Research On Flexible Stress Sensor Based On Graphene

In order to ensure the safety of roads,bridges and transportation vehicles during their service,a large number of stress sensors are required to monitor their structural health in time,and the traditional fiber-optic stress sensors commonly used have the disadvantages of fragility and high cost.Therefore,stress sensors with high structural stability,low cost,and high sensing performance need to be studied.And graphene has received extensive attention and research in high-performance stress sensor applications due to its excellent performance.However,limited by the traditional preparation technology and electrical conductivity of graphene,most of the current stress sensors based on graphene require wire connection during testing,which makes it inconvenient to install and maintain,and unable to obtain sensing data in time.Therefore,based on high-performance graphene materials,designing of the wireless mechanical stress sensors with high mechanical stability,low cost,and flexibility is of great significance.In this thesis,based on the excellent electrical and mechanical properties of the multilayer graphene film,several graphene flexible wireless stress sensors have been studied and designed,which are summarized as follows:1.The synthesis methods of traditional graphene are summarized,and based on the conductivity model of the single-layer graphene,its conductivity in the microwave to millimeter wave band is analyzed and discussed.In the end,the preparation technology of high-conductivity multilayer graphene material and its microscopic characterization are introduced.And the excellent electrical and mechanical properties of the multilayer graphene material are studied in detail,laying a foundation for the study of the graphene based flexible and wireless stress sensors.2.Based on the graphene frequency selection surface,a single-layer and double-layer stress sensors are designed respectively.In the design of a graphene based single-layer frequency-selective surface stress sensor,a detailed simulation analysis of the structure and electromagnetic performance of the sensor is applied.And the tensile force is directly applied to the sensor during the wireless stress sensor experiment,where the resonance frequency has a shift of 600 MHz with stress increased from 0 to 40 N.Thus the single-layer graphene based stress sensor has advantages of wireless sensing,flexibility and stretchability.Secondly,a graphene based double-layer frequency selection surface stress sensor is designed.And when there is stress,the coupling characteristics between two layers will be changed.The simulation and experimental results are consistent with the theoretical analysis.And the measured results show that in the linear dynamic range of 5mm,the sensor has sensitivity of 370MHz/mm and 280MHz/mm in the two directions.Namely the sensor can achieve two-dimensional sensing,and has advantages of large linear sensing range,high sensitivity and structural flexibility.3.Based on the graphene radio frequency identification tag,a chipped and a chipless tag are designed.First,based on a flexible multi-layer graphene film with high conductivity,a flexible RFID tag antenna working at 912 MHz is designed and fabricated,and it is applied to a wireless stress sensor.The change of electromagnetic performance of the bended tag antenna with bending angle changing from 0 to 2π is analyzed in detail.With the change of the power received by the reader as the indicator,and through wireless communication between the reader and the tag antenna,wireless reading of sensor data is realized,which also simplifies wireless sensing operation.Secondly,in order to further reduce the cost of the sensor,the polarization characteristics and resonance characteristics of the C-shaped resonator is used to design a chipless radio frequency identification tag based wireless stress sensor.And the identity information and the sensing information are encoded in the frequency spectrum.The simulated results of the stress sensor show that in the sensing process,only the frequency of the sensing information linearly shifts with the stress,and the frequency of the identity information almost does not change with the stress.Thus the chipless RFID tag based stress sensor can realize independent control of the identity information and the sensing information,and has advantages of flexibility,Simple structure and low cost.