Nickel-black solar absorber coatings

Energy Convers. Mgmt Vol. 24, No. 4 pp. 255 - 258, 1984

(1196-8904/84 $3.00+0.00 Copyright ~) 1984 Pergamon Press Lid

Printed in Great Britain. All rights reserved

NICKEL-BLACK SOLAR ABSORBER COATINGS K. N. SRINIVASAN, N. V. SHANMUGAM, M. SELVAM, S. JOHN and B. A. SHENOI Central Electrochemical Research Institute, Karaikudi - 623 006, India

(Received 2 July 1983) Abstract--A new electrolyte has been proposed for the deposition of nickel-black selective coatings for use in flat-plate solar collectors. The authors have studied the influence of various ingredients and operating parameters on the appearance of the black coatings so obtained with special reference to their optical values (c~, ~). The coating exhibits better corrsion resistance than the well known black-nickel coatings (which contains nickel, zinc and sulphur) apart from having thermal stabilityupto 380°C. With no zinc in the deposit, this can be used in place of black nickel coatings with less corrosion problems. Solar absorber coatings Deposit of nickel-black

Nickel-black Selection coatings Metal colouring Solar energy

INTRODUCTION The use of selective coatings has been widely established as an industrial practice to enhance the efficiency of flat plate collectors used to absorb solar thermal energy. Such coatings are characterised by high solar absorptance c~ > 90% and low thermal emittance ~ < 20%. They are normally coated over the collectors by electroplating a thin deposit of black chrome [1-11] or black nickel [12-21]. Other known black coatings are black copper, [22] black aluminium [23-25] and blackened electroless nickel [2627]. Even though better optical properties are offered by black nickel, its utility is questionable in humid conditions. Whereas, the other successful competitor, namely black chrome, involves operating difficulties for a full sized collector owing to the use of high current densities of the order of 400 A/ft 2 at low temperature (15-20°C). Hence, a developmental effort has been initiated in this laboratory to develop better selective coatings than black nickel which resulted in the nickel black process without the presence of zinc. In this article, the authors report on the characteristics of various ingredients used in the electrolyte, operating parameters and their impact on optical properties and other tests carried out on the deposits so produced. An optimised electrolyte composition has been proposed to obtain a nickel-black deposit having high absorptance (a = 0.87 - 0.90) and low emittance (~ = 0.09 - 0.12).

Black coatings

Flat plate collectors

the panels were applied with a nickel-black finish using the following composition: Nickel chloride, 100 g/l; ammonium chloride, 25g/1; ammonium thiocyanate, 10 g/l. The grey coloured deposits so obtained were dipped in 5% v/v nitric acid solution to get black coloured deposits of desirable optical properties. All the chemicals used were of laboratory reagent grade. The suggested composition of operating parameters were arrived at with preliminary Hull cell studies. The panels were tested for their optical values using the alphatometer and emissometer manufactured by M/s Devices and Services Co., U.S.A. The thermal stability of the coatings was tested by keeping the panels in an electric oven and the temperature was raised from 30"C to 300°C within half-an-hour and then maintained for the next 8 h. The oven was switched off and the panels were allowed to cool overnight. This was repeated for 7 days. The effect of this test on optical values was also measured. Corrosion studies were made by subjecting the panels to 5% sodium chloride neutral salt spray test for a duration of 100 h. The change of optical values with time were recorded at every 8 hour interval along with visual microscopic inspection at a magnification of 100 x.

RESULTS AND DISCUSSIONS EXPERIMENTAL

Influence of nickel chloride concentration

Copper or mild steel panels of size 100 x 100ram were nickel plated, after subjecting them to conventional cleaning pretreatment, to a thickness of 10 i~m using either a low concentration nickel [28] electroiyte or conventional Watt's type electrolytes. Later, 255

Figure I shows the code used for recording the Hull cell pattern obtained in various experiments, after subsequent immersion in acid. Figure 2 shows the Hull cell pattern of panels for various concentrations

SRINIVASAN et aL:

256

Grey

Burnt

Streoky

NICKEL-BLACK COATINGS

Block

Brownish

Interference

~_~

Fig. 1. Code for recording Hull cell pattern.

