Synthesis and electromagnetic wave absorption of CoNi@SiO2@ZnO core–bi-shell microspheres

Cobalt-Nickel (CoNi) alloy particles are promising electromagnetic wave absorbers due to their strong magnetic loss and tunable morphology. To further enhance the performance, a multilayer core-shell-shell strategy was employed to construct a CoNi@SiO₂@ZnO composite. This design integrates magnetic and dielectric components to optimize impedance matching and introduce multiple interfacial polarizations. The synthesis involved a multi-step process: first, flower-like CoNi cores were synthesized via a polyol-mediated solvothermal process, whose branched architecture provides a large surface area and multiple reflection sites for enhanced microwave dissipation; then, the cores were amino-functionalized with APTES to facilitate the uniform deposition of a SiO₂ layer via a TEOS-based sol-gel process; finally, a seed-mediated method was employed. The resulting CoNi@SiO₂ particles were used as seeds, which were redispersed in an ethanol solution containing zinc ions and sodium hydroxide (NaOH) and subjected to a secondary solvothermal reaction. This successfully induced the epitaxial growth of a ZnO shell, ultimately yielding the final CoNi@SiO₂@ZnO composite material. The interlayer SiO₂ shell acts as a dielectric spacer, protects the magnetic core, and creates a distinct interface with the outer ZnO shell, collectively boosting interfacial polarization loss. The synergistic effect between the magnetic CoNi core and the dielectric double shells results in superior absorption performance. With a thickness of only 2.0 mm, the composite achieves a minimum reflection loss of −31.95 dB at 14.05 GHz and an effective absorption bandwidth (RL < −10 dB) of 6.59 GHz (11.41–18.00 GHz). This work demonstrates a rational route toward lightweight, high-efficiency, broadband absorbers under low-thickness conditions.