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Influence of nanoporous aluminum oxide interlayer on the optical absorptance of black electroless nickel–phosphorus coating
Thin Solid Films 592 (2015) 88–93
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influence of nanoporous aluminum oxide interlayer on the optical absorptance of black electroless nickel–phosphorus coating Fatemeh Ebrahimi ⁎, Saeed Shirmohammadi Yazdi, Mehdi Hosseini Najafabadi, Fakhreddin Ashrafizadeh Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 29 August 2015 Accepted 2 September 2015 Available online 5 September 2015 Keywords: Anodized aluminum Nonoporous Electroless Ni–P Absorption coefficient Emission coefficient
a b s t r a c t This paper introduces a technique to make an ultra-black surface by employing nanoporous anodized aluminum oxide as a template and deposition of nickel–phosphorus nanowires by the electroless process. The optical properties were compared with two other processes; a conventional black Ni–P deposition and a nickel electrocoloring process, on aluminum substrate. Surface morphologies of the samples were examined by field emission scanning electron microscope and elemental analysis of the coatings was performed by the energy dispersive spectroscopy method. Optical properties of surfaces were determined by spectrophotometry and infrared spectroscopy techniques. In addition, optical characteristics of the coated surfaces were evaluated by calculation of absorption and emission coefficients of the surfaces. The results showed that ultra-black duplex coating possessed an absorption coefficient higher than 99%, while emission coefficient decreased about 6% compared with simple black electroless Ni–P. Calculation of ξ factor indicated that a value of 5.1 proved that optical properties in the duplex coated sample had a significant improvement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Solar energy in the form of heat absorbed by a black surface can be use in photo thermal systems. The main part of the collector utilized for the production of solar energy is the absorber and the needed black absorber surface can be produced by deposition of an appropriate film on a metal substrate [1]. The absorber may be selective or nonselective; selective solar absorber coatings form the basis of a wide range of high performance optical coatings and interference filters. A selective surface is achieved by applying a thin coating of high absorptivity on a metal surface of low emissivity, obtained by several techniques [2]. Since aluminum has light weight, low cost, good thermal conductivity and low emittance, it has been used extensively as a perfect substrate for optical coatings [3,4]. Anodizing, a conventional process to improve surface properties of aluminum alloys, has been used to produce black surfaces [5]. Since the porous structure of anodic oxide layer formed on the surface of aluminum is used as a template to make such structures as nanowires and nanotubes, it could be considered as an interlayer for impregnation of various pigments [6]. A black anodized surface may be produced by electrolytic coloring techniques. In this process, metal particles are deposited into the pores of the anodic oxide coating during the half cycle
⁎ Corresponding author. E-mail addresses: [email protected] (F. Ebrahimi), [email protected] (S.S. Yazdi), [email protected] (M.H. Najafabadi), ashrafi[email protected] (F. Ashrafizadeh).
http://dx.doi.org/10.1016/j.tsf.2015.09.004 0040-6090/© 2015 Elsevier B.V. All rights reserved.
of alternating current. Coloring of aluminum oxide may be achieved by organic dyes or by chemical or electrochemical deposition of inorganic compounds inside the pores. Various black anodizing processes such as inorganic dyes, electrolytic coloring, precipitation pigmentation, or combinations of organic dyeing and electrolytic coloring have been studied for optical applications [7,8]. In these processes, several elements including nickel, copper and tin are used as pigments; due to high absorption, low cost and ease of manufacture, nickel layers have been used more than other coatings. Electrolytic coloring pigmented aluminum oxide solar absorber was first suggested in 1979 [9]. Although it has a good optical performance, the absorber is sensitive to abrasion. Electroless plating is an appropriate technique to deposit nickel layer, regardless of geometry of the substrate, ideal for a wide range of applications in metal finishing industries. Nickel electroless coating has a combination of properties such as corrosion resistance, hardness and lubricity [10]. Electroless plating of nickel is an autocatalytic process for deposition of metal coatings on certain catalytically active substrates using a controlled chemical reaction [11]. Many selective solar absorber designs are possible, but here the selection must be focused on a composite coating over a metallic substrate of sufficiently high infrared (IR) reflectance. In this research, an electroless deposition was applied onto anodized surface of aluminum substrate in order to construct a duplex layer. The duplex coating was compared with electro-colored anodizing and electroless Ni–P in terms of optical properties. Surface properties and optical characteristics of the coatings were evaluated and absorption coefficients of the coatings were studied. The optimum black surface was introduced in
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Fig. 1. Duplex coatings of different interlayer thicknesses; (a) very thin, (b) thick, (c) optimized.
terms of ξ factor; an appropriate absorber should be selective to possess high solar absorptance (α) and low thermal emittance (ε). 2. Experimental details
transform infrared spectroscopy (FTIR) was performed by a spectrometer model 680 PLUS Jasco for determination of emission of the coatings; the radiation reflected by the samples was measured in the wavelengths 2500–25,000 nm.
