Reinforced Concrete‐Inspired Multiscale Hierarchical Metamaterial Composite for Synergistic Enhancement Across Thermal, Electromagnetic, and Mechanical Domains

Abstract Radiative cooling offers a compelling passive thermal management solution. However, conventional materials like aerogels and polymer films overlook essential domains such as mechanical integrity and electromagnetic interference control, which are critical for scalable applications. Inspired by reinforced concrete—where a lattice framework and bulk matrix synergistically deliver mechanical robustness and wave energy dissipation— a multiscale hierarchical metamaterial composite (MHMC) is developed that integrates a carbon black‐based metamaterial (CBM) with a cellulose acetate aerogel (CAA). Thermally, the gradient porous structure of CAA and high mid‐ and long‐wave infrared emissivity of the CBM enable efficient passive daytime radiative cooling, achieving a solar reflectance of 95.8% and emittance up to 93.1%. Outdoor testing confirms a maximum temperature reduction of −10.75 °C, outperforming pure CAA and Styrofoam. Electromagnetically, impedance matching of the CAA layer minimizes reflection across the C–Ku band (5.8–18 GHz), while interfacial polarization with CBM‐CAA enhances dielectric loss, yielding broadband absorption up to 95.2% and effective absorption bandwidths exceeding 11 GHz. Mechanically, aerogel reinforcement augments energy dissipation, reaching a peak energy absorption of 450.87 kJ m ^{− 3} , substantially surpassing that of the individual constituents. The MHMC design strategy marks a promising milestone toward the paradigm shift from single‐functional materials to advanced multifunctional composites.