Multi–scale Construction Model And Absorbing Performance Optimization Based On Porous Carbon

To meet the needs of electronic equipment and aircraft for light-weight absorbing materials,porous carbon materials have been studied in this work.Various absorbing mechanisms and related influencing factors have been analyzed in depth,which is in favor of designing and optimizing carbon materials with light-weight and broadband absorption characteristics at micro（nm）,meso（μm）and macro（mm）scales.Research shows,at the microscopic scale,dipole polarization and conductivity have an important influence on the absorbing performance.However,previous studies lack attention to the establishment of micro-models and theoretical analysis of key influencing factors.The first-principles are used to construct a nitrogen-doped carbon model to explore the relationship between dipole polarization,conductivity and dielectric properties.According to the N/C model,a nitrogen-doped carbon material with different N/C molar ratio has been prepared by the carbonization of melamine sponge.The analysis of absorbing performance indicates that the main absorbing mechanism consists of conduction loss and polarization loss.Besides,the actual change of the conductivity is almost consistent with the simulation results.However,when the carbonization temperature reaches 900℃,the measured dielectric characteristics are inconsistent with the simulated dielectric characteristics.Based on the theoretical analysis,it is necessary to pay attention not only to the influence of conductivity,but also to the influence of relaxation intensity.After the finite element simulation prediction,formula derivation and experimental optimization of the nitrogen-doped carbon materials,the nitrogen-doped carbon materials show light weight characteristics(4.34 mg·cm^-3)and broadband characteristics.The effective absorption bandwidth is 8.32 GHz,which basically covers the X-band and Ku-band.On the mesoscopic scale,interface polarization is also the main factor affecting the absorption performance.How to quantitatively analyze the contribution of the interface to the dielectric properties and how to maximize the interface polarization through technological means has always been difficult.Under the catalysis of a small amount of metallic nickel,a porous carbon material with a hierarchical structure has been successfully synthesized.When the carbonization temperature increases from 600℃to900℃,the porous carbon material exhibits an adjustable hierarchical structure.The test of absorbing performance proves that different hierarchical structures exhibit different frequency absorbing characteristics.Porous carbon materials with different hierarchical structures show the absorption characteristics in the target frequency band（preparing various carbon materials with the absorbing frequency of 5～8 GHz,8～12 GHz,12～18GHz,respectively,RL<-10 d B）at a thickness of 2.8 mm.Dielectric performance analysis and finite element simulation have proved that interface polarization is the key factor that affects the dielectric performance.To optimize the effect of interface polarization,the Cole-Cole dielectric model has been revised,and an improved dielectric performance formula has been proposed to extract the interface influence factor.The porous carbon materials with light weight(5.36 mg·cm^-3)and high strength（RL=-76.59 d B）can be acquired by designing and optimizing the materials based on the interface influence factor.On the macroscopic scale,the main concern is the effect of resonance loss on the absorbing performance.Due to the diversity of periodic structure,it is unrealistic to rely on a large number of experiments to try different types of periodic structure absorbers.At the same time,previous studies only discussed the relationship between the macrostructure and the absorbing performance,and did not analyze in detail how the resonance loss attenuates electromagnetic waves.Therefore,the transmission characteristics of any structure can be reflected by the genetic algorithm with global optimization characteristics and finite element simulation,which can also optimize the macro model of the periodic structure absorber.According to the optimization results,a flexible and broadband absorption material with wide incidence angle has been obtained by room temperature vulcanization and overmolding,in which the graphite nanosheets are utilized as the absorber and silicone rubber acts as the matrix.The research results show that the effective absorbing bandwidth can cover 8～24 GHz and the bandwidth can reach 16 GHz.The excellent broadband characteristics can be attributed to the newly introduced resonance loss and good impedance matching.Meanwhile,the resonance loss can improve the interface polarization and conductivity polarization of the absorber,which make the loss type of the periodic structure absorber similar to the dipole loss.Therefore,a more broadband wave absorbing performance can be obtained.Moreover,the absorbing properties of this periodic structure absorber are also insensitive to the incidence angle.Especially when the metamaterial absorber bends,the effective bandwidth of the absorber still increases.Using this optimization algorithm to optimize the macrostructure of porous carbon,it is discovered that that there is a corresponding optimal model for each material,and that the broadband absorption range is developing toward lower frequencies.