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Oxidation and diffusion in black chrome selective solar absorber coatings
Thin Solid Films, 177 (1989) 95-105 ELECTRONICS AND OPTICS
95
OXIDATION AND DIFFUSION IN BLACK CHROME SELECTIVE SOLAR ABSORBER COATINGS* P. H. HOLLOWAY,K. SHANKER,G. A. ALEXANDERAND LI DE SEDAS Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 (U.S.A.)
(ReceivedJuly 7, 1989)
Black chromium selective solar absorber coatings deposited by sputtering and :lectrodeposition have been studied after heat treating. Both sputter-deposited .~,1203-Cr and electrodeposited C r 2 O a - C r black chromium coatings degrade at temperatures below 400 °C by oxidation of chromium to Cr2Oa in the coating. At higher temperatures, degradation by diffusion of the substrate occurs when the C r 2 O a - C r coating is deposited on mild steel, nickel or stainless steel. Electrodeposited coatings on substrates of tin, copper, and tantalum degraded at temperatures below 450 °C by oxidation of the substrate.
1. INTRODUCTION Black chrome has frequently been used as a selective solar absorber coating 1"2 which has high absorptance over the visible spectrum but low emittance over the I R region where black body radiation occurs at low temperatures. Black chrome has been deposited a number of ways, but the two most c o m m o n methods are electroplating and sputter deposition 1'2. In either case, it is well established that the deposited layer consists of a mixture of metallic chromium and oxide (perhaps with some chromium hydroxide present in electrodeposited coatings), usually with a gradient from high metallic content near the substrate to high oxide content near the interface with the ambient. While black chrome performs well near room temperature over lifetimes of m a n y years, at elevated temperatures (about 300-350 °C) a degradation of solar absorptance and emittance was observed in early films after only a few hundred hours in air 3'4. This degradation has been shown to result in electrodeposited films from oxidation of metallic chromium to C r 2 0 3 5.6. Once the chromium concentration decreased below about 30~o, the absorptance decreased, resulting in less desirable selective solar absorber properties. Since black chrome coatings are used in concentrator systems, this lifetime was too short. In a series of reports, Pettit and Sowell 3'~ and Shanker and Holloway 6 showed how the electrochemical bath conditions could be modified to improve the * Paper submitted for the Memorial Issue of Thin SolidFilms in honour of Dr. John Thornton and Professor Christian Weissmantel, posthumously;paper not included owing to editorial error. 0040-6090/89/$3.50
© ElsevierSequoia/Printed in The Netherlands
96
P . H . HOLLOWAY e t
al.
lifetime of electrodeposited films. However, factors limiting the lifetime of sputterdeposited films have not been studied in such depth; neither have the degradation mechanism(s) been identified. The purpose of this research was to investigate these degradation mechanisms in sputter-deposited black chrome, and further to determine the temperature at which substrate diffusion became a problem for both sputter-deposited and electrodeposited coatings. 2. EXPERIMENTAL DETAILS
The sputter-deposited samples were supplied by Telic Corporation and consisted of a 6.75 cm x 3.75 cm glass substrate covered by 1500 A of low emittance magnetron-sputter-deposited copper. The absorber layer consisted of A1203-Cr cermet about 1000/~ thick. The cermet was graded with a chromium-rich layer over the first 500 A and an increasing AI20 3 content over the last 500/~. The coating was co-deposited from two cylindrical magnetron sources operated in either the r.f. (A1203) or the D.C. (chromium) mode. The solar absorptance was 0.9 and the thermal emittance was 0.09. For electrodeposition, coatings were deposited onto substrates of tin, copper, tantalum, mild steel, bright nickel, nickel with a chromium flash, and stainless steel at 175 A f t -2 at 20-26 °C using a Harshaw Chemical Company Chromonyx bath with carefully controlled Cr 3 + concentrations. Details of the electroplating are described elsewhere 3'4. In all cases, the samples were heated in an open tube furnace in air to temperatures between 150 and 600 °C for a standard time of 24h. After heat treatment, the samples were examined visually and analyzed using sputter profiling Auger electron spectroscopy (AES). Sputter profiling was accomplished in a PHI Thin Film Analysis system using primary electrons of 3 keV with a peak-topeak modulation voltage of 6 eV. Argon ions (2 keV) were used for sputter depth profiling with the chamber backfilled to a pressure of 4 x 10- 5 Torr. 3.
