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

Abstract Passive daytime radiative cooling (PDRC) 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. Existing solutions also underutilize the broad mid-IR (8–20 µm) spectrum, restricting space applications where no atmosphere aids cooling. To address these challenges, we present an all-inorganic broadband emitter comprising SiO 2 -overlaid periodic Si 3 N 4 /Al 2 O 3 high-index pair (HIP) backed by AlN-protected Ag reflector over quartz substrate via magnetron sputtering. HIP broadens mid-IR emission via constructive interference enabled by its high refractive-index—presumed unattainable in prior inorganic approaches. SiO 2 interface induces strong impedance mismatch through low-/high-refractive-index contrast to enhance atmospheric-window emissivity. This multilayer structure achieves 96.04% solar reflectivity, 91.06% atmospheric-window emissivity, and a remarkable 90.03% broadband emissivity, yielding 225 W·m -2 cooling power with a 19.8 K sub-ambient drop—an apex in inorganic metrics. TG-DSC confirmed thermostability and chemical inertness up to 1450°C, confirming film's stability for extreme environments. Outdoor tests despite localized feedback show sub-ambient drops of 9.7°C (horizontal) and 3.2°C (vertical) under 1100 W·m -2 and 426 W·m -2 solar irradiance, respectively, validating real-world efficacy. This scalable design overcomes material scarcity, structural complexity, and spectral trade-offs, paving the way for terrestrial and extra-terrestrial thermal management.