Preparation And Electromagnetic Wave Absorption Properties Of Fe₃O₄ Nanocomposites

The development of electromagnetic waves has brought significant changes to human life.In contrast,the increasing electromagnetic radiation and interference problems have become increasingly prominent,posing a threat to human health and affecting the precise operation of devices.In the military dimension,effectively reducing the enemy’s electromagnetic detection signals and improving the weapon system’s survival ability has become an essential issue in weapon development.Fe₃O₄is a magnetic spinel-structured ferrite that is widely used in the field of electromagnetic wave absorption.However,a single Fe₃O₄ absorbent material cannot meet the demand for high-performance absorbent materials,as its absorption performance is generally influenced by impedance mismatching and low-loss characteristics.Recent discoveries of new carbon materials such as carbon nanotubes,porous carbon,and graphene have offered opportunities to develop high-performance absorbent materials with lightweight,high electrical conductivity,large specific surface area and abundant content.This work focuses on synthesizing carbon/Fe₃O₄ composites that meet ideal absorbent material criteria through a simple and mild synthesis process that controls impedance matching and loss characteristics.By adjusting the structure,components and morphology of the carbon/Fe₃O₄ composites,we prepared the materials that can meet the ideal criteria,including lightweight,thin thickness,strong absorption,and wide frequency band,of absorbent materials.The main research content of this paper is as follows:（1）The spatial structure of the material has a significant impact on its absorption performance.For example,hollow structures can reduce the material’s density and facilitate its multiple scattering,thereby improving its loss characteristics.In this study,hollow Fe₃O₄ particles and hollow Fe₃O₄/carbon nanotube（CNT）composite materials were prepared using a hydrothermal method.The average diameter of the Fe₃O₄ hollow particles are 400 nm.The hollow spheres of Fe₃O₄in Fe₃O₄/CNT composites are partially damaged and the size is uneven.It was found that although the Fe₃O₄ hollow spheres had a perfect hollow morphology,their dielectric loss was weak,and its absorbing performance is poor.Combining the CNT with the Fe₃O₄ hollow spheres can significantly enhance its absorption performance.The optimal absorption was-29.8 d B,and the adequate absorption bandwidth was 2.47GHz（7.2-9.67 GHz）.This indicates that the addition of dielectric materials contributes more to the absorption properties of the composites.（2）CNT has the characteristics of low density,good conductivity,and excellent dielectric properties.CNT/Fe₃O₄ composite absorbing materials were prepared by hydrothermal method,and the content of CNT in the material was controlled to reveal the change of electromagnetic wave absorption performance of CNT/Fe₃O₄composites from the perspective of impedance matching.In this chapter,Fe₃O₄/CNT composites were prepared using a hydrothermal method.When the CNT contents in the materials were 17.4wt%,the composites exhibited the lowest reflection loss value of-50.37d B,When the CNT contents in the materials were 17.4wt%,the effective bandwidth was 3.83 GHz（13.75-17.58 GHz）.The results show that the absorption property of the composite can be optimized by adjusting the content of the intermediate electric material.（3）Kj C has a unique hollow spherical morphology,high specific surface area,ultra-high conductivity,and unique branching structure.By applying Kj C to the absorbing materials for the first time,we examined the absorption performance of the Kj C/Fe₃O₄ composites.We prepared the Kj C/Fe₃O₄ composites with a core-shell structure via a one-step evaporation-reflux method.This composite used Kj C as the core and Fe₃O₄ nanoparticles as the shell,with an average diameter of about 6～8 nm.It was found that this composite exhibits good impedance matching performance and excellent electromagnetic wave absorption performance.The maximum reflection loss of the composite reached-64.21 d B and the widest effective bandwidth reached 4.17GHz.This work also used a safer,more convenient,and more suitable industrial promotion reaction kettle method to prepare Kj C/Fe₃O₄ core-shell structure composite materials by controlling experimental parameters.This Kj C/Fe₃O₄ composite material with an effective absorption width of 4.41 GHz（12.82-17.23 GHz）was obtained.（4）A graphene aerogel composed of graphene structures as the basic unit can not only solve the problem of graphene’s easy aggregation but also gives the material an extremely low density and a relatively high specific surface area owing to its highly porous 3D structure.Fe O_x/r GO composite aerogel was prepared using a hydrothermal method in this work.The graphene nanosheets interconnected with each other in a three-dimensional space,forming a porous network structure.The Fe O_xparticles formed in the composite were mainly Fe₂O₃ which are distributed evenly on the graphene nanosheets.The microwave absorption tests were carried out by mixing Fe O_x/r GO composite（at 5 wt%）with paraffin.It was found that the best effective absorption bandwidth of all the prepared samples was above 6 GHz.When the theoretical Fe O_x contents is 15 wt%,the sample had the best reflection loss of-50.76d B and the effective absorption bandwidth reached 7.99 GHz（10.01-18.00 GHz）.When the theoretical Fe O_x contents is 20 wt%,the lowest reflection loss of-61.47 d B was abtained.Compared with recently reported graphene-based composite materials,the composite prepared in this chapter achieved a strong absorption capacity and broad effective absorption bandwidth with a filling percentage of 5 wt%only.This study indicates that the Fe O_x/r GO aerogel composite prepared in this work is a promising material in microwave absorption.