Synthesis And Electromagnetic Response Characteristics Of The Nanocomposite Particles

Core/shell-type nanocapsules, including metal/carbon, metal/silica, metal/(B₂O₃/H₃BO₃) and metal/polyaniline, have been synthesized by a DC arc-discharge method and a simple chemical polymerization, respectively. The microstructure, magnetic properties and electromagnetic (EM) characteristics in the frequency range of 2-18 GHz were systematically investigated by using of X-ray diffraction (XRD), transmission electron microscopy (TEM/HRTEM), differential thermal analysis thermogravimetry (TG/SDTA), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM), quantum design superconducting quantum interference device (SQUID) magnetometer and Agilent 8722ES network analyzer.Carbon-coated Fe, Co, Ni and Cu (metal/C) nanocapsules were synthesized by the DC arc-discharge method. TEM/HRTEM results show that nearly spherical shape particles with a small size distribution of around 20-50 nm in diameter were obtained in the as-made powders, and almost all the particles own a core/shell structure, with the inner metal cores and the graphite shells. Different from the Fe/C, Co/C and Ni/C nanocapsules, Cu/C nanocapsules consist of a mass of hollow graphites shell because of a migration process of Cu cores. Raman spectroscopies of the metal/carbon nanocapsules show the D, G and 2D peak at 1321, 1571 and 2650 cm^-1, respectively, indicating a high defect density of the graphite shells. TG/SDTA results show the graphite shells begin to oxidize and decompose at 269℃, which is lower than that of bulk graphite (846℃), C₆₀(420℃) and carbon nano-tubes (700-750℃), etc., further indicating that the graphite shells contain a lot of defect points with high energy levels, which favor to be oxided. EM characteristics reveal that the defective sites in the graphite shells play a role of permanent dipolar polarization centers in the presence of EM wave, enhancing the dielectric response property. Thus it can accumulate a mass of space charges between the core/shell interfaces, producing the interfacial polarization phenomenon. Due to the high surface anisotropy, the relatively complex permeabilities of the metal/carbon nanocapsules present strong natural resonance phenomena at 2-18 GHz, respectively, and split to multi-resonance for the Fe/C and Co/C nanocapules. The natural resonance is dominant among microwave magnetic losses for the metal/carbon nanocapsules because the graphite shells with so high defective properties exhibit a high resistivity that effectively suppresses eddy current loss phenomena. The nanocapsule-paraffin composites exhibit excellent microwave absorption properties based on the simulation of transmit-line theory.SiO₂-coated Fe, Co, Ni and Cu (metal/silica) nanocapsules were synthesized in a gas mixture of H₂ and Ar by the DC arc-discharge method. Co and SiO₂ powders of several tens of microns in size were homogeneously mixed with 5:1 weight ratio of Co to SiO₂, and pressed into a shape to serve as an anode, while a tungsten rod as a cathode. TEM/HRTEM results reveal that the as-prepared products were almost nanocapsules with a core/shell structure containing a metal core and a SiO₂ shell, while some of them displayed a rod-like silica configuration. Based on the conventional vapor-liquid-solid (VLS) mechanism, we firstly proposed an extended concept of the conventional VLS mechanism, which was defined as "the stoichiometric oxygen-assisted VLS (SOA-VLS) mechanism". This mechanism can open a new way to the synthesis of a series of new core/shell type metal/oxide nanocapsules. The relatively complex permeabilities of the metal/silica nanocapsules are similar to that of the metal/carbon nanocapsules, indicating the natural resonance phenomena are only affected by the magnetic cores. The imaginary part of the relatively complex permittivities of the metal/silica nanocapsules approximately approach to zero, showing a low dielectric loss, which is mainly due to the insulating property of silica shells. Therefore, metal/silica nanocapsules exhibit weaker microwave absorption properties because of an unbalanced EM match, compared with that of the metal/carbon nanocapsules.B₂O₃/H₃BO₃-coated Fe and Ni [metal/( B₂O₃/H₃BO₃)] nanocapsules were synthesized in the gas mixture of H₂ and Ar by arc discharge evaporating a mixture of Fe micro-powders and B₂O₃ micro-powders. The results revealed that metallic nanoparticles were uniformly coated with B₂O₃/H₃BO₃ shells, which further confirmed the SOA-VLS mechanism. TG/SDTA and XRD results reveal the B₂O₃ shells coated on the surface of nanocapsules can be partly transformed into H₃BO₃ through hydrolyzation by the moister in air, which can be decomposed to B₂O₃ and H₂O when the temperature increases to 126℃. In-situ TEM observation records the real-time states of H₃BO₃→B₂O₃. The EM characteristics and microwave absorption properties also have been discussed in detail.Ni/polyaniline (Ni/PANi) nanocapsules with different contents of the PANi (6.1, 7.4, 15.6, 21.6 wt. %) were prepared by a simple chemical polymerization. Firstly, the surface-cleaned Ni nanoparticles were modified by a chemical graft procedure using 4-aminobenzoic acid (ABA), and then a polymerization reaction between the grafted amine of ABA and aniline occurred in the aqueous solution of aniline with the ABA-grafted Ni nanoparticles. FTIR spectroscopy reveals that the Ni/PANi nanocapsules present the absorption peaks with a slight red shift, indicating an electric charge transfer phenomenon between the core/shell interfaces. The EM characteristics show dual Debye-type dielectric relaxations with increasing content of PANi to over 15.6 wt %, implying a relatively complete Ni/PANi interface. A phenomenological model according to the Cole-Cole semicircle approach was adopted to explain the dual dielectric relaxation phenomena. The complex permeability of all the samples identically presented natural resonance peaks in 2-18 GHz because of a higher surface anisotropy, similar to the Ni/C, Ni/SiO₂ and Ni/(B₂O₃/H₃BO₃) nanocapsules. It was emphasized that the Ni/PANi nanocomposite with 15.6 and 21.6 wt. % PANi provided favorable dielectric losses, which contribute to excellent microwave absorption properties.