Effect of Annealing on the Microwave-absorption Properties of Ni/Al2O3 Nanocomposites

Ni/Al2O3 nanocomposites were prepared by the mechanochemical synthesis method.The annealing process enlarges the grain size of both the metal Ni and insulating Al2O3 in the as-milled nanocomposite and leads to an increase of the saturation magnetization and a decrease of the surface anisotropy.An optimal reflection loss(RL) of 23 dB is obtained in the as-milled nanocomposite at 17.8 GHz for an absorber thickness of 6.6 mm.The annealed sample exhibits a RL exceeding 20 dB in the whole K u-band for an absorber thickness of 6.6-9.7 mm with an optimal RL of 54.7 dB at 13.2 GHz for a layer thickness of 9.3 mm.The excellent microwave-absorption properties are a consequence of a proper match of the dielectric and magnetic losses.