Solar Energy Materials 22 (1991) 293-302 293 North-Holland

Black cobalt solar absorber coatings

S. John Central Electrochemical Research Institute, Karaikudi 623 006, Tamil Nadu, India

N. Nagarani and S. Rajendran Physics Department, Alagappa University, Karaikudi 623 003, Tamil Nadu, India

Received 25 August 1990

A new electrolyte has been proposed for the deposition of black cobalt selective absorber coatings. These coatings are used in solar collectors for photothermal conversion of solar energy. We have studied the influence of electrolyte composition and operating parameters on the properties of the black cobalt coatings including optical (a, ¢) and electrical properties. Thermal stability and corrosion resistance tests showed good durability of black cobalt selective coatings for high temperature applications.

I. Introduction

Solar thermal collectors represent a wide-spread type of system to convert solar energy. Flat-plate and parabolic collectors are used in heating or preheating water for domestic and industrial purposes. Radiation, convection and conduction are strongly coupled energy transport mechanisms in solar collector systems. The economic viability of lower temperature applications of solar energy may be improved by increasing the quantity of usable energy delivered per unit area of collector. This is achieved by the use of selective black coatings which have a high degree of solar absorption, maintaining high energy input to the solar system, while simultaneously suppressing the emission of thermal infrared radiation.

Considerable effort has been expended on developing coatings and absorber materials having a high conversion efficiency [1-5]. Electroplated black chrome coatings have been shown to be one of the most efficient and commercially useful absorbers [6-8]. Production of such coatings however requires high current densities of the order of 20 A / d m 2 and coating facilities to maintain the electrolyte tempera- ture between 15 and 20 ° C. These conditions increase the production cost of the coating. Black nickel [6-8] also possesses good optical properties, but its corrosion resistance is moderate under humid conditions due to degradation. Hence consider- able work has been done and is still being done, to develop less expensive and durable coatings for solar energy conversion [9-13].

Cobalt oxide selective coatings have been proposed as potential candidates for

0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

294 S. John et al. / Black cobalt solar absorber coatings

high temperature solar applications due to their good thermal stability [14-21]. Black cobalt has been produced by spray pyrolysis, electroplating, chemical conver- sion, thermal oxidation and sputtering techniques. Among these methods, the electroplating technique is well established and commercially adopted for producing black cobalt for use in evacuated tube collectors [22-24]. Black cobalt has been deposited from an electrolyte containing 25 g / l cobalt chloride and 25 g /E potassium thiocyanate [20,21].

In this paper the development of a practical formulation for the deposition of black cobalt from a sulphate-acetate electrolyte is reported. We studied the effects of solution composition and operating conditions on the appearance and optical properties of black cobalt coatings.

2. Experimental

Copper plates of size 100 × 100 mm were mechanically polished, degreased with a solvent, electro-cleaned in alkali, rinsed, pickled in acid, rinsed and nickel-plated to a thickness of 10/~m from the following bath:

nickel sulphate 300 g /g , nickel chloride 25 g/E, boric acid 35 g /g , saccharin 0.5 g / l , pH 4.0, current density 3 A / d m 2, temperature 50 o C.

The panels were rinsed with tap water and then with distilled water and black cobalt deposition was carried out under the following conditions:

cobalt sulphate 10-100 g/E, ammonium acetate 10-40 g/Y, pH 5.5-9.0, current density 3-6 A / d m 2, temperature 20-60 ° C.

All the chemicals used were of laboratory reagent grade. Distilled water was used for solution preparation. To judge the quality of the black deposit over a wide range of current densities, Hull cell experiments were carried out in a standard 267 ml cell at 2 A current for 30 seconds [25-28]. The Hull cell is shown in fig. 1. This is a miniature trapezoidal plating cell made of PVC or acrylic plastic (perspex) in which the cathode is kept at an inclined position with respect to the anode. Hence one end of the cathode is closer to the anode and receives high current densities and the other end is away from the anode and receives lower current densities. A spectrum of current densities are thereby generated at the cathode and hence this is a useful technique for plating shops to evaluate bath composition and operating parameters. This cell was chosen in our study to evaluate solution composition and to optimize operating conditions including pH, current density and temperature. The code for recording a Hull cell pattern is given in fig. 2. Optical properties (a, ~) of the black

T

a

S. John et aL / Black cobalt solar absorber coatings

SOLUTION LEVEL

l ,

b I-, 12S --i

Fig. 1. 267 ml Hull cell (all dimensions in n~llimeters): (a) isometric view, (b) top view.

