The optical characteristics of black chrome solar selective films coated by the pulse current electrolysis method

Solar Energy Materials and Solar Cells 29 (1993) 149-161 North-Holland

Solar Energy Materials and Solar Cells

The optical characteristics of black chrome solar selective films coated by the pulse current electrolysis method Tai K. Lee, Dong H. Kim and P. C h u n g m o o A u h Korea Institute of Energy Research, P.O. Box 5, Taeduk Science Town, Taejon, South Korea Received 25 September 1992 The application of the pulse current method for black chrome oxide electrocrystallization has been investigated. Plating parameters to optimize the optical properties of the solar selective film included the bath composition, current density, plating time, duty cycle and substrate. The bath composition was 250-300 g / { of chromic acid, 10-15 g / / of propionic acid and 0.5 g / / of a proprietary additive. It has been observed that the black chrome coatings exhibited reasonable optical properties for commercialization when the plating parameters were properly controlled; absorptance (a) was 0.944 and 0.94, and emittance (e) was 0.084 and 0.15 for nickel and copper substrates, respectively. Thermal stability of the black chrome coatings was also studied by aging at 300°C and 450°C in air for 24 h. This study implies that the pulse current electrolysis method could enhance the optical properties of black chrome solar selective coatings for practical solar applications.

I. Introduction

Solar collector systems generally require solar selective coatings for the efficient utilization of thermal energy from the solar spectrum. Efficient selective coatings are indicated as high absorptance (> 0.9) over the spectral range of 0.3-2.0 Ixm and low emittance (< 0.1) in the infrared to lessen radiative heat loss. There are numerous techniques for preparing solar selective coatings such as sputtering, CVD, spray, chemical oxidation and electrodeposition. Among these techniques, the direct current electrodeposition process for solar selective coatings has been commonly used due to its simplicity and good economics. Most studies of black chrome selective films have focused on developing suitable bath compositions and optimum optical properties by utilizing the direct current electroplating method [1-10]. Especially Ignatiev et al. [3], Pettit et al. [4,6], Holloway et al. [5], Lampert et al. [9] and Lampert [10] observed the microstructural surface of their black chrome coatings in detail to investigate the relationship between the optical properties and surface structure of the films. Correspondence to." T.K. Lee, Korea Institute of Energy Research, P.O. Box 5, Taeduk Science Town, Taejon, South Korea. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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T.K. Lee et al. / Black chrome solar sensitive films

It has been reported by Avila et al. [11] and Perger et al. [12] that pulse current electrolysis has advantages over direct current electrolysis for general electrodeposition. Ibl [13] discussed the effect of pulse current on the electrodeposition of metal surfaces in general. Recently, Lee et al. [14] have utilized pulse current electrolysis to deposit black chrome solar selective coatings on a bright nickel substrate with preparing a new propionic acid bath solution. In this study, we have concentrated on the production of solar selective black chrome films at a low cost, and the development of a thin film production process for commercialization by reducing the average current density reported by Lee et al. [14]. The primary target was to obtain a black chrome coating with absorptance ( a ) of 0.9 or greater and emittance (E) of 0.1 or less. Black chrome oxide thin films were deposited on nickel and copper substrates of about 30 cm 2, and thin films were thermally treated in the air for 24 h at 300°C and 450°C to observe the change of the optical properties. The concentration of chromic acid in the plating bath solution in this work was compared to commercial Chromonyx, Econochrome and Tetrachromate reported by Driver and McCormick [2]. Attempts have been made to obtain proper electroplating conditions for black chrome thin films with suitable optical properties for commercialization.

2. Experimental 2.1. Material Black chrome selective surfaces were prepared by utilizing the pulse current electrolysis method on nickel and copper substrates. Since the optical properties of coated films are dependent upon the surface condition of the substrates, it is important to create uniform selective coatings. Therefore chemical polishing was performed to remove impurities from the substrate surfaces. Both nickel and copper bare substrates were immersed in a solution of 200 g/C of chromic acid in distilled water for 30 min and rinsed with acetone and finally with distilled water. This polishing procedure was repeated if necessary. Nilaco Co. supplied both nickel and copper sheet.

