Fe2O3 films on stainless steel for solar absorbers

Fe2O3 films on stainless steel for solar absorbers

Renewable and Sustainable Energy Reviews 58 (2016) 574–580 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journa...

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Renewable and Sustainable Energy Reviews 58 (2016) 574–580

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Fe2O3 films on stainless steel for solar absorbers Sean Wu a,n, Chin-Hsiang Cheng b, Yu-Jen Hsiao c, Rei-Cheng Juang d, Wen-Fu Wen e a

Department of Electronics Engineering and Computer Sciences, Tung Fang Design Institute, Kaohsiung City 82941, Taiwan Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan City 70101, Taiwan National Nano Device Laboratories, National Applied Research Laboratories, Tainan City 70101, Taiwan d Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan e Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung City 80778, Taiwan b c

art ic l e i nf o

a b s t r a c t

Article history: Received 16 September 2014 Received in revised form 22 November 2015 Accepted 27 December 2015

This paper reviews solar-selective coatings for concentrating solar power (CSP) applications. CSP systems require direct sunlight and solar tracking and utilize solar absorbers to convert sunlight to thermal electric power. Because this system receives direct sunlight which operating temperatures higher than 600 °C, heat-resistance new materials are needed to cope with. This paper presents a simple and lowcost process for depositing the high-temperature solar absorber. The high selective absorbing Fe2O3 films deposited on stainless steel (SS304) substrates to be absorbers by high thermal process at 850–1050 °C. The crystalline structure, surface microstructure and optic properties of the films were determined by Xray diffraction (XRD), scanning electron microscopy (SEM) and UV/visible spectroscopy (UV–vis–NIR Spectrophotometer, 0.25–2.5 μm). Optimal Fe2O3 films on SS304 substrates at (900–1000 °C) displayed high absorptivity (α) (0.909–0.922) and their emittance values(ε) are relatively low (0.18–0.38). This study proved the possibility of preparing high-temperature solar selective absorbing coatings with high solar absorptance and low emittance by using a simple thermal oxidation process. Those films have very good prospects for solar absorber because of simple process, low-cost, large-area and good performance. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Solar absorber Stainless steel Fe2O3

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of solar absorbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recent developments in high-temperature solar absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The oxidation process on stainless steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iron oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Structural characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Centralized receiver concentrating solar power (CSP) systems convert sunlight to electric power by using flat mirrors to focus sunlight upon a centralized receiver (or power tower). The radiant heat is absorbed by the tower and utilized to heat the steam and n

Corresponding author. E-mail address: [email protected] (S. Wu).

http://dx.doi.org/10.1016/j.rser.2015.12.263 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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push an steam turbine electric generator to produce electricity. In order to maximize the amount of light absorbed, the receiver is coated with a material (solar absorber) that absorbs as much of the solar radiation as possible [1,2]. The primary technology for concentrated solar power (CSP) system is that the ideal solar spectral selective absorbing collector should have high absorptance (α) in the wavelength range of 300–2500 nm and low emittance (ε) (λ≧2.5 μm) in the infrared region at high operating temperature. The operating temperature range for solar applications can be

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classified as low temperature (T o100 °C), mid temperature (100 °C o To400 °C), and high-temperature (T 4400 °C). The CSP systems usually work at mid- and high-temperatures. Under this working condition, a high solar absorptance and a low thermal emittance at high-temperatures are needed [3]. In recent years, scientists have devoted to obtaining superior selective solar absorbing thin films, and have been developed using various processes such as spray pyrolysis, multilayer cermet, oxidation, graded cermet and multilayer absorbers at many kinds of substrates [4–8]. Stainless steels (SS) are widely used in kitchen utensils, exhaust pipes, building materials and medical equipment…etc. And stainless steels are easy to get, cheap, heat-resisting and acidresistant. SS are also the main built materials of the centralized receiver (or power tower). Therefore, high-temperatures solar absorber on SS is an very important research [9]. The current coating technology for central receivers, Pyromark hightemperature paint, had a solar absorptance (α) in excess of 0.95 but a thermal emittance (ε) greater than 0.8, which results in large thermal losses at high-temperatures [10]. American Sandia National Laboratories developed film deposition methods onto stainless steel (SS304) coupons for the high-temperature solar absorbing techniques. Spinel oxides were chosen for their inherent high-temperature and oxidation stability and their amenability to doping and substitution of a large number of transition metal cations coated. The oxide spinel materials continue to show promise as intrinsic solar-selective absorptant materials. They displayed relatively high absorptivity (α) (0.90– 0.92) but their emittance values(ε) were still high (0.49–0.60) at 80 °C [11]. Furthermore, Sandia National Laboratories reviewed a variety of central receiver for concentrating solar power applications with high-temperature 4 650 °C requirement [12]. Some of the unique challenges associated with high-temperature receivers include the geometric designs, materials and processes that maximize solar irradiance and absorptance, and have high reliability of thermal cycles. In this study, a relatively simple and low-cost method was used to grow Fe2O3 films on stainless steel (SS304) substrates for solar absorber. It was the first time to use very high-temperatures (T 4800 °C) to grow Fe2O3 films on the applications of the solar absorbing techniques. The process of depositing Fe2O3 films was a feasible way to apply on different geometric substrates for concentrating solar power designs. The grown films and stainless steel substrates are very suitable to act the candidates in the hightemperature applications due to the temperature tolerance. In this paper, optical properties, microstructure of the films have been studied by using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectrophotometer (UV–visible).

