| The rapid development of high frequency and high speed 5G communication technology and highly integrated thin and intelligent electronic products has greatly facilitated people’s production and life,and the seriousness of electromagnetic radiation has also been highlighted.The normal operation of precision electronic devices will not only be affected by electromagnetic radiation,but also produce environmental pollution,and human health will be harmed,so it is necessary to seize the time to develop highperformance absorbing materials.Due to the high porosity,ultra-high specific surface area and adjustable topological structure,metal-organic frameworks can be used as ideal precursors for the preparation of carbon based magnetic composites.Carbon based magnetic composites derived from iron based MOFs have the advantages of simple preparation method,strong magnetic loss performance,good catalytic graphitization effect of iron and controllable morphology.Carbon matrix porosity,abundant heterogeneous interface structure and the synergy of dielectric loss and magnetic loss is expected to generate good impedance matching and microwave attenuation effect,by introducing dielectric zinc oxide,conductive graphene,type changed calcination temperature and salt,realize effective control of composite wave absorption performance,for MOFs provides the theory basis for application in the field of microwave absorption.Based on single/double/three metal MOFs precursor synthesized by four different kinds of magnetic carbon composites was studied to introduce dielectric ZnO conductive graphene change calcination temperature salt type on the preparation of composite materials by phase structure degree of graphitization microstructure of the influence of the magnetic electromagnetic parameters and microwave absorbing properties,and to explore the microwave attenuation mechanism of the absorbing agent.It provides theoretical basis and scientific value for the synthesis of mono/bi/trimetal MOFs derived magnetic carbon composites:(1)In the second chapter,ZnO with flower-like morphology was firstly synthesized,then ferric metal-organic frameworks(Fe-MOFs)was grown on ZnO with flower-like ZnO template,and ZnO/Fe-MOFs precursor was synthesized by solvothermal method.Finally,ZnFe2O4/C composite was obtained by high temperature pyrolysis.The electromagnetic parameters and microwave absorption properties of the composites could be effectively adjusted by changing the amount of ZnO in the precursor.The results showed that the composites have a unique flower structure,most of the regular octahedral carbon framework were evenly attached to the surface of the flower morphology,and a few of the regular octahedral carbon framework were embedded in the interior of the flower morphology.It was found that the addition amount of ZnO has a great influence on the electromagnetic parameters and microwave attenuation performance.When ZnO content was 60 mg,the composite exhibits excellent microwave absorption performance,that was,when the filling ratio was 45 wt.%,the minimum reflection loss(RLmin)reached-60.0 dB at 1.85 mm thickness.The effective absorption bandwidth(EAB)was up to 5.3 GHz at 1.67 mm thickness.The excellent microwave absorption performance was mainly attributed to the synergy of impedance matching,magnetic loss,conductance loss,dipole polarization and interface polarization.(2)In the third chapter,Fe3O4/C composites derived from FeZn bimetallic organic framework were synthesized by solvotherm-high temperature pyrolysis using bimetals(Fe3+ and Zn2+)as metal sources on the basis of single metal-organic framework in Chapter second.The effects of calcination temperature on the composition,phase structure,graphitization degree,microstructure,magnetic properties,electromagnetic parameters and microwave absorption properties of the materials were studied.It was found that by adjusting the calcination temperature,the morphology of carbon frame could gradually change from the traditional regular octahedron to porous spherical shape,and at the same time,the static magnetic properties and electromagnetic parameters of the composite could be adjusted,so as to effectively regulate the microwave absorption properties of the composite.When the calcination temperature was 700℃,when the filling ratio was 40 wt.%and the thickness was 2.5 mm,the RLmin reached-60.0 dB,and the X-band(8.0-12.0 GHz)full absorption was achieved at this thickness.Under the thickness of 1.85 mm,the microwave absorption intensity was less than-10.0 dB in the range of 12.6-18.0 GHz,and the EAB reached 5.4 GHz,achieving 90%coverage of Ku band(12.0-18.0 GHz).By adjusting the calcination temperature and matching thickness,the synergy of impedance matching,interfacial polarization,dipole polarization,magnetic resonance loss and other physical mechanisms could be realized to achieve effective attenuation of microwave and effective absorption of microwave in different bands.(3)In the fourth chapter,carbon materials were introduced on the basis of the shortcomings of the first two chapters:narrow effective absorption bandwidth and relatively high filling.Graphene oxide(RGO)as template,Ferric chloride hexahydrate(FeCl3 6H2O)and Zinc chloride(ZnCl2)as metal salt precursor,terephthalic acid(H2BDC)as organic ligand,N,N-dimethylformamide(DMF)as organic solvent.FeZn bimetal MOFs derived Fe3O4/C modified graphene composite absorbing material was prepared.The results showed that the morphology of carbon framework changes gradually from regular octahedron to pomegranate shape by simply changing the molar ratio of Fe and Zn in the precursor.At the same time,the microwave absorption properties of the composites were excellent,with low filling ratio,thin matching thickness,high absorption intensity,wide absorption band,and easy to control the absorption properties.When the filling ratio was 20 wt.%,under the thickness of 2.76 mm,the RLmin reached-79.0 dB.Under the thickness of 1.82 mm,the microwave absorption intensity in the range of 12.2-18.0 GHz was below-10.0 dB,that was,the EAB reached 5.8 GHz,almost covering the Ku band.At the same time,the effective absorption of microwave in different wavebands can be achieved by adjusting the molar ratio of iron salt and zinc salt in the precursor and the matching thickness.(4)Chapter the fifth builds on the previous three chapters on the single/bimetallic organic framework,the FeNi alloy/carbon composite derived from trimetallic organic framework was synthesized by solvothermal and high temperature pyrolysis by combining different types of three metal salts,the three kinds of metal salts of different kinds of salt were combined,the synthesis of three metal organic framework to derive the magnetic carbon composites,of different kinds of salt with metal salts on the morphology and the influence of microwave absorption performance.The results showed that the different kinds of metal salts have a great influence on the morphology of the product by combining the three kinds of metal salts.It was found that when the three metal salts were ferric chloride tetrahydrate(FeCl2·4H2O),zinc nitrate hexahydrate(Zn(NO3)2·6H2O),and nickel nitrate hexahydrate(Ni(NO3)2·6H2O),the product had a unique hollow core-shell structure.In this chapter,the influence of calcination temperature on composite absorbing materials was also studied.It was found that different calcination temperature has great influence on the morphology and microwave absorption properties of samples.When the three metal salts were FeCl3·6H2O,ZnCl2 and NiCl2·6H2O respectively,and the calcination temperature was 900℃,the morphology of the composite was porous microspheres on the surface,and it has realized the characteristics of thin thickness,strong absorption and wide frequency band.At this time,the microwave absorption performance was the best.In other words,RLmin reached-66.4 dB under 1.35 mm thickness,and strong absorption could be achieved under low thickness.At the same time,under the thickness of 1.4 mm,the microwave absorption intensity was below-10.0 dB in the range of 12.3-18.0 GHz,and the EAB reached 5.7 GHz,almost completely covering the Ku band.It was found that the effective absorption of microwave in different wave bands could be realized by changing the salt type,calcination temperature and matching thickness.Meanwhile,the synergistic effect of dielectric loss,conductance loss and magnetic loss could be effectively enhanced through interface polarization,dipole polarization,defect polarization and other mechanisms.Figure[67]Table[5]Reference[141]... |
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