Electromagnetic Characteristics Modulation Of The Graphitized Carbon Composites And Metamaterials

Graphitized carbon-based materials have been regarded as a promising lightweight electromagnetic wave absorbing material because of its excellent electrical conductivity,adjustable geometric structure,abundant surface groups and unsaturated bonds and strong composite modification ability.In general,composite materials with perfect graphene structures tend to show poor electromagnetic modulation performance as a result of the impedance mismatches.In this thesis,carbon-based composites with graphitized structure,used as the matrix material,was modified by different metal structures,which realized the regulation of electromagnetic spectrum in different wavebands.The details are as follows:(1)Based on the advantages of hollow structure,excellent electrical conductivity and easy functionalization,a new composite composed of thin-walled carbon nanotubes coated with cobalt nanoparticles(Co@NCNTs)in the tip was designed and synthesized through high temperature pyrolysis process.The geometrical and electronic structures of the products were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy.Among them,Co@NCNTs-700 with flammulinus-like structure presents the multiple dielectric polarization relaxation,high magnetic properties and good impedance matching characteristics,showing the strongest dielectric loss,magnetic loss and electromagnetic wave absorption performance.When the thickness is 2mm and the filling amount is only 10%,the minimum reflection loss and effective absorption bandwidth at 4.96 GHz are-21.0d B and 5.28 GHz,respectively.(2)The composite of magnetic nanoparticles with semiconductor metal oxides has the advantages of high conductivity,strong polarization process and easy preparation,and is considered as an ideal absorbing material.In this work,a Co-Zn O binary MOF(ZIF-67,ZIF-8)-based carbon composites were prepared by a simple two-step process.After detailed analysis,the nanocomposites exhibited the best electromagnetic parameters and excellent electromagnetic wave absorption ability.The results showed that Co-Zn O-C-800 with a thickness of 2.9 mm presented a maximum reflection loss of-44.8 d B at 9.2 GHz and an effective absorption bandwidth of 6.95 GHz(6.59 GHz-13.54 GHz).The mechanism analysis of samples mainly includes dielectric loss,dipole polarization and interface polarization.This work provides a simple strategy for the design and synthesis of new lightweight electromagnetic wave absorbing materials with wide band and strong absorption.(3)A class of porous carbon materials(HYS-PC-700,HYS-PC-800,HYS-PC-900)with honeycomb structure were prepared by a simple and rapid method using biomass as raw material.The dielectric properties of HYS-PC-800 show that the material has the best impedance matching characteristics and the strongest electromagnetic wave absorption performance due to the perfect honeycomb porous structure and abundant surface functional groups.In the frequency of 2-18 GHz,the minimum reflection loss of the material is-55.78 d B,and the effective absorption bandwidth is 3.82 GHz,covering the whole Ku band.The mechanism analysis showed that the optimal absorption performance of electromagnetic wave mainly comes from the balance between conductance loss and dipole loss and the synergistic effect between dielectric properties and geometric structure.The results of this work will lay a foundation for the design of lightweight microwave absorbent.(4)Based on the interference elimination principle,a graphene-based optical modulator design scheme wass proposed,which finally realized the modulation characteristics of the THz band electromagnetic field with significant plasma-induced transparency effect.By adjusting the physical and structural parameters of the graphene-patterned structural unit,the structure obtained a good optical wave electromagnetic field modulation effect and can be used in the optical tuning modulator.It is found that,compared with the discontinuous graphene structure.The structure can be tuned to the light wave electromagnetic field without changing the structural parameters.In addition,a plasma-induced transparency(PIT)graphene metamaterial structure is achieved by the periodic arrangement of graphene strips(as bright modes)and edged microsheets(as bright modes).Due to the interaction between the graphene strip as the bright mode(dipole)and the sideband as the dark mode(quadrupole),the dual PIT was formed by destructive interference,and the resulting slow light coefficient reaches 0.236 ps.The data fitted by theoretical calculation are in good agreement with the results of numerical simulation.The related work shows a great promise in the research of tunable optical modulator.