2. Characterization of solar absorbers According to a recent report, among the various solar technologies, the CSP is primarily suited for larger scale installations, while photovoltaic-based technologies are better matched for smaller-scale. Solar thermal technologies have geographical limitations, and can potentially be economically viable only in regions that has high direct normal irradiation to ensure high energy yields. The performance of a candidate solar absorber can be characterized by its solar absorptance and thermal emittance. Using Kirchoff’s law, spectral absorptance can be expressed in terms of total reflectance ρ(λ,θ) for opaque materials, α(λ,θ) ¼  ρ(λ,θ) and ε(λ,T) ¼ α(λ,T), where ρ(λ,θ) is the sum of both collimated and diffuse reflectance, λ is the wavelength, θ is the incidence angle of light, and T is the given temperature. Development of spectrally

575

selective materials depends on reliable characterization of their optical properties. Emittance is a surface property and depends on the surface condition of the material, including the surface roughness, surface films, and oxide layers [13]. Coatings typically replicate to some degree the surface roughness of the substrate. Therefore in order to facilitate development it is important to measure the emittance of each coating–substrate combination as well as the uncoated substrate when developing a solar selective coating. Furthermore, selective coatings would degrade at hightemperatures because of thermal load (oxidation), high humidity or water condensation on the absorber surface (hydratization and hydrolysis), atmospheric corrosion (pollution), diffusion processes (interlayer substitution), chemical reactions, and poor interlayer adhesion [14]. Calculating the emittance from spectral data taken at room temperature assumes that the spectral characteristics do not change with increasing temperature. This is only valid if the material is invariant and does not undergo a phase change (as do some titanium containing materials), breakdown or undergo oxidation (as do paints and some oxide coatings) at higher temperatures. It is important before using high-temperature emittance calculated from room temperature data that the calculated data is verified with high-temperature emittance measurements for each selective coating. The key for high-temperature usage is low ε, because the thermal radiative losses of the absorbers increase proportionally by the fourth power of temperature; therefore, it is important to measure the emittance at the operating temperatures and conditions [15]. In addition to the initial efficiency, long term stability is also an important requirement for absorber coatings. At high-temperatures, thermal emittance is the dominant source of losses, and the requirement of low emittance often leads to complex designs that are frequently susceptible to degradation at the working temperature.

3. Recent developments in high-temperature solar absorbers To identify potential high-temperature absorbers, the literature was reviewed for medium- to high-temperature absorber coatings [3]. Several materials have the appropriate optical properties and should be durable at operating temperatures above 500 °C. Selective absorber surface coatings can be categorized into six distinct types: (a) intrinsic, (b) semiconductor-metal tandems, (c) multilayer absorbers, (d) multi-dielectric composite coatings, and (e) textured surfaces, presented in Table 1. Absorber Materials Various transition metals – particularly those formed from the refractory metals of groups IVA, VA, and VIA and their binary and ternary compounds—have been suggested for high-temperature applications because of their high melting point and chemical inertness [16]. The titanium, zirconium, or hafnium metal boride, carbide, oxide, nitride, and silicide materials have some of the highest melting points in nature, with HfC having the highest melting point at 3316 °C. These materials also have a high degree of spectral selectivity, high hardness, improved wear, corrosion, and oxidation resistance [17,18]. A double-cermet film structure has been developed that has higher photothermal conversion efficiency than surfaces using a homogen cermet layer or a graded film structure [19]. Surface texturing is a common technique to obtain spectral selectivity by the optical trapping of solar energy. The emittance can be adjusted (higher or lower) by modifying the microstructure (microcrystallites) of the coatings with ion-beam treatments [20]. However, there is a trade-off between a highly absorbing coating and one with low emittance. Highly absorbing coatings appear rough, porous, and absorb solar energy; coatings with low emittance are very smooth, dense, highly reflective, and mirror-like to thermal energy. Combining several concepts, a hightemperature solar-selective coating could be developed from

