A SURVEY OF SELECTIVE SOLAR ABSORBERS AND THEIR LIMITATIONS

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AND THEIR LIMITATIONS

D. Mattox, R. Sowell

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque Cl, suppZ6ment au nO1, Tome 42, janvier 1981 page Cl-19

A SURVEY O F S E L E C T I V E SOLAR ABSORBERS AND T H E I R L I M I T A T I O N S

("1

D.M. Mattox and R.R. Sowell

S a n d i a N a t i o n a l L a b o r a t o r i e s (M), A l b u q u e r q u e , New Mexico 87185, V. S.A.

A b s t r a c t . - A number of m a t e r i a l s and techniques have been used t o form s e l e c t i v e s o l a r absorber c o a t i n g s . Each of t h e c o a t i n g pro- c e s s e s and t h e c o a t i n g s formed has i t s own l i m i t a t i o n s which a r e g e n e r a l l y p a o r l y defined. The poor s t a t e of knowledge on t h e l i m i - t a t i o n s of t h e c o a t i n g p r o c e s s e s and t h e r e s u l t i n g c o a t i n g s poses a problem f o r t h e component design and t h e m a t e r i a l s engineer. This paper reviews t h e s t a t e of knowledge of 9 o f t h e c o a t i n g p r o c e s s e s and m a t e r i a l s being used and of p o t e n t i a l u s e f u l n e s s i n mid-temperature s o l a r - t h e r m a l c o l l e c t o r s . A p r i n c i p l e p o i n t t h a t i s made i s t h e l a c k of production experience and knowledge of t h e d e g r a d a t i o n modes f o r most of t h e c o a t i n g t e c h n o l o g i e s and coa- t i n g m a t e r i a l .

1. 1nZrroduction.- S o l a r r a d i a t i o n i s a low i n t e n s i t y energy s o u r c e whose u t i l i z a t i o n r e q u i r e s e f f e c t i v e c o l l e c t i o n and r e t e n t i o n t o b e e c o - nomically f e a s i b l e . O p e r a t i o n a l temperature ranges of s o l a r c o l l e c t o r systems may be c a t e g o r i z e d a s : low-temperature ( T < 100°C), mid-tem- p e r a t u r e (100 < T < 400°C), and high-temverature (T > 400°C)

.

To o b t a i n mid and high o p e r a t i o n a l temperatures, some degree of c o n c e n t r a t i o n of t h e i n c i d e n t r a d i a t i o n i s r e q u i r e d /l/. I n t h e mid-temperature range, t h e c o n c e n t r a t i o n ( r a t i o of a p e r t u r e a r e a t o r e c e i v e r a r e a ) w i l l gene- r a l l y be from 5 t o 50 w h i l e i n t h e high-temperature c o l l e c t o r s t h e con- c e n t r a t i o n may exceed 1000.

C o l l e c t o r d e s i g n s u t i l i z e a r e c e i v e r s u r f a c e t o absorb t h e i n c i - d e n t s o l a r r a d i a t i o n /l/. I n t h e " c o a t i n g r e c e i v e r 1 ' , t h e r a d i a t i o n i s absorbed i n a c o a t i n g o r s u r f a c e l a y e r and t h e h e a t i s conducted through t h e c o n t a i n e r w a l l t o t h e h e a t t r a n s f e r media.

I n t h e " c o n t a i n e r r e c e i v e r " , t h e r a d i a t i o n i s absorbed w i t h i n t h e con- t a i n e r w a l l s while t h e " f l u i d r e c e i v e r " , t h e r a d i a t i o n i s absorbed i n

(U) This work i s supported by t h e U . S . - Department of Energy under Con- t r a c t DE-AC04-76-DP00789.

( W ) A U.S. DOE f a c i l i t y .

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the heat transfer fluid. In each of these designs, the optical properties of the construction materials must be optimized for maximum absorption efficiency and minimum radiation losses. The most common design is that of the coating receiver but the fluid receiver design is of inte- rest for low-temperature collectors where radiation losses are minimal. In each of these collector systems, a stagnation temperature, which is much higher than the operational temperature, map be attained when the heat transfer fluid is not flowing.

