Synthesis And Electromagnetic Performance Study Of Co,FeCo Magnetic Dielectric Composites

The metallic powders（Fe,Co,Ni）and their alloy have been studied extensively as microwave absorbents because of the advantages of high permeability and high saturation magnetization.However,the oxidizability,high density and the narrow absorption bandwidths restrict its practical application in microwave absorption field.Moreover,the single component material can hardly obtain an optimal microwave absorption property due to the weak impedance matching or the low attenuation abilities.Through embedding the metallic magnetic materials into the dielectric materials can be a well strategy to prepare microwave absorbents.Therefore,to obtain the strong reflection loss（RL）in wide frequency range,we combined the metallic magnetic materials with the carbon materials to synthesize multi-components absorbents.The main research contents and results are as follows:（1）As the shape of the flaky shape can weaken the Snoek limit and avoid the vortex effect at high frequency.Especially when the size of the nanoparticles is close to or less than the skin depth.Thus,we prepared the flaky Co@C composite through pyrolyzing a precursor by different calcination temperature under an inert atmosphere.The core-shell nanostructure ensure the composites equipped with the magnetic loss and the dielectric loss simultaneously.When the calcination temperature is 600^oC,the sample showed the best impedance matching degree and attenuation abilities.The maximum reflection loss reached-42.68 dB at the frequency of 15.88 GHz and the thickness was only 1.4 mm.Also the frequency bandwidth covered 4.2 GHz over the frequency of 2-18 GHz.（2）To achieve the strong reflection loss in wide frequency range,FeCo alloy was applied as the magnetic core to construct the magnetic-dielectric composites.The FeCo/C composites within different molar ratio of Fe/Co（3:7,4:6,6:4）were prepared by carbon thermal reduction method under N₂ atmosphere.The Z_in/Z_o values of the FeCo/C composites were calculated to investigate the effect of the Fe/Co molar ratio and the structure on the impedance matching.The results show that the cactus-like porous samples were equipped with an optimal impedance matching and advanced microwave absorption performance.The cactus-like porous structure can also induce multiple scattering and extend the travel path of the waves,which is in favor of the electromagnetic loss.In this way,the effective frequency bandwidth（RL<-10 dB）of sample S2 can be broadened as large as 6GHz（from 11.36 to 17.36 GHz）at the thickness of 2 mm when the Fe/Co molar ratio is 4:6.（3）Alloy Fe₅₀Co₅₀0 has significant application prospect in the field of high temperature radar stealth because of the advantages of high Curie temperature.Thus,we studied deeply about the electromagnetic performance of the Fe₅₀Co₅₀0 based composites under different heat preservation period.And the sample calcinated at 600℃ for 4 h showed the optimal microwave absorption performances.The unique 3-D honeycomb-like structure is beneficial to induce multiple reflection and scattering,which is beneficial to attenuate the electromagnetic wave efficiently.The maximum reflection loss reached-54.6 dB at the thickness of 2.3 mm and the frequency bandwidth covered 5.3GHz in the frequency range of 2-18GHz.