Bead-like cobalt nanoparticles coated with dielectric SiO₂ and carbon shells for high-performance microwave absorber

The development of high-efficiency microwave absorption materials with both strong absorption intensity and wide absorption bandwidth is still a significant challenge. In this work, the bead-like cobalt nanoparticles of 50 nm with strong magnetic loss capability are prepared by hydrogen plasma-metal reaction. To further regulate the dielectric parameters, the carbon, SiO₂, and SiO₂/carbon shells are coated on the bead-like cobalt cores by in-situ polymerization of silica and phenolic resin to obtain the Co@C, Co@SiO₂, and Co@SiO₂@C nanocomposites, respectively. The Co@SiO₂@C nanocomposite possesses the best electromagnetic wave (EMW) absorption performances among the samples. At the thickness of only 1.7 mm, the minimum reflection loss (RL) value of -39.6 dB at 13.5 GHz and the effective absorption bandwidth (EAB) of 7.6 GHz for RL < -10 dB are simultaneously obtained. Surprisingly, the absorption bandwidth (RL < -20 dB) is as wide as 14.2 GHz (3.8-18 GHz) with the thickness of 1.3-5.0 mm. The excellent microwave absorption performances are ascribed to the strong magnetic loss of the bead-like Co, the synergistic effect between multiple components, as well as the multiple polarization and multiple scattering induced by core-shell structure. As a consequence, the Co@SiO₂@C nanocomposite can serve as an ideal candidate for high-performance microwave absorption.