In most recekver designs, the radiatfon absorbing surface &S pro- tected by a transparent cover (glazing) to reduce convective heat losses. In the most advanced designs, this cover provides a means to con- trol the environment to which the absorber surface is exposed. In less advanced designs, the glazing may serve to trap undesirable elements such as moisture or outgassing products and create a harsh environment for the absorber surface /2/. The environments to which absorber sur- 5aces may be subjected vary greatly but include : 1) greater than operational temperatures, 2) thermal shock and fatigue, 3) corrosive and oxidizing agents, 4) ultraviolet radiation, 5) moisture, 6) preassembly and assembly contamination and damageand7) particle abrasion and ac- cumulation (unprotected surfaces). Economics will often determine to what extent the environment will be controlled.

In any collector design, an effort is made to maximize the collector efficiency by minimizing reflectance losses from reflectors and glazing, minimizing absorption losses in the glazing and to maximize the absorption of solar radiation at the absorber surface. For low temperature applications, the thermal radiation losses may not be important because of the low operational temperature while for high temperature applications radiation may not be important because 0.fthe low receiver area.

In

the mid-temperature range, thermal radiation losses may be appreciable though for some "cavity receiver" designs much of the emitted thermal radiation may be trapped and absorbed. For applications where thermal losses need to be minimized, an absorber surface which has a high solar absorptance (as) and a low thermal emittance at the operational temperature ( E ~ ) is desirable. Such a material is cal- led a selective absorber /1,3,4,5/. Typically, the designer will strive for an as = 0.95 and an ET as low as possible for the high as (typically E 3 0 0 ° ~ < 0.2).

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Fig. 1.- S o l a r spectrum e x t e r n a l t o t h e e a r t h ' s atmosphere (AMO) and a f t e r p a s s i n g through two s t a n d a r d a i r masses (AM2). Black body r a d i a - t i o n s p e c t r a f o r s e v e r a l temperatures and a t a b l e o f r a d i a n t energy contained i n each spectrum.

The human eye i s o n l y s e n s i t i v e t o about 4 0 % o f t h e energy c o n t a i n e d i n t h e s o l a r spectrum. F i g u r e 1 a l s o shows t h e r a d i a t i o n s p e c t r a from black b o d i e s a t s e l e c t e d temperatures. The t a b l e w i t h t h e f i g u r e i n d i c a t e s t h e amount of energy contained i n each o f t h e r a d i a t i o n s p e c t r a . The t o t a l r a d i a t i o n e m i t t e d from a s u r f a c e i s p r o p o r t i o n a l t o t h e s u r f a c e a r e a , t h e e m i t t a n c e and t h e s u r f a c e temperature t o t h e f o u r t h power.

I n o r d e r f o r a s o l a r c o l l e c t o r t o have a high e f f i c i e n c y and main- t a i n a high e f f i c i e n c y o v e r i t s o p e r a t i o n a l l i f e , it i s necessary t h a t t h e s t r u c t u r e and d e g r a d a t i o n modes of t h e v a r i o u s components be understood. The s e l e c t i v e s o l a r absorber s u r f a c e w i t h i t s e x a c t i n g o p t i c a l p r o p e r t i e s i s one of t h e a r e a s most s e n s i t i v e t o degradation. Unfortu- n a t e l y , t h e degradat5on mechanisms o f t h e s e s u r f a c e s i s poorly understood.

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p e r i e n c e a t t h e p r e s e n t time f o r many a p p l i c a t i o n s .

ANIIREFLTCIION SURFACE MORPHOLOGY

LOW THERMAL EMIlTANCE coRRo.0. R E v s m c k

F i g . 2.- Typical s e l e c t i v e absorber c o a t i n g c o n t a i n i n g and absorbing l a y e r on a low e m i t t a n c e s u b s t r a t e w i t h a r a d i a t i o n t r a p p i n g s u r f a c e .

F i g u r e 2 shows one type of s e l e c t i v e s o l a r absorbing s u r f a c e which i s g e n e r a l l y formed by some c o a t i n g o r r e a c t i o n technique /7/. The ab- s o r b e r c o n s i s t s o f a n a n t i r e f l e c t i n g s u r f a c e which may be produced by having a low index of r e f r a c t i o n m a t e r i a l i n c o n t a c t with t h e gas o r vacuum environment o r by having a s u r f a c e morphology which g i v e s r a - d i a t i o n t r a p p i n g . The t r a n s m i t t e d r a d i a t i o n i s absorbed i n a l a y e r having a high e x t i n c t i o n c o e f f i c i e n t f o r t h e s o l a r r a d i a t i o n spectrum and which h a s a low volume e m i t t a n c e f o r thermal r a d i a t i o n . This absorbing

l a y e r i s formed on a l a y e r which h a s a low thermal e m i t t a n c e and p r o v i - des c o r r o s i o n p r o t e c t i o n f o r t h e u n d e r l y i n g s t r u c t u r e where necessary. This s e l e c t i v e absorber c o a t i n g c o n f i g u r a t i o n should have a high t h e r - mal c o n d u c t i v i t y t o keep t h e c o a t i n g s u r f a c e temperatures a s low a s p o s s i b l e .

