Research On Polymer-based Capacitive Flexible Humidity Sensor

Humidity is closely related to our life,which refers to the concentration of water vapor in the air.Humidity has been widely used in agriculture,climate monitoring,the use and protection of precision instruments,biomedical,industrial production and daily life skin humidity monitoring and other fields.The development of traditional rigid humidity sensors have reached a more mature stage,but with the development of flexible electronics and wearable technology,the traditional rigid humidity sensors gradually reveal the limitations of application.Flexible humidity sensors have the advantages of flexible stretch,low manufacturing cost,low energy consumption,simple manufacturing process and easy integration.Therefore,flexible humidity sensors have a more extensive application.Among the flexible humidity sensors,flexible humidity sensors using polymer materials as the most sensitive materials are the most common.The polymer humidity sensors are mainly divided into two categories: capacitive and resistive.This article chooses to study the polymer capacitance type flexible humidity sensor.The polymer capacitive humidity sensors have the characteristics of good linearity,high sensitivity,small hysteresis,wide test range,good stability,low power consumption and so on,so they have broad prospects in the field of humidity sensors.The capacitive humidity sensors are divided into two categories: interdigitated structure and sandwich structure.Humidity sensors with interdigitated structure and sandwich structure have their own advantages and disadvantages.In this paper,based on the research and analysis of the moisture sensing mechanism of the humidity sensor,firstly,a flexible PVDF/PVA/LiCl humidity sensitive film was prepared,and then based on the humidity sensitive film,two types of electrode structures,interdigital and sandwich structures,were produced.The main work of this paper was as follows:(1)Preparation and research of flexible PVDF/PVA/LiCl humidity sensitive film.The hydrophobic polymer polyvinylidene fluoride(PVDF)was used as the matrix material,which was modified by blending polyvinyl alcohol(PVA)and lithium chloride(LiCl)to form PVDF/PVA/LiCl solution,and then the flexible PVDF/PVA/LiCl humidity sensitive film was prepared by casting and scraping process.It was found thatthe hydrophilicity of PVDF/PVA composite film after blending PVA was improved compared with that of pure PVDF film,while the hydrophilicity of PVDF/PVA/LiCl composite film was not further improved after further blending LiCl.It was found that PVA and LiCl could increase the pores of the films,PVA could increase the pores of the films,while LiCl could increase the pores of the films.(2)The preparation and performance of interdigital humidity sensor based on flexible PVDF/PVA/LiCl humidity sensitive film.The interdigital electrode was prepared on the flexible PVDF/PVA/LiCl humidity sensitive film by screen printing,and the flexible PVDF/PVA/LiCl humidity sensor with interdigital structure was obtained.The flexible PVDF/PVA/LiCl film was not only a humidity sensitive material but also a flexible substrate.The humidity test platform was built,and the linearity and sensitivity,humidity hysteresis,response time and repeatability,temperature influence and other performance parameters of the humidity sensor were tested.According to the test results,it was found that the capacitance of the sensor changes nonlinearly with the humidity;and it has high sensitivity,if the ratio of capacitance values was used to express the sensitivity,the maximum sensitivity was 894.92;it had good repeatability and stability;the humidity hysteresis and temperature coefficient were small,the humidity hysteresis was about 2.1%RH;and the response time was about 15 s.Finally,the working mechanism was analyzed according to the test results.(3)The preparation and performance of sandwich structure humidity sensor based on flexible PVDF/PVA/LiCl humidity sensitive film.The sandwich structure flexible PVDF/PVA/LiCl humidity sensors were fabricated on the flexible PVDF/PVA/LiCl films by screen printing and spraying silver nanowire dispersion respectively.The humidity performance of the two kinds of flexible capacitive humidity sensor was tested,and then the working mechanism was analyzed.Finally,according to the results of humidity performance test,three kinds of flexible capacitive humidity sensors were compared and analyzed.It was found that the interdigital humidity sensor had faster response time and less humidity lag,while the sandwich structure had better linearity.