| In practical application,the novel microwave absorption materials should have the wave-absorption performance requirements of "thin thickness,light weight,wide band and strong absorption".Among the many materials,graphene exhibits a series of advantages such as large specific surface area,light weight,high conductivity,good chemical stability,etc.In addition,its dielectric and electrical conductivity are easy to control so that it can be used as a microwave absorption material.However,the loss mechanism of pure graphene is single and the impedance mismatch is serious,so the microwave absorption performance is not ideal.At present,the research on graphene-based materials mainly focuses on the multi-component composite,that is,the introduction of dielectric materials or magnetic components to enrich the electromagnetic loss,but it also loses the low density and stability.In order to solve the above problems,we improved the electromagnetic absorption performance(EMA)of graphene and its composites through multi-scale structural optimization and light-weight non-metallic diatomic doping strategies.Specifically,we used graphene oxide(GO)as the matrix,carbon nanotubes(CNTs)as the functional phase,and nitrogen(N)/sulfur(S)/boron(B)atoms as target heteroatoms for doping,preparing a series of three-dimensional graphene oxide materials via hydrothermal method and freeze-drying technology.X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),transmission electron microscope(TEM),Raman spectroscopy(Raman),and other traditional materials characterization methods were utilized to study the phase composition and microstructure of materials.The vector network analyzer(VNA)was used to test the electromagnetic parameters of materials.We systematically study the influence of microstructure changes and non-metallic atomic doping on the absorbing performance.The main work of this paper includes:(1)Nitrogen doped reduced graphene oxide with multi-scale structure was obtained by one-step hydrothermal method and water bath treatment(N-rGO@CNTs).Carbon nanotubes are used to regulate the microstructure of materials and introduce nitrogen atoms and a large number of defective structures through hydrothermal reactions.The results show that the dielectric property and electrical conductivity of N-rGO@CNTs are obviously improved,which makes the composite have good impedance matching,rich loss mode and strong attenuation ability.Among them,the effective absorption bandwidth of N-rGO@CNTs0.3 is up to 7.0 GHz in the thickness of 2.6 mm,the reflection loss is up to-49 dB,and the load is only 2 wt%.The multi-scale construction of graphene-based composites further expands the research and development ideas of pure carbon-based electromagnetic absorption materials.(2)The Nitrogen/Boron co-doped reduced graphene oxide(N/B-rGO)with high porosity and network structure was successfully prepared by a two-step hydrothermal synthesis and freeze-drying process,and exhibited excellent electromagnetic absorption performance at a loading capacity of only 5 wt%.When the thickness of N/B12-rGO is 2.8 mm,the minimum reflection loss reaches-52 dB@11.6 GHz,and the effective absorption bandwidth(EAB)is 6 GHz(9.7-15.7 GHz).The mechanism of N/B co-doping and the effects of various atomic bonds on the absorption properties were studied in detail.The results show that the C-N bond with the properties of electron donor(mainly pyridinic N and pyrrolic N),the C-B bond with the properties of electron acceptor and the C-N-B bond as the center of polarization have synergistic effects on improving the electromagnetic absorption performance.The diatomic doping has important guiding significance for exploring the electromagnetic absorption field.(3)Nitrogen,Sulfur co-doped reduced graphene oxide(NS-rGO)and Sulfur,Boron co-doped reduced graphene oxide(SB-rGO)were prepared by one-pot hydrothermal method and freeze-drying process.The experimental results and theoretical calculations show that the charge distributions of N,S,B atoms and their adjacent C atoms change significantly,which effectively regulate the electron transport and polarization states,thus affecting the electrical conductivity and dielectric response of the materials.Among them,N10S3-RGO and S20B10-RGO exhibit excellent MA performance due to their richer loss modes,stronger attenuation capacity and better impedance matching.Their RLmin was-51.9 dB and-54.2 dB,respectively,and their effective absorption bandwidth was up to 6 GHz(7.8-13.8 GHz,3.6 mm)and 4 GHz(10.1-14.1 GHz,2.6 mm),while filling volume was only 2 wt%and 1.5 wt%,respectively.This indicates that the non-metallic diatomic doping strategy is very feasible and reliable for the development of light and efficient pure graphene-based microwave wave-absorption materials. |
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