The Preparation And Performance Study Of YSZ Solid Electrolyte Materials Doped With Bi₂O₃

Yttria stabilized zirconia (YSZ) is the most successful electrolyte materials currently used in solid oxide fuel cell (SOFC) and the oxygen sensors. It exhibits high oxygen ionic conductivity, excellent oxidative resistance and corrosion resistance, and non-reactive with electrode material. However, the high sintering temperature (usually greater than1400℃and operation temperature (greater than800℃) hinder the civilian development process. Therefore, improving the performance of YSZ material according to doping dopants, improving the molding method and optimizing the sintering process in order to decrease the sintering and operation temperature have been a hot spot in materials research. In this article, different content of bismuth oxide doped in3YSZ and8YSZ and then sintered under different sintering process. The electrical properties, the phase components and the microstructure morphology were analyzed to study the solid solution mechanism and the optimum fraction of Bi2O3-Y2O3-ZrO2three-phase system. It aims to provide valuable data for doping mechanism in YSZ to promote the application research of electrolyte material.A significant enhancement in sintering activity of commercial yttria stabilized zirconia (3YSZ) powder doped with1mol%-12mol%bismuth oxide (BiiO3) was detected when the mixed powder was sintered at an extremely low sintering temperature between850℃and1100℃. The effect of sintering temperature and the amount of Bi2O3doped have been studied using XRD for the phase compositions measurement and scanning electron microscopy (SEM) for the microstructure of the sintered samples. It is shown that the bulk compacts doped with3mol%Bi2O3can be densified above95%by pressureless conventional sintering method at1000℃. The Bi2O3dopants induce a transformation between tetragonal phase and monoclinic phase in ZrO2and refine the crystalline grain of ZrO2. The insoluble Bi2O3exists among the ZrO2grain boundary, part of which is in the form of δ-Bi2O3. The oxygen ionic conductivity of Bi2O3-doped YSZ bulk were detected at different temperatures, and it shows that the oxygen ion conductivity can effectively be improved owing to the increase of oxygen ionic vacancy density which attribute it to δ-Bi2O3and atoms replacement in the three-phase system solid solution. The tubular oxygen sensor, which consists of Bi2O3and3YSZ or8YSZ, was fabricated by sol-gel method. The switching temperature of the oxygen sensor was detected. It founds that the switching temperature can effectively be decreased to about400℃because of the doped Bi2O3, but the thermal shock resistance may degrade with the increasing amount of Bi2O3.3YSZ doped with3mol%Bi2O3is the most suitable material for the oxygen sensor in consideration of all aspects of performance.The two-step sintering method (TSS) was used in the sintering process of Bi2O3-doped8YSZ. The density, composition and microstructure analyses show that the two-step sintering method can effectively reduce the grain size of the fired bulks, the grain size of2mol%Bi2O3-8YSZ sintered by the conventional sintering at1100℃can be reduced from2μm to1μm using TSS method (T1=1150℃,12=900℃, hold for10h). Consequently, the mechanical property including thermal shock resistance of8YSZ is improved.