| The modification of ferrites doped or compounded to improve their microwave electromagnetic loss have been a subject of concern. In recent years, the high frequency property, microwave absorption and magnetic coupling of Z-type hexagonal ferrite have been becoming a hot and active topic in the fields of materials, condensed matter physics, information etc. In this thesis, the microwave absorption properties of Z-type strontium cobalt ferrite were researched, which was doped at Sr site or Co site and compounded with multiferroics Bi0.9La0.1FeO3, respectively.The crystalline powder samples of Sr3-xGdxCo2Fe24O41and Sr3(CuZn) xCo2-xFe24O41(x=0、0.1、0.2、0.3、0.4、0.5) were prepared by sol-gel method, and the composite of (1-x)Sr3Co2Fe24O41/xBi0.9La0.1FeO3(x=0、0.2、0.4、0.6、0.8、1) were prepared by secondary sol-gel method. The formation, crystal structure, particle morphology and element content of these samples were characterized by TGA-DSC, XRD, SEM, and EDS respectively. The hysteresis loops of the samples were measured by PPMS-9T-type physical properties measurement system. The resistivity of the samples were measured by cylinder method. The results showed that the samples of Sr3-xGdxFe24O41and Sr3(CuZn)xCo2(1-X)Fe24O41calcined above1250℃for5hours were single Z phase ferrites which had a particle morphology of micro-hexagonal flake shape, a good soft magnetic property and a resistivity in the range of semiconductor.Bi0.9La0.1FeO3had a single perovskite structure,a particle morphology of micro-cobblestone shape and a weak ferromagnetism.The composites of (1-x)Sr3Co2Fe24O41/xBi0.9La0.1FeO3was a two-phase mixture, for which the resistivity decreased with increase of x, the saturation magnetization and residual magnetization had the same order of magnitude with ferrite phase,and the magnetic coercive force had a great improvement than the single-phase.All of the above samples had a pure chemical composition and their formation went through several stages such as water evaporation, thermal decomposition and crystalline growth etc.The complex permittivity and complex permeability were measured by microwave vector network analyzer in the frequency range of2～18GHz, and the reflection coefficient and loss tangent were calculated according to measurements. The effects of doping content or composite mass fraction and thickness on microwave absorption of them were analysed and the microwave loss mechanism were discussed. The results showed that the absorption peak of Sr3-xGdxFe24O41(x=0.4) with2.30mm thickness was19.8dB at11.0GHz and its bandwidth above10dB was5.6GHz, the peak of Sr3(CuZn)xCo2(1-X)Fe24O41(x=0.3) with2.50mm thickness was29dB at11.4GHz and the bandwidth was7.7GHz. The above two kinds of materials have an effect of strong and wide microwave absorption from both certain dielectric loss and strong magnetic loss. The microwave absorption properties of Z-type strontium ferrite can be improved by doping an appropriate amount of rare earth or transition elements and because of the hexagonal flak morphology. The absorption peak of composite system (1-x)Sr3Co2Fe24O41/xBi0.9La0.1FeO3(x=0.4) with2.70mm thickness was14.8dB at13.0GHz and the bandwidth above lOdB was4.7GHz.The microwave absorption properties of the composite was superior to a single-phase phase and the strong magnetic loss and dielectric loss were complementary in different frequency bands.Therefore,a composite of ferromagnetic and ferroelectric matter is an important way to improve microwave absorption properties of a single ferroelectric or ferromagnetic material. |
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