Fabrication and enhanced microwave absorption properties of Al2O3nanoflake-coated Ni core–shell composite microspheres

Core–shell composite microspheres with Ni cores and Al2O3 nanoflake shells have been successfully fabricated by the hydrothermal deposition method. The electromagnetic parameters of the Ni microspheres and Ni/Al2O3 composites are measured by a coaxial line method. The tangent losses for the Ni/Al2O3 composite are larger than those of the Ni. A significant enhancement of electromagnetic absorption (EMA) performance of Ni microspheres coated by the alumina shells was achieved over the 1–18 GHz. The reflection loss (RL) less than −10 dB of the composite was obtained over 7.5–18.0 GHz by tuning an appropriate sample thickness between 1.3 and 2.2 mm, and an optimal RL of −33.03 dB was obtained at 9.2 GHz with a thin absorber thickness of 2.0 mm. The coating of the dielectric alumina shell significantly enhanced the microwave absorption performance due to the enhancement of interface polarization between the metals and dielectric interfaces, the synergetic effect between the dielectric loss and magnetic loss and unique flake-like dielectric alumina.