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Spectrally selective black nickel coating prepared by a conversion process
Thin Solid Films, 213 (1992) 80 85
8(t
Spectrally selective black nickel coating prepared by a conversion process S. K. S h a r m a a n d N. C. M e h r a ~ National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012 (India)
(Received August 1, 1991: revised October 29, 1991: accepted November 11, 1991)
Abstract Black nickel coatings have been prepared on zincated and zinc-electroplated aluminium substrates by a conversion process. The zincating and zinc electroplating parameters have been optimized for preparing zinc-coated aluminium substrates for the deposition of black nickel coatings. The optical properties and the morphology of these coatings have been investigated in respect of the morphology of the zinc surface underneath. It has been observed that the substrate preparation and morphology of the zinc coatings play a vital role in determining the surface morphology and the optical properties of the final solar selective black coatings, Thermal stability and humidity tests of these coatings have been carried out and it has been observed that these coatings do not degrade on thermal annealing up to 200 c'C.
I. Introduction Black nickel coatings have been known for the past four decades as solar selective surfaces for the conversion of solar radiation into useful heat. These coatings possess good optical properties but the durability shown is often poor. These solar selective coatings are mainly prepared by chemical conversion processes [1-4] or electrodeposition techniques [5-8]. For practical applications a selective absorber should have good optical properties, low cost, good thermal stability and ease of large-scale production. Keeping these considerations in view, selective black nickel coatings have been widely prepared by chemical conversion techniques. Gogna and Chopra [1] have studied black nickel coatings prepared on zinc surfaces by a chemical conversion process. They have observed that these coatings are stable and have high selectivity (~/E = 10-12) where and e are the solar absorptance and thermal emittance of the coatings respectively. Cathro [2, 3] has also investigated black nickel coatings and has found that these coatings are stable up to 200 c'C. The results of durability reported by different workers cannot be compared as the work reported has been carried out under different environmental and climatic conditions. It is therefore desirable to carry out more systematic work in this direction. In the present communication, the authors have reported the optimization study of plating parameters to
~Present address: University Science University of Delhi, Delhi 11000% India.
0040-6090/92/$5.00
Instrumentation Centre,
achieve black nickel coatings with optimum optical properties. The surface morphology and the elemental analysis of the coatings have been investigated with a scanning electron microscope and an electron probe microanalyser respectively. The optical properties have been discussed on the basis of the surface microstructure and composition of the coatings.
2. Experimental details Black nickel coatings were prepared on commercially available aluminium sheets. The sheets were cut into pieces (5 x 8 cm 2) and were first subjected to electropolishing in a fluoboric acid bath as described earlier [9]. The electropolished sheets were then coated with zinc by zincating or zinc electroplating by using the following baths. The zincating bath contained: sodium hydroxide, 525 g 1-~; zinc oxide, 200 g 1-~; Rochelle salt, 10 g I ~. The zinc electroplating bath contained: zinc oxide, 43.5 g I ~; sodium cyanide, 102 g 1-~; sodium hydroxide, 52.5 g I ~. After a zinc coating had been prepared on the aluminium sheets either by zincating or by zinc electroplating, a black nickel coating was formed by the chemical conversion process described earlier [9]. The solar absorptance :~ of these coatings was determined by averaging solar spectral absorptance data over the spectral range from 0.3 to 2.5 gm using the air mass 2 solar spectrum. A Hitachi spectrophotometer fitted with an integrating sphere attachment was used for reflectance measurements. In order to evaluate the thermal emittance E, near-normal (about 10~) specular
,.c 1992 - - Elsevier Sequoia. All rights reserved
81
s. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
reflectance measurements were carried out in the wavelength range from 2.5 to 25 gm using a P e r k i n - E l m e r I R spectrophotometer. The method of selected ordinates [ 10] has been used to calculate thermal emittance values at room temperature. In the experimental set-ups the temperature and current densities have been measured with an accuracy of +_1 °C and +_0.1 A dm -2 respectively. A J E O L scanning electron microscope model JSM-35CF has been used to study the surface microstructure of the coatings. The elemental analysis has been carried out with the help of an X-ray energy-dispersive spectrometer attachment to the scanning electron microscope.
