THERMAL DEGRADATION OF CERMET SOLAR SELECTIVE ABSORBERS

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THERMAL DEGRADATION OF CERMET SOLAR SELECTIVE ABSORBERS

I. Ritchie, J. Spitz

To cite this version:

I. Ritchie, J. Spitz. THERMAL DEGRADATION OF CERMET SOLAR SELEC- TIVE ABSORBERS. Journal de Physique Colloques, 1981, 42 (C1), pp.C1-301-C1-307.

�10.1051/jphyscol:1981120�. �jpa-00220670�

JOURNAL D E PHYSIQUE

CoZZoque CZ, suppZ6rnent au nO1, Tome 42, janvier 1981 page fl-301

THERMAL DEGRADATION OF CERMET SOLAR S E L E C T I V E ABSORBERS

I . T . Ritchie and J. S p i t z

C e n t r e d f E t u d e s ~ u c l e a i r e s d e G r e n o b l e , 85 X

-

38041 G r e n o b l e c e d e x , F r a n c e .

R&sumd.- P l u s i e u r s absorbeurs s & l e c t i E s de type cermet o n t 6 t 6 e l a - bores s o i t p a r S l e c t r o l y s e , s o i t p a r des techniques de d6pbt sous vide. Les p r i n c i p a u x mdcanismes de d d g r a d a t i o n thermique de c e s cermets o n t 6 t d e t u d i e s . Pour l e chrome n o i r , une m o d i f i c a t i o n de l a forme des p a r t i c u l e s semble S t r e

a

l ' o r i g i n e d e l a ddgradation.

Pour l e s c a r b u r e s e t s i l i c i u r e s m d t a l l i q u e s , on pense que l a degra- d a t i o n p r o v i e n t d ' i n t e r a c t i o n s chimiques e n t r e l e s d i v e r s compo- s a n t s du cermet. C e t t e 6tude permet de d e f i n i r l a s t r u c t u r e i d e a l e d ' u n absorbeur s d l e c t i f de type cermet.

A b s t r a c t . - S e v e r a l d i f f e r e n t types of cermet s e l e c t i v e a b s o r b e r s have been produced by both e l e c t r o c h e m i c a l and vacuum d e p o s i t i o n techniques. T h e i r most important modes of thermal d e g r a d a t i o n have been s t u d i e d . For e l e c t r o p l a t e d chromium b l a c k s , t h e change i n t h e shape of t h e p a r t i c l e s seems t o be r e s p o n s i b l e f o r t h e d e g r a d a t i o n . For m e t a l l i c c a r b i d e s and s i l i c i d e s , d e g r a d a t i o n i s presumed t o be due t o chemical r e a c t i o n s between t h e componentsof t h e cermet. Then, t h e s t r u c t u r e of an i d e a l cermet s e l e c t i v e a b s o r b e r i s given.

1. I n t r o d u c t i o n . - S o l a r s e l e c t i v e absorbing s u r f a c e s a r e g e n e r a l l y con- s i d e r e d t o f a l l i n t o t h r e e major c a t e g o r i e s , depending upon t h e mecha- nism which i s r e s p o n s i b l e f o r t h e s e l e c t i v e a b s o r p t i o n . An a b s o r b e r may have an i n t r i n s i c s p e c t r a l r e f l e c t a n c e p r o f i l e which l e a d s t o a high so- l a r absorptance ( a s ) and .low thermal e m i t t a n c e ( e T ) , have s u r f a c e rough- ness on such a s c a l e t h a t s h o r t e r wavelengths a r e absorbed and l o n g e r wavelengths r e f l e c t e d , o r i t may be a m u l t i l a y e r i n t e r f e r e n c e f i l t e r with t h e t h i c k n e s s e s and r e f r a c t i v e i n d i c e s o f t h e l a y e r s chosen t o pro- duce t h e e f f e c t . I n t e r f e r e n c e f i l t e r s with e x c e l l e n t s o l a r s e l e c t i v e a b s o r p t i o n p r o p e r t i e s can be produced by d e p o s i t i n g a f i l m which i s ab- s o r b i n g i n t h e s o l a r s p e c t r a l r e g i o n but r e l a t i v e l y t r a n s p a r e n t i n t h e thermal i n f r a r e d onto a h i g h l y r e f l e c t i n g metal underlayer. Metal-insu- l a t o r composites, i n which metal p a r t i c l e s a r e embedded i n an i n s u l a t i n g m a t r i x ( a cermet) a r e among t h e r e l a t i v e l y few c l a s s e s of m a t e r i a l s which can be used a s t h i s absorbing l a y e r , however due t o t h e i r inhomo- g e n e i t y t h e y i n t r o d u c e a f u r t h e r d e g r a d a t i o n mechanism i n t o t h e s o l a r absorber.

