All-Inorganic HIP Multilayer with Excellent Thermostability for High-Performance Broadband Radiative Cooling

Passive daytime radiative cooling enabling energy-free thermal regulation by solar reflection and thermal emission remains unrealized for widespread adoption owing to rapid photodegradation of organic materials and inorganic material’s narrowband spectral response. Current solutions also restrict space applications by underutilizing the broad mid-IR spectrum (8–20 μm), where no atmosphere hinders cooling. To address these challenges, we present an all-inorganic broadband emitter comprising a SiO₂-overlaid periodic Si₃N₄/Al₂O₃ high-index pair (HIP) on an AlN-protected Ag reflector over quartz substrate via magnetron sputtering. HIP broadens mid-IR emission via constructive interference enabled by its high refractive-indices─presumed unattainable in prior inorganic approaches. The SiO₂ interface induces an impedance mismatch via low-/high-refractive-index contrast to enhance atmospheric-window emissivity. This structure achieves 96.39% solar reflectivity, 91.06% atmospheric-window emissivity, and a remarkable 90.03% broadband emissivity, yielding 151.48 W·m^–2 cooling power with a 13.2 K subambient drop─an apex in inorganic metrics. TG-DSC confirms thermostability and chemical inertness up to 1450 °C, confirming the film’s stability for extreme environments. Outdoor tests despite localized feedback show subambient drops of 9.7 °C (horizontal) and 3.2 °C (vertical) under 1100 and 426 W·m^–2 solar irradiance, respectively, validating practical efficacy. This approach overcomes material scarcity, structural complexity, and spectral trade-offs, paving the way for terrestrial and extraterrestrial thermal regulation.