Fabrication Of Flexible Pressure Sensor Based On Porous MWCNTs/PU Composite Films

Pressure sensor is one of the most widely used sensors,and it is an important bridge connecting the physical world and the digital world.Compared with the pressure sensors made of rigid materials,flexible pressure sensors have better adaptability and can broaden the scope of use of sensors and apply to life and health fields such as wearable medical device.However,the flexible pressure sensor still has some problems to be solved,such as low sensitivity,narrow measurement range and poor linearity.To solve these problems,this thesis develops a flexible pressure sensor based on multi-walled carbon nanotubes（MWCNTs）/polyurethane（PU）composite film and microstructure control strategy,and explores the technical approaches to improve its range,sensitivity and linearity.Firstly,MWCNTs/N,N-dimethylformamide（DMF）dispersion was prepared,and then the casting solution was prepared by blending the PU and MWCNTs dispersion.Then the MWCNTs/PU composite film was obtained using a phase separation method.Finally,the flexible pressure sensor was assembled with the composite film as the active layer and the performance of the sensor was tested.The performance of the sensor was explored and optimized by changing the content of PU,the concentration of MWCNTs and the proportion of solvent in the coagulation bath,and the prepared flexible sensor was tested for human motion/vibration signal monitoring.The main work and results of this thesis are listed as follows.i.)Porous MWCNTs/PU films were prepared by solution blending and non-solvent induced phase separation.The effects of key parameters（PU concentration,MWCNTs content,DMF solvent ratio in coagulation bath）on the tensile properties and structural morphology of the films were systematically studied.The results show that there is a certain correlation between PU content and the pore size of the film.With the increase of PU content,the pore size of the porous PU film becomes smaller,and the distribution uniformity is better.In addition,there is a good continuity in the structure,which improves the tensile properties of the film.The content of MWCNTs has a great influence on the tensile properties of the films.With the increase of MWCMTs,the tensile properties of porous films are significantly enhanced.When excessive MWCNTs were added,the tensile properties of the composites decreased,resulting in the decrease of the overall tensile properties of the composites.The solvent composition in the coagulation bath has an important influence on the formation of pores in the film.With the increase of the solvent proportion in the coagulation bath,the pores in the film gradually decrease and the tensile properties are improved.ii.)The flexible pressure sensor was assembled with flexible porous MWCNTs/PU composite film as active material,and the piezoresistive effect of porous MWCNTs/PU composite film was analyzed.The results show that when the sensor is subjected to external pressure,the walls of the pores in the film are close to each other,and the MWCNTs on the upper and lower surfaces of the pore walls contact each other to form a new conductive channel.Under different pressures,the I-V curves of the sensor have good linearity;with the increase of pressure,the slope of I-V curve increases.Test the performance of the sensor.The result show that in the range of about 20 KPa,the sensitivity of sensor is 1.24 KPa^-1;in the range of about 20～160 KPa,the sensitivity is 0.32 KPa^-1.The linearity R² is 0.96.the detection limit is 0.23KPa.The response time is 320ms and the recovery time is 60ms.The sensor remained stable after 1500 cycles of pressure loading.iii.)The sensor performance was explored and optimized by adjusting the key parameters such as PU concentration,MWCNTs content and DMF solvent ratio in the coagulation bath.The results show that for sensors with different PU concentrations,the sensitivity of the sensor has almost no change in the range of 0KPa～30KPa.When the pressure is greater than 30KPa,the sensitivity of the sensor decreases with the increase of PU concentration.The content of MWCNTs has a key influence on the sensitivity of the sensor.The experiments set the content of MWCNTs as 3wt%,5wt%and7wt%for comparative analysis.It was found that the sensor had similar sensitivity when the MWCNTs content was 3wt%and 5wt%,and the sensitivity increased rapidly when the MWCNTs content was 7wt%.Setting the proportion of DMF solvent in the coagulation bath,with the increase of the proportion,the sensitivity of the prepared sensor is decreasing,but the linearity of the sensor is improved.When the proportion of DMF is 20wt%,the linearity reaches the maximum,R2 is 0.995.iv.)In this thesis,a simple application test of flexible pressure sensor based on MWCNTs/PU film was carried out.The sensor was adhered to the wrist pulse,chest,throat,finger joint and other positions,and the pulse signal,respiratory signal,sound signal,swallowing motion and finger bending degree were monitored.The test results show that the flexible pressure sensor can detect human motion and pulse information,which can be used for flexible wearable devices.