Preparation,Properties And Application Of Carbon Nanomaterial-Based Flexible And Wearable Multi-Functional Sensors

Recently,flexible and wearable sensing technologies have sparked increasing interest in the fast-developing fields of artificial intelligence,electronic skin,and health monitoring.Nanomaterials are widely utilized in the fabrication of wearable devices because of their small size,excellent electrical conductivity flexibility,chemical stability,and simple preparation.Among them,carbon nanomaterials(carbon nanotubes(CNTs),graphene,carbon nanocoils(CNCs).e.g.)are frequently used as sensing mediums for strain sensors because of their high sensitivities,large sensing ranges,and high resolutions.But the mechanism of electrical conductivity changes under strain is still lacking.Moreover,in addition to strain sensors,there is an urgent need to develop multifunctional devices for sensing another stimulus such as temperature or humidity.Particularly,in an extreme environment such as savage or in vivo environment,a self-powered sensing system that could be functioned by harvesting the energy from the detected object or environment without external power sources is of significant importance.Therefore,in this dissertation,we focus on the research of mechanisms of wearable strain sensors based on CNTs,further,developed self-powered multifunctional devices by utilizing the strain sensing and thermoelectric properties of CNCs and flexible polymers,respectively.Here,the main contents are as follows:(1)To investigate the conductive sensing mechanism,a two-dimensional(2D)conductive network was constructed by one-dimensional CNTs.The different densities of CNTs(7.2-152/?m2)were obtained by changing growth conditions to form quasi-2D conductive network films with a chemical vapor deposition method.The CNTs were simply transferred to a flexible substrate.The theoretical model is established based on experiment results and the strain sensing mechanism is detailly studied.It is found that the sensing mechanism of the conductive networks changes with the increase of the strain.Under CNT with high density,the conducting film accords with a traditional 2D percolation principle.With the decrease of CNT density,the conductance gradually transitions from 2D percolation to tunneling mechanism.When the CNT density decreases further,its conductance is dominated by the tunneling mechanism.The strain sensitivity of the low-density CNTs is up to 50 while the strain range of high-density CNTs is 100%.The strain sensor based on the CNT network maintains high repeatability and detects human joints,muscles,and other physical movements.This study highlights a facile technique to prepare highly stretchable strain sensors and fundamental principles for the structural design of the CNT network-based strain sensors.(2)A novel self-powered strain-temperature dual-functional sensor was fabricated based on the composite films of CNCs and conductive elastomeric blends of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)and waterborne polyurethane(WPU).The high stretchability and dispersibility of helical-structured CNCs improve strain sensing.The developed sensor exhibited a large strain detection range of 0-50%and a high sensitivity with a gauge factor of 25.Based on the Seebeck effect arising from the conducting polymer of PEDOT:PSS,the Seebeck coefficient of the composite film is 13.3 ?V/K.the temperature difference detection limit is 0.5 K and the strain deformations of 1-10%can be detected in a self-powered circumstance.To demonstrate its practical application as a wearable device,the senor was directly attached to the human skin.It not only detected subtle human pulse,motion but also distinguish the stimuli from either a warm or a cold object by their temperature signals difference.The integrated functions of self-powered temperature and strain dual-sensing are obtained.This study provides practical candidates for fabricating flexible and wearable multifunctional sensors on materials,structures,and techniques.(3)Based on the research(2),we fabricated a series of free-standing self-powered temperature-strain dual sensors by replacing WPU with poly(vinyl)alcohol and combined with PEDOT:PSS and CNCs.The Seebeck coefficients of the composite films were measured to be 19 ?V/K.The minimum detection limit for the temperature difference was 0.3 K.The strain range was measured to be 1-30%.The thermoelectric output voltage of the device is more than three times higher than that of the previous sensor in(2)at the same temperature difference because the device does not need to be encapsulated by low thermal conducting layers.The mechanical rigidity of the composite film was improved due to hydrogen bonding interaction between PEDOT:PSS and PVA and the addition of CNC.which results in the stable output strain sensing signals in the self-powered circumstance.The strains from 1 to 10%were detected without any external power supply under a constant temperature gradient.Moreover,due to the independent action of the temperature and strain sensing,the thermoelectric voltage which is generated by a constant temperature difference is maintained under different strains.The composite films can be easily made into an array to detect the temperature of the fingers and motions of the wrist by attaching it to the human wrist directly.The research provides valuable references for the fabrication of free-standing,self-powered devices.At last,the study of strain sensors based on carbon nanomaterials and self-powered multifunctional sensors based on a composite of nano-materials and polymers will provide effective theoretical and technical references for the fabrication of flexible and wearable sensing devices.This research has great application prospects in medical monitoring,movement detections,soft robots,and e-skin technics.