Structural Design And Electromagnetic Properties Of Macroscopic Assemblies Based On Two-Dimensional Nanomaterials

The rapid development of modern information technology as well as the popularization of electronic devices has greatly facilitated people’s lives.However,they also cause serious electromagnetic pollution that poses a threat to human health and the normal operation of electronic equipment.Traditional electromagnetic protection materials can hardly meet the demand of portable electronic devices and increasingly complex application environments.Thus,it is urgent to develop new alternative electromagnetic protection materials.In recent years,two-dimensional（2D）nanomaterials,such as graphene and MXene（2D transition metal carbides or nitrides）,have become a research hotspot in the field of electromagnetic protection materials and shown great application potential because of their unique layered structure,high specific surface area,and good conductivity.However,there are some problems in the preparation process of macro-scale electromagnetic protection materials from micro-scale 2D nanomaterials,such as uneven dispersion,disordered orientation,impedance mismatch,and large skin depth,which makes it difficult to make full use of the advantages of 2D nanomaterials and limits the improvement of electromagnetic protection performance.In this paper,2D nanomaterials were used as basic building blocks to construct macroscopic assemblies with different architectures（film and foam）.Based on transmission line theory,the structure and composition of these macroscopic assemblies were regulated by different methods.The effects of microstructure and phase composition on the electromagnetic protection performance of the macroscopic assemblies were explored,and their microwave shielding or absorption mechanisms were also investigated.This work would provide technical support for the structural design of new electromagnetic protection materials and enhance the practical application value of 2D nanomaterials.The main research contents and results of this paper are as follows:（1）To shorten the skin depth of graphene,GO was used to prepare graphene films with high crystallinity through vacuum filtration,chemical reduction,and subsequent graphitization process;then,highly conductive metal Ag was deposited on the surface of graphene films by magnetron sputtering to construct Ag/graphene/Ag sandwich structure.The electromagnetic interference（EMI）shielding performance and mechanism of the sandwich film were investigated in this paper.After depositing Ag on the surface of graphene film,the conductivity increased by nearly two orders of magnitude,which made skin depth decrease from 35.6μm to 4.3μm.Moreover,the sandwich structure enhanced the multiple reflections of electromagnetic waves and thus lengthened their transmission path in the film,thereby attenuating more electromagnetic waves.The shielding effectiveness（SE）of Ag/graphene/Ag sandwich film with a thickness of 26μm could reach65 d B in the frequency range of 4-18 GHz,which was much higher than that of pure graphene film with the same thickness（24 d B）.（2）To solve the problem of secondary radiation caused by the impedance mismatch between graphene and air,Fe₃O₄ magnetic nanoparticles were in-situ decorated on GO nanosheets during the assembly process of GO film,and then Fe₃O₄@graphene film was prepared at mild thermal treatment temperatures with the aid of catalytic graphitization effect of Fe₃O₄.The effect of Fe₃O₄ nanoparticles on the graphitization degree of the Fe₃O₄@graphene film was studied by X-ray diffraction and Raman spectroscopy.The EMI shielding performance of the Fe₃O₄@graphene film was tested by the waveguide method and its shielding mechanism was analyzed.When the thermal treatment temperature reached1000℃,Fe₃O₄ presented a significant catalytic effect on the graphitization of amorphous carbon in the GO,which improved the graphitization degree and conductivity of the film.The introduction of Fe₃O₄ nanoparticles also improved the relative permeability of the film,which optimized the impedance matching between the film and air.The SE of the prepared Fe₃O₄@graphene film could reach 49 d B in the X band（8.2～12.4 GHz）at the thermal treatment temperature of 1100℃,which was twice that of the pure graphene film prepared under the same conditions.Moreover,the contribution of absorption to SE reached 28 d B,which was higher than that of reflection（21 d B）.（3）To improve the impedance matching between MXene and air,a three-dimensional macroporous foam composed of Ti₃C₂T_x hollow microspheres was constructed by sacrificial template strategy.The effect of cavity size（hollow sphere diameter）on the complex permittivity,impedance matching,and microwave absorption（MA）performance of the foam was investigated in the frequency range of 2-18 GHz.Furthermore,the MA mechanism of the Ti₃C₂T_x hollow-sphere foam was also clarified in this paper.After assembling Ti₃C₂T_x nanosheets into hollow microspheres,the impedance matching performance was improved due to the scattering effect of the spherical surface on electromagnetic waves.The unique hollow-sphere structure endowed the foam with strong electromagnetic wave resonance capability,which enhanced the attenuation of electromagnetic waves.Furthermore,with the decrease of cavity size,the scattering and resonance effects of the foam on electromagnetic waves were enhanced,and the microwave absorption performance was further improved.The minimum reflection loss(RL_min)of the Ti₃C₂T_x hollow-sphere foam with a cavity size of 2μm was only-49.2 d B,which was much superior to that of Ti₃C₂T_x nanosheets（-7.4 d B）.And the foam could achieve effective absorption in the frequency range of 9.4-11.5 GHz（the reflection loss is less than-10 d B）.（4）To prevent MXene from being oxidized in the air,Fe₃O₄/Ti₃C₂T_x/reduced graphene oxide（RGO）hybrid foam was prepared through freeze-drying and sacrificial template strategies.Stable and flexible RGO nanosheets were used to wrap up Ti₃C₂T_x hollow microspheres to isolate them from the air,which improved the stability of Ti₃C₂T_x hollow microspheres.At the same time,Fe₃O₄ nanospheres were used to modify the electromagnetic parameters of Ti₃C₂T_x hollow microspheres and RGO nanosheets,which further optimized the impedance matching performance of the foam.The micromorphology,crystal structure,electromagnetic parameters,and MA performance of Fe₃O₄/Ti₃C₂T_x/RGO hybrid foam,Ti₃C₂T_x/RGO hybrid foam,Fe₃O₄/RGO hybrid foam,and RGO foam were compared systematically.The RL_min and effective absorption bandwidth（EAB）of Fe₃O₄/Ti₃C₂T_x/RGO hybrid foam with a thickness of 2 mm could reach-50.5 d B and 6.8GHz,respectively.After being exposed to the air for 3 months,the RL_min and EAB of the foam still retained 72.9%and 78.8%of the initial values,much higher than those of the Ti₃C₂T_x hollow-sphere foam without the protection of RGO nanosheets（45.3%and 61.9%）.