Interfacial Control And Electromagnetic Properties Of Co-based Bimetallic Heterostructures

Electromagnetic?EM?wave absorbing materials are a kind of important functional material,which can absorb and attenuate the energy of incident EM wave via converting EM energy into heat energy or making EM wave disappear because of interference.These materials have important applications in EM radiation protection and military stealth.Owing to many problems,such as high density,heavy weight and narrow absorption band,the traditional magnetic absorbing materials are difficult to meet the high requirements of"light weight,thin thickness,strong absorption and wide bandwidth"for effective absorbing materials.Therefore,we propose to prepare effective and light weight microwave absorbing materials,by using plasmon resonance enhancement effect to improve EM parameters,reduce the filling ratio and reduce the surface density or selecting materials with low density and/or hollow structure as absorbers.In this paper,three kinds of Co–based bimetallic magnetic absorbing materials were designed and prepared.The regulation methods of morphology,composition,structure and surface-interface and their effects on magnetostatic and microwave absorption properties were investigated.The mechanism of plasmon resonance enhanced electromagnetic and microwave absorption was clarified,the structure and composition of the materials were optimized,and excellent absorbents were obtained.The specific contents of these studies are as follows:1.The plasmon resonance enhanced electromagnetic and microwave absorption properties of strawberry–like Co/C/Fe/C core–shell hierarchical flowersA two-step method has been used to synthesize strawberry–like Co/C/Fe/C core–shell hierarchical flowers?CSHFs?consisting of separated Fe/C nanoparticles?NPs?anchoring on a Co HFs surface by decomposing Fe?CO?₅ in the presence of Co HFs under N₂.The diameter and interparticle spacing of Fe/C NPs were modulated through a carefully devised kinetically tuned procedure,in which the decomposition speeds of Fe?CO?5 and CO could be easily adjusted by the volume and decomposition temperature of Fe?CO?₅,ultimately giving rise to a strawberry–like surface.The Co/C/Fe/C CSHFs with the interparticle-spacing-to-diameter ratio of 1.36–0.76 exhibited significantly enhanced permittivity and permeability with negative values because of the surface plasmon resonance and coupling.An optimal reflection loss?R_L?of-45.06 was found at 17.92 GHz and the frequency range?R_L?-20 dB,99%absorption?was over 2–18 GHz.Our findings demonstrated that optimally designed plasmonic heterostructures could be fabricated to improve microwave absorption performances for future applications.2.Controllable preparation and ultra-wideband absorbing properties of CoNi alloy hollow spheresA series of bimetallic FCC Co_xNi_1-x?x=1,0.858,0.813,0.714,0.662,0.137?alloy hollow microspheres?AHMs?with continuously tunable composition and wall thickness were successfully synthesized through a one-pot liquid phase reduction procedure.The composition was adjusted by controlling Co²⁺/Ni²⁺molar ratio and wall thickness could be continuously tuned via changing Co²⁺/Ni²⁺molar ratio,metal ion concentration,reaction time and reaction temperature.The results manifested the formation of FCC CoNi AHMs via an inside-out Ostwald ripening process.The saturation magnetization?Ms?decreases linearly with increase of Ni content.The exploration of microwave absorbing performances demonstrated that CoNi AHMs are eminently suitable for microwave absorbing materials with strong absorption,broad bandwidth,and light weight.Hereinto,the wax-based composites containing 35 wt%Co_0.81Ni_0.19,45 wt%Co_0.86Ni_0.14 or 50 wt%Co_0.66Ni_0.34 AHMs exhibited the significantly enhanced absorption capability with the minimum R_L values of-35.3 dB,-47.3 dB,-54.6 dB and the effective absorption bandwidth?RL?-10 dB?of 8.16 GHz,9.2 GHz,10.08 GHz corresponding to a layer thickness of 1.9 mm,1.8 mm,2.6 mm,respectively.3.Research on Controllable Preparation and Electromagnetic Characteristics of Co fibers and Co/Cu bimetallic heterostructure fibersThe Co/Cu bimetallic heterostructure fibers were synthsized by a two–step method of hydrothermal-liquid phase reduction using Co fibers and Cu?CH₃COO?₂?H₂O as precursors and Cu source,respectively.The influences of Cu²⁺/Co²⁺molar ratio???,reaction time and magnetic field on the size and morphology of the Co fibers were investigated.The Cu²⁺/Co²⁺molar ratio???can effectively be used to adjust the surface morphology and composition of the Co/Cu bimetallic fibers,therefore modulating their static magnetic and microwave absorbing properties.The M_s value of the samples decreased with the increasing?,and the coercive force?H_c?value first increased and then decreased with a maximum value of 131.3Oe at?=3:10.The Co/Cu bimetallic heterostructure fibers formed at?=0.5:10 exhibited superior microwave absorption performances.The wax composites containing 20 wt%Co/Cu obtained at?=0.5:10 exhibited the bandwidth?R_L?–10 dB?of 6.64 GHz and a minimum RL value of-52.5 dB at 11.92 GHz,corresponding to a layer thickness of 2.6 mm.The excellent microwave absorption performances are attributed to the conductive network formed by heterogeneous fibers,interface polarization,orientation polarization and plasmon resonance enhanced electromagnetic and microwave absorption caused by metal–dielectric structure.Our data demonstrate that the microwave absorption performances can be improved when a small amount of Cu is composed with Co fibers.