Research On Regulation Of Impedance Matching Of Porous Carbon-based Electromagnetic Absorbing Composites

With the rapid development of information technology,electromagnetic radiation pollution is becoming increasingly serious.In addition,rapid technological advances in radar detection have made the competition between electromagnetic stealth and anti-stealth technologies fierce.These problems have put forward higher requirements for ultra-lightweight,multi-band,strong-loss electromagnetic absorption materials.Porous carbon materials with light weight and strong loss as a distinctive feature and attracted much attention.Carbon materials with high intrinsic conductivity possess strong ability to absorb electromagnetic waves,but that is not conducive to its broadband electromagnetic wave absorption properties.This is because too high a conductivity can lead to an impedance mismatch,which produces intense reflections on the material surface.In the past,the research on carbon materials mainly focused on the absorption mechanism,and relatively little research was done on impedance matching.Therefore,based on impedance matching theory,this paper proposes three strategies to modulate the impedance of carbon-based materials to achieve multi-band absorption characteristics:porous carbon skeleton with magnetic particles loaded,orientation of reduced graphene oxide aerogel,porous carbon skeleton with phase-change materials loaded.According to the impedance matching theory,when the material’s dielectric constant and permeability are equal,the impedance matching between the material and air is optimal,so the combination of carbon and magnetic materials is the most direct strategy.A relatively inexpensive agricultural by-product,coconut shell,with oriented multilayer structure was selected as the carbon source,and the square magnetic particles were compounded.After carbonization,a porous carbon/magnetic particle composite was produced.The square magnetic particles were attached to the surface of the porous laminated carbon matrix,while the magnetic particles mainly consist of numerous Fe/Fe₃C magnetic nanoparticles and carbon shells covered with them.The permittivity of this porous carbon/magnetic particle composite showed an increasing trend with the increase of pyrolysis temperature,among which the broadband electromagnetic wave absorption properties of the material after 550°C treatment was the strongest,with an EAB of 7.94 GHz（10.06 to 18.00 GHz）at 3 mm thickness.Suitable carbonization and the introduction of magnetic materials facilitated the impedance matching and the internal loss capability was also enhanced.The loss was mainly realized by the joint effect of dielectric relaxation loss,conductivity loss and eddy current loss.In addition to composition modulation,impedance matching can be achieved by microstructure modulation and macrostructure assembly.We regulated the dielectric properties of reduced graphene oxide（r GO）aerogel materials by modulating their microstructures.impedance gradient structures were obtained by using the assembly of non-homogeneous bilayers.Firstly,the oriented porous graphene aerogels with different concentrations were prepared by directional freezing and freeze-drying methods.And then the aerogels were assembled into bilayer structures.By testing its electromagnetic parameters,the ability of the density and structure to regulate the dielectric constant was clarified.High concentration of GO was beneficial to increase density and the dielectric constant,while structural anisotropy made the material possess anisotropy of electromagnetic parameters.And horizontal orientation generally lead to higher dielectric constant than that of vertical orientation.Based on these results,a multilayer multi-degree-of-freedom structural design of r GO aerogel was carried out.A non-homogeneous bilayer structure with progressive impedance was assembled.The results show that multilayer aerogels possessed higher designability of properties and higher upper limit of wave absorption capacity than single-layer aerogels.In the double-layered aerogel,when the thickness was 10 mm,the upper layer was a vertically oriented 5 mg/m L low-concentration aerogel and the lower layer was a higher concentration 10 mg/m L laterally oriented aerogel.The sample finally exhibited excellent broadband electromagnetic wave absorption properties,with the effective microwave absorption in the range of 3.95～18 GHz and the effective absorption bandwidth was up to 14.05 GHz.The surface layer of low dielectric constant possessed good surface impedance matching characteristics,while the bottom layer of higher dielectric constant possessed strong loss capability.The surface impedance matching design guaranteed the weak initial reflection,while the strong internal loss suppressed the backward propagation of electromagnetic waves,achieving the combined effect of surface impedance matching and strong internal absorption characteristics,so that this multilayer structure showed excellent broadband electromagnetic wave absorption properties.There are particularly large differences in dielectric constant before and after phase transition for some materials,which provides new ideas for effective tuning of the electromagnetic impedance matching properties of carbon materials.Biomass starch,which was easy to assemble and compound,was selected as the carbon source,and the porous carbon framework was formed by starch fermentation,freeze-drying and carbonization at 500°C.A material with room temperature phase change characteristics was selected as the tuning material.And an aqueous solution of this material was used to infuse into the interior of the carbon framework,and the porous carbon/phase change material composite was obtained after vacuum drying.The phase-transition temperature was measured to be between 17°C and 21°C.Therefore,10°C and 30°C were selected as the control temperature points,and the temperature rise and fall between 10°C and 30°C could be achieved within 4 s for a 5-mm-thick sample.The maximum expansion of the effective absorption bandwidth could reach 7.51 GHz,the strongest reflction loss could be enhanced from-10.11 d B to-28.11 d B,and the position of absorption peak could drift by 2.19 GHz.The analysis of the capability of broadband expansion showed that was mainly due to the enhancement of the attenuation ability of the material at the middle and high frequencies.