- Email: [email protected]
Al2O3 multilayer high temperature solar selective absorbing coating: Microstructure, optical properties and failure mechanism
Solar Energy Materials and Solar Cells 203 (2019) 110187
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A novel TiC–ZrB2/ZrB2/Al2O3 multilayer high temperature solar selective absorbing coating: Microstructure, optical properties and failure mechanism
T
Xiang-Hu Gaoa,b,∗∗, Xiao-Li Qiua,b, Yong-Qian Shenc, Cheng-Yu Hea,b, Gang Liua,b,∗ a
Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Multilayer Solar absorber coating Optical properties Failure mechanism
A novel multilayer nanostructured TiC–ZrB2/ZrB2/Al2O3 solar absorber coating has been developed for Concentrated Solar Power. The TiC–ZrB2 and ZrB2 work as the absorptance layers. The SS/TiC–ZrB2/ZrB2/Al2O3 solar absorber coatings exhibit a high spectral selectivity of 0.92/0.10. The sample is annealed in vacuum to test the thermal stability. After keeping the as-deposited samples and the samples annealed in vacuum at room temperature for two years, the coatings show different colors. Through the SEM, XPS and Raman analysis, we have discussed the corresponding failure mechanism. For the purpose of clarifying the failure mechanism more clearly, corresponding failure model is established.
1. Introduction Looking ahead to the new century, human beings need to find a balance between increasing energy demand and environmental protection. Solar energy, one of the promising renewable energy, has many applications in industrial heating, desalination, and electricity generation [1–4]. In order to effectively realize the photo-thermal conversion, a spectrally selective surface is required [5–7]. Concentrated Solar Power (CSP) is a new kind of power supply, which has gained much attention from both academia and industry. According to the Carnot efficiency(ηth-e = 1-T0/TA, where T0 is the ambient temperature and TA is the working temperature of the solar absorber), the solar absorber coatings should be stable in vacuum at high temperature with a longer time [8,9]. Transition metals borides and carbides (TiB2, ZrB2, TiC, HfC et al.) are usually called Ultra-High Temperature Ceramics (UHTCs), which have many applications in extreme environments of aerospace and nuclear industry [10,11]. Recently, the potential spectral selectivity of UHTCs and its application in concentrating solar power have been extensively explored [12]. On the other hand, our recent research has made an effort to develop spectrally selective solar absorbers based on
carbides of early transition metals, such as SS/TiC/Al2O3 [13,14], SS/ TiC-WC/Al2O3 [15], SS/TiC–ZrC/Al2O3 [16], which exhibit a good spectral selectivity and a good thermal stability. However, the multilayer solar absorber coatings deposited by using transition metals diborides and carbides are still an interesting research topic. Herein, a TiC–ZrB2/ZrB2/Al2O3 multilayer solar absorber coating is deposited on the stainless steel (SS). The optimized SS/TiC–ZrB2/ZrB2/ Al2O3 solar absorber coatings exhibit a high spectral selectivity of 0.92/ 0.10. It must be emphasized that after keeping the as-deposited samples and the samples annealed in vacuum at room temperature for two years, the color and optical properties of the coatings are investigated. The detailed failure mechanism model is also provided for this attractive phenomenon. 2. Experimental The SS/TiC–ZrB2/ZrB2/Al2O3 coating is deposited by magnetron sputtering (Lab 18, USA), which consists of three layers: TiC–ZrB2 (absorption layer), ZrB2 (absorption layer) and Al2O3 (anti-reflective layer). TiC (99.99%), ZrB2 (99.99%) and Al2O3 (99.99%) targets are used to deposite the coatings. The vacuum chamber is pumped down to
∗ Corresponding author. Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. ∗∗ Corresponding author. Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail addresses: [email protected] (X.-H. Gao), [email protected] (G. Liu).
https://doi.org/10.1016/j.solmat.2019.110187 Received 31 May 2019; Received in revised form 15 September 2019; Accepted 16 September 2019 Available online 30 September 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
a base pressure of 2.1 × 10−6 Torr and the sputtering pressure is 7.8 × 10−3 Torr. The TiC–ZrB2 layer is obtained by co-sputtering technique. And then the ZrB2 and Al2O3 are deposited on the SS/ TiC–ZrB2 coating successively. During the deposition process, the TiC target is used by DC sputtering, the ZrB2 and Al2O3 targets are used by RF sputtering. The power densities of the TiC, ZrB2 and Al2O3 targets are 6.58, 4.38 and 5.48 W/cm2, respectively, and the Ar flow rate is 33 sccm. The SS/TiC–ZrB2/ZrB2/Al2O3 coatings are annealed in vacuum at 500 oC and 600 oC for 100 h, respectively. UV/Vis/NIR spectrometer (Perkin Elmer Lambda 950) with an integration sphere (module 150 mm) and FT-IR Spectrometer (Bruker TENSOR 27) are used to measure the reflectance spectra in the range of 0.3–25 μm. The thermal emittance value is obtained at 82 oC. The optical properties of the coatings are obtained using the reflectance spectra. The microstructure of the coating is investigated by Scanning electron microscope (SEM, JEM-100CX). X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical structure of the coating (ESCA3000). Raman analysis is performed with a confocal-micro-Raman spectroscopy (DILOR-JOBINYVON-SPEX).
