- Email: [email protected]
A novel multilayer high temperature colored solar absorber coating based on high-entropy alloy MoNbHfZrTi: Optimized preparation and chromaticity investigation
Solar Energy Materials & Solar Cells 209 (2020) 110444
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
A novel multilayer high temperature colored solar absorber coating based on high-entropy alloy MoNbHfZrTi: Optimized preparation and chromaticity investigation Hui-Xia Guo a, *, Cheng-Yu He a, Xiao-Li Qiu b, Yong-Qian Shen c, Gang Liu b, Xiang-Hu Gao b, ** a
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou, 730070, PR China Research and Development Center for Eco-Chemistry and Eco-Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China c State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Key Laboratory of Nonferrous Metal Alloy and Processing, Ministry of Education, School of Materials Science & Engineering, Lanzhou University of Technology, Lanzhou, 730050, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: High-entropy alloys films Magnetron sputtering Deposition parameters Absorption edge Chromaticity diagram
High-entropy alloys films with the composition of MoNbHfZrTiN and MoNbHfZrTiON are fabricated by a reactive RF magnetron sputtering, which have a good solar spectral selectivity and are used for solar thermal absorption coatings. By changing the flow rate of Ar/N2/O2, the metallic content of each layer of the coatings can be controlled, which forms the MoNbHfZrTiN layer with high metallic content and the MoNbHfZrTiON layer with low metallic content. By tailoring the deposition parameters, a novel Al/MoNbHfZrTiN/MoNbHfZrTiON/ SiO2 coating has been successfully deposited on SS substrate for the purpose of shifting the absorption edge to the higher wavelength. The coating exhibits a high absorptance of 0.935 and a low emittance of 0.09. The colors and optical properties of layer-added coatings are studied, and the color coordinates are drawn in the chromaticity diagram. By controlling the thickness of MoNbHfZrTiON and SiO2 layers, the colored Al/MoNbHfZrTiN/MoN bHfZrTiON/SiO2 coatings are fabricated.
1. Introduction In the last ten years, high-entropy alloys (HEAs) with complex multiple components made their properties different from a single dominant constituent, which have aroused strong research interests [1–3]. HEAs are commonly defined as alloys that have more than five principal elements and each principal element should beyond 5%, which are put forward by Yeh with an aim to obtain phase stabilization through entropy maximization [4–8]. Unlike conventional alloys, HEAs have favorable features such as outstanding structural stability, excellent anti-corrosion and oxidation resistance, which benefit from four core effects such as: (1) structures: severe lattice distortion; (2) properties: cocktail effects; (3) thermodynamics: high-entropy effects and (4) ki netics: sluggish diffusion [3,9–12]. Benefited from many excellent fea tures, the high-entropy alloy nitrides (HEANs) show the promising application prospects [13]. Yeh et al. have deposited the nitride coatings through a single AlCrNbSiTiV HEA target, which exhibits an excellent
thermal stability and a high hardness [14]. In addition, they also demonstrate that the AlCrSiTiZr)100-xNx film shows superior corrosion resistance in H2SO4 [15]. Shen et al. have developed a solar thermal energy storage material based on TiZrHfMoNb high-entropy alloy (HEA) owing to the excellent performance of hydrogen absorption [16]. Chen et al. have proved that the AlCrTiSiN coating annealed at 700 � C shows an outstanding self-hardening [17]. Concentrating solar power (CSP) works as an exceedingly promising solar energy thermal technology, which can produce electricity in the night or during cloudy days when the sun is not available [18,19]. As a key component in solar collector, various spectrally selective absorber coatings have been developed over the recent decades, which should exhibit a high absorptance (α > 0.90) in the solar spectrum range (0.3–2.5 μm) and a low emittance (ε < 0.10) in the wavelength range of 2.5–25 μm [20–22]. Benefited from high melting point, excellent selectivity, and superior oxidation resistance, transition metal nitrides together with oxynitrides arouse increasingly interest in solar absorber
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-X. Guo), [email protected] (X.-H. Gao). https://doi.org/10.1016/j.solmat.2020.110444 Received 12 June 2019; Received in revised form 13 November 2019; Accepted 3 February 2020 Available online 12 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
coatings. For instance, WAlN/WAlON/Al2O3 [23], WSiAlNx/WSiA lOyNx/SiAlOx [24], ZrSiN/ZrSiON/SiO2 [25], NbMoN/NbMoON/SiO2 [26], Cr/CrNx/CrNxOy/SiO2 [27], TiAlSiN/TiAlSiON/SiO2 [28] and AlCrN/AlCrNO/AlCrO [29] SSACs have been reported. Recently, Shen et al. have developed a NbTiAlSiNX high-entropy films, which shows a high absorptance in the solar spectrum and a good thermal stability up to 700 � C [30]. However, the thermal emittance is not considered, which is a key factor to achieve a high spectral selectivity for solar absorber coatings. To the best of our knowledge, there is no report about the solar absorber coatings with high spectral selectivity based on high-entropy alloy. In this work, we have successfully deposited a novel Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coating on stainless steel using the magnetron sputtering method. This coating consists of four layers, namely Al acts as the infrared reflective layer, MoNbHfZrTiN (HMVF) acts as the main absorber layer, MoNbHfZrTiON (LMVF) acts as the interference absorptance layer and SiO2 works as an anti-reflection layer. By controlling the Ar/N2/O2 atmosphere, a compositional gradient layer is produced, in which the metallic content is gradually decreased from substrate to surface for the purpose of increasing solar absorption. By controlling the deposition time of the MoNbHfZrTiON and SiO2 layers, we have fabricated four different colored coatings, which exhibit good solar spectral selectivity.
temperature.
2. Experimental
x¼
X XþY þZ
(4)
y¼
Y XþY þZ
(5)
3. Chromaticity All existing colors can be marked in the chromaticity diagram that is based on the 1931 CIE color matching functions, x(λ), y(λ) and z(λ), which provides the numerical description of the chromatic response of the observer [31]. Equations (1)–(3) can be used to calculate tristimulus values X, Y and Z � � Z X¼ PðλÞ⋅x λ dλ (1) Z
� � PðλÞ⋅y λ dλ
(2)
Z
� � PðλÞ⋅y λ dλ
(3)
Y¼
Z¼
In this experiment, D65 works as illuminant, PðλÞ ¼ D65(λ) RðλÞ can be introduced, in which the reflectance spectrum RðλÞ is measured by a PerkinElmer Lambda 950 UV/Vis/NIR Spectrometer in the visible range (380–780 nm), Equations (4)–(6) were used to define the color co ordinates x, y, z
The Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 spectrally selective absorber coating was deposited on polished stainless steel (SS) sub strates (model 304, dimensions 50 mm � 50 mm) by a reactive DC/RF magnetron sputtering system (Kurt J. Lesker, USA) with constant target current, substrate bias and substrate temperature. Firstly, all substrates were respectively cleaned with alcohol, acetone and de-ionized water in an ultrasonic agitator. The base pressure was pumped down to 5 � 10 6 mtorr by a cryopump. The Al layer was deposited by the non-reactive sputtering of Al (Φ76.2 mm � 4 mm, 99.99% purity) target in Ar at mosphere. The Ar/N2/O2 gas mixture were employed to deposit MoN bHfZrTiN, MoNbHfZrTiON and SiO2 layers using the reactive sputtering of MoNbHfZrTi (Φ76.2 mm � 4 mm, 99.99% purity) and Si (Φ76.2 mm � 4 mm, 99.99% purity) targets. The detailed optimization deposition parameters are listed in Table 1. A PerkinElmer Lambda 950 UV/Vis/NIR Spectrometer with an integration sphere (module 150 mm) was employed to measure the reflectance spectra of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coatings in solar spectrum range (0.3–2.5 μm). A Bruker TENSOR 27 FTIR Spectrometer equipped with an integrating sphere (A562-G/Q) using a gold plate as a standard for diffuse was used to obtain reflectance spectra in the infrared region (2.5–25 μm). The thickness of the coating was characterized by scanning electron microscopy (SEM) (SU8200, Tokyo, Japan). To evaluate the thermal stability, the coatings were heated in vac uum in a tubular furnace at the temperature from 300 to 500 � C, respectively. The Pfeiffer turbopump was used to generate high vacuum. The accuracy of the set temperature was �1 � C. Annealing involved increasing the temperature to the desired temperature at a slow rate of 5 � C/min and maintaining the desired temperature for the designed time. Subsequently, the samples were naturally cooled down to room
(6)
z ¼ 1-x-y
The color coordinates of the absorber films were acquired using a PerkinElmer Lambda 950UV/Vis/NIR Spectrometer. According to the chromaticity diagram, the name of color can be defined by color chart function in TFCalc software [32]. 4. Results and discussion The Ar/O2/N2 flow rates, sputtering power and deposition time have been proved to be the key factors that influence the spectral property in the optimization process. Hence, it is necessary to find how the spectral properties of the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings are affected by deposition parameters. Fig. 1 shows the schematic illustration of Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings deposited on SS substrates. The detailed opti mization process will be discussed subsequently. 4.1. Deposition parameters 4.1.1. Deposited parameters regulation of Al layer The change of absorption edge and the value of optical properties (absorptance and emittance) are the important factors for judging the deposition parameters. During the experiment, aluminum as an infrared reflective layer should have a low emittance. A large number of trials have been done with an aim to achieve suitable emittance. The reflec tance spectra deposited at 200, 250 and 300 W have been shown in Fig. 2
Table 1 Optimized deposition parameters of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coatings. layer Al MoNbHfZrTiN MoNbHfZrTiON SiO2
gas flow rate (sccm) Ar
N2
40 40 40 40
0 1 8 0
deposition time (min) O2
target power density (W/cm2) Al
0 0 4 8
15 23 28 110
5.48 0 0 0
2
MoNbHfZrTi 0 3.96 3.96 0
thickness (nm) Si 0 0 0 1.76
114 30 21 50
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
absorption. When increasing the target power, the absorption edge shifts to a low wavelength, as can be shown in Fig. 2(b). The connection be tween absorptance, emittance and target power can be described in Fig. 2(c). When increasing the target power, the absorptance and emittance are all increased. The date obviously demonstrates that the coating fabricated at 250 W possesses a low emittance (0.08), mean while, keeps the higher absorptance (0.524). The Argon flow rate also has an influence on the reflectance spec trum of Al layer, which has been depicted in the Fig. 2(d). In order to choose the best infrared reflector, the target power is maintained at 250 W and Ar flow rate is varied between 30 and 50 sccm. Fig. 2(e) shows that absorption edge shifts to a long wavelength when the Ar flow rate is increased. The absorptance can be improved from 0.524 to 0.721 by increasing the Ar flow rate from 40 to 50 sccm. But the corresponding emittance is also enhanced from 0.08 to 0.12. On the other hand, by decreasing the Argon flow rate from 40 to 30 sccm, the emittance has been decreased from 0.08 to 0.04, however, the absorptance has also been sharply decreased from 0.524 to 0.248, as shown in Fig. 2(f). Further decreasing the Argon gas flow rate could also severely damage the solar absorptance. In addition, increasing the Argon gas flow rate could also produce a high emittance. Eventually, the optimal thickness of Al layer is 114 nm when the Argon flow rate is 40 sccm and target power is 250 W. Based on the above discussion, in an attempt to obtain a high infrared reflection, the IR layer should be fabricated with a low Argon flow rate and a higher target power density. The sputtering pressure is proportional to the Argon gas flow. The high IR reflectance can be achieved by reducing the sputtering pressure and enhancing the target power density, which is consistent with the result reported by Wu [23].
