Synthesis And Microwave Absorption Properties Of N-doped Graphene/magnetic Nanocrystals Hybrids

Graphene,as the world's lightest two-dimensional material,has several remarkable physical properties,such as light weight,specific surface area and high dielectric constant.These unique properties make graphene have potential applications in the fields of supercapacitors,fuel cells,absorbing materials and so on.Especially,the graphene-based materials have attracted ever-increasing attention as microwave absorbers.In order to meet the requirements of light weight,thin thickness,absorption frequency bandwidth and strong absorption for novel absorbing materials,this thesis focus on the fabrication of nitrogen-doped graphene decorated by magnetic nanoparticles,which is promised to be beneficial to improve the dielectric loss and magnetic loss of materials.Besides,doping nitrogen into the graphene can effectively tune the complex permittivity of hybrids,which is helpful to achieve better impedance matching conditions and enhance the microwave absorption properties of hybrids.(1)Synthesis and microwave absorption properties of nitrogen-doped graphene nanosheetsNitrogen-doped graphene nanosheets(N-GN)was successfully prepared by a simple hydrothermal method,and their morphologies,crystal structure and chemical elements were characterized in detail.Electron microscopy shows that N-GN and graphene(GO)are transparent with paper-like structure.In comparison with GO,the curl wrinkles of the edge for N-GN are significantly increased,which is attributed to the enhanced defect structure caused by doping nitrogen atoms.The microwave absorption results show that the maximum reflection loss(RL)of N-GN/paraffin composites with 30 wt%of N-NG can reach-34.8 dB at 6.0 GHz with a thickness of 3.5 mm.Besides,an effective absorption bandwidth(RL<-10 dB)of 14.6 GHz(3.4-18 GHz)can be achieved when the thickness of absorber is in the range of 1.5-5.0 mm.(2)Synthesis and microwave absorption properties of nitrogen-doped graphene/Fe3O4 hybridsNitrogen-doped graphene/ferrite(N-GN/Fe3O4)hybrids were fabricated using a simple hydrothermal method and their microwave absorption properties are investigated.Their morphologies,crystal structure were characterized by XRD,SAED and TEM.The microstructure of graphene was analyzed by Raman spectroscopy.Their surface chemical composition and elemental state were investigated by XPS.Their magnetic properties were analyzed by VSM.Furthermore,the electromagnetic parameters were characterized by vector network analyzer.These results show that Fe3O4 nanoparticles are uniformly dispersed on the surface of N-GN.The microwave absorption properties indicate that the N-GN/Fe3O4 hybrids show greatly enhanced microwave absorption properties in comparison with pure Fe3O4 nanocrystals.The N-GN/Fe3O4 hybrids show maximum RL value of-65.3 dB at 6.8 GHz with a thickness of 3.4 mm,and the effective absorption bandwidth(RL<-10 dB)is 14.5 GHz(3.5-18 GHz)with the absorber thickness of 1.5-5.0 mm.(3)Synthesis and microwave absorption properties of nitrogen-doped graphene/FeNi3 hybridsNitrogen-doped graphene/FeNi3(N-GN/FeNi3)hybrids were prepared by a simple hydrothermal method.The hybrids were characterized and analyzed systematically by TEM,STEM,XRD,Raman and XPS.The magnetic properties of the composites were analyzed by VSM,and the electromagnetic parameters of the composites were characterized by vector network analyzer.In the hybrids,FeNi3 nanostructures are spherical and hollow shape,which are relatively uniformly dispersed on the surface of N-GN.Combining the magnetic loss from magnetic FeNi3 nanoparticles and dielectric loss from N-GN,the hybrids show greatly improved microwave absorption properties.For the N-GN/FeNi3 hybrids,a maximum RL value of-8.6 dB can be obtained at 12.8 GHz when the matching thickness is only 1.9 mm.Besides,the effective absorption bandwidth(RL<-10 dB)can reach 14.7 GHz(3.3-18 GHz)when the thickness of absorber is in the range of 1.5-5 mm.In summary,we reported a simple solvothermal method for the fabrication of nitrogen-doped graphene nanosheets,nitrogen-doped graphene/Fe3O4 hybrids and nitrogen-doped graphene/FeNi3 hybrids.The results indicate that the doping nitrogen atoms into the graphene nanosheets have several advantages for improving the microwave absorption properties.First,the doping nitrogen can enhance the defects on the graphene nanosheets,leading to high dielectric loss abilities.Second,the doping nitrogen atoms can tune the complex permittivity,which may create better impedance matching condition.Furthermore,combining the synergistic effect between magnetic nanocrystals and nitrogen-doped graphene,the hybrids show enhanced microwave absorption properties.