RESULTS AND DISCUSSION
Auger data showed that the surfaces of as-deposited black chrome films were always contaminated with carbon, nitrogen and sulfur, while chlorine, calcium, potassium, sodium, iron and silicon were observed occasionally. Most of these contaminants were restricted to the surface since very short sputter times (about 30 s) were sufficient to cause their removal. The sputter depth profiles from sputter-deposited Al2Oa-Cr films are shown in Fig. 1 for the as-deposited and heat-treated conditions. The as-deposited film consisted of a pure AI20 a outer layer, a mixed Al2Oa-Cr region, and a region adjacent to the copper where very little oxidation of the chromium occurred during deposition. After heating to 395 °C for 24 h, the coatings took on a gray (rather than the initial dark black) appearance, and the data in Fig. l(c) show that oxidation of the chromium has taken place, similar to degradation of electrodeposited black chrome. It should be noted that the A120 a surface and copper interface layers are still intact. However, after treatment at 450°C for 24h, not only is chromium converted to Cr20 a, resulting in a gray appearance, but the copper layer was severely oxidized, as evident from the serious flaking and peeling of the coating from
97
BLACK CHROME SELECTIVE SOLAR ABSORBERS
the glass substrate and the observation of black to green copper oxides adhering to the back of the coating and to the substrate. It should be noted that while the copper layer is obviously being oxidized at 395 °C (as evident from the broadened peak and higher oxygen Auger peak-to-peak height in this region) the copper is not diffusing into or through the cermet• Thus degradation of this sputter-deposited black chromium coating appears to proceed by conversion of metal to oxide similarly to the electrodeposited coatings at low temperature, but by oxidation of the copper layer at higher temperatures.
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Fig. 1. Sputter profiles of Al2Oa-Cr on copper on glass (a) as deposited and after heating for 24 h in air at (b) 350 °C, (c) 395 °C and (d) 450 °C.
For black chrome electrodeposited on tin, the coatings were thermally stable at temperatures below the melting point of tin (232 °C). No tin was detected on the surface of the as-deposited or heated (200 °C or above) black chromium• Above the melting point, tin was found distributed through the depth of the coating. This could be due either to penetration of pores in the coating by liquid or to diffusion of tin through the coating• The former explanation is considered the most likely. Tantalum was a stable substrate for temperatures below 450 °C. At temperatures greater than 540 °C, both the tantalum and the black chromium were oxidized. The tantalum was converted to a white powder which crumbled easily when touched• The black chromium was converted to a translucent solid with a green color (presumably Cr203). The reason for this catastrophic conversion of tantalum
98
P.H. HOLLOWAYet al.
to oxide is unknown, although tantalum is reported to form non-protective oxides for T > 500 °C 1, s Data illustrating the thermal stability of copper as a substrate for black chromium coatings are shown in Figs. 2 and 3. In Fig. 2, AES spectra are shown for the surface of black chromium coatings on a copper foil in the unheated condition and after heating for 24h at 355 and 396 °C. In the unheated condition, the black chromium was contaminated with a very low level of iron, but no copper was detected on the outer surface. At 355 °C copper was just detectable on the outer surface, which was also contaminated with some silicon and carbon. After heating for 24 h at 396 °C, the outer surface was completely copper oxide; no chromium was detectable on the outer surface. After heating for 24 h at 450 °C, the black chromium coating was observed to spall from the copper foil. The AES spectra from the outer surface and the black surface of the black chromium coating and from the surface of the copper foil are shown in Fig. 3. These data show that the outer surface of the black chromium was completely covered with copper oxide, as was the case after heating for 24 h at 395 °C. In addition, the back surface of the chromium oxide was
C
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x5
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FeFe 82
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I 500 Electron
I 1000 Energy
(eV)
Fig. 2. Auger electron spectra from black chromium deposited on copper foil: spectrum a, unheated; spectrum b, heated in air to 355 °C for 24 h; spectrumc, heated in air to 396 °C for 24 h.