[28]

[27]

[26]

[24,25]

Research in intrinsic absorbers has not been very productive because there are no ideal intrinsic materials Semiconductor films of high porosity or antireflection coatings are needed Computer modeling can easily compute the optical properties given by an optimum multilayer design of candidate materials The solar absorptance can be boosted with a suitable choice of substrates and AR layers Textured surfaces were needle-like, dendritic, or porous microstructure. It is structurally stable and finding increasing use as a component in high-temperature absorber Si, Ge, and PbS Absorb short-wavelength radiation, and the underlying metal provides low emittance Using multiple reflections between layers to absorb light and Using different metals (e.g., Mo, Ag, Cu, Ni) can be tailored to be efficient selective absorbers. and dielectric layers (e.g., Al2O3, SiO2,) Alumina layer on metal (e.g., Ni, V, Cr, Co, Cu, To offer a high degree of flexibility, and the solar selectivity Mo, Ag) To obtain high solar absorptance Si0.8Ge0.2

materials with intrinsic solar selectivity and high-temperature stability using multiple cermet layers, along with appropriate surface texturing and incorporating multiple antireflective (AR) coatings. The optical properties of the refractory metal compounds have a high degree of flexibility; with further research, multiplelayer cermets with noble metals could be viable hightemperature absorbers for the CSP program. At this point, none of the existing commercial coatings have proven to be stable in air at 400 °C. Achieving the goal for a solarselective coating that is stable in air at temperatures greater than 450 °C requires high thermal and structural stabilities for both the combined and individual layers, excellent adhesion between the substrate and adjacent layers, suitable texture to drive the nucleation and subsequent growth of layers with desired morphology, enhanced resistance to thermal and mechanical stresses, and acceptable thermal and electrical conductivities. Other desirable properties are good continuity and conformability over the tube, as well as with the oxidation product, including any secondary phases present, over long periods of time at elevated temperature. Selecting materials with elevated melting points and large negative free energies of formation can meet these objectives. Stable nanocrystalline or amorphous materials are the most desirable (and practical) for diffusion-barrier applications, especially in light of material and process limitations. However, there will be a trade-off in the microstructure between a highly oxidation resistant coating (i.e., amorphous or nanocrystalline) and a solar-selective coating with both high absorption (i.e., columnar or porous microstructure) and low emittance (i.e., smooth or highly dense). High thermal stability is manifested by high melting points, single-compound formation, and lack of phase transformations at elevated temperature. Incorporating improved AR coatings, cermets, and texturing the surface should further improve the solar-selective coating; however, trade-offs exist between simultaneously obtaining both low emittance and high absorptance. There is significant uncertainty regarding the real property values of the modeled selective coatings, and the key issue is making the coating and testing its actual properties.

4. The oxidation process on stainless steel Specular stainless steel (SS304) coupons (dimension: 58 mm  58 mm  1.2 mm) were used to the experimental substrates in this study. 304 stainless steel is an austenitic alloy with 18–20 wt% Cr, 8– 10 wt% Ni and balance Fe. Those substrates were cleaned by the ultrasonic cleaning with acetone, isopropanol and ultrapure water for 10 min at first. All the substrates were thermal oxidized to grow Fe2O3

Multi-dielectric composite coatings Textured surfaces

Multilayer absorbers

Semiconductor–metal tandems

W,HfC,TaB2,In2O3, V2O5,LaB6 Intrinsic

Challenges/research needs Benefits Materials Types

Table 1 Summary of Mid- to High-Temperature Solar Selective Absorber Materials.

[21–23]

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Refs

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Fig. 1. X-ray diffraction patterns of stainless steel (SS304) at different temperatures.

