Multilayer passive radiative selective cooling coating based on Al/SiO_2/SiN_x/SiO_2/TiO_2/SiO_2 prepared by dc magnetron sputtering

摘要

A multilayer passive radiative selective cooling coating based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 prepared by dc magnetron sputtering is presented. The design was first theoretically optimized using the optical constants, refractive index and extinction coefficient, of thin single layers. The spectral optical constants in the wavelength range from 0.3 to 27 mu m were calculated from the transmittance and reflectance data of thin single layers deposited on silicon and glass substrates. The samples were characterized by Scanning Electron Microscopy, X-ray diffraction, Fourier-transform Infrared Spectroscopy and UV-VIS-NIR spectroscopy. It is shown that the TiO2 layer presents a partially ruffle phase polycrystalline structure and a higher refractive index than amorphous SiO2 and SiNx layers in the spectral range from 0.3 to 2.5 mu m. The cooling device was deposited on copper substrates and a thin low-density polyethylene foil with high transmittance in the 8 to 13 mu m spectral range was used as convection cover material. The device is characterized by both low reflectance (high emittance) in the sky atmospheric window (wavelength range from 8 to 13 mu m) and high hemispherical reflectance elsewhere, allowing for temperature drops of average 7.4 degrees C at night-time in winter, which corresponds to a net cooling power of similar to 43 W m(-2). Further, a temperature drop of 2.5 degrees C was obtained during winter daytime.