Microstructure Design And Electromagnetic Property Of Porous Carbonaceous Microwave Absorbers

With the rapid development of multi-station and multi-band radar detection technology,higher requirements for stealth performance of future fighters are put forward.The development of broadband(8?18 GHz)and highly effective(reflection coefficient?-10 d B)microwave absorbing materials is an important way to realize all-round stealth of future fighters.It is generally acknowledged that absorbing materials need to meet the requirements of"wide","strong","light"and"thin".In addition,the service environment of absorbing materials for stealth fighters requires that they have low to high temperature cooperative stealth performance.To meet the above requirements,it is necessary to improve the stealth performance under variable temperature environment by micro-structure design of microwave absorbing materials based on the microwave loss mechanism.The microwave loss mechanism mainly includes magnetic loss,conductive loss and polarization loss.The magnetic loss materials will lose their magnetism at the temperature higher than Curie point,which leads to the disappearance of the microwave loss ability and limits their practical application.The conductive loss carbon materials have the advantages of high-temperature resistance(service temperature can reach more than 3000 ^oC),light weight(1.6?1.8 g/cm³),adjustable microstructure and dielectric properties.They are the most popular candidate materials for designing microwave absorbing materials at present.However,how to balance the conductive loss and polarization loss of carbon materials is a key core scientific issue to optimize their absorbing properties.Besides,the microwave absorbing materials consist of microwave transparent matrix and microwave absorbers.Compared with the whole microwave absorbing materials,the structure and phase composition of microwave absorbers are simple,which facilitates the study of the structure-activity relationship between micro-structure and microwave loss mechanism.Therefore,in this paper,conductive loss domianted porous carbon absorbers are taken as the research object.The polarization loss of porous carbon absorbers is enhanced by introducing nano-scale,micro-scale,nano/micro multi-sacle heterogenous interface into the porous carbon absorbers through microstructure design.The conductive loss of porous carbon absorbers is adjusted by annealing process.The relationship between the microstructure of carbon microwave absorbers and their microwave absorbing performance at different service temperatures is systematically studied.The main research contents and results are as follows:(1)The effect of nano/micro carbon-air interface on the dielectric properties and microwave absorbing properties of porous carbon absorbers at room temperature was studied.Solid core carbon spheres,hollow carbon spheres and porous hollow carbon spheres with uniform size and different carbon-air interface were prepared by template-carbonization-etching method.The dielectric properties show that the real part of permittivity,imaginary part of permittivity and dielectric loss of porous carbon absorber increase with the increasing of carbon-air interface at nano/micro level,and the contribution of carbon-air interface at nano level is more obvious.Porous carbon microwave absorbers with nano-scale carbon-air interface and micro-scale carbon-air interface have the best microwave absorption performance.The EAB can cover up to 4.2 GHz(8.2-12.4 GHz)and the RC_min can reach-82 d B.The EAB of porous absorbents with only micro carbon-air interface is only 0.6 GHz and the strongest absorption is-25 d B.Compared with the absorbents with nano/micro carbon-air interface,the EAB and RC_min are reduced by 85.7%and 69.5%,respectively.(2)The effects of amorphous carbon-graphite carbon heterogeneous nanoscale interface on the dielectric properties and microwave absorption properties of porous carbon absorbers at room temperature were studied.A series of graphene hollow spheres(HGS,hollow graphene sphere)embedded in porous amorphous carbon(PAC,porous amorphous carbon)matrix were prepared by adjusting the heat treatment process and the ratio of coordination ion Co²⁺/Zn²⁺in bi-MOF(HGS@PAC).The results show that the real part of permittivity,imaginary part of permittivity and dielectric loss of porous carbon absorber increase with the increase of nano-scale amorphous carbon-graphite carbon heterogeneous interface,which indicates that the microwave loss ability of porous carbon absorber increases.When the number of graphite carbon spheres is adjusted,the effective absorption bandwidth(EAB)can cover up to 4.2 GHz(8.2-12.4 GHz)and the strongest absorption(RC_min)can reach-32 d B.When the thickness of graphite carbon spheres is adjusted,the EAB can cover up to 5.6 GHz(12.4-18 GHz)and the RC_min can reach-56 d B.The microwave loss mechanism investigation shows that the interface polarization is greatly enhanced by introducing nano-sacle amorphous carbon-graphite carbon heterogeneous interface.In addition,the N atom doping and residual oxygen-containing functional groups enhance the dipole polarization of materials.First-principle calculation results show that N atom doping enhances the dipole polarization of porous carbon absorbers as:N>graphene N>pyrrole N.(3)The effect of nano/micro carbon-air interface on the dielectric properties and microwave absorbing properties of porous carbon absorbers at different working temperature was studied.Porous carbon hollow microspheres with blood cell like morphology(RBC-PCHMs)were prepared by the template-carbonization-etching method.The red blood cell like morphology increased the nano/micro carbon-air interface significantly.The results of dielectric properties show that the real part of permittivity,the imaginary part of permittivity,and the loss tangent of porous carbon absorbers increase with the increasing of the test temperature.When the test temperature rises from room temperature to 523 K,the average values of the real and imaginary parts of the dielectric constant of RBC-PCHMs pyrolyzed at800 C increase by 33.9%and 54.9%,respectively,which indicates that the micrwove loss capacity increases faster than the microwave storage capacity of porous carbon absorbers.Based on Debye theory and least square method,the interface polarization loss and conductivity loss of porous carbon absorbers were obtained.With the test temperature rising from room temperature to 523 K,the interface polarization loss decreased from 1.42 to 1.01(28.8%)and the conductivity loss increased from 3.47 to 6.23(79.5%).The decrease of interfacial polarization loss can balance the increase of conductive loss,and therefore,improve the temperature-dependent microwave absorbing performance.The EAB of RBC-PCHMs in the range of room temperature to 523 K is always large than 3 GHz.On this basis,a coordinated control model for polarization loss and conductive loss of carbon materials is proposed to guide the design porous carbon materials working at room to 523K environment.(4)The effect of the nano/micro heterogenous interface and the macro periodic structure on the dielectric properties and ultrabroad band microwave absorbing properties of porous carbon was studied.Cellular carbon nanotubes/cellulose nanofibers foams(CNTs/CNF foam)were prepared by ice template method.The dielectric properties show that the CNTs/CNF foams with CNTs/CNF nanoscale heterointerface,CNTs/air microscale heterogeneous interface and cellualr periodic structure exhibit excellent broadband absorbing properties.The EAB of the CNTs/CNF foam reaches 29.7 GHz(2.3?18 GHz,26?40 GHz)with the thickness of 20 mm.The investigation of microwave absorbing mechanism of the CNTs/CNF foam shows that the dielectric loss caused by the foam at high frequency range and the electromagnetic resonance loss caused by the periodic structure is the main reason for its excellent broadband absorbing properties.On this basis,the ultra-broadband absorbing model combing the low-frequency absorbing ability caused by perodic structure and the high-frequency absorbing caused by the dielectric material itself is proposed.In addition,the dielectric properites of the CNTs/CNF foam at 293 K to 873 K is investigated by intruding the Si₃N₄ into the CNTs/CNF foam through chemical vapor infiltration(CVI)process.The CNTs/CNF/Si₃N₄ composites show excellent microwave absorbing properties at room temperature to 873 K.With the increase of test temperature,the RC_min decreased from-31.7 d B to-39.9 d B,indicating that the microwave absorbing properites at high temperature was more excellent.The EAB of the composite can cover the whole X-band in the whole test temperature range.