Available online at www.sciencedirect.com www.elsevier.com/locate/solener ScienceDirect Solar Energy 111 (2015) 350–356 The spectral selective absorbing characteristics and thermal stability of SS/TiAlN/TiAlSiN/Si3N4 tandem absorber prepared by magnetron sputtering Junxiao Feng a,b, Shuo Zhang a, Yu Lu a,c,d, Hongwen Yu d, Limin Kang a, Xianyang Wang a, Zongming Liu c, Haicheng Ding c,d, Yun Tian a, Jun Ouyang a,b,⇑ aKey Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China bKey Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China cSchool of Materials Science and Engineering, Jinan University, Jinan 250022, China dShandong Solar Energy Corp., Jinan 250014, China Received 1 August 2014; received in revised form 4 November 2014; accepted 4 November 2014 Communicated by: Associate Editor Antoine Bittar Abstract Spectrally selective TiAlN/TiAlSiN/Si3N4 multilayer coatings were deposited on stainless steel (SS) plate using a reactive direct cur- rent magnetron sputtering technique. In this tandem absorber system, TiAlN, TiAlSiN and Si3N4 act as the main absorbing layer, the semi-absorbing layer and the antireflection layer, respectively. An average absorptance (a) of 0.938 and emittance (e) of 0.099 were achieved in coatings prepared under optimized conditions, which exhibit a fine-grained morphology and an amorphous microstructure as evidenced by SEM and XRD analysis, respectively. Absorptance and emittance data, as well as Raman spectra were collected for coatings exposed to different levels of thermal stresses (2 h in air @200 �C, 300 �C, 400 �C, 500 �C, 600 �C). These heat treated coatings showed negligible to small degradations in their selective absorbing capabilities as compared with the as-grown ones. There was no sig- nificant change of the coatings in their morphology or composition after the heat treatments, as evidenced by the SEM and Raman spec- tra analysis, respectively. The spectral selectivity of the coatings remained stable after a heat-treatment at 272 �C in air for 300 h. � 2014 Elsevier Ltd. All rights reserved. Keywords: Solar selective absorbing coating (SSAC); TiAlN/TiAlSiN/Si3N4; Spectral properties; Thermal stability 1. Introduction The solar selective absorbing coatings (SSACs) used in the solar heat collectors incident and convert solar radia- tion into thermal energy. A good SSAC should have both a high absorptance (a P 0.90) in the wavelength range of http://dx.doi.org/10.1016/j.solener.2014.11.005 0038-092X/� 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China. Tel.: +86 531 88392439. E-mail address:
[email protected] (J. Ouyang). 0.3–2.5 lm and a low emittance (e 6 0.10) in the infrared region (k > 2.5 lm) (Karthick Kumar et al., 2013; Soum- Glaude et al., 2013; Chang et al., 2013). Among the different techniques to prepare SSAC, mag- netron sputtering is the most popular one in both research labs and industry. Based on the types of heat collectors, two fabrication routes have been proposed and broadly accepted. One is to coat a glass tube (the “inner tube”) with tandem absorber layers in a vacuum chamber configured with multiple rotational cylindrical targets, followed by packaging with a second glass tube (the “outer tube”), http://dx.doi.org/10.1016/j.solener.2014.11.005 mailto:
[email protected] http://dx.doi.org/10.1016/j.solener.2014.11.005 http://crossmark.crossref.org/dialog/?doi=10.1016/j.solener.2014.11.005&domain=pdf 1 Reflectances are measured in a wavelength range of 0.3–2.4 lm, and absorptances are computed from the reflectance spectra. This range or similar wavelength ranges (0.25–2.5 lm, etc.) are commonly used in engineering practices in solar water heater industry and adopted by most researchers in this field. Please see references (Karthick Kumar et al., 2013; Liu et al., 2012; Du et al., 2011). The absorption values we measured are not exact values, but good approximations of them (just like most of the data reported in literature). We chose this commonly used wavelength range in order to make a fair comparison of our data on selective absorbing properties with those reported in literature. J. Feng et al. / Solar Energy 111 (2015) 350–356 351 degassing and assembling to make a solar heat collector of the “vacuum-tube” type (Zhang, 2000). The other is to coat flat-panel substrates (usually Al, Cu or