Design And Preparation Of Hierarchical Hybrid Carbon-based Composites And Study On Their Microwave Absorption Properties

Along with the rapid development of wireless communication and the extensive usage of electronic devices,the problem of electromagnetic pollution becomes increasingly serious,which is a threat to human health,device performance and even national security.。Nowadays,microwave absorption materials（MAMs）have been invoked to solve this issue,by converting the incident microwave energy into heat and other energy.In general,conventional MAMs have considerable drawbacks,such as high mass density,narrow absorption bandwidth as well as weak reflection loss,which severely impede their further practical applications to some extent.Hence,it is of great scientific importance and practical value for design and fabrication of ideal MAMs with lightweight,wide frequency bandwidth,strong absorption and low absorber thickness.Carbon materials including carbon nanotubes（CNTs）,graphene and carbon black,are regarded as potentially valuable MAMs,due to their low density,stability physicochemical properties and unique electrical characteristics.However,monocomponent carbon materials usually have a single attenuation mechanism and a weak impedance matching,resulting in poor microwave absorbing（MA）property.There are two feasible solutions,one of which is incorporating carbon materials with magnetic metals（e.g.,Fe,Co,Ni,）and their oxides.Benefiting from the synergetic effect of dielectric/magnetic loss and good impedance matching,carbon-based composites exhibit excellent comprehensive MA ability.And the other is to construct special structures,involving hollow,core-shell and three-dimensional network structures,which are in favor of multiple reflection/scattering of microwave and interface polarizations,thus prolonging microwave propagation paths and then improving MA performance.Accordingly,carbon materials are cited as research objects.From the view of morphology and component,novel carbon-based composites with hierarchically porous hybrid structure are designed and fabricated in this work.Meanwhile,their MA properties and MA mechanisms are further characterized and studied in detail.The main research contents in this work are as follows:1.The hierarchical cobalt nanocrystal/carbon sponge（Co/CS）composite was synthesized by utilizing melamine-formaldehyde sponge（MFS）as a template for growth of Co-based metal-organic frameworks（ZIF-L-Co）and further carbonizing this ZIF-L-Co-decorated MFS.Co/CS composite consisted of three-dimensional carbon skeleton,one-dimensional carbon rod and zero-dimensional Co nanocrystal.Notably,carbon nanorod uniformly,compactly and vertically grew on the surface of carbon skeleton.The microstructure,composition proportion,magnetic and electromagnetic parameters of Co/CS composite is significantly affected by carbonization temperature and time,and thus its MA property could be optimized by controlling carbonization conditions.Because of impedance matching and strong dielectric/magnetic loss good,the Co/CS composite which was prepared at 800°C for 2 h,possessed the best MA ability with a minimum reflection loss of-38.7 d B at a matching thickness of 4.0 mm,as the filler loading was 15 wt%.Besides,as for the Co/CS composite synthesized at 800°C for 1 h,its MA ability improved first and then decreased with the increase of filler loading in the range of 10-20 wt%,so the optimal filler loading was 15 wt%.The corresponding maximal effective absorption bandwidth of 6.4 GHz（11.3-17.7 GHz）was obtained at a matching thickness of 4.0mm.Benefiting from hierarchically hybrid structures,Co/CS composite showed better MA performance than Co/C and CS composites.The results offer new insights for designing and fabricating magnetic carbon-based composites with hierarchical structure for high-performance MA.2.Interface polarizations occured at the interfaces among multi-components,exert a positive effect on MA attenuation.The existence of band hybridization in bimetallic metal-organic frameworks（ZIF-B）derived carbon-based composites would inevitably influences the electromagnetic properties.Hence,ZIF-B nanorod arrays in situ grew on the surface of MFS through dip-coating method.And then the ZIF-B-decorated MFS was carbonized to synthesize cobalt-iron alloy/cobalt nanocrystal/carbon sponge（Co Fe/Co/CS）composite with a hierarchically structure,which consisted of three-dimensional carbon network,one-dimensional carbon nanorod,zero-dimensional Co Fe alloy and Co nanocrystal.The mole ratio of Co²⁺to Fe²⁺,carbonization temperature and time played an important part in regulating the morphology,saturation magnetization,electromagnetic parameters,MA ability and so on.While the filler loading increased from 5 to 15 wt%,the MA performance increased at first and then decreased,therefore the perfect filler loading is 10 wt%.As the mole ratio of Co²⁺to Fe²⁺is 10:1,the Co Fe/Co/CS composite which prepared at 1000°C for 1 h showed the outstanding MA property with the minimum reflection loss of-52.3 d B at a matching thickness of 1.88 mm,the corresponding specific RL values,those were,SRL_l and SR_Lld values reached up to-523 and-278,respectively.Comparing with Co Fe-based composites,Co-based composites and bimetal composites,Co Fe/Co/CS composite exhibited more excellent comprehensive MA performance,which resulted from the coordination effect of its hierarchically porous structure and various electromagnetic loss mechanisms.This work sheds new light on the design of high-performance ZIF-B derived carbon-based MAMs.3.Specific MOF derived Metal/N-Doped CNTs composites are expected to show good MA ability,owing to the synergistic effect of magnetic loss and dielectric loss.Herein,A novel hierarchical Co nanocrystals/N-doped CNTs/carbon sponge（Co/CNTs/CS）composite with a hollow skeleton was successfully fabricated via a simple and facile combining of dip-coating with carbonization.The intertwined CNTs grew on the outer surface of the hollow carbon microtubes,and metallic Co nanocrystals were encapsulated at