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Delineating the role of surface characteristics on the solar selectivity of colored chromium oxide coating on 304 stainless steel substrate
Solar Energy Materials and Solar Cells 182 (2018) 354–361
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Delineating the role of surface characteristics on the solar selectivity of colored chromium oxide coating on 304 stainless steel substrate Angshuman Sarkara, Surajit Sinhaa, Debajyoti Palaia, Arjun Deyb, Amitava Basu Mallicka, a b
T
⁎
Department of Metallurgy and Materials Engineering, IIEST, Shibpur, Howrah 711103, India Thermal Systems Group, Vimanapura Post, Bangaluru 560017, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar absorptance IR-emittance Selectivity factor Nanopores Pore-density Surface roughness
Solar selective colored chromium oxide coatings were prepared on the SS304 substrate through the electrochemical deposition route to provide more options and flexibility in aesthetically design solar absorbers with acceptable energy performance. The variation of colors in the coatings was achieved by varying the coating thickness as a function of deposition time. Color changes in the coatings were also evident in the samples that were annealed under vacuum. Electron microscopic images show that nanometer sized pores are present on the coating surfaces. The pore-density, pore-size distribution and surface roughness exhibit significant changes with variations in deposition time and annealing temperature. Initially, the selectivity factor of 6.08 was recorded in the as prepared sample coated for 30 min, which improved to 7.36 on annealing the samples under vacuum at 700 °C, however, it drops to 4.22 when annealed at 900 °C. A similar trend was also observed for samples that are coated for 60 min. The results indicate that the pore-density, pore-size distribution and surface roughness play a dominant role in governing the solar selectivity of the coatings. Nanoindentation tests were performed on the coated samples and the results show that mechanical properties of the solar absorber coatings degrades with the increase in coating thickness but improve significantly with increase in annealing temperature.
1. Introduction The search for solar selective absorber material is becoming increasingly prominent with the growing demand of solar energy harvesting. Solar selective absorbers play an important role in all major solar thermal technologies e.g. concentrated solar power (CSP), solar thermo-electric generators (STEG), and solar thermo-photovoltaic's (STPV) for harvesting energy from the sun [1–3]. The most important criteria for the solar absorber material are to absorb sunlight without losing heat to the environment. In this regard an ideal solar selective absorber should have very high solar absorptance and low infrared (IR) emittance properties [4–6]. The past decade has witnessed an extensive research on developing ideal solar absorber material in the form of thin film coatings to attain the desirable solar selectivity by employing different coating techniques (e.g. electrochemical, sputtering, spray coating, spray pyrolysis, chemical vapour deposition, electroplating, anodizing, sol-gel, spin coating, dip coating etc.) [7]. Among these various solar absorber materials, metal-dielectric composites and cermet(s) received special attention due to their high solar absorptance, low IR emittance and good thermal stability [7,8]. Till date the most widely used and commercially available cermet
⁎
absorber material is black chromium oxide (Cr2O3) based cermets which possesses the ability to trap and expand the optical path of the solar energy, which induces a high solar absorptance in it [7,9,10]. Though it is evident that black surfaces deliver the highest solar absorptance, they are not always considered aesthetically acceptable in all cases, e.g. facade integration [11–13]. During the past few years, interest on the development of colorful solar selective absorber is seen with the objective of expanding the usage of solar technologies with a colorful architectural integration. Many studies have been reported on colored solar absorber using multilayer structures of various metal-dielectric composites [8,14–16], but availability of reports on the research work focused on the development of colorful Cr2O3 based absorber are limited and more importantly detailed study on the thermal stability, mechanical property and the effect of surface morphology and microstructure of the colored Cr2O3 coatings on the solar thermal property of the material is yet to be reported. Considering the importance of the Cr2O3 based colored absorber, the present study attempt to electrochemically develop a colored Cr2O3 based cermets coating over the stainless steel substrate. The electrochemical deposition route was selected due to its low complexity, high control over the process parameters, low cost, good reproducibility and possibility of scale up.
Corresponding author. E-mail address: [email protected] (A.B. Mallick).
https://doi.org/10.1016/j.solmat.2018.03.043 Received 27 July 2017; Received in revised form 20 December 2017; Accepted 26 March 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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size and their distributions were measured from the FESEM images recorded from different locations of the sample surface. Atomic force microscope, AFM (Dimension Edge, Bruker, Germany) was used to measure the roughness of the coating surfaces. The AFM images were acquired in the peak force tapping mode, using a silicon nitride tip at room temperature. The roughness values were evaluated after image processing of 100 × 100 µm2 scan areas from several locations and are expressed as the root mean square roughness (Rq). The coefficient of surface skewness (Sq) and kurtosis (Sk) was also determined from the AFM images. All AFM image processing work was carried out with the help of Nanoscope analysis software package (Version 1.4). Thermogravimetric and differential thermal analysis were performed on the as coated samples under static air medium with the aid of a thermal analyzer (TG-DTA, model Diamond, Perkin Elmer, US) in the temperature range of 30°–900 °C at a heating rate of 10 °C/min. The mechanical properties of coatings were investigated with the help of nanoindentation workstation (NHTX 55-0019, CSM instruments, USA) equipped with a berkovich diamond indenter. All nanoindentation tests were performed under a maximum load of 1 mN with 10 s of dwell time. The low load was selected to avoid the influence of the substrate. For each sample, a minimum of 10 (e.g. 5 × 2 array) indents were taken at randomly selected locations on the coating surfaces. The results were averaged and presented with the standard deviation. The thermo-optical properties viz., solar absorptance (αs) of the samples were determined by a solar spectrum reflectometer (SSR-E, Devices and Services Co., USA) for the entire solar region (200–2500 nm) as per ASTM C1549-09. An average hemispherical IRemissivity (εIR) of the samples were also measured by an emissometer (AE, Devices and Services Co., USA) at infrared (IR) region (3–30 µm) as per ASTM C1371-04a [18].
