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Structure, optical properties and thermal stability of Al2O3-WC nanocomposite ceramic spectrally selective solar absorbers
Optical Materials 58 (2016) 219e225
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Structure, optical properties and thermal stability of Al2O3-WC nanocomposite ceramic spectrally selective solar absorbers Xiang-Hu Gao a, b, Cheng-Bing Wang c, Zhi-Ming Guo c, Qing-Fen Geng a, Wolfgang Theiss d, Gang Liu a, * a Research & Development Center for Eco-chemistry and Eco-materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b University of Chinese Academy of Sciences, Beijing, 100049, China c National Engineering Research Center for Technology and Equipment of Green Coating, Lanzhou Jiaotong University, Lanzhou, 730070, China d W. Theiss Hard- and Software, Dr.-Bernhard-Klein-Str. 110, D-52078, Aachen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
Traditional metal-dielectric composite coating has found important application in spectrally selective solar absorbers. However, fine metal particles can easily diffuse, congregate, or be oxidized at high temperature, which causes deterioration in the optical properties. In this work, we report a new spectrally selective solar absorber coating, composed of low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC layer) and Al2O3 antireflection layer. The features of our work are: 1) compared with the metal-dielectric composites concept, Al2O3-WC nanocomposite ceramic successfully achieves the all-ceramic concept, which exhibits a high solar absorptance of 0.94 and a low thermal emittance of 0.08, 2) Al2O3 and WC act as filler material and host material, respectively, which are different from traditional concept, 3) Al2O3-WC nanocomposite ceramic solar absorber coating exhibits good thermal stability at 600 C. In addition, the solar absorber coating is successfully modelled by a commercial optical simulation programme, the result of which agrees with the experimental results. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solar absorber Nanocomposite ceramic Structure Optical properties Thermal stability
1. Introduction Access to clean, affordable, and reliable energy has been a cornerstone of the world’s increasing prosperity and economic growth since the beginning of the industrial revolution. Nowadays, solar absorbers find applications in such as concentrated solar power, solar thermophotovoltaic, solar thermionic, and solar thermoelectric systems [1e4]. In this regard, there is an increasing demand for spectrally selective coatings for mid- and hightemperature solar thermal applications. The main requirements for the solar selective coatings are high solar absorptance (a) in the solar spectral range (0.3e2.5 mm) and low thermal emittance (ε) in the infrared region (2.5e25 mm) [5,6]. Selective absorber surface coatings can be categorized into five distinct types: a) intrinsic coatings, b) metal-dielectric composite coatings, c) multilayer absorbers, d) semiconductor-metal tandems,
* Corresponding author. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.optmat.2016.05.037 0925-3467/© 2016 Elsevier B.V. All rights reserved.
e) textured surfaces. The highly absorbing metal-dielectric composite, or ceramic, consists of fine metal particles in a dielectric or ceramic matrix, or a porous oxide impregnated with metal [7]. These films are transparent in the thermal IR region, while they are strongly absorbing in the solar region because of interband transitions in the metal and the small particle resonance. According to the double interference absorptive theory, the solar selective absorbing coating from substrate to surface consists of an IRreflective metallic layer, a high metal volume fraction (HMVF) ceramic layer and a low metal volume fraction (LMVF) layer (as the double interference absorption layers), and a transparent ceramic antireflection (AR) layer [8]. In a coating with this structure, due to the gradual variation of refractive index, solar radiation is efficiently absorbed internally, and by phase interference between the double ceramic and the AR layers. For the metal-dielectric composite coating, the solar selectivity can be optimized by proper choice of constituents, coating thickness, particle concentration, size, shape, and orientation [9]. So far, solar absorbers based on ceramic coatings: PteAl2O3 [10],
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NieAl2O3 [11], MoeAl2O3 [12], WeAlN [13], MoeSi3N4 [14], AgeAl2O3 [15], AlNieAl2O3 [16], WeNieAl2O3 [17] have been reported. Angelantoni ENEA (Italy) has commercialized MoeSiO2 and WeAl2O3 coatings, while Siemens (Germany) commercializes MoeAl2O3 and WeAl2O3 coatings, which are stable in the 350e500 C temperature range [18e21]. These coatings exhibit high solar selectivity and thermal stability in vacuum. However, fine metal particles can easily diffuse, congregate, or be oxidized, in particular, in high temperature application. It causes deterioration in the optical properties of the coating at high temperature. In this regard, exploring selective coatings with excellent stability at hightemperature solar thermal applications are of technological important from the practical applications point of view. During the past few years, much effort has been focused on the metal or alloy doped into ceramic or dielectric materials. The WC transition-metal carbide ceramic has excellent high temperature strength and good corrosion resistance, being chemically and thermally stable even at the high temperatures. In addition, it is typically metallic in their electrical and optical properties [22]. In the present work, we demonstrate a new nanocomposite ceramic spectrally selective solar absorber coating with a high solar absorptance (0.94) and a low thermal emittance (0.08) by dopping Al2O3 into WC. In contrast to typical metal-dielectric composite ceramic structures that are filled with particles of one metal or alloy type, our ceramic layers based on WC ceramic host material are filled with Al2O3 prepared by co-sputtering. Based on the double ceramic layer structure, we have prepared low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC) layer and Al2O3 antireflection layer by sputtering. The structure, optical properties and thermal stability of Al2O3-WC based spectrally selective tandem solar absorbers are investigated in detail. For our nanocomposite ceramic spectrally selective solar absorber coating, the inexpensive and commercial transition-metal carbides ceramic offers high temperature strength and stability as well as good corrosion resistance while Al2O3 greatly improves the spectral selectivity. The as-deposited tandem solar absorber coating is successfully modelled by a commercial optical simulation programme. The as-deposited tandem solar absorber coating also shows good thermal stability and may have a promising application for high temperature application. 2. Experimental section The coating was deposited onto a mechanically polished SS substrate (50 50 mm) using a commercial magnetron sputtering equipment (Kurt J. Lesker). The purity, diameter, and thickness of WC and Al2O3 targets are 99.99%, 76.2 mm, and 6 mm, respectively. Prior to the deposition process, the chamber was evacuated to lower than 3 106 Torr. Co-sputtering was used to deposit the ceramic layers. Al2O3 and WC were deposited using RF and DC magnetron sputtering, respectively. The Al2O3 fill fractions of the ceramic layers were controlled by independent input power control to the WC targets. The complete deposition process was performed in an argon plasma environment at a pressure of 3 mTorr. For the as-deposited tandem solar absorber coating SS/Al2O3(L)WC/Al2O3(H)-WC/Al2O3, the sputtering parameters are the following: ceramic 1: Al2O3(L)-WC with a DC power density of 3.29 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3; ceramic 2: Al2O3(H)-WC with a DC power density of 1.75 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3; ARC: Al2O3 with a RF power density of 6.14 W/cm2. For the SS/WC/WC/Al2O3 coating, the sputtering parameters are the following: ceramic 1: WC with a DC power density of 3.29 W/cm2 for WC; ceramic 2: WC with a DC power density of 1.75 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3.