SS) continuously in large area horizontal in-line architectural vacuum coaters, followed by assembling with tube arrays, transparent cover and insulating materials, to make a solar heat collector of the “flat-panel” type (Zhiqiang, 2005). The “vacuum tube” solar heat collectors have dominated the Chinese market of solar water heaters, due to the ease of fabrication and assembly since its introduction in China in the mid-80s. However, with the development of urbanization, the flat- panel solar heat collector emerged as a better choice of green heat energy, due to its better safety, higher efficiency, longer life and broader applications, as compared with the “vacuum tube” heat collector. A variety of tandem coatings were studied and reported in literature for the applications of SSAC, including M- AlN (Zhang, 1998) AlNi–Al2O3 (Xue et al., 2013), TiAlN/TiAlON/Si3N4 (Barshilia et al., 2006), TiAlSiN/ TiAlSiON/SiO2 (Rebouta et al., 2012), TiAlN/AlON (Barshilia et al., 2008a), etc. These multilayers consist of ceramic-like top layers and metal-like bottom layers with gradient layers grown in between. A graded optical refrac- tivity is characteristic of such multilayers and accounts for their selective absorbing capability of solar radiation (Zhang, 2001). Among these multilayer coatings, the most-widely used two systems for the “vacuum tube” and “flat-panel” solar water heaters are AlN/Al–N/Al and TiO/TiON/TiN, respectively. While both materials have been commercially produced, the AlN/Al–N/Al coatings have dominated the Chinese market of solar water heaters for decades (Zhang, 1998), making it difficult for the TiON coatings to grow out of competition for resources and equipment. In China, diffusion pumps are used in most of the SSAC sputtering processes. Such a tooling choice is based on the oxygen-free composition of the AlN/Al–N/Al system and the lower cost of the diffusion pump than that of a turbo- molecular one. Therefore, introduction of oxide or oxynit- ride coatings into the existing production lines will raise concerns on oxidation of the oil in diffusion pumps, which will reduce its pumping speed and service life, hence creat- ing severe adverse effects on the product quality and pro- duction efficiency. In this work, we prepare oxygen-free TiAlN/TiAlSiN/ Si3N4 tandem absorber coatings under current tooling con- straints, in order to help accelerate the transition to “flat plate” of the Chinese solar water heater industry. We focus on the middle- to high-temperature thermal stability char- acteristics as well as regular selective absorbing properties of the coatings, under the performance requirements of a flat plate solar heat collector used in middle- to low-tem- perature ranges. In the proposed SSAC structure, TiAlN acts as the main absorber layer, TiAlSiN acts as the semi-absorber layer and Si3N4 acts as the antireflection layer. TiAlN and TiAlSiN coatings have already attracted much attention from researchers due to the excellent high temperature stability and low friction coefficients (Münz, 1986; Zhu et al., 2012; Zou et al., 2011; Zhang et al., 2011; Dobrzański et al., 2004; Wang et al., 2011; Savkova and Blahova, 2011). Con- sequently, they have been mainly used as hard coatings. While TiAlN had been already used as the main absorber layer and diffusion barrier layer in SSACs (Barshilia et al., 2008a,b), TiAlSiN and TiAlSiN/TiAlN coatings were rarely investigated for SSAC applications. In this paper, we pres- ent the optical properties (absorptance a and emittance e) of TiAlN/TiAlSiN/Si3N4 tandem absorbers heat-treated in air at different temperatures. Shimadzu UV-3600 spectro- photometer (measurement and data extraction configured in a wavelength range of 0.3–2.4 lm),1 Calorimetric emiss- ometer, X-ray diffractometer (XRD), scanning electron microscope (SEM) and micro-Raman spectrometer were used to characterize the tandem absorbing coatings. 