On the other hand the high thermal conductivity, excellent corrosion resistance property and the low emissivity are the reasons behind the selection of stainless steel as substrate. The mechanical properties of the coating were determined by using nanoindentation technique. Studies on the thermal characteristics and stability of the coatings were carried out at elevated temperature and the effects of the surface morphology and microstructure on the solar thermal property was also investigated 2. Experimental 2.1. Development of coating A commercial grade SS304 sheet of thickness 75 µm, with the weight percent composition; Cr- 18.36, Ni- 8.06, Mn- 0.93, Si- 0.49, S0.01, C- 0.06, P-0.04, Mo-0.36, Cu-0.43, N-0.08 and balance Fe was used as the substrate. For colorization, 30 mm x 62 mm size samples were cut from the SS304 sheet and each of themwereultrasonically degreased for 10 min in acetone followed by rinsing in distilled water prior to the colorization treatment. The cleaned samples were colored electrochemically by immersion in a mixture of hot (70 °C) aqueous 5 mol/L H2SO4 and 2.5 mol/L CrO3 solution under a constant current density of 3.8 mA/cm2. The electrochemical bath consists of SS304 sheet (cathode), a reference platinum electrode (anode), a DC power supply and a heating mantle to raise the temperature of the bath to 70 °C. During the colorization treatment, an oxide film was formed on the SS304 substrate by dissolution and precipitation mechanism according to the following reaction sequences [17],
Anodic reaction: M → M z +
(1)
+ e−
Cathodic reaction: HCrO4− +
7H+ +
3e− →
Cr 3 + +
4H2 O
3. Result and discussion
(2)
3.1. Effect of coating thickness
Hydrolysis reaction: pMz + + qCr 3 + + rH2 O → Mp Crq Or + 2rH+ (zp + 3q = 2r )
(3)
The surface of the coated samples was scratched with a sharp razor blade and the scratched section was studied under the FESEM for determining the thickness of the coating. Fig. 1 shows the representative FESEM images of the samples with the coating thickness and the results are presented in Table 2. The results indicate that the coating thickness of the samples increases with the increase in deposition time. A conspicuous change in color from dark blue to lime gold was also observed with the increase in coating thickness, which can be ascribed to the variations in the refractive index of the coating promoted by the changes in the coating thickness. Annealing the samples at 500 °C causes marginal change in the color and thickness of the coating surfaces, because of the simultaneous evaporation of physically attached water and reaction between unreacted metal surfaces with the residual acidic solution which remained trapped in the pores of the oxide coating. However, the change in color and reduction of thickness becomes very prominent when the samples were annealed at higher temperature. The reduction of coating thickness at temperatures above 500 °C is mainly instigated by the elimination of lattice bound water molecules, trapped acidic solutions in the pores and may also be caused by the collapsation of pore walls. The corresponding FESEM images show that no distinct interface boundary exists, indicating an excellent bonding between the coating and the substrate, which continues to exist even after annealing at 900 °C.
where, M represents the alloying elements present in the SS304 sample viz. Fe, Cr, Ni, Mn etc. and Mz+ is the corresponding ions formed by anodic dissolution reaction, viz. Fe3+, Cr3+, Ni2+, Mn3+. The cathodic reduction of chromic acid yields Cr3+ ions and finally, a heavily hydrated oxide coating was formed by the hydrolysis reaction between the metal ions produced by dissolution of the SS304 steel and Cr3+ ions formed by reduction of chromic acid, leading to precipitation of the oxide layer on the substrate. The thickness of the coating was varied by varying the coating time for 30 min and 60 min. The stability and characteristics of the coating surfaces with respect to elevated temperatures were studied after annealing the coated samples in vacuum (5 × 10−5 Torr) at 500 °C, 700 °C and 900 °C for 1 h. The sample designations with respect to the processed conditions are shown in Table 1. 2.2. Characterization The microstructure and surface morphology of the coated films were studied with the help of field emission scanning electron microscope, FESEM (JSM-7610F, JEOL, Japan). The thickness of the coating, pore Table 1 Sample designation with respect to their processing conditions. Sample specification
As coated 500 °C annealed 700 °C annealed 900 °C annealed
Sample identity
3.2. Surface morphology
30 min deposition
60 min deposition
C30 500C30 700C30 900C30
C60 500C60 700C60 900C60
The representative microstructural features of the coated surfaces of the samples are shown in Fig. 2. Presence of nanopores on the coated surfaces is observed in all samples. Formation of nanopores arises because, during the electrochemical deposition process, when the Cr3+ concentration at the metal/solution interface exceeds the solubility 355
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Fig. 1. Coating thickness of (a) C30, (b) C60, (c) 900C30 and (d) 900C60 samples.
nanometric length scale and the pore-size distribution profiles change, when the samples are annealed at different temperatures. The pore size variation in the as coated sample C60 is wider than that in the sample C30, which seems to be caused by the increase in the thickness and the amount of conversion oxides formed, leading to pore growth at sites that offer favourable condition for growth. On annealing the as coated samples at 500 °C and 700 °C, the sample C30 exhibit a decreasing trend in the width of the pore size distribution profile. An increase in the number of smaller sized pores is also visible. In comparison to C30, the sample C60, when annealed at 500 °C, the reduction in the width of the distribution profile is not evident when compared with the corresponding as coated sample, but an increase in the number of pores is seen. However, the increase in the number of smaller pores and the reduction in the width of the pore size distribution profile becomes very prominent when the sample C60 is annealed at 700 °C. The above observations can be explained by considering the fact that during the annealing treatment, loss of trapped acidic solution and physically
limit, chromium oxide is deposited over the metal substrate, excluding the grain boundaries, twin boundaries and slip planes where anodic dissolution is predominant [17]. At the initial stages of deposition, due to the larger volume ratio of the conversion oxide coating with respect to the SS304 metal substrate, compressive stresses are generated at the metal/coating interfaces. The generated compressive stresses induce formation of pores and at a later stage lead to an upward movement of the pore-wall, which grows with further deposition in the direction of the applied electric field. Therefore, it is apparent that a change in deposition time and annealing temperature would cause variation in the magnitude of the generated compressive stress, which would eventually lead to variations in pore-size distribution and density. The pore-size distribution and pore-density of the coated samples was estimated from the FESEM images by using the image processing software (Image-J, Version 1.51j8) [19]. The pore-size distribution profile of the as coated and annealed samples are shown in Fig. 3 where, it can be clearly seen that the pore diameters are in the Table 2 Thickness and corresponding color of the samples. Sample id
C30
500C30
700C30
900C30
C60
500C60
700C60
900C60
Thickness (nm) Colored Samples
170
192
98
89
347
328
280
273
Color
Dark blue
Steel blue
Azure
Grey
Lime gold
Lime
Lime green
Grey
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Fig. 2. Microstucture of the coatings (a) C30, (b) C60, (c) 700C30, (d) 700C60, (e) 900C30 and (f) 900C60 samples.