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ max 2400/PC diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKa radiation (l51.5406 Å). The surface morphologies were observed by ultra-high resolution scanning electron microscope (SU8200, Tokyo, Japan). The surface imaging of the sample was carried out by atomic force microscope (Dimension Icon, Bruker). The TEM studies on tandem solar absorber coating were performed using a high-resolution transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI). Reflectance spectra in the wavelength interval 0.3e2.5 mm were measured on a Perkin Elmer Lambda 950 UV/Vis/NIR spectrometer with an integration sphere (module 150 mm), while reflectance spectra in the wavelength interval 2.5e25 mm were obtained on a Bruker TENSOR 27 FT-IR Spectrometer, equipped with an integrating sphere (A562-G/Q) using a gold plate as a standard for diffuse reflectance. In order to test the thermal stability, the coatings were heated in vacuum (5 103 Pa) tubular furnace at 550e700 C for 5 h. The accuracy of the set temperature was ±1 C. Annealing process involved increasing the temperature of the samples from room temperature to the desired temperature at a slow heating rate of 5 C/min and maintaining the desired temperature for 5 h. Subsequently, the samples were naturally cooled down to room temperature. 3. Results and discussion The configuration of Al2O3-WC multilayer tandem solar absorber coating is schematically illustrated in Fig. 1. Compared with the traditional metal-dielectric composite system, Al2O3 is doped into WC ceramic host material and forms low ceramic volume fraction absorber layer and high ceramic volume fraction absorber layer. The volume fraction of Al2O3 and thickness of each layer are shown in Table 1. Except for the metal substrate, there is no metal in absorber layer and antireflection layer, which may refer to the all-ceramic concept. 3.1. Structure properties The morphology and structure of as-deposited Al2O3-WC multilayer tandem solar absorber coating are examined by scanning electron microscope (SEM), atomic force microscope (AFM)
Fig. 1. Schematic of Al2O3-WC multilayer tandem solar absorber coating deposited onto stainless steel (SS) with a double ceramic layer: low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC) layer and Al2O3 antireflection layer.
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 Table 1 Summary of volume fraction of Al2O3 and thickness of each layer. Layer
Volume fraction of Al2O3 (%)
Thickness (nm)
Al2O3(L)-WC Al2O3(H)-WC Al2O3
19 27 e
67 93 103
image, and X-ray diffraction (XRD). As shown in Fig. 2A, the asdeposited multilayer tandem solar absorber surface depicts dense, uniform, and fine-grained. It is reported that the holes, cracks, and ruptures of the macro-droplets result in the destruction of the outer coating, and cause the inner coating losing protection against oxidation at high temperature, which tends to increase the risk of failure of the coating [23]. Obviously, there is no hole, cracks and ruptures on the surface, which may make contribution to the structural and thermal stability. The cross-sectional scanning electron microscopy micrograph of the tandem absorber deposited on a Si substrate is shown in Fig. 2B. From the micrograph, the layer thicknesses of Al2O3(L)-WC, Al2O3(H)-WC, and Al2O3 layers are found to be approximately 67, 93, and 103 nm, respectively. The Al2O3(L)-WC and Al2O3(H)-WC layers mainly consist of columnar grains, which is separated by high-angle boundaries and the columnar grains throughout the two absorber layers. The columnar grains grow along the normal direction of the substrate surface. In addition, the surface morphology of the coating is further investigated by AFM, and the typical images are depicted in Fig. 2C. The as-deposited coating exhibits a root mean square (RMS) roughness of 1.5 nm, and shows a columnar microstructure with a texture surface. The obtained columnar structure with surface texture could increase light absorption, thus increasing solar absorption and thermal stability [24]. Normally, the increased roughness could enhance the diffusion reflectance, and inclines to cause the increase of the thermal emittance. Thermal emittance from the surface provides considerable contribution and depends on the condition of the materials surface, roughness, and the surface oxide layers [7]. For the as-deposited tandem solar absorber
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coating, the relatively low surface roughness (1.5 nm) is beneficial to the low thermal emittance. To examine the structure, the asdeposited multilayer tandem solar absorber coating is characterized by X-ray diffraction (XRD) as shown in Fig. 2D. In the diffraction patterns from the as-deposited samples, only the (Fe, C) diffraction peaks from the substrate are observed indicating the amorphous nature of the as-deposited tandem solar absorber layer. And there is only a very weak Al2O3 peak in the pattern. To give further insight about the morphology and structure of the as-deposited multilayer tandem solar absorber, ultra highresolution transmission electron microscope (HRTEM) images associated with select area electron diffraction (SAED) are shown in Fig. 3. The overview of the cross-sectional region (Fig. 3A) shows clearly that the coating consists of three layers: Al2O3(L)-WC layer, Al2O3(H)-WC layer, and the Al2O3 antireflection layer. The film thickness of each layer is consistent with the SEM result. By carefully examination we find that the interface between Al2O3(L)-WC/ Al2O3(H)-WC layers is clearly distinct. The magnified micrograph in Fig. 3B shows a magnified HRTEM pattern, which is marked in Fig. 3A. There is a clear interface between absorber layer and antireflection layer. The magnified micrograph in Fig. 3C shows a HRTEM pattern, which is also marked in Fig. 3A. It is worth to note that the absorber layer region and the Si substrate region have a clearly interface. The SiO2 layer seen in the micrograph is due to the formation of oxide layer on the pristine Si substrate. Fig. 3D shows the selected area electron diffraction (SAED) micrograph of enlarged region marked in Fig. 3A. There is no spotty ring pattern in SAED pattern, indicating an amorphous nature. This result is in agreement with the XRD result, which further confirms that the asdeposited multilayer has an amorphous structure. 3.2. Optical properties In order to investigate the optical properties and make a comparison, the layer-added film samples are fabricated, respectively. The reflectance spectra of the layer-added film samples are shown in Fig. 4. In order to achieve a high solar selectivity, the coatings
Fig. 2. Structure characterization of the as-deposited tandem solar absorber: A) SEM image of top-view surface morphology, B) cross-sectional SEM micrograph, C) AFM image of the surface, and D) XRD patterns of the sample: (a) SS, (b) SS/Al2O3(L)-WC, (c) SS/Al2O3(L)-WC/Al2O3(H)-WC, (d) SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3.
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Fig. 3. The cross-sectional transmission electron microscopy micrographs of tandem solar absorber: A) cross-sectional view, B) enlarged region marked in A showing the interface between Al2O3(H)-WC layer and Al2O3, C) enlarged region marked in A showing the interface between Si and Al2O3(L)-WC layer, and D) selected area diffraction pattern of crosssectional.