Fig. 1. Schematic illustration of Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings deposited on SS substrates.
(a). In this process, deposition time and Argon flow rate are maintained at 15 min and 40 sccm, respectively. The coating fabricated at 300 W possesses a high reflectance (>90%) from 1.2 to 2.5 μm, which con tributes the low emittance in the solar spectral range. However, the high reflectance from 0.5 to 1.1 μm leads to a low absorptance in the solar spectral range. Furthermore, the coating fabricated at 200 W exhibits a low reflectance (<30%) in the visible light spectrum, which gives rise to a high absorptance. However, the coating also has a lower reflection in infrared spectral region, which is not suitable for infrared reflective layer. The absorption edge is a transition between the high reflection and the low reflection, and is a good criterion for spectrally selective
Fig. 2. Change in (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance of Al layer at different target powers. Variation of (d) reflectance spectra, (e) absorption edge, (f) absorptance and emittance with Ar flow rate on Al layer.
Fig. 3. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with deposition time on MoNbHfZrTiN layer. 3
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
shows the lower reflectance from 450 to 800 nm, which is the most densely radiated area of solar energy. Therefore, the MoNbHfZrTiN layer contributes to a high absorptance for the coating. As deposition time is increased from 22 to 24 min, the MoNbHfZrTiN layer has a higher reflectance from 400 to 1200 nm, which results in the lower absorptance. Fig. 3(b) shows that the absorption edge of the coating shifts to the higher wavelength when deposition time is extended from 22 to 24 min. Fig. 3(c) shows the relationship among absorptance, emittance and deposition time of the coatings. In this process, the emittance is kept constant at 0.08. Numerous experiments are performed to study the influence of N2 flow rate on MoNbHfZrTiN layer. During the sputtering process, the deposition time is kept constant at 22 min and the N2 flow rate is varied. Fig. 4 shows the reflectance spectra of SS/Al/MoNbHfZrTiN coating with a different N2 flow rate. The MoNbHfZrTiN layer deposited at 2 sccm has a high reflectance in the visible wavelength range. Hence, the coating has a more lower absorptance compared with the coating deposited at 2 sccm. Experimental results indicate that further increased N2 flow rate will result in a more lower absorptance. Therefore, the SS/ Al/MoNbHfZrTiN coating shows a maximum absorptance (0.77) and a lowest emittance (0.08) when the deposition time is 22 min and the N2 flow rate is 1sccm. The ultimate thickness of MoNbHfZrTiN layer is 30 nm.
Fig. 4. The reflectance spectra of SS/Al/MoNbHfZrTiN at 1 and 2 sccm. Table 2 The coatings deposited at the different N2/O2 flow rate. N2 O2
S1 (sccm)
S2 (sccm)
S3 (sccm)
S4 (sccm)
4 2
6 3
8 4
10 5
4.1.3. Deposited parameters regulation of MoNbHfZrTiON layer To further improve the solar absorptance, the MoNbHfZrTiON layer is introduced as an interference absorption layer, which is deposited on the surface of the SS/Al/MoNbHfZrTiN coating. The high spectral selectivity can be achieved when there is a lower metal content layer that enhance absorptance through extinction interference absorption. As a result, the MoNbHfZrTiON layer should be deposited at a higher gas flow rate than the MoNbHfZrTiN layer. The MoNbHfZrTiON layer is fabricated with the MoNbHfZrTi target power at 180 W. The increased MoNbHfZrTi target power could change the graded metal content property of the multilayer coatings when depositing the MoNbHfZrTiON layer. Hence, the MoNbHfZrTi target power is maintained at 180 W during the preparation process of the MoNbHfZrTiON layer. Numerous coatings are prepared by changing the gas flow rate of N2/ O2. During the deposition process, the MoNbHfZrTi target power is
4.1.2. Prepared parameters regulation of MoNbHfZrTiN layer As a main absorber layer, the MoNbHfZrTiN layer should contribute to the largest absorptance for the coating. With an aim of achieving this goal, the deposition parameters of the MoNbHfZrTiN layer are investi gated. A large number of samples are fabricated by changing the deposition time between 20 and 24 min. The target power is maintained at 180 W and the Nitrogen gas flow is kept at 1 sccm. To consider the influence of pressure, the Argon flow rate is also preserved at 40 sccm. Fig. 3(a) displays the reflectance spectra of these coatings obtained by varying deposition time. The MoNbHfZrTiN layer deposited at 20 min possesses a low absorptance due to the higher reflectance from 500 to 1000 nm. Correspondingly, the MoNbHfZrTiN layer deposited at 22 min
Fig. 5. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with N2/O2 flow rate on MoNbHfZrTiON layer.
Fig. 6. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with O2 flow rate on SiO2 layer. 4
Solar Energy Materials and Solar Cells 209 (2020) 110444
H.-X. Guo et al.
Fig. 7. (a) Reflective spectra of layer-added coatings in the solar spectra range; (b) absorptance edge shifts to long wavelength with the increased layers; (c) Reflective spectra of layer-added coatings in infrared regime.
Fig. 8. Surface topography of (a) Al, (b) Al/MoNbHfZrTiN, (c) Al/MoNbHfZrTiN/MoNbHfZrTiON and (d) Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 on the stainless steel. (f) Chromaticity diagram of the layer-added absorber coatings.
preserved at 180 W and deposition time is maintained at 28 min. The reflectance spectra of these coatings are a function of N2/O2 flow rate. Table 2 lists the different N2/O2 flow rate. From the Fig. 5(a–b), the absorption edge of those samples is located at 598 nm, 826 nm, 864 nm, 995 nm for the S1, S2, S3 and S4, respectively. Fig. 5(c) shows the relationship among absorptance, emittance and N2/O2 flow rate. The S3 has the highest solar absorptance. When the gas flow rate is further increased, the coating has a lower absorptance. With an aim to satisfy the requirement of the spectral selective absorption performance, the optimized N2 and O2 flow rate for SS/Al/MoNbHfZrTiN/MoN bHfZrTiON coating are 9 sccm and 4 sccm, respectively. The optimized SS/Al/MoNbHfZrTiN/MoNbHfZrTiON coating illustrates the highest absorptance of 0.875 and a lowest emittance of 0.09. The ultimate thickness of MoNbHfZrTiON layer is 21 nm.
