Research About Preparation And Properties Of Co-coated Hollow Carbonyl Iron Particles By Electroless Plating Method

Carbonyl iron powder is widely applied as a kind of microwave absorbing material because of its good microwave absorption properties,low cost,relatively high permittivity.However,its high density and poor high temperature oxidation resistance are the weaknesses which limit its application.Aiming to the disadvantages,hollow carbonyl iron was prepared via pit corrosion method,and then it was coated by Co via electroless plating.In this paper,the surface morphology,phase composition,magnetic property and apparent density of initial and processed particles before and after heat treatment were studied.The complex permittivity and complex permeability of raw carbonyl iron,hollow carbonyl iron and Co-coated hollow carbonyl iron particles were measured and the mechanisms of varying electromagnetic parameters were analyzed.The cobalt ion source was cobalt sulfate hexahydrate?CoSO₄·7H₂O?,sodium hypophosphite?NaH₂PO₂·H₂O?was used as reducing agent,sodium citrate?C₆H₅Na₃O₇·2H₂O?was used as complexing agent and ammonium chloride?NH₄Cl?was used as stabilizer.The influences of types of pH regulators,temperature,concentration of cobalt salt,concentration of reducing agent and pH on morphology of samples after electroless plating were studied respectively.The best composition of plating solution and reaction conditions were found by analyzing coating layer with the aid of SEM and analyzing the high temperature oxidation resistance via XRD.The electroless plating conditions are cobalt salt concentration of 20g/L,reducing agent concentration of 25g/L.pH is adjusted to 9.5 by ammonia and the temperature is 90^oC.Finally,Co-coated hollow carbonyl iron with high temperature oxidation resistance and good completeness of Co coating layer was prepared.After coating,the carbonyl iron particles show good antioxidation property as well as soft magnetic characteristics.The saturation magnetization is 113.58 emu/g and the coercivity is53.55 Oe.The particles were homogeneously dispersed in paraffin and pressed into a toroidal shaped sample for electromagnetic properties test.Complex permittivity and complex permeability of the composites were measured by the coaxial line method in the range of 2-18 GHz using a network analyzer.The reflection loss was calculated based on the transmission line theory.The result shows that core-shell structure formed by electroless plating Co on the surface of hollow carbonyl iron contributes to increase in interface polarization which makes permittivity increase.the magnetism of cobalt is lower than iron,which result in the real part of permeability decreasing after coating.The minimum RL value can reach to-19.5 dB at 9.3 GHz with the thickness of 5 mm and reflection loss bandwidth?RL<-10 dB?can reach to 3.5 GHz.