The Design And Performance Investigation Of Carbon-based Electromagnetic Wave Absorbers

The rapid development of communication technology and electronic equipment has caused serious electromagnetic pollution problems.Electromagnetic pollution can not only interfere with the normal work of highly sensitive electronic equipment but also threaten human health and lead to unpredictable diseases.In the military field,along with radar detection and weapon guidance technology,warplanes,tanks,warships,and other weapons and equipment are more likely to be detected,tracked,and attacked by the enemy,which puts higher requirements on the radar stealth ability of weapons and equipment.Electromagnetic wave（EMW）absorbers,which through absorbing electromagnetic energy and convert it into thermal energy,play a key role in solving electromagnetic pollution and radar stealth problems.Therefore,in recent years,researchers have devoted themselves to developing EMW absorbers with characteristics such as broad absorption bandwidth（broad）,strong loss capacity（strong）,lightweight mass（lightweight）,and thin matching thickness（thin）.Carbon materials have received a lot of attention in the field of EMW absorption materials because of their high electrical conductivity and light weight.However,due to the single loss mechanism and poor impedance matching,it is difficult to meet the requirements of current EMW absorption materials.Nowadays,the most common method to improve the EMW absorption property of carbon materials is to compound carbon materials with magnetic materials to prepare carbon/magnetic composite EMW absorbers.The introduction of magnetic materials can improve the impedance matching of carbon materials,and the synergistic effect between carbon and magnetic components can further enhance the EMW loss ability of the material at the same time.In the past time,much work has been devoted to the preparation of shape-controlled,highly stable,and well-defined carbon/magnetic particle composites.However,the magnetic loss ability of magnetic materials is relatively weak compared to the dielectric loss ability of carbon materials,while the magnetic materials are denser,and the introduction of magnetic materials will increase the density of the composites,and the electromagnetic loss ability of carbon/magnetic particle composites will even decrease compared to that of pure carbon materials at the same filling ratio.Therefore,the EMW absorption property of carbon/magnetic particle composites still cannot fully meet the requirements of"broad,strong,lightweight,and thin".In view of the current situation that the magnetic properties of magnetic materials are difficult to improve significantly,it is necessary to find new methods to improve the electromagnetic loss of carbon/magnetic particle composites in order to obtain better EMW absorption properties and to meet the requirements of"broad,strong,lightweight and thin"use of EMW absorption materials.According to the above-mentioned problems,four kinds of EMW absorbers were specially designed and synthesized with a component and morphology design strategy in order to further improve the EMW absorption property.Firstly,the hollow porous carbon base layer by layer cladding EMW absorbers（polypyrrole@magnetic particles@hollow porous carbon,PPy@Co/CoFe₂O₄@HNBC）were prepared by the template method combined with in situ polymerization.In the composite material,the hollow porous structure of HNBC facilitates to enhance the interaction between EMW and the absorber;Co/CoFe₂O₄ magnetic particles can improve the impedance matching of the material while providing magnetic loss;and the introduction of PPy further improves the dielectric loss capability of the material.In addition,the multilayer cladding structure,as well as introducing more interfaces,can further extend the EMW transmission path,which is favorable to the attenuation and loss of the incident EMW.As a result,PPy@Co/CoFe₂O₄@HNBC exhibits excellent EMW absorption performance in the frequency range of 2.00-18.00 GHz,with the minimum reflection loss(RL_min)of up to-61.85 dB at a filling ratio of 25.00 wt%（base is paraffin）,the thickness of 1.90 mm,and frequency of 12.80 GHz.Moreover,the broadest effective absorption bandwidth（EAB）of 5.60 GHz is achieved by adjusting the thickness to 1.70mm.The introduction of porous structure effectively improves the EMW absorption performance of carbon matrix absorbers,but for carbon materials,the pore size will also have a certain impact on the EMW loss capacity of the material.In order to further improve the EMW absorption performance,the hierarchical porous carbon/magnetic particle composite（HPPC/CoNi）was prepared by salt template method and in situ growth method.In HPPC/CoNi,the hierarchical porous structure not only reduces the density of the material,but also increases the interaction between EMW and the composite through multiple reflection and scattering effects.At the same time,the hierarchical porous structure also brings rich interface,which is conducive to enhancing the interfacial polarization of the material.In addition,the introduction of CoNi magnetic particles gives the material a strong magnetic loss capacity,and the synergistic interaction between magnetic and dielectric components also contributes to the rapid loss of incident EMW.As a result,with a filling ratio of 35.00 wt%（base is paraffin）,HPPC/CoNi exhibits excellent EMW absorption performance with the RL_min of-62.66 dB（1.90 mm,13.12GHz）,and the broadest EAB of 5.84 GHz（1.70 mm）.The enhancement of EMW absorption performance of carbon matrix absorbers is limited by simply changing the type of pore structure in the material.Therefore,the use of introducing heteroatomic defects in carbon materials while maintaining the porous structure of the absorbers will be expected to further enhance the EMW absorption performance of the carbon matrix absorbers.The heteroatoms doped porous carbon and magnetic particles composite absorbers（NSPC-Co）were prepared by the combination of the impregnation method and the high-temperature pyrolysis method.The N、S heteroatoms in the materials provide stronger polarization loss ability for NSPC-Co.Meanwhile,the porous structure of the materials can also enhance the interaction between EMW and composite materials.In addition,the introduced Co magnetic particles give the material magnetic loss ability while also improving the impedance matching of the composite material,resulting in efficient EMW absorption.At a filling ratio of 20.00 wt%（base is paraffin）,NSPC-Co achieves the RL_min of-64.31 dB with a frequency of 15.68GHz and a thickness of 1.70 mm.And the broadest EAB of the material is 6.16 GHz（11.28-17.44 GHz）when the thickness is adjusted to 1.90 mm.The EMW absorption performance of carbon/magnetic particle absorbers have been greatly improved by using porous morphology modulation and heteroatom doping of carbon,and the EMW absorption performance of the material is further enhanced by trying to regulate the crystallinity of carbon.The porous carbon and magnetic particle absorbers（NBC/Co）were prepared by a solution method combined with high-temperature pyrolysis.The conductivity loss ability of NBC/Co is significantly enhanced by the catalysis of nanoparticles,and the porous structure also brings a richer micro-nano structure and interface to the material,which makes the material have a stronger interfacial polarization.The heteroatom defects in carbon materials also facilitate stronger dipole polarization.Meanwhile,the synergistic interaction between the carbon material and Co magnetic particles endows the NBC/Co with more excellent impedance matching performance and electromagnetic loss capability and exhibits excellent EMW absorption performance.When the filling ratio is 15.00 wt%（base is paraffin）,the RL_min of NBC/Co-0.21 to the EMW with a frequency of 12.72 GHz reaches-67.51dB（1.90 mm）.And adjusting the thickness to 1.70 mm,the broadest EAB of the material is 6.24 GHz（11.76-18.00 GHz）.To sum up,this paper mainly from the perspective of morphology and composition design,to develop four high performance EMW absorbers by designing the hollow porous structure,hierarchical porous structure,the introduction of heteroatomic defects in carbon materials,enhancement of crystallinity of carbon materials.And the absorption properties and mechanism of the material were also studied.Through the research of this paper,the EMW loss characteristics of carbon/magnetic particle composites can be further improved,and its EMW absorption performance can be further improved,and finally,the high performance absorbers that meet the requirements of"broad,strong,lightweight,and thin"characteristics can be obtained.