Fig. 4. Reflectance spectra of the layer-added film sample of A) the as-deposited SS/WC/WC/Al2O3 sample: (a) SS, (b) SS/WC, (c) SS/WC/WC, (d) SS/WC/WC/Al2O3, B) the asdeposited SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 sample: (e) SS/Al2O3(L)-WC, (f) SS/Al2O3(L)-WC/Al2O3(H)-WC, and (g) SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3.
should exhibit low reflectance in visible light region but high reflectance in the IR region. As shown in Fig. 4A, there is only one lowest point in the reflectance spectra from 200 nm to 2000 nm for the as-deposited multilayer SS/WC/WC/Al2O3 coating (see curve (d)) and the reflectance rises sharply from 500 nm, which contributes the relatively low solar absorptance and high thermal emittance. But the as-deposited multilayer SS/Al2O3(L)-WC/ Al2O3(H)-WC/Al2O3 exhibits the low reflectance in the wavelength region from 200 to 2000 nm, and high reflectance in the wavelength region from 2000 nm to 25 mm in Fig. 4B (see curve (g)). The reflectance spectra shows two lowest point from 200 to 2000 nm and rises sharply from 2000 nm. This reflectance spectra is similar
to the metal-dielectric composite coating system. The reflectance spectra exhibits a small rebounce in the wavelength region from 380 nm to 780 nm, which contributes the blue color of the coating. Table 2 gives the spectral properties, including solar absorptance, thermal emittance, and solar selectivity of the samples. The solar absorptance and thermal emittance of the sample are calculated according to the reflectance spectra of which the curve is showed in Fig. 4A and B. A polished stainless steel exhibits a solar absorptance of 0.36 and a thermal emittance of 0.11. The SS/ Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating exhibits a high solar absorptance of 0.94 and a low thermal emittance of 0.08. Compared with the SS substrate, the SS/Al2O3(L)-WC absorber layer gives the
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 Table 2 The absorptance and emittance value of the layer-added film. Samples
Absorption(a)
Emittance (ε)
a/ε
SS SS/WC SS/WC/WC SS/WC/WC/Al2O3 SS/Al2O3(L)-WC SS/Al2O3(L)-WC/Al2O3(H)-WC SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3
0.36 0.68 0.70 0.87 0.81 0.80 0.94
0.11 0.06 0.23 0.11 0.20 0.08 0.08
3.27 11.33 3.04 7.90 4.05 10 11.75
largest contribution to the solar absorptance, which is increased to 0.81 from 0.36, but induces a rising of the thermal emittance up to 0.20 from 0.11. The second absorber layer Al2O3(H)-WC greatly decreases the thermal emittance of the coating from 0.20 to 0.08. The solar radiation is mainly absorbed through two methods: the intrinsic absorption in the absorber layer and optical interference absorption between the double absorptive layers. The first absorber layer (Al2O3(L)-WC) makes the largest contribution to the solar absorptance. The second absorbing layer (Al2O3(H)-WC) incorporating with first layer plays an important role in the interference to enhance the solar absorptance further. The presence of the Al2O3 antireflection layer makes the solar absorptance increased to 0.94 from 0.80 due to the antireflection effect, and does not make more increase in thermal emittance, which keeps still at 0.08. Compared with the SS/WC/WC/Al2O3 coating, the SS/Al2O3(L)-WC/Al2O3(H)WC/Al2O3 exhibits a higher solar absorptance and a lower thermal emittance with the existance of Al2O3. It is worth to note that the Al2O3 can greatly improve the solar absorptance from 0.68 (SS/WC) to 0.81 for the SS/Al2O3(L)-WC layer, and decrease the thermal emittance from 0.23 (SS/WC/WC) to 0.08 for the SS/Al2O3(L)-WC/ Al2O3(H)-WC absorber layer, which further indicates that the Al2O3 can greatly improve the spectral selectivity.
3.3. Optical simulation We next investigate the optical properties of the as-deposited multilayer tandem solar absorber coating, the experimental result is modelled by a commercial optical simulation programme (CODE) [25]. The WC single layer is deposited on glass. A theoretical analysis of transmittance and reflectance of films can be used to obtain the optical constant [26,27]. Prior to simulate the as-deposited multilayer solar absorber coating, the optical constant of glass, WC, and stainless steel are modelled by the CODE software. Firstly, the transmittance and reflectance of glass substrate is modelled in a good agreement with the measured spectra (Fig. 5) and the fit parameters are fixed. For the stainless steel, a good agreement between the simulated and experimental data is obtained and the
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obtained refractive index n and extinction coefficient k are plotted in Fig. 6. In case of stainless steel, it is observed that there is an increase of refractive index and extinction coefficient with the wavelength, which is a characteristic behavior of metallic films. The optical constant of Al2O3 is adapted from database of CODE software. Fig. 7 shows the simulated and measured reflectance, transmittance and refractive index (n) and extinction coefficient (k) of WC on glass. Using the obtained optical constants, the three layer tandem absorber (SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3) is designed and the optical performance is simulated as shown in Fig. 8. The simulated solar absorber coating exhibits a high solar absorptance of 0.9334 and a low thermal emittance of 0.0533, which shows a good agreement with the measured reflectance spectra (Fig. 8). The simulated layer thickness and volume fraction of Al2O3 also exhibit a good agreement with the experiment results. 3.4. Thermal stability For high-temperature (T > 400 C) application, ceramic based coatings are considered to be good candidates due to their high solar absorptance, low thermal emittance, and good thermal stability because of the high temperature stable ceramic host [5,7]. In order to evaluate the thermal stability, the as-deposited coating is heat-treated in vacuum at temperatures in the range of 550e700 C for 5 h. The solar absorptance and thermal emittance values of the heat-treated SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 absorber are shown in Table 3. Fig. 9 shows the reflectance spectra of SS/ Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating annealed at different temperatures in vacuum for 5 h. The tandem absorber exhibits good thermal stability in vacuum at 600 C for 5 h. It is reported that the state-of-the-art high temperature solar selective coatings, for example, exhibits thermal stability at 580 C in vacuum for 30 days. (e.g., a ¼ 0.94; εo580C ¼ 0.13 on SS for MoeSiO2 coating) [18]. For our work, the long-term (e.g., years) high temperature stability of the solar absorbers still unknown at present, and needs further tests including thermal cycling and thermal shock experiment. 4. Conclusions In summary, we have developed and modelled a spectrally selective solar absorber based on low ceramic volume fraction absorber layer (Al2O3(L)-WC) and high ceramic volume fraction absorber layer (Al2O3(H)-WC) with antireflection layer (Al2O3) on a mechanically polished stainless steel. The as-deposited multilayer tandem solar absorber shows the amorphous structure and has a high absorptance of 0.94 and a low thermal emittance of 0.08. The existence of Al2O3 can greatly improve the spectral selectivity. A good agreement between the simulated and experimental data for the as-deposited multilayer solar absorber coating (SS/Al2O3(L)-
Fig. 5. A) Simulated and measured reflectance spectra, B) simulated and measured transmittance of glass substrate.
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Fig. 6. A) Simulated and measured reflectance spectra, B) refractive index (n) and extinction coefficient (k) of stainless steel.
Fig. 7. A) Simulated and measured reflectance spectra, B) simulated and measured transmittance spectra, C) refractive index (n) and extinction coefficient (k) of WC on glass.
Fig. 8. Simulated and measured reflectance spectra of SS/Al2O3(L)-WC/Al2O3(H)-WC/ Al2O3 coating.
Fig. 9. Reflectance spectra of SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating annealed at different temperatures in vacuum for 5 h.