2. Experimental details TiAlN/TiAlSiN/Si3N4 tandem absorber coatings were deposited on SS substrate using the reactive direct current magnetron sputtering technique. Ti–Al (50/50) and Si tar- gets with a diameter of 75 mm and a 4N purity were used for sputtering deposition of the coatings. Before being loaded into the deposition chamber, the SS substrates were ultrasonically cleaned for 15 min in acetone and ethanol baths, respectively. A base pressure of 5 � 10�4 Pa was achieved in the deposition chamber and the substrates were pre-sputtered in-situ in an argon plasma for 15 min under a DC bias of �600 V. The substrate holder can be rotated in the chamber to sequentially deposit each layer of the SSAC. All layers were sputtered in a mixed Ar/N2 atmo- sphere with a fixed Ar flow rate at 40 sccm, and the TiAl- SiN layer was deposited by co-sputtering of both targets. The sputtering parameters for the deposition of TiAlN/ TiAlSiN/Si3N4 are listed in Table 1. The reflectance spectra were measured by using a Shi- madzu UV-3600 spectrophotometer based on the reference spectrum AM 1.5, and then absorptances a were calculated from the reflectance spectra (Xue et al., 2013). The total hemispherical emittances e were measured by using a Calo- rimetric emissometer at 75 �C (Tang et al., 2008). Struc- tural analysis was conducted by using a commercial Rigaku Dmax-rc XRD diffractometer equipped with a Ni-filtered Cu Ka radiation source. Table 1 Processing parameters for the optimized TiAlN/TiAlSiN/Si3N4 tandem absorber. Layer Operating pressure (Pa) Operating temperature (�C) Substrate bias (V) N2 flow rate (sccm) Power (W) Deposition time (min) TiAlN 5.5 � 10�1 35 100 3 110 10 TiAlSiN 9.0 � 10�1 35 100 20 100 15 Si3N4 9.0 � 10�1 35 0 20 100 20 352 J. Feng et al. / Solar Energy 111 (2015) 350–356 In order to test the thermal stability, which is of great significance to a coating’s working lifetime (Cheng et al., 2013), the TiAlN/TiAlSiN/Si3N4 tandem absorbers depos- ited on SS substrates were heat treated in air in a resistive furnace at five different temperatures between 200 �C and 600 �C. Such a process was started at room temperature by heating up a sample at a rate of 10 �C/min, followed by a two-hour annealing of the sample at the temperature of choice, and then the sample was cooled down at a rate of 5 �C/min to room temperature (Barshilia et al., 2008a; Nuru et al., 2014). Surface morphologies and chemical compositions of the as-grown and heat-treated samples were analyzed by using scanning electron microscopy (SEM) and micro-Raman spectroscopy, respectively (Nuru et al., 2014). Optical properties of the annealed coat- ings were measured and compared with those of the as- deposited ones. Furthermore, we adapted the PC (Perfor- mance Criterion) nomenclature defined by the Solar Heat- ing and Cooling Program of the International Energy Agency to quantitatively evaluate thermal stability of the coating (Köhl et al., 2004). Based on the absorptance and emittance values of the as-grown coating, an annealing temperature of 272 �C was determined for extended hours of heat treatment (Brunold et al., 2000). 3. Results and discussions 3.1. Spectral properties of as-grown coatings The process parameters including target power density, N2 flow rate and layer thicknesses were optimized to obtain a TiAlN/TiAlSiN/Si3N4 tandem absorber with high absorptance and low emittance. The absorptance and emit- tance values for SS substrate, SS/TiAlN, SS/TiAlN/TiAl- SiN and SS/TiAlN/TiAlSiN/Si3N4 are given in Table 2. The absorptance increased by 0.479 and the emittance decreased by 0.118 after deposition of the main-absorber TiAlN layer on SS substrate. The absorptance increased further to 0.919 after adding the TiAlSiN semi-absorber layer and the overall emittance slightly increased to Table 2 Absorptance, emittance and solar selectivity of the layers of the optimized TiAlN/TiAlSiN/Si3N4 tandem absorber. Layer a e a/e SS 0.337 0.200 1.7 SS/TiAlN 0.816 0.082 9.9 SS/TiAlN/TiAlSiN 0.919 0.092 10.0 SS/TiAlN/TiAlSiN/Si3N4 0.945 0.094 10.0 0.092. Finally, after depositing the Si3N4 antireflective layer, the absorptance arrived at 0.945 and emittance reached 0.094. It is noted that a BaSO4 standard white plate was used for correction of the adsorption data. The reflectance spectra of SS substrate, SS/TiAlN, SS/TiAlN/ TiAlSiN and SS/TiAlN/TiAlSiN/Si3N4 are shown in Fig. 1. 3.2. XRD analysis XRD analysis was used to detect possible crystalline structures of the TiAlN and TiAlSiN layers, and the results are shown in Fig. 2. The as-deposited TiAlN coating con- tained a small amount of crystalline phase, which showed a cubic TiN-like structure with a preferred orientation of (111). In contrast, the TiAlSiN layer showed an amor- phous structure, consistent with the result for a TiSiN coat- ing prepared in a similar low temperature PVD process (Kim et al., 2002). 