Fundamental Research On The Planar Zirconia Automotive Oxygen Sensor

At present, due to their small size, short response time, low power consumption, easy to integrate and operating stability in the rough environment, planar zirconia automotive oxygen sensors have been the dominating products in the market. According to the manufacturing process flow of oxygen sensors, issues such as the powder preparation technology, the sintering techniques, the component manufacturing and testing technology, the characterization of materials, etc. were investigated in this PhD dissertation.Firstly, the chemical co-precipitation was employed to prepare the ultra fine powders of 8mol% Yttria-Stabilized Zirconia (8YSZ) in this study. The characteristics of the powders were analyzed by laser granulometer, TG-DTG, XRD and SEM. The mechanism and influential factors of agglomeration of ultra fine 8YSZ powder prepared by co-precipitation method have been analyzed. The effects of the process parameters including solution concentration, pH value, precipitation method and temperature, type of washing liquid, calcination temperature and surfactant on particle size of 8YSZ were investigated. The optimum process parameters were obtained.Secondly, on the basis of the spherical pore model, pore micro structure models for the intermediate and final stages of sintering have been set up while densification equations are derived for the above stages of sintering. The 8YSZ powders with different grain size and agglomeration degree were first pressed under various pressure then pressureless sintered under various sintering process. The microstructure, density and line shrinkage were measured. Combining the models with the densification equations derived in this study, the effects of the sintering temperature, the size of powder particle, green density and agglomeration degree of powder on the densification were discussed. The rule of the normal grain growth for single-phase system and the effect of the doped second phase (Al₂O₃) and pore size distribution on the grain growth were analyzed by combining the grain growth kinetics and experiment. The optimum sintering process of the ultra fine 8YSZ powders were obtained.Thirdly, the mixed electronic and oxygen ion-conduction materials, La_1-xSr_xMnO₃ (LSM) powders were prepared from La₂O₃, MnO₂, and SrCO₃ by the method of solid state reaction synthesis. The reaction steps were described as follows. First, La₂O₃+SrCO₃→La₂SrO_x+CO₂, and then, La₂O₃+MnO₂→LaMnO₃, finally we obtained La_1-xSr_xMnO₃. A type of limiting current oxygen sensor with dense diffusion barrier was fabricated with the platinum paste bonding method. It was helpful to solve these questions that the sensor was easy to be occluded, change the dimension, suffer from a drift during long term operation and relatively expensive for traditional sensor with a small diffusion hole or porous ceramic layer. The testing system for the sensor was developed. The test results showed that the sensors have good oxygen-sensitiveness in whole range of air-file ratio while there was a good linear relationship between the limiting current and oxygen concentration. Moreover, the sensor had many good performances such as low resistance, high output current, and wide measure range.Fourthly, a novel potentiometric oxygen sensor employing the oxygen storage material CeO₂ as a solid-state reference was developed. It overcame the problems of the traditional oxygen sensor such as complex fabrication process, high cost, trouble to miniaturize, etc. To improve the Oxygen Storage Capacity (OSC) of solid-state reference, the CeO₂-ZrO₂ and CeO₂-ZrO₂-Y₂O₃ compounds were prepared by chemical co-precipitation. The effects of high temperature on phase structure, BET surface area and OSC was studied. Tests on the sensor have been performed with various A/F value mixture gas at various operating temperature. The experimental result showed that the output voltage of the sensor was negative at lean-burn and was positive at rich-burn, different from the traditional oxygen sensor with 1000¹00mv.Finally, the structure of a thick-film amperometric sensor with dense diffusion barrier was designed. The structure of heater was optimized by the method of finite element analysis. Transient heat transfer analysis and thermal stress analysis were performed. The design results showed that the designed sensor had fast response, even temperature distribution and acceptable thermal stress. It was believed that the modeling techniques and computational approaches incorporated in this study were useful and can be applied for the development of similar products.