Table 3 Effect of annealing at different temperatures (in vacuum) on optical properties of the as-deposited multilayer solar absorber coating (SS/Al2O3(L)-WC/Al2O3(H)-WC/ Al2O3).
compared with existing solar absorber structures. This nanocomposite ceramic concept may provide a new avenue and concept for development of spectrally selective solar absorber, especially for the high temperature application.
Temperature ( C)
Duration (Hrs)
a
ε
As-deposited 550 600 650 700
e 5 5 5 5
0.94 0.94 0.94 0.93 0.89
0.08 0.08 0.10 0.12 0.14
WC/Al2O3(H)-WC/Al2O3) is demonstrated. The thermal annealing tests indicate that the solar absorbers are stable at 600 C for 5 h. The new configuration of the solar absorber coating which is achieved by dopping ceramic into ceramic are unprecedented
Acknowledgements This work is financially supported by NSFC (51402315, 51261014, E020603), the Western Light Talents Training Program of CAS and the Science and Technology Support Program of Gansu Province (Grant 1304GKCA025). References [1] 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 for
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 concentrating solar power, Nano Energy 8 (2014) 238e246. [2] A. Lenert, D.M. Bierman, Y. Nam, W.R. Chan, I. Celanovic, M. Soljacic, E.N. Wang, A nanophotonic solar thermophotovoltaic device, Nat. Nanotechnol. 9 (2014) 126e130. [3] J.W. Schwede, I. Bargatin, D.C. Riley, B.E. Hardin, S.J. Rosenthal, Y. Sun, F. Schmitt, P. Pianetta, R.T. Howe, Z.X. Shen, N.A. Melosh, Photon-enhanced thermionic emission for solar concentrator systems, Nat. Mater. 9 (2010) 762e767. [4] D. Kraemer, B. Poudel, H.P. Feng, J.C. Caylor, B. Yu, X. Yan, Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. Ren, G. Chen, High-performance flat-panel solar thermoelectric generators with high thermal concentration, Nat. Mater. 10 (2011) 532e538. [5] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. 7 (2014) 1615e1627. [6] N. Selvakumar, S.B. Krupanidhi, H.C. Barshilia, Carbon nanotube-based tandem absorber with tunable spectral selectivity: transition from near-perfect blackbody absorber to solar selective absorber, Adv. Mater. 26 (2014) 2552e2557. [7] C.E. Kennedy, Review of Mid- to High-temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, 2002. [8] Q.-C. Zhang, D.R. Mills, Very low-emittance solar selective surfaces using new film structures, J. Appl. Phys. 72 (1992) 3013e3021. [9] 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) 1e23. [10] T.K. Vien, C. Sella, J. Lafait, S. Berthier, Pt-Al2O3 selective cermet coatings on superalloy substrates for photothermal conversion, Thin Solid Films 126 (1985) 17e22. [11] T.S. Sathiaraj, R. Thangaraj, H. AlSharbaty, O.P. Agnihotri, Optical properties of selectively absorbing R. F. sputtered Ni-Al2O3 composite films, Thin Solid Films 195 (1991) 33e42. [12] Q.-C. Zhang, Y. Yin, D.R. Mills, High efficiency Mo-Al2O3 cermet selective surfaces for high-temperature application, Sol. Energy Mater. Sol. Cells 40 (1996) 43e53. [13] Q.-C. Zhang, Y.G. Shen, High performance W-AlN cermet solar coatings designed by modeling calculations and deposited by DC magnetron sputtering, Sol. Energy Mater. Sol. Cells 81 (2004) 25e37. spedes, M. Wirz, J.A. Sa nchez-García, L. Alvarez-Fraga, R. Escobar-Galindo, [14] E. Ce C. Prieto, Novel Mo-Si3N4 based selective coating for high temperature concentrating solar power applications, Sol. Energy Mater. Sol. Cells 122
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(2014) 217e225. [15] H.C. Barshilia, P. Kumar, K.S. Rajam, A. Biswas, Structure and optical properties of Ag-Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering, Sol. Energy Mater. Sol. Cells 95 (2011), 1707e1705. [16] Y. Xue, C. Wang, W. Wang, Y. Liu, Y. Wu, Y. Ning, Y. Sun, Spectral properties and thermal stability of solar selective absorbing AlNi-Al2O3 cermet coating, Sol. Energy 96 (2013) 113e118. [17] F. Cao, D. Kraemer, T. Sun, Y. Lan, G. Chen, Z. Ren, Enhanced thermal stability of W-Ni-Al2O3 cermet-based spectrally selective solar absorbers with tungsten infrared reflectors, Adv. Energy Mater. 2 (2015) 1401042. [18] S. Esposito, A. Antonaia, M.L. Addonizio, S. Aprea, Fabrication and optimization of highly efficient cermet-based spectrally selective coatings for high temperature, Thin Solid Films 517 (2009) 6000e6006. [19] Q.-C. Zhang, D. Mills, A. Monger, Thin film solar selective surface coating, US Patent 5523132A, 1996. [20] Q.-C. Zhang, Y. Yin, D.R. Mills, High efficiency Mo-Al2O3 cermet selective surfaces for high-temperature application, Sol. Energy Mater. Sol. Cells 40 (1996) 43e53. [21] T. Kuckelkorn, W. Graf, C. Hildebrandt, A. Georg, Radiation selective absorber pipe, absorber pipe with said coating, and method of making same, US Patent 0121225A1, 2008. [22] M.S. El-Eskandarany, M. Sherif El-Eskandarany, Fabrication and characterizations of new nanocomposite WC/Al2O3 materials by room temperature ball milling and subsequent consolidation, J. Alloys Compd. 391 (2005) 228e235. [23] D. Gong, H. Liu, G. Luo, P. Zhang, X. Cheng, B. Yang, Y. Wang, J. Min, W. Wang, S. Chen, Z. Cui, K. Li, L. Hu, Thermal aging test of AlCrNO-based solar selective absorbing coatings prepared by cathodic arc plating, Sol. Energy Mater. Sol. Cells 136 (2015) 167e171. [24] V. Teixeira, E. Sousa, M.F. Costa, C. Nunes, L. Rosa, M.J. Carvalho, M. CollaresPereira, E. Roman, J. Gago, Spectrally selective composite coatings of Cr-Cr2O3 and Mo-Al2O3 for solar energy applications, Thin Solid Films 392 (2001) 320e326. [25] M. Theiss, Hard and Software for Optical Spectroscopy, 2013. Dr. BernhardKlein-Str. 110, 52078 Aachen, Germany, www.mtheiss.com. [26] C.M. Maghanga, G.A. Niklasson, C.G. Granqvist, Optical properties of sputter deposited transparent and conducting TiO2:Nb films, Thin Solid Films 518 (2009) 1254e1258. [27] A. Solieman, M.A. Aegerter, Modeling of optical and electrical properties of In2O3:Sn coatings made by various techniques, Thin Solid Films 502 (2006) 205e211.