TABLE 2. Effect of immersion time in black nickel bath on the optical properties of black nickel coating" Immersion time in black nickel bath (s)
Absorptance c~
Emittance •
20 15 30 35 40 45 50 60
0.89 0.90 0.92 0.93 0.94 0.94 0.93 0.91
0.08 0.08 0.09 0.10 0.10 0.12 0.12 0.14
"Constant conditions: zincating temperature, 30 °C~ zincating time, 75 s; black nickel bath temperature, 40 '~C.
3. Results and discussion The optical properties and the surface morphology of the black nickel coatings prepared on zincated and zinc-electroplated aluminium sheets under different conditions were measured. The results obtained are discussed in the following sections. 3.1. Black nickel coatings on z & c a t e d aluminium
First the optimum parameters to deposit zinc on aluminium substrates were determined so that the subsequent nickel coating gave the best optical properties. For this purpose, zinc has been deposited on aluminium substrates by varying zincating time and temperature while keeping the parameters of the black nickel bath constant. Table 1 shows the results for absorptance and emittance of the coatings prepared in this way. It can be seen from Table 1 that an increase in either zincating time or temperature of the zincating bath results in coatings having higher values of absorptance and emittance. It is evident from the table that a zincating time of 75 s at 30 °C yields the best values of c~ and 4. Later black nickel coatings were prepared by varying the immersion time in the black nickel bath keeping the zincating parameters constant (time, 75 s;
TABLE 1. Effect of zincating parameters on the optical properties of black nickel coating~ Zincating time (s) 45 60 75 90
E
25 b
30
35
25
30
35
0.89 0.90 0.92 0.92
0.90 0.91 0.94 0.94
0.90 0.92 0.94 0.94
0.08 0.09 0.10 0.11
0.09 0.09 0.10 0.13
0.10 0.11 0.13 0.15
~Constant conditions of the black nickel bath: temperature, 40 °C; immersion time, 40 s. bZincating temperature (°C).
temperature, 30 °C) and Table 2 shows the absorptance and emittance of the coatings thus prepared. From Table 2 it is observed that an increase in the immersion time in the black nickel bath first increases the absorptance and a point is reached after which a further increase in the immersion time results in a decrease in the absorptance values, while the emittance values showed an increasing trend. The best coatings have been prepared for a 40 s immersion time in the black nickel bath and have ~ = 0.94 and E = 0.10. The reflectance of black nickel coatings thus prepared showed variations with wavelength as depicted by curve a in Fig. 1. Figures 2(a) and 2(b) show the morphology of the optimum black nickel coating and a coating prepared for an immersion time of 60 s in the black nickel bath. It is observed from the micrographs that, with an increase in the immersion time in the black nickel bath from the optimum value, further growth of particles and their coalescence take place, thereby resulting in a comparatively rough surface. 3.2. Black nickel coatings on zinc-electroplated aluminium
The effects of various parameters for the zinc electrodeposition on the optical properties of the black nickel coatings have been studied. Black nickel coatings were prepared by varying the time of deposition, temperature and current density during zinc electrodeposition while keeping the parameters of nickel deposition constant. Table 3 shows the results for the coatings prepared by varying the zinc electroplating time and current density while keeping all other plating parameters constant as given in the caption. It can be noticed that an increase in plating time during zinc electrodeposition increases both ~ and E. The same results are obtained if the current density during zinc electrodeposition is increased. These parameters control the thickness of the zinc layer which seems to be very important
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
82
IO0 80 lid
u
60
U
~LL 4o W
2O I
I
I
I
I
0.3 0.4 0.5 0.7 0.9 I,I 2.0
|
I
5,0
I0.0
I
15,0
I
20,0
25,0
WAVE L E N G T H jura .--D.
Fig. I. Variation of reflectance with wavelength for optimum black nickel coatings: curve a, black nickel on zincated aluminium; curve b, black nickel on zinc-electroplated aluminium.
