Ferrites have tremendous applications at megahertz and gigahertz frequencies because of their good magnetic properties, high resistivity and chemical stability. By changing the anisotropy field in ferrites, the dispersion of permeability and permittivity can be easily adjusted. Although metallic fillers have better magnetic properties, such as large initial permeability, large saturation and low coercivity, they have low resistivity and large eddy current at gigahertz frequencies. The large eddy current leads to small skin depth in metallic fillers. Since the size of metallic filler needs to be smaller than skin depth, nanosized metallic powder is preferred for electromagnetic (EM) applications at gigahertz frequencies.
However, nanosized metallic powders have oxidization problem in air. To address this problem, we proposed two methods. One is to use core/shell structures, where a thin insulation layer is deposited on the surface of nanosized metallic powders. On one hand, the presence of the insulation layer will increase the resistivity of composites made from the core/shell structured powders. On the other hand, the layer is able to prevent the nanosized metallic powers from being oxidized. Another solution is to prepare hollow core structure or metal coated hollow microsphere.
Together with the size of hollow core or hollow microsphere, the total size of composite particle is at the scale of micrometer and the oxidization of metallic filler is neglectable.