NiCrAlO/Al2O3 solar selective coating prepared by direct current magnetron sputtering and water boiling

doi:10.1016/j.solmat.2020.110807

Solar Energy Materials and Solar Cells

Volume 219, January 2021, 110807

https://doi.org/10.1016/j.solmat.2020.110807 Get rights and content

Highlights

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NiCrAlO/Al₂O₃ coating prepared by DC magnetron sputtering and water boiling.
•
Al is oxidized to dielectric while NiCr keep metallic after water boiling.
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High solar absorptance of 0.964 and low thermal emittance of 0.066.

Abstract

NiCrAlO/Al₂O₃ solar selective coating prepared by direct current (DC) magnetron sputtering and water boiling is demonstrated in this paper. Firstly, the coating consisting of tandem layers of Au/NiCrAl/Al with different NiCr volume fractions in NiCrAl sublayers is deposited solely by DC magnetron sputtering. Then the coating is boiled by deionized water and dehydrated by vacuum heating. The surface morphology, the depth distributions and relative chemical states of elements of the coating are characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It is found that Al in the coating is totally oxidized while most of NiCr and Au keep metallic after water boiling and vacuum dehydration, which forms the effective solar selective coating consisting of Au/NiCrAlO/Al₂O₃ with different metallic NiCr volume fractions. High near normal solar absorptance of 0.964 and low near normal thermal emittance of 0.066 are demonstrated by the reflectance measured by UV-VIS-IR spectrometer. The combination of DC magnetron sputtering and water boiling is very promising for fabrication of solar selective coating based on the dielectric aluminum oxide, as it is environmentally friendly and circumvents the radio frequency (RF) magnetron sputtering which needs RF automatic matching.

Introduction

Solar selective coatings based on cermet, which consists of metal and insulator particles, have been widely used in solar thermal applications, such as solar water heating [1], solar air conditioning [2], concentrated solar power plants [[3], [4], [5], [6]], etc. Alumina is a good antireflection dielectric for solar radiation and the alumina based cermet can efficiently absorb solar radiation. Alumina based solar selective coatings, such as Mo/Pt–Al₂O₃/Al₂O₃ [7], W/W–Al₂O₃/Al₂O₃ [8], Al/Ni–Al₂O₃/SiO₂ [9] and W/WNi–Al₂O₃/Al₂O₃/SiO₂ [10], exhibiting excellent optical properties, have been demonstrated. The magnetron sputtering and solution chemical method are two main techniques to prepare alumina based solar selective coatings. However, the two techniques have obvious disadvantages. For the magnetron sputtering method, the alumina is usually deposited by radio frequency (RF) sputtering [7,[10], [11], [12]], which has lower deposition rate and higher equipment cost compared to direct current (DC) sputtering [[13], [14], [15]]. For the solution chemical method, the preparation of precursor solutions and disposal of waste liquid always lead to environmental pollution [9,16,17].

Fortunately, an alumina dielectric can be obtained by water boiling of aluminum film [18,19]. In this paper, such method is adopted to prepare the high performance NiCrAlO/Al₂O₃ solar selective coating. The designed NiCrAlO/Al₂O₃ solar selective coating consists of three layers, i.e., the metallic Au infrared reflection layer, the NiCrAlO cermet solar absorption layer with different metal volume fractions and the insulator Al₂O₃ solar anti-reflection layer. Firstly, the Au/NiCrAl/Al multilayer coating is deposited by DC magnetron sputtering. Secondly, the as-deposited coating is transferred to Au/NiCrAlOH/AlOOH by water boiling. Finally, the proposed solar selective coating of Au/NiCrAlO/Al₂O₃ is obtained by dehydrating the as-boiled coating. Herein, the method combining DC magnetron sputtering and water boiling instead of RF magnetron sputtering is used to prepare alumina dielectric which has higher effective deposition rate and lower cost of power. In addition, the water solution for boiling is non-toxic, abundant and cheap. Therefore, this process is a very promising method for alumina based solar selective coating as it overcomes the disadvantages of high cost of RF magnetron sputtering and pollution of solution chemical method.

Section snippets

Experimental section

The tandem films of Au/NiCrAl/Al are deposited on the polished stainless steel (SS) substrates using a multi-target magnetron sputtering system. The sizes of all the targets are 200 × 60 × 3 mm³. An Au (99.99% purity) target, and an Al (99.99% purity) target are used to deposit the Au metal layer [20] and Al layer, respectively. The NiCrAl layers are prepared by sputtering of a NiCr alloy (Ni₄Cr, 99.95% purity) and Al targets alternately. The polished SS substrates are cleaned with acetone

Near normal solar absorptance and thermal emittance

Fig. 1 shows the near normal reflectance spectra of the as-deposited, as-boiled and dehydrated coatings. The as-deposited Au/NiCrAl/Al coating has high reflectance both at solar radiation and thermal emission wavelength ranges, which results to low near normal solar absorptance (α_sn) of 0.132 and thermal emittance (ε_n) of 0.013. Such bad spectral selectivity is mainly attributed to the top Al layer deposited by DC magnetron sputtering. On the contrast, the spectral selectivity of the coating is

Conclusions

High-performance NiCrAlO/Al₂O₃ solar selective coating is fabricated by DC magnetron sputtering and water boiling. Al is oxidized into porous Al₂O₃ while most of NiCr and all Au keep metallic after water boiling and vacuum dehydration of the Au/NiCrAl/Al prepared by DC magnetron sputtering. Consequently, the effective solar selective coating is formed, consisting of Au/NiCrAlO/Al₂O₃ with different metal volume fractions. Such solar selective coating has a high near normal solar absorptance of α

CRediT authorship contribution statement

Yuping Ning: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft, Writing - review & editing, Project administration. Jian Wang: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Supervision, Funding acquisition. Changzheng Sun: Methodology, Formal analysis, Funding acquisition. Zhibiao Hao: Formal analysis, Funding acquisition. Bing Xiong: Methodology, Formal analysis. Lai Wang: Formal analysis, Funding acquisition. Yanjun Han:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Key Research and Development Program of China (2017YFA0205800); the Science Challenge Project (TZ2016003); the National Natural Science Foundation of China (61991443, 61822404, 61875104, 61904093, 61927811, 61975093, 61974080); the Tsinghua University Initiative Scientific Research Program (20193080036); the Key Lab Program of BNRist (BNR2019ZS01005); the Collaborative Innovation Centre of Solid-State Lighting and Energy-Saving Electronics.

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NiCrAlO/Al2O3 solar selective coating prepared by direct current magnetron sputtering and water boiling

Highlights

Abstract

Introduction

Section snippets

Experimental section

Near normal solar absorptance and thermal emittance

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Sol. Energy Mater. Sol. Cells

Appl. Energy

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Alloys Compd.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Vacuum

Opt. Mater.

Vacuum

NiCrAlO/Al₂O₃ solar selective coating prepared by direct current magnetron sputtering and water boiling