Research And Application Of Intrinsic Elastic Conductors With Buckled Three-Dimensional Network

The elastic conductor is crucial in wearable electronics and soft robotics.The ideal intrinsic elastic bulk conductors show uniform three dimensional conductive network,high conductivity,and stable resistance during large stretch.Conventional conductive materials(such as metal nanowires,carbon-based nanomaterials and conductive copolymer)are rigid and not-stretchable;while soft elastomeric materials are nonconductive.The fabrication of stretchable conductors have been achieved successfully by dispersing unifirmly conductive fillers into soft substrates.Such composites have continueous 3D conductive network,high conductivity and large deformation,but the conductivity decreases sharply with the applied strain due to the decreased contact area between the nanomaterials,resulting in restriction of applications in flexible electronics.Attaching conductive film with stretchable structures such as nets,buckles on elastic substrates,enabling stable resistance during large deformation,is the alternative method.However,such a stretchable conductor did not have either high conductivity and uniform bulk structure,or has to be prepared by a complicated fabrication process.So it is difficult to use when an all-directional interconnect or an uniform bulk structure is required,such as in electromagnetic interference(EMI)shielding and radar wave absorption.As such,it is still a challenge to obtain a bulk elastic conductor that show constant resistance during deformation in low cost and easy processing.The crucial strategy is to introduce buckled structure into 3D conductive network to take advantage of the resistance stablance of the buckled structures during deformation and the uniform bulk structure,as well as the high conductivity of the 3D conductive network.In this paper,an intrinsic elastic conductor with 3D buckled conductive network was successfully fabricated,using polypropylene(PP)foam as elastic matrix and single walled carbon nanotubes(SWNTs)as conductive phase.SWNT formed a three-dimensional network by self-assembly on the interconnecting pore surface of a pre-stretched polypropylene foam,releasing the pre-stretch resulted in buckling of the SWNT network.The electrical and mechanical properties of buckled SWNT/PP foam was investigated.The main research work is as follows:(1)The dependencies of electrical and mechanical properties on fabrication strain under indentical dip-coating cycles for buckled SWNT/PP foam.With the increase in the biaxial(uniaxial)fabrication strain from 0% to 35%(0% to 80%),bi(uni)-directional buckles density increased,and the resistance change monotonically decreased from 15.1% to 3.8%(15.1% to 2.0%)with 70% uni-directional stretch.(2)The dependencies of electrical and mechanical properties on the number of dipcoating cycles for buckled SWNT/PP foam.With increasing m from one to seven,SWNT contents increased and the distance between the buckles of the SWNT layer increased.The resistance change increased from 3.8% to 6.3% while the maximum available strains decreased from 70% to 40%.(3)Stretching a biaxial and a uniaxial buckled SWNT/PP foam with one dip-coating cycle in different directions to investigate the resistance change.No significant resistance difference was observed by stretching a biaxial buckled SWNT/PP foam to 70% strain along each of the pre-strain axis or at 45° angle with the pre-strain axis;while stretching the uniaxial buckled SWNT/PP foam in the pre-strain direction showed smaller resistance change.(4)Finite element analysis illustrated different patterns of two-dimensional buckling structures could be obtained due to the imperfections in the conductive layer.SEM images demonstrated both unfolding of buckles and flattening of micropores contributed to the stability of the resistance under deformation.(5)The application of buckled SWNT/PP foam as all-directional interconnects,stretchable electromagnetic interference shielding and electrothermal tumor ablation was demonstrated.