Fig. 2. The reflectance spectra of (a) SS, (b) SS/TiC–ZrB2, (c) SS/TiC–ZrB2/ZrB2 and (d) SS/TiC–ZrB2/ZrB2/Al2O3.
exhibits a columnar microstructure, which is the typical feature of sputtering coating.
3. Results and discussions 3.1. The morphology analysis
3.2. Optical properties
Fig. 1(a) shows the schematic illustration for the SS/TiC–ZrB2/ZrB2/ Al2O3 solar absorber coating. The SS/TiC–ZrB2/ZrB2/Al2O3 coating contains three layers from the bottom to top: TiC–ZrB2 (absorption layer), ZrB2 (absorption layer) and Al2O3 (anti-reflective layer). Fig. 1(b) shows the photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating, which exhibits a blue color. The surface of the coating (Fig. 1(c)) is homogeneous, fine-grained and dense, which indicates a high deposition quality. Fig. 1(d) shows the cross-section morphology of the asdeposited coating. Clearly, there are clear interfaces among the absorption layers and anti-reflective layer, which demonstrate the multilayer structure. The thickness of TiC–ZrB2, ZrB2 and Al2O3 layers are 76.4, 15.9 and 30.8 nm, respectively. The cross-section of the coating
To better understand the optical properties and confirm the contribution and role of each layer to the SS/TiC–ZrB2/ZrB2/Al2O3 multilayer solar absorber coating, the corresponding coatings of SS/ TiC–ZrB2, SS/TiC–ZrB2/ZrB2, SS/TiC–ZrB2/ZrB2/Al2O3 are deposited, respectively. The reflective spectra of each corresponding samples are obtained by using UV/Vis/NIR spectrometer and FT-IR spectrometer as shown in Fig. 2, and the corresponding value of α/ε is 0.36/0.11, 0.76/ 0.11, 0.79/0.10, 0.92/0.10, respectively. The reflectance spectra of stainless steel substrate which observed from Fig. 2(a) shows a lowest point at 65% (reflectance value), which indicates a low absorptance (0.36) and an emittance (0.11). After the deposition of TiC–ZrB2, the reflectance spectra form two reflectance minimum at 400 and 1100 nm
Fig. 1. (a) Schematic diagram of the SS/TiC–ZrB2/ZrB2/Al2O3 coating (b) The photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating (c) The morphology of surface and (d) cross-sectional of the Si/TiC–ZrB2/ZrB2/Al2O3 solar absorber. 2
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
photograph of the Sample A and Sample B. It is worth to note that the color of Sample A has changed from blue to yellow in the edge of the samples. In the center of the Sample A, the color of the coating still is blue. The coatings deposited on stainless steel partially delaminate with the exposed substrate. On the whole, a desertification like process gradually happens from the edge to the center. On the contrary, the Sample B still maintains an initial blue color. The Sample A exhibits a low absorptance and a high emittance, indicating the degradation of the coating. The Sample B shows an absorptance of 0.88 and a low emittance of 0.20, which is the same as just annealed in vacuum. This result indicates a good stability in air. 3.5. Discussion of failure mechanism With an aim to clarify why the samples exhibit different color after keeping in air for two years, the following are considered. The XPS spectra of the single TiC–ZrB2 layer are shown in Fig. 5. Two peaks centered at 182.4 and 184.7 eV are originated from ZrO2 (Fig. 5(a)) [19,20], and two peaks centered at 458.5 and 464.3 eV are attributed to TiO2 (Fig. 5(b)) [21,22]. Considering the XPS analysis, we can confirm that the as-deposited samples are all oxidized into TiO2 and ZrO2, which results in the different color and the degradation of optical properties. During the past few years, our research group have fabricated a series of transition metal carbides based high temperature solar absorber coatings such as: SS/TiC/Al2O3 [13,14], SS/TiC-WC/Al2O3 [15] and SS/TiC–ZrC/Al2O3 [16]. The samples are all annealed in vacuum and corresponding degradation mechanism are discussed. The surface image of the coating annealed at 600 oC is displayed in Fig. 6. Clearly, a more dense surface structure is formed and the grain boundary disappears, which is a typical graphite process. Raman spectra of the coating which is annealed at 600 oC is shown in Fig. 7. Two peaks centered at 1358 cm−1 and 1581 cm−1 are the typical features of amorphous carbon. According to the previous reports, the peaks at 1358 cm−1 and 1581 cm−1 are D peak and G peak, which respectively represent the bond stretching of sp3 C atoms and the breathing modes of sp2 C atoms [23,24]. Generally speaking, the relative ratio of the D peak's intensity to G peak's intensity (ID/IG) can be described as the sp2/sp3 ratio. Taking into account of the SEM and Raman analysis, it could be confirmed that when the annealed temperature is increased, the sp2 bond content increases. In other words, high graphitization degree which comes from the increased sp2 bond content results in the degradation of the optical properties. This degradation mechanism of optical properties is consistent with our previous work. To intuitively understand the color change of Sample A and Sample B, we put forward the following failure mechanism based upon the aforementioned analysis, as depicted in Scheme 1. During the vacuum annealing process, the C element gradually diffuses from bottom to the surface of the coating through the void of grain and form a more dense C layer, which acts as an isolation belt to prevent the diffusion of oxygen in the air. The Sample B still keeps the blue color through kept in air for almost two years. Considering the optical properties and color, we can confirm that the formed C layer plays two roles: on one hand, the formed C layer can lead to the degradation of optical properties, on the other hand, this C layer can prevent the diffusion of oxygen, which is beneficial to the blue color. On the contrary, the oxygen can immerse
Fig. 3. The reflectance spectra of the coating annealed in vacuum at 500 oC and 600 oC, respectively.