Fig. 6(b) that the absorption edge could be easily adjusted within a narrow range of O2 flow rate. By increasing the O2 flow rate from 8 to 10 sccm, the absorptance decreases from 0.935 to 0.908 and the emittance increases from 0.09 to 0.11. When the O2 flow rate is decreased from 8 to 6 sccm, the absorptance is decreased from 0.935 to 0.920 while the emittance still keeps at 0.09, which has been shown in Fig. 7(c). It is necessary to stress that the deposition time of MoNbHfZrTiON and SiO2 layer should be taken into consideration which will be demonstrated in section 4.4. 4.2. Spectral selectivity of layer-added coating The as-deposited spectral selective absorption coatings should exhibit a good optical selectivity. The reflectance spectra in the visible
4.1.4. Effect of O2 flow rate on SiO2 layer A dielectric layer with lower metal content is necessary to serve as the antireflection that can further improve the solar absorptance by decreasing reflectance in solar spectrum range. Therefore, the SiO2 layer is introduced on top of SS/Al/MoNbHfZrTiN/MoNbHfZrTiON coating. During the deposition process, the O2 flow rate is varied within 6–10 sccm. The target power is preserved at 80 W and deposition time is maintained at 110 min. Fig. 6(a) illustrates that the coatings all possess a low reflectance from 0.4 to 0.7 μm, which contribute to a high absorp tance. But the coatings deposited at 6 sccm and 10 sccm have the higher reflectance from 1 to 2.2 μm, which contributes to the lower absorptance compared with the coatings deposited at 8 sccm. It is obvious from the
Table 3 Absorptance, emittance and spectral selectivity of stainless steel and layer-added coating. Layer SS SS/Al SS/Al/MoNbHfZrTiN SS/Al/MoNbHfZrTiN/ MoNbHfZrTiON SS/Al/MoNbHfZrTiN/ MoNbHfZrTiON/SiO2
5
Absorptance (α)
Emittance (ε)
Selectivity (α/ε)
0.382 0.524 0.771 0.870
0.13 0.08 0.08 0.09
3.12 6.03 9.64 9.67
0.935
0.09
10.39
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
Fig. 9. Reflectance spectra of the colored coatings, (a) bluish green coating (b) purplish pink coating and (c) greenish yellow coating. The inset shows structure and surface topography of those samples. (d) illustrates the chromaticity diagram of the colored Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 absorbing coatings. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
and the near-IR region (0.3–2.5 μm) are used to evaluate the absorption of solar radiation, while the reflectance spectra in the far-IR regions (2.5–25 μm) are investigated to evaluate thermal emittance, as depicted in Fig. 8(a, c). The optical properties of layer-added samples are given in Table 3. The sunlight is absorbed due to intrinsic absorption of the main absorber layer together with optical interference absorption between the double absorptive layers [33]. In order to demonstrate the absorp tance performance of each layer together with their interference effect, the layer-added samples are investigated. It is obviously can be seen that the absorptance and emittance value of the bare stainless steel are 0.382 and 0.13, respectively. The Al layer is deposited on SS, which gives a rise to the value of absorptance from 0.382 to 0.525 and decreases the value of emittance from 0.13 to 0.08. For single Al layer, an interference minimum exists in 0.5 μm with 31% reflectance. The deposition of MoNbHfZrTiN layer on the SS/Al coating significantly improves the value of absorptance from 0.524 to 0.771,
keeping the value of emittance at 0.08. For the SS/Al/MoNbHfZrTiN coating, only one interference minimum shifts towards longer wave lengths, which locate at about 732 nm with 7% reflectance. The coating exhibits the low reflectance (<20%) in the spectral range of ~0.5–1 μm, which contributes to a high absorptance for the coating. The MoNbHfZrTiON layer acts as the interference absorption layer, which makes the absorptance reach the value of 0.870 and the emittance increase to 0.09. The reflectance of the SS/Al/MoNbHfZrTiN/MoN bHfZrTiON coatings is drastically decreased to 10% from 0.56 to 1.2 μm. Specifically, the reflectance is almost zero from 0.6 to 0.9 μm. After the incorporation of SiO2 layer, the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/ SiO2 solar absorber coatings exhibit a high absorptance of 0.935 and a low emittance of 0.09. The reflectance spectra of the SS/Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 coatings show two interference minima at 0.6 and 1.5 μm, respectively. One of the values of interference minima is 0, and the other is 2%. A sharp absorption edge is yielded at
Table 4 Optical properties of all coatings. sample name 1 2 3 4 5 6 7 8
color
layer thickness
x-value
y-value
α
ε
Rvis
gray light blue purplish blue purple bluish green purplish pink greenish yellow purple
114 114/30 114/30/21 114/30/21/50 114/30/19/50 114/30/21/54 114/30/19/42 114/30/21/46
0.3115 0.2333 0.1886 0.2010 0.2188 0.3998 0.4422 0.312
0.3145 0.2362 0.1451 0.1002 0.3197 0.2530 0.4668 0.156
0.524 0.771 0.870 0.935 0.921 0.911 0.902 0.914
0.08 0.08 0.09 0.09 0.13 0.11 0.09 0.10
3.23 8.45 7.65 2.96 5.65
6
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
During the deposition process, the layer-added coatings present different colors, such as gray, light blue, purplish blue and purple, as shown in Fig. 8(a–d). Fig. 8(f) shows chromaticity diagram of the layeradded coatings. The point with equal coordinates value, i.e. x ¼ y ¼ z ¼ 0.333, is named as ‘‘Neutral’’, As a matter of fact, the region around the ‘‘Neutral’’ is corresponded to a neutral color [34]. The greater the dis tance to the “Neutral” is, the darker the color of the coating becomes. As shown in Fig. 7. (a), the interference minimum of SS/Al (sample 1) and SS/Al/MoNbHfZrTiN layer (sample 2) are located at 410 nm and 620 nm, respectively. Both of the samples possess low reflectance in visible wavelength range, in which no peaks are found. Therefore, they are all located at “Neutral”. But the sample 2 is at the edge of “Neutral”, which is due to the lower reflectance than sample 1 in visible wave length range. When the SS/Al/MoNbHfZrTiN layer is covered by MoNbHfZrTiON layer (sample 3), the color of the surface becomes remarkably different. The coating appears purplish blue appearance. Only one interference minimum is observed in the visible region, which is almost zero. The reflectance is 25% at 380 nm. As a result, the sample shows purplish blue appearance. However, the reflectance spectrum becomes obviously different when the SiO2 (AR) is added to the SS/Al/ MoNbHfZrTiN/MoNbHfZrTiON coating (sample 4). Only one peak is found at 0.86 μm in the solar spectrum. Additionally, the reflectance of 0.38 μm is 15%, and Rvis is 3.23%. Thus, the color of the coating be comes purple.
color of the coating will change. To prepare the colored solar absorber coating, the deposition time of Al and MoNbHfZrTiN layers remain unchanged, and the deposition time of the MoNbHfZrTiON and SiO2 layers are varied. In this section, we will discuss how the colors of the samples are affected by shifting peaks in visible wavelength range. During the deposition process, an interesting phenomenon is found that the color of the coating is very sensitive to the deposition time. Based on the preparation parameters in Table 1, when the deposition time of MoNbHfZrTiON layer is decreased from 28 min to 25 min, the color of the coating turns from purple to bluish green (sample 5). As illustrated in Fig. 9(a), two peaks are observed in the solar wavelength range, but only one peak is located at the visible region, that is 17% at 0.57 μm. The reflectance (2.3%) is extremely low at 0.38 μm, and Rvis is 8.45%. Thus, the coating manifests the green. On the other hand, the reflectance of the coating is below 16% from 0.3 to 1.8 μm, in particular, the reflectance is under 4% from 0.3 to 0.5 μm, which results in a high solar absorptance (0.921). Furthermore, when λ > 1.5 μm, the reflec tance dramatically increases, when λ > 3.0 μm, the reflectance is more than 53%, which brings in an emittance of 0.13. Fig. 9(b) illustrates part of characteristics of purplish pink sample (sample 6) that can be fabricated by changing the deposition time of the SiO2 layer from 110 min to 115 min. The reflectance of the coating fells to 0 when the wavelength increases from 0.3 μm to 0.5 μm. Another interference minimum value (0) is observed at 1.2 μm, which results in a high absorptance of 0.910. Then the reflectance remarkably rises and is beyond 80% when λ > 5 μm, which results in a low emittance of 0.11. Moreover, the peak (15%) reflectance in the visible is extremely high at 0.69 μm, the reflectance is 18% at 0.38 μm, and Rvis is 7.65%. As a result, the color of the coating is purplish pink. When the deposition time of the SiO2 layer is reduced from 110 to 100 min and the deposition time of MoNbHfZrTiON layer is decreased from 28 to 25 min, the surface of this coating turns out to be greenish yellow, which has been illustrated in Fig. 9 (c). Two interference mini mum are observed at 420 nm and 980 nm, which shift towards the lower wavelengths. As a result, the coating exhibits a lower absorptance of 0.902. Besides, the reflectance sharply enhances when λ > 1.1 μm, and is more than 60% when λ > 2.5 μm. Thus, the coating has a lower emit tance (0.08). In addition, the peak is 7.21% in the visible range, the reflectance (1.21%) is extremely low at 390 nm, and Rvis is 2.96%. Therefore, the coating has a greenish yellow appearance. Fig. 9 (d) illustrates all the colored coatings in the CIE tristimulus chromaticity diagram. Those coatings are all far away from the “Neutral” in the chromaticity diagram, indicating that the coatings have been produced with obvious colors by tailoring the thicknesses of MoNbHfZrTiON and SiO2 layer. Table 4 illustrates some characteristics of the eight coatings, i.e. absorptance, emittance, visible reflectance, xvalue and y-value. The x-value can be changed between 0.18 and 0.43, the y-value can be changed between 0.18 and 0.45. Therefore, more colored coatings could be fabricated by choosing suitable thickness of MoNbHfZrTiON and SiO2 layers. It is necessary to note that all the colored coatings have a high absorptance and a low emittance.
4.4. Regulation of the colored coating
4.5. Thermal stability of the coatings
Fig. 10. The reflectance spectra of the coatings annealed at different temperatures.
1452 nm, which makes low reflectance from 0.3 to 2 μm. It can be observed that the absorption edge of successive layer-added coatings shifting to a higher wavelength, as illustrated in Fig. 7(b). 4.3. Investigation on chromaticity of the layer-added coatings
Theoretically, when the peak shifts in visible wavelength range, the
The thermal stability of the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/
Table 5 Effect of 2 h (in vacuum) annealing on the optical properties of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coating. Temperature � С 300 350 400 450 500
ԑ
α As-deposited
Annealed
0.935 0.935 0.935 0.935 0.935
0.919 0.918 0.916 0.913 0.850
Δα 0.013 0.017 0.019 0.025 0.085
7
As-deposited
Annealed
0.09 0.09 0.09 0.09 0.09
0.12 0.13 0.13 0.15 0.17
Δԑ 0.03 0.04 0.04 0.06 0.08
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
SiO2 coatings at different temperature are evaluated in vacuum with different thermal annealing temperatures for 2 h and the corresponding reflectance spectra are shown in Fig. 10. The corresponding absorptance and emittance values of the annealed coating are listed in Table 5. It is worth to note that the coatings are thermally stable to 450� С for 2 h. At 500� С, it can be seen that the absorptance (Δα ¼ 0.085), emittance values (Δԑ ¼ 0.08) and reflectance spectra of the coatings changed significantly, which indicates the degradation of the SS/Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 coatings. The thermal stability of this coating is not good compared to the NbTiAlSiNX high-entropy films [30].
