Study On Humidity And Gas Sensing Of Surface Acoustic Wave Sensors Based On PAA/PVA

Surface acoustic wave(SAW)sensor utilizes the characteristic that most of the energy of SAW is concentrated on the surface of the substrate,and a very sensitive gas sensor can be prepared.Polyacrylic acid(PAA)is very suitable as a sensitive film for SAW sensors due to its low cost,simple process and a large number of hydroxyl and carboxyl groups.In this thesis,SAW ammonia sensors based on PAA and PAA / polyvinyl alcohol(PVA)were prepared,and gas sensing performances of the sensors for ammonia were studied.There are a large number of hydroxyl and carboxyl groups on the surface of PAA,which makes it suitable to adsorb water molecules and ammonia gas molecules.The SAW ammonia gas sensor based on PAA is sensitive to ammonia gas and has an excellent detection limit.Under the conditions of 25°C and 30% relative humidity,the PAA SAW ammonia sensor has a frequency shift of 11.5 kHz to 20 ppm ammonia gas,and the detection limit can reach 0.5 ppm.We have studied the sensing mechanism of SAW sensor based on PAA sensitive film,and concluded that the increase of the elastic modulus of the sensitive film is the main reason for the positive frequency shift.The prepared PAA SAW ammonia sensor not only has excellent sensitivity and lower detection limit,but also has excellent selectivity,short-term stability,and long-term stability.Meanwile,response and recovery time are linearly related to the ammonia gas concentration.Sensing mechanism analysis shows that the increase of the elastic modulus of the sensitive film is the main reason for the positive frequency shift.Gas characteristics of PAA/PVA ammonia gas sensors with different proportions are studied.By comparing the experimental data of PAA SAW ammonia sensor and PAA/PVA SAW ammonia sensor,it is found that the introduction of PVA greatly enhances the response of PAA SAW sensor(also at 25? and 30% relative humidity).Under the conditions,PAA/PVA SAW ammonia gas sensor has a frequency shift of 28.5 kHz for20 ppm ammonia gas.This is because the surface structure of the PAA sensitive film is changed.However,the introduction of PVA will cause the baseline of the sensor to move after each ammonia gas detection,because an irreversible chemical reaction occurs.Moreover,through controlled experiments,it is found that only PAA has detection effect to ammonia gas,and the introduction of PVA increased the response time and recovery time of the original PAA SAW ammonia sensor.However,the two sets of ammonia sensors have excellent performances in terms of selectivity,short-term repeatability and long-term stability.In addition,by setting different humidity environments,the experimental results show that a high humidity environment helps to enhance the PAA/PVA SAW ammonia sensor's response to ammonia.This is because water molecules change the interchain hydrogen and conductivity.