2.1. Formation of coatings
3. Results and discussion
Commercially pure aluminum sheets (Al-1100) were cut into 20 × 20 × 1 mm samples and cleaned ultrasonically in acetone bath for 5 min. The samples were degreased using 10% sodium hydroxide at 60–70 °C for 5 min, rinsed and immersed for 1 min in nitric acid at room temperature. Anodizing process was carried out in an electrolyte containing 165 g/L H2SO4. A range of anodization times and dc voltages were selected in order to prepare coatings of different thicknesses. The temperature was in the range 13 to 15 °C. Thickness of the porous anodic oxide films was determined by an eddy current thickness meter model Salu Tron®D2. To produce ultra-black surfaces, three methods were applied; first, conventional electroless Ni–P coating was applied directly on aluminum substrate in a commercial Schloetter solution; SLOTONIP 70A with a pH of 5 at 85–90 °C and 750 rpm agitation speed. The deposition rate in this solution was between 18 and 22 μm/h. After deposition, blackening process was performed by immersion of samples in 9 M nitric acid solution at 40 °C for 20 s. In the second method, electroless Ni–P film was applied over an anodized interlayer to achieve the required surface. Finally, the third set of samples was prepared by electrolytic coloring applied onto anodized interlayer for 10 min using ac voltage of 15 V in nickel bath with a pH of 5.6 at 25 °C; the electro-coloring bath contained 37 g/L NiSO4·6H2O, 22 g/L MgSO4·7H2O, 65 g/L (NH4) SO4 and 26 g/L H3BO3 with stainless steel 316 L used as a counter electrode at a constant current of 0.45 A/dm2. In the two latter processes containing interlayers, the thickness of anodized aluminum oxide (AAO) was optimized based on the quality of black coatings.
3.1. Anodized coating A range of voltages and time intervals were used to prepare several anodized films on the aluminum substrate as interlayer. In the electroless deposition of these samples, some of the anodized surfaces failed to give a uniform black surface; some thicker layers quickly changed to a nickel appearance surface with remarkable fine cauliflower morphology as observed by SEM. These samples did not produce a black
2.2. Evaluation of coatings Coating morphologies were characterized by scanning electron microscope (SEM), Philips model XL30, as well as field emission scanning electron microscope (FESEM), Hitachi model S4160. Energy dispersive spectroscopy (EDS) analysis was performed by INCA Oxford Instrument for determination of chemical composition of the coatings; incident electron beam energy of 21 keV was selected at a take-off angle of 35° and a measurement time of 50 s. The phases present in the coatings were analyzed by X-ray diffraction using Philips Expert-MPD equipped with monochromator at 40 kV, 30 mA and CuKa radiation (λ = 1.542 Å). Measurement configuration was Bragg–Brentano (θ–2θ) geometry and the scan step was 0.5° per second. Optical properties of the surfaces, including absorption, emission and ξ factor were evaluated from the results of spectrophotometry (Jasco model V-570). The radiation reflected by the samples was measured in the wavelength range 360–1700 nm. Fourier
Fig. 2. FESEM micrographs of the electroless coating before and after blackening.
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surface, although could be blackened by immersing in nitric acid; a typical blackening process used for electroless Ni–P coatings. On the other hand, very thin interlayers of AAO did not result in uniform black surface due to severe interaction of the solution near the edges; Fig. 1 shows three specimens of duplex coatings; only specimen (c) with the optimized thickness of interlayer has a uniform black surface. The optimized thickness of anodic coating to achieve a uniform black surface was about 40 μm. This thickness was obtained by anodizing at 25 V for 30 min with an increasing rate of voltage of 2 V/min in sulfuric acid; the voltage started from zero and increased at a rate of 2 V/min up to a final value of 25 V and the total anodizing time was 30 min.
3.2. Structure and morphology of coatings Surface morphology of the electroless Ni–P coatings, before and after blackening, is presented in Fig. 2; a typical cauliflower structure was observed on the deposited surface. After blackening, cauliflower interfaces were corroded with pitting corrosion at many points, while, the top surface of the cauliflowers underwent less corrosion. The uniformity of the coatings was decreased after the blackening process. Previous study has shown that when samples were placed in acid solution, some nickel atoms from the surface react with oxygen and form nickel oxides (NiO, Ni2O3) [12]. In this mechanism, oxygen enters into the coating structure and, due to nickel selective resolution and low solubility of phosphorus rich phases, the phosphorus content of surface is increased. After blackening, the interface layers of nickel and phosphorus were corroded leading to a higher surface roughness. High absorption coefficient of this coating has been attributed to this layer of high roughness and the presence of nickel oxides on the surface [11]. Fig. 3(a) shows
Fig. 3. SEM images of the anodized and electro-colored anodized surfaces.
