Preparation And Characterization Of FM(Co, FeNi, FeCo)/SiO₂ Core-shell Structured Nanoparticles

Core-shell structured magnetic nanocomposites, magnetic nanoparticles embedded in a insulate matrix, have attracted some research interest for their extensive applications as microwave absorbing, magnetic shielding materials and electromagnetic devices. Due to low resistivity, the eddy current generation of magnetic metallic alloys limits their applications at high frequencies. The soft magnetic ferrites materials possess low saturation, which limits electron device miniaturization. Therefore, it is need to exploit the new soft magnetic materials which possess high permeability, high saturation and low magnetic loss to satisfy the developmental requirements of electronic miniaturization and high-frequency.In this paper, the core-shell structured magnetic nanocomposites were prepared by the improved sol-gel combined with hydrogen reduction and co-precipitation combined with hydrogen reduction method. Core-shell structured magnetic nanocomposites with the different morphology were prepared by controlling prepared conditions.The Co/SiO₂ nanospheres and nanorods with core-shell structure have been prepared by the improved sol-gel combined with hydrogen method. This study investigated the effect of reduction temperature, drying temperature, dispersant concentration and SiO₂ content on the phase, the morphology, the size and magnetic properties, respectively. The Co₃O₄ reaction process: Co₃O₄→CoO→Co(hcp)→Co(fcc). IR confirmed the chemical bond of Si-O-Co at the interface between the core and the shell. The spherical core-shell structured Co/SiO₂ nanoparticles which were dried at 80℃and reduced by H₂ at 700℃are of small size. The average size of nanoparticles was well controlled with appropriate dispersant. Experimental result indicated that polyethylene glycol (PEG 8000) could act as a more efficient dispersive reagent than citric acid monohydrate (CAM). The Co nanoparticles diameter reaches about 30nm with PEG 8000 concentration of 50mg/mL. The average size of Co core decreases with the increase of SiO₂ content, but the saturation magnetization decreases. Furthermore, after the Co was coated with silica, the oxidative stability of Co/SiO₂ nanoparticles can be improved.Theγ-FeNi(FeNi₃)/SiO₂ nanoparticles with core-shell structure were prepared by co-precipitation combined H₂ reduction method. The result shows that the pH is an important factor for the formation of core-shell nanostructure. The nanoparticles are of approximately spherical core-shell structure at a pH of about 9. The average saturation magnetization of magnetic atoms decreases with the increase of SiO₂ content. Because the presence of the nonmagnetic silica coating lead to reduction of exchange coupling interaction between neighboring magnetic nanoparticles. The value of saturation magnetization obviously increases with the increase of reduction temperature, but nanopartcile size increases little and the coercive force decreases.The FeCo/SiO₂ one dimensional nanomaterials with core-shell structure were prepared by co-precipitation combined H2 reduction method. The study investigated the effect of surfactant, reduction temperature on the morphology, magnetic properties, respectively. The agglomerational granule structures were obtained without surfactant. Core/shell nanomaterials with different structure (e.g. nanorods, nanotubes and nanowires) were obtianed by adding surfactant sodium dodecyl benzene sulfonate (SDBS), and the growth mechanism of one dimension nanostructure formation was preliminarily explained. The experiment indicated that the surfactant sodium dodecyl benzene sulfonate (SDBS) is as dispersant and adjuvant. The value of saturation magnetization increases with the increase of reduction temperature, but the coercive force decreases.