Research On Frequency Modulation Type Humidity Sensors Based On Piezoelectric Quartz Crystal

Monitoring and controlling humidity levels are critical in a variety of fields,including human comfort,agricultural production,aerospace,and warehouse storage.Humidity sensors are used to monitor and control humidity levels,and they generally consist of moisture-sensitive materials and transducers.The demand for highperformance humidity sensors has driven the development of materials science and various transducer technologies at home and abroad.So far,researchers have developed multiple types of humidity sensors,such as capacitive,resistive,impedance,optical fiber,and frequency-modulation,based on different operating principles.The humidity sensors based on piezoelectric quartz crystal resonators(QCR)attracted much attention in humidity sensors because of their simple structures,low cost,simple operation,digital signal output,simple back-end processing circuit,strong anti-interference capability,and real-time monitoring.Quartz crystal microbalance(QCM)humidity sensors and interdigitated electrode-piezoelectric quartz crystal(IDE-PQC)humidity sensors are two typical frequency-modulation piezoelectric quartz crystal humidity sensors.This dissertation focuses on how to enhance the sensitivity of piezoelectric quartz crystal humidity sensors and proposes a new concept of enhancing sensitivity by optimizing its device structure.i.e.,introducing fringing field scattering of QCM humidity sensors,adjusting the BVD equivalent parameters of IDE-PQC humidity sensors,and optimizing the topology circuits of the piezoelectric quartz crystal humidity sensor.The final goal of enhancing the sensitivity of the piezoelectric quartz crystal humidity sensor is achieved.The main contributions of this article are as follows:First,the influence of different metal electrode materials on the sensitivity of the QCM humidity sensor was investigated.Starting from the constitutive equation of piezoelectric quartz crystal,the piezoelectric vibration of QCM in the electrode and nonelectrode regions was analyzed.The mass sensitivity distribution of QCM with different electrode materials was obtained by solving the mass displacement amplitude.The numerical theoretical calculation results showed that the QCM sensor with gold electrode has the highest sensitivity.Then,the theoretical calculation results were verified by experiments.Finally,the finite element simulation analysis reveals that Young’s modulus and density of electrode material are the main factors affecting the sensitivity of QCM humidity sensors.Second,the influence of electrode structure on the sensitivity of the QCM humidity sensor was investigated.The sensitivity of QCM humidity sensors was enhanced by constructing an asymmetric electrode structure.The traditional symmetric electrode structures are not conducive to fringing field scattering of the QCM,so two types of asymmetric electrode structures were constructed(i.e.ringed electrode QCM and n-m electrode QCM).The electrical parameters of the sensitive material in different humidity environments were introduced into the electromechanical coupling of the QCM to enhance the sensitivity of the QCM humidity sensor.Then,chitin nanofibers and lignin were used as sensitive materials for experimental verification.A BVD equivalent circuit model of the QCM humidity sensor with asymmetric electrode structure was proposed.The enhancement mechanism of the QCM humidity sensor with asymmetric electrode structure was explained by combining finite element simulation and Advanced Design System(ADS)circuit simulation.Third,a novel IDE-PQC humidity sensor was constructed,and the influence of the quartz crystal resonator’s electrode area and wafer thickness on its sensitivity was studied.Firstly,the sensitivity principle of IDE-PQC humidity sensor was analyzed from the perspective of impedance frequency modulation.Secondly,ADS software was used to simulate the influence of various equivalent circuit parameters of piezoelectric quartz crystal on the resonant characteristics of IDE-PQC humidity sensors.Then,the sensitivity of IDE-PQC humidity sensors is higher with the increase of electrode area and the decrease of quartz wafer thickness for quartz crystal resonator by MATLAB software calculation.Finally,chitin nanofibers were used as sensitive materials for experimental verification.Fourth,after analyzing that the frequency of QCM and IDE-PQC humidity sensors decreases with the increase of humidity level,this dissertation studied the influence of serial connection of sensor on the sensitivity of PQC-based humidity sensors and designed a high-performance IDE-QCM humidity sensor.Firstly,the equivalent circuit models of QCM,IDE-PQC,and IDE-QCM were analyzed.Secondly,ADS software was used to simulate and calculate that the frequency change of IDE and QCM is equal when they are used as sensitive components simultaneously,which indicates that using both IDE and QCM as sensitive elements can enhance the sensitivity of the piezoelectric quartz crystal humidity sensor.Thirdly,chitin nanofibers were used as sensitive materials for experimental verification.Finally,the moisture-sensitive mechanism of the IDE-QCM humidity sensor was described.In summary,this dissertation focused on optimizing the device structure of piezoelectric quartz crystal humidity sensors to enhance their sensitivity.A series of highsensitivity piezoelectric quartz crystal humidity sensors were constructed,and the sensitive mechanisms of each sensor were discussed by combining finite element simulation analysis and equivalent circuit models.This doctoral dissertation provides some reference and guidance for the selection of sensitive materials,device performance optimization,and mechanism analysis of piezoelectric quartz crystal humidity sensors.It also provides a reference for constructing other piezoelectric quartz sensors and their further applications in biochemistry.