Surfactant-free synthesis and electromagnetic properties of Co–Ni–B composite particles

The Co–Ni–B composite particles with different mol ratio of Co to Ni were composited of spheres, spheres in-pair, hierarchical assemblies of dentrites, which were surfactant-free synthesized by chemical reduction method in aqueous solution. The complex permeability of the Co–Ni–B composite particles indicated reverse resonant peak at the frequency range of 8–16 GHz, where the complex permittivity showed the positive resonant peak and the μr″ of particles showed negative values, caused by the transformation between electric and magnetic energy. The imaginary parts of relative permeability (μr″) of Co–Ni–B composite particles indicated one broad resonant peak over the 2–8 GHz range for the high effective anisotropy. A slight decrease in complex permittivity resulted in an excellent impedance matching property. The Co–Ni–B composite alloy particles with mol ratio of 7:3 exhibited reflection loss less than −20 dB in frequency range of 4.0–14.5 GHz for the absorber thickness of 1.1–3.2 mm, and an optimal RL of −32.4 dB was obtained at 12.8 GHz with thickness of 1.2 mm. The broadest bandwidth of reflection loss less than −10 dB from 13.0 to 17.0 GHz, covering almost the whole Ku-band, was obtained for a thickness of 1.1 mm layer.