coated samples, it can be seen that the variations in pore-density with respect to the deposition time is insignificant, however, a rise in poredensity was observed in samples that are annealed below 900 °C. This has arisen due to removal of physically attached water molecules from the coating surface, which causes opening up of new pores on the surface, leading to the rise in pore-density. However, annealing at temperature beyond 700 °C causes elimination of large amount of lattice bound water and simultaneous removal of trapped species of acidic solutions from the conversion oxide coating, which triggers the porewall collapse and as an after effect; it leads to a sharp drop in poredensity. The topographic AFM images of the coating shown in Fig. 4 revealed, deposition of the coating took place perpendicular to the
attached water in the pores is dominant, causing opening up of new pores in the coating that were previously covered with the liquid and remained undetected. These observations are in agreement with the TGDTA results, discussed in later Section 3.3. The difference in the width of the pore size distribution profiles of sample C30 and C60 at 500 °C seems to have arisen due to the increase in the coating thickness and differential nature of pore growth at favourable locations. The sample annealed at 900 °C, show a wide distribution of pore sizes, which is caused by the complete dehydration of the hydrated conversion oxide coating leading to pore-wall collapse and morphological changes in the coating surface as evidenced in the Fig. 2(e and f). The pore-density of the as coated and annealed samples evaluated from the FESEM micrograph is summarized in Table 3. In the case of as 357
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Fig. 3. Pore-size distribution profile of (a) 30 min and (b) 60 min deposition samples. Table 3 Pore-density and corresponding roughness parameters. Sample id Pore-Density (per um2) RMS Roughness Rq (in nm) Skewness (Sq) Coefficient of Kurtosis (Sk)
C30
500C30 3
1.89 × 10 × 10 86.34 − 0.38 5.98
3
700C30 3
900C30 3
1.98 × 10
2.2 × 10
0.33 × 10
76.70 − 0.97 4.31
73.14 − 0.59 3.97
106.34 − 1.44 1.21
surface in the direction of the applied electric field. The statistical analysis of roughness, including root mean square (rms) roughness, peak to valley ratio, viz. surface skewness and kurtosis values shown in Table 3, indicate that the roughness of the sample increase with the increase in deposition time. This is attributed to the fact that with the increase in deposition time, the morphological features viz. pores and pore-walls become more distinct. Annealing at temperatures below 900 °C causes a nominal reduction in roughness due to relaxation of compressive stress at the surface, grain growth, removal of physically attached water present inside the pores and partial dehydration of lattice bound water molecules which appear to make the surface smooth as evident in Fig. 2(c) and (d). However, when the samples are annealed at 900 °C, a distinct change in morphological features is witnessed due to the pore collapse and coalescence caused by the complete loss of both physically attached and lattice bound water from the coating surface and delamination of the coating caused by unequal expansion and contraction of the oxide coating and the substrate, due to the wide differences in their coefficient of thermal expansion. The negative skewness values shown in Table 3 indicate that the
C60 3
1.71 × 10 94.60 − 0.63 7.57
500C60 3
700C60 3
1.84 × 10
1.86 × 10
89.30 − 1.21 5.29
87.22 − 0.79 5.17
900C60 3
0.25 × 103 124.42 − 1.06 2.06
surface comprises of predominant valleys on the coating surfaces. On the other hand, the transformation in kurtosis distribution from leptokutoric (kurtosis > 3) to platekutoric (kurtosis < 3) in the coating samples after annealing at 900 °C arises due to widening of pore-size distribution, as evident in Fig. 3(a) and (b). 3.3. Thermal analysis Fig. 5 shows the representative TG-DTA (thermo gravimetric- differential thermal analysis) plot of the as coated sample designated as C60. A non uniform trend in weight loss is clearly evident with rising temperature in the specified temperature range of 30–900 °C. The initial weight loss in the temperature range of 30–100 °C originates from the loss of physically attached water present in the nanopores of the coating. However, in reference to the general weight loss tendency of the sample, sudden weight gains (marked as A, B, and C) are visible in the thermogram. The sudden weight gain in the temperature range of 100–190 °C marked as “A” can be assigned to the reaction between the unreacted 304 SS surface, trapped residual species of acidic solution and air medium which lead to the formation and deposition of
Fig. 4. AFM 3D images of (a) C30 and (b) C60 samples. 358
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selectivity factor. The highest selectivity factor of 7.36 and 6.75 was recorded respectively in 700C30 and 700C60 annealed samples. The roughness analysis of the samples shows that the roughness value of the as coated and annealed samples at 500 °C and 700 °C (refer Table 4) are much lower than the IR-wavelength [4] and is responsible for the lower emissivity value in the annealed samples. However, an increase in poredensity of the samples annealed at temperatures up to 700 °C; enhance the solar trapping on the surface leading to better solar absorptance. The combined effects of these two factors lead to the improved selectivity of the sample. But since on annealing the samples at 900 °C, a significant drop in pore-density of the coating is observed, a sudden drop in absorptance is witnessed and subsequently the selectivity of the material is reduced. 3.5. Mechanical property of the coating Representative load–displacement curves of the coated samples obtained from nano indentation measurements are shown in Fig. 6. The unloading segment of the curves for the as coated samples was found to be shifted to the right, indicating greater penetration and a lower hardness value when compared to the annealed samples. It has been also found that with the increase in deposition time the coating thickness increases and consequently the depth of penetration increases, which is evidenced by further displacement of unloading curve to the right. However, a clear tendency of left shift in the unloading curves was noticed in the annealed samples. This behaviour emphasizes that the annealing treatment makes the coating much harder than the as coated samples. The indentation hardness (H), and elastic modulus (E), values of all the samples were calculated using the Oliver and Pharr approach [21] and the comparative graphical representations of the results with standard deviations are shown in Fig. 7(a) and (b). The results indicate that the hardness and the elastic modulus of the coating decrease with the increase in coating thickness, which implies that the effect of the substrate decreases and the coating is softer than the substrate. However, it can be also seen from Fig. 7(a) and (b) that both hardness and elastic modulus depend strongly on the annealing temperature. The hardness and elastic modulus gradually improve with the increase in annealing temperature and the maximum hardness was recorded in the samples, which were annealed at 900 °C The area underneath the load-displacement curves, viz. total elasticplastic deformation (WTotal) and the plasticity index (H/E ratio) of the all samples were calculated and was plotted with respect to their annealing temperature [refer Fig. 7(c) and (d)], respectively. In case of the as coated samples, it is seen that as the thickness increases, the WTotal value also increases without any significant change in the plasticity index. But after annealing, the coating becomes harder and as a result, the value of WTotal decreases whereas the H/E ratio increases, as seen in Fig. 7(c) and (d).
Fig. 5. Thermal analysis (TG-DTA) plot of sample C60.
conversion metal oxides/salts inside the pores. The findings, corroborate the variations in the pore size distribution profile of the coating previously shown in Fig. 3(a-b). At the later stage in the temperature range of 190–355 °C, a significant drop in the weight is observed which can be attributed to the simultaneous elimination of lattice bound water and evaporation of the residual acidic solution from the coating. In the temperature range between 355 and 840 °C, pore-wall collapse exposes fresh metallic surfaces in the static air environment leading to further oxidation and weight gain in the sample. However, the endothermic nature of the DTA plot suggests that elimination of lattice bound water is a predominant feature in the above mentioned temperature range. The weight change at temperatures beyond 840 °C appears to be insignificant mainly because of the absence of residual species of acidic solution, lattice bound water and availability of unreacted 304SS exposed surfaces.
3.4. Solar absorptance and thermal emittance The thermo-optical properties such as solar absorptance (αs), IR emittance (εIR) and the selectivity factors (η = αs/εIR) of the as coated and annealed samples are presented in Table 4. The results clearly show that the thermo-optical property of the as coated sample C30 is better than C60 with respect to the solar selectivity. Although the emissivity of the samples remain nearly the same, a drop in αs from 0.73 in C30to 0.70 in C60 is observed, which is assigned to the reduction of poredensity in the sample C60. The reduction in αs with the decrease in pore-density can be linked to the following reasons: i) higher poredensity improves the effective sunlight trapping inside the pores and enhances multi reflection inside the pore-walls and ii) reflections from the coating surface are weakened due to variations in the dielectric constant [20] in different locality of the coated surface which can be attributed to the presence of a large number of different size air filled nanopores. Thus, the selectivity factor viz. the ratio between solar absorptance and the IR-emittance increases with the decrease in the deposition time. The samples annealed at 500 °C and 700 °C, show an increase in the solar absorptance with the increase in temperature without any significant change in IR-emittance. This results in a positive change in the
4. Conclusion Colored chromium oxide based solar selective coating was electrochemically deposited on 304SS substrate. By manipulating the deposition time and annealing temperature, variations in color were accomplished vis-a-vis changes in the coating thickness. The thermo gravimetric analysis confirms the elimination of large amount of lattice
Table 4 Thermo-optical properties of all samples. Thermo-optical properties
C30
500C30
700C30
900C30
C60
500C60
700C60
900C60
αs εIR η
0.73 0.12 6.08
0.80 0.11 7.27
0.81 0.11 7.36
0.59 0.14 4.22
0.70 0.13 5.38
0.79 0.13 6.07
0.81 0.12 6.75
0.52 0.16 3.25
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Fig. 6. Load displacement plot of samples with coating time (a) 30 min and (b) 60 min.
