Research Of The Surface Acoustic Wave Sensor Used To Detect The Nitrogen Dioxide Concentration

As one of the major air pollutants, nitrogen dioxide (NO2) is seriously harmful to humans and the environment. The development of surface acoustic wave (SAW) sensors provides great technical reserves for manufacturing miniaturized NO2sensor with high sensitivity, high stability and fast response. This paper is devoted to the development of SAW NO2sensor with high sensitivity.Firstly, two-port Rayleigh wave resonator is chosen as the nitrogen dioxide sensing unit. The concentration change of NO2is detected by monitoring the frequency of SAW. In order to make the frequency detecting system of SAW sensors easy to carry, oscillating circuits and signal processing circuits are designed and fabricated for SAW devices based on the principle of positive feedback. The stability of the oscillating circuits is improved and the fluctuation of sensor output frequency is controlled within40Hz in ten thousand seconds.Secondly, CuPc (phthalocyanine copper) is chosen as the main sensitive material. In order to increase the sensitivity to NO2at relatively low temperature, multi-walled carbon nanotubes (MWNTs) and metal oxide nanoparticles are doped into the sensitive materials. Meanwhile, several common used methods of sensitive film fabrication are investigated. CuPc/MWNTs film, CuPc/MWNTs doped nano zinc oxide(ZnO) film, CuPc/MWNTs doped nano tungsten trioxide (WO3) film and single nano ZnO thin film was prepared successfully on the SAW devices using spray coating method.Thirdly, a detecting experiment platform is built to precisely control the environmental variables, automaticly collect and store the sensor output data. Based on this platform, researches are carried out to investigate the influence of temperature, humidity, and air flow to the sensor. Then, the detecting performances of the SAW sensors with different sensing films for nitrogen dioxide are tested at50℃. By comparing the experimental results, we come to the conclusion that doping metal oxide semiconductor ZnO and WO3into CuPc/MWNTs sensitive material can significantly improve the sensitivity of the sensor for nitrogen dioxide. The sensor shows good linearity and repeatability to1-80ppm NO2and the sensitivity is50.5Hz/ppm.300ppb nitrogen dioxide is detected at50℃with a200Hz response. And the role of bulk heterojunction structure in improving the sensor response is analyzed.This paper develops a SAW nitrogen dioxide sensor with high sensitivity at50℃, which provided a new way to improve the sensitivity of SAW sensors at a relatively low working temperature.