Multi-layered nano-hollow spheres for efficient electromagnetic wave absorption

Abstract

Ferrite nano-hollow spheres (NHS) are of great significance to improve electromagnetic (EM) wave absorption performance. Herein, the deposition of dielectric SiO2 and ferrimagnetic CoFe2O4 (CFO) layers on MnFe2O4 (MnFO) NHS are found as an effective strategy to enhance EM wave attenuation. EM wave absorption properties of as-synthesized bare and bi-layered samples are investigated within a widely-used frequency range of 1–17 GHz. MnFO@CFO bi-layered NHSs exhibit an excellent reflection loss (RL) of −47.0 dB at only 20 wt% filler content with an effective broad bandwidth (BW) of ∼2.2 GHz (frequency region for RL < −10 dB). The attenuation constant is observed to increase from 191.6 Np m−1 to 457.8 Np m−1 for bare MnFO and MnFO@CFO NHSs respectively. Larger interfacial area, additional pairs of dipole, higher magnetic anisotropy, internal reflections and scattering from NHSs are responsible for superior absorption properties of MnFO@CFO NHSs. Moreover, the best impedance matching, ∣Z in /Z 0 ∣ ∼ 1, promotes the optimum RL in MnFO@CFO at 5.96 GHz. MnFO@SiO2 bi-layered NHSs result in a sufficiently high RL ∼ −30.0 dB with a composite absorber of a thickness of only 3 mm. Analysis from the λ/4 model for best matching thickness (t m ) displays a good agreement between experimental and simulated t m values. This study demonstrates optimized MnFO@CFO NHS as a highly promising low-cost and lightweight EM wave absorber suitable for practical high-frequency applications.