Study On Fabrication And Working Mechanism Of Graphene Piezoresistive Sensor

With the increasing demand for large-strain piezoresistive sensors,researchers have tried to design piezoresistive sensors with large strain working range and high sensitivity.Nanocomposites have become a popular choice.Nanocomposites with various microstructures have been successfully used in the preparation of piezoresistive sensors.However,the fabrication of piezoresistive sensors with the features of large strain working range,high sensitivity,and linear piezoresistive response is still a challenge.Therefore,this research applied graphene materials to design and fabricate a piezoresistive sensor with large strain working range,high sensitivity,and linear response.First,this dissertation introduces that circular ring-like graphene aerogels were fabricated by a novel method;which used the graphene oxide colloid drop to impact the coagulation,leading to the circular ring-like graphene aerogels with multi-layered structure.The whole impacting process combines a dynamic process of the graphene oxide droplet and the coagulation of the graphene oxide.The core of this idea is the wet-spinning fiber in the coagulation.This method can easily obtain graphene aerogel with a uniform diameter(6 mm)and mass(0.04 mg).At present,this method,which can be used to assemble graphene sheets,has not been reported yet.This material shows excellent physical and chemical features,such as good conductivity(292.0 S/m)and a highly specific area(BET value:76.6 m²/g).At the same time,the circular ring-like graphene presents satisfaction sensitivity in the pressure sensor.Second,this dissertation introduces the method of graphene vein membrane,which was fabricated by that the vein of the waste leaves of the plant has been used as a skeleton to support the graphene sheets.By applying this method,it is easy to obtain the graphene vein membrane with fractal level structure.The fractal level structure plays an important role in responding to the continuous variation of strain,which ranged from free strain to finite large strain(0-12%).The graphene vein membrane applied as the strain sensor shows excellent piezoresistive features,including high gauge factor(238.09),strictly linear response(linear coefficient is 99.4%),well long-term repeatability(0.0078%s^-1),and strong ability to resist the disturbance of the environment temperature.The working temperature fluctuates from 30 to 70,while the resistance of the graphene vein membrane varies from 154 Ohm to 161 Ohm.The piezoresistive sensor with large strain working range,high sensitivity,and excellent linear piezoresistive response.Last,the first principles simulation based on the density functional theory can be used to deal with the electronic structure of the solid.It can be conducted by Quantum Espresso which is an open-source software package.This approach can obtain the electronic energy band and distribution of the electronic cloud of the crystal structure of graphene with and without boron doping.Based on the analysis of the first principles simulation results,the graphene shows a very good gauge factor when it is subjected to the exterior mechanical strain along the zigzag-direction.The gauge factor of boron-doped graphene and pure graphene are 43.5 and 29.0,respectively.This explains the piezoresistive working mechanism at the atomic scale.The circular ring-like graphene and the graphene vein membrane shows good piezoresistivity performance,respectively.Especially,the graphene vein membrane can bear the large deformation while its relative resistance change can linearly respond to the exterior mechanical strain.This dissertation also presents three piezoresistive working mechanisms of the graphene to explain the high stability,high linearity,and high sensitivity of the graphene vein membrane sensors.The first is due to the geometry deformation of multi-layered fractal structure in the graphene vein membrane at the macroscopic scale,the second is the cracks and their opening and contacting of the microstructure of the material at the mesoscopic scale,and the third is the distortion of the electronic energy band at the atomic scale.In general,the piezoresistive working mechanism ascribes to the synergistic effects of above mentioned three aspects.The above clear mechanism analysis provides an effective approach to further enhance the piezoresistive sensor performances.