for obtaining coatings with high ~ and low E. It can be noted from Table 3 that the desired coatings are formed at a current density of 2 A d m -2 for a 30°C bath temperature during zinc electrodeposition. After optimizing the zinc electroplating parameters, selective coatings were prepared by varying the black nickel bath parameters. Table 4 shows the optical properties of the black nickel coatings thus prepared. It can be observed that, with increase in immersion time or temperature of the black nickel bath, c~ first increases and then attains a maximum value after which any further increase in these parameters causes a decrease in absorptance. However, the emittance showed continuous increase with the increase of bath temperature and immersion time. The following operating conditions for zinc electroplating and subsequent black nickel plating were found to give best values of ~ = 0.93 and E = 0.09. For the zinc plating bath, the time of deposition was 60 s, the current density was 2 A dm 2 and the temperature was 30 °C. For the black nickel bath, the temperature was 34 ~'C and the immersion time was 30 s. The reflectance of the black nickel coatings prepared under the above conditions was measured as a function of wavelength and is depicted by curve b in Fig. 1. Figures 3(a) and 3(b) show the surface morphology of a zinc layer deposited under optimum plating conditions and the optimum black nickel coating respectively. It has been observed that, for short immersion times in the black nickel bath, the coating follows the needle-shape morphology of the substrate with increase in the size of the needles. The length of the needles varies from 0.6 to 0.9 lam and the breadth from 0.1 to 0.2 rtm for an immersion time of 30 s. As the dipping time is further increased, the needle-shape particles change in shape and become spherical as depicted in Fig. 4. Further increase in the immersion time results in
(a)
(b) Fig. 2. Scanning electron micrographs depicting the surface morphology of black nickel coatings: (a) prepared under optimum conditions; (b) prepared at 60 s immersion time.
the formation of bigger particles of spherical shape as shown in Fig. 5. It has been observed that for an increase in immersion time from 60 to 90 s the diameter of the spherical particles changes from 0.18 to 0.20 lain and the optical selectivity of the coatings decreases.
3.3. Elemental analysis of black nickel coatings It has been observed that black nickel coatings with the best values of solar absorptance and thermal emittance show equal amounts of nickel and sulphur in an energy-dispersive X-ray spectrum. The energy-dispersive X-ray analysis of a typical black nickel coating
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
83
T A B L E 3. Effect of zinc electroplating time and current density on the optical properties of black nickel coating" Current density (A dm 2)
Time (s)
Absorptance ~
Emittance E
1
30 60 90 30 60 90 30 60 90 30 60 90
0.87 0.88 0.89 0.87 0.89 0.91 0.92 0.94 0.94 0.92 0.94 0.94
0.08 0.08 0.08 0.08 0.09 0.09 0.09 0.13 0.15 0. I 1 0.15 0.16
1.6
2.0
2.4
"Constant conditions: zinc electroplating temperature, 30"C; black nickel bath temperature 40 °C; immersion time, 40 s.
(a)
T A B L E 4. Effect of immersion time and temperature in the black nickel bath on the optical properties Immersion time (s)
20 30 40 50 60 90
30 a
34
40
30
34
40
0.90 0.92 0.92 0.93 0.93 0.92
0.90 0.93 0.93 0.94 0.93 0.92
0.91 0.93 0.94 0.94 0.93 0.91
0,08 0.09 0.10 0.12 0.13 0.15
0.08 0.09 0.10 0.12 0.14 0.16
0.09 0.12 0.13 0.13 0.15 0.15
"Temperature of the black nickel bath (°C).
with ~ =0.93 and e =0.09 is shown in Table 5. The energy-dispersive X-ray analysis of a black nickel coating prepared for an immersion time ( 120 s) greater than the optimum value (30 s) in the black nickel bath is shown in Table 6. It can be seen in the table that, as the immersion time in the black nickel bath is increased, the nickel and sulphur contents of the coating also increase. It has been shown by Raja Gopalan et al. [11] that in black nickel coatings the sulphur present is in the compound form and not in the elemental form. These coatings become more semiconducting owing to the formation of sulphides of nickel and zinc as the reaction proceeds. The metallic content of the coating decreases thereby decreasing the metal-to-metal-sulphide ratio. This result is in agreement with those of various authors [12-14] who have shown that selective coatings can be characterized in terms of the metal-to-metaloxide or -sulphide ratio and a decrease in this ratio correlates with the decrease in values of the optical properties of the coatings. In the case of black nickel coatings, it appears that a decrease in absorptance (after the saturation point)
(b) . Fig. 3. Scanning electron micrographs showing the surface morphology of (a) a zinc layer deposited under optimum plating conditions and (b) a black nickel coating prepared on the layer shown in (a) under optimum conditions.
with increase in immersion time above the optimum value may be due to the combined effect of changes in surface morphology and composition.