the tips of CNTs.The morphology,microstructure,component and electromagnetic parameters of Co/CNTs/CS were closely associated with carbonization conditions,thus its MA capability could be easily adjusted by changing carbonization temperature and time.When the filler loading was 10 wt%,the Co/CNTs/CS composite prepared at 800°C for 1 h showed a wide absorption bandwidth of 5.4 GHz（11.4-16.8 GHz）and a small reflection loss of-31.3 d B at a matching thickness of 2 and 1.63 mm,respectively.As the carbonization temperature increased to 900°C,Co/CNTs/CS exhibited a eminent MA ability with an optimal reflection loss of-54.6 d B at the matching thickness of 2.48 mm.Moreover,the MA ability of Co/CNTs/CS composite was also affected by the filler loading.While the filler loading decreased to 5 wt%,its low reflection loss could reach up to-17.9 d B.Remarkably,the matching thickness,reflection loss and absorption bandwidth of Co/CNTs/CS were superior to those of state-of-the-art absorbers.Hence,this study provides reference to the design of efficient MAMs with thin thickness,lightweight,wide frequency bandwidth,and strong absorption.4.Hollow structure not only gives rise to multiscattering/reflection of the incident microwaves,but also facilitates good impedance matching and interface polarizations.Thus,polymeric microcapsuleas（MCs）with poly（urea-formaldehyde）as shell and toluene as material core material were prepared through in situ polymerization in oil-in-water emulsion.And then superhydrophobic N-doped hollow carbon microsphere（N-HCMs）were fabricated by calcining these hollow core-shell structural MCs.Near-spherical N-HCMs exhibited hollow core and hierarchical surface.Their structures,specific surface areas and even electromagnetic parameters mainly depended on calcination conditions,which further impacted their MA abilities.As the filler loading was 20 wt%,with the decrease of carbonization temperature or the increase of carbonization time,the MA property of N-HCMs gradually enhanced.Herein,N-HCMs synthesized at 800°C for 4 h had the strongest MA performance with a minimum reflection loss of-59.1d B at a thin thickness of only 1.61 mm.In addition,the filler loading of N-HCMs aslo showed significant influence on their MA capability.As for N-HCMs prepared at 700°C for 1 h,an optimal minimum reflection loss of-61.2 d B and a broad effective absorption bandwidth of 6.8 GHz were achieved at the filler loading of 15wt%.Apparently,by comparison with magnetic hollow carbon and even porous carbon,the resultant N-HCMs had a better competitive MA ability with the SRL_lvalue of-408 and SRL_ld value of-182.Meanwhile,the N-HCMs exhibited a superhydrophobic behavior owing to the hierarchical micro/nanostructures and the decomposition for hydrophilic groups of precursor MCs during carbonization.Hence,constructing hierarchical surface structure and hollow structure is an effective strategy to endow materials with superhydrophobicity and MA capacity.5.Integrating carbon materials with magnetic materials（e.g.,Fe₃O₄）could satisfy impedance matching and induce magnetic loss,and thus promote the MA performance.Therefore,MAMs with superior competitive MA are fabricated by incorporating Fe₃O₄ nanoparticles with N-HCMs.Accordingly,oleic acid-coated Fe₃O₄ nanoparticles were firstly synthesized by using a chemical precipitation method.Afterward,magnetic poly（urea-formaldehyde）microcapsules（MMCs）containing Fe₃O₄ nanoparticles were prepared by in situ polymerization.Finally,the resultant MMCs were directly carbonized to acquire superhydrophobic magnetic hollow carbon microspheres（SMHCMs）.SMHCMs had nearly spherical shape with hollow core,micro/nano hierarchical coarse surfaces and nano-pores.Fe₃O₄nanoparticles were well interspersed in carbon shell.The diameter,composition,structure as well as electromagnetic parameters of SMHCMs can be controlled effectively by adjusting carbonization temperature,agitation rate and Fe₃O₄ content,which further crucially affected their corresponding MA performance.When the filler loading was 10 wt%,SMHCMs produced at the agitation rate of 1000 rpm and carbonization temperature of 1000°C,possessed the superior MA ability with a low reflection loss of-60.6 d B at a thickness of 1.94 mm,which is attributed to good impedance matching and strong microwave attenuation ability.As the carbonization temperature decreased to 700°C,an effective absorption bandwidth of up to 6.4GHz（7.2-13.6 GHz）was acquired at a matching thickness of 3.5 mm.Notably,the SRL_l and SRL_ld values of SMHCMs could reach-606 and-312,respectively,which were far higher than those of other Fe₃O₄-based composites and hollow composites,indicating SMHCMs had excellent comprehensive MA ability.Furthermore,SMHCMs showed a good tolerance towards the acidic,alkaline and high-saline environments.And the SMHCMs-bound coating exhibited the self-cleaning property.Hence,as the novel composites with exceptional superhydrophobicity and MA performance,SMHCMs had an extensive perspective application.In summary,by utilizing three dimensional porous sponges and hierarchical polymeric microcapsuleas as main carbon sources,and then incorporating monometallic/bimetallic MOF or magnetic nanoparticles,novel hierarchical hybrid carbon-based composites were designed and fabricated via the simple and controllable strategies including dip-coating,in situ polymerization and carbonization.The as-prepared composites showed excellent MA ability,which overcame the disadvantages of conventional MAMs,involving high mass density,narrow absorption bandwidth and poor reflection loss.In addition,N-HCMs and SMHCMs exhibited good superhydrophobicity,and thus had a promising prospect in the fields of self-healing and self-cleaning.This work not only provides research approaches and references for designing and fabricating advanced MAMs which meet the requirements of thin thickness,lightweight,wide frequency bandwidth,and strong absorption,but aslo plays a great role in the overall application of MAMs,implying its important scientific significance and application value.