2.2. Experimental set-up As shown in fig. 1, the pulse current electrodeposition system consists of a pyrex bath container, a water cooling glass container, a magnetic stirrer, a thermometer, an anode and a cathode. The anode was a P b - S n alloy, and the cathode was nickel or copper. The area ratio of anode to cathod was 2:1. The magnetic stirrer was employed to prevent the concentration over voltage effect near the surface of the cathode in the bath solution during the operation. The teflon sample holder is displayed in fig. 2. The cylindrical shape sample holder was employed to inhibit the edge current effect, and eventually to form

T.K. Lee et al. / Black chrome solar sensitit,e films

151

" Pulsecurrent "~

Water Out --mum ~

D

B

E

Electrolytic Solution C

- - W a t e r In I

I

o

o

l

A: Heater and Stirrer B: Water C: Magnetic Stirrer D: Anode E: Cathode F: Thermometer

Fig. l. Schematic diagram of the pulse current electrodeposition system. uniform surfaces. In the preliminary experiment, it was found that thicker selective coatings were formed near the edges without this sample holder. The sample was cut in shape and inserted between the cylindrically shaped front panel and the back panel having a groove. Thus, the black chrome coatings were deposited on only one face of the sample. The distance between the anode and cathode was kept constant, 15 cm. The bath t e m p e r a t u r e (20°C) was checked by the thermome-

Fig. 2. Teflon sample holder.

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T.K. Lee et al. / Black chrome solar sensitive films

Current Density

hTon---T~H

TIME

To. : pulse on Tff : pulse off time i p : pulse current density io : average current density Fig. 3. Schematic diagram of the applied pulse current.

ter and controlled by the water cooling system. The square pulse current as shown in fig. 3 was generated by a model HCP-301H power supply made by Hokuto Denko Co. in Japan. Pulse parameters are also shown in fig. 3.

2.3. Electroplating conditions There are numerous variables to affect the optical properties of the black chrome coatings. The bath composition used in this work along with commercial

Table 1 Bath compositions used in this work along with commercial ones Bath

Composition

Chromonyx

Chromic acid Chromonyx MR Chromonyx AA Barium carbonate

340 g / f 26 g / / 270 m l / t 7.5 g / {

Econochrome BK

Econochrome BK Black chrome salts

500 g / t

Tetrachromate

Chromic acid sodium hydroxide sucrose Fluorosilicic acid

400 g / / 60 g / / 2.5 g / { 0.85 g / /

This bath

Chromic acid Propionic acid Second additive

250 g / / or 300 g / / 10 g / t or 15 g / g 0.5 g / 1

T.K. Lee et al. / Black chrome solar sensitive films

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Table 2 Experimental conditions for electrodeposition of the black chrome coating on nickel substrates Run number

Peak current (A)

Pulse on time (ms)

Duty cycle

Plating time (min)

n-1 n-2 n-3 n-4 n-5 n-6 n-7 n-8 n-9 n-10

60 60 60 60 62 70 70 70 70 70

1 1 1 1 1 1 1 1 1 1

1/10 1/16 1/16 1/32 1/16 1/12 1/16 1/16 1/16 1/24

8 6 8 8 8 8 5 8 12 8

chromic bath solutions are listed in table 1. The bath is composed of chromic acid, propionic acid and a proprietary additive. Other parameters, such as peak current, pulse "on" time, pulse "off" time, duty cycle and plating time are listed in tables 2 and 3 for the nickel and copper substrates, respectively. The solar absorptance (a) of films was measured by utilizing an air mass (AM) 2 solar spectrum over the wavelength range 0.2 p~m-2.5 ~m. To evaluate the emittance (E) at 100°C the spectral reflectance was measured by a Perkin-Elmer 882 IR spectrophotometer in the wavelength range 2.5 i~m-25 txm and calculated by the standard equation reported in ref. [15].