3.3. Thermal stability in air Ambient thermal stability of the TiAlN/TiAlSiN/Si3N4 tandem absorber coatings was investigated. Five groups of samples were heated in air separately to different tem- peratures (200 �C, 300 �C, 400 �C, 500 �C, 600 �C), and each was annealed at its maximum temperature for 2 h. The absorptance and emittance values, and representative reflectance spectra of the heat-treated coatings are shown in Table 3 and Fig. 3, respectively, in comparison to those of the as-grown coatings. Lastly, SEM images of the heat- Fig. 1. Reflectance spectra of (a) SS substrate, (b) SS/TiAlN, (c) SS/ TiAlN/TiAlSiN and (d) SS/TiAlN/TiAlSiN/Si3N4. Fig. 2. XRD spectra of SS substrate, SS/TiAlN, SS/TiAlSiN. Fig. 3. Reflectance spectra of the SS/TiAlN/TiAlSiN/Si3N4 coating before and after heat treatment, (a) at 500 �C for 2 h, (b) at 600 �C for 2 h, (c) at 272 �C for 300 h. J. Feng et al. / Solar Energy 111 (2015) 350–356 353 treated coatings are shown in Fig. 4, in comparison with that of the as-deposited coating. The absorptance and emittance values of the tandem absorbers did not show significant changes up to a heat treat- ment temperature of 500 �C, nor did we observe any notice- able changes of the coating’s surface in the SEM images (all films showed fine-grained surface morphologies with similar grain sizes). However, the sample heat-treated at 600 �C showed a substantial increase in emittance (from �10% to �15%). These results can be understood by comparing the corresponding reflectance spectra. As are shown in Fig. 3a and b, the two spectra before and after heat treatment are close to each other in the wavelength range from 300 nm to 1000 nm, indicating a good absorptance stability for our coatings up to a heat treatment temperature of 600 �C. For the sample annealed at 500 �C, its reflectance spectra before and after heat treatment showed small differences for the whole measuring range of wavelengths, indicating a good thermal stability of its emittance performance. However, the sample annealed at 600 �C showed an earlier ramp in its reflectance spectrum as compared with that of the as- grown coating. This rapid increase in reflectance starts from a wavelength k somewhere between 1000 and 1250 nm for the annealed sample, while that of the as-growth sample starts near k = 2000 nm beyond which the reflectance rap- idly exceeds 10%. This big change in reflectance spectrum beyond 1000 nm well explains the substantial increase in emittance for the sample annealed at 600 �C. Table 3 Effect of 2 h heat treatment (in air) on the absorptance and emittance of the optimized TiAlN/TiAlSiN/Si3N4 tandem absorber. Heat treatment temperature (�C) a e Before After Da Before After De 200 0.942 0.936 �0.006 0.102 0.119 +0.017 300 0.934 0.924 �0.010 0.104 0.129 +0.025 400 0.945 0.932 �0.013 0.099 0.124 +0.025 500 0.930 0.913 �0.017 0.085 0.094 +0.009 600 0.937 0.910 �0.027 0.106 0.150 +0.044 Fig. 4. Scanning electron microscope images of the TiAlN/TiAlSiN/Si3N4 tandem absorbers, (a) as-deposited, (b) after heat treatment at 200 �C for 2 h, (c) after heat treatment at 300 �C for 2 h, (d) after heat treatment at 400 �C for 2 h, (e) after heat treatment at 500 �C for 2 h, and (f) after heat treatment at 600 �C for 2 h. Fig. 5. Raman spectra of the TiAlN/TiAlSiN/Si3N4 tandem absorbers (a) as-deposited, (b) after heat treatment at 200 �C for 2 h, (c) after heat treatment at 300 �C for 2 h, (d) after heat treatment at 400 �C for 2 h, (e) after heat treatment at 500 �C for 2 h, and (f) after heat treatment at 600 �C for 2 h. 354 J. Feng et al. / Solar Energy 111 (2015) 350–356 The degradation in emittance performance of the sample annealed at 600 �C is most likely due to its shift of thermal re-radiation spectrum to lower wavelengths at this high temperature (Peterson and Ramsey, 1975). Increase of sur- face defects at this high temperature, as observed by SEM analysis, may also play a significant role on the increase of emittance (Xinkang et al., 2008). On the other hand, for the sample annealed at 500 �C, its similar reflectance spectrum to that of the as-grown coating resulted in very small deg- radation in its absorptance and emittance