PROCEDURES FOR OBTAINING BLACK CHROME SOLAR SELECTIVE COATINGS WITH REPRODUCIBLE RADIATION PROPERTIES

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PROCEDURES FOR OBTAINING BLACK CHROME SOLAR SELECTIVE COATINGS WITH

REPRODUCIBLE RADIATION PROPERTIES

C. Pisoni, C. Isetti, M. Battuello, T. Ricolfi

To cite this version:

C. Pisoni, C. Isetti, M. Battuello, T. Ricolfi. PROCEDURES FOR OBTAINING BLACK CHROME

SOLAR SELECTIVE COATINGS WITH REPRODUCIBLE RADIATION PROPERTIES. Journal

de Physique Colloques, 1981, 42 (C1), pp.C1-195-C1-201. �10.1051/jphyscol:1981114�. �jpa-00220664�

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JOURNAL DE PHYSIQUE

CoZZoque GI, suppt6ment au nO1, Tome 42, janvier 1981 page CI-195

PROCEDURES FOR O B T A I N I N G BLACK CHROME SOLAR S E L E C T I V E COATINGS W I T H REPRODUCIBLE R A D I A T I O N PROPERTIES

C . P i s o n i , C . I s e t t i , M. B a t t u e l l o (+) and T . R i c o l f i (+) I s t i t u t o d i F i s i c a Tecnica, ~ n i v e r s i

t s

d i Genova, I t a l y (+) CNR-Istituto d i Metrologia, "G. C o l o n n e t t i " , Torino, I t a l y

R6sumE.- On a effect& une recherche expgrimentale dans le but de trouver des proc6dEs standard pour obtenir des revgtements s6lec- tifs de chrome noir ayant des valeurs reproducible de l'absorptivite solaire as et de l'Emissivit6 thermique cth. Puisque les propriSt6s as et cth des revetements obtenus au moyen de traite- ments Blectrochimiques dependent soit de la nature des sous-couches, soit des ~aramstres regissant le traitement, on a essay&

differentes sous-couches,

a

savoir laiton poli, fer et alumi- nium nickel6s, tout en Etablissant des valeurs differentes pour la durEe du traitement et pour la densit6 de courant. On a pu constater que le r6le de cette dernisre n'est pas significatif dans des limites trSs larges,tandis aue l'on peut obtenir des groupes continus de valeurs as et cth en agissant seulement sur la durge du traitement. Les r6sultats prsliminaires montrent que les valeurs d'absorptivite et d16missivit6 peuvent &tre reprodui- tes entre 0,02 et 0,03 unites respectivement.

Abstract.- An experimental investigation was made with the aim of finding standard procedures for obtaining selective black chrome coatings with reproducible values of the solar absorptance as and thermal emittance ~ t h . AS the properties a and E ~ - , of elec- troplated coatings depend both on the nature o? the substrates and on the parameters which govern the plating process, different substrates, namely, polished brass and nickel plated aluminum and iron, were tried, while setting different values for the plating time and current density. It was seen that, while the latter does not play any significant role within wide limits, continuous sets of as and E~~ values can be obtained just acting on the plating time. Prelimnary results showed that the absorptance and emittance values can be reproduced within 0.02 and 0.03 units, respectively.

1. Introduction.- It is a well established fact that the efficiency of a solar collector is lowered when increasing the temperature of its absorbing surface. Since this is mainly due to radiation losses, the use of selective surfaces showing low thermal emittances is the best way to increase the efficiency, particularly when operating at relati- vely high temperatures.

In most cases, however, every decrease of the thermal emittance ~~h is obtained at the expense of the solar absorptance as, which is in turn

lowered to some extent. Thus, for selective surfaces of the same type, proper adjustments of the parameters which govern the manufacturing

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981114

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JOURNAL DE PHYSIQUE

p r o c e s s may l e a d t o d i f f e r e n t combinations o f as and sth v a l u e s , among which one should choose t h e most convenient ones i n view o f t h e maximum e f f i c i e n c y t h a t can be provided.

