| In recent years,according to the problems of electromagnetic interference and radiation,and military requirements,electromagnetic wave(EMW)absorption field grows in leaps and bounds.The basic theory is to absorb and attenuate incident EMWs,adiating them in the main form of heat energy.The loss mechanisms of absorber encountered essentially of three groups:magnetic loss,dielectric loss and compound loss.In this paper,magnetic-loss type Fe3O4 hollow spheres,compound-loss type Fe3O4 hollow spheres/rGO,modified compound-loss type KH550@Fe3O4 hollow spheres/rGO,and compound-loss type Ni particle/rGO as EMW absorbing materials are studied in detail.First,in chapter 3,a kind of unique monodisperse Fe3O4 hollow sphere is prepared by the solvothermal method.As magnetic-loss type absorbing material,it absorbs and attenuates incident EMWs through hysteresis loss,natural resonance,eddy current loss and so on The density of Fe3O4 hollow sphere is significantly lower than that of corresponding solid sphere,making it lightweight under the same volume condition,which meets the requirement of light about absorber.Generally,there are four excellent features of "light","thin","strong" and "wide" on absorber.As a transition metal oxide,the dielectric loss of Fe3O4 hollow sphere is usually low,leaving room for further improvement on wave absorbing performance.Two-dimensional sheet structure of graphene determines its excellent properties.As a dielectric-type absorbing material,EMWs are mainly attenuated through dipole polarization.However,the attenuated way of graphene is sigle with unsatisfactory results.Therefore,in chapters 4,5,and 6,compound-loss type composites are designed and prepared,which through both magnetic loss and dielectric loss to be outstanding in absorbing performance.The composites can overcome the drawbacks such as high density,excessive matrix addition,be prone to corrosion and low high-temperature resistance of traditional absorbers.Fe3O4 hollow sphere,as a small-size magnetic particle,is easy to agglomerate.As a result,in Chapter 5,the silane coupling agent KH550 is introduced by a physical-stirring method to cover the sphere with a thin film,which can solve the above problem,making it disperse well and with uniform morphology.In Chapter 6,Ni particle,as a magnetic material,is selected to substitute Fe3O4 hollow sphere.Then,it is composited with graphene by solvothermal method.This experiment may help to explore the rule of magnetic material/graphene composites.The conclusions are drawn from the above studies:1.The size(700?220nm)and surface of Fe3O4 hollow spheres can be adjusted by different urea contents,which realizes the morphology control of "big to small" and "smooth to rough".Fe3O4 hollow spheres prepared with 0.067g/ml urea(F2)achieved a maximum reflection loss(RL)of-56.90 dB at 8.32 GHz with the broadband of 4.68 GHz.Besides,Fe3O4 hollow sphere may be widely applied not only in EMW absorption but also in energy storage and biomedical field.2.Fe3O4 hollow sphere/rGO composites are prepared with different proportions and reacting time.The sample FG21-7 has the best performance.The compound-loss type composites have both magnetic loss and dielectric loss,EMW absorption is improved.3.In order to further promote the sphere dispersion and adhesion to rGO matrix,the silane coupling agent KH550 is selected as a surface modifier on Fe3O4.The sample FG21-7 has the best performance with a maximum reflection loss(RL)of-57.87 dB at 14.96 GHz with the broadband of 4.5 GHz(from 13.1 GHz to 17.6 GHz).In particular,the thickness is only 1.48 mm,demonstrating the ultra-thin and high-intensity EMW absorption capability.This special core-shell/coating structure can be used for devising new and effective applications.4.Ni particle/rGO composites are prepared with different proportions and reacting temperature.The sample NG3 has the best performance.At different thicknesses of 4.51,3.40,and 2.90 mm,NG3 has maximum reflection losses(RL)of-47.73 dB,-53.27 dB,and-52.49 dB,respectively.The effective absorption bandwidth(?10 dB)spans C and X wave band,showing wide frequency and controllable thicknesses to high EMW absorption performance.Figure[48]table[13]references[109]... |
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