295

coatings were evaluated using Alphatometer and Emissometer (Devices and Services Co., USA).

The black cobalt coating produced under the optimised conditions were tested for adhesion by means of a tape test. In this test, a clear 25 mm width cellophane tape was pressed evenly on the thin coating and pulled off suddenly with a swift rapid motion. If the deposit does not come off on the tape, then the adhesion of the deposit is good.

NO PLATING. ~ BLACK.

GREYISH BLACK. ~ STREAKY.

GREY. GREENISH PRECIPITATE.

Fig. 2. Code for recording Hull cell pattern.

296 S. John et a L / Black cobalt solar absorber coatings

Thermal stability of the coating was evaluated by a thermal cycling test wherein the black coated specimens were heated to 280 °C for eight hours. The specimens were allowed to cool to room temperature overnight. This test was conducted for 12 consecutive days. The optical properties (a, c) were measured before and after the test.

The corrosion resistance of the coating was evaluated by an accelerated salt spray test using 5% w / v sodium chloride neutral solution on copper panels plated with 10 ttm nickel as undercoat. Optical properties (a, c) were measured at eight hours interval. From the changes in optical properties the corrosion resistance of the black coating was evaluated.

3. Results and discussion

3.1. Influence of electrolyte composition

Initial experiments were carried out from the electrolyte containing only cobalt sulphate. The coatings produced were greenish in colour. When 40 g/Y of am- monium acetate was added to the electrolyte with 30 g / l cobalt sulphate uniform black coatings were produced. Fig. 3 shows the Hull cell pattern with 2 A cell current for 30 seconds deposition time with and without the addition of ammonium acetate. A uniform black coating was obtained above a current density of 2.5 A / d m 2.

3.2. Influence of cobalt sulphate concentration

The influence of varying the cobalt sulphate concentration was studied at 30 o C, while keeping the concentration of ammonium acetate constant at 40 g / l . Lower concentrations of cobalt sulphate (10 g / d ) produced streaky coatings, while higher

CURRENT DENSITY, A /dm 2 8 6 5 4 3 21.6 0.8 0.4

i I i i I I I i i

0 0 0 0 0 0 0 0 © 0 0 0 0 0 0 0 0 0 0 0 0 © 0 0

(a)

(b) Fig. 3. Hull cell pattern with and without ammonium acetate: (a) cobalt sulphate 30 g / d , pH 5.5, 30 o C, 2 A current, 30 s; (b) cobalt sulphate 30 g / d , ammonium acetate 40 g / d , pH 5.5, 30 o C, 2 A current,

30 s.

S. John et a L / Black cobalt solar absorber coatings 297

concentrations (50 g / g ) gave rise to black coatings only at high current densities (> 6 A/dm2). Fig. 4 shows the Hull cell pattern at different concentrations of cobalt sulphate. Based on this study the optimum concentration of cobalt sulphate was chosen as 30 g /g .

3.3. Influence of ammonium acetate concentration

Ammonium acetate concentration was varied from 10 to 40 g / g while keeping the cobalt sulphate concentration at 30 g / g at 5.5 pH and 20 ° C. Fig. 5 shows the Hull cell pattern at different concentrations of ammonium acetate. At lower concentrations ( < 10 g / g ) the black coating current density range is narrow (2.5-5.5 A / d m 2) and in the low current density areas the coating is predominantly greyish. A uniform black coating was obtained at a concentration of 40 g /g . Hence the optimum concentration of ammonium acetate was chosen as 40 g /g .

3.4. Influence of pH

The pH of the black coating electrolyte is important as it determines the different constituents present in the coating. The pH of the solution was adjusted using acetic acid and ammonia and was varied from 5.5 to 9.0 for an electrolyte containing 30 g / g cobalt sulphate and 40 g / g ammonium acetate at 30 o C. Fig. 6 shows the Hull

CURRENT DENSITY, A / d i n 2. 8 6 5 4 3 2 1.6 0.8 0.4

I ~ - - I i I i I I I i I

(a )

I l l i (b)

t I I Cc)

(d) Fig. 4. Hull cell pattern at different concentrations of cobalt sulphate: ammonium acetate 40 g / l , pH

5.5, 30 o C, 2 A current, 30 s. (a) 10 g /g , (b) 30 g /g , (c) 50 g/Y and (d) 100 g /g .

