MATERIALS PROBLEMS IN EVACUATED SOLAR ENERGY COLLECTORS

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MATERIALS PROBLEMS IN EVACUATED SOLAR ENERGY COLLECTORS

G. Harding, B. Window

To cite this version:

G. Harding, B. Window. MATERIALS PROBLEMS IN EVACUATED SOLAR ENERGY COLLEC- TORS. Journal de Physique Colloques, 1981, 42 (C1), pp.C1-173-C1-189. �10.1051/jphyscol:1981112�.

�jpa-00220662�

JOURNAL DE PHYSIQUE

CoZZoque C1, suppzdment au nO1, Tome 42, janvier 1981 page C1-173

SURFACES SELECTIVES

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MOYENN: TEMPERATURE.

MATERIALS PROBLEMS I N EVACUATED SOLAR ENERGY COLLECTORS

G.L. Harding and B. Window

S c h o o l of P h y s i c s , U n i v e r s i t y o f S y d n e y , N.S.W. 2006 A u s t r a l i a

Abstract.- An i n v e s t i g a t i o n of t h e absorptance and e m i t t a n c e o f s e l e c t i v e absorbing s u r f a c e s based on r e a c t i v e l y s n u t t e r e d metal c a r b i d e f i l m s i s d e s c r i b e d . These s u r f a c e s e x h i b i t good s t a b i l i t v a t 300°C i n vacuum. Novel techniques have been developed f o r eva- l u a t i o n o f absorptance and temperature dependent e m i t t a n c e of se- l e c t i v e s u r f a c e s i n c o r n o r a t e d i n assembled c o l l e c t o r s . Thermal conduction l o s s e s i n evacuated c o l l e c t o r s due t o low p r e s s u r e s of H 0 , N 2 , He, A r , H2 and CO have been determined. Methods f o r i m - p$oving t h e e f f i c i e n c y of evacuated ' c o l l e c t o r s , by using a n t i r e - f l e c t i o n l a y e r s on t h e g l a s s enveloye o r s e l e c t i v e s u r f a c e s of enhanced absorptance a r e d i s c u s s e d .

1. I n t r o d u c t i o n . - E f f i c i e n t h e a t e x t r a c t i o n from non-concentrating so- l a r c o l l e c t o r s a t temperatures 100-150°C may be achieved by vacuum i n - s u l a t i o n of t h e absorber element and by u t i l i z i n g an absorbing s u r f a c e of high s e l e c t i v i t y . Evacuated t u b u l a r c o l l e c t o r s a r e c u r r e n t l y under development a t Sydney U n i v e r s i t y . I n t h i s paper, r e s e a r c h connected with a s p e c t s of manufacture and t e s t i n g o f t h e c o l l e c t o r i s reviewed, with p a r t i c u l a r a t t e n t i o n p a i d t o t h e choice and t r e a t m e n t of m a t e r i a l s used i n t h e c o l l e c t o r c o n s t r u c t i o n .

2. C o l l e c t o r design and s e l e c t i v e s u r f a c e production.- Prototype eva- c u a t e d t u b u l a r s o l a r energy c o l l e c t o r s under development a t Sydney Uni- v e r s i t y a r e manufactured from oyrex g l a s s t u b e s of l e n g t h

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^{1 . 4 m, ab-}

s o r b e r t u b e o u t e r diameter e i t h e r 22 mm o r 30 mm and envelope o u t e r d i a - meter e i t h e r 30 mrn o r 38 mm. The two t u b e s a r e assembled c o n c e n t r i c a l l y ,

(Fig.1) s i n c e t h i s p r e s e n t s t h e minimum problem f o r mass production.

The envelope i s dimpled a t t h r e e p o i n t s t o su?port t h e absorber t u b e i n t h e c o n c e n t r i c p o s i t i o n . This design f e a t u r e o b v i a t e s t h e need f o r in- troducing f o r e i g n m a t e r i a l s i n t h e form of m e t a l s n r i n g s o r c l i p s t o s u p p o r t t h e a b s o r b e r tube.

The p r e s e n t production procedure i n v o l v e s baking t h e a b s o r b e r t u b e i n d r y a i r a t

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500°C p r i o r t o a p p l i c a t i o n of t h e s e l e c t i v e s u r f a c e , t o reduce s u r f a c e (and wossibly bulk) contaminants of t h e g l a s s . This oro- cedure improves t h e p r o s p e c t s f o r r a o i d evacuation d u r i n g t h e f i n a l as- sembly of t h e c o l l e c t o r when t h e g l a s s and s e l e c t i v e s u r f a c e a r e baked a t

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^500°C.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981112

JOURNAL DE PHYSIQUE

Sdnev Uwersitv Evacuated Cdledor.

Fig. 1.- S e c t i o n a l view of Sydney U n i v e r s i t y Evacuated C o l l e c t o r

A s e l e c t i v e s u r f a c e developed a t Sydney U n i v e r s i t y s p e c i f i c a l l y f o r a l l g l a s s evacuated c o l l e c t o r s c o n s i s t s of a r e a c t i v e l y s p u t t e r e d metal-carbide f i l m d e p o s i t e d on s p u t t e r e d conper. This s e l e c t i v e s u r -

f a c e was developed i n small s c a l e s p u t t e r c o a t i n g svstems / I , 2, 3 / . A c y l i n d r i c a l magnetron s p u t t e r i n g system f o r c o a t i n g h a t c h e s of 20 t u - bes h a s now been operated s u c c e s s f u l l y f o r 1 8 months, nroducing a t o t a l of 1,200 t u b e s /4, 5/. Figure 2 shows a schematic diagram of t h e s n u t t e r c o a t e r . The b a t c h of twenty tubes i s mounted on a c a r o u s e l which produ- c e s p l a n e t a r y motion of t h e t u b e s about two magnetron s p u t t e r i n g sour- ces. NO change i n t h e o p e r a t i n g c o n d i t i o n s o f t h e c o a t e r a r e r e q u i r e d when l a r g e (30 rnm diameter) t u b e s a r e c o a t e d i n p l a c e of t h e 22 mm d i a - meter t u b e s , hence b a t c h e s c o n t a i n i n g a mixture of both l a r g e and s m a l l diameter tubes have been coated s u c c e s s f u l l y . Two magnetron s p u t t e r i n g e l e c t r o d e s , 38 mrn diameter and 1 . 7 m long a r e s u s ~ e n d e d from t h e t o 9 p l a t e o f t h e chamber. One e l e c t r o d e c o n s i s t s of c o m e r t u b e , t h e o t h e r

Fig. 2.- Schematic diagram of t h e magne- t r o n s ~ u t t e r c o a t i n g system. An a x i a l ma- g n e t i c f i e l d of

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^{0.02 T} i s a n p l i e d d u r i n g s p u t t e r i n g o p e r a t i o n s by a f i e l d c o i l wound on t h e vacuum chamber ( n o t shown).

