Preparation And Characterization Of Encapsulated Fly-ash Cenospheres Microwave Absorbing Powder

The bigger density of traditional microwave absorbing materials restricts their application. Fly-ash cenospheres possess lower density and regular morphology. Thereby, the dissertation focuses on research of preparation of composite powders with "core-shell" structure through using fly-ash cenospheres as core.The cenospheres coated by Ni were prepared by electroless plating approach. We studied the influence of different activation steps to coating effect. The results indicate that coating effect and stability of APTS modification activation are better than sliver colloid. The cenospheres coated by Ni have superior electromagnetic loss property, compared with raw materials cenospheres. The lowest reflection rate is -8dB in 17.2GHz when the thickness of coating is 6mm.All different types of xerogels and respective auto-combustion powders were characterized by XRD, TG-DTA, and IR. And then we found out the mechanism of sol-gel auto-combustion method. NH₄NO₃ in the xerogels is fundament of auto-combustion method. The essence of auto-combustion method is a chain reaction which is formed by mutual promotion of NH₄NO₃ decomposition and citric acid combustion.The NiFe₂O₄ powders with a few impure phases （α-Fe₂O₃, FeNi₃） were synthesized by sol-gel auto-combustion method. Heat treatment can remove impure phases. The results of electromagnetic parameter show that microwave loss ability of auto combustion powders is more excellent than powders treated at 950℃between 2-18GHz. The lowest reflection rate of auto combustion powder is -8.5dB when coating thickness is 5mm.NiFe₂O₄/cenospheres composites were prepared through sol-gel auto-combustion method. The results demonstrate that continuous NiFe₂O₄ coatings with a thickness of 400nm on the cenospheres surface have shaped. Auto-combustion composite powders have better electromagnetic loss ability compared with heat treatment powder. On the whole, the microwave loss ability of both is weak. The result like this is ascribed to the drawback of simulation calculation which does not take the loss stepping from "core-shell" structure into account.BaFe₁₂O₁₉/cenospheres composites were prepared by sol-gel auto-combustion method. The integral coatings had formed. The original coating phase （γ-Fe₂O₃） is converted into BaFe₁₂O₁₉ andα-Fe₂O₃ after treated at 850℃. The coatings possess high BaFe₁₂O₁₉ content when the proportion of Fe/Ba is 1:6. In high temperature condition, Ba can react with glass phase of cenospheres, which results in the scarcity of Ba and the production of massiveα-Fe₂O₃. There is no obvious difference in electromagnetic property between BaFe₁₂O₁₉ and composites, but their electromagnetic loss ability should be improved.