Effect of Ni fillers on microwave absorption and effective permeability of NiCuZn ferrite/Ni/polymer functional composites

Abstract This work investigated the concentration dependence of the nanosized Ni fillers on crystallographic structure, electrical transport, effective permeability, constitutive electromagnetic property, and microwave-absorbing behavior of the insulating polymer matrix embedded with NiCuZn ferrites (NCZFs). The effective permeability of the percolation ferrite composites which are composed of three-phases, 0.55 NCZF/( x ) nano Ni/(0.45- x ) polymer, increases with x till x  = 0.1. It is due to the Ni fillers acting as magnetic bridge to enhance magnetic exchange coupling and to decrease the demagnetization field between ferrite granules. The electrical resistivity decreases abruptly above the percolation threshold ( x  = 0.02). It is ascribed to the narrowed polymer gaps and the significant decrease in the grain boundary resistance after the Ni additions. The frequency response of the microwave absorption in ferrite composites is attributed to its macroscopic magnetic loss and dielectric loss correlated with domain-wall motion, spin resonance, and dipolar relaxation. Various multilayer microwave absorbers terminated by perfect metal conductor (PMC) are systematically designed utilizing the constitutive electromagnetic properties of ferrite composites. When the Ni concentration is increased, the microwave return loss is greatly enhanced and the absorption band becomes narrower towards the lower frequency. The maximal return loss of single-layer absorbers composing of 7.2 mm-thick ferrite composites with x  = 0.15 is estimated as −53.9 dB with an absorption bandwidth 1.72 GHz. Due to the decreasing products of matching thickness and frequency at maximal return loss, the incorporation of Ni fillers in ferrite composites facilitates to reduce the absorber thickness. Additionally, the microwave absorbing behaviors of double-layer microwave absorbers with different lamination sequences of Ni-encapsulated ferrite composites were also investigated.