Enhanced electromagnetic wave absorption of Ni–C core-shell nanoparticles by HCP-Ni phase

The strategy of phase control has been playing an important role in optimizing the electromagnetic wave (EMW) absorption properties. Here, Ni nanoparticles (NPs) with two phases, hexagonal close-packed Ni (HCP-Ni) and face-centered cubic (FCC-Ni), coated by C shells are synthesized by metal-organic chemical vapor deposition. The average size of the Ni nanocores and thickness of C shells are about 10 and 4 nm, respectively. After blended with paraffin wax at a mass ratio of 1:5, the Ni–C core–shell NPs exhibit excellent EMW absorption properties with an optimal reflection loss of −52.6 dB and a large effective bandwidth ($B{W}_{eff}$) of 9.3 GHz. The core–shell NPs annealed in vacuum to remove the metastable HCP-Ni exhibit degraded EMW absorption properties. Comparative analyses reveal that the HCP-Ni phase in the core–shell NPs can enhance the dielectric loss and thus improve the impedance matching, suggesting that phase control should be a practicable strategy for optimizing the properties of Ni-based EMW absorption materials. (paper)