Core-shell Type Metal@C Microwave Absorption Materials:Synthesis And Electromagnetic Properties At Gigahertz Band

As the high-speed development of electronics,the applied frequencies have been explored to gigahertz,for examples,mobile phones(0.8?1.5 GHz),internet and communications(2.45,5.0,19.0,22.0,60.0 GHz),and radar systems(11.7?12.0 GHz).These serious electromagnetic(EM)interference problems have sparked a considerable attention towards research on EM wave absorption materials,which is one of the most effective solutions.Electromagnetic wave absorption materials are usually related to the components and microstructures of absorbers.The microwave absorption capacity is related to the electromagnetic matching between the complex permeability(?r=?'+i?")and permittivity(?r=?'+i?"),and could be enhanced by integrating magnetic and/or dielectric compositions and specific structural units.Ideal electromagnetic wave absorption materials should possess simultaneously properties of both high dielectric loss and high magnetic loss,while in the studies already reported these two terms are highly opposite,thus limiting the design of electromagnetic impendence match.Based on above,we herein demonstrate an alternative microwave absorption material,including three parts in details:(1)Metal@C nanocomposites were synthesized by in-situ arc-discharging in a mixture of methane(CH4)and argon(Ar),which consist of the particle sizes of 10?100 nm in diameter and the graphitic shells of 1?10 nm in thickness.Core-shell structure were very complete.(2)A simple annealing strategy for the formation of electric dipoles into graphitic layers by a controllable oxidation process.The substitutional oxygen defects in graphitic shells,that can formed at<249?,result in the formation of asymmetric microstructure at atomic scale,which giving rise to the local electronic interaction and forming permanent electric dipoles.These dipoles thus provide a strong dielectric polarization phenomenon and contribute an enhancement of dielectric losses.(3)The effective integration of nickel and carbon components can also improve the electromagnetic impedance match.According the transmission theory,the Ni@C nanocomposites exhibit enhanced microwave absorption ability at 2-18 GHz,in particular for the one annealed at 200?.Ascribed to the substitutional oxygen defects in graphitic shells,more than 90%of the microwave power can be attenuated for a planar absorber with a thickness down to 1?1.5 mm at 7.6-13.8 GHz.