A : copper e l e c t r o d e ; B : S t a i n l e s s s t e e l ( o r chromium) e l e c t r o d e ; C : Carousel ; D : Twenty g l a s s t u b e s d r i v e n i n p l a n e t a - r y motion d u r i n g s p u t t e r i n g o p e r a t i o n s ; E : S a m ~ l e tube with

lane

g l a s s s l i d e s a t t a c h e d ; F : Water cooled s c r e e n ; G : Reactive g a s i n l e t pipe.

c o n s i s t s of t y p e 3 1 6 s t a i n l e s s s t e e l tube. A copper t u b e coated with

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^{1.5 mm} of e l e c t r o p l a t e d chromium h a s a l s o been used i n g l a c e of t h e s t a i n l e s s s t e e l e l e c t r o d e . There i s s u f f i c i e n t m a t e r i a l s t o r e d i n each of t h e s e e l e c t r o d e s t o c o a t some 15,000 g l a s s tubes. The c o s t of g a s e s consumed d u r i n g s p u t t e r i n g i s n e g l i g i b l e s o t h e m a t e r i a l s c o s t f o r t h e s e l e c t i v e s u r f a c e used i s < 2 c e n t s p e r tube. The e l e c t r o d e s a r e sepa- r a t e d by a water cooled s c r e e n which p r e v e n t s t h e i r c r o s s contamination.

The uniformity of t h e s p u t t e r e d c o a t i n g along t h e g l a s s t u b e s i s n o t p e r f e c t due t o s p u t t e r i n g e l e c t r o d e end e f f e c t s . The e l e c t r o d e s a r e so- mewhat l o n g e r t h a n t h e g l a s s t u b e s t o reduce t h e s e e f f e c t s . F i g u r e 3 i l l u s t r a t e s t h e d e p o s i t i o n r a t e f o r co9per along a g l a s s tube.

Fig. 3 . - Deposition r a t e R "vs. " d i s t a n c e X along a g l a s s tube f o r co?- p e r d e p o s i t e d a t e l e c t r o d e c u r r e n t 20A and 600V. X = 0 corresponds t o t h e lower end of t h e tube when i n t h e s p u t t e r i n g chamber.

S l i g h t l y lower r a t e s a r e o b t a i n e d a t e i t h e r end.

The s t a i n l e s s s t e e l ( o r chromium) e l e c t r o d e i s f i r s t s p u t t e r e d f o r

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⁴ minutes a t 18A and 65017 i n argon a t Dressure 0.5 Pa. This s t e p c l e a n s t h e e l e c t r o d e s u r f a c e of metal-carbide comwounds formed d u r i n g t h e f i n a l s t a g e s of t h e s e l e c t i v e s u r f a c e d e n o s i t i o n i n t h e ~ r e v i o u s c o a t i n g run, and oroduces a l a y e r of reasonable adhesion on t h e g l a s s tubes. The copper e l e c t r o d e i s t h e n m u t t e r e d i n argon f o r 8 minutes a t

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^{2 0 A} and 620V t o wroduce a 320 nrn t h i c k f i l m of low emittance copoer on t h e tubes. The s t a i n l e s s s t e e l ( o r chromium) e l e c t r o d e i s t h e n snut- t e r e d i n an argon-acetylene mixture which r e s u l t s i n d e p o s i t i o n of a metal c a r b i d e f i l m o n t o t h e copper coated tubes. A s p u t t e r i n g c u r r e n t of 17.5A a t d i s c h a r g e v o l t a g e

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600V produces a f i l m of t h i c k n e s s

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^90nm

on t h e twenty t u b e s i n 6-7 minutes. Thus an average c o a t i n g r a t e of 1 tube p e r minute f o r t h e comnlete s u r f a c e i s achieved i n t h i s nroduction system, which i s l i m i t e d by t h e power sunwly p r e s e n t l y a v a i l a b l e . Ho-

Cl-176 JOURNAL DE PHYSIQUE

wever, e v a l u a t i o n of t h e p r o p e r t i e s of t h e s u r f a c e s produced i n t h i s c o a t i n g system should g i v e a r e a l i s t i c guide t o t h e s u r f a c e p r o p e r t i e s o b t a i n e d i n production c o a t i n g systems where c o a t i n g r a t e s of f o u r o r f i v e t u b e s p e r minute a r e envisaged.

The p r e s e n t c o a t i n g system i s c o n s e r v a t i v e l y capable of one run p e r hour o r

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500 t u b e s p e r day on a t h r e e s h i f t b a s i s . Larger c o a t i n g systems which a r e e f f e c t i v e l y an e x t r a p o l a t i o n of t h e Sydney U n i v e r s i t y system could be designed w i t h i n a

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^{1.8 m} diameter h o r i z o n t a l l y moun- t e d chamber /6/. Figure 4 i l l u s t r a t e s two ? o s s i b l e d e s i g n s incornora- t i n g one hundred and f o r t y 30 mm diameter t u b e s i n a s i n g l e tube c i r c l e o r two hundred and f o r t y t u b e s i n a double t u b e c i r c l e .