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110444. References [1] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science 345 (2014) 1153–1158. [2] Y.F. Ye, Q. Wang, J. Lu, C.T. Liu, Y. Yang, High-entropy alloy: challenges and prospects, Mater. Today 19 (2016) 349–362. [3] H. Huang, Y. Wu, J. He, H. Wang, X. Liu, K. An, W. Wu, Z. Lu, PhaseTransformation ductilization of brittle high-entropy alloys via metastability engineering, Adv. Mater. 29 (2017) 1–7. [4] B. Gludovatz, A. Hohenwarter, K.V.S. Thurston, H. Bei, Z. Wu, E.P. George, R. O. Ritchie, Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures, Nat. Commun. 7 (2016) 1–8. [5] S. Shafeie, S. Guo, P. Erhart, Q. Hu, A. Palmqvist, Balancing scattering channels: a panoscopic approach toward zero temperature coefficient of resistance using highentropy alloys, Adv. Mater. 31 (2019) 1–12. [6] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [7] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299–303. [8] Y. Bu, Z. Li, J. Liu, H. Wang, D. Raabe, W. Yang, Nonbasal slip systems enable a strong and ductile hexagonal-close-packed high-entropy phase, Phys. Rev. Lett. 122 (2019) 75502. [9] M.H. Tsai, J.W. Yeh, High-entropy alloys: a critical review, Mater. Res. Lett. 2 (2014) 107–123. [10] Y.H. Jo, S. Jung, W.M. Choi, S.S. Sohn, H.S. Kim, B.J. Lee, N.J. Kim, S. Lee, Cryogenic strength improvement by utilizing room-temperature deformation twinning in a partially recrystallized VCrMnFeCoNi high-entropy alloy, Nat. Commun. 8 (2017) 1–8. [11] S. Shafeie, S. Guo, Q. Hu, H. Fahlquist, P. Erhart, A. Palmqvist, High-entropy alloys as high-temperature thermoelectric materials, J. Appl. Phys. 118 (2015). [12] Z. Li, S. Zhao, R.O. Ritchie, M.A. Meyers, Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys, Prog. Mater. Sci. 102 (2019) 296–345. [13] K.-H. Cheng, C.-H. Lai, S.-J. Lin, J.-W. Yeh, Structural and mechanical properties of multi-element (AlCrMoTaTiZr)Nx coatings by reactive magnetron sputtering, Thin Solid Films 519 (2010) 3185–3190. [14] P.K. Huang, J.W. Yeh, Effects of nitrogen content on structure and mechanical properties of multi-element (AlCrNbSiTiV)N coating, Surf. Coating. Technol. 203 (2009) 1891–1896. [15] H. Te Hsueh, W.J. Shen, M.H. Tsai, J.W. Yeh, Effect of nitrogen content and substrate bias on mechanical and corrosion properties of high-entropy films (AlCrSiTiZr)100-xNx, Surf. Coating. Technol. 206 (2012) 4106–4112. [16] H. Shen, J. Zhang, J. Hu, J. Zhang, Y. Mao, H. Xiao, X. Zhou, X. Zu, A novel TiZrHfMoNb high-entropy alloy for solar thermal energy storage, Nanomaterials 9 (2019) 248. [17] W. Chen, A. Yan, X. Meng, D. Wu, D. Yao, D. Zhang, Microstructural change and phase transformation in each individual layer of a nano-multilayered AlCrTiSiN high-entropy alloy nitride coating upon annealing, Appl. Surf. Sci. 462 (2018) 1017–1028. [18] A. Dan, H.C. Barshilia, K. Chattopadhyay, B. Basu, Solar energy absorption mediated by surface plasma polaritons in spectrally selective dielectric-metaldielectric coatings: a critical review, Renew. Sustain. Energy Rev. 79 (2017) 1050–1077. [19] H.L. Zhang, J. Baeyens, J. Degr�eve, G. Cac�eres, Concentrated solar power plants: review and design methodology, Renew. Sustain. Energy Rev. 22 (2013) 466–481. [20] J.J.C.S. Santos, J.C.E. Palacio, A.M.M. Reyes, M. Carvalho, A.J.R. Freire, M. A. Barone, Concentrating solar power, Adv. Renew. Energ. Power Technol. 1 (2018) 373–402. [21] X.H. Gao, H.X. Guo, T.H. Zhou, G. Liu, Optical properties and failure analysis of ZrC-ZrOx ceramic based spectrally selective solar absorbers deposited at a high substrate temperature, Sol. Energy Mater. Sol. Cells 176 (2018) 93–99. [22] X.H. Gao, Z.M. Guo, Q.F. Geng, P.J. Ma, G. Liu, Structure, optical properties and thermal stability of TiC-based tandem spectrally selective solar absorber coating, Sol. Energy Mater. Sol. Cells 157 (2016) 543–549. [23] A. Dan, J. Jyothi, K. Chattopadhyay, H.C. Barshilia, B. Basu, Spectrally selective absorber coating of WAlN/WAlON/Al2O3 for solar thermal applications, Sol. Energy Mater. Sol. Cells 157 (2016) 716–726. [24] A. AL-Rjoub, L. Rebouta, P. Costa, L.G. Vieira, Multi-layer solar selective absorber coatings based on W/WSiAlNx/WSiAlOyNx/SiAlOx for high temperature applications, Sol. Energy Mater. Sol. Cells 186 (2018) 300–308. [25] Y. Ning, W. Wang, L. Wang, Y. Sun, P. Song, H. Man, Y. Zhang, B. Dai, J. Zhang, C. Wang, Y. Zhang, S. Zhao, E. Tomasella, A. Bousquet, J. Cellier, Optical simulation and preparation of novel Mo/ZrSiN/ZrSiON/SiO2 solar selective absorbing coating, Sol. Energy Mater. Sol. Cells 167 (2017) 178–183. [26] Y. Wu, C. Wang, Y. Sun, Y. Xue, Y. Ning, W. Wang, S. Zhao, E. Tomasella, A. Bousquet, Optical simulation and experimental optimization of Al/NbMoN/
5. Conclusion In summary, we have created a new field of research that fabricates a novel Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coating through DC combine with RF magnetron sputtering. The deposition parameters are controlled to achieve a high absorptance of 0.935 and a low emittance of 0.09. The Al layer acts as an infrared reflectance layer, which contrib utes to a low emittance and a high absorptance for the coating. By reducing the flow of Ar or increasing the target power, the emittance of Al layer is decreased. On the other hand, the absorptance enhances with the addition of Ar flow rate or the reduction of target power. The main absorption layer (MoNbHfZrTiN) and interference absorption layer (MoNbHfZrTiON) make the absorptance of the coating intensely in crease from 0.525 to 0.870. The SiO2 works as an AR layer, which de creases the reflectance in the solar spectrum. Compared with other recent works, we prepared all kinds of colored coatings possessing excellent spectral selectivity, and investigated color coordinates of those coatings in the ICE chromaticity diagram. However, the thermal stability of the coating is just about 450 � C in vacuum, which does not reach as our expected. Further investigation will be focused on the failure mechanism of this coating and provide a theoretical support for design the high temperature solar absorber coatings based on high entropy alloy. Author contribution statement For our manuscript entitled " A novel multilayer high temperature colored solar absorber coating based on high-entropy alloy MoN bHfZrTi: Optimized preparation and chromaticity investigation ", HuiXia Guo and Xiang-Hu Gao have designed the multilayer structure prepared by high entropy alloy, discussed the experimental results and write this manuscript. Cheng-Yu He, Xiao-Li Qiu and Yong-Qian Shen have done the experimental, discussed the experimental results. Gang Liu has discussed the experimental results. So, all the authors have made contribution to this manuscript. No conflict of interest exists in the submission of this manuscript, and manuscript is approved by all au thors for publication. All the authors listed have approved the manu script that is enclosed. Declaration of competing interest No conflict of interest exists in the submission of this manuscript. Acknowledgments This work was financially supported by the Youth Innovation Pro motion Association CAS (2018455), the National Natural Science Foundation of China (No. 51402315), the Key Research & Development Program in Gansu (18YF1GA125), Major Subject of State Grid Corpo ration of China (No. 5216A01600W0) and Scientific Research Project of Colleges and Universities in Gansu Province (No. 2018D-03). We acknowledge critical and quantity of testing work supported by Beijing Zhongkebaice Technology Service Co., Ltd. 8
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
NbMoON/SiO2 solar selective absorbing coatings, Sol. Energy Mater. Sol. Cells 134 (2015) 373–380. [27] J. Liu, Z.Q. Sun, H. Wang, Design and characterization of solar absorbing multilayer stack based on Al/Cr-N-O/SiO2 layers, Sol. Energy Mater. Sol. Cells 188 (2018) 18–26. [28] L. Rebouta, P. Capela, M. Andritschky, A. Matilainen, P. Santilli, K. Pischow, E. Alves, Characterization of TiAlSiN/TiAlSiON/SiO2 optical stack designed by modelling calculations for solar selective applications, Sol. Energy Mater. Sol. Cells 105 (2012) 202–207. [29] X. Wang, X. Zhang, Q. Li, J. Min, X. Cheng, Spectral properties of AlCrNO-based multi-layer solar selective absorbing coating during the initial stage of thermal aging upon exposure to air, Sol. Energy Mater. Sol. Cells 188 (2018) 81–92.
[30] W.J. Sheng, X. Yang, J. Zhu, C. Wang, Y. Zhang, Amorphous phase stability of NbTiAlSiNX high-entropy films, Rare Met. 37 (2018) 682–689. [31] A. Schüler, C. Roecker, J.L. Scartezzini, J. Boudaden, I.R. Videnovic, R.S.C. Ho, P. Oelhafen, On the feasibility of colored glazed thermal solar collectors based on thin film interference filters, Sol. Energy Mater. Sol. Cells 84 (2004) 241–254. [32] Software spectra: TFCalc 3.5, available at, http://www.sspectra.com/. [33] C. Wang, J. Shi, Z. Geng, X. Ling, Polychromic Al-AlN cermet solar absorber coating with high absorption efficiency and excellent durability, Sol. Energy Mater. Sol. Cells 144 (2016) 14–22. [34] D. Zhu, S. Zhao, Chromaticity and optical properties of colored and black solarthermal absorbing coatings, Sol. Energy Mater. Sol. Cells 94 (2010) 1630–1635.