the morphology of a typical anodized layer; anodized surface has a porous structure with pore diameter of about 10 nm. As mentioned, this porous structure is considered an appropriate substrate for pigmentation. Nickel electro-colored surface is shown in Fig. 3(b). When examined at higher magnifications, there was no significant change in the overall surface morphology of anodized sample before and after electro-coloring process, but anodized surface before electro-coloring appeared slightly smoother and more uniform. In achieving a black surface by the electro-coloring process, Ni pigments partially fill the pores of the anodized surface [13]; partial filling of the pores did not produce any significant change between the two surfaces. Fig. 4 shows surface morphology of duplex (anodized-electroless Ni–P) coating. It is evident that there is a rough surface full of voids and pores with a network of nanowires; this structure could be a proper surface to trap light and to act as a perfect absorber. At higher magnification, Fig. 4(b), it is clear that small zones consist accumulation of Ni–P nanowires with diameter between 10 and 90 nm. During the electroless deposition, Ni–P nanowires, after filling the pores of AAO, continued to deposit on the surface, agglomerated to form islands of about 1 μm around the holes. This interesting phenomenon probably arose as a result of the high surface energy of the nanowires. Under these conditions, the common cauliflower structure was not produced, instead, when the electroless deposition continued to change the surface from black to silver, nanostructured cauliflower morphology was obtained. Fig. 5(a) shows such a structure where it is evident that this morphology is much finer than conventional electroless black surface shown in Fig. 2(b). Nucleation and growth of Ni–P in nanopores of anodized interlayer are responsible for this fine structure. In this process, the growth of Ni–P deposition continues until it completely fills the pores and, then, covers the surface. This mechanism was verified by polishing the surface as is shown in Fig. 5(b).
Fig. 4. SEM images of the duplex surface.
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Fig. 5. SEM images of as-blackened thick duplex surfaces before and after polishing.
Fig. 6 presents SEM image of fractured cross section of duplex coating after partial dissolving of alumina in NaOH. It can be seen that nanowires are embedded inside the pore channels of the anodized interlayer. EDS elemental analysis of duplex coating, shown in Fig. 7(a), confirms the presence of Ni–P. Oxygen and aluminum elements, which appeared in the spectrum, are related to Al2O3 interlayer. Results of microanalysis proved the formation of Ni–P nanowires inside and over the pore channels of the template. Elemental analysis of the electrocolored sample, shown in Fig. 7(b), revealed deposition of nickel at
Fig. 7. EDS spectra of the duplex and electro-colored surfaces.
the bottom of pores, although there was no evidence on the surface of sample in SEM micrograph. X-ray diffraction pattern of electro-colored black specimen is shown in Fig. 8(a). Diffraction lines which correspond to Al and Ni indicate that nanowires in the sample contain crystalline nickel, whereas Al peaks belong to aluminum substrate. As mentioned, in obtaining a good black surface, nickel pigments do not completely fill the pores. Diffraction patterns of Ni–P nanowires proved an amorphous state in this duplex process with no crystals of nickel, Fig. 8(b). Similar findings have been reported by other researchers [14]. Although Ni–P nanowires have amorphous structure, their growth in the template with the morphologies obtained on the surface, make them good candidate in pigmentation on alumina template to produce an ultra-black surface for optical applications.
3.3. Optical properties of coatings Fig. 6. Cross section image of the duplex coating after partial dissolution of the template in NaOH.
An ideal selective absorber is a material which absorbs all the solar spectrum with no emission. In practice, a selective absorber is
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Fig. 8. X-ray diffraction patterns of the duplex and electro-colored samples.
considered good if it has a hemispherical emissivity ε b 0.2 and absorptivity α N 0.9 [15]. The fraction of radiation incident on the surface of absorbed material as a function of reflection of the material, R, at a given wavelength is defined as absorptance of the solar collector surface: Z αsol ¼
2
0:3
Isol ðλÞð1−RðλÞÞdλ : Z 2 Isol ðλÞdλ
ξ ¼ α=ε: ð1Þ
0:3
The direct normal solar irradiance, Isol, has been defined according to ISO standard 9845-1 (1992) where the air mass of 1.5 is used [16]. Similarly, normal thermal emittance is an equally weighted fraction but between emitted radiation and the Planck black body distribution, Ip, at 100 °C, Z εt ¼
25
2:5
Ip ðλ; TÞð1−RðλÞÞdλ : Z 25 Ip ðλ; TÞdλ
above 2 μm. An ideal selective absorber material absorbs the maximum amount of incident solar radiation, and re-emits a minimum amount of the absorbed energy [17]. The ratio of solar absorptance to thermal emittance is a main parameter to select an appropriate absorber surface, which is defined as;
ð2Þ
2:5