BLACK CHROME SELECTIVE SOLAR ABSORBERS
99
heavily contaminated with copper oxide, although some chromium was still detected at the back surface. The data also show that the surface of the copper foil was completely oxidized, although several contaminants, including sulfur, chlorine, carbon and nitrogen, were detected on the copper foil after oxidation. Thus the thermal stability of black chromium coatings on copper substrates is poor. For extended lifetimes, absorbers with black chromium on copper would have to be used at temperatures well below 350 °C.
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Fig. 3. Auger electron spectra after heating a black chromium coating deposited on copper foil at 450 °C for 24 h in air: spectrum a, front surface of black chromium; spectrum b, back surface of black chromium when spalled from the copper substrate; spectrum c, oxidized surface of the copper foil substrate after the black chromium spalled.
Black chromium coatings electrodeposited onto nickel, mild steel or stainless steel all exhibited good stability with respect to diffusion of the substrate material. However, in every case, heating to a temperature of 450 °C or more for 24 h caused the black film to change to a brown-gray color, indicating that the absorptance had decreased. This change resulted from oxidation of chromium to Cr20 3 5.6 rather
P.H. HOLLOWAY et al.
100
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I 1200
;
Fig. 4. Auger electron spectra from black chromium deposited on bright nickel: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
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BLACK CHROME SELECTIVE SOLAR ABSORBERS
101
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I 0
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01
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Electron Energy (eV) Fig. 5. Auger electron spectra from black chromium depostted on nickel with an electrodeposited chromium flash: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
102
P.H. HOLLOWAYet aL
than substrate diffusion (see below). Thus, for these substrates, oxidation caused degradation before substrate diffusion occurred. However, it is still of interest to determine the temperature at which such diffusion will occur. Auger spectra for black chromium on electrodeposited bright nickel are shown in Fig. 4. At room temperature, some chlorine, carbon and iron contamination was observed on the black chromium surface. The iron contamination resulted from handling procedures used in preparing the samples for heat treatment. Spectra in Fig. 4 show that the black chromium on bright nickel was stable after 24 h at 550 °C. However, spectrum c in Fig. 4 shows that nickel was observed on the surface of the black chromium after 24 h at 650 °C. Black chromium was electrodeposited onto nickel with an intermediate electrodeposited chromium flash to determine whether such a barrier layer at the interface might stabilize the nickel substrates towards diffusion (Fig. 5). Iron contamination was detected on the unheated surface, but no nickel was detected after heat treating for 24 h at 550 °C in air. After heat treating for 24 h at 650 °C in air, iron and coper oxides were observed on the surface with very little or no nickel. It appears that the substrate was contaminated during electrodeposition of the chromium flash, presumably by deposition of iron and copper, and these species have preferentially diffused to the black chromium surface. It should be noted that the AES spectrum in Fig. 5, spectrum c, indicates that no chromium is present on the surface after heat treatment. Sputter profiling was used to determine qualitatively the thickness of the iron and copper oxide on the black chromium surface. Even after sputtering for 5 min (approximately 500.~) there was no detectable black chromium layer so the contaminant layer was thick. Mild steel was a much less stable substrate than was nickel for the black chromium coating. The data in Fig. 6 show that iron oxide was formed on the surface after heating for 24 h in air at temperatures of 500 °C or higher. Thus iron will leave the mild steel substrate and diffuse to the surface to form an oxide at much lower temperatures than for a nickel substrate. Finally, stainless steel proved to be the most stable substrate for the black chromium coatings. Data in Fig. 7 show that the non-heated black chromium film on stainless steel again had small contamination levels from iron and carbon. The spectrum in Fig. 7 shows that, after heating for 24 h in air at 550 °C, no segregation of iron to the black chromium surface was detected. There was an extremely small amount of manganese detected on the surface after this heat treatment, and Fig. 7, spectrum c, shows that the concentration of manganese increased after heat treatment for 24 h at 650 °C. However, chromium was still detected on the surface after the 650 °C heat treatment, and the concentration of manganese was relatively low (approximately one monolayer). This contrasts with the results of all the other substrates where at 650 °C the surface contamination due to oxide formation of the substrate material was sufficient to reduce the surface chromium concentration to a lower level than for stainless steel substrates. 4. SUMMARY Degradation of sputter-deposited
A1203-Cr and electrodeposited Cr203-fr
BLACK CHROME SELECTIVE SOLAR ABSORBERS
103
/-4o Fe
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I I 800 Energy {eV)
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Fig. 6. Auger electron spectra from black chromium deposited on mild steel and heated for 24h at 550 °C (spectrum a) and 650 °C (spectrum b).