9
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
A novel multilayer high temperature colored solar absorber coating based on high-entropy alloy MoNbHfZrTi: Optimized preparation and chromaticity investigation Hui-Xia Guo a, *, Cheng-Yu He a, Xiao-Li Qiu b, Yong-Qian Shen c, Gang Liu b, Xiang-Hu Gao b, ** a
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou, 730070, PR China Research and Development Center for Eco-Chemistry and Eco-Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China c State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Key Laboratory of Nonferrous Metal Alloy and Processing, Ministry of Education, School of Materials Science & Engineering, Lanzhou University of Technology, Lanzhou, 730050, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: High-entropy alloys films Magnetron sputtering Deposition parameters Absorption edge Chromaticity diagram
High-entropy alloys films with the composition of MoNbHfZrTiN and MoNbHfZrTiON are fabricated by a reactive RF magnetron sputtering, which have a good solar spectral selectivity and are used for solar thermal absorption coatings. By changing the flow rate of Ar/N2/O2, the metallic content of each layer of the coatings can be controlled, which forms the MoNbHfZrTiN layer with high metallic content and the MoNbHfZrTiON layer with low metallic content. By tailoring the deposition parameters, a novel Al/MoNbHfZrTiN/MoNbHfZrTiON/ SiO2 coating has been successfully deposited on SS substrate for the purpose of shifting the absorption edge to the higher wavelength. The coating exhibits a high absorptance of 0.935 and a low emittance of 0.09. The colors and optical properties of layer-added coatings are studied, and the color coordinates are drawn in the chromaticity diagram. By controlling the thickness of MoNbHfZrTiON and SiO2 layers, the colored Al/MoNbHfZrTiN/MoN bHfZrTiON/SiO2 coatings are fabricated.
1. Introduction In the last ten years, high-entropy alloys (HEAs) with complex multiple components made their properties different from a single dominant constituent, which have aroused strong research interests [1–3]. HEAs are commonly defined as alloys that have more than five principal elements and each principal element should beyond 5%, which are put forward by Yeh with an aim to obtain phase stabilization through entropy maximization [4–8]. Unlike conventional alloys, HEAs have favorable features such as outstanding structural stability, excellent anti-corrosion and oxidation resistance, which benefit from four core effects such as: (1) structures: severe lattice distortion; (2) properties: cocktail effects; (3) thermodynamics: high-entropy effects and (4) ki netics: sluggish diffusion [3,9–12]. Benefited from many excellent fea tures, the high-entropy alloy nitrides (HEANs) show the promising application prospects [13]. Yeh et al. have deposited the nitride coatings through a single AlCrNbSiTiV HEA target, which exhibits an excellent
thermal stability and a high hardness [14]. In addition, they also demonstrate that the AlCrSiTiZr)100-xNx film shows superior corrosion resistance in H2SO4 [15]. Shen et al. have developed a solar thermal energy storage material based on TiZrHfMoNb high-entropy alloy (HEA) owing to the excellent performance of hydrogen absorption [16]. Chen et al. have proved that the AlCrTiSiN coating annealed at 700 � C shows an outstanding self-hardening [17]. Concentrating solar power (CSP) works as an exceedingly promising solar energy thermal technology, which can produce electricity in the night or during cloudy days when the sun is not available [18,19]. As a key component in solar collector, various spectrally selective absorber coatings have been developed over the recent decades, which should exhibit a high absorptance (α > 0.90) in the solar spectrum range (0.3–2.5 μm) and a low emittance (ε < 0.10) in the wavelength range of 2.5–25 μm [20–22]. Benefited from high melting point, excellent selectivity, and superior oxidation resistance, transition metal nitrides together with oxynitrides arouse increasingly interest in solar absorber
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-X. Guo), [email protected] (X.-H. Gao). https://doi.org/10.1016/j.solmat.2020.110444 Received 12 June 2019; Received in revised form 13 November 2019; Accepted 3 February 2020 Available online 12 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
coatings. For instance, WAlN/WAlON/Al2O3 [23], WSiAlNx/WSiA lOyNx/SiAlOx [24], ZrSiN/ZrSiON/SiO2 [25], NbMoN/NbMoON/SiO2 [26], Cr/CrNx/CrNxOy/SiO2 [27], TiAlSiN/TiAlSiON/SiO2 [28] and AlCrN/AlCrNO/AlCrO [29] SSACs have been reported. Recently, Shen et al. have developed a NbTiAlSiNX high-entropy films, which shows a high absorptance in the solar spectrum and a good thermal stability up to 700 � C [30]. However, the thermal emittance is not considered, which is a key factor to achieve a high spectral selectivity for solar absorber coatings. To the best of our knowledge, there is no report about the solar absorber coatings with high spectral selectivity based on high-entropy alloy. In this work, we have successfully deposited a novel Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coating on stainless steel using the magnetron sputtering method. This coating consists of four layers, namely Al acts as the infrared reflective layer, MoNbHfZrTiN (HMVF) acts as the main absorber layer, MoNbHfZrTiON (LMVF) acts as the interference absorptance layer and SiO2 works as an anti-reflection layer. By controlling the Ar/N2/O2 atmosphere, a compositional gradient layer is produced, in which the metallic content is gradually decreased from substrate to surface for the purpose of increasing solar absorption. By controlling the deposition time of the MoNbHfZrTiON and SiO2 layers, we have fabricated four different colored coatings, which exhibit good solar spectral selectivity.
temperature.
2. Experimental
x¼
X XþY þZ
(4)
y¼
Y XþY þZ
(5)
3. Chromaticity All existing colors can be marked in the chromaticity diagram that is based on the 1931 CIE color matching functions, x(λ), y(λ) and z(λ), which provides the numerical description of the chromatic response of the observer [31]. Equations (1)–(3) can be used to calculate tristimulus values X, Y and Z � � Z X¼ PðλÞ⋅x λ dλ (1) Z
� � PðλÞ⋅y λ dλ
(2)
Z
� � PðλÞ⋅y λ dλ
(3)
Y¼
Z¼
In this experiment, D65 works as illuminant, PðλÞ ¼ D65(λ) RðλÞ can be introduced, in which the reflectance spectrum RðλÞ is measured by a PerkinElmer Lambda 950 UV/Vis/NIR Spectrometer in the visible range (380–780 nm), Equations (4)–(6) were used to define the color co ordinates x, y, z
The Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 spectrally selective absorber coating was deposited on polished stainless steel (SS) sub strates (model 304, dimensions 50 mm � 50 mm) by a reactive DC/RF magnetron sputtering system (Kurt J. Lesker, USA) with constant target current, substrate bias and substrate temperature. Firstly, all substrates were respectively cleaned with alcohol, acetone and de-ionized water in an ultrasonic agitator. The base pressure was pumped down to 5 � 10 6 mtorr by a cryopump. The Al layer was deposited by the non-reactive sputtering of Al (Φ76.2 mm � 4 mm, 99.99% purity) target in Ar at mosphere. The Ar/N2/O2 gas mixture were employed to deposit MoN bHfZrTiN, MoNbHfZrTiON and SiO2 layers using the reactive sputtering of MoNbHfZrTi (Φ76.2 mm � 4 mm, 99.99% purity) and Si (Φ76.2 mm � 4 mm, 99.99% purity) targets. The detailed optimization deposition parameters are listed in Table 1. A PerkinElmer Lambda 950 UV/Vis/NIR Spectrometer with an integration sphere (module 150 mm) was employed to measure the reflectance spectra of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coatings in solar spectrum range (0.3–2.5 μm). A Bruker TENSOR 27 FTIR Spectrometer equipped with an integrating sphere (A562-G/Q) using a gold plate as a standard for diffuse was used to obtain reflectance spectra in the infrared region (2.5–25 μm). The thickness of the coating was characterized by scanning electron microscopy (SEM) (SU8200, Tokyo, Japan). To evaluate the thermal stability, the coatings were heated in vac uum in a tubular furnace at the temperature from 300 to 500 � C, respectively. The Pfeiffer turbopump was used to generate high vacuum. The accuracy of the set temperature was �1 � C. Annealing involved increasing the temperature to the desired temperature at a slow rate of 5 � C/min and maintaining the desired temperature for the designed time. Subsequently, the samples were naturally cooled down to room
(6)
z ¼ 1-x-y
The color coordinates of the absorber films were acquired using a PerkinElmer Lambda 950UV/Vis/NIR Spectrometer. According to the chromaticity diagram, the name of color can be defined by color chart function in TFCalc software [32]. 4. Results and discussion The Ar/O2/N2 flow rates, sputtering power and deposition time have been proved to be the key factors that influence the spectral property in the optimization process. Hence, it is necessary to find how the spectral properties of the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings are affected by deposition parameters. Fig. 1 shows the schematic illustration of Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings deposited on SS substrates. The detailed opti mization process will be discussed subsequently. 4.1. Deposition parameters 4.1.1. Deposited parameters regulation of Al layer The change of absorption edge and the value of optical properties (absorptance and emittance) are the important factors for judging the deposition parameters. During the experiment, aluminum as an infrared reflective layer should have a low emittance. A large number of trials have been done with an aim to achieve suitable emittance. The reflec tance spectra deposited at 200, 250 and 300 W have been shown in Fig. 2
Table 1 Optimized deposition parameters of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coatings. layer Al MoNbHfZrTiN MoNbHfZrTiON SiO2
gas flow rate (sccm) Ar
N2
40 40 40 40
0 1 8 0
deposition time (min) O2
target power density (W/cm2) Al
0 0 4 8
15 23 28 110
5.48 0 0 0
2
MoNbHfZrTi 0 3.96 3.96 0
thickness (nm) Si 0 0 0 1.76
114 30 21 50
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
absorption. When increasing the target power, the absorption edge shifts to a low wavelength, as can be shown in Fig. 2(b). The connection be tween absorptance, emittance and target power can be described in Fig. 2(c). When increasing the target power, the absorptance and emittance are all increased. The date obviously demonstrates that the coating fabricated at 250 W possesses a low emittance (0.08), mean while, keeps the higher absorptance (0.524). The Argon flow rate also has an influence on the reflectance spec trum of Al layer, which has been depicted in the Fig. 2(d). In order to choose the best infrared reflector, the target power is maintained at 250 W and Ar flow rate is varied between 30 and 50 sccm. Fig. 2(e) shows that absorption edge shifts to a long wavelength when the Ar flow rate is increased. The absorptance can be improved from 0.524 to 0.721 by increasing the Ar flow rate from 40 to 50 sccm. But the corresponding emittance is also enhanced from 0.08 to 0.12. On the other hand, by decreasing the Argon flow rate from 40 to 30 sccm, the emittance has been decreased from 0.08 to 0.04, however, the absorptance has also been sharply decreased from 0.524 to 0.248, as shown in Fig. 2(f). Further decreasing the Argon gas flow rate could also severely damage the solar absorptance. In addition, increasing the Argon gas flow rate could also produce a high emittance. Eventually, the optimal thickness of Al layer is 114 nm when the Argon flow rate is 40 sccm and target power is 250 W. Based on the above discussion, in an attempt to obtain a high infrared reflection, the IR layer should be fabricated with a low Argon flow rate and a higher target power density. The sputtering pressure is proportional to the Argon gas flow. The high IR reflectance can be achieved by reducing the sputtering pressure and enhancing the target power density, which is consistent with the result reported by Wu [23].
