A multilayer passive radiative selective cooling coating based onAl/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 µm 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 rutile phase polycrystalline structure and a higher refractive index than amorphous SiO2 and SiNx layers in the spectral range from 0.3 to 2.5 m. The cooling device was deposited on copper substrates and a thin lowdensity polyethylene foil with high transmittance in the 8 to 13 µm 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 µm) and high hemispherical reflectance elsewhere, allowing for temperature drops of average 7.4 °C at night-time in winter, which corresponds to a net cooling power of ~43 W m -2 . Further, a temperature drop of 2.5 ºC was obtained during winter daytime.

In this work, the effect of increasing Si content in the absorber layers (CrAlSiN x /CrAlSiO y N x) of solar selective absorbers upon their selectivity and thermal stability are studied. The two optical stacks presented consist of four magnetron sputtered layers on stainless steel substrates. In both cases, tungsten is used as back-reflector, CrAlSi x N/CrAlSiO y N x as absorber layers with different Si/(Cr+Al+Si) atomic ratio (0.15 and 0.30, respectively) and finally the SiAlO x antireflection layer. The structures were theoretically designed by SCOUT software depended on experimental transmittance (T) and reflectance (R) of thin single layers deposited on glass substrates. It is observed that optical stack coatings with higher silicon content show better selectivity values, high solar absorptance, α= 95.9%, and low emissivity, ε= 9.7 % (calculated for 400 ºC), with higher thermal stability at 600 ºC in vacuum, for 650 h. Additionally, with the annealing at 600 ºC an increase of surface roughness was found, which was smaller for sample with higher Si content.

Solar thermal energy has been used as a renewable green energy source. Here we present a design of highly selective solar thermal absorber coating deposited by dc magnetron sputtering, which is four layers based on W/AlSiTiNx/SiAlTiOyNx/SiAlOx. The coating revealed an excellent average solar thermal absorbance α = 95.5 % with very low emittance ε = 9.6 % (calculated for 500 °C) together with an excellent thermal stability at 500 °C, in air for 350 h, and 630 °C in vacuum for 220 h.

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