The low thermal e m i t t a n c e u n d e r l a y e r s u r f a c e i s g e n e r a l l y a metal such a s N i , MO, Cu o r A 1 which has a thermal e m i t t a n c e of about 0 . 1 a t 300°C /8/. These metal c o a t i n g s m a y be f a b r i c a t e d by a number of tech- n i q u e s such as e l e c t r o p l a t f n g , s p u t t e r d e p o s i t i o n , vacuum e v a p o r a t i o n , o r may be t h e s t r u c t u r a l m a t e r i a l of t h e r e c e i v e r . For non-corrosive environments, t h i s u n d e r l a y e r mav be v e r y t h i n b u t where t h e u n d e r l a y e r must provide c o r r o s i o n p r o t e c t i o n f o r t h e s t r u c t u r e , t h e l a y e r must have an a p p r e c i a b l e t h i c k n e s s which w i l l depend on t h e s e r v i c e condi- t i o n s . For example, e l e c t r o p l a t e d n i c k e l on mild s t e e l should be grea- t e r t h a n 0.00127 cm (0.5 m i l ) when used i n a n air/moristure environment i n o r d e r t o provide c o r r o s i o n p r o t e c t i o n i n our t e s t c o n d i t i o n s .

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Semiconductor a b s o r b e r s which have been f a b r i c a t e d i n c l u d e s i l i - con, germanium, and l e a d s u l f i d e . Since t h e s e semiconductors a l l have high r e f r a c t i v e i n d i c e s , a n t i r e f l e c t i o n s u r f a c e s must be used t o p r e v e n t e x c e s s i v e r e f l e c t a n c e l o s s e s . Amorphous s i l i c o n prepared by plasma de- composition of s i l a n e has good a b s o r p t i o n c h a r a c t e r i s t i c s , though when hydrogen i s i n c o r p o r a t e d i n t o t h e d e p o s i t t h e a b s o r p t i o n edge i s s h i f - t e d f u r t h e r from t h e near i n f r a r e d /g/. Chemically vapour d e p o s i t e d

(CVD) s i l i c o n h a s good a b s o r p t i o n p r o p e r t i e s b u t a t temperatures above 500°C, t h e a - s i l i c o n s t r u c t u r e

i s

u n s t a b l e and t h e o p t i c a l p r o p e r t i e s

w i l l

change. The a d d i t i o n of carbon t o t h e CVD-Si s t a b i l i z e s t h e s t r u - c t u r e t o > 700°c /10/. Evaporated germanium i s a good s e l e c t i v e absor- b e r /11/ b u t l i t t l e h a s been r e p o r t e d on i t s thermal and chemical s t a - b i l i t y . Evaporated l e a d s u l f i d e i s a l s o a good s e l e c t i v e absorber /11, 1 2 / b u t c o n v e r t s t o t h e s u l f a t e when exposed t o oxygen i n t h e p r e s e n c e of u l t r a v i o l e t r a d i a t i o n /11/.

Layered s t r u c t u r e s may be f a b r i c a t e d by e v a p o r a t i o n , s p u t t e r de- p o s i t i o n o r e l e c t r o p l a t i n g . One of t h e most s u c c e s s f u l such c o a t i n g s i s t h e A1203-MO-A1203 (MA) i n t e r f e r e n c e t y p e s e l e c t i v e a b s o r b e r coa- t i n g /13, 1 4 , 15/. The topmost A 1 2 0 3 c o a t i n g ( 8 0 m) a c t s a s an a n t i - r e f l e c t i o n c o a t i n g w h i l e t h e MO ( o r Moox) (10

nm)

and t h e underlying

A 1 2 0 3 (30 nm) a c t a s t h e absorber. This l a y e r e d s t r u c k u r e d e p o s i t e d on an MO underlayer h a s y f e l d e d an as = 0.95 and €(20°C) = 0.07. This t y p e of s t r u c t u r e may degrade by o x i d a t i o n o f t h e MO a b s o r p t i o n l a y e r o r by i n t e r d i f f u s i o n of t h e l a y e r s .

