Study On The Construction And Performance Of Conductive Network Of Fiber-based Flexible Force Sensitive Sensor

As a new type of flexible electronic device,flexible force-sensitive sensors have received tremendous attention from researchers due to their excellent sensing performance,flexibility and wearability.However,the issue of how to balance between sensitivity,working range are still critical challenges for the development of high-performance flexible force-sensitive sensors.The active layer is a key component of the flexible force sensitive sensor to realize the conversion of mechanical deformation to electrical signal.The conductive network structure of active layer directly determines the sensing performance of flexible force sensitive sensors.Therefore,we focus on the force-electric coupling mechanism to address the main challenges of flexible force sensitive sensors in this work.Based on the common fiber network structure,we respectively constructed bamboo-like conductive network structure,double-layered conductive network structure and multi-level micro-protruding conductive network structure through combining the electrospun technology,layer-by-layer assembly technology and embossing technology.Furthermore,we investigated the critical factors in the construction of conductive network structures and established the constitutive relationships between these structures and sensing performance.In addition,we have also elucidated the force-electric coupling mechanism of different conductive network structures and revealed the modulation effect of conductive network structures on sensing performance.The main research contents are as follows:（1）Conductive network structure of carbon nanofibers constructed based on electrospun technology:PAN nanofiber mat fabricated by electrospun technology were used as the prefabricated network,and were then converted into carbon nanofiber（CPAN）mat with intrinsic conductivity borrowing the preparation technology of carbon fibers.Furthermore,the CPAN mat was further used as the active materials of CPAN-0#flexible force sensitive sensor.In this section,we focus on the influence of the intrinsic conductivity and macroscopic shape size of CPAN mats on the sensing performance of the sensor,and its force-electric coupling mechanism.The results of the sensing performance tests are as follows:The increase of intrinsic conductivity of CPAN mat contributes to the increase of sensitivity of sensor;The increase of CPAN mat thickness contributes to the increase of its sensitivity,but reduces its working range;The increase of CPAN mat width contributes to the simultaneous improvement of tis sensitivity and working range.CPAN-0#flexible force sensitive sensor shows excellent sensing performance,such as its sensitivity up to 127.9,working range up to 60%strain,a fast response time,and excellent dynamic stability.The force-electric coupling mechanism shows that the introduction of microcrack structure enables the CPAN mat to form an interconnected“island-bridge”structure,thus giving the sensor favorable sensing performance.In addition,the sensor also shows great potential for application in terms of practical applications.（2）Bamboo-like conductive network structure constructed based on templated electrospun technology:Inspired from moso bamboo structure,a PAN nanofiber mat with bamboo-like structure was prepared by templated electrospun technology.After carbonization,the CPAN mat with bamboo-like structure was used as the active materials of CPAN-7#flexible force sensitive sensors.In this section,we focus on the influence of tensile direction（ts//and ts⊥）on the sensing performance of the sensor and its force-electric coupling mechanism.Furthermore,a biaxial flexible force sensitive sensor with orientation recognition function was fabricated.The results of the sensing performance tests are as follows:The CPAN-7#flexible force sensitive sensor exhibits a ultrahigh sensitivity（GF up to 5430.4）and a large working range（50%strain）in the ts//tensile direction,while it exhibits an ultralow sensitivity（GF up to 7.98）,a large working range（60%strain）and a good linear response in the ts⊥tensile direction.The force-electric coupling mechanism shows that the bamboo-like structure of the CPAN mat is the essential reason for the differentiation of the force electric response behavior of the sensor under different tensile directions,in which the oriented arrangement of the fibers gives the sensor an ultrahigh sensitivity and the randomly oriented arrangement of the fibers gives the sensor a large working range.In addition,the biaxial CPAN-7#flexible force sensitive sensor shows a great application potential in the field of direction recognition.（3）Double-layered conductive network constructed based on layer-by-layer assembly technology:Based on the difference in conductivity between Ag NWs and CNTs,a double-layered conductive network structure consisting of Ag NWs layer with high conductivity and TPU/CNTs layer with low conductivity was constructed by layer-by-layer assembly technique.In this section,we focus on the influence of double-layered conductive network on the sensing performance of the sensor,and its force-electric coupling mechanism.The results of the sensing performance tests are as follows:The TPU/CNTs/Ag NWs-150 flexible force sensitive sensor exhibits excellent sensing performance,such as a high sensitivity（GF up to 1477.4）,a wide working range（strain up to 150%）,a fast response time（88ms）,favorable dynamic stability（over 7000 times）and excellent wearing comfort.The force-electric coupling mechanism shows the high conductivity and microcrack structure of Ag NWs layer gives the sensor ultrahigh sensitivity and the low conductivity and high stretchability of TPU/CNTs layer gives the sensor large working range.In addition,TPU/CNTs/Ag NWs-150 flexible force sensitive sensor,as a smart wearable device,is demonstrated to be able to monitor complex human physiological and motion signals,showing a large application potential.（4）Multi-level micro-protruding conductive network structure constructed based on embossing technology:Inspired from embossing patterns,an embossing paper with micro-protruding structure on the surface were prepared by using metal mesh as the embossed mould.After carbonatization,the carbonized embossed paper was used as active material of CEP flexible force sensitive sensor.In this section,we focus on the influence of the embossed paper with both micro-protruding and porous structures on the sensing performance of the sensor,and its force-electric coupling mechanism.Furthermore,we demonstrated the monitoring potential of the CEP force sensitive sensor in practical applications.The results of the sensing performance tests are as follows:The CEP-50M flexible force sensitive sensor shows excellent sensing performance,such as a high sensitivity(S up to 70.14 k Pa^-1),a wide working range（120 k Pa）,ultra-low detection limit（1.5 Pa）,a fast response time（60ms）,excellent bending response,and favorable fatigue resistance（10000 times）.The force-electric coupling mechanism shows that the introduction of surface micro-protruding structure changes the types of contact between papers assembled face-to-face,thus giving the sensor a high sensitivity and ultralow detection limit.The CEP-50M flexible force sensitive sensor,as a smart wearable device,was demonstrated to have a large application potential through the practical application of single-point and array sensors.All in all,the conductive network structure of active layer of fiber-based flexible force sensitive sensor is the critical factor in the regulation of its sensing performance.In this work,we take the conductive network structure as the breakthrough point to break the trade-off effect among sensing performance at technical and theoretical levels.The research results are expected to provide new research concepts for the construction of conductive network structure,and theoretical and technical basis for the development of high-performance and multi-functional fiber-based flexible force sensitive sensors.