Preparation of silica-coated ferrite nanoparticles for micro-wave absorber

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Ferrite materials, which possess highly permeable and insulating, are attracting growing interest, because they might be used at high frequencies as inductors for micro-magnetic devices or wave absorbers for EMI suppression.In this study, silica-coated ferrite nanoparticles (SiO_2-coated magnetite (Fe_3O_4), SiO_2-coated cobalt-ferrite (Co_xFe_<3-x>O_4) and SiO_2-coated zinc-ferrite (ZnxFe_<3-x>O_4) were prepared using a water-in-oil microemulsion consisting of water, polyoxyethylen (15) cetylether (surfactant) and cyclohexene (oil). After preparing Fe_3O_4 particles in the solution, an aqueous solution of cobalt chloride or zinc chloride (CoCl_2 or ZnCl_2) was added into the microemulsion containing Fe_3O_4 nanoparticles, and Co-Fe hydroxide complex or Zn-Fe hydroxide complex were formed on the surface of the Fe_3O_4 nanoparticles. TEOS and diluted NH_4OH solution were charged into the microemulsion, and subjected to hydrolysis to produce SiO_2.From TEM observations, a ferrite particl … More e was located nearly at the center of each spherical SiO_2 particle. The particle size of Fe_3O_4 and the thickness of the SiO_2 layer were very uniform and varied to within the range of 8.8 nm-13.4 nm and 7.0 nm-20.0 nm, respectively ; furthermore, the size distribution of the Fe_3O_4 particles was very narrow. As the Fe_3O_4 content increased from 3.6 wt% to 10.9 wt%, the saturation magnetization improved from 2.0 emu/g to 6.9 emu/g. Furthermore, the solid solution of Co ions and Zn ions into Fe_3O_4 to form Co_xFe_<3-x>O_4 and Zn_xFe_<3-x>O_4 nanoparticles during air calcination led to an increase in the magnetic properties of the SiO_2-coated ferrite nanoparticles. The coercivity of the SiO_2-coated Co_xFe_<3-x>O_4 nanoparticles increased with increasing amounts of added Co, obtaining a maximum value of 780 Oe around a Co/Fe ratio of 0.3-0.4. The saturation magnetization of the SiO_2-coated Co_xFe_<3-x>O_4 nanoparticles increased with increasing amounts of added Zn, and the maximum value was obtained around a Zn/Fe ratio of approximately 0.1. Less