Study On Gas And Humidity Sensing Of Surface Acoustic Wave Sensors Based On ZnO Composite Film

Surface acoustic wave（SAW）sensors have great potential for development in the sensing field due to their high sensitivity,short detection time,good stability,small size,and low cost.As an important metal oxide semiconductor material with simple synthesis process,high electron mobility,controlled morphology and vacancy defects,zinc oxide（ZnO）is very suitable to be used as a sensitive layer of the sensors for modification of gas and humidity sensitive performances.In this thesis,the ST-cut quartz with good thermal stability was used as the piezoelectric substrate,and temperature frequency factor（TCF）value of 0.12 ppm/℃was obtained.ZnO nanoflowers prepared by hydrothermal method were used as a sensitive film for SAW sensor for gas and humidity test,and the test results were used as a control group.In addition,ZnO was compounded with amino-functionalized multi-walled carbon nanotubes（MWCNTs-NH₂）and sodium alginate（SA）to prepare ZnO/MWCNTs-NH₂ composite membrane for ammonia sensor and ZnO/SA composite membrane based sensor for humidity sensor,respectively.The sensor based on ZnO/MWCNTs-NH₂ composite film has better ammonia sensing performances than that of the ZnO-based sensor due to increased specific surface area,pore space and vacancy defects.In the gas sensing test,its frequency response reached-2.7 kHz at 100 ppm NH₃concentration with a minimum detection limit of 0.1ppm,faster response and recovery time,better selectivity,short-term repeatability and long-term stability.The mechanism for the change of frequency shift to different directions during the reaction phase was investigated.The positive frequency response at low NH₃ concentration is due to the small amount of NH₃ molecules and H₂O molecules in air adsorbed into the pores inside the sensitive film at the first stage,which changes the elastic modulus of the membrane and plays a dominant role in the elastic loading response.The negative frequency response under high concentration of NH₃ occurs because of the decreased pores inside the membrane and adsorbed NH₃ molecules on the surface of the composite membrane,which increases the mass of the membrane and thus the mass loading effect plays the main role.The ZnO/SA humidity sensor prepared by compounding ZnO with SA improved the moisture-sensitive sensing performances compared with the ZnO-based SAW sensor,and the increased functional groups on the composite film facilitated the adsorption of more H₂O molecules.The ZnO/SA based humidity sensor with high sensitivity produced a frequency shift of-26.4 kHz in the humidity variation range of 30%RH-90%RH,which was linearly related to each other.The main factors leading to frequency response variation are the electrical loading effect and the mass loading effect.The former is due to the charge transfer within the membrane and ionization between water and water,and the latter is due to the H₂O molecules adsorbed by the composite membrane to form hydrogen bonds and increase the mass of the membrane.