| The new demands of thin thickness, light weight, broad bandwidth, strong absorption for radar absorbing materials (RAMs) has been established in modern military and civil field. To meet with researchers'expectations, we should control and tailor electromagnetic parameters of magnetic absorbents. Because of high permeability and high magnetic loss, magnetic micropowder (MMP) becomes one of commercial and common absorbents. On account of high permittivity, their impedance match needs to be improved. Therefore, increasing permeability and reducing permittivity are able to enhance microwave absorbing properties at low frequency and improve impedance match.It is common knowledge that the flaky magnetic alloy particle is advantageous to acquiring high permeability because shape anisotropy can raise its natural resonance. This dissertation chose FeCo alloy, which has good saturation magnetization, to mechanical alloying (MA) and mechanical grinding (MG), to get flake-like absorbents. The aspect ratios of flake-like particles are increasing with grinding time. When grinding time exceeded 24h, the aspect ratio was between 10 and 25.3. The real part of permeability rose to 4.39 at 0.5GHz and the maximumμ″was 2.27 at 6.19GHz for 84h grinding. Increasing with grinding time, natural resonance frequency shifted to low frequency, but permittivity was increasing because of strong polar mechanism and conducting electron. The real part of permittivity was increasing from 8.68~10.80 to 17.62~24.59 consistently, which deteriorated impedance match property. So microwave absorption properties are optimized at low frequency and deteriorated at high frequency.To reduce permittivity of flaky magnetic alloys,improve its impedance match, and enhance its absorption properties at high frequency, in this dissertation, we tried to coat flake-like particles with alumina by surface-deposition method. The method we used is effective and simple and the process is easy to control. We found that the pH value of suspension, addition of aluminum sources and the reaction time have important influences on forming core-shell structure and reducing its permittivity. The maximum reduction ofε′was 4.18. Microwave absorbing properties was improved. -10dB bandwidth went beyond 8GHz and the minimum reflectivity approached or got above -40dB. Meanwhile, the stability of flaky absorbents increased 20~40℃, and the core-shell structure absorbents can be used in complex ambience. Alumina precursor gradually deposited to the surface of flake particles then transited to amorphous alumina. alumina precursor form on its surface by heterogeneous nucleation. We found the grow mechanism of alumina particles by the change of absorbent particles surface morphology. Alumina precursor attached on absorbent particles as sphere-like, then needle-like, stick-like and worm-like. Afterwards, worm-like precursor particles linked to network, and alumina precursor continued to absorb on naked surface, until absorbent particles are coated completely. Core-shell structure absorbents could reduce its permittivity, optimize its impedance match, enhance its microwave absorbing properties, and improve its stability.The results indicate that the electromagnetic parameters can be well adjusted by controlling composition, microstructure, particle structure (flake structure and core-shell structure), morphology and size of absorbent particles. Particle structure is important to improving the microwave absorbing properties of magnetic absorbents. So results and ideas of changing its particle structure in the dissertation are important to improve absorbing properties. |
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