Research On Ni-based Nanoparticles/Expanded Graphite Materials For Anti-information-leakage (Electromagnetic Shielding)

Metals or alloys are usually used as anti-information-leakage (electromagnetic shielding) materials, such as Cu, Fe, Ni, Fe-Si-B alloy, et al. These materials are widely used in many fields, however also have some defects, as big density, complex for construction, hard to shape, et al. Expanded graphite (EG) has small density with soft quality and good thermal-stability and chemical-stability, and particularly EG has excellent electrical conductivity, expressing high shielding effectiveness (SE) especially against electromagnetic wave with comparatively high frequency (over 30 MHz). However, for graphite is antimagnetic, its electromagnetic shielding effectiveness at low frequencies is not satisfactory. In order to improve the SE in low frequency range, we can disperse magnetic metal or alloy particles into the pores of EG, making a good balance between electricity and magnate, thus a kind of material with fine SE in wide frequency range will be obtained. Our previous work has indicated that the loading of magnetic metal or alloy nanoparticles on EG promotes the SE against electromagnetic wave with the frequency of 300 kHz, from 43 dB of pure EG to 53～72.5 dB of composites, however makes no significant difference to the SE at the high frequency of 1.5 GHz. In addition, the composites with about 30% weight content of metals or alloys have a better SE at low frequency of 300 kHz. In this paper, we respectively made use of the methods of reduction heated in H2 atmosphere and chemical plating to prepare Ni-Fe/EG and Ni-P/EG composites, and analyzed the composites using scanning electron microscope (SEM), X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM), and determined the SE in the frequency range of 300 kHz～1.5 GHz by a Agilent E5062A vector network analyzer. We mainly researched some factors as follow effecting the electromagnetic SE of composites:1) Take 10 g EG to adequately impregnate into an ethanol solution containing quantitative iron nitrate, nickel acetate, and acetic acid. Then the mixture was heated in a water bath at 353 K to evaporate the solvent, followed by reduction in H2 flow at 723 K for a period of time. After cooling in N2 flow, Ni-Fe/EG composite was finally prepared. We fixed the weight content of metals as 30% and m (Ni):m (Fe)=8:2, and merely changed reduction time to determine the effect of reduction time on the SE at 300 kHz. We found that the SE at 300 kHz corresponding to 600 min was better, up to about 65.8 dB.2) We still took the weight content of Ni-Fe fixed as 30%, the reduction temperature and time fixed as 450℃and 600 min, merely changed Ni content in Ni-Fe, preparing Nix-Fe1-x/EG composites by the same method as what given by 1). The Ni content x was found to have obvious effects on phase structure, morphology, magnetic properties and electromagnetic SE of the composite. According to SEM images, metal particles had nano-scale diameters. When the amount of Ni was less than that of Fe (except x=0), metal particles distributed assembly like fish-scales on EG, otherwise distributed separately. The XRD patterns of the composites show that the metal particles were mainly in metallic phases, except pure Fe/EG composites contained a few of iron oxides and iron carbides. While x=0.50～0.85, almost no Fe peaks are found which indicated that some Fe and Ni atoms might form alloys. In order to access the existing of NiFe alloys, we have test the Mossbauer spectra of Ni-Fe/EG, which showed the overlap of a magnetic sextet and a nonmagnetic central singlet. The results of VSM showed thatσr/σs for the nanoparticles in EG were from 0.12 to 0.50. The addition of magnetic Ni-Fe nanoparticles significantly improved the SE at low frequencies, however made no significant difference to the SE at the high frequency of 1.5 GHz. At the frequency of 300 kHz the SE values were prompted from 43 dB of pure EG to 52～70 dB. According to SE-x curve at 300 kHz of Nix-Fei-x/EG, except x=0, the SE at 300 kHz went better as Ni content increased. The composite with pure Ni nanoparticles in EG (x=1.00) reached the SE of 70-105 dB at frequencies from 300 kHz to 1.5 GHz.3) We still made use of the method of reduction heated in H2 atmosphere to prepare Ni-Co/EG composites. The weight content of Ni-Co in composites was remained 30%, and we still fixed the reduction temperature and time as 450℃and 600 min, merely changed Ni content in Ni-Co expressed in the form of Nix-Co1-x. Ni content was found to have no obvious effect on SE at 300 kHz, the latter just fluctuated in narrow range of 69.3～70.2 dB.4) Ni-P alloy was loaded on the surface of expanded graphite (EG) by chemical plating to prepare a Ni-P/EG composite with electromagnetic radiation shielding effectiveness. Scanning electron microscope (SEM) results showed that the Ni-P layer was adhered tightly to the surface of EG. The X-ray diffraction (XRD) patterns and the magnetic measurements of the composite revealed that this chemical plated Ni-P layer was amorphous and had a low saturation magnetization. After crystallized at 500℃for 3h in N2 atmosphere, the Ni-P layer demonstrated multi-phase components and its saturation magnetization increased and coercivity decreased, presenting soft magnetism. The electromagnetic shielding effectiveness (SE) measured according to the SJ20524-95 standard for crystallized Ni-P/EG composite with areal density of 0.96 kg/m2 was 54～75 dB in the range of 300 kHz～1.5 GHz.