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Application of CuCoMn[O.sub.x] coat by sol gel technique on aluminum and copper substrates for solar absorber application.

Abstract Solar thermal heaters are used widely in domestic and industrial applications. The main part of solar thermal heaters is the absorber surface which must have a maximum absorptivity ([alpha) and minimum emissivity ([epsilon]) of solar radiation. This is achieved by application of selective coating on the absorber surface. In the present work, solar selective CuCoMn[O.sub.x] spinel films are deposited by sol gel technique using a dip-coating technique on copper and aluminum sheets. The precursor's ratio Co:Cu:Mn applied is 1:3:3. Different precursor molar ratios were combined with a fixed amount of solvents for the coating process. Process parameters such as withdrawal rate, heat treatment, and substrate materials on the coat characteristics and optical properties were studied. The coated metallic samples were heat treated at 450[degrees]C for 30 min in the case of aluminum and at 200[degrees]C at different times in the case of copper. Optical properties of the coatings, namely absorptivity ([alpha]) and emissivity ([epsilon]) were measured and the deposition process parameters were optimized in order to produce the maximum selectivity ([alpha]/[epsilon]) values. The deposition parameters were found to influence both the thickness and surface roughness of the coatings. As the coating thickness decreases, the absorptivity increases while the emissivity decreases irrespective of the substrate material. It was also observed from the results that when applying the coat on aluminum substrates, a maximum selectivity value of ([alpha]/[epsilon]) = 31 was realized while for the copper substrates a maximum value of ([alpha]/[epsilon]) = 81.8 was obtained. The deposited coatings were analyzed using SEM, XRD, and AFM.

Keywords Sol-gel, CuCoMn[O.sub.x] selective coating, Optical properties, Surface characteristics

Introduction

Solar energy is among the most important sources of renewable clean energy. In order to obtain solar radiation in a most efficient manner, selective absorber surfaces for the solar spectrum are necessary. Such surfaces are usually made of metallic materials onto which a layer of selective coating is applied. Thus, a selective coat is required to have a high absorption ([alpha]) in the solar wavelength range and an accompanying low thermal emissivity ([epsilon]) in the near infrared (NIR) and far-infrared (FIR) wavelength ranges. These requirements correspond to a material that has a reflectance of less than 10% in the UV range and greater than 90% in the IR range (wavelength >2.5 [micro]m). (1) By combining these optical attributes, a selectivity factor can be defined by the absorptivity/ emissivity ratio ([alpha]/[epsilon]) (2); the higher the selectivity ratio the more promising the material will be for such solar applications.

Solar absorber coatings have attracted a great deal of interest because the efficiency of any solar collector system is strongly dependent on the ability of its absorber to convert solar radiation into heat. There are many techniques for applying selective coating such as electroplating, (3-5) sol gel, (2,6-9) and CVD. (9) In a recent review, PVD selective coating based on transition metal nitrides, oxides, and oxinitrides was found to have promising potential for high temperature selective coating applications. (10)

Today, sol-gel thin films are used for various applications such as optics, energy conversion, electronics, or sensor devices. (11) The main advantage of the sol-gel film deposition process is the ease with which coatings are formed by simply dipping of the substrate into an appropriate colloidal solution and withdrawing in a controlled manner, thus avoiding more expensive deposition techniques. (8) An additional benefit is that the sol-gel technique allows for the deposition of thin films with a uniform chemical composition, as the dipping solution is explained in reference 12. However, the application of sol gel for selective coating remains an uncommon practice likely due to the fact that inadequate film thicknesses are realized. (12)

In the last few years, solution-chemistry or sol gel science has been proven to be a suitable method for the synthesis of nanoparticle composites appropriate for thermal solar absorber applications. (6,13) Since metals have relatively low thermal emissivity values, absorber coatings are prepared mainly on metallic substrates (Al, Cu), which inherently have high thermal conductivity and good corrosion resistance. (2)

