Study On Structure Design And Piezoresistive Properties Of Active Layers Based On Graphene And MXene

Two-dimensional materials are ideal for manufacturing pressure sensors because of their excellent mechanical strength,high electrical conductivity,high carrier mobility and stability,among which graphene and MXene are the hot spots of current research.The active layer material is an important component of flexible piezoresistive sensors,and the active layer structure is a key factor affecting the performance of the active layer.Therefore,selecting a suitable active layer material and designing a reasonable active layer structure are the keys to preparing high-performance piezoresistive sensors.In view of the construction and regulation of the conductive network structure of the active layer material of the high-performance piezoresistance sensor,graphene and MXene are selected for the structural design and preparation of high conductivity voltage resistance active layer in this thesis.The active layer is endowed with excellent conductive,mechanical,and piezoresistance properties by regulating the multistage conductive network structure that regulates the sheet spacing of two-dimensional materials and improving the preparation method of the piezoresistance active layer.And then a series of flexible piezoresistance sensors with excellent performance are designed and prepared.By studying a series of sensing characteristics of flexible piezoresistive sensors and their sensing mechanism,it is expected to prepare high-performance flexible piezoresistive sensors with high sensitivity and wide detection range,thus promoting the development of flexible sensor technology.The contents and innovations of the research are as follows.（1）To address the issue of how to effectively construct a swollen conductive network structure between graphene sheet layers,a piezoresistive active layer with a laser-reduced GO porous expanded layer as the conductive network and a flexible GO layer as the support layer was built by employing a simple,efficient,and controllable laser technique to forcefully expand the GO layer during the laser reduction of graphene oxide（GO）while suppressing the escape of small molecular gases such as water vapor and CO₂.Laser line-sweep reduction causes the Lr G layer to exhibit a regular peak-like oriented microfold structure with about 100 nm spacing between adjacent peaks.Simultaneously,the Lr G layer expands several times,and the maximum expansion thickness can be increased from about 5.26μm before the reduction to 28.64μm,which effectively reduces the interlayer stacking.To investigate the piezoresistance sensing characteristics of Janus expanded structured graphene,it was used as an active layer material and assembled into a flexible piezoresistance sensor with an interdigital electrode.After optimizing the laser reduction conditions,the sensor exhibits a pressure detection range of0.098-4.9 k Pa,sensitivity up to 65.48 k Pa^-1,detection limits down to 98 Pa,short signal response and recovery times（200/150 ms）,and reliable cycling stability performance.In addition,the sensor can be used for human activity detection with greater pressure such as detecting wrist flexion,which is expected to be used in wearable electronic devices.However,the sensing performance needs to be improved in order to adapt to more applications.Meanwhile,it is shown that the hydrophobicity on different sides of the Janus expansion structure.The contact angle of the GO layer with water is 35°,while that of the Lr G layer is 95°.The hydrophobicity difference and Janus structure make excellent bending response to hot and humid environment.Testing of its hydrothermal drive performance revealed that this Janus expanded structure graphene active layer exhibits excellent bending response in the relative humidity（RH）range of 20%to 90%and temperature range of20℃to 50℃.Among them,the maximum bending angles can reach approximately 850°and 460°,respectively.Moreover,it has a rapid bending response in the above wet and heat range,and cyclic stability.The bending response and recovery time are 25/45 s and 25/30 s respectively.Through mechanical analysis and simulation,the bending response mechanism when it is subjected to force is analyzed so that the response direction and response position can be accurately predicted.Designing different laser reduction patterns results in a series of hydrothermal actuators with controlled position and orientation response.These drivers can respond to changes in heat and humidity,with high mechanical accuracy,and can be widely used in intelligent robots,intelligent sensors,bionic equipment and other cutting-edge fields.（2）To address the problem of how to improve the piezoresistive sensing performance of Janus expanded structured graphene active layer,a graphene-based pressure sensor with a gradient expanded structure was prepared using a combination of laser reduction method and"spatial confinement".The"spatial confinement"set in this paper is the space where the gradient thaθis about 0.0028,and the expansion of GO by laser reduction is limited by it and shows a gradient.The"spatial confinement"causes the reduction of GO to vary in a gradient,which is inversely proportional to the thickness of its expansion.In addition,the expanded porous microstructure effectively increases the contact area of the conductive channel,while the presence of the gradient structure allows the active layer to respond to the pressure with a gradient.With the synergistic effect of the two structures,the performance of the pressure sensor prepared using this active layer is effectively improved,extending the pressure detection range to 16.7 k Pa,increasing the sensitivity to 127.8 k Pa^-1,reducing the pressure detection limit to 33 Pa,and the mechanical stability over 7000 loading/unloading cycles.Thanks to the presence of the gradient structure,the sensor has area response characteristics with potential applications in two-dimensional planar position detection.（3）To address the issue of how to improve the flexibility and stretchability of graphene-based piezoresistive sensors,a laser-reduced graphene piezoresistive active layer with a gradient wrinkle structure was successfully prepared using a combination of pre-stretching and laser reduction techniques.The wavelengths of the gradient wrinkles on this active layer varied in the range of approximately 40-600μm using pre-stretch ratios of 50%,80%and 100%.Laser reduction adds a unique three-dimensional raised expanded porous structure to the gradient wrinkle film and constitutes a linear conductive structure on it.Also,its width varies with the gradient,fluctuating in the range of about 30μm to 300μm.The piezoresistive properties of the active layer were optimized by modulating the multistage structure on the active layer by varying the pre-stretch ratio.The flexible piezoresistive sensor prepared from this active layer has the highest sensitivity of 125.87 k Pa^-1in a low-pressure range of less than 0.33 k Pa,and 42.76 k Pa^-1in a larger pressure range of 0.33 k Pa to 6.67 k Pa,and can also detect small pressures as low as 33 Pa.Moreover,the sensor exhibits excellent mechanical stability and maintains reliable signal output after more than 5,000 cycles of fatigue testing.The pre-stretching gives the active layer stretchability and excellent flexural properties,making it more suitable for bending and strain applications.Therefore,piezoresistive sensors prepared from this active layer have better potential for applications in the field of monitoring human bending activities.（4）To address the issue of how to further improve the pressure detection range and sensitivity of piezoresistance sensor,a flexible pressure sensor with a highly conductive MXene paper-based fiber film（MX@PF）as the conductive active layer material was prepared by using the hydrogen bonding between MXene and paper-based fibers by vacuum filtration,and by adjusting the amount of MXene.Benefiting from MXene’s excellent electrical conductivity,the porous network structure of fiber laps on the MX@PF paper-based active layer can effectively increase the contact sites and conductive pathways when under pressure,which greatly contributes to the performance of the flexible piezoresistive sensor.By adjusting the use of MXene,when the mass ratio of MXene to paper-based fiber is 0.0668:1,the MX@PF active layer has the highest conductivity of 0.45 S/m,and can show the best piezoresistance sensing performance.After testing,the maximum pressure detected by the prepared sensor can reach 196 k Pa and has linear sensitivity as high as 154.5 k Pa^-1in the overwide pressure range of 4.9 k Pa to196 ka.In addition,the piezoresistance sensor can maintain good mechanical stability after about 6000 pressure loading/unloading cycles at a large pressure of 196 k Pa.The MX@PF paper-based piezoresistive sensor with excellent piezoresistive performance has tremendous advantages in human activity detection and can be used for the almost full pressure range of human activity detection.In addition,paper-cutting technology is applied in the MX@PF paper-based piezoresistive sensor,giving it stretchability for more applications.