Synthesis And Electromagnetic Wave Absorbing Performance Of Magnetic Metal/Dielectric Composites

Magnetic metals exhibit strong absorbing ability due to both large magnetic and dielectric loss induced by the high saturation magnetization and conductivity.However,the application of magnetic metals as electromagnetic wave absorbers is restrained by the following issues:(i)the permeability and working frequency are restricted by the Snoek’s limit;(ii)poor impedance accompanied with large proportion of reflected waves due to the high conductivity;(iii)large density hindering the achievement of lightweight absorber.In this work,FeCo and Co nanoparticles with high saturation magnetization have been introduced as the magnetic components,which are combined with dielectric materials including MoS2,ZnO and V2O3 coporated with designed features such as flower-like,porous and hollow structures.The composites exhibit high permeability and magnetic loss since the particle is dispersive and the size is close to single domain size.Also the composites show enhanced impedance matching and reduced density.The main contents are as follows:(1)FeCo@MoS2 nanoflowers have been prepared by hydrothermal/coprecipitation/hydrogen reduction processes.Large surface area and sufficient voids provided by the MoS2 nanoflowers enable uniformly dispersed FeCo nanoparticles with single domain size for enhanced permeability and natural resonance.Meanwhile the permittivity and dielectric loss are adjustable through controlling the FeCo content.When the molar ratio of FeCo:MoS2 is 1:3,the composite exhibits excellent performance with a minimum reflection loss of-64.64 dB and a broad effective absorption bandwidth(RL<-10 dB)of 7.2 GHz at a small thickness of 2 mm.Excellect impedance matching is achieved for the composite with the molar ratio of 1:3 due to the synergistic effect between the magnetic and dielectric loss,giving rise to the maximum entrance of the EM waves.The EM energy can be consumed via the natural resonance of FeCo nanoparticles,the interfacial polarization between FeCo and MoS2,as well as the conductive loss.Meanwhile multiple reflections of the entered waves can be induced between the flower-like structure for enhanced absorption.(2)FeCo/ZnO porous nanosheets have been fabricated by "bubble blowing"/air calcination/hydrogen reduction processes.The density of the composites can be significantly reduced due to the unique porous feature.The particle size of pure FeCo nanosheets is larger than 100 nm,while the particle size decreases to approx.20 nm for the FeCo/ZnO composite with molar ratio of 1:1 due to the confinement effect of ZnO.Such reduced FeCo particles increases the surface anisotropy for enhanced natural resonance frequency,giving rise to improved contribution of the magnetic loss in the testing frequency range.Permittivity and dielectric loss are also tunable via interruption of the Fe7Co3 conductive network,the composite with the FeCo:ZnO molar ratio of 1:1 exhibits a broad effective absorption bandwidth of 7.92 GHz at 2.3 mm.Such enhanced absorption performance can be attributed to the improved impedance matching,which gives rise to maximum entrance of the EM waves.The natural resonance of FeCo nanoparticles,interfacial polarization between FeCo and ZnO,as well as the conductive loss consume the EM energy.Also multiple reflections can be induced between the nanosheets for enhanced absorption.The method is extendable since other nanosheets such as FeNi/ZnO and FeCo/MnO can be synthesized via changing the nitrate species.(3)Co/C@V2O3 hollow spheres have been designed and synthesized through a facile solvothermal/coprecipitation/annealing method.The composite possesses both magnetic and dielectric loss with low density due to the rational combination of the hollow V2O3 spheres and porous Co/C.Optimal performance can be achieved for VC2 via adjusting the Co/C content,with a minimun reflection loss of-40.1 dB and a broad effective absorption bandwidth of 4.64 GHz at a small thickness of 1.5 mm.Enhanced impedance matching leads to the maximum entrance of the EM waves.The wave energy can be consumed by the natural resonance of Co nanoparticles,interfacial polarization between Co,C and V2O3,as well as the conductive loss.The unique hollow structure also benefits for inducing multiple reflections for enhanced absorption.