Fabrication And Sensing Properties Of Micro-structured Inverted Pyramid On Flexible Conductive Composites Surfaces

The introduction of micro-structure on the surface of sensing element is an effective way to improve the sensitivity of flexible sensors.Thermoplastic polyurethane/short carbon fiber/carbon nanotube（TPU/SCF-CNT）composites are prepared by an extruder in this dissertation,the conductivity and flexibility of the composites are investigated.By using compression molding,the replicas with micro-structured inverted pyramid arrays on the surfaces are molded from composites.Then flexible sensors are prepared by assembling the composite replica with the corresponding composite flat plate face-to-face.The effects of micro-structured inverted pyramid arrays,flexibilities of composites and the number of microstructural sensing element on the sensitivity of flexible sensors are studied.The SCF and CNT are overlapped with each other to form the conductive path together.The melt strength and shear viscosity of composites are increased with the addition of conductive filler,but flexibilities decrease.Using TPU with the lowest hardness and 10 wt%SCF and 3 wt%CNT,prepared composite（TPU3/S10-C3）,which has the highest flexibility.The contact resistance between the replica and flat plates of the sensor under pressure is reduced due to the deformation of the bottom edges of the inverted pyramid.Thus,the sensitivity of flexible sensors with micro-structured inverted pyramid arrays are higher than that of the corresponding planar structure sensors.The sensor prepared with the composite replica and the composite flat plate with CNT content of 3 wt%exhibits the highest sensitivity(sensitivity of 3CNT-m is 0.106 k Pa^–1,0～7 k Pa),its response time and the hysteresis coefficient are 0.07 s and 7.94%,respectively.Moreover,it can maintain a relatively stable resistance response（stability error is 6.15%）.The simulation results show that the force is concentrated on the bottom edges of the inverted pyramid when the micro-structured inverted pyramid arrays is under pressure.Although the sensor prepared with the composite replica and the composite flat plate molded from TPU3/S10-C3 composite（Sensor-3）has a slightly larger hysteresis coefficient（10.04%）,it exhibits a higher sensitivity(0.32 k Pa^–1)in the linear region of 0～2.5 k Pa.This is because TPU3/S10-C3 exhibits the highest flexibility,the deformation of the bottom edges of the inverted pyramid and the increase of conductive path inside the composite are larger.Both sensors exhibite short response times stemming from rapidly changing contact resistance caused by the deformation of the bottom edges of the inverted pyramid.In addition,the sensors prepared in this work can maintain a relatively stable resistance response during 500 s（approximately 1580 cycles）of cyclic compression/release tests（under a peak pressure of～3k Pa）.Due to the increase of the contact resistance change and its proportion in the total resistance change,the sensitivity and stability of the sensors increase and the hysteresis decreases with the increase of the number of microstructural sensing element.The sensitivity,stability error and hysteresis coefficient of the sensor with triple microstructural sensing element are 0.36 k Pa^–1,14.52%and 4.15%,respectively.