Dielectric loss enhancement induced by the microstructure of CoFe2O4 foam to realize broadband electromagnetic wave absorption

Abstract

CoFe₂O₄ has been widely used for electromagnetic wave absorption owing to its high Snoek limit, high anisotropy, and suitable saturation magnetization; however, its inherent shortcomings, including low dielectric loss, high density, and magnetic agglomeration, limit its application as an ideal absorbent. This study investigated a microstructure regulation strategy to mitigate the inherent disadvantages of pristine CoFe₂O₄ synthesized via a sol–gel auto-combustion method. A series of CoFe₂O₄ foams (S0.5, S1.0, and S1.5, corresponding to foams with citric acid (CA)-to-Fe(NO₃)₃·9H₂O molar ratios of 0.5, 1.0, and 1.5, respectively) with two-dimensional (2D) curved surfaces were obtained through the adjustment of CA-to-Fe³⁺ ratio, and the electromagnetic parameters were adjusted through morphology regulation. Owing to the appropriate impedance matching and conductance loss provided by moderate complex permittivity, the effective absorption bandwidth (EAB) of S0.5 was as high as 7.3 GHz, exceeding those of most CoFe₂O₄-based absorbents. Moreover, the EAB of S1.5 reached 5.0 GHz (8.9–13.9 GHz), covering most of the X band, owing to the intense polarization provided by lattice defects and the heterogeneous interface. The three-dimensional (3D) foam structure circumvented the high density and magnetic agglomeration issues of CoFe₂O₄ nanoparticles, and the good conductivity of 2D curved surfaces could effectively elevate the complex permittivity to ameliorate the dielectric loss of pure CoFe₂O₄. This study provides a novel idea for the theoretical design and practical production of lightweight and broadband pure ferrites.