Research On Design And Structure-Activity Relationship Of Fabric-based Electromagnetic Interference Shielding Composites

The rapid development of mobile communication technology and intelligent interactive products drives new electronic devices shift towards high frequency,high speed and miniaturization,which not only brings convenience to life,but also makes the space artificial electromagnetic energy grow rapidly and continue to jump energy levels.Therefore,higher requirements are put forward for electromagnetic interference（EMI）shielding materials.Therein,the EMI shielding fabric with flexibility,light weight and wearability has attracted extensive attention due to its strong adaptability and numerous application scenarios.Emerging EMI shielding fabrics take into account the EMI shielding function and wearability by integrating the unique functions of electrical or magnetic nanoparticles and the inherent properties of fabric substrate,and have gradually replaced traditional rigid metals and been widely applied in wearable human health protection and high-intensity electromagnetic countermeasure fields.Nevertheless,the research and development of the existing EMI shielding fabrics still have shortcomings and is mainly presented in:1)integrating different materials/multi-scale functional nanoparticles into fabrics inevitably affect the inherent properties of textile substrates;2)The weak interface interactions between functional layer and textile substrates makes it easy to fall off the material,break the conductive path and can not effectively shield the electromagnetic wave during long-term use;3)the current EMI shielding fabrics with ultrahigh conductivity designed based on impedance mismatch theory can shield EMI waves mainly by reflection,thus resulting in secondary EMI radiation pollution,which is difficult to meet the requirements of safe and green EMI protection.Herein,aiming at the urgent demand for flexible EMI shielding fabrics in the field of electronic information,based on microwave interface conduction theory,the design,construction and formation mechanism of the conductive and magnetic double-effect structure on the fiber surface were analyzed in detail.A series of fabric-based EMI shielding composites with high shielding efficiency,absorption-dominated shielding mechanism and robust combination of functional structure and substrate were prepared.The electrical,magnetic and EMI shielding properties,as well as the structure-activity relationship were discussed in detail.The main research contents are:1.Based on the thiol-ene click chemistry,and the intertwined characteristic of the fabric,a thin,continuous and robust conductive network was constructed on the fabric surface by covalent bonding.Reduced graphene oxide（M-r GO）modified by 3-Mercaptopropyl triethoxysilane（MPTES）was used as conductive filler,cotton fabric modified using MPTES was used as a substrate;the pre-synthesized alkenyl-terminated waterborne polyurethane（WPU）prepolymer was used as a crosslinking agent.A series of efficient and robust EMI shielding fabrics were prepared by UV initiated synchronous thiol-ene click reaction.The results show that a strong covalent bond is formed between M-r GO and WPU through thiol-ene click reaction,which helps to form a continuous conductive network on the surface of cotton fabric.The introduction of click reaction and the increase of M-r GO content cause the obvious enhancement of conductivity and EMI shielding performance.When the thickness of M-r GO/WPU is～100μm,and the M-r GO content is 2 wt%,the obtained composite fabric exhibits high conductivity（～20 S/m）and EMI shielding effectiveness（SE,47.0 d B）,which is superior to most reported carbon-based EMI shielding fabrics.In addition,the formed covalent bond between M-r GO/WPU layer and cotton fabric also endows them excellent functional durability,so that it still retains high EMI shielding performance（EMI SE retention rate>90%）after repeated water washing,bending and friction tests.This EMI shielding fabric prepared via thiol-ene click reaction can effectively avoid the problems of functional layer falling off and conductive path fracture in long-term use,and this study provides an effective strategy for the design of high-efficient and robust EMI shielding fabric.2.Based on the electromagnetic wave interface conduction theory,the conductive network is doped with magnetic components,and its influence on EMI shielding performance and shielding mechanism is investigated.Nickel ferrite nanoparticles（Ni Fe₂O₄ NPs）were introduced into M-r GO/WPU system by a liquid phase in-situ synthesis,and then a conductive and magnetic double-effect structure was constructed on the surface of polyester fabric by combining with surface chemical modification and synchronous thiol-ene click reaction.The results show that Ni Fe₂O₄ NPs（diameter:～30 nm）can be uniformly and stably dispersed into M-r GO/WPU system by hydrogen bond and electrostatic interaction with WPU.The size effect of microstructures forces Ni Fe₂O₄ NPs embedded into the lamellar M-r GO,thus resulting in the enhancement of magnetic properties of composite fabrics as the increasing Ni Fe₂O₄ NPs content,while conductivity shows a slight downward trend.In view of this,the interaction mechanism among electrical,magnetic and EMI shielding properties was further explored.The