298 S. John et al. / Black cobalt solar absorber coatings

CURRENT DENSITY, A / d i n 2 8 6 5 3 2 1.6 0.8 0.4 I I I I I I

JIJll (a)

(b)

Jt i r lU/ I ] I I I I

(c) Fig. 5. Hull cell pattern at different concentrations of ammonium acetate: cobalt sulphate 30 g /F , pH

5.5, 30 ° C, 2 A current, 30 s. (a) 10 g /E, (b) 25 g / E and (c) 40 g / l .

cell pattern at different pH values. At 5.5 pH, a quality black coating was produced in the current density range of 2.5-8 A / d m 2. When the pH was 9 the deposit produced is predominantly streaky in nature and the coating is not black. Hence a pH of 5.5 was considered as optimum for producing a uniform cobalt black coating.

3.5. Influence of temperature

The temperature of the electroplating solution affects the formation of the black deposit as it influences the reactions responsible for the formation of the coating. The temperature of the plating solution was varied from 20 to 60 °C and the Hull cell pattern produced is given in fig. 7. An operating temperature in the range of

2 CURRENT D E N S I T Y A / d m

B 6 4 2 1.6 0.8 0.4 I I I i I

I l l [ [ 1 1 1 I I I I

I I 1 I I I

Ca)

Cb) Fig. 6. Hull cell pattern at different pH values: cobalt sulphate 30 g /E, ammon ium acetate 40 g / g ,

30 o C, 2 A current, 30 s. (a) pH = 5,5, (b) pH = 9.

S. John et aL / Black cobalt solar absorber coatings 299

CURRENT DENSITY, Aldm2. 6 5 I, 3 2 1.6 0.8 0.4 I I I I i I I

I I

(Q)

I I I I I I

(b )

(¢)

(d) Fig. 7. Hull cell pattern at different electrolyte temperatures: cobalt sulphate 30 g/g , ammonium acetate

40 g/g, pH 5.5, 2 A current, 30 s. (a) 20°C, (b) 30°C, (c) 40°C, (d) 60°C.

20-30°C yielded quality black coatings over a wide current density range (1.8-8 A/dm2). Higher temperatures (> 40 ° C) reduced the current density range of the black coating. In the low current density region a greyish black coating was obtained.

3.6. Evaluation of adhesion of black coating

The black coatings produced under optimum conditions were tested for adhesion by the tape test and found to possess good adhesion to the substrate. Exfoliation or peeling of the coating was not noted.

3. 7. Thermal stability test

When the thermal stability test was performed the absorptance of the panels did not show any change. This test establishes the good stability of black cobalt coatings for collectors operating at least to 280 ° C.

3.8. Corrosion resistance

The black coating did not show any change in optical properties after 96 hours of salt spray which indicates good performance of black cobalt in corrosive environ- ments.

3 0 0 S. John et al. / Black cobalt solar absorber coatings

i.-- , " r

i..i.1 EL o

n

.~ O2 < (...) I.-- EL o 0

0

04 l

o,C

I I I i

10 15 20 25 30

PLATING TIME IN SECONDS. Fig. 8. Effect of plating time on optical properties (a, ~): cobalt sulphate 30 g/E, ammonium acetate 40

g / f , pH 5.5, 30 ° C, 3 A / d m e.

3.9. Influence of plating time on optical properties (a, ~)

After standardising the electrolyte composition and operating conditions using Hull cell studies, standard test coupons were prepared by plating for different durations at 30 ° C, 5.5 pH and a current density of 3 A / d m 2. The influence of plating time on optical properties is shown in fig. 8. It is evident from the figure that increased plating time gives rise to a higher value of both solar absorptance (a) and thermal emittance (~). The solar absorptance is an increasing function of plating time (increasing from 0.87 to 0.96 for plating times of 10 to 30 seconds). From fig. 8 one can see that the emittance of the coating is less than 0.20 for plating times up to 30 seconds. The emittance of the coating increases almost linearly with plating time. Black cobalt plated for 25 seconds gives the best combination of optical properties (i.e. a = 0.96 and ~ = 0.12).

4. Topography studies

A scanning electron micrograph of the coating's surface structure is shown in fig. 9. Like many other electrodeposited selective black coatings, this coating has a particulate structure with micro-cavities which enhance absorption.