The standard spectral solar flux incident was limited to the range between 0.3 and 2.5 μm (UV/vis/NIR wavelength ranges) with the maximum solar intensity around 0.55 μm, whereas the optical properties of a real body in the infrared wavelength range can be characterized by its thermal emission compared to the ideal blackbody in wavelengths
ð3Þ
The greater value of ξ indicates better absorber surface. Reflections of the samples versus wavelength are shown in Fig. 9; as can be seen in Fig. 9(a), duplex samples have lower reflection data. By using this reflection curve and Eq. (1), absorption coefficient was calculated in Matlab. While the electro-colored sample has an absorption coefficient of 0.965, this value is 0.992 for the duplex and 0.985 for black electroless sample. It is known that electroless Ni–P coating is more uniform than electro-coloring and produced higher absorption coefficient. The electroless process on anodized surface not only fills the pores of surface but also constructs new nanowire network on the surface; consequently the duplex coating has higher absorption coefficient than the electro-colored surface. Difference between solar absorption of black electroless and duplex coating is due to their structures and morphologies. This means that nanoporous AAO interlayer forms an appropriate pattern for electroless coating to construct nanostructured surface in order to absorb maximum wavelengths. The emission curves of electro-colored, black electroless, and duplex samples are shown in Fig. 9(b). It is observed that FTIR curve of duplex
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Table 1 Optical properties of absorber surfaces. Coatings
Absorptivity (α)
Emissivity (ε)
ξ=α/ε
Electroless Ni–P Electrocolored Ni Duplex
0.985 0.965 0992
0.268 0.254 0.193
3.7 3.79 5.1
process, in electro-coloring, to prepare a proper black surface, nickel only partially fills the pores. In constructing a duplex coating, electroless process deposits amorphous Ni–P nanowires over nanoporous aluminum oxide. Duplex coating has an absorption coefficient of 0.992 which is higher than electrocolored and black electroless surfaces with absorption coefficients of 0.965 and 0.985, respectively. Moreover, results show that duplex sample had an emission coefficient of 0.193 which is lower than electro-colored and black electroless samples (0.254 and 0.268, respectively). By comparison of ξ factors, duplex coating has the optimum optical characteristics among the three surfaces. Such desirable optical properties are related to good absorption of black electroless Ni–P together with a nanoporous anodized aluminum oxide sublayer.
References
Fig. 9. Reflection of the samples vs. wavelength in the solar range and infrared region.
sample is located approximately 10% above that of the other samples. Emission coefficient of the samples was calculated by Eq. (2) using FTIR data; the results revealed that duplex sample has an emission coefficient of 0.193 while electro-coloring and conventional black electroless produced values of 0.254 and 0.268, respectively. These results show that duplex coating had the best emission coefficient among the three methods. Reduction of emission is due to deposition of amorphous Ni–P on nanopores, this amorphous structure can transmit IR radiation better than crystalline nickel; aluminum substrate could reflect the infrared wavelength of light, consequently, would reduce the emission. ξ factor, used to compare optical characteristics of the surfaces, obtained 3.8 and 3.7 for electrolytic colored and black electroless Ni–P samples, respectively, while this value increased to 5.1 for duplex sample. Table 1 shows optical properties of absorber surfaces. Comparison of these experimental results indicates that duplex coating has a unique nanostructure and morphology; it possesses a higher absorption, a lower emission and overall better optical properties, so it can be considered as an appropriate solar selective absorber. 4. Conclusions Electroless Ni–P coating was employed as a method to fill the pores of anodized aluminum to produce a surface with unique optical properties. In this process, at the first stage, Ni–P fills the pores of the anodized surface and, then, nanowires grow on the surface. High surface energy of nanowires caused their accumulation to form a network on the surface with plenty of voids which help trapping the light. Despite this
[1] A. Rabl, Active Solar Collectors and Their Applications, Oxford University Press, 1985. [2] C.E. Kennedy, Review of Mid- to High-temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory Golden, Colorado, 2002. [3] M. Joly, Y. Antonetti, M. Python, M. Gonzalez, T. Gascou, J.-L. Scartezzini, A. Schüler, Novel black selective coating for tubular solar absorbers based on a sol–gel method, Sol. Energy 94 (2013) 233–239. [4] R. Kunič, M. Mihelčič, B. Orel, L. Slemenik Perše, B. Bizjak, J. Kovač, S. Brunold, Life expectancy prediction and application properties of novel polyurethane based thickness sensitive and thickness insensitive spectrally selective paint coatings for solar absorbers, Sol. Energy Mater. Sol. Cells 95 (2011) 2965–2975. [5] S. Süzer, F. Kadirgan, H. Söhmen, A. Wetherilt, İ.E. Türe, Spectroscopic characterization of Al2O3-Ni selective absorbers for solar collectors, Sol. Energy Mater. Sol. Cells 52 (1998) 55–60. [6] S. Shirmohammadi Yazdi, F. Ashrafizadeh, A. Hakimizad, Improving the grain structure and adhesion of Ni–P coating to 3004 aluminum substrate by nanostructured anodic film interlayer, Surf. Coat. Technol. 