Tailoring microwave absorption performance through foam density and filler localization in nickel-coated carbon filled polyurethane foams

Abstract

Radar absorbing materials (RAMs) have become increasingly important in modern stealth technologies due to the growing need to minimize radar signatures. However, most RAMs suffer from high weight and limited structural flexibility, which restrict their practical use in lightweight and conformal applications. Therefore, developing lightweight and structurally adaptable absorbers has become a critical research priority, and this study addresses this need by evaluating the effects of polyurethane foam (PUF) density and morphology on their electromagnetic absorption performance. PUF composites with different matrix densities (30, 80, and 120 kg/m³) were prepared by incorporating nickel-coated carbon fillers at varying loadings. In addition, to examine the effect of filler localization, additional samples with identical composition were fabricated using three preparation strategies, introducing the filler into the polyol phase, the isocyanate phase, or equally dividing it between both. These approaches resulted in distinct filler distributions, allowing the evaluation of how spatial arrangement influences electromagnetic behavior. As expected, increasing foam density progressively enhanced absorption performance, with notable attenuation observed only at densities of 80 kg/m³ and above. A well-balanced cellular morphology was obtained by equally dividing the filler between both phases, resulting in homogeneous localization across both cell interiors and walls. This strategy facilitated more regular foam formation compared to polymeric methylene diphenyl diisocyanate (pMDI) based dispersion and offered comparable absorption performance, while clearly outperforming the polyol-only method in efficiency and thickness. As a key outcome, a PUF with an experimental density of 0.553 g/cm³ achieved a minimum reflection loss (RL _min ) of −41.80 dB at 4.3 cm thickness, while maintaining Reflection Loss (RL) values below −10 dB across the entire X-band. Overall, the results demonstrate that both foam density and filler localization serve as effective design parameters to tune dielectric behavior and optimize absorption in lightweight polyurethane-based RAMs.