4A zeolite based daytime passive radiative cooling material

Abstract Materials used for radiative cooling usually have high spectral reflectivity in the solar radiation band (0.3–2.5 µm) and high spectral absorptivity (or emissivity) in the infrared band (2.5–25 µm), particularly in the atmospheric window (8–14 µm). In this work, 4A zeolite was first used as a functional material for radiative cooling. The effects of heat treatment temperature on the microstructures and optical properties of 4A zeolite were investigated by X-ray diffraction, scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy and spectroscopic methods. 4A zeolite was found to transform to low-carnegieite NaAlSiO4 and nepheline NaAlSiO4 after being annealed at 800℃ and 1000℃, respectively. The results showed that the 4A zeolite heat-treated at 800°C (4A-800) demonstrated a high solar reflectivity of 92% and a high emissivity of 0.94 in the range of 8–14 µm, which is higher than most of the materials reported in the literature. Then the 4A-800 was incorporated into a coating to further study its cooling performance. The results showed that the as-prepared 4A zeolite coating after being annealed at 800℃ achieved a temperature decrease of ∼15°C compared to a pure aluminum plate at noon and a decrease of ∼4°C compared to a TiO2 coated aluminum plate under direct sunlight. The present work demonstrates that 4A zeolite is a promising energy-saving material for daytime passive radiative cooling, which is significant for green and environment protection.