Electromagnetic And Microwave Absorption Performances Of Ferrites And Sillenites

In recent years,electromagnetic pollution has seriously affected human health,and the function of microwave absorption has attracted more attentions in military field.It is urgent to develop new microwave absorption materials with high efficiency and low cost.Among the reported synthesis processes of microwave absorption materials,the oxidation-precipitation method has been widely used because of its high yield,simple equipment,controllable products,safety and reliability.In this paper,nanostructured Fe₃O₄,Co_xFe_3–xO₄ and Bi₁₂MnO₂₀ were prepared by oxidation-precipitation methods.Their compositions,microstructures,electromagnetic performances and microwave absorption mechanisms were fully discussed.Single-crystal Fe₃O₄ with various shapes were prepared by surfactant-assisted processes.A broad size distribution of 30-200 nm for Fe₃O₄ nanoblocks and a uniform size around 50 nm for Fe₃O₄ nanospheres were obtained respectively,while Fe₃O₄nanowires showed non-uniform length-to-diameter ratios.In electromagnetic analysis,Fe₃O₄ nanoblocks exhibited higher saturation magnetization and increased coercivity in contrast to Fe₃O₄ nanowires and nanospheres.The reflection loss of Fe₃O₄ nanowires and Fe₃O₄ nanoblocks achieved the minimum value of-29.7 dB and-22.6 dB,respectively.The increased attenuation constant of Fe₃O₄ nanospheres was obtained at higher frequency due to the effective electromagnetic matching.For Co_x Fe_3–x–x O₄nanocrystals,their electromagnetic performances can be controlled by changing the cobalt/iron?Co/Fe?ratios.Dielectric relaxation peaks were originated from orientation polarization and affected by oxygen vacancy densities.Magnetic resonance peaks were shifted to higher frequency due to the increased magnetocrystalline anisotropy at higher Co/Fe ratios.Due to the electromagnetic matching of dielectric relaxation and magnetic resonance,Co_xFe_3–xO₄ exhibited excellent microwave absorption performance.At last pure Bi₁₂MnO₂₀ microparticles with average sizes of 11.5-16.8?m were prepared by microwave-heating process.Their microstructures were characterized by the shapes of single or multiple intersecting tetrahedrons and the massive nanocubes in ordered arrays on each tetrahedron surface.In electromagnetic determination,noticeable resonances both in permittivity and permeability were revealed beyond relaxations.Only one cone-like reflection loss peak was obtained with the minimum value of-39.1 dB and a broad effective microwave absorption bandwidth covering 12.7-14.6 GHz.