results indicate that the electro-magnetic synergy improve surface impedance and microwave loss performance,thus the reflection is reduced while absorption is significantly enhanced,where the enhanced absorption is much higher than the reduced reflection,and causing the enhancement of EMI SE.In particular,when the M-r GO content is 2 wt%,the Ni Fe₂O₄ NPs content is 10 wt%,and the coating thickness is～100μm,the EMI SE value of composite fabric reaches 54.6 d B,in which the contribution of reflection to EMI SE is only8.8%,thus displaying an absorption-dominated shielding mechanism.In addition,the finishing process basically retains the flexibility,light weight,air permeability,mechanical properties and thermal stability of substrate,and the obtained composite fabric also exhibits good performance stability of conductivity and EMI shielding.In this study,the low reflection and high shielding were realized by doping the magnetic component into conductive network,which effectively avoids the problem of secondary pollution of electromagnetic wave.3.Study on controllable assembly of conductive and magnetic double-effect structure on textile surface and its EMI shielding performance.Although the loss mechanism of microwaves can be regulated by using inorganic carbon materials and magnetic nanoparticles,it is still worth exploring how to carry out controllable assembly on the surface of textiles.Herein,carbon nanotubes（MWCNTs）and Ni Fe₂O₄ NPs were used as electrical and magnetic components,relatively.And then the conductive and magnetic double-effect structure was directly constructed on the surface of cotton fabric by the surface modification and electrostatic induced directional assembly process.The results show that the p H value and zeta potential of assembly solution play a key role in the directional deposition and microstructure self-assembly of MWCNTs/Ni Fe₂O₄ functional layer.In the experiment,MWCNTs/Ni Fe₂O₄ layer deposits in sequence on the fiber surface and forms a densified film,and its mass change on the fabric surface per unit area displays a linear growth behavior with the assembly cycles（R~2=0.9976）.Finally,polydimethylsiloxane（PDMS）with low surface energy was dip coated to further endow this fabric with hydrophobicity,self-cleaning and durability.The results indicate that PDMS coated outside the conductive and magnetic double-effect structure without affecting the internal electron transmission and electromagnetic coupling ability.The electrical,magnetic and EMI shielding properties of the composite fabric show a similar tendency with the mass growth of functional layer.Therefore,the controllable EMI shielding performance can be realized by adjusting the assembly process.The composite fabric（thickness～0.319 mm）assembled 20 cycles possesses high conductivity（～16.5 S/m）and EMI SE（37.0 d B）,which meets the application requirements of commercial shielding materials.Based on Maxwell equations,the real protective effect of this fabric against electromagnetic radiation around human body was further simulated;and the results show that its shielding effect on electric and magnetic field strength is more than 99.9%,which is consistent with the experimental results.4.In order to satisfy the application requirements of 5G,6G and high-intensity electromagnetic countermeasure fields,the study on preparation of high-performance magnetic metallized fabric and its application in EMI shielding and radar stealth was carried out.Nickel（Ni）,cobalt（CO）and iron（FE）were selected as plating metals,the high-performance polyimide（PI）fabric was used as substrate,and then the Ni-Co-Fe-P/PANI/PI fabric with"sandwich"cladding structure was prepared by aniline in-situ polymerization and following electroless plating process.The results indicate that Ni²⁺concentration is the key factor affecting the quality and electromagnetic property of metal layer;with the decrease of Ni²⁺concentration in plating solution,the conductivity of the plated fabric decreases gradually,while the magnetic properties and EMI shielding properties increase first and then decrease.Especially,when CNi2+:CCo2+:CFe2+=2:1:1,a continuous and dense flake alloy layer is successfully formed on the PI fiber surface,and the plating thickness is about 400 nm;the corresponding X-band EMI shielding effectiveness is up to 69.5 d B,which can shield more than99.9%of electromagnetic waves and is much higher than that of carbon-based or conductive polymer coated fabrics.Compared with traditional metallized textiles,the contribution of absorbing part to its total shielding effectiveness almost to 91.9%,which indicating the absorption-dominated shielding mechanism.The main reason is that the electromagnetic adjustability of Ni-Co-Fe-P alloy improves its surface impedance matching,and resulting in more microwaves penetrating into the plated fabric;then the synergistic effects of the dielectric-magnetic loss of Ni-Co-Fe-P alloy,the dielectric loss of PANI layer,and the multiple reflection loss of Ni-Co-Fe-P/PANI/PI promote microwaves absorbed and dissipated rapidly.Finally,the radar stealth and microwave absorption functions of the plated fabrics were further explored through CST simulation and theoretical calculation.This research provides a new strategy for realizing the unification of high-efficiency EMI shielding and microwave absorption characteristics.