Graphene Based Flexible Strain Sensors

The development direction of modern sensor technology is to achieve intelligence,multifunction,miniaturization,flexibility and humanization,greatly broadening the range of sensor applications.Graphene,as one of the newly emerging carbon nano-materials with unique sturture feature,defect-affected electrical property and multidimensional spatial adaptability,has become one promising candidate for highly sensitive sensing material and it is expected to be used in flexible sensors.In this thesis,to develop a highly sensitive and flexible strain sensor to detect small deformation,graphene woven fabrics(GWFs)was synthesized and then transferred onto a flexbile substrate to assemble the strain sensor.Through rational design of its macro woven structure and micro polycrystalline morphology,the strain sensing response of graphene was improved,leading to a high sensitivity with gauge factor(GF)of 500 under 2%strain.The related strain sensing properties,working mechanism,performance optimization strategies as well as potential applications were investigated.The mechanism of signal acquisition and conversion for GWFs as strain sensing element was systematically investigated.The resistance variation of GWFs under deformation was not caused by either the intrinsic conductivity change due to graphene lattice deformation or the contact state change between the graphene and the electrodes,but caused by the the contact resistance variation between adjacent graphene sheets.According to the comparison of the strain response results among graphene micro strip,flat graphene grid and GWFs,it was inferred that the ultrahigh sensitivity of GWFs was attributed to its unique macro and micro structures,i.e.,the polycrystalline stacking cracking on the microscopic level and a weak overlap between the interlaced graphene ribbons on the macroscopic level.Three factors affecting the performance of the GWFs based flexible strain sensor were studied:graphene itself,substrate and encapsulation.The results indicated that the thickness reduction,aspect ratio increasement of GWFs,and the stiffness reduction,oxygen plasma treatment of substrate would improve the sensor sensitivity.While the thickness increasement,aspect ratio reduction of GWFs.stretching at an angle of 45 degree,larger substrate stiffness,prestretching substrate as well as encapsulation would improve the working range.Generally,sensitivity and range are a pair of mutual restricted parameters,thus reasonable process strategies would be required to get a satisfactory compromise according to the specific occasion in practical applications.The performance of the GWFs based flexible strain sensor in practical appplications was explored.The sensor could collect,identify and transmit a variety of micro signals including strain,vibration and pressure,etc.In addition,the sensor could monitor the physiological state of the body(such as pulse,gesture recognition,smile,voice,etc.),demonstrating its application prospects in the fields of electronic skin,intelligent robots and human-computer interaction.Furthermore,the strain sensor was integrated with a stretchable electrochromic module to realize the visualization of the strain sensing,which would enhance the versatility of the sensor.