| As 5G technology has been widely used in various fields,electromagnetic pollution has aroused wide public concern.As the most frequently used band in 5G technology,GHz frequency band poses increasingly serious threats to communication signals of electronic devices and human health.Based on this status quo,researchers have conducted in-depth research on GHz band electromagnetic wave(EMW)absorbing materials.Traditional EMW absorbing materials such as conductive polymers,metal oxides and metal elements have shortcomings such as narrow effective absorption bandwidth(EAB),high density and complicated preparation process,which limit their further application.Carbon materials are widely used in the preparation of EMW absorbing materials due to their advantages of light density,high conductivity and stable chemical properties.However,current studies have shown that carbon materials have shortcomings such as high conductivity and poor impedance matching in the absorption performance of GHz band EMW,which limit their further application.In this paper,cheap melamine foam was used as the precursor of carbon materials,and carbon foam(CF)with lower density was prepared by nitrogen annealing.By introducing nano-alloy particles,ferrite,sulfide and other means into CF,a matching layer with lower permittivity can be formed on the surface of CF,which can effectively improve the impedance matching of the material.In this way,we can prepare the EMW absorbing material with strong absorption intensity and wide EAB.Furthermore,by analyzing the surface chemical state,composition proportion and microstructure of the material,the mechanism of absorbing EMW by the material was deeply explored.The following are the main research contents:(1)CF was prepared by high-temperature annealing in nitrogen atmosphere with melamine foam as the precursor.Then,CuNi nanoparticles with a particle size of about 20?600 nm were modified on the surface of CF by a co-precipitation and annealing method to prepare CuNi/CF composite materials.The research results show that,compared to pure carbon materials and single-metal modified EMW absorbing materials,CuNi11 had a maximum reflection loss of-50.20 dB at a thickness of 1.6 mm,and an EAB of 3?18 GHz at a thickness of 1?5 mm.The research results show that the appropriate proportion of CuNi nanoparticles can adjust the electromagnetic parameters of the material.The study of EMW absorption mechanism shows that the improvement of the EMW absorption performance of CuNill/CF composite material is mainly attributed to that the appropriate proportion of CuNi can effectively adjust the conductivity of the material and then adjust the impedance matching of the material.(2)In the previous chapter,the gap between the permeability and permittivity of composite materials was shortened by introducing magnetic materials.In this chapter,an attempt was made to introduce another kind of material-dielectric material into CF.By forming a double-layer gradient material with high permittivity in the inner layer and low permittivity in the outer layer,the gap between the permittivity of the composite and the free space can be reduced and the absorbing performance of the material can be improved.In this chapter,with CF as the matrix,MoS2/CF composite materials were prepared by growing 1T/2H phase MoS2 with a size of about 100 nm on the surface of CF by hydrothermal method.The composite material had a maximum reflection loss of-45.88 dB at 10.2 GHz and an EAB of 5.68 GHz at a thickness of 2.2 mm.The results show that without the introduction of magnetic materials,1T/2H phase MoS2 with lower permittivity was coated on the surface of CF as a matching layer,forming the EMW absorbing material with gradient permittivity.EMW can enter the material as much as possible,which effectively improves the impedance matching of the composite material and gives full play to the high attenuation characteristics of CF.(3)Based on the previous two chapters,by introducing ferrite material with low permitivity and large permeability into CF,the permeability and permittivity of the material were balanced to form a double-layer composite material with magnetic loss,dielectric loss and gradient permittivity.In this chapter,with CF as the matrix,one-dimensional NiCo2O4 nanoneedles were grown on the surface of CF by a two-step method to prepare NiCo2O4/CF composite materials.The material had a maximum reflection loss of-63.06 dB at a thickness of 2.8 mm and an EAB of 4.72 GHz at a thickness of 1.3 mm.The research results show that the introduction of magnetic loss enriches the loss types of the material,and the balance of magnetic loss and dielectric loss also makes impedance matching possible.In addition,the material produced as many dipoles as possible when interacting with EMW because of the easy polarization of one-dimensional materials.The EMW absorption performance of the material was then improved due to dipole polarization under the action of EMW.(4)Inspired by the previous chapter,by introducing ferrite-containing porous carbon into CF,the density of the material can be further reduced without changing the properties of the composite material.With CF as the matrix,ZIF-67 nanosheets with a thickness of 200?500 nm were grown on the surface of CF by dipping method,and then ZIF-67 nanosheets were converted into carbon nanosheets containing Co3O4 by low-temperature annealing in air atmosphere,to obtain the Co3O4/CF composite material.The material had a maximum reflection loss of-46.58 dB at 10.72 GHz,and an EAB of 5.4 GHz.The research results show that the low-crystallinity Co3O4/C composite nanosheets play the role of dielectric material.The introduction of Co3O4/Ccomposite nanosheets can separate CF in the radial direction,thereby reducing the conductivity of the material.At the same time,compared with single component nanosheets,the rich interfacial polarization in Co3O4/CF composite nanosheets can make up for the poor attenuation performance caused by low dielectric loss to some extent.(5)Compounding with conductive polymer is an important means to improve the electromagnetic wave absorption properties of carbon materials.Nano-conical polyaniline/CF composites were prepared by polymerization of aniline on the surface of carbon foam in hydrochloric acid with aniline as monomer and ammonium persulfate as initiator.The effect of reaction time on the electromagnetic wave absorption properties of the composite was proved by analyzing its electromagnetic parameters.The test results of the electromagnetic wave absorption properties of the sample show that the polyaniline/CF composites prepared under the polymerization time of 24 h had a maximum reflection loss of-51.96 dB at 17.52 GHz at a thickness of 1.3 mm,and an effective absorption bandwidth of 14.4 GHz at a thickness of 1?5 mm.The study of the electromagnetic wave absorption mechanism shows that the dielectric loss of polyaniline/CF composites is mainly composed of conduction loss.The longer the relaxation time of the sample at high frequencies,the greater the polarization loss of the sample.The above research results divide the types of dielectric loss inside the material,and also provides a way to further explore the electromagnetic wave absorption mechanism of the material. |
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