Multilayered Structure Design And Properties Of Aramid Nanofiber Electromagnetic Interference Shielding Films

With the wide application of electronics and electrical equipment,electromagnetic pollution is becoming more and more serious.Higher requests for electromagnetic interference（EMI）shielding materials are brought forward in aerospace,flexible wearable electronics and so on.The optimal EMI shielding materials should be low density,excellent mechanical strength,flexibility,outstanding environmental stability and high shielding effectiveness（SE）.In recent years,conductive polymer composites（CPCs）composed of polymer matrix and conductive fillers have become the focus of research in EMI shielding materials with lightweight,high strength and performance.However,due to the high percolation threshold of CPCs,it usually requires high filler content and large thickness to obtain ideal EMI SE value.Aramid nanofiber（ANF）,one of the most preeminent polymer matrices,has outstanding 1D structure,mechanical properties and splendid thermal stability.In this work,high conductive metal nanofillers,including silver nanowire（AgNW）and copper nanoparticle（Cu NP）,combined with flexible substrate ANF to prepare composite films with excellent EMI shielding performance and mechanical properties by multilayered structural design.The main research contents are shown as follows:1)Preparation and properties of flexible multilayered ANF/AgNW composite EMI shielding films.Multilayered ANF/AgNW EMI shielding composite films with 25 wt%AgNW content were prepared by alternate pressure assisted filtration film-forming process,in which ANF was the protective layer and AgNW was the conductive layer.The results shown that that the multilayered structure composite films formed under the hydrogen bonding between the ANF layer and the AgNW layer exhibited excellent mechanical properties and EMI shielding properties.With the increased of the number of layers,the mechanical properties and EMI SE of the multilayered composite film were further enhanced.The 7layer ANF/AgNW composite film assembled by 4 layer ANF and 3 layer AgNW showed the best mechanical properties,with the tensile strength of 79.0 MPa,EMI SE of 63.3 d B,while the thickness only 23.3μm.At the same time,due to the protection of the ANF layer,the 7layer composite film also exhibited excellent thermal stability.After heat treatment at 400°C,the original EMI shielding performance was still maintained.2)Preparation and properties of sandwich structure Cu@ANF/MXene composite EMI shielding films.Firstly,the ANF/MXene composite films were prepared by the pressure assisted filtration film-forming process,and then the Ag⁺in AgNO₃ solution was reduced to nano Ag by MXene.Subsequently,the Cu NP layers were deposited on the surface of the ANF/MXene film by electroless deposition under the catalysis of Ag seed.The Cu@ANF/MXene with high mechanical properties was obtained with the MXene/ANF ratio of 1:10,With the increasing of electroless deposition time,the conductivity and EMI SE of the composite films increased gradually.When the deposition time increased to 75 min,the ultrthin composite film（27μm）with tensile strength of 149.7 MPa,conductivity of 118200 S m^-1 and EMI SE of 102.7 d B was ultimately achieved.The excellent EMI shielding performance is due to the high conductivity of Cu NP and the sandwich structure.Besides,the composite film possessed excellent thermal conductivity with the in-plane thermal conductivity of 40.7W m^-1 K^-1.