Piezoresistive Properties And Mechanisms Of CNTs-CB/Epoxy And Nickel Powders/Epoxy Composites

Epoxy resin could be used as the base material for fabricating epoxy-based conductive composites reinforced with conductive fillers owing to its excellent mechanical properties,good water resistance,strong corrosion resistance,low cost,convenient preparation process and wide deformation range.Meanwhile,epoxy-based conductive composites could be used as piezoresistive strain sensors in the field of civil engineering health monitoring,which is suitable for the service environment where corrosion resistance and large deformation are required.At present,the commonly used conductive fillers mainly include nanoscale carbon-based and metal-based fillers,but they are often need complex dispersed processes due to their thermodynamic agglomeration.Based on this,two kinds of conductive fillers with large volume were selected,including electrostatic self-assembly carbon nanotubes-carbon black(CNTs-CB)and spiny spherical nickel powders,to fabricate CNTs-CB/epoxy and Ni/epoxy conductive composites with good mechanical properties,good stability,high strain sensitivity and wide strain monitoring range.The mechanical properties,electrical properties,piezoresistive properties and its formation mechanism of two kinds of epoxy-based conductive composites were studied,respectively,which could provide theoretical and material basis for the applications of piezoresistive strain sensing.The specific work and conclusions of this paper are as follows:(1)Different contents of CNTS-CB composite fillers were dispersed into epoxy resin matrix by mechanical blending to prepare CNTs-CB/epoxy conductive composites.And their microstructures and mechanical properties were tested.The results exhibit that the CNTs-CB composite fillers could be dispersed uniformly in the epoxy resin matrix within a certain content range via simple mechanical blending dispersion method.The addition of CNTs-CB composite fillers could significantly improve the axial compressive properties of epoxy-based conductive composites,and the effect of improving in the compressive yield strength and elastic modulus is gradually enhanced with the CNTs-CB composite fillers increasing.(2)The effects of CNTs-CB composite fillers contents on the electrical and piezoresistive properties of epoxy-based conductive composites were studied.The piezoresistive responses of CNTs-CB/epoxy conductive composites under different compression loads were investigated.The conductive and piezoresistive mechanisms of conductive composites were described according to effective medium equation,percolation conduction theory and tunnel conduction theory.The results exhibit that adding CNTs-CB composite fillers could significantly improves the electrical properties of conductive composites.They exhibit brilliant piezoresistive properties under both monotonic and cyclic loading,and the stability,repeatability and strain sensitivity of the piezoresistivity were gradually improved with the increasing of CNTs-CB contents.Under monotonic compression,the piezoresistive properties of conductive composites show a wide strain sensing range(around 12000 με),and their piezoresistive responses could be divided into three stages corresponding to different piezoresistive mechanisms.(3)The Ni/epoxy conductive composites with different contents of nickel powders were prepared by mechanical blending method.Their microstructures and mechanical properties were tested.The results show that the nickel powders could be dispersed effectively and evenly in the epoxy matrix and their needle-like surface could be wrapped integrally in the epoxy matrix.The introduction of nickel powder can significantly enhance the mechanical properties of Ni/epoxy conductive composites,including increasing the compressive yield strength and elastic modulus,and reducing the poisson’s ratio.With the increase of the Ni powders contents,the mechanical strengthening effect becomes more significant.According to the anelasticity analysis,the larger the compression load amplitude is applied on the composites,the longer the strain response time is,that is,the more obvious the strain hysteresis effect is.(4)The effects of nickel powders contents on the electrical properties and piezoresistive properties of epoxy-based conductive composites were also studied.The influences of different compression loads on the piezoresistive response of Ni/epoxy conductive composites were investigated.The conductive and piezoresistive mechanism and its theoretical model were explored based on the effective path model,tunnel conduction,field emission conduction and percolation conduction effect.The results show that the resistivity of Ni/epoxy conductive composites exhibits the same percolation phenomenon as CNTs-CB/epoxy conductive composites.The Ni/epoxy conductive composites revealed remarkably stable and repeatable piezoresistive responses subjected to both monotonic and cyclic loading.The excellent piezoresistive response of Ni/epoxy conductive composites is mainly derived from the change of the number of effective conductive paths of the composite under compression.The experimental datas were in good agreement with piezoresistive theoretical model which is based on the tunnel conduction.