Thin films of cobalt oxide (CO) and tin oxide (TO) were separately prepared by the dipping sol-gel method on glass, stainless-steel (SS), and nickeled SS substrates. (12) A multi-layer coating on SS substrates consisting of a layer of cobalt oxide onto which was deposited another layer of nickel then a final layer of tin oxide resulted in [alpha] = 0.72 and [epsilon] = 0.037. (12)

Cobalt oxides, copper oxides, and manganese oxides are of the most interest in the research field of selective coatings using sol-gel technique (2,3,7-9) for solar heating applications due to ease of application and attractive optical properties. Although with little or no processing details, in reference 2, a CuCoMn[O.sub.x] layer was prepared using sol-gel coating technique on aluminum substrate and then annealed at 450[degrees]C for 30 min. Good optical properties were obtained with [alpha] = 0.83 and [epsilon] = 0.01. Reports in reference 8 showed that sol-gel coating of CuCoMn[O.sub.x] spinels on aluminum substrates resulted in [alpha] = 0.855 and [epsilon] = 0.045, although more than one dipping and annealing cycles were applied. Also, titanium doped and undoped CuCoMn[O.sub.x] spinel films on aluminum substrates with Si[O.sub.x] protective overcoat resulted in [alpha] = 0.71-0.95 and [epsilon] < 0.03. (8) A selectively absorbing surface of nickel nanoparticles mixed with alumina on aluminum substrate using spin coating was investigated. (14) The optimal coating composed of 65% Ni, having a thickness of 0.1 pm and particle size about 10 nm, resulted in [alpha] = 0.83 and [epsilon] = 0.03. Addition of antireflection silicon dioxide coating increased the absorptance to 0.93 and the emittance to 0.04. (14) The application of silica sol antireflection coats on glass cover plates reduced the reflectance losses and increased the abrasion resistance. (15) This was found to be also effective on painted absorbers where absorptance increased by 0.01-0.02 without affecting the thermal emittance at 100[degrees]C. (16) A highly selective coat based on Cu-Mn-Si-O oxides on aluminum substrates showed absorptance of 0.95 and emittance of 0.035. (17,18) In a recent report, (19) it was shown for a multi-layered Cu-C-Mn-Si-O based nanocrystalline thin films which was produced by sol gel on SS substrate that a maximum absorptance of 0.95 and emissivity of 0.12 at 100[degrees]C can be realized where each layer was between 50 and 100 nm. (19) Here, the layers produced were stable in air at 360[degrees]C.

The aim of the present work is to study the effect of concentration of the solution, deposition parameters, and post-heat treatment on the characteristics and optical properties of CuCoMn[O.sub.x] coating. Deposition was done by sol-gel dip-coating technique on both aluminum and copper substrates. A key benefit of this technique was the simultaneous coverage of both sides of the substrates. Aluminum and copper were selected as substrate materials in this study due to their high thermal conductivity which is preferable for most non-concentrating solar collectors. It is noteworthy to mention that there is yet no literature reports on the use of copper as a substrate for sol-gel technique while it is used for black nickel electroplating and electroless plating techniques. (4,5,20) The deposited coatings were characterized by measuring coating thickness, surface roughness, surface topography, and determining microstructural states using scanning electron microscopy (SEM) as well as X-ray diffraction (XRD) analysis.

Methodology

Precursor solution was prepared with Mn(II)acetate tetrahydrate, Cu(II) nitrate trihydrate, and Co(II)acetate tetrahydrate. The molar ratio of Co:Cu:Mn was 1:3:3. (2) Cu(II) nitrate trihydrate was first dissolved in ethyleneglycol (50 mL) and then added to the solution of Mn(II)acetate tetrahydrate dissolved in absolute ethanol (60 mL). This was followed by additions of the Co(II)acetate tetrahydrate and the ethyl acetoacetate (EAA). The thermal hydrolysis was performed at 60-70[degrees]C until volume of the sol decreased to half of its initial value. The precursor was added with different molar ratios (divided by 40, 50, and 60) to a fixed amount of solvent in order to study the effect of solution concentration.