Cermet l a y e r s have been t h e most widely s t u d i e d group of absorp- t i o n l a y e r s * Cermets may be f a b r i c a t e d by e l e c t r o p l a t i n g , anadizing- impregnating, i n e r t o r r e a c t i v e s p u t t e r d e p o s i t i a n arid vacuum evagara- t i o n . E l e c t r o p l a t e d "black chrome" i s

a

widely used s e l e c t i v e absorber f o r t h e lower mid-temperature ( < 300"cf range /16, 1 7 , 1 8 , 19/, This c o a t i n g

is

comprised o f C r parePclles i n

a

Cr,O, ntatrin i n

a

ztather open

s t r u c t u r e d c o a t i n g /16, 1 7 / . The chromium c o n t e a t i n c r e a s e s toward t h e s u b s t r a t e and t h e p r i n c i p l e d e g r a d a t i o n m6des seem t o be i n t e r n a l obi- d a t i a n and/or d i f f u s i o n from t h e e l e c t r o p l a t e d underlav, The b l a c k chrome absorber

i s

e a s i l y prepared b u t s u f f e r s thermal i n s t a b i l i t y above

250'6 w i t h o u t c o n t r o l of t h e 601m~6sition of t h e crlectrOplating b a t h /20, 21/. Table I shows t e s t r e s u l t s

f o r

samples prepared with d i f f e r i n g b a t h compositions /21/, By. u s i n g a low dclrrcentration of 6xt3 i n t h e b a t & a c o a t i n g which

is

s t a b l e t o 300QC i n a i r

can

be produaed. Far t h e low

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Table I.- Thermal Aging of E l e c t r o d e p o s i t e d Black Chrome /20/ P l a t i n g T e a t Time a C o n d i t i o n s T e s t Temp.

-

( h r )

-

S

-

~ ( 1 0 0 ~ ~ c ( 3 0 0 ~ ~ ) 1 6 g/; ~ r +As ~P l a t e d 0 0.95 0.09 0.15 3.5 m i n . 250°c 64 0.93 0.08 0.14 350°c 64 0.89 0.07 0.12 1 2 g/L AS p l a t e d 0 0.97 0.21 0.31 3.5 m i n . 350°c 1 5 7 0.97 0.17 0.23 1849 0.96 0.17 0.22 3908 0.95 0.16 0.22

I

REFECTANCE Of THERMALLY UNSTABLE BUCK CHROME ELECTRODEWSIT

I

"1

-

AS DEWSITED

---

AnER MAT TREATMEN TO 4m"C FOR 227 HRS.

WAVELENGTH (nml

Fig. 3 . - Typical r e f l e c t a n c e spectrum f o r thermally u n s t a b l e e l e c t r o d e - p o s i t e d black chrome showing i n c r e a s e r e f l e c t a n c e (decreased a b s o r p t i o n ) i n t h e 1000-1500 nrn range on thermal degradation.

The more s t a b l e c o a t i n g s ( 8 g / l ~ r + ~ ) seem t o be comprised of l a r g e r

+

3 p a r t i c l e s than t h e less s t a b l e c o a t i n g s (16 q / l C r 1 .

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Table I1 summarizes a number o f s e l e c t e d systems a s t o t h e f a b r i - c a t i o n t e c h n i q u e and t h e d e g r a d a t i o n t e m p e r a t u r e and mode where known. T a b l e 11.- S e l e c t e d S e l e c t i v e S o l a r Absorbers Material Coating/Substrate Fabrication Process Sputtering Sputtering Sputtering Sputtering Sputtering Sputtering Sputtering Electrodep. Electrodep./ Thermal Operati nq Temp. Air lUc) Vac (uC)

Degradation Mode References 22.23 24 26 27-29 31.32 Diffusion 13,14 33 Ox/Diffusion 16-21,34,35, 43.48.49 Morp. Change 36.37 A R / a s i ( t ~ ) / ~ o O ~ CVD 0.91/-

