Fabrication Of Magnetic Carbon-based Materials And Study For Electromagnetic Wave Absorption

With the rapid development of radar detection technology and electronic communication,the issues caused by electromagnetic wave（EMW）radiation are becoming increasingly serious,which not only interferes with the operation of surrounding electronic equipment,but also does harm for human health.The traditional EMW absorbers are still lack in weight,thickness,absorption bandwidth and strength,etc.Herein,it is urgent to design and prepare electromagnetic wave absorbing materials with excellent performance.Carbonaceous materials have attracted much attention due to their strong dielectric loss capabilities,light weight,and stable chemical properties.As one of the earliest materials used in the field of EMW absorption,magnetic metals have the advantages of strong magnetic loss and easy synthesis.Combining the advantages of carbonaceous materials and magnetic metals,advanced composites can be obtained as excellent EMW absorbing candidates.Based on this,we designed magnetic carbon-based composites with different microstructure which have multiple loss mechanisms and adequate impedance matching,as follows:1.We successfully synthesized a series of ultralight three-dimensional nickel-modified nitrogen-doped reduced oxide graphene aerogels（N-rGA/Ni）with the aid of ethylenediaminetetraacetic acid（EDTA）.Ni nanoparticles（NPs）are proved to be evenly and firmly distributed on the graphene sheet.The calcination temperature has a direct impact on the graphitization degree,the amount of nitrogen doping,the particle size and magnetic properties of Ni,which affects the dielectric loss and magnetic loss of N-rGA/Ni,and further impacts the EMW absorbing properties.Among,the composite N-rGA/Ni（600）obtained at the annealing temperature of 600°C showed the best performance.The minimum reflection loss(RL_min)gets-60.8 dB at 13.7 GHz when the coating layer thickness is 2.1 mm,corresponding to an effective absorption band（EAB）of 5.1 GHz.The amazing EMW absorption performance can be described as follows:（1）The large number of pore structures of aerogel facilitates the conduction and dissipation of EMWs,while enhancing the interface polarization and improving the impedance matching ability.（2）Moderate N-doping intensifies the dipole polarization capability of the graphene layer.（3）The introduction of Ni NPs not only increases the magnetic loss,but also improves the impedance matching ability of N-rGA/Ni.The composites calcined at 500°C and 700°C also exhibited unique EMW absorption characteristics,such as a broader EAB and a thin coating.The above results illustrate that the N-rGA/Ni can meet different electromagnetic wave absorption requirements and is an ideal choice for multifunctional collaborative wave absorbing materials.2.Spray drying technology is one of the most attractive and highly efficient ways to synthesize nano-/micro-particles.We prepared a series of pleated hollow nitrogen-doped Ni/carbon spheres（N-Ni/C）by spray drying and annealing progress.N-Ni/C displays a wrinkled hollow carbon sphere which amounts of nickel nanoparticles are embedded on the surface of the carbon sphere.By regulating the calcination temperature of the precursor,a series of products with different morphologies,degree of graphitization,nitrogen doping amount,and crystallinity were obtained.We have found that the higher the calcination temperature,the stronger the EMW absorbing ability.The sample of N-Ni/C（800）obtained under the condition of calcination temperature of 800°C manifests the best absorption performance,which RL_min value reaches-52.6 dB at 8.0 GHz,and the EAB achieve 3.27 GHz.And N-Ni/C（800）also have strong absorption ability in the lower frequency region.The results show that N-Ni/C expresses strong absorbing performance through the synergistic effect of carbon materials and magnetic metals.