Fig. 7. Comparative graphical representation of (a) Hardness, (b) Elastic modulus, (c) Wtotal, and (d) H/E ratio data as a function of annealing temperature
lattice bound water from the coating. The hardness of the coating deposited for 60 min is lower than the coating deposited for 30 min.
bound water from the coating. The changes in coating thickness became very prominent at temperatures above 700 °C. The interfacial bond between the coating and substrate appear to be very strong even after annealing at 900 °C. The microstructural images confirm the presence of nanopores on the coated surfaces in all samples. The nanoporous surface plays an important role in tuning the thermo-optical properties of the samples. Annealing below 900 °C causes increase in pore-density of the samples with nominal reduction in surface roughness. Higher pore-density causes enhancement of solar trapping and induces a higher solar absorptance. The conspicuous drop in solar thermal property accompanied with a color change and increase in surface roughness in the samples annealed at 900 °C arises due to elimination of large amount of
Acknowledgement This work was supported by the Centre of Excellence (COE), TEQIPII under MHRD, Government of India [F.No. 16-6/2015-TS-VII]. References [1] N. Wang, L. Han, H. He, N.-H. Park, K. Koumoto, A novel high-performance photovoltaic–thermoelectric hybrid device, Energy Environ. Sci. 4 (2011) 3676–3679, http://dx.doi.org/10.1039/c1ee01646f.
360
Solar Energy Materials and Solar Cells 182 (2018) 354–361
A. Sarkar et al.
[2] D. Kraemer, B. Poudel, H. 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 flatpanel solar thermoelectric generators with high thermal concentration, Nat. Mater. 10 (2011) 532–538, http://dx.doi.org/10.1038/nmat3013. [3] D. Mills, Advances in solar thermal electricity technology, Sol. Energy 76 (2004) 19–31, http://dx.doi.org/10.1016/S0038-092X(03)00102-6. [4] X. Wang, X. Wu, L. Yuan, C. Zhou, Y. Wang, K. Huang, S. Feng, Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel, Sol. Energy Mater. Sol. Cells 146 (2016) 99–106, http://dx.doi.org/10. 1016/j.solmat.2015.11.040. [5] Z. Chen, T. Boström, Electrophoretically deposited carbon nanotube spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 144 (2016) 678–683, http://dx. doi.org/10.1016/j.solmat.2015.10.016. [6] J. Feng, S. Zhang, X. Liu, H. Yu, H. Ding, Y. Tian, J. Ouyang, Solar selective absorbing coatings TiN/TiSiN/SiN prepared on stainless steel substrates, Vacuum 121 (2015) 135–141, http://dx.doi.org/10.1016/j.vacuum.2015.08.013. [7] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. (2014), http://dx.doi.org/10.1039/ c3ee43825b. [8] H.C. Barshilia, N. Selvakumar, K.S. Rajam, A. Biswas, Structure and optical properties of pulsed sputter deposited Cr xOy/Cr/Cr2O3 solar selective coatings, J. Appl. Phys. 103 (2008), http://dx.doi.org/10.1063/1.2831364 (23507-1-11). [9] Y. Yin, Y. Pan, L.X. Hang, D.R. McKenzie, M.M.M. Bilek, Direct current reactive sputtering Cr-Cr2O3 cermet solar selective surfaces for solar hot water applications, Thin Solid Films (2009), http://dx.doi.org/10.1016/j.tsf.2008.09.082. [10] S. Khamlich, M. Maaza, Cr/α-Cr2O3 monodispersed meso-spherical particles for mid-temperature solar absorber application, Energy Procedia (2015), http://dx.doi. org/10.1016/j.egypro.2015.03.229. [11] F. Chen, S.-W. Wang, X. Liu, R. Ji, Z. Li, X. Chen, Y. Chen, W. Lu, Colorful solar selective absorber integrated with different colored units, Opt. Express 24 (2016) A92–A103, http://dx.doi.org/10.1364/OE.24.000A92. [12] A. Dan, K. Chattopadhyay, H.C. Barshilia, B. Basu, Colored selective absorber
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20] [21]
361
coating with excellent durability, Thin Solid Films 620 (2016) 17–22, http://dx.doi. org/10.1016/j.tsf.2016.08.070. Z.C. Orel, M.K. Gunde, M.G. Hutchins, Spectrally selective solar absorbers in different non-black colours, Sol. Energy Mater. Sol. Cells 85 (2005) 41–50, http://dx. doi.org/10.1016/j.solmat.2004.04.010. L. Zheng, F. Zhou, Z. Zhou, X. Song, G. Dong, M. Wang, X. Diao, Angular solar absorptance and thermal stability of Mo-SiO2 double cermet solar selective absorber coating, Sol. Energy 115 (2015) 341–346, http://dx.doi.org/10.1016/j. solener.2015.02.013. X.K. Du, C. Wang, T.M. Wang, B.L. Chen, L. Zhou, N. Ru, Magnetron sputtering high temperature Mo-Al2O3 Cermet Solar Selective Coatings, Mater. Sci. Forum 546–549 (2007) 1773–1776, http://dx.doi.org/10.4028/www.scientific.net/MSF.546-549. 1773. K.S. Klepikova, N.P. Klochko, G.S. Khrypunov, V.R. Kopach, V.M. Lyubov, V.V. Starikov, Solar selective absorber based on zinc oxide-nickel cermet, Appl. Phys. (YSF) 117 (2015) 1, http://dx.doi.org/10.1109/YSF.2015.7333187 2015 Int. Young Sci. Forum.. T.E. Evans, Film formation on stainless steel in a solution containing chromic and sulphuric acids, Corros. Sci. 17 (1977) 105–124, http://dx.doi.org/10.1016/0010938X(77)90012-9. D. Palai, M. Bhattacharya, A. BasuMallick, P. Bera, A.K. Sharma, A.K. Mukhopadhyay, A. Dey, Microstructural, thermo-optical, mechanical and tribological behaviours of vacuum heat treated ultra thin SS304 foils, Mater. Res. Express 3 (2016) 96501. M. Haeri, M. Haeri, ImageJ plugin for analysis of porous scaffolds used in tissue engineering, J. Open Res. Softw. 3 (2015) 2–5, http://dx.doi.org/10.5334/jors.bn. D. Ding, W. Cai, Self-assembled nanostructured composites for solar absorber, Mater. Lett. 93 (2013) 269–271, http://dx.doi.org/10.1016/j.matlet.2012.11.070. W. Oliver, G. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement-sensing indentation systems, J. Mater. Res. 7 (1992) 1564–1583, http://dx.doi.org/10.1557/JMR.1992.1564.
