All-Inorganic HIP Multilayer With Excellent Thermostability For High-Performance Dual-Selective Radiative Cooling

Passive daytime radiative cooling (PDRC) enables sustainable, energy-free thermal management by reflecting sunlight and emitting heat through Earth's atmospheric window (8–13 µm) into outer space. However, this potential remains unrealized, as organic coolers degrade rapidly under sunlight, while inorganic ones face high cost, structural intricacy, suboptimal emissivity, and struggle to balance high solar reflectivity. Most also underutilize the broader mid-infrared (8–20 µm) spectrum, restricting space applications where no atmosphere aids cooling. To address these challenges, we propose a simple, low-cost, all-inorganic dual-selective multilayer structure comprising SiO ₂ capping alternating Si ₃ N ₄ /Al ₂ O ₃ high-index pair (HIP) atop an AlN-protected Ag reflector via magnetron sputtering over quartz substrate. The SiO ₂ /HIP interface induces strong impedance mismatch through low/high refractive index contrast, enhancing atmospheric emissivity, while the HIP structure broadens mid-IR emission. The AlN barrier prevents Ag oxidation while sustaining emissivity. Optical analysis of the MATLAB-optimized structure reveals 96.04% solar reflectivity, 91.06% atmospheric emissivity, and a 19.8 K sub-ambient temperature drop, yielding ~225 W·m ^-2 net cooling power—the highest value reported for inorganic multilayers. Notably, full mid-IR emissivity reaches 90.03%, surpassing conventional inorganic radiative coolers. SEM images confirm distinct, well-aligned columnar layers, validating deposition fidelity. The thermal inertness by TG-DSC analyses confirms 0.01% mass loss at 1450°C, confirming film's stability and suitability for extreme environments. Outdoor test confirms real-world efficacy, achieving up to 5.2°C sub-ambient temperature reduction under 960 W·m ^-2 solar irradiance despite cloud cover and wind. This scalable design addresses material scarcity, structural complexity, and spectral trade-offs, paving the way for terrestrial and extraterrestrial PDRC applications.