Static And Microwave Magnetic Propcrties Of Cobalt Flakes And Thcir Compositcs

With the increasing of frequency for microwave communication, the problem of the electromagnetic interference and electromagnetic radiation is serious. The requirement for microwave absorber is arising. However, the traditional ferrite is not suit for microwave absorbing. Recently, the magnetic metal absorber is hot. Snoek limit is the product of permeability and natural resonance frequency. The magnetic metal flakes are characterized with high saturation magnetization and un-axis shape anisotropy field which pose better microwave absorbing properties. The magnetic metal flakes are potential materials for high natural resonance frequency and high permeability. The follows are the conclusions of this paper.1. We fabricated a series of cobalt flakes with the thickness varied from50nm to440nm with hydrazine reduction method. The microwave properties of the flakes were investigated. The relationship between the thickness and Snoek limit was studied. With the decreasing of the thickness of cobalt flakes, the shape anisotropy field and the natural resonance frequency are increasing. However, the initial permeability of the flakes is decreasing for the passivated layers on the surface of cobalt flakes. The Snoek limit of flakes is higher the spherical one. The highest Snoek limit is observed in the flakes with a thickness of210mm.2. To suppressing the eddy current, the flakes were coated with silica layers by the hydrolysis method and their microwave properties are investigated. The real and image parts of silica coated flakes is lower than the uncoated one. The impedance match is improved and the magnetic tangent loss is enhanced. A better performance of microwave absorbing is achieved. Nevertheless, silica is a non-magnetic material. The increasing of mass percent for silica would lead to the decreasing of permeability of the composite. When the mass ratio of silica varied from10%to50%, the microwave properties of the composites are better than the pure cobalt sample.3. To increasing the permeability of the coated cobalt flakes, we coated the flakes with NiZn ferrite instead of silica and investigated the microwave properties of the samples with modified co-precipitation method. The results show that the initial permeability of the samples with a mass ratio of48%for NiZn ferrite is about2.8. The value is lower than the uncoated flakes (3.4), higher than silica coated flakes (2). The permittivity of NiZn ferrite coated flakes is drastically reduced, which leads to the improvement of the impedance match. The max RL is-24.7dB for10%NiZn ferrite coated flakes with absorber of2mm which is higher than10%silica coated one(-10dB) and uncoated flakes(-2.8dB). With the increasing of mass ratio of NiZn ferrite, the magnetic loss increases and the electric loss decreases, the frequency for maximum of RL move to high frequency band.4. The Ni ferrite/FeNi alloy/Ni ferrite composites were fabricated by partially reducing Ni ferrite. The microwave properties of the composites were investigated. The results show that the max RL is-20dB with thickness of2mm which is higher than that of Ni ferrite. The large magnetic loss arises from FeNi alloy and good impedance match is achieved. The preparation method is suit for mass product with low cost.