Structural Regulation And Microwave Absorption Properties Of Carbon-Coated Ferrite-Based Composites

The development of electromagnetic technology has created many convenient conditions for people’s lives,but the accompanying electromagnetic pollution is also becoming more and more serious.The development of effective electromagnetic wave absorbing materials is one of the most important means to solve this problem.The magnetic/dielectric composite material has excellent performance in the absorbing material system,because it not only has the advantage of magnetic/dielectric synergistic loss,but also can optimize the impedance matching conditions.However,usually due to the lack of structural design,it is difficult to further improve its absorbing performance.Effective structural modulation can not only control and improve the absorbing properties of materials,but also further clarify the electromagnetic wave loss mechanism.This thesis mainly takes carbon-coated ferrite-based composites as the research object.Several structural design strategies,such as the rational regulation of crystalline/amorphous heterointerfaces,the construction of 3D porous structures,the modulation of carbon shell thickness,and the in-situ construction of intercrystalline heterointerfaces and defects,were used to control the wave-absorbing properties of composite materials,and the related wave-absorbing mechanisms were analyzed in depth.Thin-layer carbon-coated Fe₃O₄nanoparticles were prepared by high-temperature oil-phase reaction and acetone vapor carbonization deposition.The carbon coated on the outside of Fe₃O₄ has an amorphous structure with a thickness of about 2 nm,and the thickness of the coated carbon layer is the same in different samples.The size of the final Fe₃O₄/C nanoparticles can be adjusted by controlling the oil-phase reaction conditions,that is,the Fe₃O₄/C crystalline/amorphous heterointerface structure can be adjusted.When the average size of the composite nanoparticles is about 52 nm,the microwave absorption performance is excellent,that is,when the thickness of the absorbing coating is 2 mm,the strongest reflection loss value reaches-58.5 d B（14.88 GHz）,and the corresponding effective absorption bandwidth is 5.63 GHz（12.37-18 GHz）.The research shows that the microwave attenuation ability of Fe₃O₄/C nanocomposites is controlled and dominated by the interface polarization loss.Fe₃O₄@C core-shell nanocomposites with three-dimensional（3D）porous structure were prepared through four processes of combustion reaction,high temperature heat treatment,surface phenolic polymerization and carbothermal reduction.Compared with zero-dimensional nanoparticles,the preparation method has optimized the structure of the material,which is beneficial to promote multiple scattering of electromagnetic waves and improve impedance matching.In addition,the tunable thickness of the amorphous carbon shell was achieved by controlling the polymerization conditions.With the increase of carbon shell thickness,the electromagnetic wave absorption performance of Fe₃O₄@C first increases and then decreases.When the average thickness of the carbon shell is 28.3nm,the Fe₃O₄@C nanocomposite has the best absorption performance,the minimum reflection loss value can reach-55.5 d B（8.3 GHz,2.5 mm）and the corresponding effective absorption band is 6.8-10.8 GHz,and its maximum effective absorption bandwidth is 5.3 GHz（8.7-14.0 GHz,2.0 mm）.Compared with the thin-layer carbon-coated Fe₃O₄ nanoparticles,Fe₃O₄@C has promoted the absorption for the intermediate frequency band on the basis of maintaining the effective absorption for other frequency bands.It is found that the modulation of the carbon shell thickness results in the promoted dipole polarization being the main contributor to the enhanced microwave loss.Based on the above four-step preparation process,the structure of the material was further optimized by introducing Mn element,and the carbon-coated Mn Fe₂O₄/Mn O heterojunction nanocomposite was prepared.The relative content of Mn Fe₂O₄ and Mn O can be controlled by adjusting the Mn/Fe molar ratio,thereby realizing the regulation and optimization of the wave-absorbing properties of the material.When the molar ratio of Mn Fe₂O₄ to Mn O is 1,the composite material exhibits the most excellent wave-absorbing properties:the strongest reflection loss at 10.45 GHz and 2.87 mm can reach-72.1 d B;the maximum effective absorption bandwidth is 5.01 GHz（8.39-13.4 GHz）and the matching thickness is 2.83 mm;the effective absorption bandwidth is higher than 4 GHz at any thickness in the range of 2-3.1 mm,which broadens the overall absorption bandwidth.The research shows that this experiment has realized the in-situ controllable construction of heterointerfaces and defects,and achieved the purpose of controllable adjustment and optimization of the wave-absorbing performance by the interface/defect-induced polarization loss.The interphase heterostructure was further modified,and the（Fe₃O₄/Zn O）@C dual-phase core@shell structure was successfully constructed by introducing Zn O with better dielectric response through the same four-step preparation method.The electromagnetic parameters and wave-absorbing properties of（Fe₃O₄/Zn O）@C can be significantly regulated by controlling the relative content of Zn O in the product.When the molar ratio of Zn to Fe is 1:2,the performance of the composite is the best,and its effective absorption bandwidth can reach 7.11 GHz when the matching thickness is 1.9 mm,achieving the largest effective absorption bandwidth in this thesis.The research and analysis show that the Fe₃O₄/Zn O dual-phase heteronucleus obtained from the in-situ carbothermic reduction on the Zn Fe₂O₄ precursor has excellent polarization loss ability,resulting in a great improvement in the wave-absorbing performance of（Fe₃O₄/Zn O）@C.