Investigation on microwave absorption properties of CuO/Cu₂O-coated Ni nanocapsules as wide-band microwave absorbers

CuO/Cu₂O-coated Ni nanocapsules with Ni nanoparticles as cores and amorphous CuO/Cu₂O as shells have been synthesized by the arc discharge method. The dielectric relaxation of the CuO/Cu₂O shell and the interfacial relaxation between the CuO/Cu₂O shell and the Ni core lead to dual nonlinear dielectric resonance. The natural resonance shifts to 5.4 GHz, due to the surface anisotropy of these Ni nanoparticles. The minimum reflection loss (RL) is less than -20 dB at 2.0-18.0 GHz with a thickness between 1.0 and 8.4 mm for paraffin-nanocapsule composites, comprising of 40 wt% Ni nanocapsules, and an optimal RL value of -47.8 dB was observed at 14.4 GHz on a specimen with a matching thickness of 1.20 mm. The frequency of the microwave absorption complies with the quarter-wavelength matching model. In the microwave S band, C band, X band and Ku band, these CuO/Cu₂O-coated Ni nanocapsules can be used as excellent lightweight microwave absorbers due to the proper combination of complex permittivity and permeability and the special microstructure of the CuO/Cu₂O-coated Ni nanocapsules.