Though i t was popular i n t h e p a s t t o base t h e c h o i c e merely on t h e l a r - g e s t as over E~~ r a t i o t h a t could be o b t a i n e d , now i t i s widely reco- gnized t h a t t h e performance o f a s e l e c t i v e s u r f a c e cannot be c o r r e c t l y r a t e d i n terms of t h i s parameter alone. According t o more e l a b o r a t e d f i g u r e s of m e r i t , which have been proposed f o r comparing t h e e f f e c t i v e - n e s s of v a r i o u s a b s o r b e r s /1-3/, a lower i s sometimes p r e f e r a b l e , provided t h a t as i s s u f f i c i e n t l y high. A s an example, P e t t i t and Sowell / 4 / r e p o r t e d t h e c a s e of a focused s o l a r c o l l e c t o r f o r which an i n c r e a - s e i n as from 0.90 t o 0.95 achieved t h e same i n c r e a s e i n e f f i c i e n c y a s d e c r e a s i n g st-, from 0.30 t o O.lO(i.e., aS/sth = 0.95/0.30 = 3.2 provided t h e same e f f i c i e n c y a s = 0.90/0.10 = 9 ) .

This would s u g g e s t t h a t , i n p r i n c i p l e , a manufacturing p r o c e s s should be provided with a c e r t a i n degree of v e r s a t i l i t y i n o r d e r t o f i t t h e r e s u l t s t o every p a r t i c u l a r a p p l i c a t i o n . To do t h i s , a previous know- ledge o f t h e r e l a t i o n s between t h e r a d i a t i o n p r o p e r t i e s and t h e pro- c e s s v a r i a b l e s i s r e q u i r e d .

With t h e aim of f i n d i n g how a c c u r a t e l y r e l a t i o n s of t h i s k i n d could be e s t a b l i s h e d , a black chrome e l e c t r o p l a t i n g p r o c e s s was i n v e s t i g a t e d . The experiment was c a r r i e d on t a k i n g i n t o account t h a t f o r p r a c t i c a l purposes it i s r e l e v a n t t h a t :

-

t h e process v a r i a b l e s a r e s e t a t t h e i r more convenient v a l u e s : f o r example, a minimum c u r r e n t d e n s i t y i s p r e f e r a b l e ;

-

t h e r a d i a t i o n p r o p e r t i e s a r e r e p r o d u c i b l e f u n c t i o n s o f t h e process v a r i a b l e s : from t h i s p o i n t of view, t o p r e s u l t s ( i . e . , t h e maximum a o r t h e minimum E ~ a r e of l i t t l e v a l u e when o b t a i n e d randomly ~ ) ;

-

s d i f f e r e n t combinations of as and cth vafues a r e obtained by trimming only one process v a r i a b l e : h o p e f u l l y , t h i s should be one easy t o con- t r o l . Though n o t completed, o u r i n v e s t i g a t i o n showed t h a t d i f f e r e n t combinations of as and E~~ v a l u e s can be o b t a i n e d i n a r e p r o d u c i b l e f a - shion j u s t a c t i n g on t h e p l a t i n g time.

2. Experimental t e c h n i q u e s and p r e l i m i n a r y r e s u . . - Black chrome was c o a t e d on r e c t a n g u l a r p a n e l s ranging i n s i z e from 0.3 t o 3 dm 2

.

^The

s u b s t r a t e m a t e r i a l s were p o l i s h e d b r a s s and n i c k e l p l a t e d aluminum and i r o n . Both t h e n i c k e l p l a t i n g and t h e mechanical p o l i s h i n g of t h e s u r - f a c e s t o be c o a t e d were performed by commercial s u p p l i e r s . This c h o i c e obeyed t o o u r p o l i c y of producing r e s u l t s r e a d i l y t r a n s f e r a b l e t o a mass production. There i s l i t t l e doubt, i n f a c t , t h a t t h i s t r a n s f e r i s favoured by t h e economic advantage connected t o t h e a v a i l a b i l i t y of s u i t a b l e commercial equipment and procedures a t every s t e p of t h e pro- c e s s .

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The b a t h composition, which had been simply d e r i v e d from t h e l i t e r a t u - r e /5/, included 250 g / l of chromic a c i d (H2Cr04) and 0.5 g / l of f l u o - s i l i c i c a c i d (H2SiF6). I n an e a r l y s t a g e o f t h e work some samples were prepared w i t h t h e c o n c e n t r a t i o n of t h e f l u o s i l i c i c a c i d changed t o 0.25 g / l . Apart from some 30 % l o n g e r p l a t i n g times t h a t were r e q u i r e d t o o b t a i n t h e same a s a n d ~ t h v a l u e s a s b e f o r e , no o t h e r s u b s t a n t i a l d i f f e - r e n c e s were found. Perhaps a somewhat l a r g e r spread of t h e lowest E~~

v a l u e s convinced us t o c o n t i n u e with t h e previous c o n c e n t r a t i o n . The b a t h was o p e r a t e d a t a temperature o f 20 +- 2OC, which was c o n t r o l l e d with a water flow.

