Structure, optical properties and thermal stability of SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber

Abstract

A tandem layer structured SS/TiC–ZrC/Al₂O₃ coating has been prepared by magnetron sputtering as a high temperature spectrally selective solar absorber. The coating consists of two layers; a TiC–ZrC layer and an Al₂O₃ layer. TiC and ZrC ceramics show inherent spectral selectivity, which is used to design spectrally selective solar absorbers. A co-sputtering technique was used to fabricate the absorptance layer (TiC–ZrC). The coating exhibits a high absorbance of 0.92 and a low emittance of 0.11, as well as good thermal stability with a selectivity of 0.92/0.13 even after annealing at 700 °C for 100 h under vacuum. The SS/TiC–ZrC/Al₂O₃ coating also shows good thermal stability at 400 °C for 5 h in air. The surface morphology, composition, structure and optical properties of the coating were characterized using SEM, XPS, XRD, Raman spectroscopy, UV-vis-NIR spectrophotometry and Fourier transform infrared spectroscopy. Benefiting from these advantageous features, the as-deposited SS/TiC–ZrC/Al₂O₃ coating can be a good candidate for concentrated solar power applications.

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1. Introduction

Concentrated Solar Power (CSP) is considered to be an environmentally friendly and sustainable technology both for electricity and solar fuels. A spectrally selective solar absorber, as a critical component for CSP, should capture maximum solar energy in the solar spectrum region of 0.3–2.5 μm and should have minimum emittance in the infrared region of 2.5–25 μm.^1–3 For practical applications, a solar absorber should have long term thermal stability at high temperatures under vacuum in order to increase the Carnot efficiency of the power generation system. However, most conventional solar absorber materials cannot withstand high temperatures and start degrading due to their poor thermal stability. It is still an urgent requirement to develop solar absorber coating that is stable at >700 °C.

In recent years, transition metal nitride, carbide and oxynitride coatings have attracted considerable research interest due to their interesting mechanical, chemical, electrical and optical properties. Various tandem and multilayer absorber coatings have been developed such as: TiAlN/TiAlON/Si₃N₄,⁴ NbAlN/NbAlON/Si₃N₄,⁵ HfMoN/HfON/Al₂O₃,⁶ Ti/AlTiN/AlTiON/AlTiO,⁷ Fe₃O₄/Mo/TiZrN/TiZrON/SiON,⁸ Ti_0.5Al_0.5N/Ti_0.25Al_0.75N/AlN,⁹ Cu/TiAlCrN/TiAlN/AlSiN¹⁰ and TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO.¹¹ However, little work has been devoted to solar absorber coatings based on composites of transition metal carbides.

Transition metal carbides, such as titanium carbide (TiC) and zirconium carbide (ZrC), whose melting points are both over 3000 °C, are typical members of the so-called ultra high temperature ceramic (UHTC) family. They are mainly employed in the aerospace industry and advanced energy systems, such as hypersonic vehicles, rocket motor nozzles, turbine blades, and nuclear fusion reactors.^12,13 These materials exhibit many favorable features, including high melting points, good wear resistance, mechanical strength, high thermal and electrical conductivity and good chemical stability. Most importantly, TiC and ZrC ceramics also show inherent spectral selectivity, which can be explained by two contributions from in-band (or Drude) and inter-band (or Lorentz) according to the rigid band model of the electronic structure of the atoms.¹⁴

In the present work, we chose TiC and ZrC ceramics to design and fabricate a high temperature spectrally selective solar absorber coating. In particular, a new tandem absorber consisting of TiC–ZrC and Al₂O₃ has been deposited on a stainless steel (SS) substrate, which exhibits an absorbance of 0.92 with an emittance of 0.11 at 82 °C and thermal stability up to 700 °C under vacuum for 100 h. The SS/TiC–ZrC/Al₂O₃ coating also shows good thermal stability at 400 °C for 5 h in air. Detailed characterization of the tandem absorber has been carried out using ultra-high resolution scanning electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-vis-NIR spectrophotometer, Fourier transform infrared spectroscopy, etc.

