Evaporated chromium black selective solar absorbers

Evaporated chromium black selective solar absorbers

Thin Solid Films, 38 (1976) 109-115 © Elsevier S e q u o i a S.A., Lausanne - - Printed in 109 Switzerland EVAPORATED CHROMIUM BLACK SELECTIVE SOLAR...

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Thin Solid Films, 38 (1976) 109-115 © Elsevier S e q u o i a S.A., Lausanne - - Printed in

109 Switzerland

EVAPORATED CHROMIUM BLACK SELECTIVE SOLAR ABSORBERS G. L. HARDING School of Physics, University of Sydney, N.S. W. 2006 (Australia)

(Received April 13, 1976} accepted June 1, 1976)

Chromium blacks made by evaporating chromium metal in low pressures of argon gas, when deposited o n t o highly reflecting copper substrates, exhibit solar selectivity. The optical and electrical properties and the structure of chromium black films are described, and the absorption mechanism is discussed. The properties of blacks prepared in argon at pressures of lh Tort and 3 Torr are compared. This selective surface is easily manufactured and may be suitable for operation at temperatures up to 200 °C in vacuum.

1. INTRODUCTION A large number of selective surfaces are now available for fiat plate and evacuated solar energy collectors. Chemical deposition or electroplating techniques offer the possibility of uniformly coating large areas of an absorber plate or tube quickly and economically. Evaporated selective surfaces, which usually consist of interference stacks, are less suitable for coating onto large areas because of critical thickness requirements for each layer deposited. The optical properties of some metal black selective surfaces depend upon structural effects in the films1; hence the requirement of deposition of precise and uniform thickness are less stringent. A n u m b e r of evaporated metal blacks have been studied by H a r r i s 2"3 . Gold and antimony blacks manufactured under certain conditions are highly selective b u t exhibit very poor stability above 100 °C. This paper describes chromium black surfaces which combine reasonable selectivity with stability at 200 °C.

2. MANUFACTURE OF CHROMIUMBLACKS Chromium metal was evaporated from a tungsten basket in an argon atmosphere of pressure 1~, 3 or 10 Tort onto polished copper substrates, onto glass slides for electrical resistance measurement, onto aluminium foil for mass thickness determination and onto nitrocellulose~oated copper grids for transmission electron microscopy. The evaporation chamber was pumped to a pressure of less than 10 -5 Tort for a period of 20 to 30 min

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G.L. HARDING

before argon was admitted, but no special precautions were taken t o remove f u r t h e r traces o f oxygen or nitrogen from the system. In each ex p e r i m e nt films o f five different mass thicknesses were obtained by successively exposing five substrates to the evaporant. The substrates were located 7 cm above or below the basket. Films o f mass thickness 1 0 - 1 0 0 ~g cm -2 were prepared in ~/~T o r t argon, and films o f mass thickness 1 0 - 1 6 0 ~g cm -2 were prepared in 3 T o r t argon. Similar to ot her metal blacks, the films deposited were e xt r em e l y fragile.

3. O P T I C A L A N D ' E L E C T R I C A L P R O P E R T I E S O F C H R O M I U M B L A C K S

T h e total reflectance o f the coated substrates was measured between 0.35 ~m and 2.5 /~m, and the specular reflectance was measured between 1 and 15/~m. Reflectances were c o m p a r e d with t hat o f polished bulk copper. Th e reflectance v e r s u s wavelength curves for chrom i um blacks prepared in z/~ T o r t argon are shown in Fig. 1. The surfaces are reasonably selective, and reflectance maxima and minima due to interference effects are evident. Th e total emittance e at r o o m temperature of blacks prepared in ~/~ T o r t argon were measured using a Gier Dunkle r e f l e c t o m e t e r which was calibrated using a gold surface.