3.4. Thermal stability and humidity resistance The thermal stability of these coatings has been studied by annealing them in air having humidity of about 50% at temperatures of 100, 150, 200 and 250 °C for 400 h. The coatings on both the substrates showed no change in their optical properties when heated to 200 °C in air. At 250 °C coatings on both the substrates showed a decrease in absorptance by 0.01 and an increase in emittance by 0.01.
84
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating T A B L E 5. Energy-dispersive X-ray analysis of a typical black nickel coating having a = 0.93 and E = 0.09 Element and line
Content (at.%)
Al K~ S K:~ Ni Kz~ Zn Ka
1.9 3.8 1.7 92.6
T A B L E 6. Energy-dispersive X-ray analysis of black nickel coating after 120 s immersion time
Fig. 4. Scanning electron micrograph depicting the change in surface morphology as the immersion time in the black nickel bath is increased from the optimum value (30 s) to 60 s.
Fig. 5. Scanning electron micrograph depicting the change in surface morphology of a black nickel coating as the immersion time in the black nickel bath is increased from 60 to 90 s.
Element and line
Content (at.%)
Al K~ S K~ Ni Kz~ Zn K~
2.7 2.0 1.0 94.3
the substrates showed numerous white spots on the surface due to corrosion. Gogna and Chopra [1] have reported a large decrease in absorptance for the coatings on a zincated aluminium substrate on heating to 200 °C for 50 h. They have also observed a decrease in absorptance from 0.90 to 0.87 for these coatings in an outdoor exposure test for 6 months. On the other hand, the coatings reported in the present work showed no change in the optical properties on heating to 200 °C for 400 h. The results of the outdoor exposure test on the zinc-electroplated substrate are consistent with the findings of Gogna and Chopra [1]. However, the difference in the results of the thermal stability study appears to be due to the difference in the chemical baths used for preparing these coatings and hence ultimately leading to the formation of selective coatings haivng different chemical compositions.
4. Conclusions
These coatings were also subjected to durability tests under ambient conditions. Some of the coatings were placed in the open under a glass cover facing the sun for a period of 1 year and their optical properties were evaluated half-yearly. During the first 6 months, the coatings on both the substrates showed practically no change in the optical properties. However, during the next 6 months, which include the summer months and the period of maximum humidity (about 98%), these coatings showed deterioration in the optical properties. These coatings showed a decrease in absorptance by 0.015 and an increase in emittance by 0.01. Moreover black nickel coatings which were relatively thin on both
Selective black nickel coatings with high selectivity (about 10) have been prepared using a relatively inexpensive dip coating process. The conditions have been determined that yield high solar absorptance while maintaining a low thermal emittance. The optical properties and morphology of these coatings have been investigated along with the morphology of the zinc surface underneath. It has been found that the morphology of the zinc surface and the morphology and composition of the final black coatings are important for getting the best values of absorptance and emittance. The thermal stability tests clearly indicate that
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
these coatings can be used for solar thermal applications up to 200 °C.
References P. K. Gogna and K. L. Chopra, Sol. Energy, 23 (1979) 405. K. J. Cathro, Sol. Energy Mater., 5(1981) 317. K. J. Cathro, Sol. Energy, 32(1984) 665. S. N. Kumar, L. K. Malhotra and K. L. Chopra, Sol. Energy Mater., 3 (1980) 519. 5 H. Y. B. Mar, R. C. Peterson and P. B. Zimmer, Thin Solid Films, 39 (1976) 95.
1 2 3 4
85
6 P. K. Gogna and K. L. Chopra, Thin Solid Films, 67(1979) 299. 7 J. D. Garrison, Sol. Energy Mater., 9 (1984) 483. 8 S. N. Patel, O. T. Inal, A. J. Singh and A. Scherer, SoL Energy Mater., 11 (1985) 381. 9 N. C. Mehra and S. K. Sharma, J. Mater. Sci. Lett., 8(1989) 707. 10 A. V. Sheklein, Geliotekhnika, 3 (1967)24. 11 S. R. Raja Gopalan, K. S. Indira and K. S. G. Doss, J. Electroanal. Chem., 10 (1965) 465. 12 O. T. Inal, J. C. Mabon and C. V. Robino, Thin Solid Films, 83 (1981) 399. 13 J. N. Sweet, R. B. Pettit and M. B. Chamberlain, Sol. Energy Mater., 10 (1989) 251. 14 I. T. Ritchie, S. K. Sharma, J. Valignat and J. Spitz, Sol. Energy Mater., 28 ( 1979-1980) 167.
