Working Stability And Multifunctionality Of Flexible Nanocomposite-Based Piezoresistive Sensors

Flexible piezoresistive sensors based on conductive nanocomposites have great potential for applications in health monitoring,human motion tracking,electronic skins,and robotics.In recent years,plenty of researches about flexible piezoresistive sensors based on conductive nanocomposites have been carried out,and most of the related studies focused on inproving the sensitivity and wearability of sensors.In practical applications,flexible wearable devices were easily affected by environmental temperature,humidity and other factors;however,the current researches were mostly concertrated on the room-temperature environment,lacking a systematic research of comprehensive performances of piezoresistive sensors in a wide temperature range.There have been few reports yet on development and design of high-performance flexible piezoresistive sensors,which were suitable for complex environmental conditions.In addition,the new generation of wearable electronic devices desired to have multifunctional integration to meet the specific requirements of sensing,energy storage,and electrothermal applications.Therefore,it would be a great challenge about how to prepare high-performance conductive nanocomposites with both versatility and working stability for the development of high-performance flexible piezoresistive sensors in the future.Therefore,this dissertation is devoted to the design,fabrication and functionalization of new-type conductive nanocomposites and their wearable piezoresistive sensors to meet the practical application requirements of flexible electronic devices.The main research contents and outcomes are presented as follows:（1）Exploring the temperature-dependent electromechanical response behaviors of conducting nanocomposites is crucial for high-precision sensing of flexible piezoresistive sensors in wide-temperature ranges.Thus,we prepare carbon nanotube（CNT）/polydimethylsiloxane（PDMS）composites,which are widely used in the field of flexible piezoresistive sensors,and then we systematically investigate their temperature-dependent electrical,mechanical and piezoresistive properties in the temperature range of-150 ^oC to 150 ^oC for the first time.At-150 ^oC,the CNT/PDMS composites become brittle and do not have reversible compressive properties and sensing performances;at the temperatures higher than 90 ^oC,the CNT/PDMS composites cause physical softening under oxygen/heating environment and do not have reversible sensing properties.In the temperature range of-60 ^oC to 90 ^oC,the CNT/PDMS composites have both reversible compressive properties and sensing performances,the modulus of compressibility and electrical resistivity gradually decrease with increasing temperature,and the sensing sensitivity also decreases gradually with increasing temperature.Besides,the results on cycling stability of the CNT/PDMS composites indicate that the composites at room temperature exhibit better mechanical and piezoresistive stability than those at low and high temperatures.Given that temperature changes have a significant effect on the sensing performances of nanoconductive composites,the effect of ambient temperature changes must be considered when flexible piezoresistive sensors are designed and fabricated.（2）In order to avoid the interference of ambient temperature on the sensor’s working stability,it is of great significance to develop a wearable piezoresistive sensor with zero temperature resistance coefficient（TCR）.In this section,graphene nanosheet（GNP）/CNT/silicone elastomer（GCE）fiber-shaped composites are successfully prepared by a simple direct-ink-writing printing technique.The CNTs in this GCE fiber-shaped composite have a negative temperature resistance coefficient（NTC）,while the GNPs have a positive temperature resistance coefficient（PTC）.By optimally adjusting the mass ratio of CNTs and GNPs,a near-zero TCR(1.14×10^-4/°C)is obtained for the GCE piezoresistive sensor.The prepared GCE piezoresistive sensor has a high sensitivity（100%,GF=14550.2）,wide strain working range（<100%）,extremely low strain detection limit（>1%）,a quick response time（170 ms）and excellent cycling stability（10,000 cycles）.In addition,the GCE piezoresistive sensors exhibit good working stability in a wide temperature range,which also verify their anti-jamming sensing applications in human motion monitoring.（3）In addition to service temperatures,flexible piezoresistive sensors are also susceptible to light,ambient moisture and other environmental factors,which would greatly affect their stability in practical application.Based on the ultrahigh conductivity of Ti₃C₂T_x MXene and the excellent biocompatibility of bamboo cellulose fibers（BCF）,MXene/BCF（MB）sponges with tunable porosity are prepared by a freeze-drying method,and the MB sponges can be encapsulated with silicone rubbers.A PDMS@MXene/BCF（PMB）flexible piezoresistive sensor is prepared,which not only has excellent piezoresistive sensing performances,that is,an ultra-wide pressure sensing range（up to 2 MPa）and a high linearity（R²=0.96）,but also exhibits excellent biocompatibility.Besides,the PMB flexible piezoresistive sensor exhibits outstanding working stability,which can be stored in air for a long time and can be resistant to temperature change,humidity/moisture erosion and UV light irradiation.Finally,the PMB flexible piezoresistive sensors have successfully demonstrated their applicability in human motion monitoring and tactile sensing for soft robots.（4）In addition to working stability,the multifunctionalization of flexible piezoresistive sensor is also an important developing direction.In this section,a MXene/bamboo fiber（BF）conductive nanocomposite is prepared by a simple vacuum filtration process,which shows various functions such as sensing,energy storage,and electric heating.Owing to the hydrogen bonding interaction between MXene and BFs,the tensile strength of the MXene/BF composite is as high as 49.46 MPa,the electrical conductivity is as high as 4.8×10³ S/m,and the electrical stability is well maintained after 1,000 cycles of stretching,bending and compression.As a result,a flexible piezoresistive sensor by stacking is fabricated based on the MXene/BF composite with a high sensitivity(0.153 k Pa^-1)and a wide linear detection range（0-2.5 k Pa）;the electric heater based on the MXene/BF composite exhibits excellent electrothermal performance.When the minimum input voltage is 2.5 V,the temperature difference between the surfaces of MXene/BF composite and the air can reach 20 ^oC,which can meet the application requirements of electrothermal physiotherapy;compared with pure MXene-based supercapacitor,the symmetric solid-state supercapacitor based on the MXene/BF composite achieves large enhancements in volumetric capacitance and gravimetric capacitance,and also shows outstanding energy storage stability under varied deformations such as bending,torsion,and folding.Therefore,the developed MXene/BF composites with the above virtues are appealing for extensive applications in multifunctional electronics,including wearable sensors,energy storage devices,electrothermal heaters,etc.