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Delineating the role of surface characteristics on the solar selectivity of colored chromium oxide coating on 304 stainless steel substrate Angshuman Sarkara, Surajit Sinhaa, Debajyoti Palaia, Arjun Deyb, Amitava Basu Mallicka, a b
T
⁎
Department of Metallurgy and Materials Engineering, IIEST, Shibpur, Howrah 711103, India Thermal Systems Group, Vimanapura Post, Bangaluru 560017, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar absorptance IR-emittance Selectivity factor Nanopores Pore-density Surface roughness
Solar selective colored chromium oxide coatings were prepared on the SS304 substrate through the electrochemical deposition route to provide more options and flexibility in aesthetically design solar absorbers with acceptable energy performance. The variation of colors in the coatings was achieved by varying the coating thickness as a function of deposition time. Color changes in the coatings were also evident in the samples that were annealed under vacuum. Electron microscopic images show that nanometer sized pores are present on the coating surfaces. The pore-density, pore-size distribution and surface roughness exhibit significant changes with variations in deposition time and annealing temperature. Initially, the selectivity factor of 6.08 was recorded in the as prepared sample coated for 30 min, which improved to 7.36 on annealing the samples under vacuum at 700 °C, however, it drops to 4.22 when annealed at 900 °C. A similar trend was also observed for samples that are coated for 60 min. The results indicate that the pore-density, pore-size distribution and surface roughness play a dominant role in governing the solar selectivity of the coatings. Nanoindentation tests were performed on the coated samples and the results show that mechanical properties of the solar absorber coatings degrades with the increase in coating thickness but improve significantly with increase in annealing temperature.
1. Introduction The search for solar selective absorber material is becoming increasingly prominent with the growing demand of solar energy harvesting. Solar selective absorbers play an important role in all major solar thermal technologies e.g. concentrated solar power (CSP), solar thermo-electric generators (STEG), and solar thermo-photovoltaic's (STPV) for harvesting energy from the sun [1–3]. The most important criteria for the solar absorber material are to absorb sunlight without losing heat to the environment. In this regard an ideal solar selective absorber should have very high solar absorptance and low infrared (IR) emittance properties [4–6]. The past decade has witnessed an extensive research on developing ideal solar absorber material in the form of thin film coatings to attain the desirable solar selectivity by employing different coating techniques (e.g. electrochemical, sputtering, spray coating, spray pyrolysis, chemical vapour deposition, electroplating, anodizing, sol-gel, spin coating, dip coating etc.) [7]. Among these various solar absorber materials, metal-dielectric composites and cermet(s) received special attention due to their high solar absorptance, low IR emittance and good thermal stability [7,8]. Till date the most widely used and commercially available cermet
⁎
absorber material is black chromium oxide (Cr2O3) based cermets which possesses the ability to trap and expand the optical path of the solar energy, which induces a high solar absorptance in it [7,9,10]. Though it is evident that black surfaces deliver the highest solar absorptance, they are not always considered aesthetically acceptable in all cases, e.g. facade integration [11–13]. During the past few years, interest on the development of colorful solar selective absorber is seen with the objective of expanding the usage of solar technologies with a colorful architectural integration. Many studies have been reported on colored solar absorber using multilayer structures of various metal-dielectric composites [8,14–16], but availability of reports on the research work focused on the development of colorful Cr2O3 based absorber are limited and more importantly detailed study on the thermal stability, mechanical property and the effect of surface morphology and microstructure of the colored Cr2O3 coatings on the solar thermal property of the material is yet to be reported. Considering the importance of the Cr2O3 based colored absorber, the present study attempt to electrochemically develop a colored Cr2O3 based cermets coating over the stainless steel substrate. The electrochemical deposition route was selected due to its low complexity, high control over the process parameters, low cost, good reproducibility and possibility of scale up.
Corresponding author. E-mail address: [email protected] (A.B. Mallick).
https://doi.org/10.1016/j.solmat.2018.03.043 Received 27 July 2017; Received in revised form 20 December 2017; Accepted 26 March 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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size and their distributions were measured from the FESEM images recorded from different locations of the sample surface. Atomic force microscope, AFM (Dimension Edge, Bruker, Germany) was used to measure the roughness of the coating surfaces. The AFM images were acquired in the peak force tapping mode, using a silicon nitride tip at room temperature. The roughness values were evaluated after image processing of 100 × 100 µm2 scan areas from several locations and are expressed as the root mean square roughness (Rq). The coefficient of surface skewness (Sq) and kurtosis (Sk) was also determined from the AFM images. All AFM image processing work was carried out with the help of Nanoscope analysis software package (Version 1.4). Thermogravimetric and differential thermal analysis were performed on the as coated samples under static air medium with the aid of a thermal analyzer (TG-DTA, model Diamond, Perkin Elmer, US) in the temperature range of 30°–900 °C at a heating rate of 10 °C/min. The mechanical properties of coatings were investigated with the help of nanoindentation workstation (NHTX 55-0019, CSM instruments, USA) equipped with a berkovich diamond indenter. All nanoindentation tests were performed under a maximum load of 1 mN with 10 s of dwell time. The low load was selected to avoid the influence of the substrate. For each sample, a minimum of 10 (e.g. 5 × 2 array) indents were taken at randomly selected locations on the coating surfaces. The results were averaged and presented with the standard deviation. The thermo-optical properties viz., solar absorptance (αs) of the samples were determined by a solar spectrum reflectometer (SSR-E, Devices and Services Co., USA) for the entire solar region (200–2500 nm) as per ASTM C1549-09. An average hemispherical IRemissivity (εIR) of the samples were also measured by an emissometer (AE, Devices and Services Co., USA) at infrared (IR) region (3–30 µm) as per ASTM C1371-04a [18].
