DONG Junle 1, ZHAO Yibo 1, HE Yuqi 1, DENG Yongjun 1, ZHAO Yong 1, LIU Yijun 2,
LI Jiayin 3 ,4, CAO Liyun 3, WANG Xuchao 3, HUANG Jianfeng 3
(1. Guangxi Mona Lisa New Materials Co., Wuzhou 543300, Guangxi, China; 2. Mona Lisa Group Co., Ltd., Foshan 528211, Guangdong, China; 3. School of Material Science and Engineering, Shaanxi Province International S & T Cooperation Foundation, Shaanxi University of Science & Technology, Xi'an 710021, Shaanxi, China; 4. Shaanxi Zhicheng Fengtec Functional Materials Co., Ltd., Xi'an 710021, Shaanxi, China)
Extended abstract:[Background and purposes] Radiative cooling is an environmentally friendly technology. SiO2 nanospheres are usually used as an emissive layer to improve the emission properties of the materials in the mid-infrared, due to the fact that the silica-oxygen bonding structure of silica exhibits significant vibrational absorption in the atmospheric window, thus improving the emission properties of the cooling materials. However, the preparation of most materials must rely on sophisticated equipment and instruments, while the preparation process is complicated. In contrast, our research team had previously prepared aluminum phosphate with triclinic and orthorhombic complex phases by using the solid phase method, which exhibited excellent solar emissivity and atmospheric window emissivity. In this study, a sol-gel method was used to synthesize SiO2@AlPO4 powder for radiation cooling applications.[Methods] SiO2@AlPO4 powder was prepared by using sol-gel method with aluminum phosphate and ethyl orthosilicate as the main raw materials. Phase composition of the powders was analyzed by using Smart Lab 9 kW X-ray diffractometer. Surface morphology was studied by using model S4800 scanning electron microscope. Highly reflective polytetrafluoroethylene white boards were used as substrates to test the reflectance in the wavelength range of 300–2500 nm by using a UV-VIS-NIR (ultraviolet-visible-near-infrared) spectral analyzer, while the emissivity measurements were conducted with Fourier transform infrared spectroscopy integrated with a gold-plated integrating sphere system (Nicolet IS50 type) by using a reflectance measurement technique.[Results] From XRD and SEM, it can be seen that SiO2@AlPO4 powder has been successfully prepared. Reflectance properties of the powder showed a decreasing and then increasing trend with increasing content of ethyl orthosilicate. This is because of the absorption properties of silica, which absorbs part of the light, thus decreasing the reflectivity of the powder. However, when the content of the silica microspheres is continuously increased, the multiple scattering between particles appeared, causing increase in reflectivity of the powder and finally reached a reflectivity of 95.8%. The emissivity of the powders in the atmospheric window increased gradually with increasing content of ethyl orthosilicate, whereas the highest emissivity was observed when the volume of ethyl orthosilicate 2 mL, which reached 90.6%. As the content of ethyl orthosilicate was further increased, no effective encapsulation could be formed, so that the emissivity could not be further enhanced. The SiO2@AlPO4 powder was used as a filler to prepare coating with aluminum foil as a substrate, showing reflectivity and emissivity of 88.4% and 0.93%, respectively. In addition, the coating achieved a cooling effect of up to 8 ℃ in daylight and up to 2.7 ℃ at night.[Conclusions] Emissivity of the powder in the atmospheric window was enhanced from 88.8% to 90.6% after aluminum phosphate was coated with silica, due to the synergistic effect of the vibrational absorption peaks of the Si-O-Si bond of silica and the vibrational absorption peaks of the P-O bond, which enhanced the emissivity of the powder at 8–13 μm. The functional powders were used as fillers to prepare coatings on the surface of aluminum foil, to study the effect of daytime radiative cooling. The coating material allowed the internal space temperature to be reduced at a maximum of 8.0 ℃, with cooling effect of up to 2.7 ℃ at night. This study is of great significance for the improvement of the performance of radiation cooling materials and their wide application.
Key words: radiation cooling; aluminum phosphate; silicon dioxide cladding