as shown in Fig. 2(b), respectively. The value of the lowest reflectance (1100 nm) almost decreases to 0, which leads to a high spectral selectivity of 0.76/0.11. With a deposition of ZrB2, the value of reflectance of the first reflectance minimum in Fig. 2(c) almost decreases to 0, which further increases the solar absorptance (0.79) of the coating. After the addition of Al2O3, three reflectance minimum appear at 373, 652 and 1730 nm, which indicate a high spectral selectivity of 0.92/ 0.10 as shown in Fig. 2(d). Compared to the reflectance minimum at 1288 nm in Fig. 2(c), the location of reflectance minimum shifts to a higher wavelength (1730 nm), which makes contribution to the high solar absorptance (0.92). From above analysis, we can confirm that TiC–ZrB2 and ZrB2 layers are absorptance layers while Al2O3 is an antireflective layer. The high spectral selectivity of SS/TiC–ZrB2/ZrB2/ Al2O3 coatings is attributed to the intrinsic absorptance of TiC–ZrB2, ZrB2 layers and interfacial interference absorption. 3.3. Thermal stability test To improve the Carnot efficiency of the CSP system, it is very important that the solar absorber coatings can suffer higher temperatures [17,18]. Herein, the as-deposited samples are annealed at 500 oC and 600 oC in vacuum for 100 h, respectively. Fig. 3 shows the corresponding reflectance spectra of the annealed samples. Table 1 lists the value of optical data. After annealing at 500 oC for 100 h, the samples exhibit a spectral selectivity of 0.91/0.11, indicating a good thermal stability. When the annealed temperature increases to 600 oC, the absorptance decreases to 0.88 and the emittance increases to 0.20, which demonstrates the degradation of the coating. 3.4. Stability in air for two years at room temperature After the deposition, we have purposelessly kept the as-deposited samples (denoted as Sample A) and the samples annealed in vacuum at 600 oC for 100 h (denoted as Sample B) in air to do the room temperature aging experiment for almost two years, respectively. Surprisingly, an interesting phenomenon occurs. Fig. 4 shows the
Table 1 The optical data of the SS/TiC–ZrB2/ZrB2/Al2O3 coatings annealed in vacuum for 100 h. Temperature o C
As-deposited 500 600
α
ε
As deposited
Annealed
Δα
As deposited
Annealed
Δε
0.92 0.92 0.92
0.92 0.91 0.88
0 −0.01 −0.04
0.11 0.11 0.11
0.11 0.12 0.20
0 0.01 0.09
3
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
Fig. 4. The photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating: (a) Sample A, (b) Sample B.
Fig. 5. XPS spectra of the Sample A: (a) Zr 3d, (b) Ti 2p.
Fig. 7. Raman spectra of the Sample B. Fig. 6. Surface morphology of the Sample B.
magnetron sputtering method. SEM image clearly demonstrates the multilayer structure, in which TiC–ZrB2 and ZrB2 are the absorptance layers while Al2O3 is the antireflective layer. This multilayer coating exhibits a high spectral selectivity of 0.92/0.10. The samples annealed in vacuum at 100 h exhibit a good thermal stability. During the annealing process in vacuum, the C element gradually diffuses from the bottom to the upper surface and forms an isolation belt between air and the surface of coatings, which leads to a blue color through the annealed sample is kept in air for two years. The as-deposited samples exhibit a different color and partially delaminate with the exposed
into the coating through the void of grain, which eventually results in the oxidation of coating and forms the TiO2 and ZrO2. With the gradually oxidation process, the Sample A exhibits a different color and partially exfoliation. 4. Conclusions In summary, we have successfully prepared a SS/TiC–ZrB2/ZrB2/ Al2O3 multilayer high temperature solar absorber coating using a 4
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
Scheme 1. The structure evolution process of Sample A and Sample B.
substrate after keeping in air for two years, which is due to the oxidation of the coating. This work provides a new insight about the long term stability of the coating in air and establishes a failure mechanism model.
