Preparation And Properties Of Carbon-based Composite Electromagnetic Wave Absorbing Materials

With the rapid development of electronic equipment,the issue of electromagnetic radiation has attracted more and more attention.The radiated electromagnetic waves will not only affect the normal operation of the equipment,but also threaten people's health and national defense security.The most effective way to solve electromagnetic pollution is to prepare electromagnetic wave absorbing materials with light weight,strong attenuation ability,thin coating,wide absorption band and excellent mechanical stability.While pursuing the excellent absorption properties of materials,it is also a major challenge to make the materials meet the needs of practical application in this field.Among many materials,carbon-based composites have attracted much attention in the field of electromagnetic wave absorption due to their adjustable electromagnetic parameters,low production cost,easy preparation,environmental friendliness and chemical inertness.Starting from the principles of simplicity,economy,greenness and easy repeatability,and through reasonable design,carbon materials are effectively combined with magnetic substances or semiconductor components to optimize impedance matching and enrich the loss pathways of electromagnetic waves.The synergistic effect of multiple loss mechanisms enhances the attenuation ability,thereby obtaining a carbon-based composite electromagnetic wave absorbing material with excellent performance.The main contents are as follows:1?Two-dimensional Ni/N co-decorated carbon sheets(Ni-NC)were prepared by coordinating of ethylenediaminetetraacetic acid with Ni²⁺firstly and the subsequent exfoliation process.Then,the carbon sheet was treated with melamine at high-temperature and one-dimensional carbon nanotubes grew on the surface of the carbon sheet under the catalytic action of nickel particles,and finally composite materials(Ni-NC/CNTs)with significantly improved electromagnetic wave absorption capacity compared with the two-dimensional carbon sheets were obtained.The composite(Ni-NC/CNTs(700))prepared at 700?,exhibits the best absorption performance.The minimum reflection loss value(RL_min)is-55.3 dB at 8.72 GHz with the thickness of2.7 mm.Besides,it can achieve effective absorption in the Ku-band(12-18 GHz)at extreme thin thickness(1.7 mm),showing an effective absorption bandwidth(EAB)of4.64 GHz.Specifically,the Ni-NC/CNTs(700)can achieve tunability of effective absorption between the whole C-band(4-8 GHz),X-band(8-12 GHz)and Ku-band by adjusting the thickness.The superior performance mainly depends on the following points:(1)Carbon nanotubes connect carbon sheets in series to form a large conductive network,which improves the electrical conductivity and promotes conductive loss.(2)A large number of gaps created by the intertwined CNTs optimizes impedance matching.(3)Electrons accumulate between different phase boundaries(CNTs,carbon sheets,nickel nanoparticles,paraffin),which promotes the interface polarization loss.(4)Heteroatoms and defects act as polarization centers,forming dipole polarization to convert electromagnetic energy into thermal energy.(5)The uniformly dispersed nickel particles form a strong magnetic coupling effect,resulting in magnetic loss.In addition,nickel particles are encapsulated in carbon nanotubes to improve stability,which makes them suitable for use in harsh environments.2?Starting from the principles of green,economical and easy access,the core-shell structure of Co₉S₈ coated carbon spheres(CS@C)was obtained by compounding semiconductor materials with humins(humins)produced by the condensation of xylose in the catalytic process as the carbon source.The Co₉S₈ prepared at different temperatures has different degrees of sulfur defects.Under the alternating electromagnetic field,the defects act as polarization centers to enhance the dipole polarization,and Co₉S₈ possesses a two-dimensional lamellar structure,which facilitates the interfacial polarization loss.when the calcination temperature reaches700?,the composite(CS@C-700)exhibits the best absorption performance due to good impedance matching and suitable loss constants.At high frequency of 13.6 GHz and thickness of 1.84 mm,the minimum reflection loss is-51.4 dB,the effective absorption bandwidth is 5.2 GHz,and the EAB is up to 5.92 GHz at ultra-thin coating thickness of 1.8 mm.Secondly,in the low-frequency C-band,the RL_min is-49.9 dB at6.4 GHz with the thickness of 3.5 mm.The above results show that by adjusting the coating thickness,CS@C-700 can show strong electromagnetic wave absorption performance and wide absorption bandwidth in both high-frequency Ku-band and low-frequency C-band,making it is an excellent electromagnetic wave absorber.When the temperature is further increased to 800?,the sample achieves a reflection loss value of-48.1 dB at a thickness of 6.5 mm,which can achieve electromagnetic wave attenuation.In general,CS@C composite material is an excellent candidate for electromagnetic wave absorber.