Synthesis and Microwave Absorption Enhancement of CoNi@SiO₂@C Hierarchical Structures

A series of hierarchical CoNi@SiO₂@C composites were successfully fabricated through a workable procedure, and the content of carbon can be adjusted by modulating the additive dose of phenolic resin precursors. The compositions, microstructures, and magnetic characters of the obtained products were characterized in detail via XRD, SEM, XPS, and VSM detailedly. The results manifested that the urchin-like CoNi alloy particles were successively encapsulated with SiO₂ and carbon coatings. The exploration into microwave absorbing performances demonstrated that wax-based hybrids containing 50 wt % CoNi@SiO₂@C-3 are endowed with improved absorbing capability possessing the maximum RL value of -46.0 dB at 10.8 GHz with an effective absorption bandwidth of 5.6 GHz (from 7.1 to 12.7 GHz) for a layer thickness of only 2.2 mm. Moreover, in the wake of layer thicknesses varying from 1.5 to 5 mm, the effective absorbing bandwidth almost overlaps the whole C, X, and K_u wave bands. The remarkable microwave absorption performances are attributed to the joint outcome of the combination of dielectric and magnetic losses, the numerous interface polarizations and dipolar relaxations provided by their hierarchically multicomponent structures and the good impedance matching.