"Core/shell" Structures And Electromagnetic Characteristics Of Metal(Fe,Ni) Composite Nanoparticles

In this work, DC arc-discharge and catalytic pyrogenation methods were employed to prepare core/shell structure composite nanoparticles, such as metal/homogeneity metal oxides, metal/heterogeneity metal oxides and Ni/C composite nanoparticles. By the analysis of the influences of the oxidation potential and surface energy on the formation of core/shell type composite nanoparticles, a thermodynamics criterion about the particle formation was brought forward. It was also investigated how the reaction temperature determine the structure of Ni/C composite nanoparticles for the catalytic pyrogenation method. The particle structures, magnetic properties and thermal stability of the samples were characterized. The electromagnetic parameters were measured in the 2-18GHz frequency range by dispersing the particles in a paraffine matrix. The relation between the particle structure and electromagnetic parameters was construed by comparison of the electromagnetic parameters for various samples.The relation was discussed between the electromagnetic characteristics and the structure of metal nanoparticles coated by homogeneity metal oxide. It was discovered that the Fe and Ni nanoparticles fabricated by DC arc-discharge method are encapsulated by Fe and Ni oxide and the oxides present enormous influences on the dielectric loss of the composite nanoparticles. The Fe oxides outside Fe nanoparticles containing Fe ions with various valence states show high dielectric loss for the oriental polarization favoring excellent microwave absorption properties, while the Ni nanoparticles possess low dielectric loss and microwave attenuation for the single valence state of Ni ions. Natural resonance peaks appear in the imaginary permeability curves of Fe, Ni and Fe-Ni alloy nanoparticles at high frequency for the enhanced surface anisotropy of the magnetic nanoparticles. The resonance frequencies shift with the alloy component, and the adjusting of alloy component can not only improve antioxidation property of the metal nanoparticles, but also achieve excellent microwave absorption property.Fe and Ni nanoparticles encapsulated in heterogeneity metal oxides were prepared by a DC arc-discharge method using the mixture compacts containing metal and oxides as precursors. The oxidation potential of Fe oxides is so low that double-oxides, Fe/Fe3-xTixO4 and Fe3-xMnxO4 formed outside the particles for the reactivity of Fe with oxygen, while AlOX shell formed due to the low oxidation potential of aluminum oxide. The oxidation potential of Ni oxide is much higher than Fe, and TiO2, Al2O3, MnO formed outside Ni nanoparticles. The electromagnetic characteristics of the composite nanoparticles were discussed based on the analysis of the metal nanoparticles coated by homogeneity oxides. The heterogeneity metal oxide not only improves the thermal stability but also change the electromagnetic characteristics. The permittivity of the composite nanoparticles present smaller values than the metal nanoparticles coated by homogeneity oxides, and constants in the whole frequency range for the synthetic action of diverse polarization machinism. The changing permittivity and permeability enable the nanoparticles show different attenuation properties, the effective absorption frequency shifts to high frequency and matching thickness increases as the lowing dielectric and magnetic loss. Differing from the Fe composite nanoparticles, the microwave attenuation properties of Ni composite nanoparticles have not improved markedly for the lower intrinsic magnetization and dielectric loss, though the permittivities reduced.With the purpose of improving the attenuation property of Ni nanoparticles, the Ni nanoparticles were annealed in methane atmosphere to synthesize C coated Ni nanoparticles by the catalytic pyrogenation method. The structures, magnetic properties, thermal stability and electromagnetic characteristics were investigated. The graphite in the composite nanoparticles exhibits different morphology for various reaction temperatures, and the Ni nanoparticle were not sintered during the reaction process. The oxidation temperatures increase and oxidation rates drop with the increasing thickness of graphit shell in the composite nanoparticles. The analysis of electromagnetic property indicates that the crystalline, thickness and structure of the graphite phase influence the dielectric prperties obviously. The defects and conductivity of the shell bring about much higher permittivity in the composite nanoparticles than in Ni nanoparticles and the Ni/C composite nanoparticles fabricated by DC arc-discharge method. The composite nanoparticles exhibit much higher dielectric loss due to the motion of free electron in graphite shells and interface polarization between cores and shells. Among the samples, the particles synthesized at 500℃exhibit highest permittivity values. The real permittivity and imaginary parts can reachs 66 and 47 in the sample containing 50wt.% composite nanoparticles. For the high dielectric loss, excellent attenuation property can be achieved in the samples with smaller additive amount of the composite nanoparticles.