| Metals or alloys with high conductivity or permeability are usually used as electromagnetic shielding materials, taking board, net, strip or paint form. But, some disadvantages of metallic materials restrict its practical application, such as high density and rigidity, poor resistance to corrosion, complex usage in construction and hard to sharping. Expanded graphite (EG) possess several advantages in electromagnetic shielding field, just as light, soft, high thermal and chemical stability. But, diamagnetism of EG determined its poor SE in low frequency range, and it’s hard to disperse additive to the gap of graphite layers because of its low density and lipophilicity. Several dispersions of magnetic nanoparticles on EG have been prepared by gas reduction in our previous work, such as Ni, Co, Fe, Fe-Ni alloys, and these materials could increase its SE in low frequency range. But high temperature (300-1000°C) and relatively long preparation period (about10hour) were prejudicial to industrial mass production, while SE should be decreased by impurities produced in high temperature environment, as Fe3O4, Fe3C.On the foundation of previous works, we prepared Fe, Ni, Cu, Ag nanoparticles dispersed on expanded graphite compound by liquid reduction, moderate temperature in reaction (0-90°C) and relatively high productiveness (10min-2h) possess advantage in industrial production. We mainly use scanned electronic microscope (SEM), X-ray diffraction (XRD), Mossbauer spectrum, vibrating sample magnetometer (VSM), flange schematic setup and Agilent E5062A network analyzer to research the surface textural and structural properties, electronic conductivity and magnetic permeability and shielding effectiveness. Then we built a system to measure and verify electromagnetic performance and shielding effectiveness of thin materials sized of ring by comparison of theoretical and actual results.The paper is mainly separated by four parts, which studied on the following contents:1) We prepared Fe and Ni magnetic nanoparticles loaded on expanded graphite for electromagnetic shielding by liquid reduction in ordinary temperature and researched the effect of complexing agent (Seignette salt) and carrier (EG) during the reaction of preparation of Fe/EG. Then we found that Seignette salt could decrease the proportion of amorphous FeB and increase the resistance to oxidation and ferromagnetic properties, EG also could enhance the crystallinity of particles distributing on the surface of layers. Both Fe/EG and Ni/EG possess the best SE value at30%of metal mass percentage, which are63.5dB and70.2dB at300kHz, comparing with pure EG, the increasing value is8.5and15.2dB.2) Leading in magnetic field of appropriate intensity during the reaction for preparation magnetic linear or chain materials is one of effective method, so we duplicate the experiment of preparation Fe/EG and Ni/EG in magnetic field and observe the effect of magnetic field in the preparation of magnetic nanoparticles and its dispersion on EG compound. The results showed that in aqueous solution, the resistance between hydrone and crystal nucleus is weak, so the nucleus could translation motion freely to form a linear of chain structure under the magnetic fields, and the draw ratio of nano chain increase while the raise of magnetic field intensity. But on the surface of EG layers, the dispersion of nanoparticles is still unordered because of the relative high resistance between the carbon surface and crystal nucleus.3) To increase the magnetic property, it’s considerable to prepare ferrated nanoparticles dispersion on EG compound by the usage of high magnetic permeability of Fe. But the result sometimes cannot show the exactly accurate structural properties by ordinary measurement, such as XRD, because of the complex valence state of the iron element. So we adopt Mossbauer spectrum to analysis the structural properties of compound of ferrated nanoparticles and EG. Through the comparison of measured Mossbauer and XRD spectrum of Fe/EG prepared by liquid reduction and NixFei_x/EG, FexN/EG prepared by gas reduction, Mossbauer spectrums could show the more accurate analysis of the composition phase of materials, which couldn’t be determined by XRD because of foggy shape or the diffraction peak nearby the Fe in alloy samples.4) We prepared the compound with non-magnetic nanoparticles (Cu, Ag) and EG by liquid reduction and through the comparison of samples with different metal mass percentage, determined that samples with30%Cu and Ag possess the best SE, which are72.4dB and69.8dB at300kHz, which shows that the implantation of particles with high conductivity also increase the SE obviously at low frequencies. So, based on the three kinds of materials, Ni/EG, Cu/EG and Ag/EG, we measure the conductivity, magnetic permeability and dielectric spectrum, and infer the theoretical electromagnetic shielding effectiveness according to the simulation. The results show that the fitness of calculation and measurement is fine; the error between both is smaller than10dB. Through the systemic measurement and calculation, we built up a system of measuring electromagnetic shielding effectiveness and analyzing of relevant parameters of thin materials sized of ring. |
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