8(t
Spectrally selective black nickel coating prepared by a conversion process S. K. S h a r m a a n d N. C. M e h r a ~ National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012 (India)
(Received August 1, 1991: revised October 29, 1991: accepted November 11, 1991)
Abstract Black nickel coatings have been prepared on zincated and zinc-electroplated aluminium substrates by a conversion process. The zincating and zinc electroplating parameters have been optimized for preparing zinc-coated aluminium substrates for the deposition of black nickel coatings. The optical properties and the morphology of these coatings have been investigated in respect of the morphology of the zinc surface underneath. It has been observed that the substrate preparation and morphology of the zinc coatings play a vital role in determining the surface morphology and the optical properties of the final solar selective black coatings, Thermal stability and humidity tests of these coatings have been carried out and it has been observed that these coatings do not degrade on thermal annealing up to 200 c'C.
I. Introduction Black nickel coatings have been known for the past four decades as solar selective surfaces for the conversion of solar radiation into useful heat. These coatings possess good optical properties but the durability shown is often poor. These solar selective coatings are mainly prepared by chemical conversion processes [1-4] or electrodeposition techniques [5-8]. For practical applications a selective absorber should have good optical properties, low cost, good thermal stability and ease of large-scale production. Keeping these considerations in view, selective black nickel coatings have been widely prepared by chemical conversion techniques. Gogna and Chopra [1] have studied black nickel coatings prepared on zinc surfaces by a chemical conversion process. They have observed that these coatings are stable and have high selectivity (~/E = 10-12) where and e are the solar absorptance and thermal emittance of the coatings respectively. Cathro [2, 3] has also investigated black nickel coatings and has found that these coatings are stable up to 200 c'C. The results of durability reported by different workers cannot be compared as the work reported has been carried out under different environmental and climatic conditions. It is therefore desirable to carry out more systematic work in this direction. In the present communication, the authors have reported the optimization study of plating parameters to
~Present address: University Science University of Delhi, Delhi 11000% India.
0040-6090/92/$5.00
Instrumentation Centre,
achieve black nickel coatings with optimum optical properties. The surface morphology and the elemental analysis of the coatings have been investigated with a scanning electron microscope and an electron probe microanalyser respectively. The optical properties have been discussed on the basis of the surface microstructure and composition of the coatings.
2. Experimental details Black nickel coatings were prepared on commercially available aluminium sheets. The sheets were cut into pieces (5 x 8 cm 2) and were first subjected to electropolishing in a fluoboric acid bath as described earlier [9]. The electropolished sheets were then coated with zinc by zincating or zinc electroplating by using the following baths. The zincating bath contained: sodium hydroxide, 525 g 1-~; zinc oxide, 200 g 1-~; Rochelle salt, 10 g I ~. The zinc electroplating bath contained: zinc oxide, 43.5 g I ~; sodium cyanide, 102 g 1-~; sodium hydroxide, 52.5 g I ~. After a zinc coating had been prepared on the aluminium sheets either by zincating or by zinc electroplating, a black nickel coating was formed by the chemical conversion process described earlier [9]. The solar absorptance :~ of these coatings was determined by averaging solar spectral absorptance data over the spectral range from 0.3 to 2.5 gm using the air mass 2 solar spectrum. A Hitachi spectrophotometer fitted with an integrating sphere attachment was used for reflectance measurements. In order to evaluate the thermal emittance E, near-normal (about 10~) specular
,.c 1992 - - Elsevier Sequoia. All rights reserved
81
s. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
reflectance measurements were carried out in the wavelength range from 2.5 to 25 gm using a P e r k i n - E l m e r I R spectrophotometer. The method of selected ordinates [ 10] has been used to calculate thermal emittance values at room temperature. In the experimental set-ups the temperature and current densities have been measured with an accuracy of +_1 °C and +_0.1 A dm -2 respectively. A J E O L scanning electron microscope model JSM-35CF has been used to study the surface microstructure of the coatings. The elemental analysis has been carried out with the help of an X-ray energy-dispersive spectrometer attachment to the scanning electron microscope.
TABLE 2. Effect of immersion time in black nickel bath on the optical properties of black nickel coating" Immersion time in black nickel bath (s)
Absorptance c~
Emittance •
20 15 30 35 40 45 50 60
0.89 0.90 0.92 0.93 0.94 0.94 0.93 0.91
0.08 0.08 0.09 0.10 0.10 0.12 0.12 0.14
"Constant conditions: zincating temperature, 30 °C~ zincating time, 75 s; black nickel bath temperature, 40 '~C.
