| With the development of 5G technology,electromagnetic(EM)capacitors for civil Internet of Things,anti-electromagnetic interference technology,and military stealth technology,microwave absorbing materials(MAMs)have received increasing attention.Carbon nanomaterials are potential MAMs due to the low density,excellent mechanical properties,good electrical conductivity and chemical stability.However,the single attenuation loss mechanism and poor impedance matching make the inferior microwave absorption performance.Metal-organic framework(MOF)is a kind of crystalline porous material with periodic.network structure,which has high porosity,large specific surface area and controllable microstructure.It has been found that carbon-based composites with good microwave absorption properties can be obtained by high-temperature pyrolysis of the MOF precursors in inert atmosphere.In this paper,four kinds of magnetic carbon composites with different morphology were prepared by the high-temperature pyrolysis of bimetallic/trimetallic MOF precursors.The effects of calcination temperature and material composition on the phase structure,graphitization degree,micromorphology,thermostability,magnetic properties,EM parameters and microwave absorption performance of the as-prepared composites were studied,and the microwave attenuation mechanisms were also explored,which could provide theoretical basis and practical reference for the design and development of MOF derived carbon-based light-weight MAMs.The main research contents were as follows:(1)In the first chapter,nitrogen-doped cobalt/cobalt oxide/carbon/reduced graphene oxide(Co/CoO/C/RGO)composites were prepared by a static aging and high-temperature pyrolysis method.The effects of pyrolysis temperature,material composition and filler content on the EM parameters and microwave absorption performance of the as-fabricated composites were explored.The results showed that the contracted rhombic dodecahedral carbon frameworks were uniformly loaded on the surfaces of crumpled sheet-like RGO,and CoO and Co nanoparticles were embedded in nitrogen-doped porous carbon frameworks.Besides,under the catalytic graphitization of metal Co nanoparticles,a lot of carbon nanotubes(CNTs)were generated in situ on the surfaces of carbon frameworks.The microwave absorption performance of composites was significantly enhanced by the addition of graphene oxide(GO).When the calcination temperature was 700℃ and the filler content was 25 wt.%(paraffin wax used as matrix),the minimum reflection loss(RLmin)value was-63.0 dB at the matching thickness of 1.6 mm and the effective absorption bandwidth(EAB)achieved 4.3 GHz at 1.5 mm.The interfacial polarization among multiple components,dipole polarization induced by nitrogen doping,the synergistic effects of antiferromagnetic CoO and magnetic Co,as well as conductive RGO and CNTs could enhance the microwave attenuation loss.(2)In the second chapter,magnetic Fe3O4 microspheres were first combined with leaf-like CoZn-MOF precursors,and then under the high-temperature pyrolysis process,nitrogen-doped CoFe alloy@C-CNTs composites were obtained.By changing the calcination temperature,the composition,phase structure,graphitization degree,magnetic properties,EM parameters and microwave absorption performance could be adjusted.The results demonstrated that the composites presented the morphology of four-pointed star stacked by leaf-like shape.When the calcination temperature was 700℃,the RLmin reached-54.5 dB at 1.4 mm and the EAB was up to 5.0 GHz at 1.6 mm.The excellent microwave absorption performance of the nitrogen-doped CoFe alloy@C-CNTs composites could be attributed to the magnetic loss from CoFe alloys,conduction loss originated from CNTs,dipole polarization induced by nitrogen doping,heterogeneous interfacial polarization(CoFe,CNTs and carbon matrix),as well as good impedance matching and moderate attenuation loss.(3)Hollow nickel/zinc oxide/carbon/reduced graphene oxide(Ni/ZnO/C/RGO)composites derived from NiZn-MOF/RGO precursors were synthesized by a solvothermal and subsequent high-temperature carbonization way in the third chapter.The influences of the additive amounts of GO in the precursors on the composition,phase structure,graphitization degree,micromorphology,magnetic properties,EM parameters and microwave absorption properties were studied in detail.The results presented that the hollow Ni/ZnO/C microspheres were uniformly loaded on the crumpled RGO surfaces.The microwave absorption performance could be effectively controlled by changing the additive amounts of GO.As the amount of GO was 60 mg,the RLmin reached-57.5 dB at a thin thickness of 1.34 mm and the EAB was 4.0 GHz.Additionally,the broader EAB could reach 5.6 GHz at 1.5 mm.The synergistic effects of unique hollow structure,abundant heterogeneous interfaces and multiple losses mechanisms could enhance the microwave attenuation performance of the composites.(4)The trimetallic NiCoZn-MOF precursor was firstly synthesized via a solvothermal method,and then the hollow core-shell NiCo alloy@C composites was fabricated by the high-temperature pyrolysis strategy in the fourth chapter.The effects of calcination temperature on the composition,phase structure,graphitization degree,micromorphology,thermostability,EM parameters and microwave absorption properties for NiCo alloy@C composites were investigated.The results depicted that with the increase of pyrolysis temperature(700℃ to 1000℃),the microstructure of the obtained composites gradually evolved from hollow structure to core-shell structure and yolk-shell structure in turn.The as-prepared composite had comprehensive excellent microwave absorption performance under the pyrolysis temperature of 900℃,that was,the RLmin value was-61.9 dB at 1.5 mm,and the EAB reached 6.0 GHz at 1.7 mm.The synergistic effects of the unique yolk-shell structure,magnetic resonance loss,dipole polarization,interfacial polarization and conduction loss could significantly enhance the microwave attenuation performance.Figure 72 Table 6 Reference 133... |
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