Preparation And Properties Of Carbon Based Electromagnetic Wave Absorbers

With the extensive application of electromagnetic technology,various electronic products are pouring into people's daily life and work,which concomitantly bring a serious electromagnetic radiation problem.Electromagnetic radiation not only interferes with the other electronic product and makes it unable to operate normally,but also harms the health of organisms.In the military,electromagnetic radiation is a primary factor to affect the radar stealth of weapons.Therefore,it is very important to develop a material which can reduce the electromagnetic radiation and efficiently absorb electromagnetic wave.The main factors affecting the electromagnetic wave absorption properties of absorbers include the impedance matching characteristic,the attenuation characteristic and micro-morphology.The impedance matching characteristic means the synergistic effect between dielectric loss component and magnetic loss component to avoid reflection of microwave on the surface of absorber.The attenuation characteristic requires strong dielectric loss capability and magnetic loss capability to realize high-efficient attenuation of incident microwave.Besides,proper design of micro-morphology is benefit to enhance the polarization effect of absorber and contribute to attenuation performence.However,nowadays,it is still a challenge to prepare an absorber which can satisfy the impedance matching characteristic and strong attenuation characteristic simultaneously.How to improve the attenuation properties by micro-morphology design needs to be further researched.In this work,two kinds of carbon based electromagnetic(EM)wave absorbers were designed and synthesized with the design of the composition components,the target is to prepare a carbon-based composite absorbing material with strong loss capability and at the same time satisfying the impedance matching characteristics.At the same time,with the adjustment of the microscopic morphology,investigate the influence of structure on the absorbing properties of materials and the mechanism of action.Firstly,sodium alginate-based carbon and Fe particles composites absobers with core-shell structure(Fe@SAC)or composite structure(Fe/SAC)were designed using natural polymer sodium alginate as raw material.With the study of the microstructure and absorption properties,it was found that the core-shell structure was more conducive to the dispersion of magnetic particles,as well as to avoid the metal crystallization during carbonization,which controlled the particles size about 50 nm and the shell thickness about 100 nm.Magnetic particles with smaller diameter were benefit to increase the crystallinity of carbon when been carbonized,contributing to a higher dielectric loss capacity for Fe@SAC than Fe/SAC.Moreover,the core-shell structure could avoid the eddy effect of magnetic particles to realize the impedance matching.At the same time,the multiple interfacial polarization of Fe@SAC can effectively enhance the interaction between absorber and microwave to improve the electromagnetic wave attenuation.Therefore,Fe@SAC exhibited a better electromagnetic wave absorption performance than Fe/SAC with the effective absorption bandwidth of 2.80 GHz(11.28~14.08 GHz)and the minimum loss of-41.59 dB at the thickness of 2 mm).While,the maximum effective absorption bandwidth of Fe/SAC was 1.53 GHz(8.56~10.09 GHz),and the minimum loss was-25.42 dB at thickness of 4 mm).Although the core-shell structure alginate-based carbon and Fe particles composites absorber exhibited an efficient electromagnetic wave absorption performance,its high filling content(45 wt%)maked it difficult to meet the requirement of light weight in application.Introducing fiber and porous structure are effective methods to decrease density of materials.Therefore,the porous carbon nanofibers and magnetic nanoparticles composite absorber(P-CNF/Fe)were prepared by electrospinning followed by stabilization and carbonization using polyacrylonitrile as carbon precursor and polymethylmethacrylate as porogen.The P-CNF/Fe exhibited an excellent attenuation characteristic with 20 wt% filler content.At the thickness of 1.5 mm,the maximum effective absorption bandwidth was 3.28 GHz and it achieved the strongest absorption with the thickness of 4.1 mm,RL min =-44.86 dB.With the introduction of porous nanostructure,the permittivity of carbon nanofibers was decreased at low frequency and the impedance matching of permittivity and permeability was realized.At the same time,the porous structure introduced more defects and interfaces to enhance the Debye relaxation and the interfacial polarization for microwave attenuation.Bsides,by adjusting the amount of PMMA,the porous morphology and pore specific area of the absorbers can be controlled to explore the effect of the porous structure on the absorption properties.With fewer pore,it failed to reduce the high permittivity of carbon fiber and couldn't realize the impedance matching.But a far decline of permittivity caused by macropore would obstruct the strong attenuation characteristic.Therefore,when design a porous carbon composites absorber,the size of proe should be adjustable in order to get excellent microwave absorption performance.