High-Performance Passive Radiative Cooling Enabled by Oriented Boron Nitride/Sodium Alginate Composites with Water Molecule-Assisted Pressure Assembly

The continuous miniaturization and increasing power density of modern electronic devices have led to severe localized heat accumulation, posing significant challenges to device performance, reliability, and lifespan. Passive radiative cooling offers a promising, energy-free solution for thermal management. However, most reported radiative cooling materials are mechanically fragile films, lacking the structural stability required for long-term integration with electronic devices. Inspired by the brick-and-mortar architecture of natural nacre structures, we report a nacre-mimetic bulk boron nitride (BN) composite that combines efficient radiative cooling with high mechanical strength. The composite is fabricated via a pressure-driven, water-molecule-assisted orientation and interfacial coupling strategy, in which sodium alginate (SA)-assisted BN dispersion and Ca2+ ionic cross-linking enable highly ordered BN nanosheet alignment and strong interfacial bonding during laminated hot pressing, resulting in a nacre-like hierarchical structure. The resulting BN-SA composite exhibits a flexural strength of 78.3 MPa and fracture toughness of 2.48 MPa·m1/2, alongside outstanding optical properties including high solar reflectance (R = 89.7%) and mid-infrared emissivity (84.5%). Consequently, net radiative cooling power reached 140.1 W·m-2 at night, lowering outdoor temperatures by 12.5 °C. This work provides a bio-inspired strategy for constructing mechanically robust radiative cooling materials, offering a viable solution for thermal management in high-heat-flux electronic devices.