3. Results and discussion The optical properties and the surface morphology of the black nickel coatings prepared on zincated and zinc-electroplated aluminium sheets under different conditions were measured. The results obtained are discussed in the following sections. 3.1. Black nickel coatings on z & c a t e d aluminium
First the optimum parameters to deposit zinc on aluminium substrates were determined so that the subsequent nickel coating gave the best optical properties. For this purpose, zinc has been deposited on aluminium substrates by varying zincating time and temperature while keeping the parameters of the black nickel bath constant. Table 1 shows the results for absorptance and emittance of the coatings prepared in this way. It can be seen from Table 1 that an increase in either zincating time or temperature of the zincating bath results in coatings having higher values of absorptance and emittance. It is evident from the table that a zincating time of 75 s at 30 °C yields the best values of c~ and 4. Later black nickel coatings were prepared by varying the immersion time in the black nickel bath keeping the zincating parameters constant (time, 75 s;
TABLE 1. Effect of zincating parameters on the optical properties of black nickel coating~ Zincating time (s) 45 60 75 90
E
25 b
30
35
25
30
35
0.89 0.90 0.92 0.92
0.90 0.91 0.94 0.94
0.90 0.92 0.94 0.94
0.08 0.09 0.10 0.11
0.09 0.09 0.10 0.13
0.10 0.11 0.13 0.15
~Constant conditions of the black nickel bath: temperature, 40 °C; immersion time, 40 s. bZincating temperature (°C).
temperature, 30 °C) and Table 2 shows the absorptance and emittance of the coatings thus prepared. From Table 2 it is observed that an increase in the immersion time in the black nickel bath first increases the absorptance and a point is reached after which a further increase in the immersion time results in a decrease in the absorptance values, while the emittance values showed an increasing trend. The best coatings have been prepared for a 40 s immersion time in the black nickel bath and have ~ = 0.94 and E = 0.10. The reflectance of black nickel coatings thus prepared showed variations with wavelength as depicted by curve a in Fig. 1. Figures 2(a) and 2(b) show the morphology of the optimum black nickel coating and a coating prepared for an immersion time of 60 s in the black nickel bath. It is observed from the micrographs that, with an increase in the immersion time in the black nickel bath from the optimum value, further growth of particles and their coalescence take place, thereby resulting in a comparatively rough surface. 3.2. Black nickel coatings on zinc-electroplated aluminium
The effects of various parameters for the zinc electrodeposition on the optical properties of the black nickel coatings have been studied. Black nickel coatings were prepared by varying the time of deposition, temperature and current density during zinc electrodeposition while keeping the parameters of nickel deposition constant. Table 3 shows the results for the coatings prepared by varying the zinc electroplating time and current density while keeping all other plating parameters constant as given in the caption. It can be noticed that an increase in plating time during zinc electrodeposition increases both ~ and E. The same results are obtained if the current density during zinc electrodeposition is increased. These parameters control the thickness of the zinc layer which seems to be very important
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
82
IO0 80 lid
u
60
U
~LL 4o W
2O I
I
I
I
I
0.3 0.4 0.5 0.7 0.9 I,I 2.0
|
I
5,0
I0.0
I
15,0
I
20,0
25,0
WAVE L E N G T H jura .--D.
Fig. I. Variation of reflectance with wavelength for optimum black nickel coatings: curve a, black nickel on zincated aluminium; curve b, black nickel on zinc-electroplated aluminium.
