Enhancement of electromagnetic wave absorption in MnFe_2O_4 nano-hollow spheres

摘要

In order to obtain a light-weight, stable, and cost-effective yet efficient electromagnetic (EM) wave absorbing material, here, we investigated EM wave attenuation properties of as-synthesized low-density MnFe_2O_4 nano-hollow spheres (NHS) in-detail, varying their sizes [mean diameter (in nm) of sample sets = 100, 220, 300, 450, and 550] within a widely used frequency range of 1-20 GHz. In addition to larger interfacial area and magnetic anisotropy of NHSs, multiple internal reflections in its hollow core promote better EM wave absorption. Therefore, tuning of NHS sizes is demonstrated as an effective strategy to achieve an excellent microwave absorber, and MnFe_2O_4 NHS of diameter ～450 nm is found to exhibit a maximum reflection loss (RL) of approximately -52.6 dB, total shielding efficiency (SE_(Total)) of approximately -39.5 dB, and a high attenuation constant (α) of ～285 Np/m due to best impedance matching, |Z_(in)/Z_0| ～ 1, along with significant dielectric and magnetic losses. Furthermore, a thickness-dependent study on 450 nm NHS composites reveals that optimum RL reached approximately -55.4 dB at 9.6 GHz for t = 5.1 mm with a broad total effective bandwidth (RL < -10 dB, i.e., attenuation >90%) of ～3.7 GHz. Moreover, the analysis from the quarter-wavelength model for best matching thickness (t_m) displays a good agreement between experimental and calculated t_m values. This study presents optimized 450 nm MnFe_2O_4 NHS at much lower filler concentration (only 20 wt. % in the epoxy resin matrix) as a highly promising low-cost and light-weight microwave absorber suitable for practical high-frequency applications.