---

>700 Crystal 10 PbS/At Evaporation 0.93/0.21 >l00 >l20 Oxidation (W) 11.12

AR/Molybdenum CVD 0.82/0.08(20)

---

39

Co30,/Ag/SS (Ox) Spray Pyro. 0.91/0.20 (20) >650

---

50

NI-Ae203/AL Anodization 0.95/0.15(20)

---

51 SS

-

Stainless Steel AR

-

Antireflection Coating Ox

-

Oxidation A n t i r e f l e c t i o n c o a t i n g s may be d e l i b e r a t e l y d e p o s i t e d u s i n g mate- r i a l s w i t h low i n d i c e s o f r e f r a c t i o n and s ~ e c i f i c t h i c k n e s s e s . A n t i r e - f l e c t i o n or r a d i a t i o n t r a p p i n g s u r f a c e s may b e i n t h e form of : 1) den- d r i t e s , 2 ) c a v i t i e s , 3) powders, o r 4) e t c h f e a t u r e s . The s p a t i a l d i - mentions of t h e f e a t u r e s must be on t h e o r d e r o f t h e wavelength o f t h e s o l a r r a d i a t i o n ( 5 0 0 nm). The t r a p p i n g morphology may b e formed i n a number of ways i n c l u d i n g : 1) i n h e r e n t growth s t r u c t u r e s , 2) chemical e t c h i n g , 3) s p u t t e r e t c h i n g , o r 4 ) d e p o s i t i o n o f f i n e p a r t i c l e s .

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produce h i g h l y absorbing s u r f a c e s o f m e t a l s / 2 3 / and germanium /11/.

The porous n a t u r e of t h e s e d e p o s i t s g i v e s high s u r f a c e temperatures and may r e s u l t i n s i n t e r i n g of t h e c o a t i n g s o r i n c r e a s e d thermal l o s s e s through r a d i a t i o n .

3 . Degradation modes.

-

Generally, t h e d e g r a d a t i o n modes f o r t h e various c o a t i n g s a r e poorly understood and a r e dependent on t h e t e s t environment and c o n d i t i o n s . Since g e n e r a l l y very s m a l l amounts of m a t e r i a l a r e involved i n any changes, i t i s g e n e r a l l y d i f f i c u l t t o unambiguously s p e c i f y t h e degradation mode. Degradation can be promoted by thermal p r o c e s s e s ( i n c l u d i n g d i f f u s i o n ) and chemical processes. Both p r o c e s s e s a r e temperature, time, environment and m a t e r i a l dependent.

The most obvious d e g r a d a t i o n mode i s l o s s of adhesion of t h e coa- t i n g g e n e r a l l y due t o poor i n i t i a l adhesion o r i n t e r f a c i a l c o r r o s i o n o r r e a c t i o n . Pinhole c o r r o s i o n l e a d s t o d e g r a d a t i o n of t h e c o a t i n g s u r - rounding t h e p i n h o l e region.

The o p t i c a l p r o p e r t i e s of t h e c o a t i n g s g e n e r a l l y degrade due t o l o s s of absorptance and n o t t o an i n c r e a s e i n thermal e m i t t a n c e . Indeed t h e r e o f t e n seems t o be a d e c r e a s e i n e m i t t a n c e w i t h a d e c r e a s e i n absorptance. The l o s s of absorptance mav be caused by changes i n :

-

s u r f a c e morphology

-

chemical o r phase composition

-

phase d i s t r i b u t i o n o r s i z e

For e l e v a t e d temperatures, d i f f u s i o n from t h e underlay c o a t i n g o f t e n seems t o be a problem. Most e l e c t r o p l a t i n g b a t h s have a number of o r g a n i c and i n o r g a n i c a d d i t i v e s which become incorworated i n t o t h e de- p o s i t and a r e subsequently a b l e t o d i f f u s e i n t o t h e c o a t i n g a l o n g w i t h t h e metal. This d i f f u s i o n w i l l be promoted by an "open s t r u c t u r e " of t h e absorbing c o a t i n g which allows s u r f a c e o r g r a t n boundary d i f f u s i o n .

