Design And Preparation Of Magnetic Particles-cotaining Graphene Hollow Microspheres And Their Microwave Absorption Performance

In recent years,with the continuous development of anti-stealth radar technology,the higher requirements have been put forward for stealth performance of stealth materials which was used on weapons and equipment.The ideal stealth materials are not only required to have strong absorption capacity,wide absorption frequency bandwidth,but also required to have thin thickness,low density,good chemical stability and excellent mechanical carrying capacity.Graphene has a series of advantages,such as large specific surface area,low density,good chemical stability,high electronic conductivity,which made graphene very promising as a kind of potential microwave-absorbing material.However,when use the graphene alone as an absorbing material,its high electronic conductivity made it mismatch with the impedance of air and resulting in electromagnetic waves can be reflected easily on its surface,which cause the unsatisfactory microwave-absorbing performance.Therefore,how to combine dielectric loss type graphene with magnetic loss type magnetic material through the appropriate technical to prepare composite absorbing materials with multi-electromagnetic loss mechanism and special structure is in today's challenging technical problems.This paper embarks from the material structure design and the reduced graphene oxide microspheres with hollow structure were designed.For the first time,we have introduced the magnetic nanoparticles into the hollow reduced graphene oxide microspheres by using self-assembly technique which combined with water-in-oil emulsion technique and high-temperature calcination process,and three kinds of graphene composite microspheres with different magnetic nanoparticles were prepared successfully(Air@rGO€Co,Air@rGO€Ni,Air@rGO€Fe3O4).X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscope(TEM).X-ray photoelectron spectroscopy(XPS)and vibrating sample magnetometer(VSM)were used to investigate the phase structure characteristics,microstructure morphology,surface chemical composition and magnetic response characteristics of reduced graphene oxide composite microspheres,respectively.Furthermore,the electromagnetic properties of the hollow composite microspheres were measured by a vector network analyzer.The influence of the types and content of magnetic nanoparticleson absorbing performance and change rule of Air@rGO€Co,Air@rGO€Oi,Air@rGO€Fe34 composite microspheres in the frequency range of-18 GHzwere discussed through Matlab software.Finally,the electromagnetic waves loss mechanism of hollow composite microspheres was studied.The results show that the prepared composite microspheres have excellent absorption properties and corrosion resistance,and their absorption intensity and effective absorption bandwidth are much larger than that of pure reduced graphene oxide and pure magnetic nanoparticles.The Air@rGO€Co composite microspheres reached optimum absorption pertformance when the matching thickness was 2.2mm,and the minimum reilection loss(RL)value reaches-68.1 dB with an absorption bandwidth of 7.1 GHz.After soaked in hydrochloric acid solution for 5 days,the saturation magnetization of Air@rGO€Co composite microspheres decreased by only 7.5%.When the thickness was 2.8mm,the absorbing performance of hydrochloric acid treated Air@rGO€Co composite microspheres was the best,the minimum RL value reached-50.4dB,and the absorption bandwidth is 6.1 GHz.The minimum RL value of the Air@rGO€Ni composite microspheres value reaches up to-59.7 dB with a matching thickness of 2.7 mm,and the absorption bandwidth is 4.8 GHz.After soaked in hydrochloric acid solution for 5 days,the saturation magnetization of Air@rGO€Ni composite microspheres was decreased by 4.8%.When the matching thickness was 3.0mm,the optimum absorption performance of hydrochloric acid treated Air@rGO€Ni composite microspheres was obtained.The minimum RL value reached-48.3dB,and the bandwidth is 4.2 GHz.Air@rGO€Fe3O4 composite microspheres exhibit good ability of microwave absorption with a match thickness of 2.8 mm,the mimmum RL value reaches-52 dB,and the absorption bandwidth(lower than-10 dB)is 7.2 GHz.The saturated magnetization of Air@rGO€Fe3O4 composite microspheres decreased by only 6.5%after immersed in 0.1mol/L hydrochloric acid solution for 5 days.When the matching thickness was 3.5mm,hydrochloric acid treated Air@rGO€Fe3O4 composite microspheres rached the optimum absorption performance.The minimum RL value reached-46dB and the absorption bandwidth(lower than-10 dB)is 6.0GHz.The main reason for the excellent microwave absorbing properties of reduced graphene oxide composite hollow microspheres with magnetic nanoparticles is that due to its unique reduced graphene oxide-embedded magnetic nanoparticles.The reduced graphene oxide has a suitable electrical conductivity,which makes it more resistance loss than the graphene oxide.The good conductivity of reduced graphene oxide suppresses the eddy current of the magnetic particles and increases the matching characteristics of the composite microspheres.The electromagnetic loss mechanism mainly includes:interfacial polarization between graphene and magnetic particles and magnetic particles and magnetic particles;resistance loss,Debye relaxation,Maxwell-Wagner relaxation,electron/ion polarization and dipole polarization of graphene;electron polarization,natural resonance and exchange resonance of magnetic nanoparticles.According to the principle of impedance gradient,the design principle of the simultaneous improvemrnt of absorbing performance,mechanical properties and weight loss function,we have builded the structural composite with gradient microwave absorbing properties,which constituted with the aramid fiber reinforced bismaleimide resin which as the through microwave layers,and the carbon fiber reinforced bismaleimide resin which as the reflecting microwave layer,and the composite microspheres with different kinds of magnetic nanoparticles/bismaleimide resin/aramid fiber composites which as the absorptive layer.The influence of types and content of omposite microsphereson absorbing performance and change rule of the composite plate were studied.The results show that the absorption intensity of the composite plate increases and then decreases with the increase of the content of the composite microspheres.The sample with the best absorption performance has a minimum value of-71.6 dB at 8.6 GHz,and RL value is less than-10 dB in frequency range of 6.5-18GHz and the effective absorption bandwidth is 11.5 GHz.The above research provides a new design idea for the preparation of "thin,light,wide,strong" absorbing composite material.