Multilayer passive radiative selective cooling coating based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 prepared by dc magnetron sputtering
Introduction
The increasing energy consumption used for air conditioning and building cooling demands more efficient and environmental friendlier approaches [1], [2], [3]. One way of achieving this is by the radiative cooling of building surfaces using optimized coatings or paint. This approach allows to tailor the radiative properties of the surfaces by decreasing or increasing the absorption, emission, or reflection of radiant energy [1,2,4]. In terms of incoming infrared radiation from the atmosphere on a surface facing the sky, the intensity is very low in the atmospheric window (wavelengths from 8 to 13 µm). Thus, efficient passive radiative cooling surfaces should show very low reflectance and no transmittance (high absorbence) in that atmospheric window, resulting in a high thermal emittance. Furthermore, the hemispherical reflectance elsewhere must be high in order to maximize heat radiative losses and to minimize the heating by radiative absorption [5,6]. In this way, the radiative heat can be transferred from the surface to the atmosphere, leading to radiative cooling of the surface [7].
Passive radiative cooling can be performed with a proper selection of materials, such as polymers pigmented paints or multilayer coatings. Several materials already used, include plastic foils containing polyethylene, ZnS [5], PbS [8], ZnSe, TiO2 white paint, ZrO2, ZnO [9, 10], SiO2 and SiC [11, 12] and BaSO4 mixed with TiO2 [13], amongst others. These materials often show limitations and poor performance under direct solar radiation.
For daytime cooling, the solar radiation reduces the performance of such systems as it is necessary a very high solar radiation reflectivity (above 94% [14]) to achieve an equilibrium temperature below the ambient temperature.
Thus, multilayer coatings as convective shields have been optimized to be functional in both day and night-times. The use of a multilayer with repetitive high index-low index periodic layers allows for increasing the average solar radiation reflectance and the mid infrared absorption, which results in significant cooling powers. Multilayers with oxides usually require a back metallic reflector [14], [15], [16] and different structures have been developed with several layers based on SiO2/S3N4 [6, 7], SiO2/HfO2 [14], SiO2/TiO2 [15, 18], VO2/TiO2 [17] and SiO2/Al2O3 [16] coated on good metal reflectors, such as Al or Ag. The multilayers of birefringent polymer pairs do not need the metallic layer, because they act as dielectric mirrors, reflecting better than metals in the wavelength range in which the solar radiation is more intense [18].
The present work reports on a magnetron sputtered Al/SiO2/SiNx/SiO2/TiO2/SiO2 multilayer design for passive radiative cooling. Materials were selected to improve the optical properties, the structure and the selectivity of the device. The multilayer system was covered by a thin polyethylene foil in order to decrease the radiator convection losses.
Section snippets
Theoretical background for selective radiative cooling
The cooling power of a selective radiative cooling system of area A (radiator + polymeric cover) can be defined by [5, 6, 8, 15]: where, Prad, Patm, Psun, Prad/cov and Pcon+conv are, respectively, the thermal radiation power emitted by the surface, the absorbed atmospheric radiation power, the absorbed solar radiation power, the radiation flux between the polymeric cover and the radiator and the power losses due to convection and
Optical properties of the single layers
The optical design of the multilayer has the objective of simultaneously maximize the reflectivity of the solar radiation and to maximize the absorption in the transparent infrared atmospheric window. Fig. 2a shows the transmittance and the reflectance spectra of SiNx, SiO2, and TiO2 single layers deposited on glass, in the wavelength range of 300 to 2500 nm, showing that all layers are transparent. Using these spectra, the thicknesses and the optical constants (refractive index and extinction
Conclusions
This work presents a multilayer design for passive selective radiative cooling based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 and prepared by dc magnetron sputtering. The design was theoretically optimized by SCOUT software using the spectral optical constants n and k of thin single layers deposited on silicon and glass substrates. The optical constants of these single layers were obtained from the transmittance and reflectance modelling. The TiO2 layer shows a polycrystalline rutile phase and a higher
CRediT authorship contribution statement
N.F. Cunha: Conceptualization, Investigation, Methodology, Writing - original draft. A. AL-Rjoub: Investigation, Writing - original draft, Methodology. L. Rebouta: Supervision, Conceptualization, Writing - review & editing. L.G. Vieira: Investigation. S. Lanceros-Mendez: Supervision, Writing - review & editing.
Declaration of Competing Interest
We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.
We confirm that we
Acknowledgments
The authors acknowledge the support of FCT in the framework of the Strategic Funding UID/FIS/04650/2013 and the financial support of FCT, POCI and PORL operational programs through the project POCI-01-0145-FEDER-016907 (PTDC/CTM-ENE/2892/2014), co-financed by European community fund FEDER.
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