Preparation And Electromagnetic Characteristics Of Magnetic Microwave Absorber With Hollow Structure

Ferrites and magnetic metal powders are widely used as microwave absorbing materials because of their low cost and good microwave absorption; however, they are quite heavy which restricts their use in the application requiring light weight. Therefore, it is necessary to explore new microwave absorbing materials to meet the requirements of thin, light weight, wide bandwidth and strong microwave absorption. Preparing materials with hollow structure is an effective way to obtain low density. Moreover, multiple reflections of electromagnetic waves may be formed in the cavity and improve the microwave absorption. Recently, much attention has been focused on the materials with hollow spherical structure, synthesized by coating metals or composite on the hollow glass spheres (or microballoon).In this paper, the hollow nickel and hollow Ni/Co spheres were prepared by template method. The phase structure, chemical composition, morphology, magnetic properties and electromagnetic characteristics were studied by means of X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and vector network analyzer. The absorbing properties were calculated by electromagnetic parameters. The main researches as follows:Using carbonyl iron as template and hydrazine hydrate as reducing agent, carbonyl iron/Ni powders were prepared by electroless plating, and then hollow nickel spheres were obtained by removing the template through corrosion.The effect of loadage of carbonyl iron on the structure and electromagnetic characteristics of hollow nickel has been studied. Results showed that the shell thickness can be controlled by adjusting the loadage of carbonyl iron template. The hollow nickel spheres exhibited good magnetic properties. The complex permittivity and permeability changed as the loadage varied. The microwave absorption performances could be tuned by changing the loadage. As the loadage was20g/L, the hollow nickel spheres exhibited the thinnest shell of0.12μm and showed the minimum reflection loss (RL) value of-27.2dB at13.4GHz with a matching thickness of1.4mm, the effective absorption band (RL<-5dB) from11.8to18GHz, covering the whole Ku-band (12.4-18GHz). Carbonyl iron/Ni composites with different corrosion stage were obtained by changing corrosion time. Results showed that the internal cavity increased as the corrosion time increased, and the saturation magnetization decreased due to the decrease of Fe content. The complex permittivity and permeability changed as the internal cavity varied. The absorbing properties also can be tuned by the internal cavity. The samples which under corrosion completely, hollow nickel powder, showed the best absorption performance.The hollow Ni/Co spheres were prepared by electroless plating on the hollow nickel spheres. The results showed that the Co layer was loose and Co particles varied from sphere to fluffy as the loadage increased. Changing the loadage has a little effect on the magnetic properties. The complex permittivity and permeability of hollow Ni/Co powders can be adjusted by changing the amount of loadage to change the content of Ni and Co. The microwave absorbing performance of the Ni/Co spheres has been improved compared with the hollow nickel. The minimum reflection loss (RL) value of-47.1dB was obtained at10.08GHz with a matching thickness of2.6mm, and the absorption band (RL<-10dB) was2.86GHz.