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Structure, optical properties and thermal stability of Al2O3-WC nanocomposite ceramic spectrally selective solar absorbers Xiang-Hu Gao a, b, Cheng-Bing Wang c, Zhi-Ming Guo c, Qing-Fen Geng a, Wolfgang Theiss d, Gang Liu a, * a Research & Development Center for Eco-chemistry and Eco-materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b University of Chinese Academy of Sciences, Beijing, 100049, China c National Engineering Research Center for Technology and Equipment of Green Coating, Lanzhou Jiaotong University, Lanzhou, 730070, China d W. Theiss Hard- and Software, Dr.-Bernhard-Klein-Str. 110, D-52078, Aachen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2016 Received in revised form 20 May 2016 Accepted 23 May 2016
Traditional metal-dielectric composite coating has found important application in spectrally selective solar absorbers. However, fine metal particles can easily diffuse, congregate, or be oxidized at high temperature, which causes deterioration in the optical properties. In this work, we report a new spectrally selective solar absorber coating, composed of low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC layer) and Al2O3 antireflection layer. The features of our work are: 1) compared with the metal-dielectric composites concept, Al2O3-WC nanocomposite ceramic successfully achieves the all-ceramic concept, which exhibits a high solar absorptance of 0.94 and a low thermal emittance of 0.08, 2) Al2O3 and WC act as filler material and host material, respectively, which are different from traditional concept, 3) Al2O3-WC nanocomposite ceramic solar absorber coating exhibits good thermal stability at 600 C. In addition, the solar absorber coating is successfully modelled by a commercial optical simulation programme, the result of which agrees with the experimental results. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solar absorber Nanocomposite ceramic Structure Optical properties Thermal stability
1. Introduction Access to clean, affordable, and reliable energy has been a cornerstone of the world’s increasing prosperity and economic growth since the beginning of the industrial revolution. Nowadays, solar absorbers find applications in such as concentrated solar power, solar thermophotovoltaic, solar thermionic, and solar thermoelectric systems [1e4]. In this regard, there is an increasing demand for spectrally selective coatings for mid- and hightemperature solar thermal applications. The main requirements for the solar selective coatings are high solar absorptance (a) in the solar spectral range (0.3e2.5 mm) and low thermal emittance (ε) in the infrared region (2.5e25 mm) [5,6]. Selective absorber surface coatings can be categorized into five distinct types: a) intrinsic coatings, b) metal-dielectric composite coatings, c) multilayer absorbers, d) semiconductor-metal tandems,
* Corresponding author. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.optmat.2016.05.037 0925-3467/© 2016 Elsevier B.V. All rights reserved.
e) textured surfaces. The highly absorbing metal-dielectric composite, or ceramic, consists of fine metal particles in a dielectric or ceramic matrix, or a porous oxide impregnated with metal [7]. These films are transparent in the thermal IR region, while they are strongly absorbing in the solar region because of interband transitions in the metal and the small particle resonance. According to the double interference absorptive theory, the solar selective absorbing coating from substrate to surface consists of an IRreflective metallic layer, a high metal volume fraction (HMVF) ceramic layer and a low metal volume fraction (LMVF) layer (as the double interference absorption layers), and a transparent ceramic antireflection (AR) layer [8]. In a coating with this structure, due to the gradual variation of refractive index, solar radiation is efficiently absorbed internally, and by phase interference between the double ceramic and the AR layers. For the metal-dielectric composite coating, the solar selectivity can be optimized by proper choice of constituents, coating thickness, particle concentration, size, shape, and orientation [9]. So far, solar absorbers based on ceramic coatings: PteAl2O3 [10],
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NieAl2O3 [11], MoeAl2O3 [12], WeAlN [13], MoeSi3N4 [14], AgeAl2O3 [15], AlNieAl2O3 [16], WeNieAl2O3 [17] have been reported. Angelantoni ENEA (Italy) has commercialized MoeSiO2 and WeAl2O3 coatings, while Siemens (Germany) commercializes MoeAl2O3 and WeAl2O3 coatings, which are stable in the 350e500 C temperature range [18e21]. These coatings exhibit high solar selectivity and thermal stability in vacuum. However, fine metal particles can easily diffuse, congregate, or be oxidized, in particular, in high temperature application. It causes deterioration in the optical properties of the coating at high temperature. In this regard, exploring selective coatings with excellent stability at hightemperature solar thermal applications are of technological important from the practical applications point of view. During the past few years, much effort has been focused on the metal or alloy doped into ceramic or dielectric materials. The WC transition-metal carbide ceramic has excellent high temperature strength and good corrosion resistance, being chemically and thermally stable even at the high temperatures. In addition, it is typically metallic in their electrical and optical properties [22]. In the present work, we demonstrate a new nanocomposite ceramic spectrally selective solar absorber coating with a high solar absorptance (0.94) and a low thermal emittance (0.08) by dopping Al2O3 into WC. In contrast to typical metal-dielectric composite ceramic structures that are filled with particles of one metal or alloy type, our ceramic layers based on WC ceramic host material are filled with Al2O3 prepared by co-sputtering. Based on the double ceramic layer structure, we have prepared low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC) layer and Al2O3 antireflection layer by sputtering. The structure, optical properties and thermal stability of Al2O3-WC based spectrally selective tandem solar absorbers are investigated in detail. For our nanocomposite ceramic spectrally selective solar absorber coating, the inexpensive and commercial transition-metal carbides ceramic offers high temperature strength and stability as well as good corrosion resistance while Al2O3 greatly improves the spectral selectivity. The as-deposited tandem solar absorber coating is successfully modelled by a commercial optical simulation programme. The as-deposited tandem solar absorber coating also shows good thermal stability and may have a promising application for high temperature application. 2. Experimental section The coating was deposited onto a mechanically polished SS substrate (50 50 mm) using a commercial magnetron sputtering equipment (Kurt J. Lesker). The purity, diameter, and thickness of WC and Al2O3 targets are 99.99%, 76.2 mm, and 6 mm, respectively. Prior to the deposition process, the chamber was evacuated to lower than 3 106 Torr. Co-sputtering was used to deposit the ceramic layers. Al2O3 and WC were deposited using RF and DC magnetron sputtering, respectively. The Al2O3 fill fractions of the ceramic layers were controlled by independent input power control to the WC targets. The complete deposition process was performed in an argon plasma environment at a pressure of 3 mTorr. For the as-deposited tandem solar absorber coating SS/Al2O3(L)WC/Al2O3(H)-WC/Al2O3, the sputtering parameters are the following: ceramic 1: Al2O3(L)-WC with a DC power density of 3.29 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3; ceramic 2: Al2O3(H)-WC with a DC power density of 1.75 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3; ARC: Al2O3 with a RF power density of 6.14 W/cm2. For the SS/WC/WC/Al2O3 coating, the sputtering parameters are the following: ceramic 1: WC with a DC power density of 3.29 W/cm2 for WC; ceramic 2: WC with a DC power density of 1.75 W/cm2 for WC and a RF power density of 6.14 W/cm2 for Al2O3.
