Enhanced Electromagnetic Wave Absorption by Bi-layered Nano-hollow Spheres

Abstract

To cope with the increasingly serious microwave radiation pollution, an extensive search for efficient microwave absorption materials (MAMs) has attracted great attention recently [1-3]. We have already reported the lightweight ferrite nano-hollow spheres (NHS) to exhibit enhanced electromagnetic (EM) wave absorption [4]. Herein, the deposition of the dielectric SiO2 layer on MnFe2O4 (MnFO) NHSs is found to be an effective strategy to enhance EM wave attenuation for a larger bandwidth. EM wave absorption properties of as-synthesized bare and SiO2 coated MnFe2O4 NHSs with different coating thicknesses are investigated within a widely-used frequency range. Larger interfacial area, higher magnetic anisotropy, internal reflections and scattering from NHSs along with higher magnetic permeability of MnFO are responsible for superior absorption properties of MnFO NHS. The higher dielectric permittivity of SiO2 coated on the surfaces of MnFO NHSs further improves microwave absorption for a large frequency bandwidth. The MnFO (~ 450 nm)/SiO2 (~ 68 nm) bi-layered NHSs (TEM micrograph shown in Fig.1) result in a sufficiently high RL ~ - 30.0 dB with a composite absorber of thickness only 3mm and filler concentration of 20% as shown in Fig.2. The above study on the variation of SiO2 thickness over MnFO NHSs helps us to optimize for maximum bandwidth and absorption with minimum composite absorber thickness. Analysis from the λ/4 model for best matching thickness (tm) displays a good agreement between experimental and simulated tm values. This study demonstrates the optimized MnFO/SiO2 NHS as a highly promising low-cost and lightweight EM wave absorber suitable for high-frequency filter applications.  Fig. 1 : Transmission Electron Micrograph of a MnFO/SiO2 NHS  Fig. 2 : Three dimensional plot of reflection loss versus frequency and composite absorber thickness for MnFO (~ 450 nm)/SiO2 (~ 68 nm) bi-layered NHSs