I n t h i s a r t i c l e we s h a l l d i s c u s s t h e thermal d e g r a d a t i o n of seve- r a l d i f f e r e n t types of cermet s e l e c t i v e a b s o r b e r s produced by both e l e c -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981120

JOURNAL DE PHYSIQUE

trochemical and vacuum deposition techniques to illustrate their most important modes of thermal degradation.

Single layer solar selective surfaces usually also emply additio- nal effects to enhance their solar absorptance, such as surface rough- ness, or a gradient in refractive index through the layer ;theseeffects may also cause a decrease in the absorber's performance after heat trea- tment and will also be discussed.

The degradation mechanisms of cermet materials may be divided into two classes

:

physical and chemical. However, although the physical or chemical nature of the cermet may change after heat treatment, signifi- cant degradation of the optical properties of the film may not occur.

Consequently it is important to identify the parameters which affect solar selective performance.

Possible physical degradation modes in cermets are

:

- Changes in metal particle shape

- Growth of particles

- Agglomeration of particles to give long chains

- Densification of the insulating matrix.

Chemical reactions can occur between the components of the cermet, or between the cermet and either the atmosphere or the substrate. These chemical changes may be facilitated if one of the components is metasta- ble, which often occurs in non-equilibrium deposition processes.

Solar selective absorbers are of most interest in collectors which produce low or medium grade heat

;

consequently they must have a life- time of at least several months at the stagnation temperature and seve- ral years at the operating temperature of the collector in which they are employed. In flat plate type collectors both these temperatures are below 200°C, but in evacuated or focussing collectors they may be as high as 400°C.

2.

Cermet films.- Three cermet systems have been chosen for this study

:

electroplated chromium black films, which are characterized by their very high solar absorptance

( >

0.95) and which have a structure which is probably fairly typical of such electroplated blacks, reactively sput- tered metal carbide and silicide surfaces deposited in a manner which produces a particulate film, and films prepared

by

vacuum co-deposition of a metal and dielectric. The latter two selective surfaces have lower solar absorptances but when deposited onto suitable substrates show very low thermal emittances

( <

0.05) .

2.1.

Fllectroplated ahromium blacks.- The selective absorbers produced by electroplating chromium black are probably the most widely commer- cialized selective surface in use on flat plate type solar collectors.

The chromium black film is deposited onto copper or electroplated nick-

e l , t o a t h i c k n e s s o f 200-300 nm /1/ and t y p i c a l l y h a s a s > 0.95 and 0.15 < e T < 0 . 2 5 .

Recent s t u d i e s o f t h e s e f i l m s have shown t h a t t h e y c o n s i s t o f nee- d l e - l i k e p a r t i c l e s o f chromium o x i d e , o r i e n t e d a p p r o x i m a t e l y normal t o t h e s u b s t r a t e /2,3/, and t h a t t h e r e i s a h i g h d e n s i t y o f s m a l l e r chro- mium m e t a l p a r t i c l e s embedded i n t h e chromium o x i d e p a r t i c l e s . The f i l m i s more dense n e a r t h e s u b s t r a t e t h a n a t t h e s u r f a c e .

Chromium b l a c k f i l m s a r e s t a b l e up t o a b o u t 3 0 0 ' ~ / I / , b u t show i r r e v e r s i b l e d e g r a d a t i o n ( d e c r e a s e s i n a, and e T ) a t h i g h e r t e m p e r a t u r e s /I/. The p r i n c i p a l d e g r a d a t i o n mechanism apGears t o b e a t r a n s f o r m a t i o n of t h e s h a p e o f t h e chromium o x i d e p a r t i c l e s from n e e d l e - l i k e t o sphe- r i c a l .

F i g . 1.- Scanning e l e c t r o n micrographs o f a chromium b l a c k f i l m ; l e f t , a f t e r p r e p a r a t i o n ; r i g h t , a f t e r 134 h o u r s a t 350°C i n a i r , a r e shown.

