Research On Capacitive Humidity Sensor Based On Polyimide Film

Humidity is an important environmental parameter,which plays an important role in daily life and production activity.At present,the most widely used polymer capacitive humidity sensor has attracted extensive attention from researchers due to its wide test range,high linearity,short response time,and long-term stability.However,with the expansion of humidity sensor applications,people have put forward higher requirements for the various characteristics of humidity sensors.Capacitive humidity sensors based on conventional polyimide(PI)humidity sensing materials have become the mainstream due to their high stability and high linearity,but their low sensitivity and large temperature coefficient limite improvement of detection accuracy and application range.Therefore,studying the working mechanism and improvement schemes of high-precision capacitive humidity sensors has important theoretical and practical value.In recent years,the fusion of nanotechnology and sensor technology has provided new ideas for the performance optimization of humidity sensors.Based on the interdigital capacitance humidity sensor structure,this thesis uses a composite film of the polymer PI and nano-titanium dioxide(Ti O₂)as the humidity sensing material,and develops a high-performance capacitive humidity sensor.According to the actual application requirements,the design,simulation,preparation,optimization,measurement and compensation calibration are completed.The main contents of the thesis are summarized as follows.1.The working mechanisms of different types of humidity sensors are introduced,and the challenges of current humidity sensor researches are analyzed.The performance characteristics of humidity sensors are described,including the static and dynamic parameters.2.Based on the rapid response advantage of the interdigital capacitor structure,a platinum metal interdigital capacitor structure with a glass substrate is designed,and the width,spacing and index of the interdigital electrodes are determined.Based on the humidity sensing mechanism of the PI and nano-Ti O₂,the PI and three kinds of PI/Ti O₂ humidity sensing materials with different doping concentrations are designed.The model of the sensor is completed by using COMSOL,and the output capacitance characteristics of the humidity sensors under different relative humidity(RH)are simulated.In addition,the quantitative relationship between the penetration depth of the electric field lines in the humidity sensitive film and the sensor sensitivity is studied.The optimized thickness of the humidity sensing film is determined to be 50?m.3.Using the Fourier Transform Infrared(FT-IR)Spectroscopy,Scanning Electron Microscopy(SEM),Atomic Force Microscopy(AFM),Contact Angle Measuring Instrument and Impedance Analyzer,the prepared PI and PI/Ti O₂ humidity sensing films are characterized and tested.The relationships between the content of Ti O₂ in the composite film and the roughness,hydrophilicity,dielectric constant and dissipation factor are investigated.A humidity sensor electrical parameter test platform is built to measure the characteristic parameters such as the sensitivity,hysteresis,response time and stability.Test results show that the sensitivity of the optimally designed sensor(PI+5 wt.%Ti O₂,50?m)is 12.84 f F/%RH,which is 97%higher than the unoptimized(pure PI,50?m)sensor(6.52 f F/%RH),and the hysteresis is 1.61%RH.4.The design and verification of temperature compensation are completed,and the maximum humidity error is reduced from 6.811%RH before temperature compensation to0.818%RH.The application module circuit of capacitive humidity sensor is designed based on the capacitive readout chip MS3110,which can realize the online compensation output from the capacitance value to the humidity value.In summary,the capacitive humidity sensor designed in this thesis has the characteristics of relatively high sensitivity and low hysteresis.In addition,an online temperature compensation function is added,which not only reduces the output error of the humidity sensor,but also provides a good reference for the application circuit design of other types of capacitive sensors,such as the acceleration sensors and inertial sensors,showing good practical values.