- ........ ASF

'1209075 60 45 I I I t I

I

3 0 18 12 t I I

5 I

1.5 I I

Fig. 3. Hull cell pattern with different pH values.

N i % 5og/I ~

~

3A/2mts

~

~

i

~

3N,O2100g/t A 12 mrs

'::.'":.'::.:::

i

i

~.-:.

i

i:!-~-~ iii ~

shown in Fig. 4. The absorptance is not altered widely in between the time limit of 10 to 60s, whereas the emittance steadily increases with time. Measurement at different temperatures further indicates that a lower temperature 30°C produces favourable optical values. Since high a b s o r p t a n c e (c( = 0.8?) and l o w

N,C,.5o0.,

ASF

emittance (, -- 0 1 ) i s obtained at a p,ating t,me of 10s, this value is chosen for operation at 30°C. I

1209075 60 45 ~ ~ ~ ~ J

50 ~

18 12 ~ ~

3 ~

15 I

Fig. 2. Hull cell pattern with different concentrations of NiCt2.

Effect of current density Figure 5 shows the effect of current desnity (16--60 A/ft 2) and optical properties for a plating time of 10 s. 10

= a

of nickel chloride, namely from 50-150 g/l, by keeping the other ingredients at a fixed concentration, at a pH of 6.0 and at room temperature (28 + 2°C). At a concentration of 100 g/l of nickel chloride, maximum current density is obtained x with a wider black current density range of 12-90 A/ft 2. Further increased concentration to 150 g/l resulted only in the decrease of useful current density range, i.e. between 12-60 A/ft 2. Hence, the nickel chloride concentration was fixed at 100 g/I to obtain quality black

08

A"-'A'--~ ""-"--

~ 04 § ° oz <

I

o

I

4o

5o

I

60

Effecl of current density on ophcol properlies (a,~)

lo~ o

=E

08

_

06--

~' ~ 04 ~o o2< 0

The influence of plating time on optical properties at two different temperatures (30 and 40°C) are

I

30

Fig. 4. Effect of plating time on optical properties (et, e) at two different temperatures.

Effect of pH

Influence of plating time

I

2o

P~otmg time (s)

coatings.

The pH of the solution was varied electrometrically between 4 to 8 by the addition of diethanolamine and the effect is shown in Fig. 3. At pH 4.0, a black coating is obtained only above 60 A/ft 2 and is limited to 90 A/ft 2. At pH 8.0, black deposits were obtained in the current density range of 3-24 A/ft 2 whereas only at pH 6.0, a wide current density range is possible, i.e. between 12-90 A/ft 2, and hence, this value is chosen as the optimum pH.

i

Io

I

I

I

20

40

60

I

c D in A SF

Fig. 5. Effect of current density on optical properties (a, c).

SRINIVASAN et al.: NICKEL-BLACK COATINGS at 30°C. A t very low current density values, the

257

Effect of

coating appears as an interference pattern, whereas

acid

concenlrotion

on

op.co~ proPerties (a,*)

at high current density regions, streaks are noted. A t

16 A/ft 2, it looks mostly as a brown finish, whereas further increase in current density produces black coloured deposits. The emittance increases with current density whereas absorptance shows a different behaviour. It reaches a m a x i m u m value of ,~=0.87 at 36 A/ft 2 along with an emittance of 0.10 and starts decreasing with further increased current density value. Decreased plating time can ensure high absorptance and low emittance at the higher current density region, but the control of time less than 10 s is very difficult in practice and hence restricted only to a current density value of 36 A/ft. 2