Fig. 4.- S e c t i o n a l views of two n o s s i b l e ho- r i z o n t a l l y mounted mass n r o d u c t i o n b a t c h coa-

DQ

t i n g systems. ( a ) Carousel c o n t a i n s a s i n g l e c i r c l e w i t h c a p a c i t y

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140 t u b e s of diameter 30 mm. (b) Carousel c o n t a i n s a double t u b e c i r c l e w i t h t o t a l c a p a c i t y

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240 t u b e s o f d i a - meter 30 mm. A : Vacuum chamber ; B : S t a i n - l e s s s t e e l e l e c t r o d e ( s ) : C : C o p ~ e r e l e c t r o -

0-

3m d e ( s ) ; D : Carousel c o n t a i n i n g t u b e s ; E : Reactive g a s i n l e t s ; F : S c r e e n ( s ) f o r ore- (a) Single Circle Carousd v e n t i o n of e l e c t r o d e c r o s s contamination (a) ;

o r f o r a d j u s t i n g r e l a t i v e c o a t i n g r a t e o n t o i n n e r and o u t e r t u b e c i r c l e s ( b ) .

The l a t t e r system (Fig. 4b) may r e s u l t i n s l i g h t l y h i g h e r e f f i c i e n c y of s p u t t e r e d m a t e r i a l u t i l i z a t i o n a s w e l l a s a h i g h e r average c o a t i n g , r a t e and b e t t e r uniformity of c o a t i n g a l o n g t h e izubes. The l a t t e r system a l - s o allows i n c o r p o r a t i o n of more than two e l e c t r o d e s . Three o r f o u r d i f - f e r e n t e l e c t r o d e m a t e r i a l s could be i n c o r u o r a t e d i f a m u l t i - l a y e r coa- t i n g were r e q u i r e d . E i t h e r system run w i t h t o t a l e l e c t r o d e c u r r e n t >

6 0 A should allow a production c a p a c i t y i n e x c e s s of 5,000 t u b e s p e r day s i n c e t h e f r a c t i o n o f production time involved i n pwnpdown would be small.

3 . P r o ~ e r t i e s of t h e s e l e c t i v e s u r f a c e s . - S p e c i m e n s o f s e l e c t i v e s u r f a c e a r e o b t a i n e d i n v a r i o u s c o a t i n g runs by i n t r o d u c i n g a samnle tube w i t h e i g h t p l a n e g l a s s s l i d e s a t t a c h e d a t e a u a l i n t e r v a l s a l o n g i t s 1.5 m l e n g t h (Fig. 2 ) . Two types of s e l e c t i v e s u r f a c e have been i n v e s t i g a t e d .

Homogeneous chromium o r s t a i n l e s s s t e e l c a r b i d e s u r f a c e s a r e pro- duced by m a i n t a i n i n g a c o n s t a n t flow of a c e t y l e n e i n t o t h e s n u t t e r i n g chamber d u r i n g t h e metal c a r b i d e d e n o s i t i o n . The a c e t y l e n e flow i s ad- j u s t e d t o produce a metal c a r b i d e f i l m of r e s i s t a n c e

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10 k O / u , and t h e s p u t t e r i n g time a d j u s t e d t o produce a r e f l e c t a n c e minimum a t 0.90 urn f o r a peak s o l a r a b s o r ~ t a n c e of a

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0.82. Techniques f o r absorntance and room temperature e m i t t a n c e measurement a r e d i s c u s s e d elsewhere / I / . Emittance v a l u e s of E

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^{2.3 % a t 300 K} a r e obtained f o r t h e homogeneous s u r f a c e s . Figure 5 shows t h e r e f l e c t a n c e "vs." wavelength f o r a t v o i c a l homogeneous chromium c a r b i d e s u r f a c e . Samules have been aged i n a con- t i n u o u s l y pumped vacuum chamber a t temperatures 200°C, 300°C and 4 0 0 ' ~ .

Fig.

m i u m Conti ne :

5.- R e f l e c t a n c e ( R ) "vs." wavelength ( A ) f o r a homogeneous chro- c a r b i d e on copper s e l e c t i v e s u r f a c e .

.nuous l i n e : a f t e r manufacture ( a = 0.81, E = 0.024) ; Broken li- a f t e r 3,800 h a t 400°C ( a = 0.78, E = 0.08).

Figure 6 shows absorgtance and e m i t t a n c e a s a f u n c t i o n of annealing ti- me f o r t y p i c a l homogeneous chromium c a r b i d e s u r f a c e s . The s u r f a c e s a r e r e l a t i v e l y s t a b l e a t temperatures up t o 300°C. A t 400°C t h e e m i t t a n c e degrades slowly due t o i n t e r d i f f u s i o n a t t h e copper-metal c a r b i d e i n - t e r f a c e . Almost i d e n t i c a l behaviour h a s been observed f o r s t a i n l e s s s t e e l c a r b i d e s u r f a c e s . F i g u r e 5 shows t h e r e f l e c t a n c e of a chromium c a r b i d e s u r f a c e a f t e r 3,800 h a t 400°C i n vacuum. The o n t i c a l t h i c k n e s s o f t h e a b s o r b i n g l a y e r h a s i n c r e a s e d probably due t o d i f f u s i o n of c a r - bon i n t o t h e cooper a t t h e i n t e r f a c e .

Graded s e l e c t i v e s u r f a c e s a r e produced by varving t h e a c e t v l e n e flow i n t o t h e s p u t t e r i n g zone d u r i n g t h e metal c a r b i d e d e p o s i t i o n . Ab- s o r p t a n c e s c o n s i d e r a b l y h i g h e r t h a n t h e 80 % c h a r a c t e r i s t i c of homoge- neous s u r f a c e s may be obtained. Figure 7 i l l u s t r a t e s q u a l i t a t i v e l v two grading p r o f i l e s f o r s p u t t e r e d s e l e c t i v e s u r f a c e s which have r e c e n t l y been i n v e s t i g a t e d i n a s m a l l magnetron s n u t t e r i n g svstem.

JOURNAL DE PHYSIQUE

k-=-.--.

^-di-

0 B O O

TIME (h)

Fig. 6.