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ max 2400/PC diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKa radiation (l51.5406 Å). The surface morphologies were observed by ultra-high resolution scanning electron microscope (SU8200, Tokyo, Japan). The surface imaging of the sample was carried out by atomic force microscope (Dimension Icon, Bruker). The TEM studies on tandem solar absorber coating were performed using a high-resolution transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI). Reflectance spectra in the wavelength interval 0.3e2.5 mm were measured on a Perkin Elmer Lambda 950 UV/Vis/NIR spectrometer with an integration sphere (module 150 mm), while reflectance spectra in the wavelength interval 2.5e25 mm were obtained on a Bruker TENSOR 27 FT-IR Spectrometer, equipped with an integrating sphere (A562-G/Q) using a gold plate as a standard for diffuse reflectance. In order to test the thermal stability, the coatings were heated in vacuum (5 103 Pa) tubular furnace at 550e700 C for 5 h. The accuracy of the set temperature was ±1 C. Annealing process involved increasing the temperature of the samples from room temperature to the desired temperature at a slow heating rate of 5 C/min and maintaining the desired temperature for 5 h. Subsequently, the samples were naturally cooled down to room temperature. 3. Results and discussion The configuration of Al2O3-WC multilayer tandem solar absorber coating is schematically illustrated in Fig. 1. Compared with the traditional metal-dielectric composite system, Al2O3 is doped into WC ceramic host material and forms low ceramic volume fraction absorber layer and high ceramic volume fraction absorber layer. The volume fraction of Al2O3 and thickness of each layer are shown in Table 1. Except for the metal substrate, there is no metal in absorber layer and antireflection layer, which may refer to the all-ceramic concept. 3.1. Structure properties The morphology and structure of as-deposited Al2O3-WC multilayer tandem solar absorber coating are examined by scanning electron microscope (SEM), atomic force microscope (AFM)
Fig. 1. Schematic of Al2O3-WC multilayer tandem solar absorber coating deposited onto stainless steel (SS) with a double ceramic layer: low Al2O3 ceramic volume fraction (Al2O3(L)-WC) layer, high Al2O3 ceramic volume fraction (Al2O3(H)-WC) layer and Al2O3 antireflection layer.
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 Table 1 Summary of volume fraction of Al2O3 and thickness of each layer. Layer
Volume fraction of Al2O3 (%)
Thickness (nm)
Al2O3(L)-WC Al2O3(H)-WC Al2O3
19 27 e
67 93 103
image, and X-ray diffraction (XRD). As shown in Fig. 2A, the asdeposited multilayer tandem solar absorber surface depicts dense, uniform, and fine-grained. It is reported that the holes, cracks, and ruptures of the macro-droplets result in the destruction of the outer coating, and cause the inner coating losing protection against oxidation at high temperature, which tends to increase the risk of failure of the coating [23]. Obviously, there is no hole, cracks and ruptures on the surface, which may make contribution to the structural and thermal stability. The cross-sectional scanning electron microscopy micrograph of the tandem absorber deposited on a Si substrate is shown in Fig. 2B. From the micrograph, the layer thicknesses of Al2O3(L)-WC, Al2O3(H)-WC, and Al2O3 layers are found to be approximately 67, 93, and 103 nm, respectively. The Al2O3(L)-WC and Al2O3(H)-WC layers mainly consist of columnar grains, which is separated by high-angle boundaries and the columnar grains throughout the two absorber layers. The columnar grains grow along the normal direction of the substrate surface. In addition, the surface morphology of the coating is further investigated by AFM, and the typical images are depicted in Fig. 2C. The as-deposited coating exhibits a root mean square (RMS) roughness of 1.5 nm, and shows a columnar microstructure with a texture surface. The obtained columnar structure with surface texture could increase light absorption, thus increasing solar absorption and thermal stability [24]. Normally, the increased roughness could enhance the diffusion reflectance, and inclines to cause the increase of the thermal emittance. Thermal emittance from the surface provides considerable contribution and depends on the condition of the materials surface, roughness, and the surface oxide layers [7]. For the as-deposited tandem solar absorber
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coating, the relatively low surface roughness (1.5 nm) is beneficial to the low thermal emittance. To examine the structure, the asdeposited multilayer tandem solar absorber coating is characterized by X-ray diffraction (XRD) as shown in Fig. 2D. In the diffraction patterns from the as-deposited samples, only the (Fe, C) diffraction peaks from the substrate are observed indicating the amorphous nature of the as-deposited tandem solar absorber layer. And there is only a very weak Al2O3 peak in the pattern. To give further insight about the morphology and structure of the as-deposited multilayer tandem solar absorber, ultra highresolution transmission electron microscope (HRTEM) images associated with select area electron diffraction (SAED) are shown in Fig. 3. The overview of the cross-sectional region (Fig. 3A) shows clearly that the coating consists of three layers: Al2O3(L)-WC layer, Al2O3(H)-WC layer, and the Al2O3 antireflection layer. The film thickness of each layer is consistent with the SEM result. By carefully examination we find that the interface between Al2O3(L)-WC/ Al2O3(H)-WC layers is clearly distinct. The magnified micrograph in Fig. 3B shows a magnified HRTEM pattern, which is marked in Fig. 3A. There is a clear interface between absorber layer and antireflection layer. The magnified micrograph in Fig. 3C shows a HRTEM pattern, which is also marked in Fig. 3A. It is worth to note that the absorber layer region and the Si substrate region have a clearly interface. The SiO2 layer seen in the micrograph is due to the formation of oxide layer on the pristine Si substrate. Fig. 3D shows the selected area electron diffraction (SAED) micrograph of enlarged region marked in Fig. 3A. There is no spotty ring pattern in SAED pattern, indicating an amorphous nature. This result is in agreement with the XRD result, which further confirms that the asdeposited multilayer has an amorphous structure. 3.2. Optical properties In order to investigate the optical properties and make a comparison, the layer-added film samples are fabricated, respectively. The reflectance spectra of the layer-added film samples are shown in Fig. 4. In order to achieve a high solar selectivity, the coatings
Fig. 2. Structure characterization of the as-deposited tandem solar absorber: A) SEM image of top-view surface morphology, B) cross-sectional SEM micrograph, C) AFM image of the surface, and D) XRD patterns of the sample: (a) SS, (b) SS/Al2O3(L)-WC, (c) SS/Al2O3(L)-WC/Al2O3(H)-WC, (d) SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3.
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Fig. 3. The cross-sectional transmission electron microscopy micrographs of tandem solar absorber: A) cross-sectional view, B) enlarged region marked in A showing the interface between Al2O3(H)-WC layer and Al2O3, C) enlarged region marked in A showing the interface between Si and Al2O3(L)-WC layer, and D) selected area diffraction pattern of crosssectional.
