| The development of all-directional and ultra-broadband radar detection weapons at sea,land and sky,plus the increasingly serious electromagnetic radiation pollution require the electromagnetic wave absorption materials to be ultra-lightweight,ultra-low frequency,ultra-broadband and omni-directional.The applications of magnetic materials in the microwave absorption field are limited owing to their high density.The 3D porous carbon materials are highly potential in application due to their advantages of light weight and high dielectric loss.However,due to the inherent contradiction between the high electromagnetic loss and impedance matching of electromagnetic wave,realizing the broadband absorption of carbon materials is still a great challenge,especially in the 1-8 GHz band(wavelength reaches the centimeter level).In order to solve the problem of low-frequency wave absorption and to obtain light broadband,omni-directional microwave absorption materials,a new method to solve the contradiction between high dielectric loss and impedance matching of electromagnetic wave is proposed in this thesis.Based on the theory of impedance gradient,the dielectric properties of carbon materials were firstly regulated,and then the impedance matching optimization design was carried out from the macro scale to prepare the three-dimensional structure absorber based on carbonized waxberry and reduced graphene oxide(r GO)sponge.The effect of macro structure on dielectric properties and microwave absorption properties of carbon materials was studied.In addition,the effect of macro structure on impedance matching performance was analyzed through experiments and simulations.Finally,the mechanism of broadband wave absorption of three-dimensional porous carbon materials was revealed from multi-scale perspective.In order to explore the effect of impedance gradient structure on the mircowave absorption property,spherical carbon materials based on carbonized waxberry were prepared by freeze-drying and carbonization method.Carbonized waxberry has natural density gradient structure and presents the characteristic of impedance gradient from outside to inside.The important influences of microstructure on electromagnetic parameters is investigated.Under the same conditions,the dielectric constant of porous skeleton is larger than that of powder sample.The metamaterial(Metamaterials in the broad sense)based on carbonized bayberry was prepared to achieve effective microwave absorption(reflection loss less than-10 d B)at 8-40 GHz,and reflection loss less than-8 d B at 1-8 GHz.Moreover,the maximum absorption strength reaches-75 d B.In addition,the excellent broadband wave absorption performance can still be maintained when the incidence angle is increased from 5°to40°.The excellent absorbing properties of carbonized waxberry metamaterial benefit from its impedance gradient structure.To further improve the absorption strength in the low-frequency band,activated carbonized waxberry with high dielectric parameters were prepared by activating with the KOH activator.KOH activation increased the specific surface area of the outer structure by 1582 times,which greatly increased the interfacial polarization loss capacity and the dielectric constant of low-frequency wave by more than 5 times.The impedance matching optimization design of activated carbonized waxberry was carried out at macro scale,which caused the 3D metamaterial achieving low-frequency microwave absorption.At 1.5-40 GHz,the reflection loss is below-10d B,and the effective absorption bandwidth reaches 38.5 GHz.Even at 2.5 GHz,the absorption intensity can reach-35 d B.Strong absorption at 1-2 GHz can also be achieved through periodic parameter regulation.In addition,the metamaterial also has the advantages of ultra-lightweight and wide-angle wave absorption:the apparent density is only 0.019 g/cm~3,and the metamaterial still retains the characteristics of ultra-broadband wave absorption when the electromagnetic wave incidence angle increases to 40 degrees.The mechanism of broadband absorption of the material is revealed from the multi-scale perspective:firstly,the electromagnetic loss capacity is improved by activation;secondly,the impedance gradient structure of the whole sphere/hemisphere cross arrangement improves the impedance matching.This reserch provides a new strategy and analysis method for solving the inherent contradiction between high intensity electromagnetic wave absorption and impedance matching.In order to further reduce the density of microwave absorption materials,r GO sponges with different shapes were prepared by freeze-drying and steam reduction of hydrazine hydrate in this paper and assembled them into metamaterials.The influence of the periodic cells shape on the dielectric properties of metamaterials was studied.The experimental and simulation results show that the macroscopic shape of periodic cells have an important effect on the low-frequency microwave absorbption performance of metamaterials,and it is achieved mainly by changing the impedance matching of metamaterials.Among them,the metamaterial with frustum pyramid-like cell structure achieves ultra-light and ultra-broadband absorption:the density of periodic cell is only 5.5 mg/cm~3.The effective absorption is realized in 2.4-40 GHz band.The absorptivity is greater than 99%in the 6.4-38.3 GHz band,and the high intensity absorption bandwidth(31.9 GHz)is much higher than that of the existing metamaterials.The average absorption intensity is-22.9 d B at 2-40 GHz.The impedance matching performance of r GO metamaterial was quantitatively analyzed based on effective medium theory,and the absorption mechanism of ultra-low frequency and ultra-wide frequency was revealed.It is proved that the macro impedance matching design plays an important role in the preparation of broadband microwave absorption carbon materials. |
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