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Absorptance ( a ) and room t e m p e r a t u r e e m i t t a n c e ( E ) " v s . " a g e i n g t i m e f o r homogeneous chromium c a r b i d e s e l e c t i v e s u r f a c e s a n n e a l e d i n vacuum a t 200°C, 300°c and 400°c. Continuous l i n e s : a b s o r p t a n c e s ; Bro- ken l i n e s : e m i t t a n c e s .

. . . . .

. .

^:

f

^..

^.

^....a

^.-..*.:.

^...a.

^{.:. - .}

DMD _d .... *...'.' -. . . . ^. ... _{. . .} ^. . ...-

^-

^{. -}

^-.a

F i g . 7.- Diagrams i l l u s t r a t i n g q u a l i t a t i v e l y two t y p e s o f g r a d i n g oro- f i l e , MD g r a d i n g and DMD g r a d i n g . The d e n s i t y of d o t s i l l u s t r a t e s t h e c o n c e n t r a t i o n o f m e t a l i n t h e f i l m and t h e g r a p h s i l l u s t r a t e t h e v a r i a - t i o n of m e t a l c o n c e n t r a t i o n (m) and d i s t a n c e (dl from t h e c o p p e r sub- s t r a t e .

These may be c l a s s i f i e d a s MD and DMD s u r f a c e s . I n MD s u r f a c e s t h e meL k a l c a r b i d e f i l m i s graded from m e t a l r i c h a t t h e conner s u b s t r a t e t o d i e l e c t r i c a t t h e f i l m s u r f a c e by i n c r e a s i n g t h e r e a c t i v e gas flow from z e r o t o a maximum d u r i n g t h e d e ~ o s i t i o n /2, 3/. Dl?D s u r f a c e s a r e graded from d i e l e c t r i c a t t h e cogper s u b s t r a t e t o metal r i c h n e a r t h e c e n t r e o f t h e absorbing l a y e r t o d i e l e c t r i c a t t h e f i l m s u r f a c e /7/. Absorp- t a n c e s a 0.92 may be o b t a i n e d f o r both grading p r o f i l e s . Only PO gra- ded s u r f a c e s have been s t u d i e d i n t h e t u b e c o a t i n g system. Reactive gas may be admitted t o t h e s p u t t e r zone of t h e b a t c h c o a t e r according t o any r e c i p e by manually ovening a f i n e needle valve. The flow r a t e of r e a c t i v e gas i n t o t h e chamber i s measured by monitoring t h e p r e s s u r e drop a c r o s s a s t a i n l e s s s t e e l c a v i l l a r y tube. A range of grading o r o f i - l e s has been i n v e s t i g a t e d . For t h e s u r f a c e s of h i g h e s t absorotance

( a

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0.93-0.95) a range of e m i t t a n c e s ( E

-

0.030 t o E

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0.045 a t 300K) may be obtained by varying t h e s p u t t e r i n g time f o r g u r e s t a i n l e s s s t e e l

( i . e . with no r e a c t i v e gas) i n t h e i n i t i a l s t a g e of t h e d e p o s i t i o n /8/.

The high e m i t t a n c e f i l m s produced i n t h i s way may be u s e f u l f o r l i m i - t i n g t h e s t a g n a t i o n temperatures of evacuated c o l l e c t o r s , p a r t i c u l a r l y when c o n c e n t r a t o r s a r e used. Figure 8 shows r e a c t i v e gas flow a s a

f u n c t i o n of time f o r d e p o s i t i o n of a s t a i n l e s s s t e e l c a r b i d e s u r f a c e of high absorptance ( a

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0.94) and r e l a t i v e l y low emittance ^{( E}

-

^0.030).

Fig. 8.- Flow r a t e of a c e t y l e n e ( f ) i n t o t h e s o u t t e r i n g chamber a s a f u n c t i o n of time ( t ) f o r d e ~ o s i t i o n o f two metal c a r b i d e s e l e c t i v e s u r - f a c e s . S o l i d l i n e : a = 0.94, E = 0.030 ; Broken l i n e : a =0.88,

E = 0.026.

This grading p r o f i l e i s u t i l i z e d i n t h e m a j o r i t y of production runs f o r both s t a i n l e s s s t e e l c a r b i d e and chromium c a r b i d e . The chromium c a r b i d e s u r f a c e s e x h i b i t marginally lower absorntance /5, 9/. Figure 8 a l s o shows t h e C2H2 flow used t o produce a very low emittance s u r f a c e ^(E

-

JOURNAL DE PHYSIQUE

0 . 0 2 6 a t 3 0 0 K ) with somewhat lower absorgtance a

-

^0.88. The r e f l e c t a n - c e s f o r both t h e s e grading p r o f i l e s a r e shown i n f i g u r e 9.

'"I

-8

Fig. 9.- R e f l e c t a n c e (R) "vs. ^" wavelength ( A ) f o r two graded s t a f n l e s s s t e e l c a r b i d e s e l e c t i v e s u r f a c e manufactured u s i n g t h e g r a d i n g p r o f i l e s i l l u s t r a t e d i n f i g u r e 8.

Small samples of s e l e c t i v e s u r f a c e have been aged i n vacuum a t 2 0 0 ° C r 30O0C, 4 0 0 ° C and 500°C. Figure 1 0 shows absorgtance and e m i t t a n c e

"vs." ageing time f o r graded chromium s u r f a c e s . The s u r f a c e i s s t a b l e a t 2 0 0 ° C and 3 0 0 ° C a f t e r an i n i t i a l s m a l l d e c r e a s e i n absorntance.

200%

0 1000 2000 3003

TIME (h)

Fig. 10.- Absorptance ( a ) and room temperature e m i t t a n c e (E) "vs." age- i n g time f o r graded chromium c a r b i d e s e l e c t i v e s u r f a c e s annealed i n va-*

cuum a t 2 0 0 ° C r 300°C, 4 0 0 ° C and 500°C. Continuous l i n e s : absorptances ; Broken l i n e s : emittances.

