Development And Application Of Stretchable Conductors Based On Metallic Nanowires And Mxene

As a new sort of electronic devices,flexible electronics can adapt to complex deformation and keep stable performance.The devices have been widely used in the motion monitoring,personal healthcare,and human-machine interface interaction.Compared with traditional inorganic and metallic materials with excellent electrical properties,its significant advantage is that it can maintain good electrical conductivity under deformation such as bending and stretching.Therefore,stretchable conductors play a vital role in the fabrication and assembly of flexible electronics,which are indispensable for keeping the electrical and mechanical properties of the devices stable under deformation.Low dimension conductive nanomaterials such as metal nanowires and MXene have stood out as competitive material choices for stretchable conductors,due to their excellent mechanical flexibility and high electrical conductivity.The applications of flexible and wearable electronics based on stretchable conductors mainly include stretchable electrodes and flexible sensing devices.A straightforward application of stretchable conductors is stretchable electrodes that can be used to connect flexible circuits and rigid device components.The electrical properties of the stretchable conductors for stretchable electrodes keep stable under various deformation and environment,which can be applied in the applications of stretchable conducting interconnects,flexible triboelectric nanogenerators,stretchable energy-storage devices,actuators,and heaters.Flexible sensing devices endow the wearable devices the ability to perceive and response to the external environment.The electrical properties of the stretchable conductors for flexible sensing devices change with the deformation and environment,which can realize applications such as strain,temperature,pressure,and humidity sensing.Stretchable conductors are most commonly fabricated by embedding conductive materials into or placing them on the elastic substrates.The stretchable conductors often own a large area of interfaces between the conductive materials and elastic substrates.When subjected to stretching,the change of interfacial bonding and microstructure between different materials determines the mechanical and electrical properties of the stretchable conductors.At present,the study mainly focuses on preparing and combining conductive materials with high conductivity and elastic substrates with good mechanical properties.A large number of stretchable conductors with different electromechanical performance have been reported.However,due to the lack of research on the interfacial bonding mechanism,it is difficult to fully clarify the fundamental reasons for the differences in the performance,which is not conducive to the further development of the stretchable conductors.In addition,the energy supply is indispensable for the electronic equipment.However,the traditional energy structures and supply patterns are incompatible with the use of flexible electronics.It is a key issue to develop suitable modes for energy harvest and conversion to support the operation of wearable electronic systems.Furthermore,the current research of the stretchable conductors in the sensing field mainly focuses on the response to mechanical stimuli such as strain or pressure.By contrast,the capability to respond to other environmental stimuli(e.g.,temperature and humidity)has been ignored.As an important parameter,temperature plays an irreplaceable role in the sensing of surroundings and people.Further studies of temperature sensors can contribute to the development of multifunctional and integrated sensing systems,In this dissertation,low dimension conductive nanomaterials such as silver nanowires(Ag NW)and MXene were utilized to fabricated dual-function conductive fibers for stretchable conducting interconnects and flexible strain sensors,fiber triboelectric nanogenerators for self-powered sensing of the mechanical stimulus,and flexible temperature sensors for temperature and proximity detection.The device performance was investigated systematically,and corresponding working mechanisms were also analyzed.Specific research works are as follows:(1)Ag NW based dual-function conductive fiberWe proposed a simple fabricating strategy for a silver nanowire/polyurethane(Ag NW/PU)composite fiber with sheath-core architecture.The interfacial bonding layer was regulated and its influence on the performance of conductive fibers was investigated,based on which an interfacial interaction model was proposed.The model underlined the effect of the embedding depth of Ag NW network on the conductivity,stretchability and variation of the resistance under applied strains.Both conductive fibers with sensitive resistance to strain at 60%strain(gauge factor up to 650)and negligible conductance degradation below the strain of 150%were obtained via interface regulating,exhibiting the potentials in the applications of wearable strain sensors and stretchable conducting interconnects.(2)Ti₃C₂T_x based fiber triboelectric nanogeneratorWe prepared a fiber-like flexible triboelectric nanogenerator based on Ti₃C₂T_xto develop a new energy supply method suitable for flexible electronics and realize self-powered sensing of mechanical stimulation.Ti₃C₂T_x was selected as a triboelectric material because its highly electronegative surface trended to attract electrons when friction happened through the contact of two materials.Owing to the addition of Ag NW and PEDOT:PSS and the stretchable structure design,the Ti₃C₂T_x hybrid fibers with low resistance(3?/cm)and excellent stability(stable conductivity below the strain of100%)were prepared.The Ag NW/Ecoflex composite twining spirally around the Ti₃C₂T_x hybrid fiber was used as another triboelectric material and electrode.When the device was subjected to tensile or compressive stress,the two triboelectric materials with different electron-attracting ability contacted and separated to realize power generation.The fiber triboelectric nanogenerator supported a high open circuit voltage up to 16 V under compression.It also exhibited a high working strain(120%)and the output signal grew with the increase of strain,indicating its potential in the self-powered human motion monitoring.(3)Ti₃C₂T_x based flexible temperature sensorIn order to carry out the research of the stretchable conductors for the applications of temperature sensing,we designed a flexible temperature sensor with tunable sensitivity and response range based on a novel Ti₃C₂T_x nanoparticle–lamellas hybrid network with polydimethylsiloxane(PDMS)as substrates.As the sensing units,Ti₃C₂T_x nanoparticles and lamellas with various morphologies and sizes can be obtained through controlling the fabrication conditions.The as-prepared sensors exhibited significant outstanding performance in the tests with a desirable incorporation of a high sensitivity(up to 986 ^oC^-1)and a wide working range(140 ^oC),which surpass most of the reported nanomaterial-based flexible temperature sensors.The sensor can be applied as e-skin to monitor temperature in both contact and proximity modes.Moreover,the temperature sensor can detect illuminations from ultraviolet to infrared light,which extends the application scope of the sensor in the noncontact detection.Finally,we integrated the sensors into a 4×4 sensing array and succeeded in demonstrating the shape and temperature distribution of diverse stimuli,indicating its great potential for applications in the personal healthcare and human–machine interactions.