Study On Surface Acoustic Wave Gas Sensor Based On Carbon Material

Surface acoustic wave（SAW）sensors use surface acoustic wave technology to convert physical parameters into electrical signals that we can study through piezoelectric and inverse piezoelectric effects.Because SAW propagates on the substrate surface,the sensor is very sensitive to the external environment.The large surface area of carbon material is conducive to gas adsorption and penetration,and its semiconductor property makes it advantageous for gas detection.In this thesis,ST-cut quartz is used as the base of SAW sensor,carbon material is selected as the main sensitive material,and the sensitive film is prepared by spinning coating method.The SAW gas sensors based on palladium-modified carbon nanotube sensitive film and palladium-modified reduced graphene oxide sensitive film are prepared,respectively.It is found that the two sensors have good selectivity,high sensitivity,good long term and short-term stability as well as good thermal stability.The SAW sensor based on palladium-modified reduced GO sensitive film has temperature coefficient of 0.11ppm/℃.The SAW gas sensor based on the palladium-modified carbon nanotube sensitive film has a NO₂ response of-1622 Hz to 50 ppm,and the detection limit is 2ppm.The electroloading effect is the main reason for the negative frequency shift,and the Schottky barrier formed by the contact between Pd and carbon nanotube semiconductor is the main mechanism of the electroloading effect.Graphene has a large specific surface area and a two-dimensional network structure that facilitates gas penetration.In order to further improve the sensitivity of the sensor,GO is prepared by Hummers method,GO is reduced by sodium borohydride,and then the treated reduced GO is modified with palladium to produce a SAW sensor based on Pd-RGO sensitive film.It is found that the sensor had a frequency response of-2456 Hz to 50 ppm NO₂ gas.The detection limit is 1 ppm,the response frequency is-22 Hz;and the response time is very fast,about 15 seconds;and the recovery time increases with the increase of NO₂ concentration.In the optimal range of concentration detection,frequency shift is linear with gas concentration,and the value of sensitivity is-150 Hz/ppm.The reason for the negative frequency change is that NO₂ first dissociates with palladium and then spills out,which makes the electrons in RGO transfer,forming more holes and increasing the conductivity.In addition,the two sensors are compared with other NO₂ gas sensors.