P . H . HOLLOWAY et al.
104
~~
r Mn
0
LU "0 Z "0
Cr Mn ~r
l° Cr+Mn
3r
I 0
I 01 400 Electron
i 800 Energy
I
I 1200
I
(eV)
Fig. 7. Auger electron spectra from black c h r o m i u m deposited on stainless steel: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
BLACK CHROME SELECTIVE SOLAR ABSORBERS
105
black chromium solar absorbed coatings by oxidation and substrate diffusion has been studied. Auger electron data shown that oxidation of chromium to Cr2Oa is the dominant low temperature degradation mechanism in both types of coatings. Deposition conditions can be changed to improve the temperature stability. At temperatures of 450°C or more after 24h in air, both sputter-deposited and electrodeposited coatings degraded by diffusion. With respect to degradation by substrate stability and diffusion, the data indicate that tin, copper and tantalum are unlikely to be useful substrate materials for black chromium selective solar absorber coatings. Mild steel is a relatively cheap substrate material for such coatings and therefore is attractive, but its thermal stability is not as good as those of nickel and stainless steel. Both nickel and stainless steel substrates exhibited little diffusion of the substrate material to the black chromium coating's surface at temperatures of 550 °C for 24 h. However, both exhibited such diffusion at 650 °C with the amount from stainless steel being much less than that from nickel. An electrodeposited chromium flash on the nickel substrate contained impurities which diffused at a temperature of 650 °C and limited its usefulness in stabilizing the coating-substrate interaction. On comparing the temperatures at which substrate diffusion occurs with that for oxidation of the metal within the black chromium coatings, it is obvious that degradation by diffusion occurs at higher temperatures. Thus oxidation is the primary degradation mechanism of the black chromium coatings. However, if the microstructure and composition of the black chromium coatings are modified to reduce the amount of oxidation which occurs, then diffusion of substrate material may become an important degradation mechanism for black chromium selective solar absorbers. ACK~NOWLEDGMENTS
The sputter-deposited black chromium sample was supplied by Dr. John Thornton when he was at Telic Corporation. The electrodeposited samples were supplied by Drs. R. Sowell and R. Petit of Sandia National Laboratories. Support by the Solar Energy Research Institute is gratefully acknowledged. REFERENCES I O.P. Agnihotri and B. K. Gupta, Solar Selective Surfaces, Wiley, New York, 1981. 2 B.O. Seraphin, in B. O. Seraphin (ed.), Topics in Applied Physics, Vol. 31, Springer, New York, 1979. 3 R.B. Pettit and R. R. Sowell, Proc. 1979 Int. Solar Energy Society Congr., A tlanta, GA, May 28-June 1, 1979. 4 R.B. Pettit and R. R. Sowell, Proc. 2nd Annu. Conf. on Absorber Surfaces for Solar Receivers, Boulder, CO, January 24, 1979, in Rep. SERI/TP-69-182m, 1979 (Solar Energy Research Institute). 5 P.H.. Holloway, K. Shanker, R. B. Penit, and R. R. Sowell, Thin Solid Films, 72 (1980) 121. 6 K. Shanker and P. H. Holloway, Thin Solid Films, 127(1985) 181.
95
OXIDATION AND DIFFUSION IN BLACK CHROME SELECTIVE SOLAR ABSORBER COATINGS* P. H. HOLLOWAY,K. SHANKER,G. A. ALEXANDERAND LI DE SEDAS Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 (U.S.A.)