Both absorptance and e m i t t a n c e d e t e r i o r a t i o n occurs above 400°C. The absorptance d e c r e a s e s due t o t h e graded f i l m becoming more homogeneous a s carbon d i f f u s e s w i t h i n t h e absorbing l a y e r . The abso-tances t e n d towards a

-.

^{0.85 which i s} more t y p i c a l of a homogeneous s u r f a c e . S t a i n - l e s s s t e e l s u r f a c e s d e t e r i o r a t e s l i g h t l y more r a p i d l y t h a n s u r f a c e s ba- s e d on chromium c a r b i d e /5/.

4. Emittance measurements a t e l e v a t e d temperature.- Emittances a t e l e - v a t e d temperature f o r s e l e c t i v e s u r f a c e s d e p o s i t e d on g l a s s t u b e s have been determined by p l a c i n g t h e c o a t e d tube w i t h i n a g l a s s t u b u l a r vacu- um chamber w i t h water cooled w a l l s , and i n s e r t i n g h e a t e r elements i n khe coatdd tube. Figure 11 shows a schematic r e p r e s e n t a t i o n of t h e an- p a r a t u s .

Fig. 11.- Aoparatus f o r measurement o f Dower r a d i a t e d o r thermally7 con- ducted between two c o n c e n t r i c t u b e s . A : O ~ e n ended g l a s s tube ( c o a t e d o r uncoated) ; B : Outer g l a s s enveloge ; C : Annular r e g i o n , evacua- t e d o r c o n t a i n i n g low n r e s s u r e gas ; D : Water j a c k e t ; E : Water i n l e t ;

F : Water o u t l e t ; G : C e n t r a l h e a t e r segment ; B : Outer h e a t e r seg- ments ; I : Thermocouple and h e a t e r l e a d s ; J : E l e c t r o n i c manometer ; K : Needle v a l v e f o r gas i n l e t ; L : To d i f f u s i o n numn.

The power i n p u t t o t h e h e a t e r s r e q u i r e d t o maintain t h e c o a t e d t u b e a t s t a t i c temperature allows e v a l u a t i o n of t h e a b s o l u t e v a l u e of emittance /8/. Figure 12 shows t h e temperature deqendence of e m i t t a n c e f o r homo- geneous chromium and s t a i n l e s s s t e e l c a r b i d e s u r f a c e s . The e m i t t a n c e s a r e r e v e r s i b l e f o r c y c l e s between room temoerature and 300°C. Heating t h e s u r f a c e s above 300°C r e s u l t s i n an i r r e v e r s i b l e i n c r e a s e i n ernit- tance. S i m i l a r l y , graded s u r f a c e s d e t e r i o r a t e a t temperatures above 300°C. F i g u r e 13 shows t h e e m i t t a n c e of a graded chromium c a r b i d e s u r - f a c e a f t e r manufacture and a f t e r c y c l i n g between 100°C and 400°C.

JOURNAL DE PHYSIQUE

Oi 160 '

I I

200 300 LOO

t

^("C)

Fig. 1 2 .

-

^{Emittance ( € 1 "vs. "} temperature ( t ) f o r homogeneous metal c a r - b i d e s e l e c t i v e s u r f a c e s . The emittance of s p u t t e r e d copaer i s included f o r comparison. S o l i d l i n e : S p u t t e r e d copper ; Long dashed l i n e : d a t a f o r chromium c a r b i d e o b t a i n e d i n f i r s t run t o 400°C. S h o r t dashed l i n e : r e v e r s i b l e curve o b t a i n e d a f t e r s e v e r a l c v c l e s between 100°c and 400°c;

Continuous l i n e with c i r c l e s : d a t a f o r s t a i n l e s s s t e e l c a r b i d e o b t a i - ned i n f i r s t run t o 4009C ; Continuous l i n e with t r i a n g l e s : d a t a f o r s t a i n l e s s s t e e l c a r b i d e o b t a i n e d i n t h i r d run t o 400°C ; Continuous li- ne w i t h s q u a r e s : d a t a o b t a i n e d i n f o u r t h run t o 4 0 0 ' ~ .

Repeated c y c l i n g r e s u l t s i n t h e E-T r e l a t i o n s h i p t e n d i n g toward a rever- s i b l e curve. S i m i l a r r e s u l t s a r e o b t a i n e d f o r s t a i n l e s s s t e e l c a r b i d e s u r f a c e s /8/. The ageing d a t a f o r s m a l l specimens and t h e E-T d a t a sug- g e s t t h a t a d i f f u s i o n mechanism, probably d i f f u s i o n of carbon i n t o t h e copper s u b s t r a t e , becomes important a t temperatures above 300°C.

Emittances of t h e s e l e c t i v e s u r f a c e i n f u l l y assembled evacuated c o l l e c t o r s (Fig. 1) have been determined by i n s e r t i n g h e a t e r elements w i t h i n t h e absorber tube and water c o o l i n g t h e e n v e l o ~ e /9/. Emittances have been determined immediately a f t e r assembly of t h e c o l l e c t o r , p r i o r t o a 50O0C bakeout, and immediately a f t e r t h e b a k e o u t and f i n a l s e a l i n g of the envelope.

Fig. 13.- Emittance ( € 1 "vs." tempera- t u r e (t) f o r a graded chromium c a r b i d e s e l e c t i v e s u r f ace of r e l a t i v e l y low emittance. The emittance of s p u t t e r e d copper i s included f o r comparison, So- l i d l i n e : s n u t t e r e d copper ; s o l i d l i n e with c i r c l e s : d a t a o b t a i n e d i n f i r s t run t o 4 0 0 ' ~ ; Long dashed l i n e : d a t a obtained i n second run t o 400°C ; S h o r t dashed l i n e ; r e v e r s i b l e curve o b t a i n e d a f t e r s e v e r a l c y c l e s between 100°c and 400°c.

Figure 1 4 shows emittance "vs." t e n o e r a t u r e b e f o r e and a f t e r f i n a l ba- keout f o r s t a i n l e s s s t e e l c a r b i d e s u r f a c e s made w i t h t h e grading p r o f i - l e s and r e f l e c t a n c e s i l l u s t r a t e d i n f i ~ u r e s 8 and 9 r e s n e c t i v e l v .