for obtaining coatings with high ~ and low E. It can be noted from Table 3 that the desired coatings are formed at a current density of 2 A d m -2 for a 30°C bath temperature during zinc electrodeposition. After optimizing the zinc electroplating parameters, selective coatings were prepared by varying the black nickel bath parameters. Table 4 shows the optical properties of the black nickel coatings thus prepared. It can be observed that, with increase in immersion time or temperature of the black nickel bath, c~ first increases and then attains a maximum value after which any further increase in these parameters causes a decrease in absorptance. However, the emittance showed continuous increase with the increase of bath temperature and immersion time. The following operating conditions for zinc electroplating and subsequent black nickel plating were found to give best values of ~ = 0.93 and E = 0.09. For the zinc plating bath, the time of deposition was 60 s, the current density was 2 A dm 2 and the temperature was 30 °C. For the black nickel bath, the temperature was 34 ~'C and the immersion time was 30 s. The reflectance of the black nickel coatings prepared under the above conditions was measured as a function of wavelength and is depicted by curve b in Fig. 1. Figures 3(a) and 3(b) show the surface morphology of a zinc layer deposited under optimum plating conditions and the optimum black nickel coating respectively. It has been observed that, for short immersion times in the black nickel bath, the coating follows the needle-shape morphology of the substrate with increase in the size of the needles. The length of the needles varies from 0.6 to 0.9 lam and the breadth from 0.1 to 0.2 rtm for an immersion time of 30 s. As the dipping time is further increased, the needle-shape particles change in shape and become spherical as depicted in Fig. 4. Further increase in the immersion time results in
(a)
(b) Fig. 2. Scanning electron micrographs depicting the surface morphology of black nickel coatings: (a) prepared under optimum conditions; (b) prepared at 60 s immersion time.
the formation of bigger particles of spherical shape as shown in Fig. 5. It has been observed that for an increase in immersion time from 60 to 90 s the diameter of the spherical particles changes from 0.18 to 0.20 lain and the optical selectivity of the coatings decreases.
3.3. Elemental analysis of black nickel coatings It has been observed that black nickel coatings with the best values of solar absorptance and thermal emittance show equal amounts of nickel and sulphur in an energy-dispersive X-ray spectrum. The energy-dispersive X-ray analysis of a typical black nickel coating
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
83
T A B L E 3. Effect of zinc electroplating time and current density on the optical properties of black nickel coating" Current density (A dm 2)
Time (s)
Absorptance ~
Emittance E
1
30 60 90 30 60 90 30 60 90 30 60 90
0.87 0.88 0.89 0.87 0.89 0.91 0.92 0.94 0.94 0.92 0.94 0.94
0.08 0.08 0.08 0.08 0.09 0.09 0.09 0.13 0.15 0. I 1 0.15 0.16
1.6
2.0
2.4
"Constant conditions: zinc electroplating temperature, 30"C; black nickel bath temperature 40 °C; immersion time, 40 s.
(a)
T A B L E 4. Effect of immersion time and temperature in the black nickel bath on the optical properties Immersion time (s)
20 30 40 50 60 90
30 a
34
40
30
34
40
0.90 0.92 0.92 0.93 0.93 0.92
0.90 0.93 0.93 0.94 0.93 0.92
0.91 0.93 0.94 0.94 0.93 0.91
0,08 0.09 0.10 0.12 0.13 0.15
0.08 0.09 0.10 0.12 0.14 0.16
0.09 0.12 0.13 0.13 0.15 0.15
"Temperature of the black nickel bath (°C).
with ~ =0.93 and e =0.09 is shown in Table 5. The energy-dispersive X-ray analysis of a black nickel coating prepared for an immersion time ( 120 s) greater than the optimum value (30 s) in the black nickel bath is shown in Table 6. It can be seen in the table that, as the immersion time in the black nickel bath is increased, the nickel and sulphur contents of the coating also increase. It has been shown by Raja Gopalan et al. [11] that in black nickel coatings the sulphur present is in the compound form and not in the elemental form. These coatings become more semiconducting owing to the formation of sulphides of nickel and zinc as the reaction proceeds. The metallic content of the coating decreases thereby decreasing the metal-to-metal-sulphide ratio. This result is in agreement with those of various authors [12-14] who have shown that selective coatings can be characterized in terms of the metal-to-metaloxide or -sulphide ratio and a decrease in this ratio correlates with the decrease in values of the optical properties of the coatings. In the case of black nickel coatings, it appears that a decrease in absorptance (after the saturation point)
(b) . Fig. 3. Scanning electron micrographs showing the surface morphology of (a) a zinc layer deposited under optimum plating conditions and (b) a black nickel coating prepared on the layer shown in (a) under optimum conditions.
with increase in immersion time above the optimum value may be due to the combined effect of changes in surface morphology and composition.
3.4. Thermal stability and humidity resistance The thermal stability of these coatings has been studied by annealing them in air having humidity of about 50% at temperatures of 100, 150, 200 and 250 °C for 400 h. The coatings on both the substrates showed no change in their optical properties when heated to 200 °C in air. At 250 °C coatings on both the substrates showed a decrease in absorptance by 0.01 and an increase in emittance by 0.01.