(ReceivedJuly 7, 1989)
Black chromium selective solar absorber coatings deposited by sputtering and :lectrodeposition have been studied after heat treating. Both sputter-deposited .~,1203-Cr and electrodeposited C r 2 O a - C r black chromium coatings degrade at temperatures below 400 °C by oxidation of chromium to Cr2Oa in the coating. At higher temperatures, degradation by diffusion of the substrate occurs when the C r 2 O a - C r coating is deposited on mild steel, nickel or stainless steel. Electrodeposited coatings on substrates of tin, copper, and tantalum degraded at temperatures below 450 °C by oxidation of the substrate.
1. INTRODUCTION Black chrome has frequently been used as a selective solar absorber coating 1"2 which has high absorptance over the visible spectrum but low emittance over the I R region where black body radiation occurs at low temperatures. Black chrome has been deposited a number of ways, but the two most c o m m o n methods are electroplating and sputter deposition 1'2. In either case, it is well established that the deposited layer consists of a mixture of metallic chromium and oxide (perhaps with some chromium hydroxide present in electrodeposited coatings), usually with a gradient from high metallic content near the substrate to high oxide content near the interface with the ambient. While black chrome performs well near room temperature over lifetimes of m a n y years, at elevated temperatures (about 300-350 °C) a degradation of solar absorptance and emittance was observed in early films after only a few hundred hours in air 3'4. This degradation has been shown to result in electrodeposited films from oxidation of metallic chromium to C r 2 0 3 5.6. Once the chromium concentration decreased below about 30~o, the absorptance decreased, resulting in less desirable selective solar absorber properties. Since black chrome coatings are used in concentrator systems, this lifetime was too short. In a series of reports, Pettit and Sowell 3'~ and Shanker and Holloway 6 showed how the electrochemical bath conditions could be modified to improve the * Paper submitted for the Memorial Issue of Thin SolidFilms in honour of Dr. John Thornton and Professor Christian Weissmantel, posthumously;paper not included owing to editorial error. 0040-6090/89/$3.50
© ElsevierSequoia/Printed in The Netherlands
96
P . H . HOLLOWAY e t
al.
lifetime of electrodeposited films. However, factors limiting the lifetime of sputterdeposited films have not been studied in such depth; neither have the degradation mechanism(s) been identified. The purpose of this research was to investigate these degradation mechanisms in sputter-deposited black chrome, and further to determine the temperature at which substrate diffusion became a problem for both sputter-deposited and electrodeposited coatings. 2. EXPERIMENTAL DETAILS
The sputter-deposited samples were supplied by Telic Corporation and consisted of a 6.75 cm x 3.75 cm glass substrate covered by 1500 A of low emittance magnetron-sputter-deposited copper. The absorber layer consisted of A1203-Cr cermet about 1000/~ thick. The cermet was graded with a chromium-rich layer over the first 500 A and an increasing AI20 3 content over the last 500/~. The coating was co-deposited from two cylindrical magnetron sources operated in either the r.f. (A1203) or the D.C. (chromium) mode. The solar absorptance was 0.9 and the thermal emittance was 0.09. For electrodeposition, coatings were deposited onto substrates of tin, copper, tantalum, mild steel, bright nickel, nickel with a chromium flash, and stainless steel at 175 A f t -2 at 20-26 °C using a Harshaw Chemical Company Chromonyx bath with carefully controlled Cr 3 + concentrations. Details of the electroplating are described elsewhere 3'4. In all cases, the samples were heated in an open tube furnace in air to temperatures between 150 and 600 °C for a standard time of 24h. After heat treatment, the samples were examined visually and analyzed using sputter profiling Auger electron spectroscopy (AES). Sputter profiling was accomplished in a PHI Thin Film Analysis system using primary electrons of 3 keV with a peak-topeak modulation voltage of 6 eV. Argon ions (2 keV) were used for sputter depth profiling with the chamber backfilled to a pressure of 4 x 10- 5 Torr. 3.
