Metal Oxide Materials For NO₂ Sensing Under Atmosphere And Automobile Exhaust

With the rapid development of human society,the emission of air pollutants has been increasing.As a result,the air quality is becoming increasingly poor,affecting human health as well as natural environment.Nitrogen dioxide is a major air pollutant with significant emissions coming from automobile exhaust.Therefore,it is very important to effectively detect NO₂ under the settings of atmosphere as well as automobile exhaust.Development of high performance NO₂ sensor has also become a major focus of scientists and engineers.Among different types of gas sensors,chemresistive sensor based on metal oxide semiconductor（MOS）has been very popular.However,MOS sensor is not stable under high temperature environment such as automobile exhaust.On the other hand,mixed potential gas sensor offers high stability and outstanding sensitivity in high temperature,compensating the drawback of MOS sensor.In this study,nano-structured manganese ferrite（MnFe₂O₄）was synthesized as the sensing material for MOS sensor and thick films of NiO composites were utilized as sensing electrode for mixed potential gas sensors,aiming to detect NO₂ under respectively different conditions of atmosphere and automobile exhaust.In the first chapter,the research background was introduced.The perniciousness,current status and sources of the NO₂ pollutant were described in detail.The research and applications of various types of NO₂ sensors were discussed.The principles of gas sensing reaction and characteristics of sensing properties were also expounded.Finally,the goal and contents of this research were listed.In the second chapter,manganese ferrite was synthesized by hydrothermal method,and utilized as resistive sensing material for NO₂.The phase and morphology were analyzed by XRD and SEM.The gas sensing properties were tested in 500 ppb NO₂,preliminarily qualifying the NO₂-sensing capability of MnFe₂O₄.In the third chapter,MnFe₂O₄ nanospheres with diameters in the range of 200-300 nm were synthesized by a solvothermal method.The gas sensing properties of the samples were studied.The sensor was tested at different temperature in the atmosphere of 250 ppb NO₂.After comparing the sensing properties at different temperatures,200℃was defined as the optimum operating temperature for the sensor.At 200℃,the detection limit is as low as 25 ppb with the response stable in a period of 70 days.Besides of that,the selectivity of the gas sensor is excellent for NO₂ over other air pollutants.Finally,the principle of sensing reaction was explored by XPS.In the forth chapter,NiO composite materials were utilized as sensing electrode to detect NO₂ at 850℃.NiO powders were mixed with other materials by ball milling.Through improvement on the sensing electrode and device structure,the sensor is capable of detecting 25-500 ppm NO₂ and the sensing properties were stable within one month.In the fifth chapter,a summary is given to list out the accomplishment and shortcomings of the present work as well as future work following the project,in a best hope to benefit other research in the field of NO₂ sensor.