XIFtm Fig. 1. T h e reflectance normalized to copper vs. wavelength curves for c h r o m i u m blacks prepared in ½ T o T a~Con: curve 1, a = 15 #g cm-2; curve 2, o = 26/~g era-9; curve 3, o = 44/~g c m - 2 ; curve 4, o = 62/~g c m - 2 ; curve 5, a = 78 ~ c m -2.

T h e total solar absorption coefficients a s at 300 K were calculated from reflectances using solar irradiance data for air mass 1. Emittances and absorption coefficients are p l o t t e d against mass thickness a in Fig. 2. These results suggest that, f or mass thickness o ~ 20 /~g cm -2, the solar absorptance as at 300 K is approximately 80%, and the emittance e is approximately 5%. Higher values o f absorptance are obtained f o r greater mass thickness b u t at the expense o f higher emittance.

CHROMIUM

BLACK

SOLAR ABSORBERS

111

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Fig. 2. T h e t o t a l solar a b s o r p t a n c e (÷) and the t o t a l e m i t t a n c e ( e ) at r o o m t e m p e r a t u r e mass t h i c k n e s s for c h r o m i u m black films prepared in 1A Tort argon.

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kl~m Fig. 3. The reflectance normalized to copper vs. wavelength curves for chromium blacks prepared in 3 T o ~ argon: curve 1, o = 14 ~ug c m - 2 ; curve 2, a = 37 /.Lg c m - 2 ; curve 3, o ffi 59 pg c m - 2 ; curve 4, (; = 8 5 / l g c m -2.

The reflectance v e r s u s wavelength curves o f typical blacks manufactured in a 3 Tort argon atmosphere are shown in Fig. 3. These surfaces are also selective, but reflectance maxima and minima are n o t evident. Relatively few results have been obtained for the chromium blacks prepared in 3 Tort argon because o f their tendency to oxidize rapidly in air. Five films were manufactured in argon at a pressure of 10 Tort. Their reflectances ave qualitatively similar to the blacks prepared at 3 Torr in that reflectance oscillations am n o t evident.

112

G . L . HARDING

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Fig. 5. Absorptance v s . wavelength curves for chromium black surfaces near their cut-on. Chromium blacks prepared in ½ T o ~ argon: o, 15 Ng c m - 2 ; e, 26 ~ c m - 2 ; +, 44 I.lg cm -2. Chromium black prepared in 3 Torr argon: A 37/Jg cm -2.

A graph o f log(1 - - a s ) v e r s u s o shown in Fig. 4 for the ~ Torr blacks is approximately linear. Such a linear relationship is typical o f a bulk absorber; however, at the largest wavelength studied ( 1 5 ~m) a relationship o f the form as = o ~ exists with fl = 2.0 ± 0.1 for b o t h the z~ Tort and the 3 Tort blacks. This relationship is typical o f an interference filter near its cut-on. In Fig. 5, the absorptance v e r s u s wavelength curves are plotted o n a l o g - l o g scale for z~ Tort chromium blacks o f mass thickness 15, 26 and 4 4 ~g c m - 2 ; as cc k -c with c = 2.0 ± 0.2 in a wavelength range near their cuton. This is again typical o f interference filters. However, considerably lower

CHROMIUM BLACK SOLAR ABSORBERS

113

values of the exponent c are obtained for the 3 Tort blacks, e.g. c = 1.2 + 0.2 for a 3 Tort black of mass thickness 37/~g cm-2 (Fig. 5). For the ranges of mass thickness studied the values of electrical resistance per square are 104-10 s f~ for blacks prepared at lh Tort and 10 sI0 s f~ for blacks prepared at 3 Torr. These resistances are considerably lower than those of highly selective gold blacks (approximately 10 z° f~) discussed elsewhere 4.

