrode with the favorable microstructure. Under this condition, the composite electrode with three intercrossing and interconnecting networks of metal, electrolyte and gas pore can form the long and continuous enough tripe phase boundary, which helps to the circulation and interchange between the electronic current and oxygen ionic current. So this structure has the optimal electrochemical catalysis.The various electrode systems of zirconia oxygen sensor are studied by electrochemical impedance spectroscopy (EIS), and on the base of it discussed the effects from the material and processing parameters on the electrochemical performance of electrode. By measuring the EIS data of electrode system under different temperatures, the apparent activation energies of the resistance of electrlysis and electrode polarization is figured out. And the relaxation frequency of polarization process apace increases with the higher measuring temperature, it is shown that the reaction of electrode process is accelerated by higher temperature. Furthermore, by contrast with the EIS data of various electrodes, the results show that under the same measuring condition the AgPd electrode and Pt/YSZ cermet composite electrode have less polarization resistance and more catalysis activity than Pt electrode. The effect of various equivalent circuits is compared, and then an appropriate equivalent circuit model is adopted. By this circuit model the EIS data of electrode systems can be exactly fitted. And the circuit elements of the model correspond with the physical and chemical processes of electrode, so the behavior of the electrode can be explained on base of the model.On the basis of the seven steps of electrode process, the continuous microstructure of the cermet composite electrode is established, and then based on it the equivalent circuit of microstructure is obtained. So a continuum theory for the polarization resistance and cermet composite electrode reaction has been developed. The electrochemistry polarization behaviors in the whole polarization range is discussed and the optimal thickness range of composite electrode has is expected. The general relationship between the overpotential of the cermet electrode and its thickness is described. Theη0- I t curve has been compared with that of a pure noble metal electrode.The thimble shape YSZ oxygen sensor and a test setup of its performance are designed and fabricated. The sensor was characterized in terms of output potential, response time and dynamic oscillograph test. When the temperature of sensor is more than 850K, the results in the veracity testing of sensor show that there is a lesser and invariable zero EMF between the output EMF and the theoretical EMF calculating from the Nernst equation. It was found that when the temperature of the exhaust gas is higher than 623K, the response time of the sensor is less than 600ms. EMF output in the rich combustion condition is higher than 850mV, and it is less than 150mV in the lean combustion condition. The controllable change period is about one second. The performances of the sensor can meet the request of the automobile emission control application. We obtained theλ-U curve in the wholeλrange on the base of chemical equilibrium between the products of combustion. This theoreticalλ-U curve is well consistent with the experimental result. And the influences of the temperature of exhaust upon EMF are discussed. The results obtained in this paper contribute to the application and a better understanding of the operation performance of Zirconia oxygen sensor.