Flexible Strain Sensors Based On Carbon Nanocoil/Carbon Nanotube Networks

In recent years,benefitting from the synergetic development of computer science,optoelectronics,material science,mechanics science and other fields,artificial intelligence technology has been advancing at an unprecedented speed.As one of the derivative directions,the intelligent wearable devices have attracted much attention.For the intelligent wearable devices,flexible strain sensors are one of the indispensable and important components.Therefore,the development of high-performance strain sensors has become one of the research hotspots currently.Owing to their stretchability and high sensitivity,flexible strain sensors have wide application prospects in many fields,e.g.health monitoring,electronic skin and joint motion control of robots.At present,the resistance-type strain sensors,which can effectively transform the physical deformation of the strain sensing medium into the resistance change so as to achieve strain sensing,are most widely investigated and used.The sensing medium of such type of strain sensors can include a variety of materials,e.g.metal nanomaterials,polymer materials and carbon nanomaterials.Among these materials,carbon nanomaterials are particularly suitable,due to their nanoscale cross-contact junctions,excellent mechanical properties and electrical conductivity.The main contents are as follows:Firstly,the sensing characteristics of a strain sensing unit on a single carbon nanocoil as substrate are studied.Because of the unique three-dimensional helical morphology,a carbon nanocoil possesses excellent elasticity in its axial direction,facilitating elastic deformation under tensile strain.In order to magnify the change of electrical conductivity of the carbon nanocoil during stretching,a highly elastic strain sensing unit which integrates a single modified carbon nanocoil as the strain matrix,a layer of titanium film as the sensing layer and an aluminium oxide film as the insulation layer in-between,is fabricated and studied.The sensing performance of the integrated sensing unit is optimized by controlling the deposition thickness of the titanium film so as to regulate the arrangement and stacking of the titanium nanoparticles.According to the results obtained by the tensile tests,theoretical analysis and finite element simulation,the strain sensing unit is found to be able to sense the strain at microscale and transform a large longitudinal tensile deformation into a small localized torsional strain.This research provides a novel method for producing the high-performance strain sensing units that can be used in micro/nano electromechanical systems.Secondly,a network consisting of carbon nanocoils is fabricated by an electrophoretic method between a single pair of parallel gold electrodes;the density of the carbon nanocoil network is controllable by changing the times of electrophoresis.The as-prepared carbon nanocoil network is then sandwiched between two thin films of PDMS to construct a flexible strain sensor.The strain sensor exhibits an ultrahigh gauge factor of approximately 10000.withstands more than 5000 cyclic tensile tests,and owns a fast response time of approximately 50 ms.The experimental results obtained from the tensile tests are successfully verified by theoretical calculations,which reveals that the change in electrical conductivity of the strain sensor during the stretching process mainly arises from the change in the number of contact points between the adjacent carbon nanocoils in the network.Thirdly,on the experimental basis of the strain sensor made of the carbon nanocoil network deposited between a single pair of parallel gold electrodes,a pressure sensor is further developed by depositing the carbon nanocoil network on multiple pairs of parallel gold electrodes with a comb-shape arrangement via the electrophoretic method.For the as-deposited carbon nanocoil network,the portion between the parallel electrodes is relatively dense,while that in contact with the electrodes is relatively weak.When pressures at different degrees are applied to the pressure sensor,different connection states of the carbon nanocoil network lead to two different sensing mechanisms,i.e.(i)the connection change between adjacent carbon nanocoils,and(ii)the connection change between carbon nanocoils and electrodes.The experimental results show that this network structure is particularly sensitive to pressure.It can not only detect a small pressure as low as 0.5 kPa,but also withstand a high pressure up to 100 kPa.This pressure sensor exhibits an ultrahigh sensitivity of up to 193/kPa,and can maintain a high sensitivity of over 150/kPa in a wide pressure range of 50-100 kPa.A high stability and reproducibility of more than 10000 cyclic pressure tests and a rapid response time as fast as 48 ms are also successfully achieved.In order to compare the sensing performance of the pressure sensor with those reported in other works,a concept of pressure optimal value is proposed.The pressure optimal value of the carbon nanocoil network-based pressure sensor is calculated to be 3.72×16,which is 300 times higher than that of most current pressure sensors.Finally,multi-walled carbon nanotubes are used as raw materials,and a dense network of carbon nanotubes(buckypaper)is prepared through the method of vacuum filtration.A strain sensor is fabricated using the as-obtained carbon nanotube network as sensing medium.When the sensor deforms,different degrees of cracks are generated on the surface of the carbon nanotube network,thus resulting in the electrical conductivity change to achieve the purpose of sensing.Under the premise that the carbon nanotube network still possesses a good electrical conductivity,the size of cracks generated by the deformation has shown to reach up to several hundred micrometers.The carbon nanotube network exhibits excellent sensing performances in the detection of both stretching and pressure.For sensing tensile strain,the gauge factor of the sensor is as high as 20216.A maximum strain of 75%can be obtained and a minimum strain of 0.1%can be distinguished clearly.Also,the sensor can withstand more than 10000 cycles of tensile test,and has a fast response time of less than 87 ms.For sensing pressure,the sensor has a wide detection range of 0-1.68 MPa,high sensitivity of 89.7/MPa,and a good stability and reproducibility of more than 3000 cycles.The optimal values of tensile strain and compressive strain of the sensor are calculated to be as high as 3.07×108 and 1.35×107,respectively,which are remarkably greater than those of most strain and pressure sensors currently available.The carbon nanotube network-based sensor exhibits good performance in joint motion detection,pulse monitoring,vibration sensing and so on,thus establishing a solid foundation for its further development and application in the areas,such as intelligent wearable devices and electronic skin.