F i g u r e 1 shows s c a n n i n g e l e c t r o n micrographs o f a chromium b l a c k f i l m a f t e r p r e p a r a t i o n and a f t e r a n n e a l i n g i n a i r a t 350°C. The change i n shape o f t h e p a r t i c l e s i s c l e a r l y v i s i b l e ; no i m p o r t a n t chemical phase t r a n s i t i o n s were o b s e r v e d , n o r was t h e r e any s i g n i f i c a n t growth i n p a r -

w lor o Z

S

w ,J 08-

F i g . 2.- R e f l e c t a n c e s p e c t r a a s a func- t i o n o f wavelength o f t h e two f i l m s i n f i g u r e 1 a r e shown (- a s p r e p a r e d :

W

.* - -

a f t e r a n n e a l i n g ) t o g e t h e r w i t h

c a l c u l a t e d s p e c t r a (-.- b e s t f i t t o a s p r e p a r e d spectrum :--

-

s i m i l a r model

06. w i t h s p h e r i c a l p a r t i c l e s ) .

O L -

02-

3 5 10 15 2 0 2 5

WAVELENGTHlum)

c1-304 JOURNAL DE PHYSIQUE

t i c l e s i z e . I n f i g u r e 2 a r e shown t h e r e f l e c t a n c e s p e c t r a of t h e s e two f i l m s , t o g e t h e r w i t h c a l c u l a t e d s p e c t r a based on a model of t h e f i l m s /2/. The changes i n t h e e s s e n t i a l f e a t u r e s of t h e experimental r e f l e c -

t a n c e spectrum a f t e r a n n e a l i n g can be w e l l reproduced i n t h e model by a l t e r i n g t h e shape of t h e oxide p a r t i c l e s .

This s t r u c t u r a l t r a n s f o r m a t i o n occurs a s t h e n e e d l e - l i k e p a r t i c l e s t r y t o d e c r e a s e t h e i r s u r f a c e energy and t h u s become more s p h e r i c a l . A s e r i e s o f f i f t y , f i v e minute h e a t c y c l e s t o 350°C d i d n o t produce any n o t i c e a b l e d e g r a d a t i o n , whereas an e q u i v a l e n t continuous t r e a t m e n t pro- duces s e v e r e d e c r e a s e s i n as and e T I which a l s o i n d i c a t e s t h a t t h e de- g r a d a t i o n mechanism has a p h y s i c a l , r a t h e r t h a n chemical o r i g i n . The t r a n s f o r m a t i o n i s a i d e d by t h e low d e n s i t y of t h e f i l m s ( p = 3.6 g/cm 3

3 3

PCr = 7.2 g/cm I PCr203 = 5.1 g/cm ) ; i t i s l i k e l y t h a t a more dense f i l m would show g r e a t e r thermal s t a b i l i t y , however such an i n c r e a s e i n f i l m d e n s i t y would i n c r e a s e t h e r e f r a c t i v e index of t h e f i l m and hence lower i t s s o l a r absorptance.

2.2. R e a c t i v e l y - s p u t t e r e d films.- Thin f i l m s of s e v e r a l o f t h e t r a n s i - t i o n metals, when s p u t t e r e d i n an atmosphere c o n t a i n i n g a s u i t a b l e r e - a c t i v e g a s (methane o r s i l a n e ) o n t o metal s u b s t r a t e , can produce s o l a r a b s o r b e r s with a t t r a c t i v e s e l e c t i v i t y and high thermal s t a b i l i t y /4-7/.

These f i l m s d e p o s i t a s a c e r m e t l i k e composite of t h e metal c a r b i d e o r s i l i c i d e i n a porous m a t r i x of carbon o r s i l i c o n /6,8/, s e e f i g u r e 3.

F f g . 3.- Transmission e l e c t r o n micrograph of a r e a c t i v e l y s p u t t e r e d T i - S1 f i l m ; t h e m e t a l l i c p a r t i c l e s i n a S i m a t r i x can be r e a d i l y i d e n t i - f i e d

.

The d i e L e c t r i c p r o p e r t i e s of t h e s e d e p o s i t s may be a l t e r e d c o n s i d e r a b l y

by changing t h e p r e s s u r e o f t h e r e a c t i v e g a s i n t h e d i s c h a r g e : low r e s i s t i v i t y m e t a l l i c f i l m s composed o f s m a l l p a r t i c l e s o f t h e m e t a l l i c phase a r e produced a t low r e a c t i v e g a s p r e s s u r e s and h i g h r e s i s t i v i t y

( a 50 ncm) d i e l e c t r i c f i l m s , where t h e m e t a l l i c p a r t i c l e s a r e embedded i n t h e i n s u l a t i n g m a t r i x , a r e produced a t h i g h r e a c t i v e g a s p r e s s u r e s .