Fig. 1. Schematic illustration of Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 solar absorber coatings deposited on SS substrates.
(a). In this process, deposition time and Argon flow rate are maintained at 15 min and 40 sccm, respectively. The coating fabricated at 300 W possesses a high reflectance (>90%) from 1.2 to 2.5 μm, which con tributes the low emittance in the solar spectral range. However, the high reflectance from 0.5 to 1.1 μm leads to a low absorptance in the solar spectral range. Furthermore, the coating fabricated at 200 W exhibits a low reflectance (<30%) in the visible light spectrum, which gives rise to a high absorptance. However, the coating also has a lower reflection in infrared spectral region, which is not suitable for infrared reflective layer. The absorption edge is a transition between the high reflection and the low reflection, and is a good criterion for spectrally selective
Fig. 2. Change in (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance of Al layer at different target powers. Variation of (d) reflectance spectra, (e) absorption edge, (f) absorptance and emittance with Ar flow rate on Al layer.
Fig. 3. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with deposition time on MoNbHfZrTiN layer. 3
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
shows the lower reflectance from 450 to 800 nm, which is the most densely radiated area of solar energy. Therefore, the MoNbHfZrTiN layer contributes to a high absorptance for the coating. As deposition time is increased from 22 to 24 min, the MoNbHfZrTiN layer has a higher reflectance from 400 to 1200 nm, which results in the lower absorptance. Fig. 3(b) shows that the absorption edge of the coating shifts to the higher wavelength when deposition time is extended from 22 to 24 min. Fig. 3(c) shows the relationship among absorptance, emittance and deposition time of the coatings. In this process, the emittance is kept constant at 0.08. Numerous experiments are performed to study the influence of N2 flow rate on MoNbHfZrTiN layer. During the sputtering process, the deposition time is kept constant at 22 min and the N2 flow rate is varied. Fig. 4 shows the reflectance spectra of SS/Al/MoNbHfZrTiN coating with a different N2 flow rate. The MoNbHfZrTiN layer deposited at 2 sccm has a high reflectance in the visible wavelength range. Hence, the coating has a more lower absorptance compared with the coating deposited at 2 sccm. Experimental results indicate that further increased N2 flow rate will result in a more lower absorptance. Therefore, the SS/ Al/MoNbHfZrTiN coating shows a maximum absorptance (0.77) and a lowest emittance (0.08) when the deposition time is 22 min and the N2 flow rate is 1sccm. The ultimate thickness of MoNbHfZrTiN layer is 30 nm.
Fig. 4. The reflectance spectra of SS/Al/MoNbHfZrTiN at 1 and 2 sccm. Table 2 The coatings deposited at the different N2/O2 flow rate. N2 O2
S1 (sccm)
S2 (sccm)
S3 (sccm)
S4 (sccm)
4 2
6 3
8 4
10 5
4.1.3. Deposited parameters regulation of MoNbHfZrTiON layer To further improve the solar absorptance, the MoNbHfZrTiON layer is introduced as an interference absorption layer, which is deposited on the surface of the SS/Al/MoNbHfZrTiN coating. The high spectral selectivity can be achieved when there is a lower metal content layer that enhance absorptance through extinction interference absorption. As a result, the MoNbHfZrTiON layer should be deposited at a higher gas flow rate than the MoNbHfZrTiN layer. The MoNbHfZrTiON layer is fabricated with the MoNbHfZrTi target power at 180 W. The increased MoNbHfZrTi target power could change the graded metal content property of the multilayer coatings when depositing the MoNbHfZrTiON layer. Hence, the MoNbHfZrTi target power is maintained at 180 W during the preparation process of the MoNbHfZrTiON layer. Numerous coatings are prepared by changing the gas flow rate of N2/ O2. During the deposition process, the MoNbHfZrTi target power is
4.1.2. Prepared parameters regulation of MoNbHfZrTiN layer As a main absorber layer, the MoNbHfZrTiN layer should contribute to the largest absorptance for the coating. With an aim of achieving this goal, the deposition parameters of the MoNbHfZrTiN layer are investi gated. A large number of samples are fabricated by changing the deposition time between 20 and 24 min. The target power is maintained at 180 W and the Nitrogen gas flow is kept at 1 sccm. To consider the influence of pressure, the Argon flow rate is also preserved at 40 sccm. Fig. 3(a) displays the reflectance spectra of these coatings obtained by varying deposition time. The MoNbHfZrTiN layer deposited at 20 min possesses a low absorptance due to the higher reflectance from 500 to 1000 nm. Correspondingly, the MoNbHfZrTiN layer deposited at 22 min
Fig. 5. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with N2/O2 flow rate on MoNbHfZrTiON layer.
Fig. 6. Variation of (a) reflectance spectra, (b) absorption edge, (c) absorptance and emittance with O2 flow rate on SiO2 layer. 4
Solar Energy Materials and Solar Cells 209 (2020) 110444
H.-X. Guo et al.
Fig. 7. (a) Reflective spectra of layer-added coatings in the solar spectra range; (b) absorptance edge shifts to long wavelength with the increased layers; (c) Reflective spectra of layer-added coatings in infrared regime.
Fig. 8. Surface topography of (a) Al, (b) Al/MoNbHfZrTiN, (c) Al/MoNbHfZrTiN/MoNbHfZrTiON and (d) Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 on the stainless steel. (f) Chromaticity diagram of the layer-added absorber coatings.
preserved at 180 W and deposition time is maintained at 28 min. The reflectance spectra of these coatings are a function of N2/O2 flow rate. Table 2 lists the different N2/O2 flow rate. From the Fig. 5(a–b), the absorption edge of those samples is located at 598 nm, 826 nm, 864 nm, 995 nm for the S1, S2, S3 and S4, respectively. Fig. 5(c) shows the relationship among absorptance, emittance and N2/O2 flow rate. The S3 has the highest solar absorptance. When the gas flow rate is further increased, the coating has a lower absorptance. With an aim to satisfy the requirement of the spectral selective absorption performance, the optimized N2 and O2 flow rate for SS/Al/MoNbHfZrTiN/MoN bHfZrTiON coating are 9 sccm and 4 sccm, respectively. The optimized SS/Al/MoNbHfZrTiN/MoNbHfZrTiON coating illustrates the highest absorptance of 0.875 and a lowest emittance of 0.09. The ultimate thickness of MoNbHfZrTiON layer is 21 nm.