[10] E. Sani, L. Mercatelli, D. Fontani, J.-L. Sans, D. Sciti, Hafnium and tantalum carbides for high temperature solar receivers, J. Renew. Sustain. Energy 3 (-1–6) (2011) 063107. [11] E. Sani, L. Mercatelli, P. Sansoni, L. Silvestroni, D. Sciti, Spectrally selective ultrahigh temperature ceramic absorbers for high-temperature solar plants, J. Renew. Sustain. Energy 4 (-1–13) (2012) 033104. [12] M. Coulibaly, G. Arrachart, A. Mesbah, X. Deschanels, From colloidal precursors to metal carbides nanocomposites MC (M=Ti, Zr, Hf and Si): synthesis, characterization and optical spectral selectivity studies, Sol. Energy Mater. Sol. Cells 143 (2015) 473–479. [13] 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. [14] X.-H. Gao, W. Theiss, Y.-Q. Shen, P.-J. Ma, Gang Liu, Optical simulation, corrosion behavior and long term thermal stability of TiC-based spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 167 (2017) 150–156. [15] X.-H. Gao, Z.-M. Guo, Q.-F. Geng, P.-J. Ma, A.-Q. Wang, G. Liu, Microstructure, chromaticity and thermal stability of SS/TiC-WC/Al2O3 spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 164 (2017) 63–69. [16] X.-H. Gao, Z.-M. Guo, Q.-F. Geng, P.-J. Ma, A.-Q. Wang, G. Liu, Structure, optical properties and thermal stability of SS/TiC-ZrC/Al2O3 spectrally selective solar absorber, RSC Adv. 6 (2016) 63867–63873. [17] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294–303. [18] Y. Li, C. Lin, D. Zhou, Y. An, D. Li, C. Chi, H. Huang, S. Yang, C. YanTso, C.Y.H. Chao, B. Huang, Scalable all-ceramic nanofilms as highly efficient and thermally stable selective solar absorbers, Nano Energy 64 (2019), https://doi.org/ 10.1016/j.nanoen.2019.103947. [19] Y. Xu, Z. Huang, Y. Liu, M. Fang, L. Yin, M. Guan, Influence of yttria addition on the phase transformations of zirconia from zircon ore by carbothermal reduction process, Solid State Sci. 14 (2012) 730–734. [20] X. Xiong, Y. Wang, G. Li, Z. Chen, W. Sun, Z. Wang, HfC/ZrC ablation protective coating for carbon/carbon composites, Corros. Sci. 77 (2013) 25–30. [21] L. Zhang, R.V. Koka, A study on the oxidation and carbon diffusion of TiC in alumina-titanium carbide ceramics using XPS and Raman spectroscopy, Mater. Chem. Phys. 57 (1998) 23–32. [22] O. Wilhelmssona, J.-P. Palmquist, E. Lewina, J. Emmerlich, P. Eklund, P.O.A. Persson, H. Hogberg, S. Li, R. Ahuja, O. Eriksson, L. Hultman, U. Jansson, Deposition and characterization of ternary thin films within the Ti-Al-C system by DC magnetron sputtering, J. Cryst. Growth 291 (2006) 290–300. [23] W. Dai, A. Wang, Deposition and properties of Al-containing diamond-like carbon films by a hybrid ion beam sources, J. Alloy. Comp. 509 (2001) 4626–4631. [24] X. Yuan, L. Cheng, L. Zhang, Controlled fabrication of TiC nanocrystal clusters on surface of Ti particles for application in electromagnetic wave absorption, J. Alloy. Comp. 622 (2015) 282–287.
Acknowledgments This work was financially supported by the Youth Innovation Promotion Association CAS (2018455), the Key Research & Development Program in Gansu (18YF1GA125) and the National Natural Science Foundation of China (No. 51402315). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110187. References [1] P. Tao, G. Ni, C. Song, W. Shang, J. Wu, J. Zhu, G. Chen, T. Deng, Solar-driven interfacial evaporation, Nature Energy 3 (2018) 1031–1041. [2] L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu, J. Zhu, 3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination, Nat. Photonics 10 (2016) 393–398. [3] S. Kalogirou, The potential of solar industrial process heat applications, Appl. Energy 76 (2003) 337–361. [4] L.A. Weinstein, J. Loomis, B. Bhatia, D.M. Bierman, E.N. Wang, G. Chen, Concentrating solar power, Chem. Rev. 115 (2015) 12797–12838. [5] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. 7 (2014) 1615–1627. [6] N. Selvakumar, H.C. Barshilia, Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications, Sol. Energy Mater. Sol. Cells 98 (2012) 1–23. [7] C.E. Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, 2002. [8] J. Moon, D. Lu, B. VanSaders, T.K. Kim, S.D. Kong, S. Jin, R. Chen, Z. Liu, High performance multi-scaled nanostructured spectrally selective coating concentrating solar power, Nano Energy 8 (2014) 238–246. [9] P. Li, B. Liu, Y. Ni, K.K. Liew, J. Sze, S. Chen, S. Shen, Large-scale nanophotonic solar selective absorbers for high-efficiency solar thermal energy conversion, Adv. Mater. 31 (2015) 4585–4591.