Consequently, t h e f i l m s may be g r a d e d i n c o m p o s i t i o n t h r o u g h t h e i r t h i c k n e s s by i n c r e a s i n g t h e p r e s s u r e o f r e a c t i v e g a s i n t h e s p u t t e r d i s - c h a r g e . T h i s h a s t h e e f f e c t o f p r o d u c i n g a c o a t i n g which i s more d i e l e c - t r i c n e a r t h e s u r f a c e t h a n n e a r t h e s u b s t r a t e , i n c r e a s i n g t h e s o l a r ab- s o r p t a n c e o f t h e f i l m /9/. Uniform f i l m s on c o p p e r s u b s t r a t e s a r e s t a b l e above 400°C /4,5/ and have as a 0.80 and e T a 0.03 w h i l s t graded f i l m s a r e s t a b l e up t o 400°C / 6 , 7 / and have a s > 0.90 and e T a 0.05.

The s t r u c t u r e o f t h i s c l a s s o f f i l m s was r e v e a l e d by e l e c t r o n m i - c r o s c o p y , and no change i n t h e f i l m s ' , s t r u c t u r e c o u l d be o b s e r v e d a f - t e r a n n e a l i n g f o r a s u f f i c i e n t l y l o n g t i m e t o c a u s e s i g n i f i c a n t changes i n t h e i r o p t i c a l p r o p e r t i e s . However, t h e r e s i s t i v i t i e s o f t h e f i l m s i n c r e a s e d s i g n i f i c a n t l y a f t e r a n n e a l i n g . S i n c e t r a n s i t i o n m e t a l c a r b i - d e s and s i l i c i d e s e x i s t o v e r a l a r g e r a n g e o f s t o e c h i o m e t r i e s , i t i s presumed t h a t h e a t t r e a t m e n t c a u s e s some o f t h e d i e l e c t r i c component o f t h e m e t a l l i c p a r t i c l e s t o p r e c i p i t a t e o u t i n t o t h e i n s u l a t i n g m a t r i x , which w i l l r e s u l t i n t h e o b s e r v e d i n c r e a s e i n r e s i s t a n c e . F i l m s which a r e more s t a b l e c o u l d presumably be manufactured by p e r f o r m i n g a p o s t - d e p o s i t i o n a n n e a l on f i l m s which were i n i t i a l l y t o o m e t a l l i c t o produce t h i s p r e c i p i t a t i o n under c o n t r o l l e d c o n d i t i o n s .

The i n f l u e n c e o f o p e r a t i n g environnement i s a l s o i l l u s t r a t e d by t h e b e h a v i o u r o f t h e s e f i l m s ; w h i l s t m e t a l s i l i d i d e f i l m s c a n be pro- duced which a r e s t a b l e i n b o t h a i r and vacuum, m e t a l c a r b i d e s a r e o n l y s t a b l e i n vacuum, d e g r a d i n g r a p i d l y when a n n e a l e d i n a i r . T h i s degrada- t i o n i s presumably due t o t h e f o r m a t i o n o f C02 on t h e c a r b i d e f i l m s and t h e s u b s e q u e n t d e p l e t i o n o f carbon from t h e f i l m , whereas t h e o x i d a t i o n o r s i l i c o n would produce a r e l a t i v e l y s t a b l e l a y e r on t h e m e t a l s i l i c i d e a b s o r b e r s .

2.3. Vacuum w = d e _ p o s i t e d cermets.- I n an e f f o r t t o a v o i d s e v e r a l o f t h e d i f f i c u l t i e s mentioned above, a s s o c i a t e d w i t h f i l m s h a v i n g a cermet s t r u c t u r e i n h e r e n t i n t h e d e p o s i t i o n method, s e v e r a l workers have pro- duced cermet f i l m s by vacuum c o - d e p o s i t i o n o f a m e t a l and a d i e l e c t r i c from s e p a r a t e s o u r c e s o n t o a m e t a l s u b s t r a t e /lo-13/. The n o b l e m e t a l s

(Au, Ag, Cu) mainly have been employed, t o g e t h e r w i t h e i t h e r MgO o r A1203 a s d i e l e c t r i c , and t h e s e c e r m e t f i l m s a r e o f t e n graded i n conpo- s i t i o n t o be d i e l e c t r i c n e a r t h e s u r f a c e and m e t a l l i c n e a r t h e s u b s t r a - t e , by changing t h e r e l a t i v e d e p o s i t i o n r a t e s o f m e t a l and d i e l e c t r i c d u r i n g t h e d e p o s i t .

