Preparation And Microwave Absorbing Properties Of Nitrogen-doped Graphene-based Composite Aerogels

With the rapid development of 5G communication technology and the wide use of electronic devices,microwave is closely related to our life.The widespread microwave not only pollutes the environment,but also does harm to our health.Therefore,the development of high-performance microwave absorbing materials is an urgent problem.Graphene aerogels are candidates for microwave absorption materials because of their unique three-dimensional porous network structure,extremely low density and high specific surface area.However,the single graphene aerogel has poor impedance matching and a single microwave absorption attenuation mechanism,which limits its application in microwave absorption field.In this paper,nitrogen-doped graphene-based magnetic/dielectric composite aerogels with three-dimensional porous network structure were obtained by hydrothermal method,solvothermal method and high temperature calcination,etc.The phase structure,elemental composition,graphitization degree,microstructure,electrical conductivity and electromagnetic parameters of the samples were characterized by changing nitrogen doping,calcination temperature and composition,etc.It provides some reference value for the construction and application of light microwave absorbing materials.The main research results of this paper are as follows:（1）Nitrogen-doped graphene/cobalt-zinc ferrite(NRGO/Co_0.5Zn_0.5Fe₂O₄)composite aerogels were prepared by solvothermal and hydrothermal two-step method.The effect of nitrogen doping amount on microwave absorption performance of composite aerogel was investigated.The phase structure,graphitization degree,microstructure,electrical conductivity and electromagnetic parameters of the obtained NRGO/Co_0.5Zn_0.5Fe₂O₄ composite aerogels were characterized,and the microwave absorption capacity was analyzed.The results show that the composite aerogel has a very low density(～12.1-14.6 mg·cm^-3)and a unique three-dimensional（3D）network porous structure.Co_0.5Zn_0.5Fe₂O₄ microspheres were uniformly loaded on the surface of the flake NRGO.In addition,the composite aerogels have excellent mechanical properties.When the nitrogen doping amount is 2.5 wt.%,the composite aerogel has the best electromagnetic wave absorption capacity.When the filling ratio was 15 wt.%（paraffin matrix）,the minimum reflection loss（RLmin）was-66.8 dB when the matching thickness is 2.6 mm.In addition,when the matching thickness is 1.6 mm,the composite aerogel has the widest effective bandwidth（EAB,RL≤-10 dB,absorption capacity reaches more than 90%）is 5.0 GHz.Electromagnetic absorption performance is the result of this enhanced nitrogen doping introduced structural defects,magnetic microspheres and graphene sheets produced by heterogeneous interface structure between interfacial polarization effect,unique 3D channel structure optimization of impedance matching and extension of the microwave transmission path and enhancing microwave energy attenuation,conductance loss,polarization loss and magnetic loss of synergy,etc.（2）Nitrogen-doped graphene/nickel ferrite（NRGO/NiFe₂O₄）composite aerogels were prepared on the basis of the relatively narrow effective bandwidth of the previous system.Firstly,NRGO/NiFe₂O₄ composite aerogels were prepared by solvothermal and hydrothermal two-step method.The effects of calcination temperature on the phase structure,graphitization degree,microstructure,electromagnetic parameters and microwave absorption capacity of the composite aerogels were further studied.The results show that the prepared NRGO/NiFe₂O₄ composite aerogels have a unique 3D network porous structure,and can expand the effective bandwidth and reduce the matching thickness.When the filling ratio is 10 wt.%and the matching thickness is 1.55 mm,the RLmin is-60.6 dB.When the matching thickness is 1.62 mm,the EAB of the NRGO/NiFe₂O₄ composite aerogel is 5.52 GHz.The excellent microwave absorption performance was mainly attributed to the synergistic effects of impedance matching,magnetic loss,conductance loss,dipole polarization and interface polarization,etc.（3）In the fourth chapter,nitrogen-doped graphene/SnO₂（NRGO/SnO₂）composite aerogels were prepared.By changing the amount of nitrogen doping in the system,the influence of nitrogen doping amount on the microwave absorption capacity of the composite aerogel was studied.The phase structure,elemental composition,graphitization degree,microstructure and electromagnetic parameters of the obtained NRGO/SnO₂ composite aerogels were characterized.The results show that the composite aerogel has a unique 3D porous network structure,and the SnO₂ nanoparticles were uniformly loaded on the NRGO lamellae.When the filling ratio is 30 wt.%and the matching thickness is 3.5 mm,the RLmin is-62.3 dB,and the EAB is 5.1 GHz（12.9-18 GHz）when the matching thickness is 1.6 mm.The excellent microwave absorption ability of composite aerogels is mainly due to the synergistic effects of conductive loss,dielectric loss,dipole polarization,unique three-dimensional pore structure optimized impedance matching,extended microwave transmission path and enhanced microwave energy attenuation,etc.（4）Based on filling in the higher shortcomings in the previous chapter,the introduction of rare earth oxides,nitrogen doped graphene/cerium oxide preparation（NRGO/CeO₂）composite aerogel microwave absorbing materials,by changing the system of mediation of electric materials research on phase structure of composite aerogel,graphitization degree,element composition,microstructure and the influence of electromagnetic parameters,etc.,it was found thatThe composite aerogel has a unique three-dimensional porous network structure.In addition,the CeO₂ nanoparticles have a cubic morphology and are anchored to the surface of the NRGO lamellae.The prepared NRGO/CeO₂ composite aerogels can reduce the filling ratio to paraffin and expand the effective bandwidth.When the filling ratio is 20 wt.%and the matching thickness is 4 mm,the RLmin is-50.0 dB,and the EAB is 5.7 GHz（12.3-18 GHz）when the matching thickness is 1.9 mm.It covers almost the entire Ku band.The excellent microwave absorption performance was mainly attributed to the synergistic effects of impedance matching,unique oxygen vacancy,conductance loss,dipole polarization and interface polarization of rare earth compounds.Figure[49]Table[8]Reference[135]...