2. Experimental details

The coatings were deposited on stainless steel (SS) substrates (dimensions 50 mm × 50 mm) using a commercial magnetron sputtering system (Kurt J. Lesker). The SS substrates were metallographically polished to get a smooth surface and the substrates were ultrasonically cleaned in isopropyl alcohol and acetone to remove grease and dust particles on the surface of the substrates. A vacuum chamber was pumped down to a base pressure of 2.1 × 10⁻⁶ Torr. High purity TiC (99.99%), ZrC (99.99%) and Al₂O₃ (99.99%) targets (diameter = 76.2 mm) were used for deposition of the coatings. Ar was introduced inside the chamber at a pressure model of 3 mTorr. The target was cleaned using Ar plasma for 10 min before deposition. A co-sputtering technique was used to deposit the TiC–ZrC layer. Then, the antireflection layer Al₂O₃ was deposited onto the SS/TiC–ZrC coating. The complete deposition process was performed in an argon plasma environment at a pressure of 3 mTorr. The optimized process parameters for the deposition of the tandem absorber are listed in Table 1.

Table 1 Optimized process parameters for the deposition of the tandem layer SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber coating

Layer	Sputtering method	Ar flow rate (sccm)	Power density (W cm^−2)	Thickness (nm)	Substrate temperature (°C)	Operating pressure (Pa)
TiC	DC	33	6.58	55	200	1.03
ZrC	RF		1.09		200	1.03
Al2O3	RF	33	6.14	46	200	1.03

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The coatings deposited on stainless steel were heat treated in vacuum and air in a tubular furnace at different temperatures. The Pfeiffer turbo pump was used to generate high vacuum. The accuracy of the set temperature was ±1 °C. Annealing involved increasing the temperature at a slow rate of 5 °C min and maintaining the desired temperature for different times. Subsequently, the samples were naturally cooled down to room temperature.

The surface morphologies were observed by a ultra-high resolution scanning electron microscope (SU8200, Tokyo, Japan). The chemical bonding state and chemical composition of the coating were defined via X-ray photoelectron spectroscopy (XPS, equipped with a standard monochromatic AlKα source 1486.6 eV, ESCALAB 210, VG scientific Ltd., UK). The XPS binding energy data was calibrated with respect to the C1s signal of ambient hydrocarbons (C–H and C–C) centered at 284.8 eV. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2400/PC diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKα radiation (151.5406 Å). Changes in the chemical composition of the solar selective coating as a result of heating were measured using Raman spectroscopy (DILOR-JOBIN-YVON-SPEX).

Reflectance spectra in the wavelength interval 0.3–2.5 μm 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 2.5–25 μm 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.

The measurements were combined to create one spectrum and the normal α_s and ε_T values were calculated using eqn (1) and (2). Solar absorbance α_s is theoretically defined as a weighted fraction between absorbed radiation and incoming solar radiation, where λ is wavelength, R(λ) is reflectance and I_s(λ) is direct normal solar irradiance. It is defined according to ISO standard 9845-1, normal radiance, AM1.5.

Normal thermal emittance ε_T is equally a weighted fraction but between emitted radiation and Planck black body distribution, I_b(λ, T), at temperature T.

Accordingly, thermal emittance values of samples are denoted as ε₈₂ in this work, when these were obtained at 82 °C.

3. Results and discussion

3.1 Surface morphology

A schematic diagram of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coating deposited on stainless steel (SS) is shown in Fig. 1. The tandem coating consists of TiC–ZrC and Al₂O₃ layers, whose thicknesses are 55 and 46 nm, respectively. Firstly, the TiC–ZrC layer was deposited onto the SS substrate using a co-sputtering technique. Through the adjustment of sputtering power, deposition pressure and deposition time, different TiC–ZrC coatings were fabricated. According to the absorbance and emittance values, the optimized process parameters were fixed. Then, an Al₂O₃ antireflection layer is deposited onto the surface of the SS/TiC–ZrC layer. The morphology and structure of the SS/TiC–ZrC/Al₂O₃ tandem spectrally selective solar absorber were examined by ultra-high resolution scanning electron microscopy. As shown in Fig. 2a, a typical FESEM image indicates that the absorber surface is dense, uniform, homogeneous, and fine-grained. The cross-sectional scanning electron microscopy micrograph of the tandem absorber is shown in Fig. 2b.

Fig. 1 Schematic diagram of the SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber coating.

Fig. 2 Surface (a) and cross-sectional (b) morphology of the SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber.