Fig. 1 4

.-

^{Emittance ( E ) "VS. "} temnerature ' O L - ( T ) f o r graded s t a i n l e s s s t e e l c a r b i d e s u r - f a c e s coated o n t o tubes and assembled i n t o evacuated modules. S o l i d l i n e : verv low emittance s u r f a c e ( s e e F i g s . 8 and 9)

,

befo- r e 500°C bakeout (lower curtre) and a f t e r .02- 500°C bakeout ( ~ ~ p e r curve) ; Broken l i n e : Surface of high absorptance and r e l a t i v e l y low e m i t t a n c e ( s e e Figs. 8 and 9 ) , b e f o r e 500°C bakeout (lower curve) and a f t e r

CU

- _ _ _ - - -

_______---

I I

500°C bakeout (uooer c u r v e ) . O 160 ' 200 300

T("C)

c1-184 JOURNAL DE PHYSIQUE

The prolonged bakeout of t h e c o l l e c t o r s a t 500-520°C f o r

-

¹

42

^hours

d u r i n g f i n a l evacuation r e s u l t s i n a s i g n i f i c a n t i n c r e a s e of e m i t t a n a e due t o d i f f u s i o n w i t h i n t h e s u r f a c e . However, a f t e r bakeout o f t h e s u r - f a c e a t 500°C, no f u r t h e r changes should occur d u r i n g continuous onera- t i o n of t h e c o l l e c t o r a t

-

100°C, o r extended s t a g n a t i o n a t up t o 300°C.

This h a s been confirmed by measuring t h e e m i t t a n c e of a c o l l e c t o r imme- d i a t e l y a f t e r manufacture, and again a f t e r seven months s t a g n a t i o n out- doors a t temperatures up t o 280°C. I n measuring t h e e m i t t a n c e a f t e r se- ven months s t a g n a t i o n , t h e c o l l e c t o r envelo?e was f r a c t u r e d t o allow removal of any gas which may have accumulated i n t h e module.

5. Absorptance measurements f o r assembled c o l l e c t o r s . - E f f i c i e n c i e s

llo of evacuated c o l l e c t o r s i s o l a t e d by a h i g h l y absorbing background have been measured by comparing t h e temnerature r i s e s of nrobes i n s e r - t e d w i t h i n t h e c o l l e c t o r b e i n g i n v e s t i g a t e d and w i t h i n a c a l i b r a t e d

" s t a n d a r d " c o l l e c t o r w h i l e exnosed t o t h e same n a t u r a l i n s o l a t i o n . Cons- t r u c t i o n of t h e temperature ?robe and the nrocedure f o r c a l i b r a t i o n of

v com- t h e s t a n d a r d c o l l e c t o r a r e d e s c r i h e d e l s e w h e r e / g /. This s t r i c t l . p a r a t i v e measurement t e c h n i a u e which o b v i a t e s t h e need f o r a Fyranome- t e r and may be used s u c c e s s f u l l y under s t e a d y s t a t e o r i n t e r m i t t e n t i n s o l a t i o n c o n d i t i o n s , a l l o w s a s e n s i t i v e e v a l u a t i o n of t h e absorptance of t h e s e l e c t i v e s u r f a c e a t any time d u r i n g t h e l i f e t i m e of t h e c o l l e c - t o r . A maximum e f f i c i e n c y

n o

= 0.835 h a s been ~ e a s u r e d f o r s t a i n l e s s s t e e l c a r b i d e s u r f a c e s w h i l e

n o

= 0.815 f o r chromium c a r b i d e . The ab- s o r b e r ? r e a used i n c a l c u l a t i o n o f

no

i s e a u a l t o t h e c r o s s s e c t i o n a l a r e a o f t h e a b s o r b e r tube. The r e l a t i o n s h i ? between e f f i c i e n c y

no

and absorptance a i s d i s c u s s e d e l s e w h e r e / 9 / . B y i n s e r t i n g t h e temperature

?robe a t v a r i o u s p o i n t s alonq an a b s o r b e r t u b e , absorptance a s a func- t i o n of p o s i t i o n h a s been e v a l u a t e d . F i g u r e 15 shows

n o

a s a f u n c t i o n of p o s i t i o n a l o n g t h e absorber tube f o r a t y p i c a l c o l l e c t o r .

Fig. 15.- E f f i c i e n c y

(no)

a s a f u n c t i o n of p o s i t i o n ( X ) a l o n g t h e absor- b e r t u b e o f an evacuated c o l l e c t o r p l a c e d i n a h i g h l y absor- bing background.

S l i g h t l y lower absorptances a r e measured a t t h e extreme ends of t h e ab- s o r b e r tube due t o end e f f e c t s a s s o c i a t e d with t h e s p u t t e r i n g e l e c t r o - des

/a/.

E f f i c i e n c y measurements on a c o l l e c t o r b e f o r e and a f t e r bake- o u t a t 500°C f o r

-

¹

^V2

hours i n d i c a t e a d e t e r i o r a t i o n i n absorvtance o f 1 %. E f f i c i e n c y measurements on a c o l l e c t o r immediately a f t e r ma- n u f a c t u r e , then a f t e r sevenmonths o f s t a g n a t i o n outdoors a t temperatu- r e s up t o 280°C i n d i c a t e an absorntance d e t e r i o r a t i o n of l e s s t h a n 1 %.

6. C o l l e c t o r d e g r a d a t i o n due t o s u r f a c e outgassing.- A f t e r nrolonged o p e r a t i o n a t e l e v a t e d temperature, gas may accumulate i n t h e evacuated space of c o l l e c t o r tubes. P o s s i b l e n r o c e s s e s causing gas accumulation a r e d e s o r p t i o n from t h e g l a s s and s e l e c t i v e s u r f a c e , decomposition of t h e s e l e c t i v e s u r f a c e and permeation of a t m o s ~ h e r i c helium through t h e g l a s s absorber t u b e w h i l e a t e l e v a t e d temperature. The o u t g a s s i n g r a t e from baked pyrex i s f a r s u ~ e r i o r t o t h a t f o r a l l okher m a t e r i a l s i n c l u - ding s t a i n l e s s s t e e l

/ l o /

s u g g e s t i n g t h e advantage of all- lass evacua- t e d c o l l e c t o r s i n com_oarison with metal a b s o r b e r s i n g l a s s envelones.

