The Strain Sensing Characteristics Of Multi-walled Carbon Nanotubes/Polymer Composites

Conductive composites composed of polymer and nano-particles are widely used in aerospace,electronics,automotive and medical fields because of their excellent mechanical,thermal and electrical properties.Carbon nanotubes(CNTs)possess the characteristics of high aspect ratio,high electron mobility and high strength.The conductive network formed in the polymer matrix is easily influenced by external force,which is expected to provide a new method for the design of high-sensitivity and flexible sensor.In this study,the piezoresistive characteristics of multi-walled carbon nanotubes(MWCNT)/polyvinylidene fluoride(PVDF)under direct current(DC)were analyzed.The RLC equivalent circuit model was built,and the force-to-current coupling characteristics of these composites under alternating current(AC)were investigated systematically as well.The main work in this study are as follows:(1)After ultrasonic dispersion and planetary stirring process,multi-walled carbon nanotube/polyvinylidene fluoride(MWCNT/PVDF)films were fabricated by solution casting method.The films were polished,and their surfaces were coated with conductive silver paste which acts as the electrodes,then strain gauges were obtained by cutting films to uniform size.SEM morphology images show that MWCNT can be dispersed well in the polymer matrix without obvious agglomeration,and they contact with each other to form a three-dimensional conductive network.(2)An elaborate testing platform was built and the strain sensing characteristics of strain gauges under DC and AC circuits were studied experimentally.The main research findings include:(a)The strain gauges made of the MWCNT/PVDF films can sustain over 20% strain without any damage.(b)In DC circuit,the strain gauge possesses piezoresistivity,but the strain-resistance relationship is nonlinear.(c)In AC circuit,the dielectric loss tangent is dependent on the test frequency and the MWCNT content at zero strain,but it has nothing to do with the test voltage.(d)When the strain is not zero,the dielectric loss tangent changes with the variation of the strain.Moreover,the change rate of the dielectric loss tangent is linearly proportional to the strain,but its slope is influenced by the testing frequency.(3)Considering the resistance,capacitance and inductance of carbon nanotubes,an equivalent RLC circuit model for CNT-based composites was established.Massive results indicate that the model can predict the effect of frequency on the material's sensitivity under AC.Three different types of composites,viz.multi-walled carbon nanotube/epoxy(MWCNT/EP),multi-walled carbon nanotube/ polydimethylsiloxane(MWCNT/PDMS)and MWCNT/PVDF,were also fabricated.Measurements certify that the proposed RLC model can predict the tan? of these three composites at different frequencies very well.This model has great potential to be applied in the design of wireless strain sensor.