The substrate samples were cut from copper sheet (0.2 mm thickness) and aluminum sheet (0.5 mm thickness) to dimensions of 10 cm x 2 cm. These were first pretreated by immersion in a degreasing solution of 15 g/L sodium phosphate and 15 g/L sodium carbonate solution in water at 80[degrees]C for 3 min in order to remove fats and oils from the surfaces. Further chemical cleaning was conducted on the metallic substrates using a solution of diluted nitric acid (1:1) for the Cu sample and 10% NaOH then (1:1) dilute HN[O.sub.3] for the A1 samples. As a necessity the samples were rinsed after each step using distilled water.

For the coating process, the samples were dipped in the sol and withdrawn at a predetermined rate. The process parameters studied for copper substrate are solution concentration, heat treatment time (15, 30, 45, and 60 min) at 200[degrees]C, and withdrawal rate (1, 2, 4, and 7 cm/min). It is to be noted that 200[degrees]C was chosen in order to prevent oxidation of the copper substrate and peeling-off of the coating that occurs at higher temperatures. Similar process parameters were applied for aluminum substrate except for the heat treatment which was constant at 450[degrees]C for 30 min based on previous work. (2) For each production condition, three samples were prepared and studied.

Some of the physical attributes, namely thickness and surface roughness ([R.sub.a]), of the coatings were determined using the devices PosiTector 600-FN1 and Qualitest TR100/TR101, respectively. At least five points were measured on each sample and the average used for the data interpretation. Surface topography and phase identifications were determined using SEM (Jeol JSM-5410 Scanning microscope), XRD (X'pert pro PANalytical) analysis, and AFM (P9 XPM Solver Next NT-MDT) operated in the semicontact mode.

The absorptivity of the coat was determined using the UV spectrophotometer PG Instruments 90+ with integrating sphere. The average reflectance was evaluated for wavelength between 230 and 700 nm. The absorptivity ([alpha]) was obtained using the equation:

[alpha] + R + t = 1, (1)

where [alpha] is the absorptivity, R is the reflectance, and t is the transmittance of the surface. Since t = 0 for opaque surfaces, then [alpha] = 1 - R.

For emissivity measurements, the infrared (IR) spectrometer Nicolet 380 FTIR was used. The emissivity ([epsilon]) given by (1--reflectance) was calculated from the reflectance measurements in the IR range (from 2500 to 25000 nm). (21) The optical properties were measured on three positions on each sample and the average value used for the data interpretation. Using the ratio of absorptivity and emissivity ([alpha]/[epsilon]) it was possible to estimate the selectivity.

Results and discussion

Effect of different solution concentrations on optical properties of coated aluminum

The aluminum substrates were used for the study on the effect of the solution concentration on the coat properties at 1 cm/min withdrawal speed. In general, visual observation revealed adequate coat adhesion to the substrate. The reflectance curves for the aluminum samples prepared by dipping in solutions obtained for precursor molar ratios 40, 50, and 60 are shown in Fig. 1. Table 1 lists the average absorptivity. The results indicate that the lowest reflectance, hence the maximum absorptivity, is obtained for the solutions with molar ratio/50 where the absorptivity reached 0.9 [+ or -] 0.01 and the emissivity 0.029 [+ or -] 0.001 resulting in a selectivity of 31, which is considered a good value. Therefore, the molar ratio/50 was selected for the rest of the work.

The thickness of the coatings is also given in Table 1 where it can be observed that the thickness of the coatings decreases with decreasing concentration of the precursors in the solution. This is due to the decrease in viscosity of the solution.

Effect of heat treatment time on copper-coated samples

Effect of heat treatment time on coat optical properties

Using a fixed withdrawal rate (1 cm/min) and a solution of molar ratio/50, copper samples were coated followed by heat treatment at 200[degrees]C for 15,30,45, and 60 min. The effect of heat treatment time was studied on samples in order to reach the maximum selectivity. The reflectance curves of this parameter are plotted in Fig. 2. Based on the reflectance results, the average absorptivity given in Table 2 indicates that the maximum absorptivity 0.9 [+ or -] 0.01, minimum emissivity 0.011 [+ or -] 0.001 is obtained for annealing time of 60 min. These results correspond to a selectivity of 81.8.