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films in the high-temperatures furnace in air. Those thermal temperatures were 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C for 2 h. The surface roughness was measured by a Veeco Dektak 6M Manual αstep surface profiler. The crystalline structure was measured by a Panalytical X’Pert Pro X-ray diffraction. The copper anticathode of the diffractometer presents a λKα(Cu) wavelength equal to 0.154184 nm with the 2θ angles in the range 30–70° in the scanning step of 0.02°. The surface microstructure was measured by a HITACHI SU8000 ultrahigh resolution scanning electron microscope. The optic properties of reflective spectra was measured by a Jasco V-760 UV–vis spectrophotometer in the wavelength range of 0.25–2.5 μm. The emittance was measured by Devices & Services Co. Emissometer Model AE1 at 80 °C.

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The XRD patterns of those experimental samples at different temperatures are shown in Fig. 1. The XRD pattern at 25 °C showed the crystalline structure of the SUS304 stainless steel coupons. The XRD pattern at 850 °C showed the relatively strong (104) plane, the weak (024) and the weak (116) planes. The XRD pattern at 900 °C showed the intensity of (104) plane became weaker. That

were also found the weak (110) and (113) planes appeared and the weak (024) and (116) planes still exited. The XRD pattern at 950 °C showed the relatively strong (110) plane, the weak (104), (113), (024), (116) planes. That were also found the weak (214) and (300) planes appeared. The XRD pattern at 1000 °C is similar to the XRD pattern at 950 °C. The intensity of (110) plane became stronger. The XRD pattern at 1050 °C showed a weak (110) plane and the other planes were almost disappeared. Peaks centered at 2θ ¼33.2, 35.6, 40.8, 49.7, 54.2, 62.7 and 64.1 correspond to (104), (110), (113), (024), (116), (214) and (300) planes, respectively of rhombohedral-hematite Fe2O3 [JCPDS, PDF-33-0664]. This clearly indicates the formation of Fe2O3 in the films prepared from 850 to 1050 °C in present work. The SEM surface morphology of those Fe2O3 films at 850– 1050 °C was very different as shown in Figs. 2–6. For the Fe2O3 films at 850 °C, there were full of irregular grains regions and irregular cracks as shown in Fig. 2(a), and the grain size was small as shown in Fig. 2(b). For the Fe2O3 films at 900 °C, Fig. 3(a) was similar to Fig. 2(a), but the grains regions were much denser than the grains regions at 850 °C. The grain size in Fig. 3(b) was bigger than the grain size at 850 °C and rhombus grains were also observed. In Fig. 4(a), it was found that the stand rose [6] on the surface of films at 950 °C. The stand rose structure was composite of complex rhombus grains as shown in Fig. 4(b). The Fe2O3 films at 1000 °C, the oxidized surface produced a morphology of flower

Fig. 2. Surface morphology of Fe2O3/SS304 at 850 °C (a) magnification of  2000 (b) magnification of  15,000.

Fig. 3. Surface morphology of Fe2O3/SS304 at 900 °C (a) magnification of  2000 (b) magnification of  15,000.

5. Iron oxide 5.1. Structural characterization

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Fig. 4. Surface morphology of Fe2O3/SS304 at 950 °C (a) magnification of  2000 (b) magnification of  15,000.

Fig. 5. Surface morphology of Fe2O3/SS304 at 1000 °C (a) magnification of  2000 (b) magnification of  15,000.

(Fig. 5(a)), and the surface of flower were full with holes (Fig. 5(b)). We found the flower becomes larger than the ones at 1050 °C as shown in Fig. 6(a). It was because the temperature was too high, the melting phenomenon occurred at the edge of the flower (Fig. 6 (b)). The roughness of the top surface was another important factor to affect the absorptance of sun lights. We also measured the surface roughness of those experimental samples and the data is shown in Fig. 7. It was found the surface roughness of specular SS was 75.5 nm. After oxidation process, the surface roughness of SS substrates increased as the working temperatures increased. The surface roughness at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C were 118.1 nm, 114.3 nm, 431.2 nm, 657.1 nm and 2954.4 nm respectively.

