| With the rapid development of technologies such as robots.Internet of Things and artificial intelligence,sensors play an increasingly important role in modern society,and are the key devices for intelligent systems to perceive the world.As one of the many types of sensors,gas sensors are used to detect gases change in the atmosphere.The core problem of gas sensor research is to develop a new type of materials with fast response,high sensitivity,good selectivity and long-term stability.Metal oxides have the advantages of rich material types,variable physicochemical properties,and easy control of microstructures.They are the focus of gas sensor research and have wide applications in industrial production and environmental testing.This paper focuses on the gas sensing properties of LBCO single crystal thin films and ZnO nanorod arrays.The gas response mechanism of detecting reducing gases are discussed.In addition,this work also found that LBCO thin film resistors have a very large temperature coefficient,and the LBCO single crystal thin film is easily fabricated to microsensors for vacuum measurement.The main research contents and results of this thesis are as follows:(1)Electrical and gas sensing properties of LBCO single crystal films:LBCO single crystal films with perovskite structure were grown by magnetron sputtering on(001)MgO single crystal substrates.The effects of ambient temperature and reducing gas on the resistance of LBCO single crystal films were studied.The results show that the resistivity of LBCO single crystal film is related to the oxygen content in the film.The temperature coefficient of resistance of LBCO single crystal film annealed by high temperature oxygen is as high as 7.86%/K,which is much higher than that of polycrystalline LBCO film and transition metal film deposited under the same conditions.It can be used to measure vacuum pressure.LBCO single crystal film has stable and repeatable ability to response to reducing gases such as ethanol,hydrogen,carbon monoxide and acetone above 250?.At 350??the gas response(Rgas/Rair)of single crystal LBCO films defined by resistance(R)changes increases with the increase of ethanol concentration(10-2000 ppm),which is 2-6 times higher than that of polycrystalline LBCO films,while the response time decreases with the increase of ethanol concentration.(2)LBCO micro-sensor:Using precision laser processing technology,LBCO single crystal thin film was fabricated into a micro-sensor with a line width of 30 to 200?m.The feasibility of using LBCO single crystal thin film for vacuum measurement and reducing gas detection was discussed.The results show that LBCO micro-sensor can make a good repeatable resistance response to the vacuum pressure change in the range of 5×10-2 to 105 Pa,and the response range is three orders of magnitude larger than that of the traditional Pirani vacuum gauge.With the decrease of linewidth,the sensitivity of the sensor decreases gradually,but it does not change obviously with the change of gas type.It is found that the electrode area of LBCO micro sensor has an important influence on its gas sensing characteristics in the process of reducing gas detection.When the electrode region is properly protected,LBCO micro-sensors with different linewidth exhibit almost identical gas sensing performance and long-term stability.The response time of LBCO micro sensor to ethanol is almost exponentially increased with the increase of air humidity,which not only shows excellent moisture resistance,but also can be used to monitor air humidity and ethanol or other reducing gases simultaneously.Based on the experimental results using air and Ar as carrier gas and the experimental data of oxygen content in LBCO material varying with temperature,it is revealed that the gas response mechanism of LBCO sensor is related to the oxidation-reduction process of LBCO film,i.e.the oxygen vacancy mechanism of metal oxide gas sensor.(3)Room temperature photoresponse of ZnO nanorod arrays and the influence of reducing gases:In order to reduce the working temperature of gas sensors to room temperature,periodic ZnO nanorod arrays were prepared on patterned sapphire substrates by using aqueous solutions of equal molar Zn(NO3)2 and C6H]2N4.The UV and visible light response of ZnO nanorod arrays with different growth conditions was studied,and the possibility to test reducing gases at room temperature was discussed.The results show that ZnO nanorod arrays can produce obvious photocurrent under the irradiation of ultraviolet or visible light.In air environment,with the increase of illumination time,the photogenerated current of ZnO nanorod arrays increases gradually,and can not reach saturation in nearly an hour.Under ultraviolet irradiation,the photogenerated current of ZnO nanorod arrays increases with the decrease of oxygen partial pressure in the environment,and there are two different response stages.When the oxygen content in the environment is lower than that in the air,the photocurrent increases sharply with the decrease of oxygen partial pressure,while when the oxygen content in the environment is higher than that in the air,the photocurrent decreases slowly with the increase of oxygen partial pressure.X-ray photoelectron spectroscopy further proved that the surface of ZnO nanorods was unstable under ultraviolet irradiation,which was closely related to the oxygen content in the environment.ZnO nanorod arrays irradiated by ultraviolet light have photocurrent response to ethanol or other reducing gases.They are promising candidate materials for room temperature gas sensors.However,gas sensitivity and response speed need to be further optimized.(4)Gas sensing properties of ZnO nanorod arrays:ZnO nanorod arrays were fabricated into two-electrode gas sensors.The effects of growth conditions and humid environment on gas sensing properties of ZnO nanorod arrays and their detection mechanism were studied.The results show that ZnO nanoflower arrays have the ability to respond to reducing gases such as ethanol,hydrogen,carbon monoxide,methane and acetone above 200?.At 300?,the sensor can respond rapidly to 10-2000 ppm ethanol,and the gas response increases with the increase of ethanol concentration.It was found that the gas sensing properties of ZnO nanorod arrays depend on the concentration of ethanol,showing two different concentration dependences.By measuring the change of sensor resistance with oxygen partial pressure and the surface state of ZnO nanorods in the absence of reducing gases,it is proved that the response of ZnO nanorod array to reducing gases may involve two mechanisms:lattice oxygen and adsorbed oxygen.At low ethanol concentration.ZnO nanorods are in high resistance statelwhich is similar to the results of high oxygen partial pressure(>4.0×102 Pa).The possible response mechanism is close to the ion adsorption model,desorption of ionized oxygen on the surface of ZnO nanorods is the reason for the increase of resistance.At high ethanol concentration,the transition of ZnO nanorods to low resistance state is equivalent to the sensor operating at low oxygen partial pressure(<1.7×102 Pa).At this time,the lattice oxygen on the surface of ZnO nanorods is reduced in large quantities.Oxygen vacancies or zinc interstitial atoms are the main reasons for the formation of low resistance.In addition,the experimental results show that the conductive network formed by the overlapping of ZnO nanorods is the main electronic transmission channel.When there is humid air in ethanol environment,the gas response of ZnO nanorod array increases with the increase of air relative humidity,but the response time does not change much.In the absence of ethanol or other reducing gases,ZnO nanorod arrays have the ability to detect air relative humidity. |
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