Preparation And Electromagnetic Wave Absorbing Properties Of Anisotropic Nanocapsules

The traditional magnetic/dielectric core/shell-nanostructured electromagnetic(EM)wave absorbing materials with isotropic geometry generally always own high input impedance due to their high relative permittivity(?_r=?_r?-j?_r?)and low relative permeability(?_r=?_r?-j?_r?),which seriously hinders their development and application.One-dimensional(1D)and two-dimensional(2D)magnetic nanomaterials show better performance than the isotropic cases,which can exceed the Snoek's limit,due to the enhanced shape anisotropy induced permeability and natural resonance.Based on the high permittivity of core/shell nanocapsules,anisotropic magnetic cores are employed and carefully studied in this work,in order to improve the relative permeability,the impedance matching,and EM wave absorption performance fundamentally.In this paper,anisotropic nanocapsules of Ni/PANI and Ni/PS nanochains,Fe₃O₄/C nanorods,Ba Fe₁₂O₁₉/g-C₃N₄ nanosheets are prepared by various liquid chemical methods,and their growth mechanism and EM wave absorption properties are studied in details.Firstly,Ni/PANI and Ni/PS nanochains coated with conductive and insulating shells are prepared by conbining a low-temperature liquid reduction method and an in-situ polymerization.Compared with Ni and Ni/PS,the interfacial dipoles enhance the dielectric attenuation of Ni/PANI nanochains,while the shape anisotropy modulated magnetic attenuation leads to a broadband absorption of 3.0GHz in X-band at a thin thickness of d=2.7 mm,and the reflection loss(RL)achieves-43.0 d B at 8.2 GHz.Secondly,the one-dimensional Fe₃O₄/C nanorods are prepared by a carbothermal reduction method.By the adjustment of the amorphous C-shell thickness and anisotropic enhancement of the 1D[111]easy magnetization direction,the nanocapsules of thinner C-shell shows strong EM wave absorbing properties in Ku band with a highest RL of-51.3 d B at f=17.0 GHz and d=5.1 mm,and the effective absorption bandwidth of 2.1 GHz.Finally,2D Ba Fe₁₂O₁₉/g-C₃N₄ nanosheets are prepared by an alkali-assisted hydrothermal method.2D dielectrics isolate the interaction between the magnetic dipoles in Ba Fe₁₂O₁₉ nanosheets and inhibit the skin-effect,which increases the??of 1%and the??of 27.2%at 1-8 GHz.However,only a sharp absorption peaks with a effective absorption bandwidth(RL<-5)of 0.2GHz and the RL of-8.79 d B at 17.4 GHz is obtained at d=7.6 mm,because of the weak interfacial dielectric polarizations affected by the core-shell gaps.The results exhibit that the anisotropic magnetic structure can enhance the magnetic attenuation performance of nanocapsules,improve impedance matching,and enhance the bandwidth and performance of EM wave absorption.It is of great theoretical and practical value for the EM wave absorbing nanomaterials with broad bandwidth and comprehensive performance.