| With the development of anti-stealth technology,the requirements of radar absorbing materials for various weapons and equipment become higher and higher.They not only require thin thickness,strong absorption,broad bandwidth,light weight,but also good chemical stability to adapt to complex environmental requirements.?'-Fe4N is a new absorbing material with great potential in research and development.Compared with carbonyl iron,it has better corrosion resistance,wear resistance and chemical stability.However,previous studies show that when ?'-Fe4N is used alone,its high conductivity leads to poor impedance matching,which limits the performance of its high loss characteristics.Therefore,?'-Fe4N composite absorbing materials with core-shell structure are constructed by combining y'-Fe4N with many dielectric materials with different conductivity,which is expected to improve the absorbingproperties of ?'-Fe4N and meet the requirements of "thin thickness,wide bandwidth,light weight and strong absorption".In this paper,the core-shell composite powder with y'-Fe4N as cores and amorphous carbon and SiO2 as shells was prepared.The morphology andelectromagnetic properties of the composite powder were studied.The effects of the coating layer and powder morphology on its magnetostatic parameters,electromagnetic parameters,absorbing performance,and loss mechanism were analyzed.The absorbing mechanism was revealed.The absorbing performance of the absorbing body with double-layers structure was designed and studied.The main research contents and conclusions are as follows:The flake-like and cubic a-Fe2O3 powder was prepared respectively by hydrothermal method.The intermediate products of polyaniline coating,flake-like?-Fe2O3@PANI,were prepared by in-situ polymerization.The intermediate products of SiO2 coating,flake-like ?-Fe2O3@SiO2 and cubic ?-Fe2O3@SiO2,were prepared by the sol-gel method.After nitriding,?'-Fe4N@C and y'-Fe4N@SiO2 composite absorbing powders with different shapes were obtained.The results of structure and morphology show that the nuclear phase of the product is basically the same,with y'-Fe4N as the main phase and containing a small amount of Fe3O4 and Fe.The different coating layer and shape will affect the relative content of the phase and the morphology of the particle surface.The surface of uncoated ?'-Fe4N particles is rough and has a lot of pores.The SiO2 coated flake composite powders(F-?'-Fe4N@SiO2)and cubic composite powders(C-?'-Fe4N@SiO2)still maintain the morphology like before nitriding,and the surface of the particles is relatively smooth.When the coating layer is SiO2,the content of Fe in the cubic product is relatively high.When the coating layer is PANI,the PANI layer is transformed into amorphous carbon layer after nitridation treatment.The shape of the product C-?'-Fe4N@C becomes irregular and the surface roughness is large.The results show that the phase composition and morphology of the composite absorbing material can be controlled by the design of the cladding material,and the absorbing performance of the composite absorbing material can be optimized.The influence of composite powder shape and coating layer on magnetostatic and electromagnetic parameters was studied by vibrating sample magnetometer(VSM)and vector network analyzer(VNA).respectively.The results show that when the particles have flake shape and coating layer,the specific surface area increases with the decrease of the particle size,which results in the decrease of saturation magnetization and the increase of coercivity.The existence of the cladding material enhances the interfacial polarization effect of ?'-Fe4N composites and adjusts the relative dielectric constant effectively.The electromagnetic loss ability and absorbing mechanism of ?'-Fe4N composite powder were analyzed in terms of dielectric loss and magnetic loss.The results show that the tangent values of dielectric loss angle and magnetic loss angle of all kinds of composite powders have resonance absorption peaks.The relative dielectric constant has a good dispersion effect,which indicates that there are several polarization relaxation processes,and the polarization relaxation loss is significant.When the frequency is less than 12 GHz,the ferromagnetic resonance plays a dominant role in the magnetic loss,and the magnetic loss plays a more important role in the enhancement of the absorbing performance than the dielectric loss.When the frequency is higher than 12 GHz,the magnetic loss mainly comes from eddy current loss.However,with the increase of frequency,the contribution of magnetic loss decreases,and the dielectric loss occupies a dominant position.Through the study of impedance matching and attenuation constant,it is found that the impedance matching property of F-?'-Fe4N@SiO2 is better than that of C-?'-Fe4N@C.However,the attenuation is inverse.The uncoated ?'-Fe4N powder has the worst impedance matching performance.The minimum reflection loss of F-?'-Fe4N@SiO2 is-37.8 dB with a thickness of 2.0 mm and the effective bandwidth of 2.9 GHz at 8.8 GHz.The minimum reflection loss of C-?'-Fe4N@C is-35.1 dB with a thickness of 2.0 mm and the effective bandwidth of 3.6 GHz at 8.9 GHz.The simulation design of the absorbing body with double-layer structure is carried out.The results show that when the composite powder with the best impedance matching is selected as the outer matching layer and the composite powder with the best attenuation performance as the inner absorption layer,the absorption performance of the composite powder with the same thickness is stronger than that of the monolayer structure.The effective absorption band width is also slightly increased.When the thickness of the absorption layer is 1.8 mm,and the thickness of the matching layer is 0.2 mm,the minimum reflection loss is-49.5 dB with effective absorption bandwidth of 3.8 GHz at 9.1 GHz.The results show that for ?'-Fe4N composite absorbing materials,the synergistic effect of the intrinsic electromagnetic properties,the shape,and the amorphous dielectric coating is the key to obtain excellent absorbing properties. |
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