On the other hand the high thermal conductivity, excellent corrosion resistance property and the low emissivity are the reasons behind the selection of stainless steel as substrate. The mechanical properties of the coating were determined by using nanoindentation technique. Studies on the thermal characteristics and stability of the coatings were carried out at elevated temperature and the effects of the surface morphology and microstructure on the solar thermal property was also investigated 2. Experimental 2.1. Development of coating A commercial grade SS304 sheet of thickness 75 µm, with the weight percent composition; Cr- 18.36, Ni- 8.06, Mn- 0.93, Si- 0.49, S0.01, C- 0.06, P-0.04, Mo-0.36, Cu-0.43, N-0.08 and balance Fe was used as the substrate. For colorization, 30 mm x 62 mm size samples were cut from the SS304 sheet and each of themwereultrasonically degreased for 10 min in acetone followed by rinsing in distilled water prior to the colorization treatment. The cleaned samples were colored electrochemically by immersion in a mixture of hot (70 °C) aqueous 5 mol/L H2SO4 and 2.5 mol/L CrO3 solution under a constant current density of 3.8 mA/cm2. The electrochemical bath consists of SS304 sheet (cathode), a reference platinum electrode (anode), a DC power supply and a heating mantle to raise the temperature of the bath to 70 °C. During the colorization treatment, an oxide film was formed on the SS304 substrate by dissolution and precipitation mechanism according to the following reaction sequences [17],
Anodic reaction: M → M z +
(1)
+ e−
Cathodic reaction: HCrO4− +
7H+ +
3e− →
Cr 3 + +
4H2 O
3. Result and discussion
(2)
3.1. Effect of coating thickness
Hydrolysis reaction: pMz + + qCr 3 + + rH2 O → Mp Crq Or + 2rH+ (zp + 3q = 2r )
(3)
The surface of the coated samples was scratched with a sharp razor blade and the scratched section was studied under the FESEM for determining the thickness of the coating. Fig. 1 shows the representative FESEM images of the samples with the coating thickness and the results are presented in Table 2. The results indicate that the coating thickness of the samples increases with the increase in deposition time. A conspicuous change in color from dark blue to lime gold was also observed with the increase in coating thickness, which can be ascribed to the variations in the refractive index of the coating promoted by the changes in the coating thickness. Annealing the samples at 500 °C causes marginal change in the color and thickness of the coating surfaces, because of the simultaneous evaporation of physically attached water and reaction between unreacted metal surfaces with the residual acidic solution which remained trapped in the pores of the oxide coating. However, the change in color and reduction of thickness becomes very prominent when the samples were annealed at higher temperature. The reduction of coating thickness at temperatures above 500 °C is mainly instigated by the elimination of lattice bound water molecules, trapped acidic solutions in the pores and may also be caused by the collapsation of pore walls. The corresponding FESEM images show that no distinct interface boundary exists, indicating an excellent bonding between the coating and the substrate, which continues to exist even after annealing at 900 °C.
where, M represents the alloying elements present in the SS304 sample viz. Fe, Cr, Ni, Mn etc. and Mz+ is the corresponding ions formed by anodic dissolution reaction, viz. Fe3+, Cr3+, Ni2+, Mn3+. The cathodic reduction of chromic acid yields Cr3+ ions and finally, a heavily hydrated oxide coating was formed by the hydrolysis reaction between the metal ions produced by dissolution of the SS304 steel and Cr3+ ions formed by reduction of chromic acid, leading to precipitation of the oxide layer on the substrate. The thickness of the coating was varied by varying the coating time for 30 min and 60 min. The stability and characteristics of the coating surfaces with respect to elevated temperatures were studied after annealing the coated samples in vacuum (5 × 10−5 Torr) at 500 °C, 700 °C and 900 °C for 1 h. The sample designations with respect to the processed conditions are shown in Table 1. 2.2. Characterization The microstructure and surface morphology of the coated films were studied with the help of field emission scanning electron microscope, FESEM (JSM-7610F, JEOL, Japan). The thickness of the coating, pore Table 1 Sample designation with respect to their processing conditions. Sample specification
As coated 500 °C annealed 700 °C annealed 900 °C annealed
Sample identity
3.2. Surface morphology
30 min deposition
60 min deposition
C30 500C30 700C30 900C30
C60 500C60 700C60 900C60
The representative microstructural features of the coated surfaces of the samples are shown in Fig. 2. Presence of nanopores on the coated surfaces is observed in all samples. Formation of nanopores arises because, during the electrochemical deposition process, when the Cr3+ concentration at the metal/solution interface exceeds the solubility 355
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Fig. 1. Coating thickness of (a) C30, (b) C60, (c) 900C30 and (d) 900C60 samples.
nanometric length scale and the pore-size distribution profiles change, when the samples are annealed at different temperatures. The pore size variation in the as coated sample C60 is wider than that in the sample C30, which seems to be caused by the increase in the thickness and the amount of conversion oxides formed, leading to pore growth at sites that offer favourable condition for growth. On annealing the as coated samples at 500 °C and 700 °C, the sample C30 exhibit a decreasing trend in the width of the pore size distribution profile. An increase in the number of smaller sized pores is also visible. In comparison to C30, the sample C60, when annealed at 500 °C, the reduction in the width of the distribution profile is not evident when compared with the corresponding as coated sample, but an increase in the number of pores is seen. However, the increase in the number of smaller pores and the reduction in the width of the pore size distribution profile becomes very prominent when the sample C60 is annealed at 700 °C. The above observations can be explained by considering the fact that during the annealing treatment, loss of trapped acidic solution and physically
limit, chromium oxide is deposited over the metal substrate, excluding the grain boundaries, twin boundaries and slip planes where anodic dissolution is predominant [17]. At the initial stages of deposition, due to the larger volume ratio of the conversion oxide coating with respect to the SS304 metal substrate, compressive stresses are generated at the metal/coating interfaces. The generated compressive stresses induce formation of pores and at a later stage lead to an upward movement of the pore-wall, which grows with further deposition in the direction of the applied electric field. Therefore, it is apparent that a change in deposition time and annealing temperature would cause variation in the magnitude of the generated compressive stress, which would eventually lead to variations in pore-size distribution and density. The pore-size distribution and pore-density of the coated samples was estimated from the FESEM images by using the image processing software (Image-J, Version 1.51j8) [19]. The pore-size distribution profile of the as coated and annealed samples are shown in Fig. 3 where, it can be clearly seen that the pore diameters are in the Table 2 Thickness and corresponding color of the samples. Sample id
C30
500C30
700C30
900C30
C60
500C60
700C60
900C60
Thickness (nm) Colored Samples
170
192
98
89
347
328
280
273
Color
Dark blue
Steel blue
Azure
Grey
Lime gold
Lime
Lime green
Grey
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Fig. 2. Microstucture of the coatings (a) C30, (b) C60, (c) 700C30, (d) 700C60, (e) 900C30 and (f) 900C60 samples.