Table 2 illustrates the thickness results of the coatings for copper samples. It can be observed that thickness of the coatings decreases with increasing heat treatment time due to higher loss of liquids of the sol (ethylene glycol evaporates at 197.5[degrees]C, ethyl alcohol at 78.3[degrees]C, and EAA at 78.3[degrees]C) and due to densification of the solid components.

Effect of coat thickness on optical properties of copper-coated samples

The decrease in coating thickness resulting from the heat treatment time on copper samples influenced both the absorptivity and emissivity. Absorptivity was found to range between 0.88 and 0.9 [+ or -] 0.01, for a coatings thickness between 2 and 11 pm, Fig. 3a. The higher absorptivity corresponds to a thickness between 2 and 7 [micro]m. Influence on the emissivity was more pronounced for the decrease in coatings thickness from 11 to 2 [micro]m. Figure 3b illustrates that the emissivity decreases by threefold reaching 0.011 [+ or -] 0.001 with this decrease in thickness. This is reflected in the large increase in selectivity illustrated in Fig. 3c which indicates that the maximum selectivity of 81.8 is obtained for the lowest coating thickness, 2 [micro]m. These results are in agreement with previous work on black Ni-Co coating on Al (17,18) where the best optical properties were obtained for a certain range of thickness. The application of a single layer of Cu-Co-Mn-O with 50-100 nm thickness on SS resulted in absorptance of 0.86 and emissivity of 0.11 (19) while with the application of a Si[O.sub.x] antireflection layer, the absorptivity increased to 0.94 with no change in emissivity (i.e., selectivity 8.5). With the addition of an intermediate layer which was a mixture of the bottom Cu-Co-Mn-O layer and the top antireflection layer the absorptivity increased to 0.96 and the emissivity to 0.12. (19) It is to be noted that selectivity values in the present work are much higher than those reported in known literature mainly due to the lowest emissivity obtained which can be affected by the emissivity of the substrate material and its surface finish. (20)

From these results, it can be said that a coating thickness of 2 [micro]m on copper substrate is preferable as it results in the maximum selectivity, 81.8.

Effect of withdrawal rate on optical properties of copper and aluminum substrates

Different withdrawal rates were used in coating copper and aluminum substrates followed by heat treatment. Figures 4a and 4b indicate that with increasing the withdrawal rate, the thickness of the coating was found to increase for both copper and aluminum substrates and the surface roughness to decrease. This is in accordance with previous work as it has been reported by Joly et al. (19) that the deposited liquid film thickness depends on the liquid viscosity, the substrate speed, the vapor surface tension, the density of the liquid and the liquid-vapor surface tension based on Landau and Levich law and that the thickness is proportional to (the withdrawal rate) (2/3).

It is also observed that the coating on copper substrate has smaller thickness and lower surface roughness than on aluminum substrate. By measuring the surface roughness of the substrates just before coating (average of 6 readings), the roughness ([R.sub.a]) was found to be 0.16 [micro]m for copper and 0.40 [micro]m for aluminum; indicating that as the substrate roughness increases the coating thickness and roughness also increase. The effect of substrate roughness on coat thickness is supported by Krechetnikov and Homsy (22) who found that a substrate with higher surface roughness results in a significant thickening of the film relative to that on a smooth substrate. This is an additional factor which was not considered in the classical Landau-Levich law. Regarding the roughness, it is also reasonable that the higher the roughness of the substrate, the higher the roughness of the deposited layer especially when only a few microns are deposited.