total normal emissivity and absorptivity were plotted as a function of different temperatures as shown in Fig. 9. The emissivity increased as the oxidation temperature increased. The absorptivities of the samples at 850 °C, 900 °C, 950°C, 1000 °C and 1050 °C were 0.848, 0.909, 0.917, 0.922 and 0.917 respectively. The emissivities of the samples at 850 °C, 900 °C, 950 °C,100 °C and 1050 °C were 0.13, 0.18, 0.25, 0.38 and 0.44 respectively. We considered the flower surface morphology was a key to enhance the absorptivity. Those Fe2O3 films on SS304 substrates at (900–1000 °C) displayed relatively high absorptivity (α) (0.909–0.922) and their emittance values(ε) are relatively low (0.18–0.38). Ambrosini et al. demonstrated relatively high absorptivity (α) (0.90–0.92) and their emittance values(ε) are still high (0.49–0.60) [11]. In this research, those properties are better than the ones of ref. [11]. Comparison of the high-temperature solar absorbers were prepared on 304 stainless steel (304SS) between the literatures and this work in Table 2. Developing spectrally selective materials also depends on reliable characterization of their composition, morphology, and physical and optical properties. The key for hightemperature usage is low emittance (ε). The protocols are being developed and the capabilities built for accurate and precise measurements of the thermal/optic properties of the selective coating at this study. As data become available, development can begin of a criterion for high-temperature selective surfaces applicable for concentrating applications.

5.2. Optical properties The absorptivity of the Fe2O3/SS304 absorber was derived from the measured normal reflectivity curve of a standard black sample. Fig. 8 gave the spectral results of the Fe2O3/SS304 under different oxidized temperature at 850–1050 °C. It was found that the reflectivity line at 300–500 nm declined, because the α-Fe2O3 energy band gap is 2.3 eV. The reflectivity line of the Fe2O3/SS304 at 1050 °C in visible region is high, because the oxidized surface films were took off, but it was unaffected in infrared region. The

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Fig. 8. Reflectance spectra of Fe2O3/SS304 at different temperatures.

Fig. 9. Absorptivity and emissivity of the Fe2O3/SS304 at different temperatures.

Fig. 6. Surface morphology of Fe2O3/SS304 at 1050 °C (a) magnification of  2000 (b) magnification of  15,000.

Fig. 7. Surface roughness of Fe2O3/SS304 at different temperatures.

6. Conclusion We proposed to utilize thermal oxidation synthesis techniques to prepare intrinsic solar absorbers for use in high-temperature. Ideal absorbers have high solar absorptance ( 40.95) in the visible region and low thermal emittance (o0.05) in the IR region, be stable in air, and be low-cost and readily manufacturable. This

Table 2 Comparison of the high-temperature solar absorbers were prepared on 304 stainless steel (304SS) between the literatures and this work. Process method

Materials Thermal curing (°C)

Absorptance (α) Emittance (ε) Refs

Dip coating Spin coating Spin coating Spin coating Spin coating Spin coating Spin coating Thermal Spray Coatings Thermal oxidation Thermal oxidation

Co3O4 CoFe2O4 CuFe2O4 Co3O4 NiFe2O4 NiCo2O4 FeCo2O4 WC-Co

400 600 600 800 800 800 800 41000

0.88 0.83 0.79 0.94 0.82 0.83 0.88 0.85

0.76 0.52 0.57 0.82 0.56 0.46 0.59 0.84–0.94

[11] [11] [11] [11] [11] [11] [11] [11]

Fe2O3

900

0.91

0.18

Fe2O3

1000

0.92

0.38

This study This study

paper has revealed a relatively low-cost method to grow Fe2O3 films using thermal oxidation on SS304 substrates for hightemperature solar absorber. This method using very hightemperatures (850–1050 °C) to grow Fe2O3 films for the applications on the high-temperature (T 4400 °C) solar absorbing techniques and the process to grow Fe2O3 films was a feasible way to apply on different geometric substrates for concentrating solar power designs. Those Fe2O3 films on SS304 substrates at (900– 1000 °C) displayed relatively high absorptivity (α) (0.909–0.922)

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and their emittance values (ε) are relatively low (0.18–0.38). Oxidation of stainless steel at high-temperatures is a simple, low-cost, large-area and effective technique for the high-temperature solar absorber. The grown films and stainless steel substrates are very suitable to act the candidates in the high-temperature applications due to the temperature tolerance. Table 2 provides a comparison of the high-temperature solar absorbers were prepared on 304 stainless steel (304SS) between the literatures and this work. The iron oxide has shown promise as a good solar selective absorber after a viable high-temperature solar-selective coating is demonstrated by thermal oxidation.

[11]

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[16]

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