Modular carbon-Ni aerogel spheres as post-processable building blocks for lightweight broadband electromagnetic metamaterials.

Aerogel microwave absorbers are attractive for lightweight electromagnetic protection, but conventional monolithic fabrication often couples porous-network formation with macroscopic shaping, limiting post-processability, scalable assembly, and device-level architectural design. Here, we report modular carbon-Ni (C-Ni) aerogel spheres as post-processable building blocks for lightweight broadband electromagnetic metamaterials. The spheres are fabricated by Ni2+-mediated coordination gelation, continuous injection, freeze-drying, and carbonization. During carbonization, the guar-gum-derived aerogel framework is converted into a hierarchical porous carbon network, while Ni species are transformed in situ into metallic Ni nanoparticles uniformly anchored within the carbon skeleton. Electromagnetic analysis and density functional theory calculations reveal that Ni-induced Ni/C heterointerfaces regulate interfacial polarization and conductive-network development, enabling an optimized balance between impedance matching and dielectric attenuation. The optimized spheres achieve an effective absorption bandwidth (EAB) of 7.6 GHz and a minimum reflection loss of -49.9 dB. The spheres can further be assembled into curved, hexagonal, and periodic architectures. A fabricated C-Ni sphere-based metamaterial exhibits an ultrabroad EAB of 35.5 GHz in the 2-40 GHz range, with an areal density of 0.52 g cm-2 and a thickness of 6.1 mm. This work provides a modular aerogel-materials strategy for transforming shape-locked porous absorbers into scalable, post-processable, and architecturally programmable electromagnetic devices.