4. S TR UC TUR E OF CHROMIUM BLACKS

The chromium black deposits on nitroceUulose~overed copper grids were studied using transmission electron microscopy. Electron diffraction patterns obtained for the blacks are typical of chromium metal. However, the high electrical resistances obtained and the tendency for chromium to undergo reactive evaporation w h e n minute quantities of oxygen are present in the evaporation chamber suggest that the blacks m a y be a low oxide of chromium or m a y consist of metal particles embedded in a chromium oxide matrix. Thin films of chromium oxide prepared by reactive evapora-

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Fig. 6. A transmission electron micrograph of a chromium black prepared in 1~ Tort argon. Fig. 7. A transmission electron micrograph of a chromium black prepared in 3 Tort argon.

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G.L. HARDING

tion o f chromium in low pressures ( 1 0 - 5 0 m T orr) o f oxygen were f o u n d to p r o d u c e electron diffraction patterns almost identical with t hat o f c h r o miu m metal. A transmission electron micrograph o f a 1/~ T o r r black o f mass thickness 15 ~g cm -2 (Fig. 6) shows clumps o f crystallites approximately 150 A in diameter o n a continuous background. These m ay be metal particles in a background o f chr om i um oxide. A micrograph of a 3 T o r r black (Fig. 7) shows clumps o f crystallites similar to the lh T o r r black; however, there is no material in the background. This may be c o n n e c t e d with the absence o f reflectance max im a and minima and the higher resistance for the 3 T o r r blacks.

5. E F F E C T O F A N N E A L I N G

IN V A C U U M

AND EXPOSURE

TO AIR

Chromium blacks o f several mass thicknesses were aged in a vacuum o f less than 10 -e T o r t at temperatures up t o 500 °C. No significant changes occurred in any o f the surfaces after 36 h at 200 °C. The reflectance o f a 26 ~g cm -2 specimen (as ~ 88%) increased after a f u r t h e r 30 h at 350 °C and 72 h at 500 °C (Fig. 8). A f t e r 30 h at 350 °C the absorptance decreased to 80%; however the emittance also decreased significantly. Prolonged exposure (8 months) o f some films t o air has resulted in a b o u t 5% decrease in reflectance between 1 and 15 ~m (and hence a considerable increase o f emittance), due t o corrosion o f the c o p p e r substrate. 6. DISCUSSION AND CONCLUSION T h e absorption o f metal black films can be calculated in certain cases, e.g. th e properties o f highly selective gold black films m ay be explained on 1-0 ~.8

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Fig. 8. The reflectance v s . wavelength curves for a 26 ~g c m

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in ½ Torr argon: ~ , reflectance i m m e d i a t e l y after preparation; - - - , reflectance after 30 h at 350 °C in v a c u u m ; , reflectance after 7 2 h at 5 0 0 °C in vacuum.

CHROMIUM

BLACK SOLAR ABSORBERS

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the basis of Mie scattering for isolated conducting spheres 4. The.reflectance data for chromium blacks suggest that interference effects play an important part in their absorption mechanism; however, at present we have insufficient knowledge of the precise structure of chromium black to construct a model of the absorption processes which result in the selective property. The deficiencies of this selective surface are its fragility, its slow deterioration in air and its moderate value of absorptance if low emittance is required. Advantages o f this t y p e o f surface are the ease of deposition and the fact that thickness o f the film is n o t a critical factor in determining its selectivity.

ACKNOWLEDGMENTS

The author would like to thank Dr. B. Window, Dr. D. R. McKenzie, Mr. C. Horwitz and Dr. J. H. Cockayne for advice and assistance during the course of this work.

REFERENCES 1 B. O. Seraphin and A. B. Meinel, in B. O. Seraphin (ed.), Solids -- New Developments, North-Holland Publ. Co., Amsterdam, 1975. 2 L. Harris, D. Jeffries and B. M. Siegel, J. Opt. Soc. Am., 38 (1948) 582. 3 L. Harris, The Optical Properties o f Metal Blacks and Carbon Blacks, The Eppley F o u n d a t i o n for Research, Newport, R.I., Monograph Series No. 1, 1967. 4 D. R. McKenzie, J. Opt. Soc. Am., 66 (1976) 249.