Microwave absorbing properties of Ag-coated Ni–Zn ferrite microspheres prepared by electroless plating

This investigation presents an electromagnetic radiation absorptive composition comprising Ag-coated ferrite microspheres dispersed in a silicone rubber matrix for use as a thin microwave absorber in GHz frequencies. Ni–Zn ferrite microspheres with an average diameter of 50 μm were prepared by spray-drying and sintering at 1130 °C. A conductive Ag layer was coated on the ferrite spheres by electroless plating. Uniform Ag coating can be obtained using the plating solution with a high AgNO3 concentration. For particle compacts of the conductive Ni–Zn ferrite spheres, electrical resistance is reduced to as low as 10−2 Ω. Rubber composites containing the Ag-plated ferrite spheres exhibit a high value of both real and imaginary parts of complex permittivity, while the complex permeability spectrum is not significantly changed with Ag plating. Due to the conductive and magnetic property of the microspheres, matching thickness can be reduced to as low as 2 mm at the frequency of 7.6 GHz, which is much thinner than non-coated ferrite absorbers.

Research highlights
► Microwave absorptive composition comprising Ag-coated Ni–Zn ferrite microspheres.
► A conductive Ag layer was coated on the ferrite spheres by electroless plating.
► The ferrite-rubber composites exhibit high dielectric constant and dielectric loss.
► A reduced matching thickness is predicted with the conductive ferrite microspheres.