RESULTS AND DISCUSSION
Auger data showed that the surfaces of as-deposited black chrome films were always contaminated with carbon, nitrogen and sulfur, while chlorine, calcium, potassium, sodium, iron and silicon were observed occasionally. Most of these contaminants were restricted to the surface since very short sputter times (about 30 s) were sufficient to cause their removal. The sputter depth profiles from sputter-deposited Al2Oa-Cr films are shown in Fig. 1 for the as-deposited and heat-treated conditions. The as-deposited film consisted of a pure AI20 a outer layer, a mixed Al2Oa-Cr region, and a region adjacent to the copper where very little oxidation of the chromium occurred during deposition. After heating to 395 °C for 24 h, the coatings took on a gray (rather than the initial dark black) appearance, and the data in Fig. l(c) show that oxidation of the chromium has taken place, similar to degradation of electrodeposited black chrome. It should be noted that the A120 a surface and copper interface layers are still intact. However, after treatment at 450°C for 24h, not only is chromium converted to Cr20 a, resulting in a gray appearance, but the copper layer was severely oxidized, as evident from the serious flaking and peeling of the coating from
97
BLACK CHROME SELECTIVE SOLAR ABSORBERS
the glass substrate and the observation of black to green copper oxides adhering to the back of the coating and to the substrate. It should be noted that while the copper layer is obviously being oxidized at 395 °C (as evident from the broadened peak and higher oxygen Auger peak-to-peak height in this region) the copper is not diffusing into or through the cermet• Thus degradation of this sputter-deposited black chromium coating appears to proceed by conversion of metal to oxide similarly to the electrodeposited coatings at low temperature, but by oxidation of the copper layer at higher temperatures.
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(d)
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80
120
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Fig. 1. Sputter profiles of Al2Oa-Cr on copper on glass (a) as deposited and after heating for 24 h in air at (b) 350 °C, (c) 395 °C and (d) 450 °C.
For black chrome electrodeposited on tin, the coatings were thermally stable at temperatures below the melting point of tin (232 °C). No tin was detected on the surface of the as-deposited or heated (200 °C or above) black chromium• Above the melting point, tin was found distributed through the depth of the coating. This could be due either to penetration of pores in the coating by liquid or to diffusion of tin through the coating• The former explanation is considered the most likely. Tantalum was a stable substrate for temperatures below 450 °C. At temperatures greater than 540 °C, both the tantalum and the black chromium were oxidized. The tantalum was converted to a white powder which crumbled easily when touched• The black chromium was converted to a translucent solid with a green color (presumably Cr203). The reason for this catastrophic conversion of tantalum
98
P.H. HOLLOWAYet al.
to oxide is unknown, although tantalum is reported to form non-protective oxides for T > 500 °C 1, s Data illustrating the thermal stability of copper as a substrate for black chromium coatings are shown in Figs. 2 and 3. In Fig. 2, AES spectra are shown for the surface of black chromium coatings on a copper foil in the unheated condition and after heating for 24h at 355 and 396 °C. In the unheated condition, the black chromium was contaminated with a very low level of iron, but no copper was detected on the outer surface. At 355 °C copper was just detectable on the outer surface, which was also contaminated with some silicon and carbon. After heating for 24 h at 396 °C, the outer surface was completely copper oxide; no chromium was detectable on the outer surface. After heating for 24 h at 450 °C, the black chromium coating was observed to spall from the copper foil. The AES spectra from the outer surface and the black surface of the black chromium coating and from the surface of the copper foil are shown in Fig. 3. These data show that the outer surface of the black chromium was completely covered with copper oxide, as was the case after heating for 24 h at 395 °C. In addition, the back surface of the chromium oxide was
C
uJ x5
Z 1o
4i
x5
Si
FeFe 82
xl
Cu
0
0
I 500 Electron
I 1000 Energy
(eV)
Fig. 2. Auger electron spectra from black chromium deposited on copper foil: spectrum a, unheated; spectrum b, heated in air to 355 °C for 24 h; spectrumc, heated in air to 396 °C for 24 h.
