Preparation And Microwave Absorbing Properties Of Magnetic Nanocomposites

The expanding electromagnetic interference and information leaking problems have been arising seriously from the ever-growing utilization of wireless communications and high frequency circuit devices in the gigahertz, has become a big issue in both military and civil application. The microwave absorbing materials are one of the effective way to solve this problem. The magnetic nanoparticles are being extensively used as microwave absorbent in a wide frequency band because of its advantages such as high dielectric and magnetic losses.The Fe, Ni nanoparticles(Fe^ Ni NPs) and carbon-coated Ni nanocapsules(Ni(C) NCs) were prepared by DC arc-discharge method. It reveals that Fe and Ni nanoparticles were coated by oxide, and the Ni(C) nanocapules were coated by carbon. The surface modification of as-prepared Fe and Ni nanoparticles was carried out by using silane coupling agents (KH-550) to improve its dispersion stability into the organic matrix. A series of Fe NPs/CFs/ER nanocomposite plates were prepared by using Fe nanoparticles as microwave absorbent, carbon fibers were used to enhance the multi reflection inside and also mechanical properties.The electromagnetic characteristics and absorption mechanism of Ni and Ni(C) nanoparticles were investigated at different wt.%(10%,20%,30% and 40%) with epoxy resin matrix. The results revealed that polarization mechanisms for Ni nanoparticles including the interface polarization, space charge polarization and dipole polarization, magnetic losses are attributed to the natural resonance and eddy current loss at high frequency. The core-shell structure of nanoparticles results in a common point at 8.2GHz in real part of complex permittivities versus frequency of composites with different Ni contents. It has been observed that electromagnetic loss of Ni nanoparticles is mainly dominated by magnetic loss. Ni(C) nanocapsules exhibited characteristics of polarization losses and its carbon layer exhibited obvious dielectric relaxation behavior. The dielectric loss was improved because of carbon shells which also enhanced the interface polarization, the space charge polarization and dipole polarization. Carbon layer would weaken the magnetization of Ni(C) nanocapsules. It revealed that electromagnetic loss of Ni(C) nanocapsules is mainly dominated by dielectric loss.It showed that microwave absorption of Fe NPs/CFs/ER nanocomposite plates increased with increasing of wt.% of Fe nanoparticles and carbon fibers. The carbon fibers enhanced the microwave absorption properties through multi-reflection inside, meanwhile, Fe nanoparticles improved impedance matching condition through changing the impedance of carbon fiber. The microwave absorption property is anisotropic, at the vertical direction of carbon fibers towards incident wave will be polarized and promoted the multi-reflection of microwave and further absorbed by the gradient dispersed Fe nanoparticles which leads to the change of input impedance. Considering the direction of carbon fibers towards incident wave an excellent microwave absorbing structure can be designed and fabricated by adjusting the concentration and gradient distribution of Fe nanoparticles. The reflection loss was calculated by using complex permeabilities and complex permittivities of Ni and Ni(C) nanoparticles, later compared with the measured reflection loss of nanocomposites plates within 2-18 GHz range. It was observed that density of nanoparticles and electromagnetic properties of the matrix would greatly affect on microwave absorption properties of nanocomposites. The microwave absorption of Ni(C) NCs/ER nanocomposites is better than Ni NPs/ER nanocomposites, and diluent needs to be added during curing process to improve fluidity of Ni(C) NCs/ER nanocomposites.