5
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A novel TiC–ZrB2/ZrB2/Al2O3 multilayer high temperature solar selective absorbing coating: Microstructure, optical properties and failure mechanism
T
Xiang-Hu Gaoa,b,∗∗, Xiao-Li Qiua,b, Yong-Qian Shenc, Cheng-Yu Hea,b, Gang Liua,b,∗ a
Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Multilayer Solar absorber coating Optical properties Failure mechanism
A novel multilayer nanostructured TiC–ZrB2/ZrB2/Al2O3 solar absorber coating has been developed for Concentrated Solar Power. The TiC–ZrB2 and ZrB2 work as the absorptance layers. The SS/TiC–ZrB2/ZrB2/Al2O3 solar absorber coatings exhibit a high spectral selectivity of 0.92/0.10. The sample is annealed in vacuum to test the thermal stability. After keeping the as-deposited samples and the samples annealed in vacuum at room temperature for two years, the coatings show different colors. Through the SEM, XPS and Raman analysis, we have discussed the corresponding failure mechanism. For the purpose of clarifying the failure mechanism more clearly, corresponding failure model is established.
1. Introduction Looking ahead to the new century, human beings need to find a balance between increasing energy demand and environmental protection. Solar energy, one of the promising renewable energy, has many applications in industrial heating, desalination, and electricity generation [1–4]. In order to effectively realize the photo-thermal conversion, a spectrally selective surface is required [5–7]. Concentrated Solar Power (CSP) is a new kind of power supply, which has gained much attention from both academia and industry. According to the Carnot efficiency(ηth-e = 1-T0/TA, where T0 is the ambient temperature and TA is the working temperature of the solar absorber), the solar absorber coatings should be stable in vacuum at high temperature with a longer time [8,9]. Transition metals borides and carbides (TiB2, ZrB2, TiC, HfC et al.) are usually called Ultra-High Temperature Ceramics (UHTCs), which have many applications in extreme environments of aerospace and nuclear industry [10,11]. Recently, the potential spectral selectivity of UHTCs and its application in concentrating solar power have been extensively explored [12]. On the other hand, our recent research has made an effort to develop spectrally selective solar absorbers based on
carbides of early transition metals, such as SS/TiC/Al2O3 [13,14], SS/ TiC-WC/Al2O3 [15], SS/TiC–ZrC/Al2O3 [16], which exhibit a good spectral selectivity and a good thermal stability. However, the multilayer solar absorber coatings deposited by using transition metals diborides and carbides are still an interesting research topic. Herein, a TiC–ZrB2/ZrB2/Al2O3 multilayer solar absorber coating is deposited on the stainless steel (SS). The optimized SS/TiC–ZrB2/ZrB2/ Al2O3 solar absorber coatings exhibit a high spectral selectivity of 0.92/ 0.10. It must be emphasized that after keeping the as-deposited samples and the samples annealed in vacuum at room temperature for two years, the color and optical properties of the coatings are investigated. The detailed failure mechanism model is also provided for this attractive phenomenon. 2. Experimental The SS/TiC–ZrB2/ZrB2/Al2O3 coating is deposited by magnetron sputtering (Lab 18, USA), which consists of three layers: TiC–ZrB2 (absorption layer), ZrB2 (absorption layer) and Al2O3 (anti-reflective layer). TiC (99.99%), ZrB2 (99.99%) and Al2O3 (99.99%) targets are used to deposite the coatings. The vacuum chamber is pumped down to
∗ Corresponding author. Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. ∗∗ Corresponding author. Research and Development Center for Eco-Chemistry and Eco-Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail addresses: [email protected] (X.-H. Gao), [email protected] (G. Liu).
https://doi.org/10.1016/j.solmat.2019.110187 Received 31 May 2019; Received in revised form 15 September 2019; Accepted 16 September 2019 Available online 30 September 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
a base pressure of 2.1 × 10−6 Torr and the sputtering pressure is 7.8 × 10−3 Torr. The TiC–ZrB2 layer is obtained by co-sputtering technique. And then the ZrB2 and Al2O3 are deposited on the SS/ TiC–ZrB2 coating successively. During the deposition process, the TiC target is used by DC sputtering, the ZrB2 and Al2O3 targets are used by RF sputtering. The power densities of the TiC, ZrB2 and Al2O3 targets are 6.58, 4.38 and 5.48 W/cm2, respectively, and the Ar flow rate is 33 sccm. The SS/TiC–ZrB2/ZrB2/Al2O3 coatings are annealed in vacuum at 500 oC and 600 oC for 100 h, respectively. UV/Vis/NIR spectrometer (Perkin Elmer Lambda 950) with an integration sphere (module 150 mm) and FT-IR Spectrometer (Bruker TENSOR 27) are used to measure the reflectance spectra in the range of 0.3–25 μm. The thermal emittance value is obtained at 82 oC. The optical properties of the coatings are obtained using the reflectance spectra. The microstructure of the coating is investigated by Scanning electron microscope (SEM, JEM-100CX). X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical structure of the coating (ESCA3000). Raman analysis is performed with a confocal-micro-Raman spectroscopy (DILOR-JOBINYVON-SPEX).
