Research Of On-board Mixed-potential Gas Sensors Based On Stabilized Zirconia And Oxide Electrode

Recently, with the strengthening of the emissions standards, development ofyttria-stabilized zirconia （YSZ） nitrogen oxides and ammonia sensor research hasbecome very urgent. Researchers find that the sensing electrode （SE） material in thesesensors is an important factor in determining the performance of the sensor.Meanwhile, the electrode structure, the device structure, the fabrication and testconditions, also play important roles. In this thesis, focusing on the electrode material,and taking account of the sintering temperature of the material and device fabricationprocess, we adjust the electrode microstructure and research the sensitive properties ofthe nitrogen oxides and ammonia sensor.First of all, the first chapter introduces the solid electrolyte and its application inthe field of sensors, as well as the research progress of yttria-stabilized zirconiasensor.Secondly, in chapter two we focus on the NO_xsensing characteristic of W-basedoxides. A series of mixed W/Cr oxides with different ratio of W and Cr （1:6,1:2and3:2） have been prepared by using polymeric precursor method. By comparing their sensitivities to20-300ppm NO₂, it was found that the sensor using oxide with3:2W/Cr gave the largest response value. For3:2W/Cr oxide, the effect of the sinteringtemperature on the electrode microstructure was also investigated. As a result, thedevice sintered at1000C showed the best performance. The response value to100ppm NO₂is51.6mV, the response time is within20s and the sensing device alsoshows an excellent selectivity against other coexisting gases. The characteristics ofSEM and TEM revealed that the special microstructure of oxide electrode formed bysintering plays a significant role in better sensing performance. Additionally, values ofthe ΔEMF to100ppm NO₂obtained with different relative humidity （10%,20%,50%,70%and90%） are less than5%difference, which shows that the water has little effecton the NO₂response for the sensor.In chapter three, we have prepared MnCr₂O₄with spinel structure by usingpolymeric precursor method. The thermal stability, grain size and surface morphologyof MnCr₂O₄calcined at different temperatures were studied using thermogravimetricanalysis and differential scanning calorimeter analysis （TG-DSC）, X-ray diffraction（XRD）, scanning electron microscopy （SEM）. From these measurements, weinvestigated the influences of electrochemical activity of sensing electrode （SE） andTPB on devices’ sensitivity. Then the YSZ-based sensors using MnCr₂O₄sintered atdifferent temperatures （800,900,1000,1100and1250C） as electrode were fabricatedand their NO₂sensing properties were investigated. Results demonstrate thatMnCr₂O₄sample calcined at1000C exhibits higher response to100ppm NO₂, whichis about73mV. It is believed that the calcined temperature affect the material’ssensing properties. It was suggested that when temperature increased, a rather pureMnCr₂O₄phase was obtained after1000C; however, taking account of the triple phase boundary, a high surface area is necessary, so too high temperature is not good.These two factors determined1000C as the optimal temperature. Additionally, thesensing characteristics such as selectivity, repeatability were investigated.In chapter four we focus on the on-board NH₃sensors. A series of tungstateMWO₄（M=Co, Zn and Ni） has been prepared by polymeric precursor method.Meanwhile, yttria-stabilize zirconia based sensors using these oxides as sensingelectrodes were investigated, and among the oxides tested, CoWO₄was found to bethe most suitable for the SE of the ammonia sensor. The sensor attached with CoWO₄shows the fast response and recovery characteristics （not more than5s respectively）and large sensitivity （-51mV/decade） at elevated temperature. The electric potentialdifference （ΔV） of the sensor varies almost linearly with the NH₃concentrations inthe examined range of50-1000ppm. The SEM observation reveals that the specialmicrostructure of CoWO₄-SE, bulky rod-like crystals coated by tiny particles, plays asignificant role in sensing performance. Although the selectivity has been improved,but it is still not perfect. The challenge of the NH₃sensor on board is great, andshould be investigated further.Finally, chapter five is summary and outlook.