Fig. 6(b) that the absorption edge could be easily adjusted within a narrow range of O2 flow rate. By increasing the O2 flow rate from 8 to 10 sccm, the absorptance decreases from 0.935 to 0.908 and the emittance increases from 0.09 to 0.11. When the O2 flow rate is decreased from 8 to 6 sccm, the absorptance is decreased from 0.935 to 0.920 while the emittance still keeps at 0.09, which has been shown in Fig. 7(c). It is necessary to stress that the deposition time of MoNbHfZrTiON and SiO2 layer should be taken into consideration which will be demonstrated in section 4.4. 4.2. Spectral selectivity of layer-added coating The as-deposited spectral selective absorption coatings should exhibit a good optical selectivity. The reflectance spectra in the visible
4.1.4. Effect of O2 flow rate on SiO2 layer A dielectric layer with lower metal content is necessary to serve as the antireflection that can further improve the solar absorptance by decreasing reflectance in solar spectrum range. Therefore, the SiO2 layer is introduced on top of SS/Al/MoNbHfZrTiN/MoNbHfZrTiON coating. During the deposition process, the O2 flow rate is varied within 6–10 sccm. The target power is preserved at 80 W and deposition time is maintained at 110 min. Fig. 6(a) illustrates that the coatings all possess a low reflectance from 0.4 to 0.7 μm, which contribute to a high absorp tance. But the coatings deposited at 6 sccm and 10 sccm have the higher reflectance from 1 to 2.2 μm, which contributes to the lower absorptance compared with the coatings deposited at 8 sccm. It is obvious from the
Table 3 Absorptance, emittance and spectral selectivity of stainless steel and layer-added coating. Layer SS SS/Al SS/Al/MoNbHfZrTiN SS/Al/MoNbHfZrTiN/ MoNbHfZrTiON SS/Al/MoNbHfZrTiN/ MoNbHfZrTiON/SiO2
5
Absorptance (α)
Emittance (ε)
Selectivity (α/ε)
0.382 0.524 0.771 0.870
0.13 0.08 0.08 0.09
3.12 6.03 9.64 9.67
0.935
0.09
10.39
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
Fig. 9. Reflectance spectra of the colored coatings, (a) bluish green coating (b) purplish pink coating and (c) greenish yellow coating. The inset shows structure and surface topography of those samples. (d) illustrates the chromaticity diagram of the colored Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 absorbing coatings. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
and the near-IR region (0.3–2.5 μm) are used to evaluate the absorption of solar radiation, while the reflectance spectra in the far-IR regions (2.5–25 μm) are investigated to evaluate thermal emittance, as depicted in Fig. 8(a, c). The optical properties of layer-added samples are given in Table 3. The sunlight is absorbed due to intrinsic absorption of the main absorber layer together with optical interference absorption between the double absorptive layers [33]. In order to demonstrate the absorp tance performance of each layer together with their interference effect, the layer-added samples are investigated. It is obviously can be seen that the absorptance and emittance value of the bare stainless steel are 0.382 and 0.13, respectively. The Al layer is deposited on SS, which gives a rise to the value of absorptance from 0.382 to 0.525 and decreases the value of emittance from 0.13 to 0.08. For single Al layer, an interference minimum exists in 0.5 μm with 31% reflectance. The deposition of MoNbHfZrTiN layer on the SS/Al coating significantly improves the value of absorptance from 0.524 to 0.771,
keeping the value of emittance at 0.08. For the SS/Al/MoNbHfZrTiN coating, only one interference minimum shifts towards longer wave lengths, which locate at about 732 nm with 7% reflectance. The coating exhibits the low reflectance (<20%) in the spectral range of ~0.5–1 μm, which contributes to a high absorptance for the coating. The MoNbHfZrTiON layer acts as the interference absorption layer, which makes the absorptance reach the value of 0.870 and the emittance increase to 0.09. The reflectance of the SS/Al/MoNbHfZrTiN/MoN bHfZrTiON coatings is drastically decreased to 10% from 0.56 to 1.2 μm. Specifically, the reflectance is almost zero from 0.6 to 0.9 μm. After the incorporation of SiO2 layer, the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/ SiO2 solar absorber coatings exhibit a high absorptance of 0.935 and a low emittance of 0.09. The reflectance spectra of the SS/Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 coatings show two interference minima at 0.6 and 1.5 μm, respectively. One of the values of interference minima is 0, and the other is 2%. A sharp absorption edge is yielded at
Table 4 Optical properties of all coatings. sample name 1 2 3 4 5 6 7 8
color
layer thickness
x-value
y-value
α
ε
Rvis
gray light blue purplish blue purple bluish green purplish pink greenish yellow purple
114 114/30 114/30/21 114/30/21/50 114/30/19/50 114/30/21/54 114/30/19/42 114/30/21/46
0.3115 0.2333 0.1886 0.2010 0.2188 0.3998 0.4422 0.312
0.3145 0.2362 0.1451 0.1002 0.3197 0.2530 0.4668 0.156
0.524 0.771 0.870 0.935 0.921 0.911 0.902 0.914
0.08 0.08 0.09 0.09 0.13 0.11 0.09 0.10
3.23 8.45 7.65 2.96 5.65
6
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
During the deposition process, the layer-added coatings present different colors, such as gray, light blue, purplish blue and purple, as shown in Fig. 8(a–d). Fig. 8(f) shows chromaticity diagram of the layeradded coatings. The point with equal coordinates value, i.e. x ¼ y ¼ z ¼ 0.333, is named as ‘‘Neutral’’, As a matter of fact, the region around the ‘‘Neutral’’ is corresponded to a neutral color [34]. The greater the dis tance to the “Neutral” is, the darker the color of the coating becomes. As shown in Fig. 7. (a), the interference minimum of SS/Al (sample 1) and SS/Al/MoNbHfZrTiN layer (sample 2) are located at 410 nm and 620 nm, respectively. Both of the samples possess low reflectance in visible wavelength range, in which no peaks are found. Therefore, they are all located at “Neutral”. But the sample 2 is at the edge of “Neutral”, which is due to the lower reflectance than sample 1 in visible wave length range. When the SS/Al/MoNbHfZrTiN layer is covered by MoNbHfZrTiON layer (sample 3), the color of the surface becomes remarkably different. The coating appears purplish blue appearance. Only one interference minimum is observed in the visible region, which is almost zero. The reflectance is 25% at 380 nm. As a result, the sample shows purplish blue appearance. However, the reflectance spectrum becomes obviously different when the SiO2 (AR) is added to the SS/Al/ MoNbHfZrTiN/MoNbHfZrTiON coating (sample 4). Only one peak is found at 0.86 μm in the solar spectrum. Additionally, the reflectance of 0.38 μm is 15%, and Rvis is 3.23%. Thus, the color of the coating be comes purple.
color of the coating will change. To prepare the colored solar absorber coating, the deposition time of Al and MoNbHfZrTiN layers remain unchanged, and the deposition time of the MoNbHfZrTiON and SiO2 layers are varied. In this section, we will discuss how the colors of the samples are affected by shifting peaks in visible wavelength range. During the deposition process, an interesting phenomenon is found that the color of the coating is very sensitive to the deposition time. Based on the preparation parameters in Table 1, when the deposition time of MoNbHfZrTiON layer is decreased from 28 min to 25 min, the color of the coating turns from purple to bluish green (sample 5). As illustrated in Fig. 9(a), two peaks are observed in the solar wavelength range, but only one peak is located at the visible region, that is 17% at 0.57 μm. The reflectance (2.3%) is extremely low at 0.38 μm, and Rvis is 8.45%. Thus, the coating manifests the green. On the other hand, the reflectance of the coating is below 16% from 0.3 to 1.8 μm, in particular, the reflectance is under 4% from 0.3 to 0.5 μm, which results in a high solar absorptance (0.921). Furthermore, when λ > 1.5 μm, the reflec tance dramatically increases, when λ > 3.0 μm, the reflectance is more than 53%, which brings in an emittance of 0.13. Fig. 9(b) illustrates part of characteristics of purplish pink sample (sample 6) that can be fabricated by changing the deposition time of the SiO2 layer from 110 min to 115 min. The reflectance of the coating fells to 0 when the wavelength increases from 0.3 μm to 0.5 μm. Another interference minimum value (0) is observed at 1.2 μm, which results in a high absorptance of 0.910. Then the reflectance remarkably rises and is beyond 80% when λ > 5 μm, which results in a low emittance of 0.11. Moreover, the peak (15%) reflectance in the visible is extremely high at 0.69 μm, the reflectance is 18% at 0.38 μm, and Rvis is 7.65%. As a result, the color of the coating is purplish pink. When the deposition time of the SiO2 layer is reduced from 110 to 100 min and the deposition time of MoNbHfZrTiON layer is decreased from 28 to 25 min, the surface of this coating turns out to be greenish yellow, which has been illustrated in Fig. 9 (c). Two interference mini mum are observed at 420 nm and 980 nm, which shift towards the lower wavelengths. As a result, the coating exhibits a lower absorptance of 0.902. Besides, the reflectance sharply enhances when λ > 1.1 μm, and is more than 60% when λ > 2.5 μm. Thus, the coating has a lower emit tance (0.08). In addition, the peak is 7.21% in the visible range, the reflectance (1.21%) is extremely low at 390 nm, and Rvis is 2.96%. Therefore, the coating has a greenish yellow appearance. Fig. 9 (d) illustrates all the colored coatings in the CIE tristimulus chromaticity diagram. Those coatings are all far away from the “Neutral” in the chromaticity diagram, indicating that the coatings have been produced with obvious colors by tailoring the thicknesses of MoNbHfZrTiON and SiO2 layer. Table 4 illustrates some characteristics of the eight coatings, i.e. absorptance, emittance, visible reflectance, xvalue and y-value. The x-value can be changed between 0.18 and 0.43, the y-value can be changed between 0.18 and 0.45. Therefore, more colored coatings could be fabricated by choosing suitable thickness of MoNbHfZrTiON and SiO2 layers. It is necessary to note that all the colored coatings have a high absorptance and a low emittance.
4.4. Regulation of the colored coating
4.5. Thermal stability of the coatings
Fig. 10. The reflectance spectra of the coatings annealed at different temperatures.
1452 nm, which makes low reflectance from 0.3 to 2 μm. It can be observed that the absorption edge of successive layer-added coatings shifting to a higher wavelength, as illustrated in Fig. 7(b). 4.3. Investigation on chromaticity of the layer-added coatings
Theoretically, when the peak shifts in visible wavelength range, the
The thermal stability of the SS/Al/MoNbHfZrTiN/MoNbHfZrTiON/
Table 5 Effect of 2 h (in vacuum) annealing on the optical properties of the Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coating. Temperature � С 300 350 400 450 500
ԑ
α As-deposited
Annealed
0.935 0.935 0.935 0.935 0.935
0.919 0.918 0.916 0.913 0.850
Δα 0.013 0.017 0.019 0.025 0.085
7
As-deposited
Annealed
0.09 0.09 0.09 0.09 0.09
0.12 0.13 0.13 0.15 0.17
Δԑ 0.03 0.04 0.04 0.06 0.08
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
SiO2 coatings at different temperature are evaluated in vacuum with different thermal annealing temperatures for 2 h and the corresponding reflectance spectra are shown in Fig. 10. The corresponding absorptance and emittance values of the annealed coating are listed in Table 5. It is worth to note that the coatings are thermally stable to 450� С for 2 h. At 500� С, it can be seen that the absorptance (Δα ¼ 0.085), emittance values (Δԑ ¼ 0.08) and reflectance spectra of the coatings changed significantly, which indicates the degradation of the SS/Al/MoN bHfZrTiN/MoNbHfZrTiON/SiO2 coatings. The thermal stability of this coating is not good compared to the NbTiAlSiNX high-entropy films [30].