The reflectance curves of coated aluminum samples prepared using different withdrawal rates are plotted in Fig. 5, where it is to be noted that the withdrawal rate of 1 cm/min gave the lowest reflectance, hence the maximum absorptivity. The average absorptivity listed in Tables 3 and 4 indicates that the absorptivity for aluminum substrate is higher for a withdrawal rate of 1 cm/min which corresponds to the lowest thickness of 11.6 [micro]m, while for copper substrate, the absorptivity is almost unchanged for withdrawal rates 1, 2, and 4 cm/ min which corresponds to a coat thickness in the range from 2 to 6.5 [micro]m. In a previous work on black Ni-Co coat, (23) it was found that the maximum optical properties are obtained for coat thickness between 5 and 7.5 [micro]m and that larger or smaller thickness has no further improvement effect on the optical properties. This indicates that the withdrawal rate and the resulting coating thickness are important parameters in designing the coating process in order to obtain maximum selectivity. (17)

On the other hand, Tables 3 and 4 indicate that the emissivity decreases with decreasing withdrawal rate, reaching the lowest value of 0.029 [+ or -] 0.01 for Al and 0.011 [+ or -] 0.001 for Cu substrates at a withdrawal rate of 1 cm/min. The lower emissivity of the copper substrate is due to its very good reflectivity in the IR wave range.

As a result, a maximum selectivity of 31 and 81.8 was obtained for both aluminum and copper substrates, respectively. These values are markedly higher than those obtained previously using a multi-layered SS substrate consisting of Ni, black cobalt oxide, and a surface layer of tin oxide where the selectivity was 19.1. (12)

Comparing the present absorptivity and emissivity values with those of aluminum substrates coated with CuCoMn[O.sub.x] using similar precursors and heat treatment cycle in reference 8, it was found that the values for the present optical properties are higher. In the previous work, the highest absorptivity was [alpha] = 0.83 and emissivity [epsilon] = 0.050, while in present work the values obtained are [alpha] = 0.90 [+ or -] 0.01 and [epsilon] = 0.029 [+ or -] 0.001, probably due to differences in process parameters and surface preparation conditions which are not mentioned in the previous work.

Therefore, the most promising results (higher a and lower [epsilon] and maximum [alpha]/[epsilon]) are obtained using a withdrawal rate of 1 cm/min for coating copper and aluminum substrates.

Effect of coat thickness and roughness on optical properties

Figure 6 shows the effect of thickness on the optical properties of copper and aluminum substrate samples as a result of different withdrawal rates. It is interesting to note that there is almost a continuous effect of coating thickness on the optical properties irrespective of the substrate material. These results are important, indicating that the coating thickness plays an important role in determining both the absorptivity and the emissivity as discussed previously and that higher absorptivity and lower emissivity are realized with decreasing the coating thickness. Figure 6c indicates that the highest selectivity is reached for copper and aluminum substrates at coating thicknesses of 2 and 11.6 [micro]m, respectively. It is also to be noted that the effect of coating thickness on the absorptivity (between 0.85 [+ or -] 0.01 and 0.91 [+ or -] 0.01) is not as significant as on the emissivity (between 0.01 [+ or -] 0.001 and 0.053 [+ or -] 0.001) which varies in a fivefold range. The thickness and the substrate composition were found to play a crucial role in UV and visible light absorptivity in Ni-Co film deposited on aluminum substrate. (24) Moreover, the present results indicate that the effect of substrate material is markedly pronounced on the emissivity compared to the absorptivity as indicated in Figs. 6a and 6b, where the variation in emissivity in the IR range is much higher than the absorptivity in the UV range.

Figure 7 illustrates the effect of the coating surface roughness on the optical properties for both copper and aluminum samples. The results indicate that for the increasing roughness range between 0.1 and 1.3 [micro]m, the absorptivity also increases, Fig. 7a, in agreement with previous results (23) while the emissivity decreases, Fig. 7b. As a result, the selectivity increases by approximately twofold with surface roughness. However, the selectivity for coatings in the copper substrate samples is always higher than for the aluminum substrates samples. This is probably due to the lower thickness of the coatings on the copper substrates and to the lower emissivity of copper. Based on reference 25, the emissivity of polished aluminum is 0.04-0.06 and that of polished copper is 0.02.

