Study On Mixed-potential-type NOx Sensors Based On LLSGM

The enhancement of auto exhaust emission standard gave birth to the demand of NOx sensor.Currently, due to the superiority on wide gas test range, simple structure, response signal testabilityetc., NOx sensors based on Yttria Stablilzed Zirconia （YSZ） have gained more attentions. However,due to the intrinsic limitation in YSZ physical/electrochemical properties, the YSZ-based gassensors can only at the temperatures work above800℃, which would increase the cost onmaterials’ selection and sensors’ production. In order to lower the working temperature of sensors,in this thesis, Sr and Mg codoped LaGaO3(La_0.9Sr_0.1Ga_0.8Mg_0.2O_x, referred to LSGM) was selectedas solid electrolyte to replace the conventional YSZ. A series of test and research on materialproperties and sensing characteristics of NO₂sensors based on LSGM solid electrolyte were alsocarried out in this thesis.La_0.9Sr_0.1Ga_0.8Mg_0.2O_xpowder prepared by sol-gel method was used as solid electrolyte. Thepowder was sintered in conventional way and its physical and electrochemical properties such asrelative density, XRD, line shrinkage rate, SEM and conductivity were characteristized. The resultsshow that optimum sintering temperature of LSGM used as solid electrolyte was around1400℃.Mixed-potential-type NO₂sensors based on LSGM that sintered at1400℃for4h wasfabricated by screen-printing method. And both the physical and sensing characteristics werestudied. The results show that, at the range of450⁷50℃, the EMF values increase with theincrease of the NO₂concentrations. However, with the continuously increase in the operationaltemperature, especially, when it is over600℃, the amplitude of the EMF response towards NO₂decreases as the NO₂concentration increasing. The results of complex impedances spectroscopyindicate that as the NO₂concentrations increasing, the resistance of electrode reaction involvingNO₂decreases gradually.In order to improve the performance of the sensor, Mixed-potential-type NO₂sensor usingLSGM-NiO composite as sensing electrode was fabricated. The physical properties of the NiO（mixed with different proportion LSGM）-SE and sensing characteristics of the sensors werestudied. The results indicate that, at600℃, the sensor using NiO（+15%LSGM） gives the largestEMF among all the samples. Additionally, a LSGM-based amperometric NO₂sensor using coupled with NiO-SE and Pt-RE was also fabricated with the similar way, and its sensing characteristics and stability wereevaluated. The results suggest that a desirable linear relationship between the variation of thecurrent change I and NO₂concentration can be seen at the range of400⁶50℃. Particularly, at450℃, the sensor shows relatively quick response rate, and the response/recovery time are50s and90s, respectively. But the stability of the sensor needs to be further improved.