232 (2013) 561–566. [7] M. Zemanova, M. Chovancova, Z. Galikova, P. Krivošík, Nickel electrolytic colouring of anodic alumina for selective solar absorbing films, Renew. Energy 33 (2008) 2303–2310. [8] C.A. Grubbs, Anodizing of aluminum, Met. Finish. 97 (1999) 476–493. [9] Å. Andersson, O. Hunderi, C. Granqvist, Nickel pigmented anodic aluminum oxide for selective absorption of solar energy, J. Appl. Phys. 51 (1980) 754–764. [10] P. Sahoo, S.K. Das, Tribology of electroless nickel coatings—a review, Mater. Des. 32 (2011) 1760–1775. [11] M. Hosseini Najafabadi, F. Ashrafizadeh, M. Monirvaghefi, Optimization of nanostructured black electroless Ni–P coating for solar absorber, International Journal of Modern Physics: Conference Series, World Scientific 2012, pp. 670–678. [12] S. Ashrafizadeh, M. Bagheri, M. Hosseini, A comparative study on optical properties of black nickel coatings and color anodizing for solar absorber, Optical Interference Coatings, Optical Society of America 2013, p. MC. 3. [13] A. Wazwaz, J. Salmi, R. Bes, The effects of nickel-pigmented aluminium oxide selective coating over aluminium alloy on the optical properties and thermal efficiency of the selective absorber prepared by alternate and reverse periodic plating technique, Energy Convers. Manag. 51 (2010) 1679–1683. [14] Y.-F. Liu, F.-H. Wang, D.-L. Guo, S.-Y. Huang, J.-P. Sang, X.-W. Zou, Effects of heat treatment on optical absorption properties of Ni–P/AAO nano-array composite structure, Appl. Phys. A 97 (2009) 677–681. [15] C.G. Granqvist, Transparent conductors as solar energy materials: a panoramic review, Sol. Energy Mater. Sol. Cells 91 (2007) 1529–1598. [16] T. Boström, E. Wäckelgård, G. Westin, Solution-chemical derived nickel–alumina coatings for thermal solar absorbers, Sol. Energy 74 (2003) 497–503. [17] A. Amri, Z.T. Jiang, T. Pryor, C.-Y. Yin, S. Djordjevic, Developments in the synthesis of flat plate solar selective absorber materials via sol–gel methods: a review, Renew. Sust. Energ. Rev. 36 (2014) 316–328.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influence of nanoporous aluminum oxide interlayer on the optical absorptance of black electroless nickel–phosphorus coating Fatemeh Ebrahimi ⁎, Saeed Shirmohammadi Yazdi, Mehdi Hosseini Najafabadi, Fakhreddin Ashrafizadeh Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 29 August 2015 Accepted 2 September 2015 Available online 5 September 2015 Keywords: Anodized aluminum Nonoporous Electroless Ni–P Absorption coefficient Emission coefficient
a b s t r a c t This paper introduces a technique to make an ultra-black surface by employing nanoporous anodized aluminum oxide as a template and deposition of nickel–phosphorus nanowires by the electroless process. The optical properties were compared with two other processes; a conventional black Ni–P deposition and a nickel electrocoloring process, on aluminum substrate. Surface morphologies of the samples were examined by field emission scanning electron microscope and elemental analysis of the coatings was performed by the energy dispersive spectroscopy method. Optical properties of surfaces were determined by spectrophotometry and infrared spectroscopy techniques. In addition, optical characteristics of the coated surfaces were evaluated by calculation of absorption and emission coefficients of the surfaces. The results showed that ultra-black duplex coating possessed an absorption coefficient higher than 99%, while emission coefficient decreased about 6% compared with simple black electroless Ni–P. Calculation of ξ factor indicated that a value of 5.1 proved that optical properties in the duplex coated sample had a significant improvement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Solar energy in the form of heat absorbed by a black surface can be use in photo thermal systems. The main part of the collector utilized for the production of solar energy is the absorber and the needed black absorber surface can be produced by deposition of an appropriate film on a metal substrate [1]. The absorber may be selective or nonselective; selective solar absorber coatings form the basis of a wide range of high performance optical coatings and interference filters. A selective surface is achieved by applying a thin coating of high absorptivity on a metal surface of low emissivity, obtained by several techniques [2]. Since aluminum has light weight, low cost, good thermal conductivity and low emittance, it has been used extensively as a perfect substrate for optical coatings [3,4]. Anodizing, a conventional process to improve surface properties of aluminum alloys, has been used to produce black surfaces [5]. Since the porous structure of anodic oxide layer formed on the surface of aluminum is used as a template to make such structures as nanowires and nanotubes, it could be considered as an interlayer for impregnation of various pigments [6]. A black anodized surface may be produced by electrolytic coloring techniques. In this process, metal particles are deposited into the pores of the anodic oxide coating during the half cycle
⁎ Corresponding author. E-mail addresses: [email protected] (F. Ebrahimi), [email protected] (S.S. Yazdi), [email protected] (M.H. Najafabadi), ashrafi[email protected] (F. Ashrafizadeh).
http://dx.doi.org/10.1016/j.tsf.2015.09.004 0040-6090/© 2015 Elsevier B.V. All rights reserved.