5. Conclusion

The best electrodeposited black cobalt solar absorber coating was deposited from an electrolyte of the following composition and operating conditions:

cobalt sulphate 30 g/E, ammonium acetate 40 g/Y,

S. John et al. / Black cobalt solar absorber coatings 301

Fig. 9. SEM micrograph of surface topography. Deposition at 3 A/dm 2, 20 s.

p H 5.5, t empera tu re 2 0 - 3 0 o C, current dens i ty 3 A / d m 2, depos i t ion t ime 20 -25 s.

Opt ica l p roper t i e s of the coa t ing p r o d u c e d unde r the o p t i m u m cond i t ions were found to be a = 0.96 and c = 0.12. The rma l cycl ing and cor ros ion tests in i t ia l ly indica te good durab i l i ty and s tabi l i ty of the coat ing.

Acknowledgement

The au thors would l ike to express their g ra t i tude to Professor S.K. Ranga ra j an , Direc tor , C E C R I , K a r a i k u d i for encouragemen t and pe rmiss ion to pub l i sh this paper .

References

[1] G.G. Libowitz and M.S. Wittinghan, Materials Science in Energy Technology (Academic Press, London, 1979) p. 215.

[2] B.O. Seraphin, Solar Energy Conversion (Springer, Berlin, 1979) p. 4. [3] D.M. Mattox, J. Vac. Sci. Technol. 13 (1976) 127. [4] O.P. Agnihotri and B.K. Gupta, Solar Selective Surfaces (Wiley, New York, 1981). [5] M.M. Koltun, Selective Surfaces for Solar Energy Converters (Allerton Press, New York, 1981).

302 S. John et al. / Black cobalt solar absorber coatings

[6] C.M. Lampert, Thin Solid Films 72 (1980) 73. [7] P.M. Driver and P.G. McCormick, Sol. Energy Mater. 6 (1982) 381. [8] O.T. Inal, J.C. Mabon and C.V. Robino, Thin Solid Films 83 (1981) 399. [9] C.E. Johnson, Metal Finishing 78, 7 (1980) 21.

[10] N.V. Shanmugam, S. John, K.N. Srinivasan, M. Selvam and B.A. Shenoi, Metal Finishing 82, 10 (1984) 91.

[11] M. Selvam, S. John, K.N. Srinivasan, N.V. Shanmugam and B.A. Shenoi, Metal Finishing 82, 4 (1984) 75.

[12] S. John, N.V. Shanmugam and S. Guruviah, Metal Finishing 87, 8 (1989) 19. [13] C.M. Lampert and J. Washburn, Sol. Energy Mater. 1 (1979) 81. [14] R.B. Gellate, Sol. Energy 4 (1960) 24. [15] G.B. Smith and A. Ignatiev. Sol. Energy Mater. 2 (1980) 461. [16] C. Choudhury and H.K. Seghal, Appl. Energy 10 (1982) 313. [17] K. Chidambaram, L.K. Malhotra and K.L. Chopra, Thin Solid Films 87 (1981) 365. [18] C. Choudhury and H.K. Sehgal, Sol. Energy 30, 3 (1983) 291. [19] P.K.C. Pillai and R.C. Aggarwal, Alternative Energy Sources 4, 1 (1982) 191. [20] B. Vitt, Sol. Energy Mater. 13 (1986) 323. [21] B. Vitt, Sol. Energy Mater. 14 (1989) 131. [22] H.O. Jungk and H. Scholz, Deutsche Often P25567162. [23] H. Horster, Ed., Wege Zum Energiesparenden Wohnhaus (Philips Fachbuchverlag, Hamburg, 1980). [24] H. Bloom, J.C. de Grijs and R.L.C. de Vaan, Philips Tech. Rev. 40 (1982) 181. [25] W. Nohse and R.O. Hull, The Investigation of Electroplating and Related Solutions with the Aid of

the Hull Cell (Robert Draper, Teddington, UK, 1948). [26] J. Jackson and D.A. Swalheim, Plating Surf. Finishing 65, 5 (1978) 38. [27] Hull tests for quality plating, American Electroplaters Society Illustrated Lecture No. 32, Florida,

USA. [28] J.B. Kushner, Electroplating Know-how II, A Home Study Course in Modern Metal Finishing

(Kushner, California, USA, 1974).