Research On Structural Design And Humidity Sensitive Properties Of Cellulose/Carbon Nanotube Composite Materials

In recent years,humidity measurements have attracted much attention in many fields including weather prediction,agricultural production,industrial manufacturing,historic preservation,and wearable electronics.As important tools for humidity detection,humidity sensors can convert virtual humidity signals into readable electrical signals,which have been widely used in people's daily lives.Currently,most commercially available humidity sensors are based on porous ceramics.However,due to their rigid structure and complex preparation process,their application in the field of emerging wearable electronics is limited.Even though sensors based on polymer matrix/conductive filler system exhibit good flexibility,the commonly used polymer substrates are petroleum-based derivatives,which have some problems such as non-biodegradability,insufficient durability,and poor compatibility with conductive fillers.To address the above problems,in this work,multiscale cellulose materials with excellent hydrophilicity,high strength,good flexibility,abundance and biodegradability were used for the preparation of cellulose and carbon nanotubes(CNTs)composites to construct cellulose/CNT-based humidity sensors.From the perspective of structural design,aiming at improving the sensitivity of the sensors,one strategy is to amplify the influence of fibers swelling on the conductivity of the CNTs network,and the other is to increase the contact area between the sensors and humidity environment for increasing water molecules adsorption capacity of the sensors.Based on the above two strategies,flexible and self-supporting cellulose nanofiber/CNT(CNF/CNT)composite film humidity sensor,flexible paper humidity sensor based on CNF/CNT dual networks,electrostatic self-assembly enabled flexible 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-oxidized cellulose fiber/CNT(TOCF/CNT)paper-based humidity sensor,porous CNF/CNT aerogel humidity sensor,and 3D printing patterned CNF/CNT scaffold humidity sensor were prepared.Also,some concepts such as CNF/CNT dual networks structure,CNF/CNT/paper fiber mechanical interlocking structure,TOCF/CNT conformal assembly structure,CNF/CNT porous structure,and patterned CNF/CNT scaffold structure were proposed.By further understanding the humidity sensitivity and the structure of the sensors,the relationship between them is clarified,which provides a new idea and direction for the fabrication of novel cellulose-based humidity sensors.The main work of this dissertation includes:(1)As is known,pristine CNTs are difficult to disperse in aqueous medium.To solve this problem,CNF was used as a dispersant to facilitate the dispersion of CNTs in water.Then,a flexible and self-supporting CNF/CNT composite film humidity sensor was obtained using the CNF/CNT dispersion through vacuum filtration method.The excellent hydrophilicity of CNF greatly enhances the water molecule adsorption capacity of the composite film,and further improves the sensitivity of the CNT-based humidity sensor.The results show that the sensitivity of the CNF/CNT composite film sensor under 95%relative humidity(RH)is as high as 69.9%(?I/I₀)when the loading of CNTs is 5 wt%.Besides,the sensor also shows good bending durability and long-term stability.Based on the sensitive performance of the sensor,the humidity sensing mechanism is analyzed and discussed.Finally,the potential application of the CNF/CNT composite film sensor in the field of human respiration monitoring is demonstrated.(2)To further optimize the humidity sensitive performance of the prepared sensors,herein,carboxylated CNTs were used as the humidity sensitive materials,and then a flexible paper-based bilayer humidity sensor(PBHS)was readily prepared by depositing CNF/CNT ink on the surface of the paper substrate using paper coating method.Due to the excellent hydrophilicity of CNF and paper fibers,the sensitivity of the PBHS under 95%RH is as high as 65.0%(?I/I₀)when the CNF/CNT loading is 0.11 g/m².It also shows excellent linearity(R²=0.995)and long-term stability(more than 3 months).Besides,the unique interlocking structure and strong bonding strength between CNF/CNT dual networks and paper fibers enable the PBHS with good bending durability.The obtained PBHS can tolerate extreme bending(maximum curvature of 22.2 cm^-1)and folding(up to 50 times)treatment without obvious changes in conductivity and sensitivity.Finally,the PBHS demonstrates applications in fields of human respiratory monitoring and air humidity monitoring.(3)Cellulose pulp was pretreated by TEMPO oxidization,and strong negative charges were introduced to the surface of the cellulose fibers in the process.CNTs were dispersed by cationic cetyl trimethyl ammonium bromide(CTAB),which introduced positive charges on the surface of CNTs.Then,a flexible TOCF/CNT(TC)humidity sensor was prepared based on the conformal assembly of positively charged CNTs on the surface of negatively charged TOCFs through electrostatic attraction.This structure endows the sensor with good conductivity and maximizes the influence of TOCFs swelling on the CNTs conductive network.When the ratio of TOCFs to CNTs is 30:1,the obtained TC sensor exhibits a maximum response value of 87.0%(?I/I₀)under 95%RH.Besides,it also exhibits excellent linearity(R²=0.995),good bending(with a curvature of 2.1 cm^-1)and folding(up to 50 times)durability,and long-term stability(more than 3 months).Finally,the sensor demonstrates potential applications in fields of human respiratory monitoring,finger humidity monitoring,and air humidity monitoring.(4)A lightweight,porous,and high-strength humidity sensor was developed based on CNF/CNT aerogel(CCA)using freeze-drying method.The dynamic water vapor adsorption results illustrate that the porous structure can increase the water vapor adsorption capacity of the CCA sensor.When the CNTs loading is 15 wt%,the obtained CCA sensor shows a maximum sensitivity of 87.3%(?I/I₀)at 95%RH,and the water vapor adsorption behavior of the CCA sensor can be well described by the pseudo-first-order kinetic model.In addition,the CCA sensor also exhibits excellent linearity(R²=0.996)and long-term stability(more than 2months).As a preliminary exploration for the application of the CCA sensor,the sensor demonstrates good response performance for human respiratory monitoring and finger humidity monitoring.(5)CNF was used as the dispersant and viscosifier to prepare printable CNF/CNT conductive ink due to its amphiphilicity,high viscosity,and shear thinning properties.Patterned CNF/CNT scaffold(CCS)humidity sensors were then obtained using the CNF/CNT ink via 3D printing combined with freeze-drying method.Benefiting from the hierarchical millimetre-sized grid structure and micron/nano-sized pore structure,the CCS sensor achieved controllable response performance.When the loading of CNTs in the printing ink is 15 wt%,the CCS sensor(with a height of 2 mm)at 15%infill density shows a high sensitivity of 526.7%(?R/R₀)at 95%RH.Besides,the sensor also shows excellent stability(more than 2 months).As a proof of concept,the CCS sensor shows great potential applications in fields of human respiratory monitoring,human voice recognition,and finger humidity monitoring.