| Due to high saturation magnetization, microwave pereability and various mechanism of energy loss, ferro-magnetic metal or alloy paricles has attracted plenty of researchers. In this thesis, flake-shaped cobalt particles were prepared by ball-milling, then heat treated in hydrogen. Besides, raw and as-milled particles were coated with silica by Stober method. Through the process, relationships between particles’ microstructure and their anti-oxidant, static magnetic and electromangetic properties were then investigated. Based on tranmission line theory and genetic algorithm, microwave absorbing properties of one and multi-layer coatings were evaluated and meanwhile mutilayer coatings for the perpose of broad or target microwave absorbing band were designed.Through the ball-milling process, flake-shaped cobalt particles of large aspect ratio for diameter to thickenss were first got and then broken into small pieces, with largest diameter about 40 μm and thickness about 1 μm. Once milled, defects increased and two phase of α- and β- lattice transformed into one phase of β-lattice in particles. This change could be reversed by heat treatment in hygrogen. This phase transformation were responsible for the change of particles’ coeivity.By repeating the process of Stober method, thickness of silica outer layer on cobalt particles were increased and its property of oxidation resistance was improved. This improvement was much better for raw sphere particles than for as-milled ones. Just once coating, raw silica coated cobalt particles started increasing weight due to oxidation at about 600 centigrade degree. This termperature were incresed to about 800 centigrade degree after particles coated with silica for six times.The test suggested that electromagnetic property of particles was related to many aspects such as particles’ shape and size, defects in particles and destribution of particles in matrix. Flake-shaped particles with larger aspect ratio of diameter to thickness possessed higher permittivity and permeability and meanwhile decreased defects which could be achived by heat treatment in particles would decrease these two value. Local conductive network(LCN) are easier to form when filling rate of particles are increased or large flake-shaped particles are used, which could cause interrupt incease of permittivity and local decrease of permeability at higher freuqnecy near to 18 GHz. This eddy current effect triggered by LCN could be inhibited by coating the particles with silica of proper thickness.Coatings containing cobalt particles with treatment of ball-milling, annealing or silica coating presented better microwave absorbing(EMA) property than that containing raw sphere particles. Particles with different milling conditions could show excellent performance in different frequency band. Besides, through adjusting filling rate of cobalt particles, various demands for EMA property could be satisfied. Better EMA property, especially the reflection loss peak, could be achived by annealing or coating cobalt particles with silica. However, the reflection loss peak would shift to higher frequency band after such treatment. Sepecifically, coatings containing 75wt% cobalt particles milled for 20 hours at 450 rpm could reach a RL value of-33dB at 8 GHz at the thickness of 2.5 mm with effective absorption band( larger than-10 d B) covering 6-10 GHz.Multilayer coating design based on genetic alorithm suggestted that this design method could make full use of various microwave absorbing characteristics of different materials to accomphlish the goals for broad band or target band EMA property. In three layer sturcture, coatings with thickness of 2.67 mm could achieve an effective absorption band covering 4.5-18 GHz. Further analysis stated that an normal gradient of refractive index along with the thickness of coatings or increased numbers of layers could extand the effectvie absorption band. Two layer structure with proper material kinds and layer thickness could achieve the goal for target band EMA property such as S-, C-, X-, Kuband in total coating thickness less than 2 mm. |
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