of alternating current. Coloring of aluminum oxide may be achieved by organic dyes or by chemical or electrochemical deposition of inorganic compounds inside the pores. Various black anodizing processes such as inorganic dyes, electrolytic coloring, precipitation pigmentation, or combinations of organic dyeing and electrolytic coloring have been studied for optical applications [7,8]. In these processes, several elements including nickel, copper and tin are used as pigments; due to high absorption, low cost and ease of manufacture, nickel layers have been used more than other coatings. Electrolytic coloring pigmented aluminum oxide solar absorber was first suggested in 1979 [9]. Although it has a good optical performance, the absorber is sensitive to abrasion. Electroless plating is an appropriate technique to deposit nickel layer, regardless of geometry of the substrate, ideal for a wide range of applications in metal finishing industries. Nickel electroless coating has a combination of properties such as corrosion resistance, hardness and lubricity [10]. Electroless plating of nickel is an autocatalytic process for deposition of metal coatings on certain catalytically active substrates using a controlled chemical reaction [11]. Many selective solar absorber designs are possible, but here the selection must be focused on a composite coating over a metallic substrate of sufficiently high infrared (IR) reflectance. In this research, an electroless deposition was applied onto anodized surface of aluminum substrate in order to construct a duplex layer. The duplex coating was compared with electro-colored anodizing and electroless Ni–P in terms of optical properties. Surface properties and optical characteristics of the coatings were evaluated and absorption coefficients of the coatings were studied. The optimum black surface was introduced in
F. Ebrahimi et al. / Thin Solid Films 592 (2015) 88–93
89
Fig. 1. Duplex coatings of different interlayer thicknesses; (a) very thin, (b) thick, (c) optimized.
terms of ξ factor; an appropriate absorber should be selective to possess high solar absorptance (α) and low thermal emittance (ε). 2. Experimental details
transform infrared spectroscopy (FTIR) was performed by a spectrometer model 680 PLUS Jasco for determination of emission of the coatings; the radiation reflected by the samples was measured in the wavelengths 2500–25,000 nm.
2.1. Formation of coatings
3. Results and discussion
Commercially pure aluminum sheets (Al-1100) were cut into 20 × 20 × 1 mm samples and cleaned ultrasonically in acetone bath for 5 min. The samples were degreased using 10% sodium hydroxide at 60–70 °C for 5 min, rinsed and immersed for 1 min in nitric acid at room temperature. Anodizing process was carried out in an electrolyte containing 165 g/L H2SO4. A range of anodization times and dc voltages were selected in order to prepare coatings of different thicknesses. The temperature was in the range 13 to 15 °C. Thickness of the porous anodic oxide films was determined by an eddy current thickness meter model Salu Tron®D2. To produce ultra-black surfaces, three methods were applied; first, conventional electroless Ni–P coating was applied directly on aluminum substrate in a commercial Schloetter solution; SLOTONIP 70A with a pH of 5 at 85–90 °C and 750 rpm agitation speed. The deposition rate in this solution was between 18 and 22 μm/h. After deposition, blackening process was performed by immersion of samples in 9 M nitric acid solution at 40 °C for 20 s. In the second method, electroless Ni–P film was applied over an anodized interlayer to achieve the required surface. Finally, the third set of samples was prepared by electrolytic coloring applied onto anodized interlayer for 10 min using ac voltage of 15 V in nickel bath with a pH of 5.6 at 25 °C; the electro-coloring bath contained 37 g/L NiSO4·6H2O, 22 g/L MgSO4·7H2O, 65 g/L (NH4) SO4 and 26 g/L H3BO3 with stainless steel 316 L used as a counter electrode at a constant current of 0.45 A/dm2. In the two latter processes containing interlayers, the thickness of anodized aluminum oxide (AAO) was optimized based on the quality of black coatings.
3.1. Anodized coating A range of voltages and time intervals were used to prepare several anodized films on the aluminum substrate as interlayer. In the electroless deposition of these samples, some of the anodized surfaces failed to give a uniform black surface; some thicker layers quickly changed to a nickel appearance surface with remarkable fine cauliflower morphology as observed by SEM. These samples did not produce a black
2.2. Evaluation of coatings Coating morphologies were characterized by scanning electron microscope (SEM), Philips model XL30, as well as field emission scanning electron microscope (FESEM), Hitachi model S4160. Energy dispersive spectroscopy (EDS) analysis was performed by INCA Oxford Instrument for determination of chemical composition of the coatings; incident electron beam energy of 21 keV was selected at a take-off angle of 35° and a measurement time of 50 s. The phases present in the coatings were analyzed by X-ray diffraction using Philips Expert-MPD equipped with monochromator at 40 kV, 30 mA and CuKa radiation (λ = 1.542 Å). Measurement configuration was Bragg–Brentano (θ–2θ) geometry and the scan step was 0.5° per second. Optical properties of the surfaces, including absorption, emission and ξ factor were evaluated from the results of spectrophotometry (Jasco model V-570). The radiation reflected by the samples was measured in the wavelength range 360–1700 nm. Fourier
Fig. 2. FESEM micrographs of the electroless coating before and after blackening.
