Preparation And Properties Of Mxene-based Nanocomposites For Electromagnetic Interference Shielding With Low Reflection

With the popularization of electronic communication equipment in modern society,the resulting electromagnetic radiation has become serious pollution that cannot be ignored.Two-dimensional transition metal carbons/nitrides（MXene）have attracted widespread attention due to their unique layered nanostructure,outstanding electrical conductivity and good solution dispersion,showing strong competitiveness in the field of lightweight and flexible electromagnetic interference（EMI）shielding materials.However,the excellent EMI shielding effectiveness caused by high conductivity leads to the shielding mechanism dominated by reflection loss,which will bring serious secondary pollution to the environment.Therefore,it is imperative to prepare a new shielding material with low reflection,high absorption and excellent electrical conductivity.Reasonable addition of magnetic filler and other components or adjustment of material structure design can not only prepare high-performance electromagnetic shielding materials,but also further reduce secondary electromagnetic radiation pollution.In this paper,utilizing the strong interaction between one-dimensional（1D）magnetic nanomaterials and two-dimensional（2D）,low-density three-dimensional（3D）porous MFC composite aerogels were prepared by directional freezing.This material not only possesses an absorption-dominated shielding mechanism,but also exhibits excellent compressibility,good thermal insulation and infrared stealth property.In addition,rationally adjusting the structural design of materials,MXene-based gradient composite films were prepared by a layer-by-layer assembly method.This gradient layered structure composed of an impedance matching absorption layer and a highly conductive layer can have good EMI shielding ability and effectively reduce the secondary EMWs pollution.The main research contents are as follows:（1）3D porous MXene-based composite aerogels with low reflection prepared by a directional freezing method:First,Fe₃O₄was uniformly coated on acidified carbon nanotubes（a MWCNTs）by co-precipitation method to prepare Fe₃O₄@a MWCNTs（FC）magnetic nanoparticles.Then,exploiting the similar hydrophilicity between 1D magnetic nanoparticles FC and 2D MXene nanosheets and the strong interaction of surface groups between them,MXene/Fe₃O₄@a MWCNTs（MFC）aerogels were prepared.The low-density(about 10 mg cm^-3)MFC aerogels still have stable shape structure and high mechanical strength.By adjusting the content of conductive,dielectric and magnetic components and rational structural design,their shielding performance can be adjusted in the range of 24.5～51.6 d B.At the same time,the reflection power coefficient（R）of the aerogel can be as low as 0.31,which means that only 31%of the EMWs in the 99.9%shielded EMWs will undergo secondary reflection,which is much lower than that of traditional MXene-based EMI shielding materials.This design provides a feasible strategy for fabricating lightweight,flexible,low-reflection,and multifunctional magnetic MXene-based composite aerogels.（2）An MXene-based gradient composite film with low reflection prepared by a layer-by-layer assembly method:By a layer-by-layer assembly method,we prepared MXene/WPU/WPU@Fe₃O₄@a MWCNTs（M-W-F）gradient composite films with positive conductivity gradient and negative magnetic gradient.By changing the number of layers of the magnetic absorption layer and the composition of the highly conductive reflective layer,the M-W-F gradient composite film can have better impedance matching and higher shielding efficiency.When the thickness of the MXene-based gradient composite film is only 450μm,the shielding effectiveness can reach 48.6 d B and the average R value can be reduced to 0.55.And the R value reaches a very low peak of 0.19,showing that the prepared MXene gradient composite films have obvious characteristics of high absorption and low reflection.In addition,the influence on the shielding performance of the multilayer gradient structure,the impedance matching layer and the high conductivity layer are discussed comprehensively.When the EMWs are incident from the magnetic layer,the microwaves can enter the composite film more.The EMWs entering the interior of the film are blocked by the highly conductive layer and reflected back to the interior of the composite film,and then the EMWs are further attenuated through multiple reflections between the film’s interfaces.