Water i s t h e predominant gas desorbed from t h e s u r f a c e and bulk of t h e g l a s s . Some of t h i s i s removed from t h e a b s o r b e r t u b e n r i o r t o c o a t i n g by a prebaking procedure i n d r y a i r . The g l a s s envelone should a l s o be baked p r i o r t o t h e f i n a l assembly o f t h e c o l l e c t o r . A n r e l i m i n a q ana- l y s i s of gas pum~ed from t h e c o l l e c t o r d u r i n g bakeout a t 500°C h a s in- d i c a t e d t h e presence of H20, C0, C02, 0 2 , N 2 , and p o s s i b l y argon and hydrogen. CO and C02 r e s u l t from r e a c t i o n of H20 w i t h t h e metal c a r b i d e c o a t i n g . Small q u a n t i t i e s of argon and hydrogen may be i n c o r p o r a t e d i n metal c a r b i d e c o a t i n g s d u r i n g t h e s n u t t e r d e n o s i t i o n . G e t t e r s have n o t y e t been u t i l i z e d i n t h e Sydney U n i v e r s i t v c o l l e c t o r , however barium- aluminium based m a t e r i a l s should g e t t e r a l l g a s e s e x c e n t argon and he- l i u m .

Thermal conduction l o s s e s between c o n c e n t r i c g l a s s t u b e s c o n t a i - ning low Dressure (0-1.5 Pa) A r , CO, H2, He, H 2 0 and N 2 have been mea- s u r e d u s i n g t h e a p p a r a t u s shown i n f i g u r e 11. I n t h i s Dressure range f o r modules of t h e dimensions d e s c r i b e d , h e a t l o s s i s n r o ~ o r t i o n a l t o g r e s s u r e f o r a l l g a s e s e x c e n t H20. The t h e o r v f o r h e a t conduction through g a s e s when t h e p r e s s u r e i s such t h a t t h e mean f r e e o a t h i s gre- t e r t h a n t h e dimensions of t h e v e s s e l n r e d i c t s a l i n e a r denendence of h e a t l o s s on t e m ~ e r a t u r e d i f f e r e n c e and p r e s s u r e /11/. Heat l o s s has been measured a s a f u n c t i o n o f temnerature f o r a n l a i n g l a s s i n n e r tube,

f o r a t u b e coated with s p u t t e r e d c o m e r , and f o r a tube with a complete gnaded s t a i n l e s s s t e e l c a r b i d e s e l e c t i v e s u r f a c e . Conduction l o s s de- pends on t h e i n t e r a c t i o n of gas molecules w i t h t h e s u r f a c e s , and hence

C1-186 JOURNAL DE PHYSIQUE

on t h e m a t e r i a l s coated o n t o t h e g l a s s tube. Figure 1 6 i l l u s t r a t e s t h e t r e n d observed f o r t h e t h r e e s u r f a c e s considered f o r helium gas. The same t r e n d i s observed q u a l i t a t i v e l y f o r a l l s i ~ gases. The s e l e c t i v e s u r f a c e r e s u l t s i n maximum thermal conduction. This t r e n d i s p a r t l v due t o t h e composition of t h e s e l e c t i v e s u r f a c e and may be n a r t l y due t o i n c r e a s i n g s u r f a c e roughness o f t h e s p u t t e r e d f i l m a s t h i c k n e s s i s i n c r e a s e d .

Fig. 16.- Thermally conducted power (P) "vs." temnerature d i f f e r e n c e (AT) between i n n e r tube and water cooled g l a s s e n v e l o ~ e , f o r low n r e s - s u r e s ( 0

-

1.5 Pa) of helium gas i n t r o d u c e d between t h e tubes. Onen c i r c l e s : p l a i n g l a s s i n n e r t u b e : T r i a n g l e s : i n n e r tube coated with 320 nm o f s p u t t e r e d copner ; Closed c i r c l e s : i n n e r tube c o a t e d with graded s t a i n l e s s s t e e l c a r b i d e s e l e c t i v e s u r f a c e .

I f t h e atmospheric p a r t i a l p r e s s u r e of helium ( - 0.5 Pa) d i f f u s e s i n t o t h e evacuated r e g i o n , ( a s it w i l l , i f t h e module s t a g n a t e s above 300°C f o r p e r i o d s o f t h e o r d e r of weeks/12/), a conduction l o s s of

_- -

25 W p e r m Z of absorber can be e ~ e c t e d f o r an o p e r a t i n g temperature of AT

-

100°C. This conduction l o s s would reduce t h e h e a t e x t r a c t i o n e f f i c i e n c y a t 100°c with 1kw/mL i n c i d e n t on t h e absorber a r e a bv about 3 %. Figure 1 7 i l l u s t r a t e s conduction l o s s "vs." AT f o r a l l s i x gases f o r t h e s e l e c t i v e s u r f a c e c o a t e d onto t h e i n n e r g l a s s tube. These re- s u l t s s u g g e s t t h a t t h e p r e s s u r e of H20 and H2 i n p a r t i c u l a r must be maintained below 0.5 Pa t o m a i n t a i n reasonable e f f i c i e n c y of c o l l e c t o r s

a t o p e r a t i n g temperatures above 100°c.

7. Methods of improving c o l l e c t o r e f f i c i e n c y . - The absorptance of s m a l l samples o f graded s e l e c t i v e s u r f a c e has been enhanced by

-

^{2 % by depo-}

s i t i n g a l i g h t c o a t i n g of carbon p a r t i c l e s on t h e s u r f a c e from a c o o l hydrocarbon flame /5/. Absorptances i n e x c e s s of 9 6 % have been o b t a i - ned,however t h e ernittance i s a l s o i n c r e a s e d b!r

-

^{1 1%.} Carbon c o a t i n g s a r e compatible w i t h metal c a r b i d e s u r f a c e s , and although extremelv f r a -

g i l e , would be s u i t a b l e f o r i n c l u s i o n i n evacuated c o l l e c t o r s .