3.2 X-ray photoelectron spectroscopy

To investigate the chemical composition and chemical state of the various elements of the tandem absorber, individual layers were characterized by XPS. Fig. 3(a–c) shows the high resolution XPS core level spectra of the TiC–ZrC layer. As shown in Fig. 3a, two groups of doublets of Zr3d can be detected in the XPS fitted spectrum, which correspond to ZrC bonds (179.1 eV and 181.5 eV) and ZrO₂ bonds (182.4. eV and 184.8 eV).^15,16 The Ti2p core level spectrum consists of six peaks centered at 455.05, 455.8, 457, 458.5, 461.3 and 464.2 eV. The peaks at 455.05 and 455.8, and 461.3 eV, originate from Ti2p_3/2 and Ti2p_1/2 electrons in TiC, respectively, while the peaks at 457 and 458.5, and 464.2 eV, originate from Ti2p_3/2 and Ti2p_1/2 in TiO₂, respectively.^17,18 The carbon core level spectrum could be resolved into five peaks centered at binding energies of 281.9, 284.3, 284.7, 285.9 and 288.5 eV. The predominant peaks at 284.3 eV and 285.9 eV correspond to sp² C–C bonds and sp³ C–C bonds, respectively.¹⁹ The low intensity peak at 285.4 eV corresponds to C–O bonds.¹⁹ The peak at 281.9 eV corresponds to Ti–C and Zr–C bonds.^17,20 The intense peak at 284.7 eV indicates carbon contamination in the film.¹¹ The oxygen attached to titanium and zirconium may be due to the presence of residual O₂ and H₂O in the vacuum chamber. Fig. 3(d and e) show the high resolution XPS core level spectra of Al₂O₃ layer. The O1s spectrum shows three peaks at 530.7 eV, 531.7 eV and 532.3 eV. The peak located at a binding energy of 530.7 eV is assigned to crystal lattice oxygen (O–Al).²¹ The other two peaks at 531.7 and 532.3 eV are characteristic of Al₂O₃.^22,23 The Al2p spectrum shows a peak centered at a binding energy of 74.7 eV, which corresponds to Al₂O₃.²⁴ Further analysis of the XPS data indicates that the composition of the TiC–ZrC layer is 0.83 at% Zr, 11.58 at% Ti and 87.6 at% C.

Fig. 3 XPS spectra of (a) Zr3d, (b) Ti2p and (c) C1s for the TiC–ZrC coating and (d) O1s, and (e) Al2p for the Al₂O₃ coating.

3.3 Solar absorbance and emittance

With an view to investigating spectral properties, the layer-added film samples of SS/TiC–ZrC and SS/TiC–ZrC/Al₂O₃ were fabricated, respectively. Table 2 gives the spectral properties, including absorbance and emittance. A polished stainless steel (SS) substrate shows an absorbance of 0.36 and an emittance of 0.11. The single layer TiC–ZrC deposited on SS substrate exhibits an absorbance of 0.76 with an emittance of 0.12. When further adding the Al₂O₃ layer to the surface of SS/TiC–ZrC, the absorbance increases to 0.92 due to the antireflection effect, while the emittance remains at 0.11. The reflectance spectra of the layer-added SS/TiC–ZrC/Al₂O₃ tandem spectrally selective solar absorber is shown in Fig. 4. The SS/TiC–ZrC/Al₂O₃ tandem coating exhibits low reflectance in the wavelength region of 0.3–2.5 μm, and high reflectance in the region of 2.5–25 μm. It is worth noting that there are two lowest points in the reflectance spectra from 300 nm to 2500 nm for the as-deposited multilayer SS/TiC–ZrC/Al₂O₃ coating, which is similar to a traditional metal-dielectric composite solar absorber coating.²

Table 2 Absorbance and emittance of different layers of the SS/TiC–ZrC/Al₂O₃ coating

Material	α	ε	α/ε
SS	0.36	0.11	3.27
SS/TiC–ZrC	0.76	0.12	6.33
SS/TiC–ZrC/Al2O3	0.92	0.11	8.36

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Fig. 4 The reflectance spectra of the layer-added coating samples of (a) SS, (b) SS/TiC–ZrC and (c) SS/TiC–ZrC/Al₂O₃.

