This study designed MgO ceramics with a gradient porous structure to optimize their spectral response in the 0.3–25 μm wavelength range.

① Materials Design and Structural Optimization: The study proposes a MgO ceramic with a gradient structure, achieving efficient solar‑light reflection and selective infrared radiation through differences in surface density and internal porosity. It delivers high performance within the 8–13 μm atmospheric window. Emissivity and it suppresses infrared radiation in the 2.5–8 μm and 13–25 μm spectral bands, thereby reducing additional heat transfer.

Figure 1. Basic principle of radiative cooling. (a) Schematic diagram of the heat flux at the surface of a radiative cooling material. P Solar refers to the absorption of solar radiation; P rad is the outward infrared radiation of the refrigeration material; P atm is downward Atmospheric Radiation Absorption. Heat conduction and convective heat transfer are neglected. (b) Ideal spectral reflectance and emissivity of broadband radiative cooling materials (BRCM) and spectrally selective radiative cooling materials (SRCM), compared with the standardized ASTM G173 solar spectrum and MODTRAN’s tropical atmospheric transmission. (c) Spectral radiative intensity distributions of BRCM and the atmosphere. (d) Assuming total Heat transfer coefficient The relationship between cooling power and the ambient temperature difference under conditions of 3 W/m²·K and an ambient temperature of 300 K.

Figure 2. Structural design and optimization of cooling ceramics. (a) Scattering efficiency of air pores in different inorganic dielectric materials, with a pore diameter of 1000. nm .(b) Infrared optical constants of MgO. (c) Infrared reflectance of an ideal MgO material. (d) Photograph of a gradient nanoporous ceramic. (e) Schematic diagram of a gradient nanoporous MgO ceramic. The nanoscale pores within the ceramic lie in the solar spectral range (0.3–8 µm). micrometer ) Provides strong scattering reflection. The phonon absorption of MgO occurs in the range of 8–13. micrometer The band provides strong infrared emission. The dense surface of the ceramic exhibits properties in the range of 13–25. micrometer Strong Reststrahlen reflections are generated within the range. (f) Different apertures in the range of 0.25–8. micrometer Simulation of the single‑hole scattering efficiency in an MgO medium across the wavelength range. The refractive index of MgO is taken from previous studies. (g) Influence of hole diameter and porosity on the solar reflectance of cooling ceramics ( R solar) and long-wave infrared window emissivity ( epsilon The influence of MIR). In the white‑highlighted region, the ceramic exhibits both high reflectance and high emissivity. (h) The effect of porosity on the Reststrahlen band of radiative cooling ceramics. As porosity increases, the intensity of the Reststrahlen band decreases.

③ Optical Performance and Cooling Capacity Testing: Outdoor testing verified the material’s cooling performance under various climatic conditions. The results indicate that the material delivers substantial daytime and nighttime cooling in hot‑and‑humid environments. Further energy simulations show that, in high‑temperature, high‑humidity settings, the material can reduce cooling energy consumption by 9.48 MJ per square meter.

Figure 3. Structural characteristics and spectral performance of the cooling ceramic. (a) SEM micrograph of a cross-section of the ceramic obtained via a pressure‑free sintering process, revealing a dense surface and a porous internal structure. (b) Internal porosity and pore size as functions of different processing parameters. The region shaded in red corresponds to a porosity of 5–15% and pore sizes ranging from 100 to 1000 nm, consistent with the theoretical design. (c) Spectral performance of the ceramic under various processing conditions compared with that of previously reported high‑performance SRCMs. (d) CT‑derived variation of porosity from the surface to the interior; porosity increases with depth and stabilizes at approximately 50 μm. The pie chart illustrates the size distribution of pores within the ceramic. (e) SEM images of the ceramic’s surface (top) and interior (bottom), along with inverse pole figure maps (EBSD‑IPF Z0); no pronounced color bias indicates the absence of dominant texture. (f) Size distribution of pore diameters within the cooling ceramic. (g) Spectral reflectance and emissivity of the cooling ceramic, normalized against the solar spectrum and tropical atmospheric transmission. (h) Angular dependence of the cooling ceramic’s solar reflectance Rsolar(θ) and long‑wave infrared emissivity εLWIR(θ).

Figure 4. Outdoor radiative cooling tests of the cooling ceramic. (a) Radiative cooling test setup. (b) Schematic of the foam box used to measure radiative cooling temperature and cooling power. (c–e) Outdoor measurement results from 11:00 to 14:00 on March 11, 2024. (f–h) Outdoor measurement results from 01:00 to 04:00 on March 12, 2024. (c) and (f): Relative humidity (RH) and wind speed. (d) and (g): Outdoor-measured temperatures of the cooling ceramic and the ambient environment. (e) and (h): Temperature difference between the cooling ceramic and the ambient environment, along with cooling power.

④ Durability and Cleanability Assessment: This ceramic exhibits excellent durability, strong resistance to UV radiation and acid erosion, and is well suited for long-term outdoor applications. Its high‑density surface structure imparts superior self‑cleaning properties; after being rinsed by rain, it can restore its cooling performance, thereby helping to reduce maintenance costs.

Figure 5. Cooling potential and self-cleaning capability of the cooling ceramic. (a) Cooling potentials of recently reported high-performance radiative cooling materials compared with those of the cooling ceramic developed in this study. Solar reflectance (Rsolar) and longwave infrared emissivity (εLWIR) are typically used to quantify cooling power, while spectral selectivity is employed to evaluate cooling temperature and all‑day radiative cooling performance. (b) Spatial cooling mechanisms of the cooling ceramic during daytime and nighttime. Yellow arrows indicate reflected sunlight, blue arrows denote thermal radiation transmitted through the longwave infrared window, and red arrows represent radiative heat exchange with the atmosphere and surrounding environment. (c) Maximum cooling temperatures achieved by previously reported SRCMs and the cooling ceramic under daytime and nighttime conditions ( Tmax ) and the maximum cooling capacity (Pmax). (d) Annual total heating, ventilation, and air-conditioning (HVAC) energy consumption in large office buildings across different climate zones before and after applying the cooling ceramic. (e) Contact angle of droplets. (f) Self-cleaning performance of the ink‑contaminated cooling ceramic.

Summary: The gradient nanoporous MgO radiative cooling ceramic developed in this study integrates structural scattering, phonon absorption, and the Reststrahlen effect, achieving high spectral selectivity with a solar reflectance of 0.96 and an infrared emissivity of 0.95. Its cooling performance excels in hot‑humid environments, while also exhibiting excellent mechanical properties and environmental robustness. Moreover, the material’s straightforward fabrication process and self‑cleaning capability endow it with the potential for large‑scale application in building cooling, offering a viable pathway to address the current energy crisis and mitigate the greenhouse effect.