BLACK CHROME SELECTIVE SOLAR ABSORBERS
99
heavily contaminated with copper oxide, although some chromium was still detected at the back surface. The data also show that the surface of the copper foil was completely oxidized, although several contaminants, including sulfur, chlorine, carbon and nitrogen, were detected on the copper foil after oxidation. Thus the thermal stability of black chromium coatings on copper substrates is poor. For extended lifetimes, absorbers with black chromium on copper would have to be used at temperatures well below 350 °C.
Ca V O
Cu
LM "ID Z qD
C
Cr
I Cu
Cu
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Cu
0
0
Cu
I 1000
500 Electron
Energy
(eV)
Fig. 3. Auger electron spectra after heating a black chromium coating deposited on copper foil at 450 °C for 24 h in air: spectrum a, front surface of black chromium; spectrum b, back surface of black chromium when spalled from the copper substrate; spectrum c, oxidized surface of the copper foil substrate after the black chromium spalled.
Black chromium coatings electrodeposited onto nickel, mild steel or stainless steel all exhibited good stability with respect to diffusion of the substrate material. However, in every case, heating to a temperature of 450 °C or more for 24 h caused the black film to change to a brown-gray color, indicating that the absorptance had decreased. This change resulted from oxidation of chromium to Cr20 3 5.6 rather
P.H. HOLLOWAY et al.
100
xlO /
a
iCrfV re Fe
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Z
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b
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Cr
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I 1200
;
Fig. 4. Auger electron spectra from black chromium deposited on bright nickel: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
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BLACK CHROME SELECTIVE SOLAR ABSORBERS
101
O xs
Cr Cr O
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01
I 800
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Electron Energy (eV) Fig. 5. Auger electron spectra from black chromium depostted on nickel with an electrodeposited chromium flash: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
102
P.H. HOLLOWAYet aL
than substrate diffusion (see below). Thus, for these substrates, oxidation caused degradation before substrate diffusion occurred. However, it is still of interest to determine the temperature at which such diffusion will occur. Auger spectra for black chromium on electrodeposited bright nickel are shown in Fig. 4. At room temperature, some chlorine, carbon and iron contamination was observed on the black chromium surface. The iron contamination resulted from handling procedures used in preparing the samples for heat treatment. Spectra in Fig. 4 show that the black chromium on bright nickel was stable after 24 h at 550 °C. However, spectrum c in Fig. 4 shows that nickel was observed on the surface of the black chromium after 24 h at 650 °C. Black chromium was electrodeposited onto nickel with an intermediate electrodeposited chromium flash to determine whether such a barrier layer at the interface might stabilize the nickel substrates towards diffusion (Fig. 5). Iron contamination was detected on the unheated surface, but no nickel was detected after heat treating for 24 h at 550 °C in air. After heat treating for 24 h at 650 °C in air, iron and coper oxides were observed on the surface with very little or no nickel. It appears that the substrate was contaminated during electrodeposition of the chromium flash, presumably by deposition of iron and copper, and these species have preferentially diffused to the black chromium surface. It should be noted that the AES spectrum in Fig. 5, spectrum c, indicates that no chromium is present on the surface after heat treatment. Sputter profiling was used to determine qualitatively the thickness of the iron and copper oxide on the black chromium surface. Even after sputtering for 5 min (approximately 500.~) there was no detectable black chromium layer so the contaminant layer was thick. Mild steel was a much less stable substrate than was nickel for the black chromium coating. The data in Fig. 6 show that iron oxide was formed on the surface after heating for 24 h in air at temperatures of 500 °C or higher. Thus iron will leave the mild steel substrate and diffuse to the surface to form an oxide at much lower temperatures than for a nickel substrate. Finally, stainless steel proved to be the most stable substrate for the black chromium coatings. Data in Fig. 7 show that the non-heated black chromium film on stainless steel again had small contamination levels from iron and carbon. The spectrum in Fig. 7 shows that, after heating for 24 h in air at 550 °C, no segregation of iron to the black chromium surface was detected. There was an extremely small amount of manganese detected on the surface after this heat treatment, and Fig. 7, spectrum c, shows that the concentration of manganese increased after heat treatment for 24 h at 650 °C. However, chromium was still detected on the surface after the 650 °C heat treatment, and the concentration of manganese was relatively low (approximately one monolayer). This contrasts with the results of all the other substrates where at 650 °C the surface contamination due to oxide formation of the substrate material was sufficient to reduce the surface chromium concentration to a lower level than for stainless steel substrates. 4. SUMMARY Degradation of sputter-deposited
A1203-Cr and electrodeposited Cr203-fr
BLACK CHROME SELECTIVE SOLAR ABSORBERS
103
/-4o Fe
LLI
O
Z "U
!S
I
~e
Fe
O
0
I
I 400 Electron
I
I I 800 Energy {eV)
I 1200
I
Fig. 6. Auger electron spectra from black chromium deposited on mild steel and heated for 24h at 550 °C (spectrum a) and 650 °C (spectrum b).
