Elsevier

Thin Solid Films

Volume 694, 31 January 2020, 137736
Thin Solid Films

Multilayer passive radiative selective cooling coating based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 prepared by dc magnetron sputtering

https://doi.org/10.1016/j.tsf.2019.137736Get rights and content

Highlights

  • Simulated and experimental design for passive cooling in day and night times.

  • Multilayer selective cooling design based on Al/SiO2/SiNx/SiO2/TiO2/SiO2.

  • Optical properties and structure of SiNx, SiO2 and TiO2 is studied.

Abstract

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 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 μm. The cooling device was deposited on copper substrates and a thin low-density 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 °C was obtained during winter daytime.

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]:Pcooling=Prad(Trad)PatmPsunPrad/covPcond+conv(Trad,Tcov,Tamb) 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.

References (28)

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