Table 3 Experimental conditions for electrodeposition of the black chrome coating on copper substrates Run number

Peak current (A)

Pulse on time (ms)

Duty cycle

Plating time (rain)

c-I c-2 c-3 C-4 C-5 a)

55 60 62 (280/15) 62 (300/15) 30 (200/10) 62 (300/15) 65 65 65 65 65 65 (300/20) 70 (300/15)

1 1 1 1 1 1 1 2 5 9 10 1 1

1/24 1/16 1/16 1/16 1/16 1/16 1/16 1/16 1/16 1/16 1/16 1/16 1/16

8 8 8 8 3 5 8 8 8 8 8 8 8

C-6 C-7 C-8 C-9 C-10 c-ll C-12

b) b) b) b)

b) b)

a) Two-step electrodeposition. b) (280/15, 300/15, 300/20); (chromic acid/propionic acid).

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154

3. Results and discussion

3.1. Nickel substrate The changes in r e f e c t a n c e of black chrome coatings deposited on nickel substrates were measured by an IR spectrophotometer as a function of wavelength and illustrated in fig. 4 for the samples of as-deposited, aged at 300°C and 450°C in air for 24 h. These samples are electrodeposited under the conditions of a peak current of 60 A, a duty cycle of 1/16, and a plating time of 8 min in the bath solution containing 250 g/C of chromic acid and 10 g/C of propionic acid and 0.5 g of a proprietary additive. The calculated emittances of the samples of as-deposited, aged at 300°C and at 450°C were 0.084, 0.08 and 0.075, respectively. This result indicates that thermal treatment decreases the emittance. The absorptance values measured by a solar spectrum reflectometer over the range 0.2 ~m-2.5 I~m were 0.944 for the black chrome coatings of as-deposited and 0.94 for aged at 300°C and 0.854 for aged at 450°C. This result shows that the absorptance of the black chrome coatings is deteriorated as the heat treatment temperature increases. The optical properties of the black chrome coatings deposited under the various electroplating conditions are listed in table 4. It is noted that n-3 and n-8 samples are electroplated under the same conditions, except for bath concentrations and the peak currents of 60 A and 70 A. Their measured absorptance values are 0.944 and 0.951, and the emittances are 0.084 and 0.172, respectively. While the absorptance and emittance of the black chrome coating deposited at the condition of a 70 A peak current are higher, the selectively ( a / e ) is much lower than that of

1.0 D

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0.9

0.8 aged at 450 °C

t't"

0.7

0.6

0.5 0

i

•

aged at 300 °C

---e--- as-deposited

i

i

10

20 Wavelength

30

(urn)

Fig. 4. Reflectance as a function of wavelength of the black chrome coatings deposited on nickel substrates (250 g / t chromic acid, 60 A, 1/16).

T.K. Lee et al. / Black chrome solar sensitive films

155

Table 4 Optical properties of the black chrome coatings deposited on nickel substrates at different electroplating conditions Run number

Absorptance (a)

Eminance (e)

Selectivity (a/E)

