| In present era,the modern science based on communication and electronic technology is developing with each passing day,which brings great convenience to people’s life,work and production.However,it also leads to severe electromagnetic pollution,causing great challenges to the anti-interference of precision equipment and human health protection.Meanwhile,the equipment electromagnetic stealth also puts forward higher requirements in military fields.Therefore,the electromagnetic wave absorption materials(EWAMs)have attracted more and more attention because they can attenuate and weaken electromagnetic waves effectively.In recent times,great progress has been made for EWAMs,however,it is still difficult for them to meet the absorption requirements of "thin,light,wide and strong",as well as the characteristics of high-temperature resistance,oxidation resistance,impact resistance and environmental friendliness,which limits further developments of EWAMs.To overcome the above problems and explore high-performance EWAMs,iu this study,a variety of composite nanofibers with high aspect ratio and fine microstructures were prepared on the basis of electrospinning technology.Focusing on the dielectric semiconductor,magnetic metals and carbon,the effects of various composite methods on absorption performances and their contributions to property improvement were explored in depth.By promoting the synergy of various components and loss mechanisms,and improving the impedance matching characteristics and attenuation abilities,the high-efficiency EWAMs with light weight,wide bandwidth and strong absorption intensity can be obtained finally.Therefore,taking the fine structure design of nanofibers and the coordination of different components as the key points,a series of nanofibers with fine structures including ZnO/Co,MnS/C,MnO/C,TiO2/C,Ni/TiO2/C,and Co/TiO2/C were prepared by electrospinning.The main contents of this article are listed as followed:(1)In order to explore the electromagnetic wave absorption mechanisms and properties of semiconductor/magnetic metal binary-composite nanofibers,hollow ZnO/Co composite nanofibers were prepared by the combination method of electrospinning,high-temperature calcination and hydrogen reduction.The nanofibers with optimized composition ratio exhibit satisfactory electromagnetic wave absorption performances.With the matching thickness of 3.0 mm,the minimum reflection loss is-68.4 dB at 11.4 GHz,With the matching thickness of 2.6 mm,the effective absorption bandwidth can reach 5.9 GHz(9.5-15.4 GHz).The electromagnetic wave absorption performances are much better than that of other composite nanofibers and pure ZnO nanofibers.The analysis indicates that the semiconductor/magnetic metals binary-composite strategy can help to achieve impedance matching optimization and attenuation enhancement at the same time.The particle combination in the nano scale can greatly enhance the interface polarization loss.And the hollow structures can reduce the material density and further optimize impedance matching conditions.The results imply that the design of semiconductor/magnetic metals binary-composite nanofibers can greatly improve the electromagnetic wave absorption performances of materials.(2)In order to simplify the nanofiber preparation process,improve the oxidation resistance and further reduce the density of materials,the semiconductor/carbon binary-composite nanofibers including MnO/C and MnS/C were prepared by replacing the magnetic metals with carbon.In structure,MnO or MnS particles are evenly embedded on the surface of fibrous carbon support.Performance analysis indicated that the nanofibers process three characteristics:light weight,strong absorption and ultra-wide bandwidth.With the matching thickness of 3.6 mm and the frequency at 11.1 GHz,the minimum reflection loss is-68.9 dB.With the thickness of 3.3 mm,the effective absorption bandwidth can reach 7.2 GH.And the wide absorption bandwidth can be achieved at almost all matching thickness.The results indicate that the excellent impedance matching condition is the most important reason for its ultra-wide absorption bandwidth.Meanwhile,sulfide semiconductor and carbon exhibit wonderful functional coordination.This study will provide a new idea for the development of high-performance EWAMs.(3)In order to solve the problem of relatively thick matching thickness,and maintain the convenient preparation method,and maintain the convenient preparation method,magnetic metals were introduced into the above binary composite nanofibers,and the TiO2/Ni/C ternary-composite nanofibers were prepared.Structurally,a layer of rough and folded TiO2 is uniformly coated on the surface of carbon nanofibers,while Ni nanoparticles are embedded both in the surface and interior of the nanofibers.In performance,with the thickness of 2.0 mm,the minimum reflection loss is-74.5 dB.With the thickness of 2.6 mm,the effective absorption bandwidth can reach 6.7 GHz.The results show that the magnetic metals can improve the attenuation ability and reduce the matching thickness.The comprehensive performances of these nanofibers are much superior than other reported ternary-composite materials,which implies a good potential as an efficient EWAMs.(4)In order to maintain the above advantages in absorption properties and material preparation,and further optimize the performance in medium and low frequency ranges,the previous idea was inherited,and the metallic Co was applied to replace Ni to prepare TiO2/Co/C ternary-composite nanofibers.Structurally,Co nanoparticles are dispersedly embedded on the surface of carbon nanofibers,while TiO2 nanocrystals with-3 nm diameter are uniformly distributed on both the surface and inside of carbon nanofibers.With the matching thickness of 2.5 mm,the minimum reflection loss is-57.2 dB,and the maximum effective absorption bandwidth reaches 6.8 GHz.The absorption performances in medium and low frequency ranges are significantly enhanced,which mainly benefits from the further optimization of impedance matching characteristics and adjustment of microstructures by metallic Co.The analysis proves that the design strategy of semiconductor/magnetic metals/carbon ternary-composite nanofibers possesses high rationality and superiority,which further demonstrates its great potential as a new-type high-efficiency EWAMs(5)In order to further study the effects of carbon matrix in the ternary-composite nanofibers,TiO2/Co/C composite nanofibers with different graphitization degrees were prepared.It is noted that with the raising carbonization temperature,the permittivity and conductivity will increase continuously,resulting in the electromagnetic wave absorption performances improve first and then decline.Further study indicates that the increased carbonization temperature will lead to the enhancement of attenuation ability and the weakened of impedance matching at the same time.It is exactly the opposite relationship between impedance matching characteristics and attenuation ability that ultimately limits the material preparation in a specific carbonization temperature range.Meanwhile,the study indicates that this law may be apply to most EWAMs obtained from organic component carbonization,which implies a high guiding significance for the exploration and design of high-performance carbon-based EWAMs. |
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