Fig. 4. Reflectance spectra of the layer-added film sample of A) the as-deposited SS/WC/WC/Al2O3 sample: (a) SS, (b) SS/WC, (c) SS/WC/WC, (d) SS/WC/WC/Al2O3, B) the asdeposited SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 sample: (e) SS/Al2O3(L)-WC, (f) SS/Al2O3(L)-WC/Al2O3(H)-WC, and (g) SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3.
should exhibit low reflectance in visible light region but high reflectance in the IR region. As shown in Fig. 4A, there is only one lowest point in the reflectance spectra from 200 nm to 2000 nm for the as-deposited multilayer SS/WC/WC/Al2O3 coating (see curve (d)) and the reflectance rises sharply from 500 nm, which contributes the relatively low solar absorptance and high thermal emittance. But the as-deposited multilayer SS/Al2O3(L)-WC/ Al2O3(H)-WC/Al2O3 exhibits the low reflectance in the wavelength region from 200 to 2000 nm, and high reflectance in the wavelength region from 2000 nm to 25 mm in Fig. 4B (see curve (g)). The reflectance spectra shows two lowest point from 200 to 2000 nm and rises sharply from 2000 nm. This reflectance spectra is similar
to the metal-dielectric composite coating system. The reflectance spectra exhibits a small rebounce in the wavelength region from 380 nm to 780 nm, which contributes the blue color of the coating. Table 2 gives the spectral properties, including solar absorptance, thermal emittance, and solar selectivity of the samples. The solar absorptance and thermal emittance of the sample are calculated according to the reflectance spectra of which the curve is showed in Fig. 4A and B. A polished stainless steel exhibits a solar absorptance of 0.36 and a thermal emittance of 0.11. The SS/ Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating exhibits a high solar absorptance of 0.94 and a low thermal emittance of 0.08. Compared with the SS substrate, the SS/Al2O3(L)-WC absorber layer gives the
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 Table 2 The absorptance and emittance value of the layer-added film. Samples
Absorption(a)
Emittance (ε)
a/ε
SS SS/WC SS/WC/WC SS/WC/WC/Al2O3 SS/Al2O3(L)-WC SS/Al2O3(L)-WC/Al2O3(H)-WC SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3
0.36 0.68 0.70 0.87 0.81 0.80 0.94
0.11 0.06 0.23 0.11 0.20 0.08 0.08
3.27 11.33 3.04 7.90 4.05 10 11.75
largest contribution to the solar absorptance, which is increased to 0.81 from 0.36, but induces a rising of the thermal emittance up to 0.20 from 0.11. The second absorber layer Al2O3(H)-WC greatly decreases the thermal emittance of the coating from 0.20 to 0.08. The solar radiation is mainly absorbed through two methods: the intrinsic absorption in the absorber layer and optical interference absorption between the double absorptive layers. The first absorber layer (Al2O3(L)-WC) makes the largest contribution to the solar absorptance. The second absorbing layer (Al2O3(H)-WC) incorporating with first layer plays an important role in the interference to enhance the solar absorptance further. The presence of the Al2O3 antireflection layer makes the solar absorptance increased to 0.94 from 0.80 due to the antireflection effect, and does not make more increase in thermal emittance, which keeps still at 0.08. Compared with the SS/WC/WC/Al2O3 coating, the SS/Al2O3(L)-WC/Al2O3(H)WC/Al2O3 exhibits a higher solar absorptance and a lower thermal emittance with the existance of Al2O3. It is worth to note that the Al2O3 can greatly improve the solar absorptance from 0.68 (SS/WC) to 0.81 for the SS/Al2O3(L)-WC layer, and decrease the thermal emittance from 0.23 (SS/WC/WC) to 0.08 for the SS/Al2O3(L)-WC/ Al2O3(H)-WC absorber layer, which further indicates that the Al2O3 can greatly improve the spectral selectivity.
3.3. Optical simulation We next investigate the optical properties of the as-deposited multilayer tandem solar absorber coating, the experimental result is modelled by a commercial optical simulation programme (CODE) [25]. The WC single layer is deposited on glass. A theoretical analysis of transmittance and reflectance of films can be used to obtain the optical constant [26,27]. Prior to simulate the as-deposited multilayer solar absorber coating, the optical constant of glass, WC, and stainless steel are modelled by the CODE software. Firstly, the transmittance and reflectance of glass substrate is modelled in a good agreement with the measured spectra (Fig. 5) and the fit parameters are fixed. For the stainless steel, a good agreement between the simulated and experimental data is obtained and the
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obtained refractive index n and extinction coefficient k are plotted in Fig. 6. In case of stainless steel, it is observed that there is an increase of refractive index and extinction coefficient with the wavelength, which is a characteristic behavior of metallic films. The optical constant of Al2O3 is adapted from database of CODE software. Fig. 7 shows the simulated and measured reflectance, transmittance and refractive index (n) and extinction coefficient (k) of WC on glass. Using the obtained optical constants, the three layer tandem absorber (SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3) is designed and the optical performance is simulated as shown in Fig. 8. The simulated solar absorber coating exhibits a high solar absorptance of 0.9334 and a low thermal emittance of 0.0533, which shows a good agreement with the measured reflectance spectra (Fig. 8). The simulated layer thickness and volume fraction of Al2O3 also exhibit a good agreement with the experiment results. 3.4. Thermal stability For high-temperature (T > 400 C) application, ceramic based coatings are considered to be good candidates due to their high solar absorptance, low thermal emittance, and good thermal stability because of the high temperature stable ceramic host [5,7]. In order to evaluate the thermal stability, the as-deposited coating is heat-treated in vacuum at temperatures in the range of 550e700 C for 5 h. The solar absorptance and thermal emittance values of the heat-treated SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 absorber are shown in Table 3. Fig. 9 shows the reflectance spectra of SS/ Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating annealed at different temperatures in vacuum for 5 h. The tandem absorber exhibits good thermal stability in vacuum at 600 C for 5 h. It is reported that the state-of-the-art high temperature solar selective coatings, for example, exhibits thermal stability at 580 C in vacuum for 30 days. (e.g., a ¼ 0.94; εo580C ¼ 0.13 on SS for MoeSiO2 coating) [18]. For our work, the long-term (e.g., years) high temperature stability of the solar absorbers still unknown at present, and needs further tests including thermal cycling and thermal shock experiment. 4. Conclusions In summary, we have developed and modelled a spectrally selective solar absorber based on low ceramic volume fraction absorber layer (Al2O3(L)-WC) and high ceramic volume fraction absorber layer (Al2O3(H)-WC) with antireflection layer (Al2O3) on a mechanically polished stainless steel. The as-deposited multilayer tandem solar absorber shows the amorphous structure and has a high absorptance of 0.94 and a low thermal emittance of 0.08. The existence of Al2O3 can greatly improve the spectral selectivity. A good agreement between the simulated and experimental data for the as-deposited multilayer solar absorber coating (SS/Al2O3(L)-
Fig. 5. A) Simulated and measured reflectance spectra, B) simulated and measured transmittance of glass substrate.
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Fig. 6. A) Simulated and measured reflectance spectra, B) refractive index (n) and extinction coefficient (k) of stainless steel.
Fig. 7. A) Simulated and measured reflectance spectra, B) simulated and measured transmittance spectra, C) refractive index (n) and extinction coefficient (k) of WC on glass.
Fig. 8. Simulated and measured reflectance spectra of SS/Al2O3(L)-WC/Al2O3(H)-WC/ Al2O3 coating.
Fig. 9. Reflectance spectra of SS/Al2O3(L)-WC/Al2O3(H)-WC/Al2O3 coating annealed at different temperatures in vacuum for 5 h.