Surface characterization of the coat

Two coated samples--aluminum and copper--were investigated using SEM and XRD to study the topography as well as the phases presented in the coatings.

Figures 8a and 8b show that the morphology of the coatings on the aluminum substrate is in the form of spherical nanoparticles with some spaces/separation distances between the particles while similar morphology but with agglomerations and larger spaces is observed in the copper-coated sample, Fig. 9. Similar morphology with spherical nano-sized structure and well-defined grain boundaries were observed in Cu-Co mixed oxides. (24,26,27) The porous/rough morphology was attributed to high evolution of oxygen gases during high temperature decomposition of Cu and/or Co oxides in order to ultimately form [Cu.sub.x][Co.sub.y][O.sub.z] system. (26) This structure seems to be more favorable for the higher absorptivity of copper samples. In the selective coating of Joly et al., (19) Cu-Co-Mn-Si-O nano-crystalline grains of 10-20 nm diameter in form of agglomerations is also found. It has been mentioned in previous work (23) that the presence of arrays of fine particles with irregular needle shape structure facilitates trapping of radiation hence enhancing the solar absorptivity. The present morphology which contains spherical nano-size particles with spaces or deep recesses is then efficient in absorbing solar radiation in addition to the reduction of the front surface reflections from the absorber surface.

The XRD analysis on aluminum- and copper-coated samples presented in Figs. 10a and 10b reveals the presence of copper oxide and copper manganese oxide, as shown in Fig. 10a. In Fig. 10b, the coat contains cobalt oxide in addition to the oxides found in Fig. 10a. Previous work also indicates the presence of the spinel (CuCoMn[O.sub.x]) in the coating. (2) Shaheen et al. (28) deposited Cu-Co mixed oxides with different Cu:Co ratios and found a CuO monoclinic phase for the ratio 3Cu:1Co when heated at 500[degrees]C and it was noticed that CuO phase disappeared gradually with the increase in Co content.

The surfaces of two examplary samples were scanned using the AFM to determine the surface topography of the aluminum surfaces before and after coating. Figures 11a and 11b illustrate surface topography of the aluminum substrates before coating using scanning area of 30 x 30 [micro][m.sup.2] indicating the traces of surface preparation with sharp dimples in one direction as well as pits. The microsurface roughness measured at a 4 x 4 [micro][m.sup.2] area indicates [R.sub.a] = 252 nm and [R.sub.q] = 347 nm.

The AFM surface topography of the aluminum-coated surface shows very fine peaks with high density in a scanned area of 4 x 4 [micro][m.sup.2] in Fig. 12a while Fig. 11b illustrates the details of the peaks with relatively sharp but rounded tips. The height profile in Fig. 11c for the area 4x4 [micro][m.sup.2] gives [R.sub.a] (the arithmetic mean value) 85.74 nm and [R.sub.q] (root mean-square-average) of 111.605 nm which are much lower than the corresponding values before coating, indicating a smoother surface after coating.

A comparison between the surface roughness on a macroscale (area covered several mm) in Fig. 4b and on the micrometer scale in Figs. 11 and 12 (area of several micrometer) reveals that after coating the roughness decreases on both the macroscale and the microscale.

Conclusions

Application of CuCoMn[O.sub.x] coat on copper and aluminum substrates using sol gel dipping technique was successful for both aluminum and copper substrates. The best optical properties, i.e., maximum absorptivity and minimum emissivity at 100[degrees]C, are obtained using a precursor molar ration/50 and a withdrawal rate of 1 cm/min followed by heat treatment at 450[degrees]C for 30 min for aluminum and at 200[degrees]C for 60 min for copper.

By decreasing the coating thickness to 2 and 11.6 pm for both copper and aluminum substrates, respectively, and increasing coating surface roughness in the submicron range, the absorptivity increases and emissivity decreases. The emissivity is much more affected than the absorptivity and varies within 3- to 5-folds.