performances, indicating that the TiAlN/TiAlSiN/Si3N4 tandem absorb- ers have a good thermal stability in air up to two hours of annealing at 500 �C. This agrees fairly well with the reported results on thermal stability of TiAlSiN (Xie et al., 2012) and Si3N4 (Chen et al., 2002) layers. The thermal stability of the tandem absorbers was fur- ther investigated by using micro-Raman spectroscopy. The Raman spectra of the as-deposited tandem absorber and of those heat-treated up to 600 �C are shown in Fig. 5. The spectrum of the as-deposited tandem absorber shows two peaks centered at 250 cm�1 and 635 cm�1, J. Feng et al. / Solar Energy 111 (2015) 350–356 355 respectively, while the 250 cm�1 band corresponds to the acoustic vibrational modes of the Ti and Al ions, and the 635 cm�1 band is related to the optical vibrations of N ions (Barshilia et al., 2008a; Barshilia and Rajam, 2004). We did not observe substantial changes in the Raman spectra of heat-treated samples up to 600 �C, indicating a stable microstructure of the SSAC. Lastly, we annealed the coating in air at 272 �C for 300 h based on the PCnomenclature (Köhl et al., 2004). The reflec- tance spectra before and after annealing are given in Fig. 3c. Changes in the spectrumbefore 1000 nmwere small, indicat- ing a stable absorptance. On the other hand, changes in reflectance after 1000 nm were substantial, indicating an increase of emittance. By comparing a and e values of both as-deposited (a = 0.936, e = 0.102) and annealed coatings (a = 0.916, e = 0.126), a small PC value was obtained (PC = Da � 0.5De = 0.032 < 0.05), which suggests that the coating was thermally stable in air at 272 �C for up to 300 h. 3.4. Discussion The TiAlN/TiAlSiN/Si3N4 tandem absorbers exhibited excellent overall solar thermal properties for a flat plate heat collector, which include an excellent solar absorbing selec- tivity (a = 0.938, e = 0.099, a/e � 9.5) and a high thermal stability (stable in air at 600 �C for 2 h, and at 272 �C for 300 h). There have been reports of excellent solar thermal properties of coatings with a similar composition. However, these coatings either contain significant amount of oxygen (Barshilia et al., 2008a,b; Liu et al., 2012), or lack of exper- imental evidence on their thermal stabilities at elevated tem- peratures for prolonged hours (Du et al., 2011). 4. Conclusions TiAlN/TiAlSiN/Si3N4 tandem absorbing coatings were prepared on SS substrate using a reactive dcmagnetron sput- tering technique. The optimized coatings exhibited a high absorptance of �0.938 ± 0.005 and a low emittance of �0.099 ± 0.007. XRD analysis of the coatings revealed dominance of an amorphous microstructure. The absorp- tance and emittance of this coating showed negligible changes after it was heat treated in air up to 500 �C for 2 h. Such an ambient thermal stability of this coating was sup- ported by SEM and micro-Raman analysis. Finally, the selective absorbing properties of the coatings underwent insignificant changes after a heat-treatment at 272 �C in air for 300 h, validating their long-time thermal stability. Over- all, the TiAlN/TiAlSiN/Si3N4 tandem absorber is a good coating candidate for the development of flat plate solar heat collectors in present China, where most of the production equipments desire an oxygen-free composition of the SSAC. Acknowledgments The authors acknowledge the financial support from the Shandong province independent innovation and achieve- ment transformation major special project (Grant No. 2013ZHZX1A0502) and Jinan city independent innovation project (Grant No. 201302089). J. Ouyang would like to thank the financial support provided by the Program for New Century Excellent Talents in University (State Educa- tion Ministry), the Independent Innovation Foundation of Shandong University (Grant No. 2011DX005) and the Key Laboratory of Inorganic Coating Materials, Chinese Acad- emy of Sciences. Y. Lu and Z. Liu would like to thank the financial support from the Ministry of Housing and Urban-Rural Development (MOHURD) (Project Grant No. 2011100, Lu Cai Jian Zhi). S. Zhang would like to thank the project supported by the Shandong province sci- entific and technological development plan (Grant No. 2012GGX10422). References Barshilia, H.C., Rajam, K., 2004. 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