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S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating T A B L E 5. Energy-dispersive X-ray analysis of a typical black nickel coating having a = 0.93 and E = 0.09 Element and line
Content (at.%)
Al K~ S K:~ Ni Kz~ Zn Ka
1.9 3.8 1.7 92.6
T A B L E 6. Energy-dispersive X-ray analysis of black nickel coating after 120 s immersion time
Fig. 4. Scanning electron micrograph depicting the change in surface morphology as the immersion time in the black nickel bath is increased from the optimum value (30 s) to 60 s.
Fig. 5. Scanning electron micrograph depicting the change in surface morphology of a black nickel coating as the immersion time in the black nickel bath is increased from 60 to 90 s.
Element and line
Content (at.%)
Al K~ S K~ Ni Kz~ Zn K~
2.7 2.0 1.0 94.3
the substrates showed numerous white spots on the surface due to corrosion. Gogna and Chopra [1] have reported a large decrease in absorptance for the coatings on a zincated aluminium substrate on heating to 200 °C for 50 h. They have also observed a decrease in absorptance from 0.90 to 0.87 for these coatings in an outdoor exposure test for 6 months. On the other hand, the coatings reported in the present work showed no change in the optical properties on heating to 200 °C for 400 h. The results of the outdoor exposure test on the zinc-electroplated substrate are consistent with the findings of Gogna and Chopra [1]. However, the difference in the results of the thermal stability study appears to be due to the difference in the chemical baths used for preparing these coatings and hence ultimately leading to the formation of selective coatings haivng different chemical compositions.
4. Conclusions
These coatings were also subjected to durability tests under ambient conditions. Some of the coatings were placed in the open under a glass cover facing the sun for a period of 1 year and their optical properties were evaluated half-yearly. During the first 6 months, the coatings on both the substrates showed practically no change in the optical properties. However, during the next 6 months, which include the summer months and the period of maximum humidity (about 98%), these coatings showed deterioration in the optical properties. These coatings showed a decrease in absorptance by 0.015 and an increase in emittance by 0.01. Moreover black nickel coatings which were relatively thin on both
Selective black nickel coatings with high selectivity (about 10) have been prepared using a relatively inexpensive dip coating process. The conditions have been determined that yield high solar absorptance while maintaining a low thermal emittance. The optical properties and morphology of these coatings have been investigated along with the morphology of the zinc surface underneath. It has been found that the morphology of the zinc surface and the morphology and composition of the final black coatings are important for getting the best values of absorptance and emittance. The thermal stability tests clearly indicate that
S. K. Sharma, N. C. Mehra / Spectrally selective black nickel coating
these coatings can be used for solar thermal applications up to 200 °C.
References P. K. Gogna and K. L. Chopra, Sol. Energy, 23 (1979) 405. K. J. Cathro, Sol. Energy Mater., 5(1981) 317. K. J. Cathro, Sol. Energy, 32(1984) 665. S. N. Kumar, L. K. Malhotra and K. L. Chopra, Sol. Energy Mater., 3 (1980) 519. 5 H. Y. B. Mar, R. C. Peterson and P. B. Zimmer, Thin Solid Films, 39 (1976) 95.
1 2 3 4
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6 P. K. Gogna and K. L. Chopra, Thin Solid Films, 67(1979) 299. 7 J. D. Garrison, Sol. Energy Mater., 9 (1984) 483. 8 S. N. Patel, O. T. Inal, A. J. Singh and A. Scherer, SoL Energy Mater., 11 (1985) 381. 9 N. C. Mehra and S. K. Sharma, J. Mater. Sci. Lett., 8(1989) 707. 10 A. V. Sheklein, Geliotekhnika, 3 (1967)24. 11 S. R. Raja Gopalan, K. S. Indira and K. S. G. Doss, J. Electroanal. Chem., 10 (1965) 465. 12 O. T. Inal, J. C. Mabon and C. V. Robino, Thin Solid Films, 83 (1981) 399. 13 J. N. Sweet, R. B. Pettit and M. B. Chamberlain, Sol. Energy Mater., 10 (1989) 251. 14 I. T. Ritchie, S. K. Sharma, J. Valignat and J. Spitz, Sol. Energy Mater., 28 ( 1979-1980) 167.