Although t h e s e a b s o r b e r s a r e more c o m p l i c a t e d t o produce t h a n t h e

c1-306 JOURNAL DE PHYSIQUE

selective surfaces discussed above, the two components can be chosen to be immiscible, and thus films which are stable up to 500'~ /12,13/

have been produced. The volume fraction of the metallic phase in these cermets can be sufficiently high to lead to particles which are almost touching and at these temperatures significant agglomeration of the me- tallic particles can occur

/14/,

especially if the film is graded in composition so that it is metallic near the substrate. Agglomeration of the metallic particles in a cermet causes it to become more metallic, as long conducting chains are formed.

Certain metal particles such as those of

W

or Mo, remain uniformly dispersed within the dielectric phase even after prolonged annealing

/15/

(although some particle growth does occur) and so cermets such as these may be sufficiently stable to be used as high temperature absor- bers. McKenzie /13/ has suggested that the relative stability of Cr-A12 O3 cermets is due to the reaction of the metal particles with the oxide matrix to form a stable oxide coating which prevents coalescence and subsequent degradation of the solar absorption properties of these films. Unfortunately, little systematic work has been done on solar se- lective properties of cermet films composed of these more refractory metals.

3.

Conclusion.- From the above examples of the types of thermal degra- dation which can occur in cermet films, the structure of an ideal cermet selective absorber emerges. The film would consist of small isolated spherical metallic particles, sufficiently refractory to resist agglome- ration, in a dielectric matrix of low refractive index which is immis- cible with the metallic material. Such a composite would be the most likely to withstand thermal degradation by the three most common mecha- nisms

:

particle shape changes (or densification), precipitation (che- mical reaction between the two phases) and particle agglomeration or growth. Additionally, the film should be deposited by a technique which is readily and economically adapted to the large area production of optical-quality films

;

though the shape of the absorber, either flat- plate or tubular, may limit this choice.

However it must be recognized that several unstable structures can increase the solar selective absorptance of cermet films

;

ellip- soidal particles and porous matrices are examples. Consequently, a tho- rough thermal characterization of any cermet selective absorber should be performed to determine its suitability for a given temperature appli- cation and collector design.

In conclusion, cermet films deposited onto metal substrates show

attractive solar selective absorption properties but their composite

structure-introduces another thermal degradation mechanism. The surfa-

ces should be carefully defined so that this thermal degradation does

not limit the lifetime of the solar collector in which it is used.

References

Spitz J., Tran Van Danh and Aubert,A., Sol. Energy Mater. 1

( 1 9 7 9 ) 1 8 9

Ritchie,I.T., Sharma,S.K., Valignat'J. and Spitz,J., Sol. Energy Mater. 2

( 1 9 8 0 ) 1 6 7

Valignat,J., Spitz,J. and Ritchie,I.T., Revue Phys. Appl.

( 1 9 8 0 )

Harding,G.L., J. Vac. Sci. ~echnol. 13

( 1 9 7 6 ) 1 0 7 0

Harding,G.L., J. Vac. Sci. Technol. 15

( 1 9 7 8 ) 6 5

Ritchie,I.T. and Harding,G.L., Thin Solid Films 57

( 1 9 7 9 ) 3 1 5

Harding,G.L. and Graig,S.,J. Vac. Sci. Technol.

1 6 ( 1 9 7 9 ) 8 5 7 -

Ritchie,I.T., in Proc. Int. Conf. Metall.

Coatings,

2 1 - 2 5

April

1 9 8 0 ,

San Diego CAI to be published in Thin Solid Films Ritchie,I.T. and Window,B., Appl.

Opt.

16

( 1 9 7 7 ) 1 4 3 8

Gittl.eman,J.I., Appl. Phys. Lett. 28

( 1 9 7 6 ) 3 7 0

Fan

,J.C.

C. and Zavracky ,P .M. , Appl. Phys .

Lett. 2

( 1 9 7 6 ) 4 7 8

Craighead,H.C. and Buhrman,R.A., Appl. Phys.

Lett. 21

( 1 9 7 7 ) 4 2 3

1,I~Kenzie~D.R.~ Appl. Phys. Lett. 34

( 1 9 7 9 ) 2 5

Abeles,B., in "Applied Solid State Science", R. Wolfe

Ed.

bcademic Press

N.Y.)

5 ,

^{1 9 7 6}

/ 1 5 /

Abeles,B., Pinch,H.L. and Gittleman, Phys.

Rev. Lett. 35

( 1 9 7 5 ) 2 4 7