Preparation And Microwave Absorption Properties Of Graphene-based Magnetic/Dielectric Composite Aerogels

In recent years,the emergence of convenience,fast and intelligent electronic equipment has greatly facilitated people’s production and life with the rapid development of 5G communication technology.But the large amount of electromagnetic radiation released by electronic equipment will not only reduce the service life of the instrument itself,but also endanger human health.Therefore,it is of great significance to design microwave absorbing materials with thin thickness,small density,strong absorption and wide absorption band under the background of the rapid development of information technology.Reduced graphene oxide（RGO）was the most common graphene matrix,which has the characteristics of two-dimensional（2D）layered structure,low density,large specific surface area and excellent mechanical strength.Moreover,the surface of RGO has some oxygen-containing groups such as hydroxyl and carboxyl groups,which can produce dielectric loss.Therefore,RGO has been considered as a potential candidate for electromagnetic attenuation.However,the electrical conductivity of RGO is too high to meet the requirements of impedance matching,resulting in poor absorption and strong reflection of incident waves,which seriously hinders its practical application in the field of microwave absorption.The studies have shown that the self-assembly of2D RGO nanosheets into three-dimensional（3D）reticular RGO aerogel can not only inhibit the aggregation of RGO nanosheets,but also reduce the bulk density and improve the electrical conductivity.Besides,there are multiple reflections and scattering inside the aerogel,and the unique porous network structure improves the impedance matching between the air and the absorber,making RGO aerogel promising as a lightweight microwave absorber.Hence,a graphene-based composite aerogel material with three-dimensional porous structure was designed and prepared in this paper.And using X-ray diffractometer,Raman spectroscopy,X-ray photoelectron spectroscopy,scanning electron microscope,Transmission electron microscope,Vibrating sample magnetometer,Vector network analyzer analytical techniques to characterize such as composite phase structure,the degree of graphitization of the aerogel,elemental composition,microstructure,magnetic properties and electromagnetic parameters.By modifying spinel ferrite with magnetic loss,zinc oxide with dielectric loss and multi-walled carbon nanotubes with electrical conductivity in a three-dimensional meshstructure,the material has multiple loss mechanisms.Under the synergistic effect ofmultiple loss mechanisms,the impedance matching is effectively improved and the effective absorption bandwidth is widened,so as to prepare high-performnce microwave absorber.The results of this paper will provide reference value for the performance regulation and structural design of excellent 3D porous graphene-based wave absorbing materials,as well as the study of wave absorbing mechanisms.The main research contents are as follows:1.In the second chapter,reduced graphene oxide/zinc oxide（RGO/Zn O）composite aerogels were prepared by one-step hydrothermal method.The aerogels had ultra-low density(8.9-9.7 mg·cm^-3)and unique three-dimensional network structure,and zinc oxide was evenly distributed on the folded graphene sheets.In addition,by exploring the effect of sodium citrate addition on the morphology and absorbing properties of Zn O,it is found that with the increase of sodium citrate addition,the morphology of Zn O gradually evolves from irregular polygon to hexagon and then to pentagon.When the addition amount of sodium citrate is 0.8 g,zinc oxide presents a regular hexagonal morphology.When the filling ratio was 20 wt.%（the matrix is paraffin）and the matching thickness was 2.49 mm,the minimum reflection loss(RL_min)achieved-79.1d B.When the matching thickness was 1.97 mm,the maximum effective absorption bandwidth(EAB_max)reached up to 7.7 GHz（10.3-18.0 GHz）.The microwave absorption mechanism of RGO/Zn O composite aerogel was investigated and found that RGO defects,hydroxyl and carboxyl functional groups on the surface of the dipole polarization,caused by space charge accumulation of heterogeneous interface between RGO and Zn O,resulting in the interfacial polarization,and electric conduction loss of synergy,enhanced composite aerogel together on the microwave attenuation.2.In the third chapter,hollow magnesium ferrite microspheres were prepared by solvothermal method,and Nitrogen-doped reduced graphene oxide/hollow magnesium ferrite（NRGO/hollow Mg Fe₂O₄）composite aerogels were prepared by hydrothermal method and the freeze-drying technology.The results show that hollow Mg Fe₂O₄microspheres inlaid evenly on graphene laminates can generate heterogeneous interface between NRGO and hollow Mg Fe₂O₄,enhance the interface polarization,which was conducive to microwave attenuation.The electromagnetic parameters and absorbing properties of composite aerogel were significantly affected by changing the amount of graphene added.When the amount of graphene added was 90 mg,the filling ratio was15 wt.%,the RL_min of composite aerogel was-55.7 d B at the matching thickness was1.98 mm,while the matching thickness was 2.29 mm,the EAB_max achieved 7.1 GHz（10.9-18.0 GHz）.In addition,the wave absorption mechanism of the composite aerogels were also discussed,including good impedance matching,moderate attenuation constant,synergistic effect of conductance loss,dielectric loss and magnetic loss.3.In the fourth chapter,hollow nickel ferrite microspheres were prepared by solvothermal method,and nitrogen-doped reduced graphene oxide/muti-walled carbon nanotubes/hollow nickel ferrite（NRGO/MWCNTs/hollow Ni Fe₂O₄）composite aerogels were prepared by one-step hydrothermal method and vacuum freeze-drying technology.The results show that the meshed NRGO,hollow Ni Fe₂O₄ microspheres and MWCNTs were connected and entangled together,and a rich heterogeneous interface was generated among the three,which greatly enhances the interface polarization.In addition,the ternary NRGO/MWCNTs/Ni Fe₂O₄ composite aerogels present the best microwave absorption performance compared with the unit NRGO aerogels,binary NRGO/MWCNTs and NRGO/Ni Fe₂O₄ composite aerogels.When the filling ratio was 12.5 wt.%and the matching thickness was 2.1 mm,the RL_min and reached up to-69.0 d B,the matching thickness was 2.0 mm EAB_max achieved 6.7 GHz（11.3-18.0 GHz）,respectively.4.In the fifth chapter,Ferric metal organic framework（Fe-MOFs）precursor was firstly prepared by the solvothermal method,and then Fe-MOFs precursor derived ferroferric oxide/iron/carbon composites were obtained by high-temperature pyrolysis.Finally,nitrogen-dopedreducedgrapheneoxide/ferroferric oxide/iron/carbon（NRGO/Fe₃O₄/Fe/C）composite aerogels were prepared by hydrothermal method.By exploring the absorbing properties of NRGO/Fe₃O₄/Fe/C composite aerogel at different filling ratios and comparing it with NRGO aerogel and Fe-MOFs precursor derived Fe₃O₄/Fe/C composites at the optimal filling ratio,NRGO/Fe₃O₄/Fe/C composite aerogel had the best microwave absorption performance when the filling ratio was 15 wt.%.When the thickness was 3.01 mm,the RL_min reached up to-48.2 d B,and when the thickness was 2.34 mm,the EAB_max was 7.9 GHz.In addition,the possible microwave absorption mechanism of NRGO/Fe₃O₄/Fe/C composite aerogels were speculated to be the synergistic effect of dipole polarization,interfacial polarization,conduction loss and magnetic loss.Figure[50]Table[8]Reference[146]...