4A zeolite based daytime passive radiative cooling material

Materials used for radiative cooling usually have high spectral reflectivity in the solar radiation band (0.3-2.5 mu m) and high spectral absorptivity (or emissivity) in the infrared band (2.5-25 mu m), particularly in the atmospheric window (8-14 mu m). 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 degrees C and 1000 degrees C, respectively. The results showed that the 4A zeolite heat-treated at 800 degrees C (4A-800) demonstrated a high solar reflectivity of 92% and a high emissivity of 0.94 in the range of 8-14 mu m, 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 degrees C achieved a temperature decrease of similar to 15 degrees C compared to a pure aluminum plate at noon and a decrease of similar to 4 degrees 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.