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surface, although could be blackened by immersing in nitric acid; a typical blackening process used for electroless Ni–P coatings. On the other hand, very thin interlayers of AAO did not result in uniform black surface due to severe interaction of the solution near the edges; Fig. 1 shows three specimens of duplex coatings; only specimen (c) with the optimized thickness of interlayer has a uniform black surface. The optimized thickness of anodic coating to achieve a uniform black surface was about 40 μm. This thickness was obtained by anodizing at 25 V for 30 min with an increasing rate of voltage of 2 V/min in sulfuric acid; the voltage started from zero and increased at a rate of 2 V/min up to a final value of 25 V and the total anodizing time was 30 min.
3.2. Structure and morphology of coatings Surface morphology of the electroless Ni–P coatings, before and after blackening, is presented in Fig. 2; a typical cauliflower structure was observed on the deposited surface. After blackening, cauliflower interfaces were corroded with pitting corrosion at many points, while, the top surface of the cauliflowers underwent less corrosion. The uniformity of the coatings was decreased after the blackening process. Previous study has shown that when samples were placed in acid solution, some nickel atoms from the surface react with oxygen and form nickel oxides (NiO, Ni2O3) [12]. In this mechanism, oxygen enters into the coating structure and, due to nickel selective resolution and low solubility of phosphorus rich phases, the phosphorus content of surface is increased. After blackening, the interface layers of nickel and phosphorus were corroded leading to a higher surface roughness. High absorption coefficient of this coating has been attributed to this layer of high roughness and the presence of nickel oxides on the surface [11]. Fig. 3(a) shows
Fig. 3. SEM images of the anodized and electro-colored anodized surfaces.
the morphology of a typical anodized layer; anodized surface has a porous structure with pore diameter of about 10 nm. As mentioned, this porous structure is considered an appropriate substrate for pigmentation. Nickel electro-colored surface is shown in Fig. 3(b). When examined at higher magnifications, there was no significant change in the overall surface morphology of anodized sample before and after electro-coloring process, but anodized surface before electro-coloring appeared slightly smoother and more uniform. In achieving a black surface by the electro-coloring process, Ni pigments partially fill the pores of the anodized surface [13]; partial filling of the pores did not produce any significant change between the two surfaces. Fig. 4 shows surface morphology of duplex (anodized-electroless Ni–P) coating. It is evident that there is a rough surface full of voids and pores with a network of nanowires; this structure could be a proper surface to trap light and to act as a perfect absorber. At higher magnification, Fig. 4(b), it is clear that small zones consist accumulation of Ni–P nanowires with diameter between 10 and 90 nm. During the electroless deposition, Ni–P nanowires, after filling the pores of AAO, continued to deposit on the surface, agglomerated to form islands of about 1 μm around the holes. This interesting phenomenon probably arose as a result of the high surface energy of the nanowires. Under these conditions, the common cauliflower structure was not produced, instead, when the electroless deposition continued to change the surface from black to silver, nanostructured cauliflower morphology was obtained. Fig. 5(a) shows such a structure where it is evident that this morphology is much finer than conventional electroless black surface shown in Fig. 2(b). Nucleation and growth of Ni–P in nanopores of anodized interlayer are responsible for this fine structure. In this process, the growth of Ni–P deposition continues until it completely fills the pores and, then, covers the surface. This mechanism was verified by polishing the surface as is shown in Fig. 5(b).
Fig. 4. SEM images of the duplex surface.
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Fig. 5. SEM images of as-blackened thick duplex surfaces before and after polishing.
Fig. 6 presents SEM image of fractured cross section of duplex coating after partial dissolving of alumina in NaOH. It can be seen that nanowires are embedded inside the pore channels of the anodized interlayer. EDS elemental analysis of duplex coating, shown in Fig. 7(a), confirms the presence of Ni–P. Oxygen and aluminum elements, which appeared in the spectrum, are related to Al2O3 interlayer. Results of microanalysis proved the formation of Ni–P nanowires inside and over the pore channels of the template. Elemental analysis of the electrocolored sample, shown in Fig. 7(b), revealed deposition of nickel at
Fig. 7. EDS spectra of the duplex and electro-colored surfaces.
the bottom of pores, although there was no evidence on the surface of sample in SEM micrograph. X-ray diffraction pattern of electro-colored black specimen is shown in Fig. 8(a). Diffraction lines which correspond to Al and Ni indicate that nanowires in the sample contain crystalline nickel, whereas Al peaks belong to aluminum substrate. As mentioned, in obtaining a good black surface, nickel pigments do not completely fill the pores. Diffraction patterns of Ni–P nanowires proved an amorphous state in this duplex process with no crystals of nickel, Fig. 8(b). Similar findings have been reported by other researchers [14]. Although Ni–P nanowires have amorphous structure, their growth in the template with the morphologies obtained on the surface, make them good candidate in pigmentation on alumina template to produce an ultra-black surface for optical applications.
3.3. Optical properties of coatings Fig. 6. Cross section image of the duplex coating after partial dissolution of the template in NaOH.