3.4 Thermal stability in vacuum

3.4.1 Surface morphology. In order to investigate thermal stability, the SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber was subjected to heat-treatment under vacuum at temperatures in the range of 600–900 °C for 5 h. Fig. 5 shows the surface morphologies of the post-annealed SS/TiC–ZrC/Al₂O₃ tandem solar absorber coating. Compared to the dense and homogeneous structure of the as-deposited sample shown in Fig. 2a, the surface structure of the post-annealed samples does not show a significant change with increasing the annealing temperature from 600 to 800 °C. When the samples are heated under vacuum at 900 °C for 5 h, the grain boundary becomes indistinct and forms a more dense structure.

Fig. 5 Surface morphology of the SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber after annealing at different temperatures: (a) 600 °C, (b) 700 °C, (c) 800 °C, (d) 900 °C for 5 h under vacuum.

3.4.2 XRD analysis. Fig. 6 shows XRD patterns of the as-deposited and post-annealed SS/TiC–ZrC/Al₂O₃ coating. It is worth noting that the prominent diffraction peaks of the as-deposited films can be assigned to stainless steel.⁸ After annealing at 600 °C, 700 °C, 800 °C and 900 °C for 5 h, there are also no other peaks observed except for that of SS, suggesting a dominant amorphous or poorly crystallized microstructure of the tandem solar absorber.

Fig. 6 XRD pattern of (a) the as-deposited SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber and after annealing at different temperatures in vacuum for 5 h: (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C.

3.4.3 Thermal stability. Thermal stability is an important evaluation parameter for the high temperature spectrally selective solar absorber coating. Increasing the operating temperature of CSP will increase the Carnot efficiency and reduce the cost of electricity. To get better thermal stability, the individual layers of the coating should have elevated melting points, large negative free energies of formation, and a lack of phase transformation at elevated temperature. The whole coating should have stable nanocrystalline or amorphous materials. The thermal stability of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings is studied by annealing the coatings under vacuum at temperatures in the range of 600–900 °C for 5 h. Fig. 7 shows the reflectance spectra of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber annealed at different temperature for 5 h under vacuum. The corresponding absorbance and emittance values of the heat-treated tandem absorber are listed in Table 3. It is worth noting that the coatings are thermally stable up to 700 °C for 5 h. At 900 °C, it is found that the absorbance (Δα = −0.14) and emittance values (Δε = −0.04) and reflectance spectra of the coatings change significantly, indicating the degradation of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings. The exact reason for the degradation of the tandem absorber at 900 °C under vacuum is not known and may be attributed to the partial delamination of the coating because of different thermal expansion coefficients of the substrate and the coating. Long term thermal stability is also important for the use of the solar absorber material in concentrated solar thermal applications. The SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings are heat treated under vacuum at 600 °C, 700 °C and 800 °C for 100 h, respectively. Fig. 8 shows the reflectance spectra of the as-deposited and annealed coatings after 100 h. The absorbance and emittance values are shown in Table 4. It is evident from the table that the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coating has good thermal stability at 700 °C.

Fig. 7 Reflectance spectra of the (a) as-deposited SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber and after annealing at different temperatures under vacuum for 5 h: (b) 600 °C, (c) 700 °C, (d) 800 °C and (e) 900 °C.

Table 3 Effect of 5 h annealing (in vacuum) on the absorbance and emittance values of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings

Temperature (°C)	α			ε
	As-deposited	Annealed	Δα	As-deposited	Annealed	Δε
600	0.92	0.92	0	0.11	0.11	0
700	0.92	0.92	0	0.11	0.11	0
800	0.92	0.92	0	0.11	0.13	0.02
900	0.92	0.78	−0.14	0.12	0.08	−0.04

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Fig. 8 Reflectance spectra of the (a) as-deposited SS/TiC–ZrC/Al₂O₃ spectrally selective solar absorber and after annealing at different temperatures under vacuum for 100 h: (b) 600 °C, (c) 700 °C and (d) 800 °C.

