Research On QCM Humidity Sensor Based On Chitosan Composites

With the gradual deepening of people's understanding of humidity,the detection and accurate control of humidity has gradually formed norms in many fields such as meteorological monitoring,industrial production,agricultural planting,national defense science and technology,aerospace and even daily life.In recent years,researchers at home and abroad have done a lot of fruitful work in the research and development of high-performance humidity sensors.Not only are more and more different types of sensors available for humidity detection,but also the devices are developing rapidly towards miniaturization,wearability and integration.However,conventional resistive humidity sensors usually require high conductivity of sensitive materials,and the high moisture resistance of materials has always been a difficult problem to solve.Therefore,many weak conductive or even insulating hydrophilic materials with good adsorption properties are limited in the field of humidity detection.Quartz crystal microbalance?QCM?is a mass-type sensor based on the piezoelectric effect of quartz crystal.Based on the special working principle,this type of device does not have strict requirements on the electrical properties of sensitive materials.In addition,QCM sensors have attracted much attention in the field of sensors due to their advantages such as high detection accuracy,easy electrode surface modification,working at room temperature,and real-time monitoring.In this paper,the biomaterial chitosan?Chitosan,Chit?was used as the matrix material,and the QCM was used as the sensor device.The composite materials with strong hydrophilicity were prepared by solution blending and surface deposition crosslinking.And the high-performance QCM humidity sensors were fabricated by drip coating,dipping and other methods.On the one hand,the oscillator circuit analysis method was used to comprehensively review the device performance from the aspects of dynamic response characteristics,repeatability,stability,selectivity,wet hysteresis characteristics,and response-recovery time.On the other hand,the electrical parameters inside the device were systematically studied by impedance analysis.The main research content and the phased results obtained are as follows:1.The construction of QCM humidity sensor detection system was completed through the design of the QCM sensor vibration circuit,the preparation of the saturated salt solution static humidity generating device,the surface of the QCM sensor functional modification and the connection and debugging of high-precision frequency meters and impedance analyzers.2.A QCM humidity sensor was fabricated using chitosan as a humidity-sensitive material.The test results show that the device exhibits good response characteristics to water molecules under various relative humidity conditions.The main reason is that the surface of chitosan is rich in hydrophilic groups such as amino?-NH₂?and hydroxyl?-OH?and the water molecules in the environment can be adsorbed on the surface of the chitosan-sensitive film by hydrogen bonding.However,it was found in subsequent experiments that the chitosan-modified QCM humidity sensor showed poor repeatability after repeated cycle tests,and there was a serious humidity hysteresis during continuous moisture absorption and dehumidification.The reason is that chitosan is a polymer material,and its own flexible chain-like structure will easily swell after absorbing water.These phenomena will lead to the sensitive film and QCM substrate cannot achieve synchronous oscillation,resulting in the device's short-term and long-term stability are affected by varying degrees.3.In view of the shortcomings of the QCM humidity sensor modified with chitosan,we aimed to improve the material structure and mechanical properties of sensitive materials.The zero-dimensional Graphene quantum dots?GQDs?was introduced into the chitosan system,and the two materials formed a composite material with a network structure?GQDs-Chit?through the close combination of electrostatic effect and hydrogen bond.The results show that the performance of the QCM humidity sensor modified by GQDs-Chit composite material has been significantly improved compared to the QCM sensor modified by chitosan,mainly in terms of increased sensitivity,reduced hysteresis,improved repeatability and enhanced long-term stability.In addition,it has been verified by impedance analysis that the QCM humidity sensor modified by GQDs-Chit composite material has a higher quality factor,which shows that the resistance that QCM devices need to overcome is significantly reduced.4.On the basis of the above work,this paper replaces GQDs materials with one-dimensional multi-walled carbon nanotubes?MWCNTs?,and silica materials with ordered mesoporous structure MCM-41.The above two materials were used as supporting framework of chitosan to prepare a composite material,and QCM humidity sensors were manufactured using the obtained composite materials,respectively.Through the analysis and comparison of various humidity sensitivity test results,it was found that the devices modified by the two composite systems not only significantly improved the phenomenon of poor short-term and long-term stability of the device modified by pure chitosan material,the sensitivity,humidity hysteresis and many other humidity-sensitive properties were also significantly improved.In addition,the principle of water molecule adsorption was analyzed and discussed by using Langmuir isothermal adsorption model,and the Gibbs free energy was calculated to verify that the adsorption reaction was spontaneous.This paper draws a conclusion from the above work:Compared with the single material,the composite with the introduction of the reinforcing phase has higher mechanical strength,better adsorption performance and less energy loss.The synergistic effect of the two single materials on the performance and structure makes the composite with more superior comprehensive moisture sensitive performance.The above research results can provide certain reference value for the mechanism analysis of QCM sensors,and can also provide some feasible ideas and methods for the selection and improvement of sensitive materials for QCM sensors and the improvement of device performance.