Contrallable Synthesis Of Magnetic Materials With Hollow Multi-shelled Structures For High-performance Electromagnetic Wave Absorbing

The wide use of wireless communication and other electronic products has promoted the great progress of modern society,however,it also brought immeasurable electromagnetic radiation,causing serious electromagnetic interference and pollution problems.Electromagnetic wave absorbing materials that transform the energy of incident electromagnetic wave into heat energy and other forms of attenuation and dissipation,is of great significance to human health,communication equipment,environment protection and military applications.As a kind of traditional wave absorbing material,ferrite has performed lots of advantages such as high permeability and low cost,but there are some defects such as high density,single loss mechanism and poor impedance matching,making it hardly meet the requirements of light,thin,poweful and wide absorbing materials at the same time.Hollow multi-shelled structure（HoMS）,a unique multi-stage cavity structure,performs well in significantly reducing the density of materials,prolonging the propagation distance of electromagnetic wave and providing rich design space for the structural control of composite materials.Thus,HoMS has great application potential in the field of electromagnetic wave absorption.In this paper,using spray drying technology based on the order template method,a series of successful preparation of magnetic HoMS（MFe₂O₄,M=Ni,Zn,Fe,Co）are reported.And through the composite carbon regulation,the composition and structure of material and the relationship between the wave absorption performance are systematically studied.The details are as follows:（1）By spray drying and followed high temperature calcination,a series of HoMS materials such as NiFe₂O₄and Co Fe₂O₄,were successfully prepared.On this basis,Fe₃O₄HoMS were prepared by thermal reduction of Fe₂O₃HoMS,while Zn-doped NiFe₂O₄HoMS were obtained by adding zinc ions to the precursor solution.By adjusting precursor solution composition,spray drying process parameters,calcination conditions and other factors,the HoMS composition,morphology,shell number,shell thickness and other geometric parameters were controlled.Besides,the macro preparation process of HoMS materials was explored,and the magnification preparation of Ni O and Fe₂O₃HoMS was preliminary realized,which laid a foundation for the large-scale production and practical application of HoMS materials.（2）The hysteresis loops of NiFe₂O₄HoMS and Co Fe₂O₄HoMS with different shells were measured by VSM.The results show that the saturation magnetization of ferrite HoMS increases and the coercivity decreases with the increase of the number of shells.Using paraffin as binder,the electromagnetic parameters of these magnetic HoMS in the frequency range of 1～18 GHz were measured by coaxial method,and the reflection loss in the thickness range of 1～5 mm was calculated according to the transmission line theory.With the increase of the number of shells,both the dielectric constant and the imaginary part of the permeability of HoMS increase.HoMS shows the characteristics of enhanced electromagnetic loss,with its magnetic loss tangent much higher than the dielectric tangent and absorption loss peak concentrated at the high frequency stage.However,due to the single magnetic component and poor impedance matching,its performance is still not satisfactory.（3）In order to improve the impedance matching of magnetic HoMS materials,the carbon-coated HoMS composites were successfully prepared by in-situ polymerization of dopamine and heat treatment under inert atmosphere.The content and thickness of carbon coating were controlled by changing the dosage of dopamine and calcination parameters.The electromagnetic wave absorption properties of the carbon composite HoMS are better than that of single component.Among them,3s-Fe₃O₄@C HoMS showed the best reflection loss,reaching-54.65 d B at the thickness of 2.7 mm and 7.81 GHz frequency.The maximum effective bandwidth is achieved by 3s-NiFe₂O₄@C HoMS,4.49 GHz（3.64～5.82 and 15.44～17.75 GHz）,while the matching thickness is 5 mm.The effective bandwidth of 3s-Co Fe₂O₄@C HoMS is up to 4.49 GHz（3.64～5.82 and 15.44～17.75 GHz）and reflection loss exceeds-5 d B（9.27～18 GHz）in almost the entire X and Ku bands with thickness is only 2.5 mm.