Fabrications And Properties Of EVA-based Smart Electromagnetic Shielding Composites

As a new type of functional material with high loss of electromagnetic waves,electromagnetic shielding materials are widely used in electronic equipment to avoid electromagnetic interference.Among them,the carbon material/polymer electromagnetic shielding composites have attracted much attention because of their light weight,corrosion resistance,high microwave absorption and low reflection.In recent years,the scientific and industrial circles have increasingly higher requirements for the flexible,wearable and intelligent controllability of electromagnetic shielding materials,which has brought new opportunities and challenges to the research and development of electromagnetic shielding materials.Shape memory poly(ethylene-co-vinyl acetate)(EVA)is a smart polymer material with good flexibility,self-healing structure and shape programmable characteristics.Therefore,electromagnetic shielding composites with shape memory effect were obtained based on using EVA advantages of intelligence and easy processing has important theoretical significance and potential applications in the intelligent response of electromagnetic shielding functional components and the conversion of electromagnetic shielding effectiveness.This subject is closely related to the development of electromagnetic shielding composites with controllable shielding effectiveness and intelligent stimulus response properties.Carbon fiber felt(CFF)and oriented graphene fiber film(GFF)were combined with EVA through the hot-pressing process to prepare an EVA-based shape memory electromagnetic shielding composites.The study results are as follows:(1)The CFF/EVA composites with Sandwich structure were successfully prepared by hot-pressing process,where CFF was used as the conductive layer and EVA was used as the shape memory functional layer.The results showed that the prepared the CFF/EVA composites have stable conductive network,and their conductivities were up to 16.69 S/cm when the CFF's surface density was 30 g/m².As the surface density of CFF increased,the average electromagnetic interference(EMI)shielding effectiveness of the CFF/EVA composites in the X-band increased from 28.2 d B to 41.1 d B,but the bending shape recovery ratio decreased from100%to 97%.The CFF₂₀/EVA composite can recover from the‘?'shape to the original shape(‘?',Inner 16 mm×40 mm/outer 30 mm×60 mm)within 21 s under 10 V.The electrothermal-driven shape memory recovery can quickly control the transition of EMI from unshielded to shielded state,which enriches the functional applications of the electromagnetic shielding materials.(2)The GFF was prepared by wet spinning and chemical reduction techniques,and the GFF/EVA shape memory electromagnetic shielding composites with anisotropic sandwich structure were successfully constructed by hot-pressing process using GFF as the conductive layer.The results showed that the shape memory performance of the GFF/EVA composites had obvious anisotropy,and the shape recovery ability in the fiber radial(90°)direction was better than that the axial(0°)direction.The shape fixity ratio,shape recovery ratio and reversible strain ratio of the GFF/EVA composites in the 90°direction were 91%,99%and 11%,respectively.The average EMI shielding effectiveness in the 0°direction was 43.2 d B,and the EMI shielding effectiveness in the 90°direction was reduced to less than 10 d B.In addition,the EMI shielding performance in the 0°direction can be regulate by applying shape memory strain to the 90°direction.(3)Based on the research of(1)and(2),the GFF/CFF/EVA shape memory electromagnetic shielding composites with multi-layer structure were prepared by the hot-pressing process.The results showed that the average EMI shielding effectiveness of the GFF/CFF/EVA composites in the 0°direction is 45.6 d B and in the 90°direction was 37.5 d B when the GFFs are arranged in parallel.The average EMI shielding effectiveness in the 0°direction was 44.7 d B and in the 90°direction was 44.0 d B when the GFFs were arranged vertically.In addition,the bending shape memory fixation ratio and shape memory recovery ratio of the GFF/CFF/EVA composites were about 99%and 95%,respectively.The above results showed that the EVA-based composites prepared in this subject have excellent electrothermal-driven shape memory and EMI shielding properties.The shape programmability of EVA-based electromagnetic shielding composites can be used to the needs of equipment with complicated shapes.Secondly,the anisotropic EVA-based smart electromagnetic shielding composite can achieve adjustable EMI shielding performance through shape memory strain or double-layer superimposed rotation,and its has potential application prospects in the fields of electric-driven actuators,EMI protection and signal recognition.In addition,the electrothermal-driven shape memory recovery effect can control the EMI shielding material to switch from unshielding to shielding area,which has potential application value in the fields of intelligent anti-electromagnetic interference systems.