Electromagnetic properties and predicted absorption performance of vapor-condensed Ni–NiO core–shell nanoparticle/polyurethane composites

In this study, electromagnetic (EM) wave absorbing composites were fabricated by dispersing Ni–NiO core–shell nanoparticles synthesized via a vapor condensation process into a polyurethane (PU) matrix. The EM properties were experimentally measured, and the absorption performance was predicted as a function of nanoparticle content. TEM and XRD analyses confirmed a core–shell structure with an average diameter of 25 nm, comprising a Ni core and a NiO shell. M–H hysteresis analysis further revealed that the NiO shell thickness remained consistent at approximately 2.3–2.7 nm, independent of nanoparticle content. SEM analysis confirmed uniform dispersion of the nanoparticles within the PU matrix. Both the permittivity and permeability of the Ni–NiO/PU composites increased with increasing nanoparticle content. The real part of the permeability increased up to dfC = − 68.0% and then decreased beyond this point, which is attributed to the lag in magnetic moment response and resonance frequency shift. EM simulation based on the measured EM properties predicted that the thickness corresponding to the maximum absorption decreased from 18.5 mm to 2.1 mm with increasing nanoparticle content, while absorption exceeding 95% was maintained across all content conditions. This is attributed to the reduction in effective wavelength of EM waves within the composite and a decrease in the thickness satisfying the λ/4 condition, both resulting from the increase in permittivity and permeability. At a nanoparticle content of 17.8 vol% and a thickness of 2.5 mm, a maximum absorption of 99% was predicted at dfC = −12.6%. The broadest absorption bandwidth was predicted at a thickness of 2.3 mm with a 90% absorption bandwidth (BW90%) of 17.9%, and at a thickness of 2.1 mm with a 70% absorption bandwidth (BW70%) of 36.8%. These results suggest that both the absorption performance and bandwidth can be controlled by adjusting the nanoparticle content and thickness. The Ni–NiO core–shell/PU composites show promising potential as lightweight, high-performance EM wave absorbing materials.