Table 3 Effect of annealing at different temperatures (in vacuum) on optical properties of the as-deposited multilayer solar absorber coating (SS/Al2O3(L)-WC/Al2O3(H)-WC/ Al2O3).
compared with existing solar absorber structures. This nanocomposite ceramic concept may provide a new avenue and concept for development of spectrally selective solar absorber, especially for the high temperature application.
Temperature ( C)
Duration (Hrs)
a
ε
As-deposited 550 600 650 700
e 5 5 5 5
0.94 0.94 0.94 0.93 0.89
0.08 0.08 0.10 0.12 0.14
WC/Al2O3(H)-WC/Al2O3) is demonstrated. The thermal annealing tests indicate that the solar absorbers are stable at 600 C for 5 h. The new configuration of the solar absorber coating which is achieved by dopping ceramic into ceramic are unprecedented
Acknowledgements This work is financially supported by NSFC (51402315, 51261014, E020603), the Western Light Talents Training Program of CAS and the Science and Technology Support Program of Gansu Province (Grant 1304GKCA025). References [1] 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 for
X.-H. Gao et al. / Optical Materials 58 (2016) 219e225 concentrating solar power, Nano Energy 8 (2014) 238e246. [2] A. Lenert, D.M. Bierman, Y. Nam, W.R. Chan, I. Celanovic, M. Soljacic, E.N. Wang, A nanophotonic solar thermophotovoltaic device, Nat. Nanotechnol. 9 (2014) 126e130. [3] J.W. Schwede, I. Bargatin, D.C. Riley, B.E. Hardin, S.J. Rosenthal, Y. Sun, F. Schmitt, P. Pianetta, R.T. Howe, Z.X. Shen, N.A. Melosh, Photon-enhanced thermionic emission for solar concentrator systems, Nat. Mater. 9 (2010) 762e767. [4] D. Kraemer, B. Poudel, H.P. Feng, J.C. Caylor, B. Yu, X. Yan, Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. Ren, G. Chen, High-performance flat-panel solar thermoelectric generators with high thermal concentration, Nat. Mater. 10 (2011) 532e538. [5] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. 7 (2014) 1615e1627. [6] N. Selvakumar, S.B. Krupanidhi, H.C. Barshilia, Carbon nanotube-based tandem absorber with tunable spectral selectivity: transition from near-perfect blackbody absorber to solar selective absorber, Adv. Mater. 26 (2014) 2552e2557. [7] C.E. Kennedy, Review of Mid- to High-temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, 2002. [8] Q.-C. Zhang, D.R. Mills, Very low-emittance solar selective surfaces using new film structures, J. Appl. Phys. 72 (1992) 3013e3021. [9] 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) 1e23. [10] T.K. Vien, C. Sella, J. Lafait, S. Berthier, Pt-Al2O3 selective cermet coatings on superalloy substrates for photothermal conversion, Thin Solid Films 126 (1985) 17e22. [11] T.S. Sathiaraj, R. Thangaraj, H. AlSharbaty, O.P. Agnihotri, Optical properties of selectively absorbing R. F. sputtered Ni-Al2O3 composite films, Thin Solid Films 195 (1991) 33e42. [12] Q.-C. Zhang, Y. Yin, D.R. Mills, High efficiency Mo-Al2O3 cermet selective surfaces for high-temperature application, Sol. Energy Mater. Sol. Cells 40 (1996) 43e53. [13] Q.-C. Zhang, Y.G. Shen, High performance W-AlN cermet solar coatings designed by modeling calculations and deposited by DC magnetron sputtering, Sol. Energy Mater. Sol. Cells 81 (2004) 25e37. spedes, M. Wirz, J.A. Sa nchez-García, L. Alvarez-Fraga, R. Escobar-Galindo, [14] E. Ce C. Prieto, Novel Mo-Si3N4 based selective coating for high temperature concentrating solar power applications, Sol. Energy Mater. Sol. Cells 122
225
(2014) 217e225. [15] H.C. Barshilia, P. Kumar, K.S. Rajam, A. Biswas, Structure and optical properties of Ag-Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering, Sol. Energy Mater. Sol. Cells 95 (2011), 1707e1705. [16] Y. Xue, C. Wang, W. Wang, Y. Liu, Y. Wu, Y. Ning, Y. Sun, Spectral properties and thermal stability of solar selective absorbing AlNi-Al2O3 cermet coating, Sol. Energy 96 (2013) 113e118. [17] F. Cao, D. Kraemer, T. Sun, Y. Lan, G. Chen, Z. Ren, Enhanced thermal stability of W-Ni-Al2O3 cermet-based spectrally selective solar absorbers with tungsten infrared reflectors, Adv. Energy Mater. 2 (2015) 1401042. [18] S. Esposito, A. Antonaia, M.L. Addonizio, S. Aprea, Fabrication and optimization of highly efficient cermet-based spectrally selective coatings for high temperature, Thin Solid Films 517 (2009) 6000e6006. [19] Q.-C. Zhang, D. Mills, A. Monger, Thin film solar selective surface coating, US Patent 5523132A, 1996. [20] Q.-C. Zhang, Y. Yin, D.R. Mills, High efficiency Mo-Al2O3 cermet selective surfaces for high-temperature application, Sol. Energy Mater. Sol. Cells 40 (1996) 43e53. [21] T. Kuckelkorn, W. Graf, C. Hildebrandt, A. Georg, Radiation selective absorber pipe, absorber pipe with said coating, and method of making same, US Patent 0121225A1, 2008. [22] M.S. El-Eskandarany, M. Sherif El-Eskandarany, Fabrication and characterizations of new nanocomposite WC/Al2O3 materials by room temperature ball milling and subsequent consolidation, J. Alloys Compd. 391 (2005) 228e235. [23] D. Gong, H. Liu, G. Luo, P. Zhang, X. Cheng, B. Yang, Y. Wang, J. Min, W. Wang, S. Chen, Z. Cui, K. Li, L. Hu, Thermal aging test of AlCrNO-based solar selective absorbing coatings prepared by cathodic arc plating, Sol. Energy Mater. Sol. Cells 136 (2015) 167e171. [24] V. Teixeira, E. Sousa, M.F. Costa, C. Nunes, L. Rosa, M.J. Carvalho, M. CollaresPereira, E. Roman, J. Gago, Spectrally selective composite coatings of Cr-Cr2O3 and Mo-Al2O3 for solar energy applications, Thin Solid Films 392 (2001) 320e326. [25] M. Theiss, Hard and Software for Optical Spectroscopy, 2013. Dr. BernhardKlein-Str. 110, 52078 Aachen, Germany, www.mtheiss.com. [26] C.M. Maghanga, G.A. Niklasson, C.G. Granqvist, Optical properties of sputter deposited transparent and conducting TiO2:Nb films, Thin Solid Films 518 (2009) 1254e1258. [27] A. Solieman, M.A. Aegerter, Modeling of optical and electrical properties of In2O3:Sn coatings made by various techniques, Thin Solid Films 502 (2006) 205e211.