coated samples, it can be seen that the variations in pore-density with respect to the deposition time is insignificant, however, a rise in poredensity was observed in samples that are annealed below 900 °C. This has arisen due to removal of physically attached water molecules from the coating surface, which causes opening up of new pores on the surface, leading to the rise in pore-density. However, annealing at temperature beyond 700 °C causes elimination of large amount of lattice bound water and simultaneous removal of trapped species of acidic solutions from the conversion oxide coating, which triggers the porewall collapse and as an after effect; it leads to a sharp drop in poredensity. The topographic AFM images of the coating shown in Fig. 4 revealed, deposition of the coating took place perpendicular to the
attached water in the pores is dominant, causing opening up of new pores in the coating that were previously covered with the liquid and remained undetected. These observations are in agreement with the TGDTA results, discussed in later Section 3.3. The difference in the width of the pore size distribution profiles of sample C30 and C60 at 500 °C seems to have arisen due to the increase in the coating thickness and differential nature of pore growth at favourable locations. The sample annealed at 900 °C, show a wide distribution of pore sizes, which is caused by the complete dehydration of the hydrated conversion oxide coating leading to pore-wall collapse and morphological changes in the coating surface as evidenced in the Fig. 2(e and f). The pore-density of the as coated and annealed samples evaluated from the FESEM micrograph is summarized in Table 3. In the case of as 357
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Fig. 3. Pore-size distribution profile of (a) 30 min and (b) 60 min deposition samples. Table 3 Pore-density and corresponding roughness parameters. Sample id Pore-Density (per um2) RMS Roughness Rq (in nm) Skewness (Sq) Coefficient of Kurtosis (Sk)
C30
500C30 3
1.89 × 10 × 10 86.34 − 0.38 5.98
3
700C30 3
900C30 3
1.98 × 10
2.2 × 10
0.33 × 10
76.70 − 0.97 4.31
73.14 − 0.59 3.97
106.34 − 1.44 1.21
surface in the direction of the applied electric field. The statistical analysis of roughness, including root mean square (rms) roughness, peak to valley ratio, viz. surface skewness and kurtosis values shown in Table 3, indicate that the roughness of the sample increase with the increase in deposition time. This is attributed to the fact that with the increase in deposition time, the morphological features viz. pores and pore-walls become more distinct. Annealing at temperatures below 900 °C causes a nominal reduction in roughness due to relaxation of compressive stress at the surface, grain growth, removal of physically attached water present inside the pores and partial dehydration of lattice bound water molecules which appear to make the surface smooth as evident in Fig. 2(c) and (d). However, when the samples are annealed at 900 °C, a distinct change in morphological features is witnessed due to the pore collapse and coalescence caused by the complete loss of both physically attached and lattice bound water from the coating surface and delamination of the coating caused by unequal expansion and contraction of the oxide coating and the substrate, due to the wide differences in their coefficient of thermal expansion. The negative skewness values shown in Table 3 indicate that the
C60 3
1.71 × 10 94.60 − 0.63 7.57
500C60 3
700C60 3
1.84 × 10
1.86 × 10
89.30 − 1.21 5.29
87.22 − 0.79 5.17
900C60 3
0.25 × 103 124.42 − 1.06 2.06
surface comprises of predominant valleys on the coating surfaces. On the other hand, the transformation in kurtosis distribution from leptokutoric (kurtosis > 3) to platekutoric (kurtosis < 3) in the coating samples after annealing at 900 °C arises due to widening of pore-size distribution, as evident in Fig. 3(a) and (b). 3.3. Thermal analysis Fig. 5 shows the representative TG-DTA (thermo gravimetric- differential thermal analysis) plot of the as coated sample designated as C60. A non uniform trend in weight loss is clearly evident with rising temperature in the specified temperature range of 30–900 °C. The initial weight loss in the temperature range of 30–100 °C originates from the loss of physically attached water present in the nanopores of the coating. However, in reference to the general weight loss tendency of the sample, sudden weight gains (marked as A, B, and C) are visible in the thermogram. The sudden weight gain in the temperature range of 100–190 °C marked as “A” can be assigned to the reaction between the unreacted 304 SS surface, trapped residual species of acidic solution and air medium which lead to the formation and deposition of
Fig. 4. AFM 3D images of (a) C30 and (b) C60 samples. 358
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selectivity factor. The highest selectivity factor of 7.36 and 6.75 was recorded respectively in 700C30 and 700C60 annealed samples. The roughness analysis of the samples shows that the roughness value of the as coated and annealed samples at 500 °C and 700 °C (refer Table 4) are much lower than the IR-wavelength [4] and is responsible for the lower emissivity value in the annealed samples. However, an increase in poredensity of the samples annealed at temperatures up to 700 °C; enhance the solar trapping on the surface leading to better solar absorptance. The combined effects of these two factors lead to the improved selectivity of the sample. But since on annealing the samples at 900 °C, a significant drop in pore-density of the coating is observed, a sudden drop in absorptance is witnessed and subsequently the selectivity of the material is reduced. 3.5. Mechanical property of the coating Representative load–displacement curves of the coated samples obtained from nano indentation measurements are shown in Fig. 6. The unloading segment of the curves for the as coated samples was found to be shifted to the right, indicating greater penetration and a lower hardness value when compared to the annealed samples. It has been also found that with the increase in deposition time the coating thickness increases and consequently the depth of penetration increases, which is evidenced by further displacement of unloading curve to the right. However, a clear tendency of left shift in the unloading curves was noticed in the annealed samples. This behaviour emphasizes that the annealing treatment makes the coating much harder than the as coated samples. The indentation hardness (H), and elastic modulus (E), values of all the samples were calculated using the Oliver and Pharr approach [21] and the comparative graphical representations of the results with standard deviations are shown in Fig. 7(a) and (b). The results indicate that the hardness and the elastic modulus of the coating decrease with the increase in coating thickness, which implies that the effect of the substrate decreases and the coating is softer than the substrate. However, it can be also seen from Fig. 7(a) and (b) that both hardness and elastic modulus depend strongly on the annealing temperature. The hardness and elastic modulus gradually improve with the increase in annealing temperature and the maximum hardness was recorded in the samples, which were annealed at 900 °C The area underneath the load-displacement curves, viz. total elasticplastic deformation (WTotal) and the plasticity index (H/E ratio) of the all samples were calculated and was plotted with respect to their annealing temperature [refer Fig. 7(c) and (d)], respectively. In case of the as coated samples, it is seen that as the thickness increases, the WTotal value also increases without any significant change in the plasticity index. But after annealing, the coating becomes harder and as a result, the value of WTotal decreases whereas the H/E ratio increases, as seen in Fig. 7(c) and (d).
Fig. 5. Thermal analysis (TG-DTA) plot of sample C60.
conversion metal oxides/salts inside the pores. The findings, corroborate the variations in the pore size distribution profile of the coating previously shown in Fig. 3(a-b). At the later stage in the temperature range of 190–355 °C, a significant drop in the weight is observed which can be attributed to the simultaneous elimination of lattice bound water and evaporation of the residual acidic solution from the coating. In the temperature range between 355 and 840 °C, pore-wall collapse exposes fresh metallic surfaces in the static air environment leading to further oxidation and weight gain in the sample. However, the endothermic nature of the DTA plot suggests that elimination of lattice bound water is a predominant feature in the above mentioned temperature range. The weight change at temperatures beyond 840 °C appears to be insignificant mainly because of the absence of residual species of acidic solution, lattice bound water and availability of unreacted 304SS exposed surfaces.
3.4. Solar absorptance and thermal emittance The thermo-optical properties such as solar absorptance (αs), IR emittance (εIR) and the selectivity factors (η = αs/εIR) of the as coated and annealed samples are presented in Table 4. The results clearly show that the thermo-optical property of the as coated sample C30 is better than C60 with respect to the solar selectivity. Although the emissivity of the samples remain nearly the same, a drop in αs from 0.73 in C30to 0.70 in C60 is observed, which is assigned to the reduction of poredensity in the sample C60. The reduction in αs with the decrease in pore-density can be linked to the following reasons: i) higher poredensity improves the effective sunlight trapping inside the pores and enhances multi reflection inside the pore-walls and ii) reflections from the coating surface are weakened due to variations in the dielectric constant [20] in different locality of the coated surface which can be attributed to the presence of a large number of different size air filled nanopores. Thus, the selectivity factor viz. the ratio between solar absorptance and the IR-emittance increases with the decrease in the deposition time. The samples annealed at 500 °C and 700 °C, show an increase in the solar absorptance with the increase in temperature without any significant change in IR-emittance. This results in a positive change in the
4. Conclusion Colored chromium oxide based solar selective coating was electrochemically deposited on 304SS substrate. By manipulating the deposition time and annealing temperature, variations in color were accomplished vis-a-vis changes in the coating thickness. The thermo gravimetric analysis confirms the elimination of large amount of lattice
Table 4 Thermo-optical properties of all samples. Thermo-optical properties
C30
500C30
700C30
900C30
C60
500C60
700C60
900C60
αs εIR η
0.73 0.12 6.08
0.80 0.11 7.27
0.81 0.11 7.36
0.59 0.14 4.22
0.70 0.13 5.38
0.79 0.13 6.07
0.81 0.12 6.75
0.52 0.16 3.25
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Fig. 6. Load displacement plot of samples with coating time (a) 30 min and (b) 60 min.
