Research On Strain Sensor Based On Carbon Nanotube/Epoxy Composites

The resistance of carbon nanotube(CNT)composites can vary with external strain,the piezoresistive properties of the composite materials can be used to make strain sensors.Although the CNT/Polymer composites' dielectric behavior under direct current(DC)have been studied by many researchers,the conductive mechanism of this type of materials in alternating electric fields still not clear.In this study,an integrated process is developed to disperse high-quality multi-walled carbon nanotubes into the epoxy resin(EP)to fabricate highly sensitive strain sensors.The composite's conductive and piezoresistive properties under DC and AC conditions were investigated by experimental test and theoretical analysis.First,both planetary agitator and ultrasonic disperser were used to prepare CNT/EP composites.The force-electrical properties of the composites were measured by a self-made precision tensile tester,and the results show that the Young's modulus of the CNT/EP composite changes with the CNT mass fraction.It increases at the first stage,followed by a descending phase as the CNT content becomes larger.When the CNT load is low,the composite behaves like nonconductive material,but it becomes conductive when the CNT content is greater than a specific value.Experimental results show that the conductive percolation threshold of the composite is 0.3% in terms of weight.The conductive behavior of the composite can be attributed to two mechanisms,viz.(1)the CNTs form a complete conductive network in the matrix,and(2)the "tunneling effect" between the carbon nanotubes.Second,this study also developed a novel method to install electrodes to CNT/EP films.By this method,the connection between the electrodes and the composite film approaches perfect so that the performance of the sensors is much more stable.Meanwhile,the efficiency of making this kind of sensors can be improved tremendously.A large number of strain gauges prepared by the proposed fabrication process were tested under DC condition in the research.The sensitivity coefficient of the strain gauges with a CNT weight load of 0.2wt% can reach more than 120,that is about 24 times larger than that of an ordinary foil strain gauge.Third,the AC dielectric behavior of the composite strain gauge was investigated.The experimental tests show that the frequency of the AC current does not affect the loss tangent of the gauge,but the change rate of the loss tangent increases gradually with the growth of strain.At the same strain level,the lower the mass fraction of the CNT is,the greater is the change rate of the loss tangent.This phenomenon is explained by an equivalent Resister-Capacitor parallel circuit model.In the AC condition,the rate of change of the loss tangent and the rate of change of the equivalent resistance have high sensitivity and stability to the strain,which means that a very small strain can be detected.By using the AC measurement method,the measurement efficiency can be improved,and the measurement range can be increased,all of which provide a viable direction and experimental basis for the manufacture of ultra-high sensitivity strain sensors.