Study On Preparation And Microwave Absorbing Properties Of W-Type Barium Hexaferrite

Ferrite is a very important class of radar absorption materials (RAM) because of high absorptivity, well impedance matching, wide bandwidth and inexpensiveness. But these capabilities lie on a thick coating and the RAM has a low absorption at S wave band(2⁴GHz). To obtain good quality ferrite absorber, a series of research works have been developed in the thesis. Substituted W-type barium hexagonal (BaCo₂Fe₁₆O₂₇) powders had been synthesized by co-precipitation method, and then been compounded with other kinds of absorbents. The complex permeability and complex permittivity of these substituted ferrites, compounds and multilayer ferrite coaxial holders were measured. And the absorption characteristics were simulated. Three-layer structure with the thickness of 1.66mm and the surface density of 4.685kg/m² was prepared and its microwave absorbing properties were measured by a net work analyzer from 2GHz to 18GHz.The results show that:1. The W-type ferrite could be synthesized from the co-precipitation producetion under a temperature of 1260℃ for 2.5h and a spot of absence of Fe is necessary to form the single-phase BaCo₂Fe₁₆O₂₇.2. The absorptivity and matching thickness increase by substitution of Zn²⁺ for Co²⁺. But the absorbing properties are weaken when the Fe³⁺ is replaced by Al³⁺. Substituting a spot of Dy³⁺ for Fe³⁺ could increase the complex permeability and decrease the complex permittivity, which are good for the impedance matching and the absorbing properties.3. The absorbing properties of compounds with different absorbents are superior to that of single ferrite. Mixing ex-carbonyl iron in to ferrite could decrease the matching thickness, increase the absorptivity and broaden the bandwidth of ferrite. Also could broaden the bandwidth and improve the impedance matching of Ex-carbonyl iron, especially the absorptivity of it at S wave band.4. The absorbing properties of multilayer coating are obviously superior to that of monolayer ferrite coating. And the absorbing properties could be influenced by different kinds of absorbents and the thickenss of each coating dramatically. Complex permittivity depression from outer to inner is better for impedance matching.5. The reflection loss of the Ba hexaferrite-based multiplayer absorber at 2¹8GHz is under -1.34dB completely, for a reflection loss under -10dB ,the bandwidth is above 7.9GHz.