Fig. 2. The reflectance spectra of (a) SS, (b) SS/TiC–ZrB2, (c) SS/TiC–ZrB2/ZrB2 and (d) SS/TiC–ZrB2/ZrB2/Al2O3.
exhibits a columnar microstructure, which is the typical feature of sputtering coating.
3. Results and discussions 3.1. The morphology analysis
3.2. Optical properties
Fig. 1(a) shows the schematic illustration for the SS/TiC–ZrB2/ZrB2/ Al2O3 solar absorber coating. The SS/TiC–ZrB2/ZrB2/Al2O3 coating contains three layers from the bottom to top: TiC–ZrB2 (absorption layer), ZrB2 (absorption layer) and Al2O3 (anti-reflective layer). Fig. 1(b) shows the photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating, which exhibits a blue color. The surface of the coating (Fig. 1(c)) is homogeneous, fine-grained and dense, which indicates a high deposition quality. Fig. 1(d) shows the cross-section morphology of the asdeposited coating. Clearly, there are clear interfaces among the absorption layers and anti-reflective layer, which demonstrate the multilayer structure. The thickness of TiC–ZrB2, ZrB2 and Al2O3 layers are 76.4, 15.9 and 30.8 nm, respectively. The cross-section of the coating
To better understand the optical properties and confirm the contribution and role of each layer to the SS/TiC–ZrB2/ZrB2/Al2O3 multilayer solar absorber coating, the corresponding coatings of SS/ TiC–ZrB2, SS/TiC–ZrB2/ZrB2, SS/TiC–ZrB2/ZrB2/Al2O3 are deposited, respectively. The reflective spectra of each corresponding samples are obtained by using UV/Vis/NIR spectrometer and FT-IR spectrometer as shown in Fig. 2, and the corresponding value of α/ε is 0.36/0.11, 0.76/ 0.11, 0.79/0.10, 0.92/0.10, respectively. The reflectance spectra of stainless steel substrate which observed from Fig. 2(a) shows a lowest point at 65% (reflectance value), which indicates a low absorptance (0.36) and an emittance (0.11). After the deposition of TiC–ZrB2, the reflectance spectra form two reflectance minimum at 400 and 1100 nm
Fig. 1. (a) Schematic diagram of the SS/TiC–ZrB2/ZrB2/Al2O3 coating (b) The photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating (c) The morphology of surface and (d) cross-sectional of the Si/TiC–ZrB2/ZrB2/Al2O3 solar absorber. 2
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
photograph of the Sample A and Sample B. It is worth to note that the color of Sample A has changed from blue to yellow in the edge of the samples. In the center of the Sample A, the color of the coating still is blue. The coatings deposited on stainless steel partially delaminate with the exposed substrate. On the whole, a desertification like process gradually happens from the edge to the center. On the contrary, the Sample B still maintains an initial blue color. The Sample A exhibits a low absorptance and a high emittance, indicating the degradation of the coating. The Sample B shows an absorptance of 0.88 and a low emittance of 0.20, which is the same as just annealed in vacuum. This result indicates a good stability in air. 3.5. Discussion of failure mechanism With an aim to clarify why the samples exhibit different color after keeping in air for two years, the following are considered. The XPS spectra of the single TiC–ZrB2 layer are shown in Fig. 5. Two peaks centered at 182.4 and 184.7 eV are originated from ZrO2 (Fig. 5(a)) [19,20], and two peaks centered at 458.5 and 464.3 eV are attributed to TiO2 (Fig. 5(b)) [21,22]. Considering the XPS analysis, we can confirm that the as-deposited samples are all oxidized into TiO2 and ZrO2, which results in the different color and the degradation of optical properties. During the past few years, our research group have fabricated a series of transition metal carbides based high temperature solar absorber coatings such as: SS/TiC/Al2O3 [13,14], SS/TiC-WC/Al2O3 [15] and SS/TiC–ZrC/Al2O3 [16]. The samples are all annealed in vacuum and corresponding degradation mechanism are discussed. The surface image of the coating annealed at 600 oC is displayed in Fig. 6. Clearly, a more dense surface structure is formed and the grain boundary disappears, which is a typical graphite process. Raman spectra of the coating which is annealed at 600 oC is shown in Fig. 7. Two peaks centered at 1358 cm−1 and 1581 cm−1 are the typical features of amorphous carbon. According to the previous reports, the peaks at 1358 cm−1 and 1581 cm−1 are D peak and G peak, which respectively represent the bond stretching of sp3 C atoms and the breathing modes of sp2 C atoms [23,24]. Generally speaking, the relative ratio of the D peak's intensity to G peak's intensity (ID/IG) can be described as the sp2/sp3 ratio. Taking into account of the SEM and Raman analysis, it could be confirmed that when the annealed temperature is increased, the sp2 bond content increases. In other words, high graphitization degree which comes from the increased sp2 bond content results in the degradation of the optical properties. This degradation mechanism of optical properties is consistent with our previous work. To intuitively understand the color change of Sample A and Sample B, we put forward the following failure mechanism based upon the aforementioned analysis, as depicted in Scheme 1. During the vacuum annealing process, the C element gradually diffuses from bottom to the surface of the coating through the void of grain and form a more dense C layer, which acts as an isolation belt to prevent the diffusion of oxygen in the air. The Sample B still keeps the blue color through kept in air for almost two years. Considering the optical properties and color, we can confirm that the formed C layer plays two roles: on one hand, the formed C layer can lead to the degradation of optical properties, on the other hand, this C layer can prevent the diffusion of oxygen, which is beneficial to the blue color. On the contrary, the oxygen can immerse
Fig. 3. The reflectance spectra of the coating annealed in vacuum at 500 oC and 600 oC, respectively.