Chemical r e a c t i o n r a t e s w i l l a l s o be i n c r e a s e d by having an "open s t r u c t u r e " t o t h e absorbing c o a t i n g . An o x i d i z i n g environment w i l l allow t h e conversion of m e t a l l i c phases t o oxides t h u s reducing t h e absorp- t a n c e of cermet o r l a y e r type a b s o r b e r s . The presence of an o x i d i n g environment enhances d i f f u s i o n of o x i d i z a b l e m e t a l s from t h e i n t e r f a c e t o t h e s u r f a c e / 4 4 / . The presence of water i n t h e environment probably i n - c r e a s e s t h e r e a c t i o n r a t e .

The most s t a b l e system f o r o x i d i z i n g environments seem t o be ones which use r e f r a c t o r y metal u n d e r l a y e r s (MO) w i t h o x i a a t i o n r e s i s t a n t

( P t , Au) metal p a r t i c l e s i n an oxide m a t r i x /26/. I t h a s been suggested t h a t a d i f f u s i o n b a r r i e r on t h e metal u n d e r l a y e r may provide i n c r e a s e d s t a b i l i t y /14/.

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f o r

a

mid-temperature absorber s u r f a c e i n a l i n e c o n c e n t r a t o r configura- t i o n . I n i t i a l development and t e s t c o n f i g u r a t i o n s were coated i n t h e l a b o r a t o r y b u t production has been done a t s e v e r a l commercial e l e c t r o - p l a t i n g firms. The d e s i r e d c o a t i n g s should have a s o l a r a absorptance of 0.95 and a n e m i t t a n c e o f c 0.20 a t t h e o p e r a t i n g temperature (%320v) The c o a t i n g s a r e n o t s u b j e c t e d t o h i g h e r s t a g n a t i o n temperatures s i n c e f l u i d t r a n s f e r f a i l u r e r e s u l t s i n t u r n i n g t h e c o n c e n t r a t o r through t o t h e s t o r e p o s i t i o n . Recent r e s u l t s with t h i s c o n c e n t r a t o r u s i n g s i l v e - red, chemically s t r e n g t h e n e d g l a s s r e g l e c t o r s and t h e improved black chrome absorber c o a t i n g h a s given a 60% c o l l e c t i o n e f f i c i e n c y a t 320°C.

I n t h e l a b o r a t o r y , a number of important d e p o s i t i o n v a r i a b l e s have been i d e n t i f i e d a s t o performance and s t a b i l i t y o f t h e black chrome c o a t i n g . The i n i t i a l b a t h which was used was t h e Harshaw chromonyxR f o r - mulation developed f o r d e c o r a t i v e black c o a t i n g s . I t was found t h a t c a r e f u l c o n t r o l of t h e t o t a l charge passed y i e l d e d an optimum s o l a r ab-

s o r p t a n c e with a minimal e m i t t a n c e /16/. The c u r r e n t d e n s i t y f o r a good 2

c o a t i n g i s i n t h e range of 150-200 A/ft a t 18-28OC f o r 4-1/2 t o 6 m i -

0

n u t e s g i v i n g a c o a t i n g t h i c k n e s s of about 2000 A a t an anode-to-catho- de r a t i o of 2-3/1. These c o n d i t i o n s may v a r y with b a t h composition.

Thermal s t a b i l i t y s t u d i e s have shown t h a t t h e ~ r c o n c e n t r a t i o n + ~ i n t h e chromonyxR b a t h h a s an important e f f e c t on t h e subsequent t h e r - mal s t a b i l i t y of t h e c o a t i n g a s shown i n t a b l e I /20/. I n o r d e r t o ob- t a i n a t h e r m a l l y s t a b l e c o a t i n g f o r use above 250°C, i t was found ne- c e s s a r y t o c o n t r o l t h e ~ r + 3 c o n c e n t r a t i o n very c a r e f u l l y . The black chrome c o a t i n g i s r o u t i n e l y d e p o s i t e d on an e l e c t r o p l a t e d n i c k e l under- l a y e r and i f t h e n i c k e l i s n o t p l a t e d immediately with t h e black chrome, t h e s u r f a c e must be a c t i v a t e d by an H C 1 e t c h . On o c c a s i o n , t h e n i c k e l i s a p p a r e n t l y n o t a c t i v a t e d completely and t h e r e s u l t i n g black chrome c o a t i n g i s n o t thermally s t a b l e , p o s s i b l y due t o poor n u c l e a t i o n of t h e e l e c t r o d e p o s i t e d black chrome on t h e n i c k e l s u r f a c e .