Table 4 Effect of 100 h annealing (under vacuum) on the absorbance and emittance values of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings

Temperature (°C)	α			ε
	As-deposited	Annealed	Δα	As-deposited	Annealed	Δε
600	0.92	0.92	0	0.11	0.10	−0.1
700	0.92	0.92	0	0.11	0.13	0.02
800	0.92	0.86	−0.06	0.11	0.16	0.05

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3.4.4 Raman analysis. In order to further investigate the microstructure, the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings were studied using Raman spectroscopy as shown in Fig. 9. The as-deposited SS/TiC–ZrC/Al₂O₃ coating exhibits four rather broad bands centered at approximately 221, 630, 1376 and 1564 cm⁻¹. The peaks at around 221 and 630 cm⁻¹ are characteristic of TiC Raman features.^25,26 The other two peaks are typical features of amorphous carbon, namely the D peak located at 1376 cm⁻¹ and the G peak located at around 1564 cm⁻¹. The G peak originates from bond stretching of all pairs of sp³ atoms in both rings and chains, and the D peak is caused by the breathing modes of sp² atoms in rings.^27,28 No obvious band shift is observed in the annealed coating up to 700 °C. At 800 °C, the intensities of the D peak and G peak drastically decrease and new peaks at 281 and 346 cm⁻¹ appear, which correspond to ZrC and TiC, respectively.²⁹ The sp²/sp³ ratio can be characterized by the relative ratio of D peak intensity to G peak intensity (I_D/I_G). It is worth noting that with an increase in annealing temperature, the ratio of I_D/I_G increases, especially at 800 °C. The relatively high content of sp² bonds represents the high graphitization degree of the coatings, which will result in the degradation of optical properties.³⁰

Fig. 9 Composite Raman spectra of: (a) the as-deposited SS/TiC–ZrC/Al₂O₃ tandem absorber and the coating heat treated at (b) 600 °C, (c) 700 °C and (d) 800 °C for 5 h under vacuum.

3.5 Thermal stability in air

In addition, we also studied the thermal stability of the as-deposited samples in air at 300 °C, 400 °C, 500 °C and 600 °C for 5 h. The SS/TiC–ZrC/Al₂O₃ coating delaminates completely at temperatures greater than 600 °C in air. The absorbance and emittance values of the heat-treated coatings are listed in Table 5. It is clear from Table 5 that for the SS/TiC–ZrC/Al₂O₃ coating, the absorbance and the emittance values do not change significantly even after heat treatment at 400 °C for 5 h. At 500 °C, the absorbance value decreases (Δα = −0.04) significantly. Therefore, the SS/TiC–ZrC/Al₂O₃ coating is thermally stable in air up to 400 °C. Fig. 10 shows the Raman spectra of the as-deposited and annealed coatings in air at 300 °C, 400 °C and 500 °C for 5 h. Clearly, with the increased annealing temperature in air, the ratio of I_D/I_G increases, particularly at 500 °C. The increased sp² bonds means a high graphitization degree of the coatings, indicating the degradation of optical properties.³⁰

Table 5 Effect of 5 h annealing (in air) on the absorbance and emittance values of the SS/TiC–ZrC/Al₂O₃ tandem solar absorber coatings

Temperature (°C)	α			ε
	As-deposited	Annealed	Δα	As-deposited	Annealed	Δε
300	0.92	0.92	0	0.11	0.11	0
400	0.92	0.92	0	0.11	0.11	0
500	0.92	0.88	−0.04	0.11	0.11	0

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Fig. 10 Composite Raman spectra of: (a) the as-deposited SS/TiC–ZrC/Al₂O₃ tandem absorber and the coating heat treated at (b) 300 °C, (c) 400 °C and (d) 500 °C for 5 h in air.

4. Conclusions

We have developed a TiC–ZrC/Al₂O₃ tandem solar absorber on a SS substrate with a high solar absorbance of 0.92 and a low thermal emittance of 0.11. The atomic bonding structures of the individual layers were investigated via XPS analysis. Thermal stability of the SS/TiC–ZrC/Al₂O₃ was investigated for different temperatures in vacuum and air. The coating shows high thermal stability up to 700 °C under vacuum for 100 h with a solar selectivity of 0.92/0.13. The SS/TiC–ZrC/Al₂O₃ coating also shows good thermal stability at 400 °C for 5 h in air. The combined spectral selectivity and high temperature stability demonstrated in our work make a contribution to the applications of transition metal carbides as a high temperature spectrally selective solar absorber coating.

Acknowledgements

This work was financially supported by NSFC (51402315).

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