Fig. 7. Comparative graphical representation of (a) Hardness, (b) Elastic modulus, (c) Wtotal, and (d) H/E ratio data as a function of annealing temperature
lattice bound water from the coating. The hardness of the coating deposited for 60 min is lower than the coating deposited for 30 min.
bound water from the coating. The changes in coating thickness became very prominent at temperatures above 700 °C. The interfacial bond between the coating and substrate appear to be very strong even after annealing at 900 °C. The microstructural images confirm the presence of nanopores on the coated surfaces in all samples. The nanoporous surface plays an important role in tuning the thermo-optical properties of the samples. Annealing below 900 °C causes increase in pore-density of the samples with nominal reduction in surface roughness. Higher pore-density causes enhancement of solar trapping and induces a higher solar absorptance. The conspicuous drop in solar thermal property accompanied with a color change and increase in surface roughness in the samples annealed at 900 °C arises due to elimination of large amount of
Acknowledgement This work was supported by the Centre of Excellence (COE), TEQIPII under MHRD, Government of India [F.No. 16-6/2015-TS-VII]. References [1] N. Wang, L. Han, H. He, N.-H. Park, K. Koumoto, A novel high-performance photovoltaic–thermoelectric hybrid device, Energy Environ. Sci. 4 (2011) 3676–3679, http://dx.doi.org/10.1039/c1ee01646f.
360
Solar Energy Materials and Solar Cells 182 (2018) 354–361
A. Sarkar et al.
[2] D. Kraemer, B. Poudel, H. 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 flatpanel solar thermoelectric generators with high thermal concentration, Nat. Mater. 10 (2011) 532–538, http://dx.doi.org/10.1038/nmat3013. [3] D. Mills, Advances in solar thermal electricity technology, Sol. Energy 76 (2004) 19–31, http://dx.doi.org/10.1016/S0038-092X(03)00102-6. [4] X. Wang, X. Wu, L. Yuan, C. Zhou, Y. Wang, K. Huang, S. Feng, Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel, Sol. Energy Mater. Sol. Cells 146 (2016) 99–106, http://dx.doi.org/10. 1016/j.solmat.2015.11.040. [5] Z. Chen, T. Boström, Electrophoretically deposited carbon nanotube spectrally selective solar absorbers, Sol. Energy Mater. Sol. Cells 144 (2016) 678–683, http://dx. doi.org/10.1016/j.solmat.2015.10.016. [6] J. Feng, S. Zhang, X. Liu, H. Yu, H. Ding, Y. Tian, J. Ouyang, Solar selective absorbing coatings TiN/TiSiN/SiN prepared on stainless steel substrates, Vacuum 121 (2015) 135–141, http://dx.doi.org/10.1016/j.vacuum.2015.08.013. [7] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energy Environ. Sci. (2014), http://dx.doi.org/10.1039/ c3ee43825b. [8] H.C. Barshilia, N. Selvakumar, K.S. Rajam, A. Biswas, Structure and optical properties of pulsed sputter deposited Cr xOy/Cr/Cr2O3 solar selective coatings, J. Appl. Phys. 103 (2008), http://dx.doi.org/10.1063/1.2831364 (23507-1-11). [9] Y. Yin, Y. Pan, L.X. Hang, D.R. McKenzie, M.M.M. Bilek, Direct current reactive sputtering Cr-Cr2O3 cermet solar selective surfaces for solar hot water applications, Thin Solid Films (2009), http://dx.doi.org/10.1016/j.tsf.2008.09.082. [10] S. Khamlich, M. Maaza, Cr/α-Cr2O3 monodispersed meso-spherical particles for mid-temperature solar absorber application, Energy Procedia (2015), http://dx.doi. org/10.1016/j.egypro.2015.03.229. [11] F. Chen, S.-W. Wang, X. Liu, R. Ji, Z. Li, X. Chen, Y. Chen, W. Lu, Colorful solar selective absorber integrated with different colored units, Opt. Express 24 (2016) A92–A103, http://dx.doi.org/10.1364/OE.24.000A92. [12] A. Dan, K. Chattopadhyay, H.C. Barshilia, B. Basu, Colored selective absorber
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20] [21]
361
coating with excellent durability, Thin Solid Films 620 (2016) 17–22, http://dx.doi. org/10.1016/j.tsf.2016.08.070. Z.C. Orel, M.K. Gunde, M.G. Hutchins, Spectrally selective solar absorbers in different non-black colours, Sol. Energy Mater. Sol. Cells 85 (2005) 41–50, http://dx. doi.org/10.1016/j.solmat.2004.04.010. L. Zheng, F. Zhou, Z. Zhou, X. Song, G. Dong, M. Wang, X. Diao, Angular solar absorptance and thermal stability of Mo-SiO2 double cermet solar selective absorber coating, Sol. Energy 115 (2015) 341–346, http://dx.doi.org/10.1016/j. solener.2015.02.013. X.K. Du, C. Wang, T.M. Wang, B.L. Chen, L. Zhou, N. Ru, Magnetron sputtering high temperature Mo-Al2O3 Cermet Solar Selective Coatings, Mater. Sci. Forum 546–549 (2007) 1773–1776, http://dx.doi.org/10.4028/www.scientific.net/MSF.546-549. 1773. K.S. Klepikova, N.P. Klochko, G.S. Khrypunov, V.R. Kopach, V.M. Lyubov, V.V. Starikov, Solar selective absorber based on zinc oxide-nickel cermet, Appl. Phys. (YSF) 117 (2015) 1, http://dx.doi.org/10.1109/YSF.2015.7333187 2015 Int. Young Sci. Forum.. T.E. Evans, Film formation on stainless steel in a solution containing chromic and sulphuric acids, Corros. Sci. 17 (1977) 105–124, http://dx.doi.org/10.1016/0010938X(77)90012-9. D. Palai, M. Bhattacharya, A. BasuMallick, P. Bera, A.K. Sharma, A.K. Mukhopadhyay, A. Dey, Microstructural, thermo-optical, mechanical and tribological behaviours of vacuum heat treated ultra thin SS304 foils, Mater. Res. Express 3 (2016) 96501. M. Haeri, M. Haeri, ImageJ plugin for analysis of porous scaffolds used in tissue engineering, J. Open Res. Softw. 3 (2015) 2–5, http://dx.doi.org/10.5334/jors.bn. D. Ding, W. Cai, Self-assembled nanostructured composites for solar absorber, Mater. Lett. 93 (2013) 269–271, http://dx.doi.org/10.1016/j.matlet.2012.11.070. W. Oliver, G. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement-sensing indentation systems, J. Mater. Res. 7 (1992) 1564–1583, http://dx.doi.org/10.1557/JMR.1992.1564.