| Ferrite is a kind of traditional material with shortcomings of high density and narrow band for absorbing electromagnetic wave, so that the study of modified ferrite is still an active topic. In addition to modification methods such as doping and compounding, special particle morphology has an important effect on the improvement of ferrite's microwave absorption properties. In this thesis, the microwave absorption properties of spinel ferrite Fe3O4 with sharp octahedral morphology and doped W-type ferrite with hexagonal flake morphology were researched.Fe3O4 powder samples were synthesized by hydrothermal method. The crystal structure, morphology and composition of these samples were characterized by X-ray diffraction(XRD), fourier transform infrared spectrometer(FT-IR), scanning electron microscopy(SEM) and energy dispersive spectrometer(EDS), and the formation mechanism of particle morphology in the samples was discussed. Their complex permittivity and complex permeability were measured by microwave vector network analyzer in the frequency range from 2 to 18 GHz, the reflection coefficient and loss tangent were calculated according to measurements, and the microwave absorbing mechanism was discussed. The results showed that, two crystalline powders of Fe3O4 with sharp octahedral and spherical morphology were got in two different hydrothermal reaction conditions respectively, and particle shape anisotropy of the material was so helpful to its microwave absorption that sharp octahedral Fe3O4 was a broadband microwave absorbing material with low reflectivity, which is better than spherical. The microwave absorption peak of sharp octahedral Fe3O4 was 35dB at 7.1 GHz frequency and its absorption bandwidth above 10dB was 7.9GHz for the sample with 2.8 mm thickness and hydrothermal growth of 12 hours. The microwave absorption was resulted in magnetic loss mainly as well as dielectric loss secondly in the low-frequency range of 2-13 GHz, and dielectric loss mainly but magnetic loss weakly in the high-frequency range of 13～18 GHz.The crystalline powder samples of Ba(Me)xCo2-2xFe16O27 (Me =MnCu or CuZn, x=0.0,0.1,0.2,0.3,0.4,0.5) were prepared by sol-gel method. The crystal structure, particle morphology and formation of these samples were characterized by XRD, SEM, differential scanning calorimetry-thermal gravity analysis(DSC-TGA) and FT-IR. Their electromagnetic parameters were measured by microwave vector network analyzer in the frequency range from 2 to 18 GHz. The relationships between microwave reflectivity and frequency, between loss tangent and frequency were calculated according to the measurements. The microwave absorbing properties and electromagnetic loss mechanism of the material were studied. The results showed that the formation of Ba(Me)xCo2-2xFe16O27 samples went through several stages such as gel water evaporation and melting, thermal decomposition and crystalline growth, the samples calcined above 1235℃for 4 hours were single W phase ferrite and the particle morphology was micro-hexagonal flake shape. MnCu and CuZn doping were so helpful to improving microwave absorption of W type barium ferrite that the absorption peak of Ba(MnCu)xCo2-2XFe16O27 (x=0.3) with 2.3mm thickness was 24dB at 10GHz and its bandwidth above lOdB was 8.8 GHz, and the peak of Ba(CuZn)xCo2-2xFe16O27 (x=0.3) with 2.3mm was 17dB at 10GHz and its bandwidth was 7.6GHz. The microwave absorption of Ba(Me)xCo2-2xFe16O27was resulted in magnetic loss mainly but dielectric loss weakly and the magnetic loss was caused by domain wall resonance, magnetization relaxation and natural resonance. |
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