Size influence on microwave properties of Ni-Co-P hollow spheres

Ni-Co-P hollow spheres (HSs) of different sizes were fabricated by the autocatalytic reduction method. With the decrease in particle size, the value of complex permittivity increases and the permeability changes in the opposite direction. The resonance peaks appear in the curves of both permittivity and permeability. The former is due to atomic and electronic polarization, and the latter is caused by natural resonance. For sample 2, with a particle size of about 200 nm, the minimal reflection loss (RL) (-52.5 dB) was predicted at 11.2 GHz with a thickness of 1.6 mm. Additionally, for Ni-Co-P HSs, size dependence of minimal RL is not linear and the excellent microwave absorption property can be obtained in the size range around 200 nm.