as shown in Fig. 2(b), respectively. The value of the lowest reflectance (1100 nm) almost decreases to 0, which leads to a high spectral selectivity of 0.76/0.11. With a deposition of ZrB2, the value of reflectance of the first reflectance minimum in Fig. 2(c) almost decreases to 0, which further increases the solar absorptance (0.79) of the coating. After the addition of Al2O3, three reflectance minimum appear at 373, 652 and 1730 nm, which indicate a high spectral selectivity of 0.92/ 0.10 as shown in Fig. 2(d). Compared to the reflectance minimum at 1288 nm in Fig. 2(c), the location of reflectance minimum shifts to a higher wavelength (1730 nm), which makes contribution to the high solar absorptance (0.92). From above analysis, we can confirm that TiC–ZrB2 and ZrB2 layers are absorptance layers while Al2O3 is an antireflective layer. The high spectral selectivity of SS/TiC–ZrB2/ZrB2/ Al2O3 coatings is attributed to the intrinsic absorptance of TiC–ZrB2, ZrB2 layers and interfacial interference absorption. 3.3. Thermal stability test To improve the Carnot efficiency of the CSP system, it is very important that the solar absorber coatings can suffer higher temperatures [17,18]. Herein, the as-deposited samples are annealed at 500 oC and 600 oC in vacuum for 100 h, respectively. Fig. 3 shows the corresponding reflectance spectra of the annealed samples. Table 1 lists the value of optical data. After annealing at 500 oC for 100 h, the samples exhibit a spectral selectivity of 0.91/0.11, indicating a good thermal stability. When the annealed temperature increases to 600 oC, the absorptance decreases to 0.88 and the emittance increases to 0.20, which demonstrates the degradation of the coating. 3.4. Stability in air for two years at room temperature After the deposition, we have purposelessly kept the as-deposited samples (denoted as Sample A) and the samples annealed in vacuum at 600 oC for 100 h (denoted as Sample B) in air to do the room temperature aging experiment for almost two years, respectively. Surprisingly, an interesting phenomenon occurs. Fig. 4 shows the
Table 1 The optical data of the SS/TiC–ZrB2/ZrB2/Al2O3 coatings annealed in vacuum for 100 h. Temperature o C
As-deposited 500 600
α
ε
As deposited
Annealed
Δα
As deposited
Annealed
Δε
0.92 0.92 0.92
0.92 0.91 0.88
0 −0.01 −0.04
0.11 0.11 0.11
0.11 0.12 0.20
0 0.01 0.09
3
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
Fig. 4. The photograph of the SS/TiC–ZrB2/ZrB2/Al2O3 coating: (a) Sample A, (b) Sample B.
Fig. 5. XPS spectra of the Sample A: (a) Zr 3d, (b) Ti 2p.
Fig. 7. Raman spectra of the Sample B. Fig. 6. Surface morphology of the Sample B.
magnetron sputtering method. SEM image clearly demonstrates the multilayer structure, in which TiC–ZrB2 and ZrB2 are the absorptance layers while Al2O3 is the antireflective layer. This multilayer coating exhibits a high spectral selectivity of 0.92/0.10. The samples annealed in vacuum at 100 h exhibit a good thermal stability. During the annealing process in vacuum, the C element gradually diffuses from the bottom to the upper surface and forms an isolation belt between air and the surface of coatings, which leads to a blue color through the annealed sample is kept in air for two years. The as-deposited samples exhibit a different color and partially delaminate with the exposed
into the coating through the void of grain, which eventually results in the oxidation of coating and forms the TiO2 and ZrO2. With the gradually oxidation process, the Sample A exhibits a different color and partially exfoliation. 4. Conclusions In summary, we have successfully prepared a SS/TiC–ZrB2/ZrB2/ Al2O3 multilayer high temperature solar absorber coating using a 4
Solar Energy Materials and Solar Cells 203 (2019) 110187
X.-H. Gao, et al.
Scheme 1. The structure evolution process of Sample A and Sample B.
substrate after keeping in air for two years, which is due to the oxidation of the coating. This work provides a new insight about the long term stability of the coating in air and establishes a failure mechanism model.