n-1 n-2 n-3 n-4 n-5 n-6 n-7 n-8 n-9 n-10

0.942 0.929 0.944 0.86 0.939 0.96 0.893 0.951 0.939 0.924

0.162 0.078 0.084 0.054 0.09 0.232 0.1 0.172 0.212 0.14

5.8 11.9 11.2 15.9 10.4 4.1 8.93 5.5 4.4 6.6

a 60 A peak current. The corresponding average current densities are about 155 m A / c m 2 and 133 m A / c m 2, respectively. Thus, it can be stated that the black chrome coating deposited at 60 A peak current seems to be better for commercialization, since the absorptance is high enough and the emittance is much lower than the one obtained at 70 A peak current. Furthermore, the bath solution for the 60 A peak current contained 250 g/I of chromic acid, 10 g/I of propionic and 0.5 g/I of a proprietary additive while the bath composition used for the 70 A peak current must be c o m p o s e d of more than 300 g/t' of chromic acid, 15 g/t e of propionic acid and 0.5 g/I of a proprietary additive. This implies that the maximum peak current might be related to the Cr 6+ concentration in the bath solution. Another achievement in this work is that a good quality of black chrome coatings was obtained with less average current density than in ref. [14]. The changes in absorptances and emittances as a function of duty cycle of the samples electrodeposited on a nickel sheet under the conditions of a 60 A peak current and a plating time of 8 min are shown in fig. 5. The absorptances at the given conditions are increased in the low duty cycle region and level off in the high duty cycle area; however, the emittances increase as the duty cycle increases. The high duty cycle means a longer pulse " o n " time and eventually a longer deposition time for the black chrome oxide onto the substrate. Thus, the absorptance and emittance are increased as the duty cycle increases. This result shows that the appropriate duty cycle at given conditions is 1/16, since the selectivity is optimum as compared to other conditions. Fig. 6 illustrates the behavior of optical properties of the selective coatings as a function of electroplating time. The electroplating conditions are a 70 A peak current and a duty cycle of 1/16. As shown in this figure, the absorptance is increased initially and becomes flat as the plating time increased above 8 min. The absorptance shows a maximum at 8 min (0.951); however, the emittance value is higher and the chromic acid concentration was higher than that of a 60 A peak

T.K. Lee et al. / Black chrome solar sensitiue films

156

0.5

1,0

-0.4 o- . . . . . . . . . . . . . . .'

oc

8

o.. . . . . . . . . .,.o

.0.3 /'

0.9 ¸

/,'

iii

/ <

- 0.2

/

-0.1

0.8

0.0

i

0.04

0.08 Duty

Cycle

Fig. 5. Changes in the optical properties of the black chrome coatings deposited on nickel substrates as a function of duty cycle (250 g/• chromic acid, 60 A, 8 min).

"0.5

1.0-

0.4 ,,o~ . . . . . . . . . . . . . . C(. ,,' /o"

...... o ...............

8

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/o°

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.£1

0.2

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0.0

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Plating Time (min) Fig. 6. Absorptance (ct) and emittance (e) behavior of the black chrome coatings deposited on nickel substrates as a function of electroplating time (300 g / / chromic acid, 70 A, 1/16).

T.K. Lee et al. / Black chrome solar sensitive films

157

1.0

0.9

0.8 ~ 0.7

aged at 450 °C aged at 300 °C

~

as-deposited

0.6

0.5

i

0

10

20

30

Wavelength (um) Fig. 7. Reflectance as a function of wavelength of the black chrome coatings deposited on copper substrates (300 g / t chromic acid, 60 A, 1/16).

current. current suitable 300 g/•

This indicates that the black chrome coating obtained by utilizing a peak of 60 A in the bath solution containing 250 g / / of chromic acid is more for commercialization that that obtained using a 70 A peak current and of chromic acid in the solution.

3.2. Copper substrate For practical application of black chrome coatings, a copper substrate was employed for economic reasons although copper has a rather lower thermal stability. The emittance obtained by utilizing an IR spectrophotometer is displayed in fig. 7. A 60 A peak current, an electrodeposition time of 8 min and a duty cycle of 1 / 1 6 with a bath solution containing 300 g / / of chromic acid, 15 g / t of propionic acid and 0.5 g of the second additive were the electrodeposition conditions utilized for these samples. The calculated values of the emittance for the samples of as-deposited, aged at 300°C and 450°C are 0.16, 0.15 and 0.12, respectively. The measured absorptance values by the solar spectrum reflectometer of these samples are 0.93, 0.92 and 0.842, respectively. This indicates that the black chrome coatings deposited on a copper substrate become rather unstable at temperatures above 300°C. The optical properties of films deposited on the copper substrate under the various conditions are listed in table 5. The effect of the concentration of chromic acid in the bath solution on the optical properties appears to be negligible as can be seen by comparing c-3 and c-4. The optical properties of the sample obtained by two-step pulse electrodeposition (c-5) were not significantly enhanced when compared with c-4. However, this

T.K. Lee et aL / Black chrome solar sensitice films

158

Table 5 Optical properties of the black chrome coatings deposited on copper substrates at different electroplating conditions Run number

Absorptance (a)

Emittance (e)

