Research On Limiting Current Oxygen Sensor Based On Sm,Nd And Ca Multi-Doped Ceria

Limit current type oxygen sensors are usually used in metallurgy,electric power,automotive and other fields,and are of great interest because of their wide range of oxygen measurement,high oxygen sensitivity,short response time,high stability,simple preparation and low cost.Oxygen sensors using Y-stabilized zirconium oxide（YSZ）as the electrolyte need to operate at high temperatures,thus limiting the sensor performance.The development of electrolyte materials with high oxygen ion conductivity at intermediate and low temperatures can improve the performance of oxygen sensors.The doped cerium oxide based material is considered as the most suitable alternative electrolyte because it has higher conductivity than all other materials at intermediate temperature.To suppress the electronic conductivity generated in this material,based on the above,this thesis investigates the properties of Sm,Nd,and Ca multi-doped cerium-based electrolytes,and selects La_0.8Sr_0.2Fe_0.7Ni_0.3O_3-δ（LSFN）as the dense diffusion barrier layer material,and prepares an oxygen sensor using a ceramic sheet composite method,and investigates its oxygen-sensitive performance.Ce_0.8Sm_0.15-xNd_xCa_0.05O_2-δ（CSNC x=0,0.05,0.10,0.15）electrolyte powder and La_0.8Sr_0.2Fe_0.7Ni_0.3O_3-δ（LSFN）mixed conductor materials were prepared by sol-gel method,respectively.The physical structure,surface chemical state,band gap width,microscopic morphology and electrical properties of CSNC samples were investigated by X-ray diffractometer（XRD）,Raman spectrometer（Raman）,X-ray photoelectron spectrometer（XPS）,ultraviolet-absorption spectrometer（UV-vis）,scanning electron microscope（SEM）and alternating current impedance（AC）techniques,respectively.The physical structure,microscopic morphology and electrical conductivity of LSFN were characterized by XRD,SEM and digital source table.The results show that the electrolyte powder with single cubic fluorite structure was synthesized by roasting CSNC series powder at 800℃,and the cell volume increased with the increase of neodymium doping.After sintering at 1300℃,electrolyte materials with relative densities greater than 95%can be obtained;among them,the electrolyte with the highest concentration of oxygen vacancies,the smallest forbidden band width and the largest conductivity at 700℃is 2.24×10^-2 S/cm when the Nd doping is5 mol%.LSFN formed a hexagonal chalcocite structure after roasting at 800℃,and its conductivity reached 359.40 S/cm at 700℃.The best performing Ce_0.8Sm_0.10Nd_0.05Ca_0.05O_1.875 was used as the electrolyte,and LSFN was used as the dense diffusion barrier layer material to prepare the limiting current type oxygen sensor by ceramic sheet composite method.The effects of different electrolyte thicknesses（0.45,0.55,0.65,0.70 mm）and different dense diffusion barrier layer thicknesses（0.65,0.70,0.75,0.85 mm）on the oxygen sensitivity characteristics of the sensors were investigated.The results show that for different electrolyte thicknesses,fixing the thickness of dense diffusion barrier layer,the limit current platform gradually increases with the increase of electrolyte thickness in the range of oxygen content from 2 to 20%,and then the limit current platform gradually decreases after reaching a certain thickness,and the limit platform reaches the optimum when the electrolyte thickness is 0.65 mm.In the oxygen content range of 2 to 20%,the ratio of oxygen concentration to ultimate current is proportional to the oxygen concentration at 650℃and 700℃,and linear to the ultimate current value at 750℃and 800℃.At the same oxygen partial pressure,the limit current value increases with temperature and the voltage interval of the limit current plateau decreases.Using repeated conversion measurements with 15%and 2%oxygen concentration,the sensor has a rise response time of approximately 8 s and a fall response time of approximately 13 s.The thickness of the fixed electrolyte is 0.80 mm,and when the thickness of the dense diffusion barrier layer varies,a good ultimate current plateau occurs at a thickness of 0.70 mm and an oxygen concentration in the range of 2 to 20%as the thickness of the dense diffusion barrier layer increases.In the range of 700～800℃,its oxygen concentration and limit current are linearly related,and at 650℃,the ratio of oxygen concentration and limit current is positively related to oxygen concentration.When the oxygen concentration is the same,the limit current value increases with the temperature and the voltage range where the limit current plateau appears decreases.The response time measured by the oxygen sensor was within 12 s when changing back and forth between a concentration of 15%and 2%.