-

F i g . 17.

-

Thermally conducted nower (P)

" v s

. ^"

temTerature d i f f e r e n c e (AT) bet- . ween i n n e r t u b e and water cooled g l a s s

e n v e l o ~ e f o r low o r e s s u r e (0-1.5 Pa) of

-

v a r i o u s g a s e s i n t r o d u c e d h e t w e e n t h e t u - 300 hes. The i n n e r tube i s coated w i t h a graded

s t a i n l e s s s t e e l c a r b i d e s e l e c t i v e s u r f ace.

The e f f i c i e n c y of evacuated c o l l e c t o r s could f e a s i b l y be enhanced by 3 % t o 5 % by a p p l i c a t i o n of a n t i r e f l e c t i o n l a v e r s t o t h e Tyrex en- veloye. A promising a n t i r e f l e c t i o n l a y e r c o n s i s t s o f s i l i c a p a r t i c l e s d e p o s i t e d from a n a l c o h o l i c s o l u t i o n when t h e g l a s s i s withdrawn from t h e s o l u t i o n a t speeds of 2-20 mm.s-' /13, 14/. A f t e r dippinq, t h e g l a s s may b e baked a t 500°C t o harden t h e f i l m , a p r o c e s s compatible w i t h t h e f i n a l bakeout o f t h e Sydney U n i v e r s i t v c o l l e c t o r . 3 e f l e c t a n c e s of 2.1 % have been o b t a i n e d f o r p l a n e g l a s s a f t e r t r e a t m e n t /13/. The r e f l e c t a n c e r i s e s t o 3.1 % a f t e r t h r e e months i n a i r , p o s s i b l v due t o t h e f i l m &- s o r b i n g water. I n evacuated t u b e s , t h e f i l m c o a t e d on t h e i n n e r s u r f a c e of t h e envelope should be ~ r o t e c t e d from such d e t e r i o r a t i o n .

8. A l t e r n a t i v e s e l e c t i v e s u r f a c e s . - The vroduction and p r o p e r t i e s of some a l t e r n a t i v e r e a c t i v e l y s p u t t e r e d s e l e c t i v e s u r f a c e s d e p o s i t e d onto g l a s s t u b e s a r e p r e s e n t l y under i n v e s t i g a t i o n . A graded metal oxycarbide s u r f a c e produced by s y t t e r i n g a s t a i n l e s s s t e e l e l e c t r o d e i n argon-car- bon monoxide h a s reasonable s e l e c t i v e n r o w e r t i e s (Fig. 181, however t h i s s u r f a c e d e t e r i o r a t e s d u r i n g a 1 hour bakeout a t 500°C i n t h e f i n a l col- l e c t o r assembly s t a g e .

Highly r e f r a c t o r y metals such a s t i t a n i u m a r e roba ably necessary t o o b t a i n a c c e p t a b l e s t a b i l i t y f o r metal oxy-carbides.

9. Conclusion.- X e t a l c a r b i d e s u r f a c e s d e n o s i t e d a t high r a t e i n a cy- l i n d r i c a l magnetron s q u t t e r i n g system have e x c e l l e n t s e l e c t i v e nroner- t i e s and good s t a b i l i t y a t 300°C i n vacuum. These s u r f a c e s may be s u i - t a b l e f o r mass production of evacuated c o l l e c t o r s . I n v e s t i g a t i o n o f thermal conduction l o s s e s i n c o l l e c t o r s c o n t a i n i n g low n r e s s u r e s of va- r i o u s g a s e s h a s i l l u s t r a t e d t h a t conduction l o s s denends on t h e mate-

JOURNAL DE PHYSIQUE

Fig.

oxy-

E =

18.- Reflectance R "vs." wavelength X f o r a graded s t a i n l e s s s t e e l , c a r b i d e s e l e c t i v e s u r f a c e . S o l i d l i n e : A f t e r manufacture ( a = 89 %

2.8 % ) ; Broken l i n e : A f t e r

-

¹ hour a t 5 0 0 ' ~ (a = 85 %, E = 3 . 3 % ) r i a l s used i n t h e absorber t u b e c o a t i n g and t h a t t o t a l gas o r e s s u r e must i d e a l l y be maintained below 1.0 Pa d u r i n g t h e l i f e t i m e o f t h e c o l l e c - t o r .

The enhancement of absorntance of s e l e c t i v e s u r f a c e s by a deno- s i t of p a r t i c u l a t e carbon may r e s u l t i n h i g h e r h e a t e x t r a c t i o n e f f i c i e n - cy f o r t u b e s running a t low temperature o r i n c o r p o r a t e d i n concentra- t o r s .

F u r t h e r r e s e a r c h i s aimed a t t h e i n v e s t i g a t i o n of t h e high r a t e production and p r o p e r t i e s o f a l t e r n a t i v e s e l e c t i v e s u r f a c e s manufac-

t u r e d i n t h e magnetron t u b e c o a t i n g system. Study o f t h e g a s e s desor- bed from s e l e c t i v e s u r f a c e s d u r i n g bakeout a t h i g h temperature and eva- l u a t i o n o f c o l l e c t o r s i n c o r n o r a t i n g a n t i r e f l e c t i o n l a y e r s on t h e g l a s s envelope i s a l s o i n p r o g r e s s .

Acknowledgements.- The a u t h o r s would l i k e t o thank t h e i r c o l l e a g u e s T. Moon, S. C r a i g and D r . D. PlcKenzie f o r t h e i r advice and a s s i s t a n c e . This r e s e a r c h was sun?orted bv t h e Sydnev U n i v e r s i t y Energy Research Centre, t h e Government o f New South Wales and Saudi Arabian i n t e r e s t s under t h e a u s n i c e s of t h e Sydney U n i v e r s i t y Science Foundation f o r Physics.

R e f e r e n c e s

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