| Graphene has the characteristics of ultra-high specific surface area,low density,high stability and good processability,so it is regarded as one of the most ideal"light and thin"microwave adsorption materials.However,pure graphene can not be directly used as absorbing materials because of its high conductivity which lead to the high reflection and low loss of microwave.The common solution is to combine with low dielectric polymer compounds or magnetic loss magnetic nanoparticles.But there is no doubt that the absorbing material will become thick and heavy,so that it can not be applied to the national defense safety and the eli mination of stray current interference in aerospace and aeronautics.How to change the structure and properties of graphene to achieve high absorption characteristics of single lightweight materials is the bottleneck of current research.Using the reductant containing hetero-atoms,the heteroatom doped graphene was prepared by partial reduction of graphene at low temperature and liquid phase in this paper.It is found that the concentration of polar functional groups and defects on the surface of reduce graphene oxide can be effectively adjusted by changing the amount of reducing agent,so that the conductivity and dielectric properties of graphene can be adjusted.By optimizing the polarization between the interfaces of graphene and the polarization of dipole orientation,the frequency band and intensity of electromagnetic wave absorption can be controlled by the optimization of the polarization between the interfaces of graphene and the dipole orientation.The quantitative relationship between the type of doped atoms,the concentration and electromagnetic parameters of graphene-based materials,the absorption frequency band and absorption capacity is established,and the controllable preparation of absorbing materials is realized.Multi-functional absorbing materials with high absorptivity,wide absorption band,light weight,thin thickness,excellent mechanical properties,high temperature resistance and low cost were prepared.（1）Using hydrazine hydrate as a reducing agent,N-doping of graphene was realized by wet chemical reduction of GO.Due to the existence of electron-rich nitrogen atom,the conductivity of N-GS increased by one order of magnitude than H-GS.The mechanism of electromagnetic loss is dielectric absorption,which is dominated by relaxation losses such as electron polarization,electric dipole polarization,and so on.With the attenuation constant and the composite dielectric constant increased,the real part of the dielectric constant varie d from 26.3 to 15.7 in the range of 2-18 GHz,and the imaginary part changed from 21.7 to 7.8.The microwave absorption ability of N-GS is higher than H-GS.In the range of 6.5-18GHz,the reflection loss is less than-10 d B,that is,it can effectively absorb more than90%electromagnetic wave,which can meet the basic requirements of practical applications.However,it is easy to dope into the structure of graphene surface layer in the reduction process because of the similar radius between N atom with C atom.And the concentration of reducing agent has little effect on the doping degree,so,as the quality of reducing agent is changed,the conductivity and absorption of N-doped graphene changed slightly.（2）B atom doping on graphene was finished by wet chemical reduction of GO with sodium borohydride as a reducing agent.The effects of the quantity of sodium borohydride on the conductivity,electromagnetic parameters and absorbing properties of graphene were investigated.XRD、FT-IR、Raman and XPS were used to study the effect of sodium borohydride addition on graphene conductivity,electromagnetic parameters and wave absorption properties.The structure,morphology and absorbing properties of B-GS were characterized by TEM and vector network analyzer.The results showed that the electron-deficient B-GS can effectively adjust the conductivity to the optimal value.Moreover,it also increased the interface polarization and achieved impedance matching at the same time.So,its microwave absorption capacity could be improved effectively.Especially,the B-GS4 materials we prepared were optimal because the impedance matching and the attenuation ability were quite excellent.As for B-GS4,at the thickness of 2.5 mm,the effective absorption bandwidth touched to 2 GHz（6.96-8.96 GHz）when the reflection loss was less than-10 d B.The optimal absorption peak appeared at 7.84 GHz with the maximum reflection loss of-43.78 d B,which means that,the microwave absorption is more than99.99%.（3）S atom doping was achieved by wet chemical reduction of GO with Na2S as reducing dopant.The effects of the amount of Na2S on the conductivity,electromagnetic parameters and wave absorption properties of graphene were investigated.The results showed that S-GS can be used to absorb in microwave C,X and Ku bands.In the range of 2-18 GHz,the reflection loss of S-GS is up to-20 d B.In other words,the absorption intensity of electromagnetic wave is up to 99.99%,which is obviously much higher than B-GS.For S-GS4,at the thickness of 2.5mm,the optimal absorption peak appears at 8.24 GHz with the maximum reflection loss of-42.2 d B and the effective absorption bandwidth reaches to 2.32 GHz（7.28-9.60 GHz）when the reflection loss is less than-10d B,which means that,the microwave absorption is more than 99.99%.（4）In order to further analyze the effect of S doping on the structures and wave absorption properties of graphene,S atom doping was realized by chemical reduction of graphene oxide with sodium polysulphide as reducing agent,and the concentration and species of S doped functional groups were mainly investigated.The effects of paraffin wax content on the absorbing properties of S x-GS were studied.The results showed that with the increase of S doping concentration and the decrease of paraffin ratio,Sx-GS gradually changed from electromagnetic shielding characteristics to electromagnetic absorption characteristics.In addition,the optimum absorption band of Sx-GS shifted from low-frequency to high-frequency as the elemental S and sulfide are converted to thiophene and sulfonic acid on the surface of graphene lamella.The optimum absorption band of Sx-GS shifted from low frequency to high frequency. |