[10] E. Sani, L. Mercatelli, D. Fontani, J.-L. Sans, D. Sciti, Hafnium and tantalum carbides for high temperature solar receivers, J. Renew. Sustain. Energy 3 (-1–6) (2011) 063107. [11] E. Sani, L. Mercatelli, P. Sansoni, L. Silvestroni, D. Sciti, Spectrally selective ultrahigh temperature ceramic absorbers for high-temperature solar plants, J. Renew. Sustain. Energy 4 (-1–13) (2012) 033104. [12] M. Coulibaly, G. Arrachart, A. Mesbah, X. Deschanels, From colloidal precursors to metal carbides nanocomposites MC (M=Ti, Zr, Hf and Si): synthesis, characterization and optical spectral selectivity studies, Sol. Energy Mater. Sol. Cells 143 (2015) 473–479. [13] 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. [14] X.-H. Gao, W. Theiss, Y.-Q. Shen, P.-J. Ma, Gang Liu, Optical simulation, corrosion behavior and long term thermal stability of TiC-based spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 167 (2017) 150–156. [15] X.-H. Gao, Z.-M. Guo, Q.-F. Geng, P.-J. Ma, A.-Q. Wang, G. Liu, Microstructure, chromaticity and thermal stability of SS/TiC-WC/Al2O3 spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 164 (2017) 63–69. [16] X.-H. Gao, Z.-M. Guo, Q.-F. Geng, P.-J. Ma, A.-Q. Wang, G. Liu, Structure, optical properties and thermal stability of SS/TiC-ZrC/Al2O3 spectrally selective solar absorber, RSC Adv. 6 (2016) 63867–63873. [17] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294–303. [18] Y. Li, C. Lin, D. Zhou, Y. An, D. Li, C. Chi, H. Huang, S. Yang, C. YanTso, C.Y.H. Chao, B. Huang, Scalable all-ceramic nanofilms as highly efficient and thermally stable selective solar absorbers, Nano Energy 64 (2019), https://doi.org/ 10.1016/j.nanoen.2019.103947. [19] Y. Xu, Z. Huang, Y. Liu, M. Fang, L. Yin, M. Guan, Influence of yttria addition on the phase transformations of zirconia from zircon ore by carbothermal reduction process, Solid State Sci. 14 (2012) 730–734. [20] X. Xiong, Y. Wang, G. Li, Z. Chen, W. Sun, Z. Wang, HfC/ZrC ablation protective coating for carbon/carbon composites, Corros. Sci. 77 (2013) 25–30. [21] L. Zhang, R.V. Koka, A study on the oxidation and carbon diffusion of TiC in alumina-titanium carbide ceramics using XPS and Raman spectroscopy, Mater. Chem. Phys. 57 (1998) 23–32. [22] O. Wilhelmssona, J.-P. Palmquist, E. Lewina, J. Emmerlich, P. Eklund, P.O.A. Persson, H. Hogberg, S. Li, R. Ahuja, O. Eriksson, L. Hultman, U. Jansson, Deposition and characterization of ternary thin films within the Ti-Al-C system by DC magnetron sputtering, J. Cryst. Growth 291 (2006) 290–300. [23] W. Dai, A. Wang, Deposition and properties of Al-containing diamond-like carbon films by a hybrid ion beam sources, J. Alloy. Comp. 509 (2001) 4626–4631. [24] X. Yuan, L. Cheng, L. Zhang, Controlled fabrication of TiC nanocrystal clusters on surface of Ti particles for application in electromagnetic wave absorption, J. Alloy. Comp. 622 (2015) 282–287.
Acknowledgments This work was financially supported by the Youth Innovation Promotion Association CAS (2018455), the Key Research & Development Program in Gansu (18YF1GA125) and the National Natural Science Foundation of China (No. 51402315). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110187. References [1] P. Tao, G. Ni, C. Song, W. Shang, J. Wu, J. Zhu, G. Chen, T. Deng, Solar-driven interfacial evaporation, Nature Energy 3 (2018) 1031–1041. [2] L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu, J. Zhu, 3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination, Nat. Photonics 10 (2016) 393–398. [3] S. Kalogirou, The potential of solar industrial process heat applications, Appl. Energy 76 (2003) 337–361. [4] L.A. Weinstein, J. Loomis, B. Bhatia, D.M. Bierman, E.N. Wang, G. Chen, Concentrating solar power, Chem. Rev. 115 (2015) 12797–12838. [5] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. 7 (2014) 1615–1627. [6] N. Selvakumar, H.C. Barshilia, Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications, Sol. Energy Mater. Sol. Cells 98 (2012) 1–23. [7] C.E. Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, 2002. [8] J. Moon, D. Lu, B. VanSaders, T.K. Kim, S.D. Kong, S. Jin, R. Chen, Z. Liu, High performance multi-scaled nanostructured spectrally selective coating concentrating solar power, Nano Energy 8 (2014) 238–246. [9] P. Li, B. Liu, Y. Ni, K.K. Liew, J. Sze, S. Chen, S. Shen, Large-scale nanophotonic solar selective absorbers for high-efficiency solar thermal energy conversion, Adv. Mater. 31 (2015) 4585–4591.
5