Excellent selective optical properties for solar absorbers are obtained with [alpha]/[epsilon] = 0.9/0.011 for copper with selectivity 81.8, and with [alpha]/[epsilon] = 0.9/0.029 for aluminum with selectivity 31.

Surface topography reveals the presence of protruding bundles of nano-scale copper oxide, cobalt oxide, and spinel copper manganese oxide with relatively rounded ends and having spaces which help entrapment of solar radiation.

The present results rank deposited black sol-gel CuCoMn[O.sub.x] spinels on copper and aluminum substrates among the promising candidates for spectrally selective absorber coatings for solar collectors applications.

DOI 10.1007/s11998-014-9592-9

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N. El Mahallawy ([mail])

Design and Production Engineering Department, Faculty of Engineering, Ain Shams University, Abasia, Cairo, Egypt

e-mail: n_mahalawy@yahoo.com

N. El Mahallawy, Y. Ali

The German University in Cairo, New Cairo City-Al Tagamoa Al Khames, Cairo, Egypt

e-mail: eng.yehia.ali@gmail.com

M. Shoeib

Central Metallurgical Research Center, ElTebbin, Helwan, Cairo, Egypt

e-mail: mshoeib@yahoo.com

Table 1: Effect of the solution precursor concentration (molar ratio)
on coated aluminum properties heat treated at 450[degrees]C for 30
min

Molar ratio/x    Thickness ([micro]m)      Absorptivity
                                        ([alpha]) (average)

Molar ratio/40          16.33                  0.874
Molar ratio/50          11.6                   0.90
Molar ratio/60           6.5                   0.862

Molar ratio/x    Emissivity ([epsilon])       Selectivity
                                          ([alpha]/[epsilon])

Molar ratio/40           0.06                    14.5
Molar ratio/50           0.029                   31
Molar ratio/60           0.03                    28.7

Table 2: Optical properties for heat-treated coated copper samples
at 200[degrees]C for different times

Heat treatment   Thickness ([micro]m)      Absorptivity
time (min)                              ([alpha]) (average)

15                       10.6                  0.896
30                        7.6                  0.887
45                        7                    0.903
60                        2                    0.9

Heat treatment   Emissivity ([epsilon])       Selectivity
time (min)                                ([alpha]/[epsilon])

15                       0.031                   28.9
30                       0.021                   42.2
45                       0.018                   50.1
60                       0.011                   81.8

Table 3: Optical properties for aluminum-coated substrates using
different withdrawal followed by heat treatment after dipping at
450[degrees]C for 30 min

Withdrawal      Thickness         Roughness,        Absorptivity
rate (cm/min)   ([micro]m)   [R.sub.a] ([micro]m)    ([alpha])

1                  11.6              1.3               0.90
2                  12.5              0.94              0.85
4                  14.5              0.7               0.847
7                  14.8              0.51              0.848

Withdrawal      Emissivity        Selectivity
rate (cm/min)   ([epsilon])   ([alpha]/[epsilon])

1                 0.029              31
2                 0.035              24.2
4                 0.0529             16
7                 0.0537             15.7

Table 4: Optical properties for copper-coated substrates using
different withdrawal followed by heat treatment at 200[degrees]C for
1 h

Withdrawal      Thickness         Roughness,        Absorptivity
rate (cm/min)   ([micro]m)   [R.sub.a] ([micro]m)    ([alpha])

1                  2                 0.67               0.9
2                  4                 0.6                0.91
4                  6.5               0.3                0.90
7                  9                 0.13               0.87

Withdrawal      Emissivity        Selectivity
rate (cm/min)   ([epsilon])   ([alpha]/[epsilon])

1                  0.011             81.8
2                  0.013             70
4                  0.014             64.3
7                  0.016             54.4


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Please note: Some tables or figures were omitted from this article.
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Article Details
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Author:El Mahallawy, Nahed; Shoeib, Madiha; Ali, Yehia
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Nov 1, 2014
Words:5610
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