Design And Electromagnetic Properties Of Three-Dimensional Graphene-based Absorbing Materials

With the increasing use of electromagnetic(EM)waves in radars,advanced detectors,and precision weapons,the EM wave pollution has become increasingly serious.Therefore,there is an urgent need in investigating EM wave absorbing materials.EM wave absorbing material is a kind of functional material that can effectively absorb the incident EM waves and convert the EM energy into heat or other forms of energy.The ideal wave-absorbing material would combine high absorption,wide frequency band,low density,thin thickness,and good environmental stability.However,the traditional ferrite-based wave-absorbing material can difficultly meet the above-mentioned requirements because of their high density,narrow absorption bands and too weak absorption.The unique two-dimensional structure of graphene gives it a large specific surface area that promotes scattering and multiple reflections of electromagnetics waves,and improves their absorption.Plenty of studies have confirmed that single-loss type or single-component absorbers can difficultly ensure high absorption in the 2-18 GHz range.Multi-loss matching and multi-component absorption are excellent strategies to possibly solve this problem.In this paper,graphene and modified graphene were combined with other types of absorbing materials to construct graphene based three-dimensional structure composite,and their microwave absoption performance has been studied.The main research contents are as follows:Firstly,porous BiFeO3 microspheres and three-dimensional porous BiFeO3 microsphere-reduced graphene oxide(RGO)composite(3D porous BiFeO3/RGO)were prepared by one-step etching processing using pure BiFeO3 particles as precursors.Electromagnetic(EM)absorption performance measurements exhibit that at low thickness of 1.8 mm,porous BiFeO3/RGO composite can achieve reflection loss(RLmax)value up to-46.7 dB and absorption bandwidth(defined by RL?-10 dB)exceeding 4.7 GHz(from 12.0 to 16.7 GHz),testifying oustanding microwave absorbing performance.Compared with pure porous BiFeO3,improved electromagnetic wave absorption ability of as-prepared porous BiFeO3/RGO composite is attributed to interfacial polarization,multiple reflections,scattering,and appropriate impedance matching.Secondly,covalent bond polyaniline(PANI)/graphene aerogel(GA)was synthesized by hydrothermal and in-situ polymerization.The chemical bonding and micromorphology between PANI and GA were examined by several characterizations.Combining impedance match,synergistic effect,hierarchical nanostructure of PANI and GA,the hybrids exhibit evidently improving microwave absorption performances compared with GA.The PANI/GA shows a strongest reflection loss(RLmax)of-42.3 dB at 11.2 GHz with a matching thickness of 3 mm,and the corresponding absorption bandwidth(RL?-10 dB)is almost 3.2 GHz(8.7-11.9 GHz).Thirdly,hollow ZnSnO3 nanocrystals grown on nitrogen-doped graphene nanosheets(ZnSnO3-NRGO)has been successfully synthesized through calcination-hydrothermal two-step route.As a synergistic microwave absorber,ZnSnO3-NRGO exhibits remarkable microwave absorbing performances compared with ZnSnO3.The maximum reflection loss(RLmax)of ZnSnO3-NRGO(with low filler loading ration of only 10 wt%)can reach up to-40.8 dB at 17.6 GHz with an absorber thickness of only 2 mm and the efficient absorption bandwidth(RL exceeding-10 dB)is 3.2 GHz.Under the thickness of 3 mm,the RLmax of ZnSnO3-NRGO can reach-33.2 dB at 13.7 GHz with high-efficiency bandwidth(RL?-10 dB)of 6.4 GHz(from 11.4 to 18 GHz).The improving performances can be attributed to more reasonable impedance matching,higher attenuation capability,proper synergistic effect and multiple interfacial polarizations.Finally,the hierarchical NiFe2O4/N-GN/ZnO composite was successfully designed by growing ZnO nanoflowers on magnetically decorated graphene through a facile hydrothermal route.The microstructure,compositions and electromagnetic parameters of the obtained samples were investigated by various techniques.Results showed that NiFe2O4/N-GN/ZnO composite showed enhanced EM wave absorption properties compared with NiFe2O4/N-GN.With a low filler loading of 20 wt%,the strongest reflection loss(RLmax)was as high as-70.7 dB at 13.5 GHz and the bandwidth(RL over-10 dB)reached 3.5 GHz(11.7-15.2 GHz)with an absorber thickness of 2.7 mm.