Fabrication,Performance And Wearable Application Of Carbon-based Voltage Output Type Flexible Humidity Sensor

With the rise and development of flexible wearable electronics,flexible humidity sensors,as a device that can convert environmental humidity changes into detectable electrical signals with excellent flexibility and bendability,have been widely used in various fields such as human and plant health monitoring management,non-contact human-computer interaction,intelligent electronic textiles,industrial and agricultural production and other fields.At present,flexible humidity sensors based on carbon materials are widely researched,but there are still some problems,such as the preparation process is more complex and costly,insufficient sensitivity,long response/recovery time,and most of them are based on the response principle of resistance,capacitance,etc.,which do not have the self-powered characteristics of voltage output or low voltage output variation resulting in insufficient sensitivity,which greatly limit their application possibilities.To address this status quo,this paper aims at wearable applications and uses carbon materials to develop a flexible humidity sensor with simpler preparation,lower cost,high sensitivity,good stability,and voltage output signal,and demonstrates its multi-functional application in wearables related humidity detection.The main research contents and work are as follows:Firstly,the CI/FP moisture-sensitive composite film was prepared by the"impregnation-drying"method using commonly used items such as cellulose filter paper（FP）and carbon ink（CI）as materials,and was assembled with a flexible polyester conductive cloth tape in an asymmetric sandwich structure（partially encapsulated on the upper surface and fully encapsulated on the lower surface）to build a voltage-type CI/FP flexible humidity sensor.The morphological tests of carbon ink showed that it contains uniformly distributed carbon nanoparticles,which can be used as a modified carbon material.X-ray energy spectroscopy and Fourier transform infrared spectroscopy tests showed that the CI/FP composite film contains hydrophilic functional groups,and the characterization of contact angle also proved its good hydrophilicity.These tests demonstrate that the CI/FP composite film can be used as a flexible moisture-sensitive sensing layer for a flexible humidity sensor.The remarkable feature of this humidity sensor is the generation of a voltage output corresponding to a wide relative humidity range（RH 11～98%）,fast response/recovery time（132/50 s）and sensitivity（1.83 m V/%RH）,and excellent flexibility（more than 1000 bends）.It can also be successfully applied to monitor respiration rate,air humidity,and non-contact humidity sensing of wet palms or objects.To further improve the moisture-sensitive performance of CI/FP humidity sensors,four inorganic salts（Li Cl,Mg Cl₂,Na Cl,KNO₃）were used to modify the CI/FP film by secondary impregnation and construct the voltage-type CI/FP/inorganic salt flexible humidity sensors with the same structure.The moisture sensitivity test results showed that all four inorganic salts enhanced the voltage output and significantly reduced the response/recovery time of the CI/FP humidity sensor at maximum humidity（RH 98%）to different degrees,among which the effect of Mg Cl₂ and Li Cl salts was more obvious.Taking Mg Cl₂ salt as an example,the response voltage value at RH 98%is improved～1.9 times;sensitivity is improved～2.4 times;response/recovery time is reduced to 40/33s,and response speed is reduced～3 times.In addition,considering the paper materials are easy to tear and have low wet strength in practical use,especially in high humidity environment.The voltage-type CI/NWF/Li Cl flexible humidity sensor was prepared using cellulose non-woven fabric（NWF）with the same porous structure,higher flexibility and greater wet strength as the base material.The test results show that the sensors exhibit good humidity-voltage response and cyclic response performance at different RH,which indicates that the cellulose fabrics represented by non-woven fabric can also be used for the preparation of carbon-based flexible humidity sensors,providing more options for the raw materials.Finally,considering that the humidity sensors based on paper and fabric substrates have problems such as poor adhesion to the upper and lower conductive fabric electrodes,poor stability due to easy moisture absorption in long-term storage and use,and still long response/recovery times.Polyvinyl alcohol（PVA）loaded with Nano carbon powder（NCP）and Mg Cl₂ was used to prepare a PVA-NCP-Mg Cl₂ film（PCMF）as a flexible moisture-sensitive sensing layer.In terms of sensor structure design,the innovative use of flexible copper and aluminum foil tapes as different electrode material layers to build a voltage-type Cu/PCMF/Al flexible humidity sensor.The results show that the humidity sensor has a more stable and solid structure,excellent moisture-sensitive performance,and voltage output.Specifically,it has an ultra-fast response/recovery time（6/11 s）,a sustained high voltage（～0.6 V）and current output（～2.3μA）over 10,000 s,and excellent sensitivity（～9.2 m V/%RH,about 5 and 2 times higher than CI/FP and CI/FP/Mg Cl₂ humidity sensors,respectively）.The sensor can be integrated into commercial masks for monitoring human breathing patterns,rates,and breathing status before and after exercise,as well as into face masks for simple recognition of human pronunciation and coughing,and has good potential for practical applications such as non-contact distance recognition of fingers or palms,switch sensing,and wet and dry skin condition detection.