Preparation of hierarchical hollow structured Ni/C microspheres with strong and broadband microwave absorption properties in Ku band

As microwave absorbers, hierarchical hollow structured magnetic metal/carbon composites exhibit good microwave absorption properties in the Ku band (12–18 GHz), while their effective absorption bandwidths still cannot cover the whole Ku band. In this paper, we prepared hierarchical hollow structured Ni/C microspheres by a two-step strategy of hydrothermal synthesis and vacuum heat treatment. The effects of different vacuum heat treatment temperatures on the microwave absorption properties of the Ni/C microspheres were investigated. The Ni/C-700 sample exhibited broader microwave absorption bandwidth than homologous composites. The minimum reflection loss (RL) of −48.74 dB was obtained at 1.60 GHz with a matching thickness of 2.7 mm. As the matching thickness decreased to 2.5 mm, the effective absorption band (RL < −10 dB) reached 7.35 GHz (10.65–18 GHz), which covered the entire Ku band. The excellent microwave absorption properties could be attributed to the optimization of high-frequency impedance matching by the synergy of dual polarization relaxation and exchange resonance. These results suggested that such Ni/C microspheres can be used for microwave absorption at the Ku band.