| Nowadays, electromagnetic radiations emanating from a wide variety of sources can interfere with electronic devices and affect the health of people. The electromagnetic shielding materials with high effectiveness are increasingly required to meet the high demand for the reliability of electronics and the health of people. EG has been found to be a highly effective electromagnetic shielding material in the range of1-2GHz because of its excellent electrical conductivity. But the shielding effectiveness of EG decreases sharply with the decrease of frequency. Our previous work have been proved that the metals with high magnetic permeability may play the role of absorbing electromagnetic radiations, especially at low frequencies. Thus the composite materials of EG and magnetic metals have exhibited both absorption and reflection losses and increase the shielding effectiveness ultimately in a wide range of frequencies. Unfortunately, it is usually difficult to prevent the metallic nano-particles from environmental oxidation, and such effects usually cause the materials to loss their excellent characters and limit the applications. The nitride metal or the encapsulation of metals inside carbon nanotube can immunize or bate the metallic species against environmental degradation and retain their intrinsic properties. Such composites are highly demanded not only for understanding their fundamental features, but also for applications such as electromagnetic interference shielding materials, bio-materials, high-density magnetic storage media, magnetic fluids, catalysts and more.In this work, we prepared and characterized the composites of magnetic iron nitrides on EG, the magnetic nanopartical in carbon nanotube and the monodisperse melamine formaldehyde resin (MF) microspheres coated with metallic Ag and Ni. Their properties of electromagnetic shielding effectiveness were further determined. The main results are summarized below.(1) The composites with nano-particles of magnetic iron nitrides dispersed on the nano-layers of expanded graphite (EG) were prepared by the impregnation of EG with ethanol solutions of metal acetates, followed by drying and reduction in H2. The iron nanopartical are treated in the NH3flow to obtain iron nitride, and the temperature must be at573-873K. With an increase of reaction temperature, iron first undergoes initial nitriding to Fe4N, and then a further nitridation occurs to form Fe2-3N and Fe2N. The spherical or square nano-particles of nitrided iron were found to be well spread on the layers of EG with particle sizes of mainly100-600nm. The addition of magnetic nano-particles of nitrided iron slightly improved the shielding effectiveness at low frequencies. The composite with30%FexN in EG reached the shielding effectiveness of68-110dB during the frequencies from300kHz to1.5GHz.(2) The composites with bamboo-like carbon nanotube encapsulated magnetic Fe, Co and Ni nano-particles were prepared by pyrolysis of melamine and metal chloride, followed by grinding, carbonization in N2and treament in6M HCl over night. The with nano-particles carbide with the size of20-200nm were found to be well coated by bamboo-like carbon nanotube with200-800nm、200-500nm and100-200nm in diameter. It was found that the bamboo-like carbon nanotube were easily dispersed in water for its hydrophilic and thus the sample for electromagnetic shielding was prepared by filtering. The composite exhibited5-35dB of shielding effectiveness at the frequencies from300kHz to1.5GHz with the areal-density of0.01g/cm2.(3) The monodisperse melamine formaldehyde resin (MF) microspheres (1～3μm) was prepared by dispersion polymerization in water firstly. After pre-treatment of sensitization and activation, the microspheres coated with metallic nickel and silver were prapared with chemical liquid deposition method. The results showed that nickel plated under alkaline condition was of high quality than that of acidic. The DTA and TG results showed that the peak of pyrolysis of MF microspheres was about420℃, while the peak of pyrolysis of Ni coated and Ag coated MF microspheres were400℃and382℃respectively, which imply the enhanced catalytic process by coated metals. In order to simplified the process of chemical deposition, a new method was tested to utilize the active surface group. The MF microspheres were preadsorbed by Fe3+, Pd2+ Ag+and then reducted. The results show that the nanoparticles of metallic Pd and Ag was formed on the surface of MF microsphere, which indicated that possibility of chemical deposition of metals without sensitization and activation. |
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