From our l a b o r a t o r y r e s u l t s , f t i s a p p a r e n t t h a t t o o b t a i n high q u a l i t y black chrome c o a t i n g s it i s important t h a t t h e r e be :

1. Continuous b a t h a n a l y s i s

2 . O p t i c a l monitoring o f p a r t s d u r i n g f a b r i c a t i o n

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o p t i c a l p r o p e r t i e s of t h e c o a t i n g s . I t was found necessary f o r Sandia personnel t o be p r e s e n t d u r i n g a l l production r u n s o f o u r product i n o r d e r t o make o p t i c a l measurements on t h e production c o a t i n g s . I t has been o u r experience t h a t t h e e l e c t r o p l a t i n g b a t h s used t o produce t h e r - mally s t a b l e c o a t i n g s w i l l d e t e r i o r a t e over a p e r i o d of time. This may be i n p a r t due t o v a r i a b l e s such a s contaminants which we have n o t iden- t i f i e d and t h u s do n o t c o n t r o l . I n a d d i t i o n , any change i n s u b s t r a t e geometry r e s u l t s i n t h e p r o c e s s having t o be a d j u s t e d t o achieve o p t i - mum r e s u l t s .

A high volume o p t i c a l c o a t i n g d e p o s i t i o n process which has proven t o be r e l i a b l e i s t h a t of s p u t t e r d e p o s i t i o n which i s widely used t o produce coated a r c h i t e c t u r a l g l a s s /45/. S e v e r a l groups a r e using sput- t e r d e p o s i t i o n t o d e p o s i t s e l e c t i v e s o l a r absorbing c o a t i n g s /32, 4 6 / .

A c o s t a n a l y s i s of t h i s p r d c e s s would seem t o i n d i c a t e t h a t t h e p r o c e s s i s c o s t competitive with e l e c t r o p l a t i n g i n high volumes / 3 1 / .

Other p r o c e s s e s such a s anodization/im~regnation /51/, thermal o x i d a t i o n , s p r a y p y r o l y s i s / 5 0 / , chemical vapour deposition/47/, o r o t h e r coating techniques may prove t o be c o s t c o m p e t i t i v e and f e a s i b l e f o r commercial scale-up b u t t h e s e techniques have y e t t o prove themselves i n a mass production environment.

5. Summary.- A number of s e l e c t i v e absorber c o a t i n g systems with high s o l a r absorptance e x i s t which may be used i n t h e mid-temperature range. Some of t h e systems a r e more chemically and t h e r m a l l y s t a b l e t h a n o t h e r s . Unfortunately, t h e r e a r e l a r g e gaps i n t h e s t a b i l i t y d a t a f o r a l a r g e number o f t h e systems.

I n an i n e r t environment, t h e p r i n c i p l e d e g r a d a t i o n mechanisms a r e i n t e r d i f f u s i o n between t h e l a y e r s o r phases and changes i n s u r f a c e morphology. These degradation mechanisms would be minimized by using refrac- t o r y m e t a l s and compounds f o r t h e absorbing l a y e r and u s i n g r e f r a c t o r y m a t e r i a l s o r d i f f u s i o n b a r r i e r s f o r t h e underlayer.

For use i n a r e a c t i v e environment, t h e choice of m a t e r i a l s i s much more r e s t r i c t i v e s i n c e i n t e r n a l chemical r e a c t i o n s can change phase com- p o s i t i o n s and i n t e r f a c i a l r e a c t i o n s can l e a d ko l o s s of adhesion.

For a c o a t i n g process t o be u s e f u l , it i s necessary t o determine w h a t p a r a m e t e r s i n f l u e n c e t h e performance of t h e c o a t i n g and t h e l i m i t s t o t h e s e parameters. This s e n s i t i v i t y " h a s a d i r e c t i n f l u e n c e on t h e production p r o c e s s c o n t r o l s necessary t o produce a good product. Again, much of t h i s type o f information i s n o t a v a i l a b l e on a number o f c o a t i n g systems.

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produce a thermally stable produce for use to 320°C has increased our cost significantly. It seems to be rather difficult to obtain the necessary production process controls. It should be noted that the black chrome electroplate is quite satisfactory for use at temperaturesbelow 250°C.

In view

of

the increased cost and complexity necessary to extend the thermal stability range of the present electrodeposited black chrome from 250°C to 350°C, it would seem that some other deposition technique might provide a significant improvement over electroplating for coatings to be used above 250°C.

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