Selectivity (a/e)

c-I c-2 c-3 c-4 c-5 c-6 c-7 c-8 c-9 c-10 c-ll c-12

0.904 0.93 0.935 0.931 0.931 0.94 0.956 0.952 0.948 0.945 0.95 0.948

0.093 0.16 0.16 0.155 0.15 0.15 0.16 0.17 0.16 0.15 0.252 0.21

9.7 5.8 5.8 6.0 6.2 6.3 6.0 5.6 5.9 6.3 3.8 4.5

process might have a high potential to improve the optical properties of the thin films with less peak current density when the plating time and concentration of chromic acid in the solution are considered. 0.5

1.0

' 0.4

°.O

....O-'O , O o" • 0.3

8

i

o

0,9' • 0.2

"0.1

0.8

i

40

so

0.0

£

8o

Peak Current (A) Fig. 8. Absorptance ( a ) and emittance (E) versus peak current of the black chrome coatings deposited on copper substrates (300 g / g chromic acid, 1/16, 8 rain).

T.IC Lee et al. / Black chrome solar sensitive films

159

-0.5

"I.0

0.4

•

Of,

,0 ......

--.....

,.,"

o

............

.2

0..

-o

0.3

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0.9

g kU

m

0.2

'0.1

0.0

0.8 ).02

0.06

O. 0

0.14

Duty Cycle Fig. 9. Absorptance (a) and emittance (e) versus duty cycle of the black chrome coatings deposited on copper substrates (300 g / / c h r o m i c acid, 65 A, 8 rain).

The effect of the peak current on the optical properties of the black chrome coatings was investigated under the conditions of a duty cycle of 1 / 1 6 and a plating time of 8 min and is shown in fig. 8. The absorptance increases as the peak current increases; however, the emittance first decreases and then increases with peak current. This seems to result from the nonuniformity of the black chrome thin films at a 70 A peak current. The black chrome coating obtained using a 65 A peak current might be of practical use at the given conditions, although the selectivity was somewhat low (6.3). The behavior of the optical properties was observed as a function of duty cycle at a fixed peak current of 65 A and a plating time of 8 min. This result is shown in fig. 9. The absorptance has a maximum at a duty cycle of 1/16 and decreases slightly as the duty cycle increases and the emittance is increased with duty cycle. A duty cycle of 1/16 gives the best results for this experimental condition. The effect of the pulse " o n " time at fixed duty cycle (1/16) on the properties of the coatings was studied and is shown in fig. 10. The optical properties of the sample with the conditions of 65 A peak current and an electroplating time of 8 min do not change significantly, regardless of the changes in the pulse " o n " times (ms). This suggests that the ratio of pulse " o n " time to pulse "off" time (duty cycle) plays an important role, rather than physical pulse " o n " time for a given plating time.

T.K. Lee et al. / Black chrome solar sensitive films

160

0.5

1.0

0.4

o- ..... ,•

,•.

.......

.

.

.

.o-.-..,

.

-o . . . . . . . . . . o . . . o

o'

0.3 o,.

0.9"

kid

<

0.2

0.1

0.0

0.8

Pulse on time (msec)

Fig. 10. Changes in the optical properties of the black chrome coatings deposited on copper substrates as a function of pulse "on" time at a fixed duty cycle (300 g / / c h r o m i c acid, 65 A, 8 min).

4. Conclusions The main purpose of this study was to produce black chrome solar selective coatings for commercialization by the pulse current electrolysis method with lower average current density than in previous work [14]. The main component of the bath solution used in this work is chromic acid, of which the amount is less than that of the commercial products summarized in table 1. The substrate area was about 30 cm 2 and efforts have been directed toward minimizing the average current density and electroplating time with suitable optical properties of black chrome coatings. The aborptance for the nickel substrate appropriate for commercialization was 0.944 and for copper substrate 0.94. However, the absorptance for copper was obtained under the conditions of a 65 A peak current in a bath solution containing 300 g / t of chromic acid and 15 g / / of propionic acid while for nickel the peak current was 60 A in a bath composition of 250 g/t of chromic acid and 10 g / t of propionic acid. The emittance was 0.084 for nickel and 0.15 for copper under the conditions stated in the above. This seems to be a function of characteristic spectral reflectance of the materials. Although the densities of the two substrates are similar, the nickel substrate is a more promising candidate for the black chrome coatings for solar applications due to its lower emittance. The thermal