P l a t i n g c u r r e n t d e n s i t i e s ranging from 16 t o 32 ~ / d m ~ were t r i e d . I t was found t h a t between t h e s e l i m i t s t h e r e s u l t s were l a r g e l y indepen- d e n t o f t h e c u r r e n t d e n s i t y . This i s i n agreement with p r e v i o u s obser- v a t i o n s /6/, according t o wh4ch t h e c u r r e n t d e n s i t y i s n o t a c r i t i c a l v a r i a b l e of t h e p r o c e s s . Below 1 6 A/dm 2 no s a t i s f a c t o r y c o a t i n g s could be o b t a i n e d , even f o r long p l a t i n g times, s o t h a t t h i s v a l u e appeared t o be a t h r e s h o l d f o r an e f f i c i e n t c o a t i n g p r o c e s s .

The r a d i a t i o n p r o p e r t i e s as andcth were i n f e r r e d from r e f l e c t a n c e d a t a measured u s i n g two d i f f e r e n t techniques. Between 0.35 and 2 pm an i n t e - g r a t i n g sphere r e f l e c t o m e t e r was used f o r measuring s p e c t r a l r e f l e c t a n - c e s from which as was d e r i v e d by i n t e g r a t i o n o v e r t h e s o l a r s p e c t r a l d i s t r i b u t i o n o f Moon /7/ f o r a i r mass 2. A s an a l t e r n a t i v e way, t h e same r e f l e c t o m e t e r could be used t o g i v e d i r e c t l y t h e i n t e g r a t e d r e f l e c - t a n c e /8/. I n both t h e o p e r a t i n g modes, which were a c t u a l l y used d u r i n g t h e i n v e s t i g a t i o n , t h e measured r e f l e c t a n c e s were r e f e r e n c e d t o a sam- p l e made of p r e s s e d BaS04 powder. A heated c a v i t y r e f l e c t o m e t e r provided f o r s p e c t r a l r e f l e c t a n c e s i n t h e wavelength i n t e r v a l from 2 t o 1 5 p . The i n s t r u m e n t had been b u i l t a c c o r d i n g t o t h e p r i n c i p l e f i r s t sugges- t e d by Gier e t a l . /9/. I t was equipped w i t h a heat-pipe blackbody which ~ r e a t l y reduced t h e thermal g r a d i e n t s which a r e known t o be one of t h e major drawbacks of t h e method. Although t h e measurements were made a t a sample temperature of about 40°C, t h e thermal e m i t t a n c e was r e f e r e n c e d t o t h e temperatures of i n t e r e s t by i n t e g r a t i n g t h e s p e c t r a l r e f l e c t a n c e d a t a over t h e a p p r o p r i a t e blackbody d i s t r i b u t i o n s .

A t t h e beginning of o u r i n v e s t i g a t i o n a p r e l i m i n a r y set o f c o a t i n g s we- r e made on both b r a s s and n i c k e l p l a t e d aluminum and i r o n . The r e s u l t s showed t h a t b r a s s was more s u i t a b l e f o r t h e subsequent r e f i n i n g s o f t h e p r o c e s s , because of t h e b e t t e r c h a r a c t e r i s t i c s of t h e c o a t i n g s . Thus, t h e work was f i r s t continued w i t h t h e b r a s s a l o n e , though it was l a t e r found t h a t t h e lower performance of t h e n i c k e l p l a t e d m e t a l s was mere- l y due t o a poor q u a l i t y of t h e n i c k e l p l a t i n g i t s e l f .

Some 40 black chrome-on-brass samples were prepared a t d i f f e r e n t current

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d e n s i t i e s and p l a t i n g t i m e s and examinated f o r t h e i r s o l a r a b s o r p t a n c e and t h e r m a l e m i t t a n c e . A s p r e v i o u s l y n o t e d , t h e c u r r e n t d e n s i t y d i d n o t a p p e a r t o p l a y any s i g n i f i c a n t r o l e , a t l e a s t f o r v a l u e s e x c e e d i n g 1 6 A/dm 2

.