An ideal selective absorber is a material which absorbs all the solar spectrum with no emission. In practice, a selective absorber is
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Fig. 8. X-ray diffraction patterns of the duplex and electro-colored samples.
considered good if it has a hemispherical emissivity ε b 0.2 and absorptivity α N 0.9 [15]. The fraction of radiation incident on the surface of absorbed material as a function of reflection of the material, R, at a given wavelength is defined as absorptance of the solar collector surface: Z αsol ¼
2
0:3
Isol ðλÞð1−RðλÞÞdλ : Z 2 Isol ðλÞdλ
ξ ¼ α=ε: ð1Þ
0:3
The direct normal solar irradiance, Isol, has been defined according to ISO standard 9845-1 (1992) where the air mass of 1.5 is used [16]. Similarly, normal thermal emittance is an equally weighted fraction but between emitted radiation and the Planck black body distribution, Ip, at 100 °C, Z εt ¼
25
2:5
Ip ðλ; TÞð1−RðλÞÞdλ : Z 25 Ip ðλ; TÞdλ
above 2 μm. An ideal selective absorber material absorbs the maximum amount of incident solar radiation, and re-emits a minimum amount of the absorbed energy [17]. The ratio of solar absorptance to thermal emittance is a main parameter to select an appropriate absorber surface, which is defined as;
ð2Þ
2:5
The standard spectral solar flux incident was limited to the range between 0.3 and 2.5 μm (UV/vis/NIR wavelength ranges) with the maximum solar intensity around 0.55 μm, whereas the optical properties of a real body in the infrared wavelength range can be characterized by its thermal emission compared to the ideal blackbody in wavelengths
ð3Þ
The greater value of ξ indicates better absorber surface. Reflections of the samples versus wavelength are shown in Fig. 9; as can be seen in Fig. 9(a), duplex samples have lower reflection data. By using this reflection curve and Eq. (1), absorption coefficient was calculated in Matlab. While the electro-colored sample has an absorption coefficient of 0.965, this value is 0.992 for the duplex and 0.985 for black electroless sample. It is known that electroless Ni–P coating is more uniform than electro-coloring and produced higher absorption coefficient. The electroless process on anodized surface not only fills the pores of surface but also constructs new nanowire network on the surface; consequently the duplex coating has higher absorption coefficient than the electro-colored surface. Difference between solar absorption of black electroless and duplex coating is due to their structures and morphologies. This means that nanoporous AAO interlayer forms an appropriate pattern for electroless coating to construct nanostructured surface in order to absorb maximum wavelengths. The emission curves of electro-colored, black electroless, and duplex samples are shown in Fig. 9(b). It is observed that FTIR curve of duplex
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Table 1 Optical properties of absorber surfaces. Coatings
Absorptivity (α)
Emissivity (ε)
ξ=α/ε
Electroless Ni–P Electrocolored Ni Duplex
0.985 0.965 0992
0.268 0.254 0.193
3.7 3.79 5.1
process, in electro-coloring, to prepare a proper black surface, nickel only partially fills the pores. In constructing a duplex coating, electroless process deposits amorphous Ni–P nanowires over nanoporous aluminum oxide. Duplex coating has an absorption coefficient of 0.992 which is higher than electrocolored and black electroless surfaces with absorption coefficients of 0.965 and 0.985, respectively. Moreover, results show that duplex sample had an emission coefficient of 0.193 which is lower than electro-colored and black electroless samples (0.254 and 0.268, respectively). By comparison of ξ factors, duplex coating has the optimum optical characteristics among the three surfaces. Such desirable optical properties are related to good absorption of black electroless Ni–P together with a nanoporous anodized aluminum oxide sublayer.
References
Fig. 9. Reflection of the samples vs. wavelength in the solar range and infrared region.
sample is located approximately 10% above that of the other samples. Emission coefficient of the samples was calculated by Eq. (2) using FTIR data; the results revealed that duplex sample has an emission coefficient of 0.193 while electro-coloring and conventional black electroless produced values of 0.254 and 0.268, respectively. These results show that duplex coating had the best emission coefficient among the three methods. Reduction of emission is due to deposition of amorphous Ni–P on nanopores, this amorphous structure can transmit IR radiation better than crystalline nickel; aluminum substrate could reflect the infrared wavelength of light, consequently, would reduce the emission. ξ factor, used to compare optical characteristics of the surfaces, obtained 3.8 and 3.7 for electrolytic colored and black electroless Ni–P samples, respectively, while this value increased to 5.1 for duplex sample. Table 1 shows optical properties of absorber surfaces. Comparison of these experimental results indicates that duplex coating has a unique nanostructure and morphology; it possesses a higher absorption, a lower emission and overall better optical properties, so it can be considered as an appropriate solar selective absorber. 4. Conclusions Electroless Ni–P coating was employed as a method to fill the pores of anodized aluminum to produce a surface with unique optical properties. In this process, at the first stage, Ni–P fills the pores of the anodized surface and, then, nanowires grow on the surface. High surface energy of nanowires caused their accumulation to form a network on the surface with plenty of voids which help trapping the light. Despite this
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