Preparation And Mechanism Study Of Bacterial Cellulose-Based Flexible Humidity Sensors

Humidity relates closely to human life,which is highly significant in meteorological,medical,agricultural and industrial fields.As a humidity detection tool,humidity sensors convey the comfort of the human living environment and reflect human health information in real time.Utilizing the constantly changing humidity field in the environment the non-contact measurement can significantly reduce the wear and tear of equipment as well as the risk of bacterial cross-contamination.Flexible and transparent insulating polymers such as PET,PI,and PE have been frequently used as substrates in humidity sensors,yet their non-degradability and the requirements for small,portable,low-power,and low-cost remain unrealized,which significantly limits their application scenarios.Cellulose is considered as an ideal support for manufacturing humidity sensors and wearable devices due to its high biocompatibility,biodegradability,and abundant hydrophilic groups inherent on its surface.In this thesis,humidity sensors/actuators with excellent humidity response performance and mechanical properties were prepared from highly hydrophilic,high-strength and degradable bacterial cellulose（BC）using synergistic interactions with conductive materials such as electrolytes,carbon nanotubes（CNT）and two-dimensional MXene.The microscopic morphology,mechanical properties and humidity sensing performance of the bacterial cellulose-based humidity sensors were thoroughly analyzed by a series of characterization methods such as scanning electron microscopy,transmission electron microscopy and mass spectrometer tests,and the differences in the sensing mechanisms of the three humidity sensors were elaborated with the help of Fourier infrared,XRD and XPS analyses.The main conclusions are as follows.（1）Bacterial cellulose films were obtained by drying bacterial cellulose gels in different ways,and humidity sensing materials with fast wet response were obtained by impregnating electrolytes.The experimental results showed that the rapid dehydration of the films under oven drying led to severe damage of the internal structure;The films were more compact under hot-pressure drying,but the structure collapsed under high pressure（10 MPa）;The replacement drying of tert-butanol effectively protected the nano structure of bacterial cellulose,with the measured maximum stress value of 62.2 MPa and the maximum strain value of 2.4%for the films,the linearity R² was Among them,the KOH concentration at 2%featured the fastest response/recovery times of 76 s and 39 s,respectively,with a hysteresis of 4.45%.（2）A two-layer paper-based humidity sensor with excellent humidity response,mechanical properties,and low cost was synergistically prepared by coating and constructing a paper-based hydrophilic coating（first layer）through hydrogen bonding between bacterial cellulose nanofibers and paper base,and carboxylated nanocellulose dispersed multi-walled carbon nanotube ink coated paper base as a conductive active layer（second layer）.The experimental results showed that the mechanical strength reached 66.3 MPa after the introduction of BC coating.94.5%maximum response variation and 6.7 times less CNT ink usage were achieved compared with the sensor without BC（at RH=98%）.The fastest response time and recovery time were 150 s and297 s,respectively,with a hysteresis of 5.4%.（3）The anti-oxidation and high-strength MXene materials were prepared by surface modification of Ti₃C₂T_XMXene which was prepared by HCl/Li F system using natural tannic acid under alkaline conditions,while BC nanofibers were used as reinforcing materials,and humidity-responsive materials with excellent humidity-driven properties and mechanical properties were prepared by ultrasonic mixing and filtration.The experimental results showed that the tensile strength reached the maximum value of 243MPa,the response time was 2.48 s,the recovery time was 4.64 s,and the maximum bending angle was 161.5°with the BC content of 40%.The response time of the composite film was 10 s with recovery time of 34 s when the BC content was 70%.