T h i s was shown by p l a t i n g some samples w i t h d i f f e r e n t c u r r e n t d e n s i t i e s , w h i l e k e e p i n g c o n s t a n t t h e p r o d u c t o f t h e c u r r e n t d e n s i t y i and of t h e p l a t i n g t i m e T . The s p r e a d of t h e as and E~~ v a l u e s was j u s t o f t h e same o r d e r a s f o r samples o b t a i n e d w i t h t h e same c u r r e n t d e n s i t y . For t h i s r e a s o n , t h e r e s u l t s w e r e grouped w i t h o u t any d i s t i n c t i o n a s t o t h e c u r r e n t d e n s i t y . They a r e summarized i n f i g u r e 1, where as and E~~

a r e p l o t t e d a s f u n c t i o n s o f t h e p r o d u c t i x T .

F i g . 1.- S o l a r a b s o r p t a n c e and t h e r m a l e m i t t a n c e o f b l a c k chrome coa- t i n g s on p o l i s h e d b r a s s .

Three c u r v e s f o r E~~ have been r e p o r t e d , a c c o r d i n g t o t h e b l a c k b o d y d i s - t r i b u t i o n s a t 50°CI 100°C and 200°C. They c l e a r l y show t h a t t h e tempe- r a t u r e dependence o f t h e e m i t t a n c e i s r a t h e r s t r o n g , as u s u a l f o r s e l e c c t i v e s u r f a c e s . The r e a s o n f o r t h i s l i e s on t h e marked v a r i a t i o n w i t h wavelength o f t h e s p e c t r a l r e f l e c t a n c e i n t h e t r a n s i t i o n r e g i o n from low t o h i g h v a l u e s , a s shown i n f i g u r e 2 , where t h e r e f l e c t a n c e c u r v e s f o r some o f t h e t e s t e d samples have been drawn. S i n c e t h e e m i t t a n c e i n c r e a s e from 50 t o 200°C i s r o u g h l y 50 %, it i s e v i d e n t t h a t i t must be t a k e n i n t o a c c o u n t f o r a p r o p e r e v a l u a t i o n o f t h e c o a t i n g performan- c e a t any o p e r a t i n g t e m p e r a t u r e .

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0

5 1 15 5 7 0 TT,

I

WAVELENGTH (!11n)

F i g , 2.- l n f r a r e d s p e c t r a l r e f l e c t a n c e s f o r some o f t h e t e s t e d samples, The smoothness of t h e curves of f i g u r e 1 would s u g q e s t a good reprodu- c i b i l i t y o f the r e s u l t s . As a m a t t e r o f f a c t , i t was found t h a t t h e as v a l u e s were r e p r o d u c i b l e Within l e s s than 0-02 u n i t s , w h i l e t h e E~~ va- l u e s , a t l e a s t i n t h e r e g i o n of p r a c t i c a l i n t e r e s t , which may be consi- dered t o l a y below 50 A'min /dm 2

,

were spread w i t h i n 0.03 U n i t s .

No such complete r e s u l t s a r e p r e s e n t l y a v a i l a b l e f o r t h e n i c k e l p l a t e d aluminum and i r o n s u b s t r a t e s . A p r e l i m i n a r y s e r i e s o f samples o b t a i n e d w i t h e l e c t r o p l a t e d n i c k e l was r e j e c t e d because t h e black chrome coatings showed poor adherence t o t h e s u b s t r a t e s and t h e e m i t t a n c e s were t o o l a r - ge. B e t t e r r e s u l t s came when t h e n i c k e l p l a t i n g was performed with an e l e c t r o l e s s chemical conversion process. Even i n t h i s c d s e , however, it h a s n o t y e t been p o s s i b l e t o f i n d a smooth v a r i a t i o n o f t h e r a d i a t i o n p r o p e r t i e s a s functkons of t h e product i x T. The b e s t r e s u l t s c o n s i s - t e d of absorptance and emittance ( a t 50°C) v a l u e s of 0 , 9 4 and 0.12, r e s - p e c t i v e l y , and w e r e o b t a i n e d a t i x T = 35 A * m h /dm 2

.

