Microwave properties of sea-urchin-like Ni nanoparticles.

Sea-urchin-like Ni nanoparticles based on radial cones with the diameters of 30-50 nm and the lengths of 150-300 nm were synthesized by a wet chemical reduction method. The microwave properties of the Ni nanoparticle-paraffin wax composites with 11.5 vol%, 18.2 vol%, and 23.3 vol% of the sea-urchin-like Ni nanoparticles were measured in the 0.1-18 GHz frequency range. The imaginary part of complex permeability shows two broad resonance peaks due to the natural and exchange resonance. The natural resonance peak appears at a relatively higher frequency of 4.3-4.7 GHz because of the large shape anisotropy of sea-urchin-like particles. The natural resonance peak shifts to lower frequency with increasing volume fraction of the Ni nanoparticles.