P . H . HOLLOWAY et al.
104
~~
r Mn
0
LU "0 Z "0
Cr Mn ~r
l° Cr+Mn
3r
I 0
I 01 400 Electron
i 800 Energy
I
I 1200
I
(eV)
Fig. 7. Auger electron spectra from black c h r o m i u m deposited on stainless steel: spectrum a, unheated; spectrum b, heated for 24 h at 550 °C; spectrum c, heated for 24 h at 650 °C.
BLACK CHROME SELECTIVE SOLAR ABSORBERS
105
black chromium solar absorbed coatings by oxidation and substrate diffusion has been studied. Auger electron data shown that oxidation of chromium to Cr2Oa is the dominant low temperature degradation mechanism in both types of coatings. Deposition conditions can be changed to improve the temperature stability. At temperatures of 450°C or more after 24h in air, both sputter-deposited and electrodeposited coatings degraded by diffusion. With respect to degradation by substrate stability and diffusion, the data indicate that tin, copper and tantalum are unlikely to be useful substrate materials for black chromium selective solar absorber coatings. Mild steel is a relatively cheap substrate material for such coatings and therefore is attractive, but its thermal stability is not as good as those of nickel and stainless steel. Both nickel and stainless steel substrates exhibited little diffusion of the substrate material to the black chromium coating's surface at temperatures of 550 °C for 24 h. However, both exhibited such diffusion at 650 °C with the amount from stainless steel being much less than that from nickel. An electrodeposited chromium flash on the nickel substrate contained impurities which diffused at a temperature of 650 °C and limited its usefulness in stabilizing the coating-substrate interaction. On comparing the temperatures at which substrate diffusion occurs with that for oxidation of the metal within the black chromium coatings, it is obvious that degradation by diffusion occurs at higher temperatures. Thus oxidation is the primary degradation mechanism of the black chromium coatings. However, if the microstructure and composition of the black chromium coatings are modified to reduce the amount of oxidation which occurs, then diffusion of substrate material may become an important degradation mechanism for black chromium selective solar absorbers. ACK~NOWLEDGMENTS
The sputter-deposited black chromium sample was supplied by Dr. John Thornton when he was at Telic Corporation. The electrodeposited samples were supplied by Drs. R. Sowell and R. Petit of Sandia National Laboratories. Support by the Solar Energy Research Institute is gratefully acknowledged. REFERENCES I O.P. Agnihotri and B. K. Gupta, Solar Selective Surfaces, Wiley, New York, 1981. 2 B.O. Seraphin, in B. O. Seraphin (ed.), Topics in Applied Physics, Vol. 31, Springer, New York, 1979. 3 R.B. Pettit and R. R. Sowell, Proc. 1979 Int. Solar Energy Society Congr., A tlanta, GA, May 28-June 1, 1979. 4 R.B. Pettit and R. R. Sowell, Proc. 2nd Annu. Conf. on Absorber Surfaces for Solar Receivers, Boulder, CO, January 24, 1979, in Rep. SERI/TP-69-182m, 1979 (Solar Energy Research Institute). 5 P.H.. Holloway, K. Shanker, R. B. Penit, and R. R. Sowell, Thin Solid Films, 72 (1980) 121. 6 K. Shanker and P. H. Holloway, Thin Solid Films, 127(1985) 181.