Nitrogen Dioxide Sensor Based On Surface Acoustic Waves And Its Stability Studies In Acid And Alkali Environments

With the rapid development of the Internet of things,intelligent monitoring and wireless sensor technology has been widely developed.As a key part of the network,sensor has always been a research hotspot.Surface acoustic wave（SAW）devices are widely used in the fields of physical,chemical and biological sensing due to their high sensitivity,miniaturization,low power consumption and wireless passivity.In recent years,the research on SAW sensors is endless.Nitrogen dioxide（NO₂）is a common air pollutant.With the improvement of environmental protection awareness,people put forward higher requirements for the development of new NO₂ sensors.However,there are still many problems in the surface acoustic wave NO₂ sensors reported at present,such as complex preparation process of sensitive layer,small response range of devices,low sensitivity,need for additional heating and so on.In addition,most SAW sensors are based on bulk piezoelectric materials,because of its stable material parameters and high electromechanical coupling coefficient.The corresponding piezoelectric thin film has many advantages,such as adjustable material parameters,compatible with semiconductor process,convenient on-chip integration,and can be deposited on various hard or flexible substrates.It is widely used in ultra-high frequency devices,gas sensors,temperature and humidity sensors,and biological sensors.It has a wide application prospect in the fields of sensor and microfluidic applications.In these applications,the piezoelectric film often needs to be in direct contact with the measured,which makes the sensor often in high temperature and humidity,toxic and harmful gases,acid-base corrosion and other environments.When working in these extreme conditions,the piezoelectric film may be irreversibly damaged,leading to device performance deterioration or even failure.There are few researches on performance and failure behavior.In view of the above problems,the research work of this paper is divided into two parts:1.Research on nitrogen dioxide sensor based on saw;2.Research on the stability of piezoelectric film saw device in acid-base extreme environment.Firstly,the surface acoustic wave NO₂ sensor based on 3D porous reduced graphene oxide-polypyrrole/Ag（r GO-PPy/Ag）composite sensitive film was designed and fabricated by optimizing the material preparation process,using noble metal doping,UV assisted irradiation and other methods.The intrinsic mechanism of noble metal doping and UV assisted irradiation to improve the sensing performance was systematically studied.A new method based on 3D porous structure,Ag nanoparticles doping,p-p heterojunction was proposed As well as the enhancement mechanism of photo generated carriers,the gas sensing process and the reason of frequency shift are described in detail,and the high-performance sensing of NO₂at room temperature is realized.The test results show that the r GO-PPy/Ag SAW NO₂ sensor based on UV assisted irradiation has high sensitivity（127.68 Hz/ppm）and fast response/recovery speed（39.7 s/58.5 s）,showing good repeatability and selectivity.Secondly,two kinds of SAW devices based on zinc oxide（Zn O）thin film and aluminum nitride（Al N）thin film are designed and fabricated in this paper.The performance and failure behavior of SAW devices based on Zn O thin film in acid and alkali harsh environment are systematically studied by using HCl solution and Na OH solution to simulate acid and alkali harsh environment.The results show that the corrosion rate of Zn O film is very fast in the acid and alkali harsh environment,the corrosion resistance time is less than 45 s,the crystal orientation of the film does not change during the reaction process,but the film defects increase,the grain size decreases,and the resonant frequency of the device decreases.Combined with the simulation,it is proved that the elastic change of the film is the main reason for the decrease of the resonant frequency of the device.In addition,In addition,the experimental results show that the resistance of Al N thin film device to acid and alkali harsh environments is better than that of Zn O thin film device,but it can not work normally for a long time.