Research On Multifunctional Flexible Sensors Based On Conductive Systems Composed Of Carbon Nanotubes/Carbon Nanocoils Or Carbon Black

With the rapid development of flexible electronics,wearable and flexible strain sensors have attracted much attention due to their potential applications in human motion monitoring,personal health monitoring,electronic skins,intelligent robots,human-machine interactions,and the Internet of Things.The scenarios of these applications and their requirements are often diversified in practical applications,which brings new opportunities and challenges to the development of flexible sensors.To date,the development of flexible strain sensors mainly faces following difficulties:(1)The trade-off between high sensitivity and large working range,making the performance of the prepared strain sensors insufficient to meet the sensing requirements for a variety of application scenarios;(2)The construction of a multi-functional flexible sensor is complicated,which requires expensive equipment and complex operation process,making it unconvinient for mass-production;(3)Signal crosstalk always occurs in multifunctional sensors,which makes it difficult for the sensor to distinguish which stimuli cause the signal change and to monitor multiple stimuli(such as strain,temperature,humidity,etc.)simultaneously;(4)Flexible sensors encapsulated by elastic polymer matrixs have poor air permeability,resulting in poor long-term wearing comfort.In response to the above challenges,this paper aims to design and manufacture a series of flexible sensors with different functions based on lightweight carbon nanomaterials,polymer elastomer and soft breathable fiber/fabric materials to meet diverse application requirements.The main research contents are as follows:1.Carbon nanotubes(CNTs)were grown in situ on the three-dimensional framework of carbon nanocoils(CNCs)by two-step chemical vapor deposition method to obtain a hierarchical composite network structure of CNT/CNC.A thin film of the composite network structure was obtained using the peeling-off transfer method and then embedded into the PDMS matrix to prepare flexible strain sensors.The influence of the length and density of CNTs on the crystallinity,conductivity and specific surface area of the composite was studied,and the tunable performance of the introduction of carbon nanotubes on the sensing capabilities was investigated.The synergistic effect of the highly stretchable CNC and the highly conductive CNT enables the strain sensor to have both a large working range(0～37.5%)and high sensitivity(GF～70).By controlling the length and density of the CNTs,the working range and sensitivity of the strain sensor are tuned from 9%to 90%and from 4.5 to 70,respectively.The structural evolution of the composite structure under tension was observed,and the sensing mechanism of the sensor was analyzed by establishing a point contact model.The advantages of the performance tunable sensor in applications of pulse monitoring and human motion monitoring are verified.2.Inspired by the composite structure of CNT/CNC,nylon/spandex weft-knitted fabric was used to construct elastic network.carbon black nanoparticles were dip-coated on the fabric to prepare high-performance flexible strain sensors.The simple fabrication process endows the textile sensor with good repeatability,which paves the way for mass production.The sensor has an excellent linear response(R~2～0.99),large tensile range(0～30%),high sensitivity(GF～63.3),excellent washing ability,cycle stability,and tensile recoverability.By observing the structure evolution of the nylon/spandex fibers,the synergetic sensing mechanism of the decrease of fiber contact area and the generation of the crack was revealed.The advantages of the fabric-based strain sensor in wearable applications were demonstrated.3.On the basis of the nylon/spandex-carbon black textile strain sensor,a simple sewing method was put forward to prepare a multifunctional textile sensor that combines capacitive and resistive sensing in one device.This sensor is sensitive to the variations of strain,temperature,and humidity.Under different external stimuli,the sensor resistance and capacitance have different changing trends.Each external stimulus can be well identified by comparing the positive or negative response of sensor resistance and capacitance.The sensing sensitivity of the strain sensor reaches 62.2.When the relative humidity and temperature increase by 1%and 1°C,the resistance of the sensor changes by 6.27%and-5.3%,respectively.Moreover,the sensor shows good working stability in 5000 tensile cycles.The mechanisms of stretching,temperature and humidity sensing of the multifunctional sensor were analyzed,and the application potential of the multifunctional sensor in the fields of human-machine interaction,personal health monitoring,and the Internet of Things was demonstrated.4.An all-fiber-based printable and air-permeable multifunctional electronic skin was designed and fabricated.We prepared breathable and elastic TPU fibrous films first by electrospinning and then dip-coated them with carbon black nanoparticles and PEDOT:PSS as strain and temperature sensing layers,respectively.An intermediary pristine TPU film was sandwiched and hot-pressed between the TPU-CB and TPU-PEDOT:PSS layers to form a capacitive pressure sensor.Since the resistance,capacitance,and voltage signals of the sensor have specific and sensitive responses to stretching,pressure and temperature,respectively,the signal crosstalk phenomenon is circumvented.The simultaneous detection of stretching,temperature and humidity multiple stimuli was successfully realized.The mechanical properties of the TPU series were investigated,and the strain,pressure and temperature sensor sensitivity,response time,detection limit and cycle stability of the sensor were systematically studied.We further used printing technology to print PEDOT:PSS and carbon black nanoparticles on the TPU membrane to prepare a multifunctional sensor array,which can detect the intensity and distribution of the pressure and temperature stimuli.In summary,this paper aims to developed a series of flexible sensors with different functions from the selection of flexible substrates and sensitive materials,the construction of conductive systems,and the design of sensor structures.It provides a new research idea and structural reference for the design and manufacture of multifunctional sensors.