T.K. Lee et al. / Black chrome solar sensitive films

l 61

stability test shows that the nickel s u b s t r a t e is m o r e suitable t h a n the c o p p e r s u b s t r a t e at high t e m p e r a t u r e s d u e to t h e s a m e r e a s o n e x p l a i n e d above. F r o m the results of this investigation, it was c o n c l u d e d t h a t we w e r e able to p r o d u c e c o m m e r c i a l i z a b l e b l a c k c h r o m e selective coatings which exhibit high solar a b s o r p t a n c e a n d selectivity with g o o d t h e r m a l stability with an a v e r a g e c u r r e n t d e n s i t y o f 130-155 m A / c m 2. This a v e r a g e c u r r e n t d e n s i t y is almost 50% less t h a n t h e value r e p o r t e d in ref. [14]. A two-stage e l e c t r o d e p o s i t i o n was c a r r i e d o u t for t h e c o p p e r substrate. T h e initial e l e c t r o d e p o s i t i o n was c o n d u c t e d at a r a t h e r low c o n c e n t r a t i o n o f c h r o m i c acid in the solution with low p e a k c u r r e n t to p r o v i d e the n u c l e a t i o n sites for the s e c o n d stage for a short time. A 62 A p e a k c u r r e n t a n d a e l e c t r o p l a t i n g time of 5 min w e r e e m p l o y e d for t h e s e c o n d stage to c o m p a r e with the o n e d e p o s i t e d with the s a m e p e a k c u r r e n t b u t 8 min e l e c t r o d e p o s i t i o n time. T h e result was not so o u t s t a n d i n g , b u t this p r o c e s s could e n h a n c e t h e optical p r o p e r t i e s with low c u r r e n t d e n s i t y a n d short e l e c t r o p l a t i n g t i m e w h e n t h e p a r a m e t e r s a r e p r o p e r l y c o n t r o l l e d .

Acknowledgement T h e a u t h o r s a r e p l e a s e d to a c k n o w l e d g e the M i n i s t r y o f Science a n d T e c h n o l ogy, g o v e r n m e n t o f K o r e a for financial s u p p o r t o f this project.

References [1] P.M. Driver, Sol. Energy Mater. 4 (1981) 179. 12] P.M. Driver and P.G. McCormick, Sol. Energy Mater. 6 (1982) 159. [3] A. Ignatiev, P. O'Neill and G. Zajac, Sol. Energy Mater. 1 (1979) 69. [4] R.B. Pettit and R.R. Sowell, J. Vac. Sci. Technol. 13 (1976) 596. [5] P.H. Holloway, K. Shanker, R.B. Pettit and R.R. Sowell, Thin Solid Films 72 (1980) 121. [6] R.B. Pettit, R.R. Sowell and l.J. Hall, Sol. Energy Mater. 7 (1982) 153. [7] R.B. Pettit, Sol. Energy Mater. 8 (1983) 349. [8] J.N. Sweet, R.B. Pettit and M.B. Chamberlain, Sol. Energy Mater. 10 (1984) 251. [9] C.M. Lampert and J. Washburn, Sol. Energy Mater. 1 (1979) 81. [10] C.M. Lampert, Thin Solid Films 72 (1980) 73. [11] A.J. Avila and M.J. Brown, Plating 58 (1970) 1105. [12] G. Perger and P.M. Robinson, Metal Finish. 77 (1979) 17. [13] N. lbl, Surface Tech. 10 (1980) 81. [14] T.K. Lee, W.B. Kim, S.H. Cho and P.C. Auh, Sol. Energy Mater. 23 (1991) 13. [15] J.A. Duffle and W.A. Beckman, Solar Engineering of Thermal Processes (Wiley, New York, 1980).