lo o: ~ o a, E¢D ~ ~, o6

~

~

8= 04 o Z=o o o2 ~

~ I 5

o

I 1o %

Influence of acid concentration

of

h 15 HNO 3

Fig. 6. Effect of acid concentration on optical properties

In 'as plated' condition, the nickel black is only grey in colour and improved blackness can be obtained by immersion in dilute acids such as nitric, sulphuric and hydrochloric acid. However, treatment in hydrochloric and sulphuric acid sometimes results in patchy, streaky and non-uniform colour, whereas use of dilute nitric acid produces uniform black colour in a controllable m a n n e r , and hence, was further examined regarding its concentration in the range of 5-15% v/v. The results obtained are shown in Fig. 6. Lower t values are obtained only at 5% nitric acid solutions, whereas higher values are obtaiped both at concentrations above and below 5% v/v nitric acid. Further, at high acid concentration, the coating possesses grey to light brown colour shades. Moreover, the absorptance value also behaves in the same way and exhibits a m a x i m u m value at the same lower acid concentration,

(a, ~).

Effect of immersion time on

opt,co= properties(o,~)

4o ^ t7

os

-

~

t to

t 2o

= ~ ~ o6 ~ 04 ~5 o § oz ,¢ -

o

I ~o

I 40

I ~o

I 6o

Immersion time (s)

Effect of time of immersion Fig. 7. Effect of immersion time on optical properties (a, ~). Figure 7 shows the effect of immersion time in 5% v/v nitric acid on optical values f o r a duration of 5-50 s. Lower time of immersion, around 5-7 s, results in lower emittance values, whereas prolonged immersion results either in the complete stripping of the coating or in imparting a brownish shade. Absorptance values are slightly altered by an increase in acid immersion time up to 50 s. The acid immersion normally produces textural discontinuities of the order of solar wave lengths and promotes increased solar absorption through scattering and multiple reflection of the incident radiation causing the surface to appear as black[29]. Simultaneously, the surface appears optically flat to IR radiation, and low values of thermal emittance can be maintained,

Corrosion resistance Copper panels coated with nickel black provided with 10 v,m nickel undercoat with initial optical values of ot = 0.87 and ~ = 0.09 were subjected to 5% neutral salt spray test to assess its corrosion resistance. Change of optical values with exposure time is shown in Table 1. It is interesting to note that

Table 1. Effect of salt spray on opticalproperties (a, ~) Hours of salt spray Initial value a

0.87 0.09

8

16

24

32

40

48

56

64

72

80

88

100

0.87 0.87 0.88 0.88 0.89 0.89 0.9 0.9 0.91 0.92 0.93 0.94 0.09 0.09 0.1 0.1 0.11 0.12 0.13 0.13 0.13 0.14 0.15 0.15

258

SRINIVASAN et al.: NICKEL-BLACK COATINGS

absorptance is increased as well as emittance. This behaviour is quite contradictory with black nickel, where absorptance normally decreases, whereas emittance increases with time of exposure. Hence, nickel-black is preferred coating rather than black nickel, as it shows an increased absorptance value in a corrosive environment. The panel reached optical values o f a = 0.94 and ~ = 0.15 after a 100 h duration. Thermal cycling test The results obtained for thermal cycling after 7 days revealed an increase in absorptance values, namely from 0,87 from 0.91, and increase in emittance value too, namely from 0.09 to 0.12. The increased values may be associated with a possible build up in oxide layer thickness.

CONCLUSION Based on the above studies, the following electrolyte composition and conditions are recommended for obtaining selective 'nickel-black' coating:nickel chloride, 100 g/l; ammonium chloride, 25 g/l; ammonium thiocyanate, 10 g/l; pH, 6.0; current density, 36 A/ft2; plating time, 10 s; anode, nickel. The deposits obtained are further etched in 5% v/v nitric acid (HNO3 acid) for a duration of 5-7 s to achieve optical values of the order of absorptance ~ = 0.87 - 0.9 and emittance ~ = 0.09 - 0.12, The coating is stable to 300°C which is sufficient for application in flat-plate collectors. The corrosion resistance is better than black nickel coating. Based on the above solution a 10 litre electrolyte has been successfully operated. Acknowledgements--The authors wish to express their sincere thanks to Dr. K. S. Rajagopalan, Director, Central Electrochemical Research Institute for his kind permission to publish this paper.

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