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110444. References [1] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science 345 (2014) 1153–1158. [2] Y.F. Ye, Q. Wang, J. Lu, C.T. Liu, Y. Yang, High-entropy alloy: challenges and prospects, Mater. Today 19 (2016) 349–362. [3] H. Huang, Y. Wu, J. He, H. Wang, X. Liu, K. An, W. Wu, Z. Lu, PhaseTransformation ductilization of brittle high-entropy alloys via metastability engineering, Adv. Mater. 29 (2017) 1–7. [4] B. Gludovatz, A. Hohenwarter, K.V.S. Thurston, H. Bei, Z. Wu, E.P. George, R. O. Ritchie, Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures, Nat. Commun. 7 (2016) 1–8. [5] S. Shafeie, S. Guo, P. Erhart, Q. Hu, A. Palmqvist, Balancing scattering channels: a panoscopic approach toward zero temperature coefficient of resistance using highentropy alloys, Adv. Mater. 31 (2019) 1–12. [6] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [7] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299–303. [8] Y. Bu, Z. Li, J. Liu, H. Wang, D. Raabe, W. Yang, Nonbasal slip systems enable a strong and ductile hexagonal-close-packed high-entropy phase, Phys. Rev. Lett. 122 (2019) 75502. [9] M.H. Tsai, J.W. Yeh, High-entropy alloys: a critical review, Mater. Res. Lett. 2 (2014) 107–123. [10] Y.H. Jo, S. Jung, W.M. Choi, S.S. Sohn, H.S. Kim, B.J. Lee, N.J. Kim, S. Lee, Cryogenic strength improvement by utilizing room-temperature deformation twinning in a partially recrystallized VCrMnFeCoNi high-entropy alloy, Nat. Commun. 8 (2017) 1–8. [11] S. Shafeie, S. Guo, Q. Hu, H. Fahlquist, P. Erhart, A. Palmqvist, High-entropy alloys as high-temperature thermoelectric materials, J. Appl. Phys. 118 (2015). [12] Z. Li, S. Zhao, R.O. Ritchie, M.A. Meyers, Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys, Prog. Mater. Sci. 102 (2019) 296–345. [13] K.-H. Cheng, C.-H. Lai, S.-J. Lin, J.-W. Yeh, Structural and mechanical properties of multi-element (AlCrMoTaTiZr)Nx coatings by reactive magnetron sputtering, Thin Solid Films 519 (2010) 3185–3190. [14] P.K. Huang, J.W. Yeh, Effects of nitrogen content on structure and mechanical properties of multi-element (AlCrNbSiTiV)N coating, Surf. Coating. Technol. 203 (2009) 1891–1896. [15] H. Te Hsueh, W.J. Shen, M.H. Tsai, J.W. Yeh, Effect of nitrogen content and substrate bias on mechanical and corrosion properties of high-entropy films (AlCrSiTiZr)100-xNx, Surf. Coating. Technol. 206 (2012) 4106–4112. [16] H. Shen, J. Zhang, J. Hu, J. Zhang, Y. Mao, H. Xiao, X. Zhou, X. Zu, A novel TiZrHfMoNb high-entropy alloy for solar thermal energy storage, Nanomaterials 9 (2019) 248. [17] W. Chen, A. Yan, X. Meng, D. Wu, D. Yao, D. Zhang, Microstructural change and phase transformation in each individual layer of a nano-multilayered AlCrTiSiN high-entropy alloy nitride coating upon annealing, Appl. Surf. Sci. 462 (2018) 1017–1028. [18] A. Dan, H.C. Barshilia, K. Chattopadhyay, B. Basu, Solar energy absorption mediated by surface plasma polaritons in spectrally selective dielectric-metaldielectric coatings: a critical review, Renew. Sustain. Energy Rev. 79 (2017) 1050–1077. [19] H.L. Zhang, J. Baeyens, J. Degr�eve, G. Cac�eres, Concentrated solar power plants: review and design methodology, Renew. Sustain. Energy Rev. 22 (2013) 466–481. [20] J.J.C.S. Santos, J.C.E. Palacio, A.M.M. Reyes, M. Carvalho, A.J.R. Freire, M. A. Barone, Concentrating solar power, Adv. Renew. Energ. Power Technol. 1 (2018) 373–402. [21] X.H. Gao, H.X. Guo, T.H. Zhou, G. Liu, Optical properties and failure analysis of ZrC-ZrOx ceramic based spectrally selective solar absorbers deposited at a high substrate temperature, Sol. Energy Mater. Sol. Cells 176 (2018) 93–99. [22] X.H. Gao, Z.M. Guo, Q.F. Geng, P.J. Ma, G. Liu, Structure, optical properties and thermal stability of TiC-based tandem spectrally selective solar absorber coating, Sol. Energy Mater. Sol. Cells 157 (2016) 543–549. [23] A. Dan, J. Jyothi, K. Chattopadhyay, H.C. Barshilia, B. Basu, Spectrally selective absorber coating of WAlN/WAlON/Al2O3 for solar thermal applications, Sol. Energy Mater. Sol. Cells 157 (2016) 716–726. [24] A. AL-Rjoub, L. Rebouta, P. Costa, L.G. Vieira, Multi-layer solar selective absorber coatings based on W/WSiAlNx/WSiAlOyNx/SiAlOx for high temperature applications, Sol. Energy Mater. Sol. Cells 186 (2018) 300–308. [25] Y. Ning, W. Wang, L. Wang, Y. Sun, P. Song, H. Man, Y. Zhang, B. Dai, J. Zhang, C. Wang, Y. Zhang, S. Zhao, E. Tomasella, A. Bousquet, J. Cellier, Optical simulation and preparation of novel Mo/ZrSiN/ZrSiON/SiO2 solar selective absorbing coating, Sol. Energy Mater. Sol. Cells 167 (2017) 178–183. [26] Y. Wu, C. Wang, Y. Sun, Y. Xue, Y. Ning, W. Wang, S. Zhao, E. Tomasella, A. Bousquet, Optical simulation and experimental optimization of Al/NbMoN/
5. Conclusion In summary, we have created a new field of research that fabricates a novel Al/MoNbHfZrTiN/MoNbHfZrTiON/SiO2 coating through DC combine with RF magnetron sputtering. The deposition parameters are controlled to achieve a high absorptance of 0.935 and a low emittance of 0.09. The Al layer acts as an infrared reflectance layer, which contrib utes to a low emittance and a high absorptance for the coating. By reducing the flow of Ar or increasing the target power, the emittance of Al layer is decreased. On the other hand, the absorptance enhances with the addition of Ar flow rate or the reduction of target power. The main absorption layer (MoNbHfZrTiN) and interference absorption layer (MoNbHfZrTiON) make the absorptance of the coating intensely in crease from 0.525 to 0.870. The SiO2 works as an AR layer, which de creases the reflectance in the solar spectrum. Compared with other recent works, we prepared all kinds of colored coatings possessing excellent spectral selectivity, and investigated color coordinates of those coatings in the ICE chromaticity diagram. However, the thermal stability of the coating is just about 450 � C in vacuum, which does not reach as our expected. Further investigation will be focused on the failure mechanism of this coating and provide a theoretical support for design the high temperature solar absorber coatings based on high entropy alloy. Author contribution statement For our manuscript entitled " A novel multilayer high temperature colored solar absorber coating based on high-entropy alloy MoN bHfZrTi: Optimized preparation and chromaticity investigation ", HuiXia Guo and Xiang-Hu Gao have designed the multilayer structure prepared by high entropy alloy, discussed the experimental results and write this manuscript. Cheng-Yu He, Xiao-Li Qiu and Yong-Qian Shen have done the experimental, discussed the experimental results. Gang Liu has discussed the experimental results. So, all the authors have made contribution to this manuscript. No conflict of interest exists in the submission of this manuscript, and manuscript is approved by all au thors for publication. All the authors listed have approved the manu script that is enclosed. Declaration of competing interest No conflict of interest exists in the submission of this manuscript. Acknowledgments This work was financially supported by the Youth Innovation Pro motion Association CAS (2018455), the National Natural Science Foundation of China (No. 51402315), the Key Research & Development Program in Gansu (18YF1GA125), Major Subject of State Grid Corpo ration of China (No. 5216A01600W0) and Scientific Research Project of Colleges and Universities in Gansu Province (No. 2018D-03). We acknowledge critical and quantity of testing work supported by Beijing Zhongkebaice Technology Service Co., Ltd. 8
H.-X. Guo et al.
Solar Energy Materials and Solar Cells 209 (2020) 110444
NbMoON/SiO2 solar selective absorbing coatings, Sol. Energy Mater. Sol. Cells 134 (2015) 373–380. [27] J. Liu, Z.Q. Sun, H. Wang, Design and characterization of solar absorbing multilayer stack based on Al/Cr-N-O/SiO2 layers, Sol. Energy Mater. Sol. Cells 188 (2018) 18–26. [28] L. Rebouta, P. Capela, M. Andritschky, A. Matilainen, P. Santilli, K. Pischow, E. Alves, Characterization of TiAlSiN/TiAlSiON/SiO2 optical stack designed by modelling calculations for solar selective applications, Sol. Energy Mater. Sol. Cells 105 (2012) 202–207. [29] X. Wang, X. Zhang, Q. Li, J. Min, X. Cheng, Spectral properties of AlCrNO-based multi-layer solar selective absorbing coating during the initial stage of thermal aging upon exposure to air, Sol. Energy Mater. Sol. Cells 188 (2018) 81–92.
[30] W.J. Sheng, X. Yang, J. Zhu, C. Wang, Y. Zhang, Amorphous phase stability of NbTiAlSiNX high-entropy films, Rare Met. 37 (2018) 682–689. [31] A. Schüler, C. Roecker, J.L. Scartezzini, J. Boudaden, I.R. Videnovic, R.S.C. Ho, P. Oelhafen, On the feasibility of colored glazed thermal solar collectors based on thin film interference filters, Sol. Energy Mater. Sol. Cells 84 (2004) 241–254. [32] Software spectra: TFCalc 3.5, available at, http://www.sspectra.com/. [33] C. Wang, J. Shi, Z. Geng, X. Ling, Polychromic Al-AlN cermet solar absorber coating with high absorption efficiency and excellent durability, Sol. Energy Mater. Sol. Cells 144 (2016) 14–22. [34] D. Zhu, S. Zhao, Chromaticity and optical properties of colored and black solarthermal absorbing coatings, Sol. Energy Mater. Sol. Cells 94 (2010) 1630–1635.
9