Below t h l s l i m i t , us suddenly decreased t o v a l u e s lower than 0.90, w h i l e nothing was gained on ^{E t h t} which remained t h e same a s above, The a v a i l a b l e d a t a we- r e i n s u f f i c i e n t t o g i v e any s a t i s f a c t o r y e x p l a n a t i o n f o r t h i s behaviour, which d i f f e r s from t h a t found w i t h t h e b r a s s s u b s t r a t e s . I n any c a s e ,

from a p r a c t i c a l p o i n t of view, t h e problem does n o t appear s o r e l e v a n t ,

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JOURNAL DE PHYSIQUE

t h e r e b e i n g no u s e i n l o w e r i n g a w i t h o u t any s i m u l t a n e o u s d e c r e a s e i n s

€ t h -

I t seem worth, i n s t e a d , t o p o i n t o u t t h a t t h e s e p r e l i m i n a r y r e s u l t s have shown how t h e n a t u r e of t h e s u b s t r a t e may i n f l u e n c e t h e c h a r a c t e r i s - t i c s of t h e c o a t i n g s . Under t h i s p o i n t o f view, t h e v e r s a t i l i t y o f a manufacturing p r o c e s s s h o u l d p r o v i d e n o t o n l y f o r d i f f e r e n t a s and ^E

t h combinations a s r e f e r r e d t o t h e same s u b s t r a t e , b u t a l s o f o r t h e b e s t c o n d i t i o n s which may be o b t a i n e d w i t h d i f f e r e n t s u b s t r a t e s .

3 . Conclusion.- Our i n v e s t i g a t i o n i n d i c a t e d t h a t t h e p r o c e s s under t e s t i s s u i t a b l e f o r o b t a i n i n g b l a c k chrome c o a t i n g s w i t h good and reprodu- c i b l e r a d i a t i o n p r o p e r t i e s . The r e p r o d u c i b i l i t y was a p a r t i c u l a r p o i n t of i n t e r e s t i n view o f t h e u t i l i z a t i o n of s u b s t r a t e s which had been t r e a t e d f o r c o a t i n g and p o l i s h i n g on a commercial b a s i s .

One f u r t h e r i m p o r t a n t f e a t u r e , which i s going t o be e v a l u a t e d , r e f e r s t o t h e envinronmental s t a b i l i t y o f t h e c o a t i n g s , which i n c l u d e s b o t h c o r r o s i o n and t h e r m a l d e g r a d a t i o n r e s i s t a n c e . I n f a c t , w h i l e it i s ge- n e r a l l y r e c o g n i z e d t h a t b l a c k chrome i s one o f t h e most s t a b l e s o l a r s e l e c t i v e c o a t i n g s , even a t t e m p e r a t u r e s up t o 300-400°C, n e v e r t h e l e s s r e c e n t works confirmed t h a t i t s r e s i s t a n c e t o c o r r o s i o n depends on t h e n a t u r e o f t h e s u b s t r a t e

/ l o / ,

w h i l e i t s t h e r m a l a g e i n g behaviouk i s l a r g e l y determined by t h e b a t h composition /11/. Thus, t h e r e s u l t s ob- t a i n e d i n a g i v e n i n s t a n c e c a n n o t be r e a d i l y a p p l i e d t o any somewhat d i f f e r e n t p r o c e s s .

T h i s work h a s been s u p p o r t e d by t h e F i n a l i z e d Energy Programme of t h e N a t i o n a l Research C o u n c i l (CNR)

.

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References

Edwards, D. K., Gier, J.T., Nelson, K.E. and Roddick, R.D., Sol.Energy

5

( 1 9 6 2 ) 1 - 8

Edwards,D.K., Solar Collector Design (Franklin Inst. Press, Philadelphia) 1977, pp. 3 1 - 3 2 Collier, R.K., Sol.Energy

23

( 1 9 7 9 ) 4 5 5 - 4 5 8

Pettit, R.B. and Sowell, R.R., J.Vac. Sci .Technol.

1 3 ( 1 9 7 6 ) 5 9 6 - 6 0 2

-

Shenoi,B.A. and Gowri,S., Met.Finish.

12

( 1 9 6 4 ) 66-68

Borzoni,J.T., Coatings for Solar Collectors Symposium, American Electroplater's Society, Atlanta ( 1 9 7 6 ) pp. 8 9 - 1 0 0

Moon, P., J. Franklin Inst.

230

( 1 9 4 0 ) 5 8 3 - 6 1 7 Ricolfi, T. and Battuello, M.

,

On the use of inte- grating sphere reflectometers for measuring the solar absorptance of selective surfaces. Paper presented at this Conference

Gier ,J.T., Dunkle, R.V. and Bevans, J .T., J.Opt

.

Soc.Arn.

9

( 1 9 5 4 ) 5 5 8 - 5 6 2

Di Bari. G.A. and Turillon, P.P., Proc. of ISES

Silver Jubilee Congress, Atlanta ( 1 9 7 9 ) pp. 1 9 1 7 - 1 9 2 1 Pettit, R.B. and Sowell, R.R., id., pp. 1 8 9 7 - 1 9 0 1