Optical and microstructural analyses of a chemically converted textured black copper selective surface

Solar Energy Materials 20 (1990) 199-214 North-Holland

199

OPTICAL AND MICROSTRUCTURAL ANALYSES OF A CHEMICALLY CONVERTED TEXTURED BLACK COPPER SELECTIVE SURFACE Purnima RICHHARIA Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi 110016, India Received 24 October 1988; in final form 22 November 1989 This paper presents optical and microstructural studies of a black copper selective surface produced by the chemical conversion technique. The surface exhibits a significantly high initial solar absorptance a = 0.97-0.98 and a relatively low thermal emittance q00 = 0.2 + 0.02. These surfaces are stable up to 250 ° C in both air and vacuum (10-5 Torr). The cross section of the film reveals a sponge-like porous, layered structure with thickness of the order of - 2.6-3.0/tm. The surface structure is indicative of a small-scale rough morphology. It consists of loosely packed needle-like long and fine grains of Cu dispersed in an oxide (CuO) matrix. The spacing between these grains are of the order of the solar wavelength. The high a is a cumulative effect of the trapping of light by various phenomena such as antireflecting effect, resonant scattering, texturing and the presence of a highly absorbing intrinsic semiconductor oxide (CuO). The low emittance is due to the quasi-metallic region and the underlying Cu substrate.

1. Introduction Spectrally selective absorbers displaying low reflectance (high absorptance) in the solar region 0.3-2.5 ~m and high reflectance (low emittance) in the infrared spectral region 2.5-50.0/~m play an important role in high efficiency photothermal devices [1,21. Most of the developed selective surfaces are based on the tandem effect between an absorbing surface layer and a highly reflecting metallic surface [1,3]. Black copper, being one of the oldest, is a well known and largely documented selective surface [1,4-11]. An account of various investigations on the selectivity of copper oxide surfaces developed by various oxidation techniques have been discussed at length by Roos et al. [4], Scherer et al. [9] and Milgram [12]. The traditional black copper surface is reported to be stable up to 200 o C. In this paper the results based on extensive investigations of the microstructure and optical properties of the chemically converted black copper selective surface which is stable up to 250 °C in both air and vacuum are presented. 2. Experimental details

2.1. Preparation conditions Black copper films were deposited on copper by chemically converting the surface in an alkaline bath of (lab grade) potassium persulphate (K2S208) - a 0165-1633/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

200

P. Richharia ' ( 'hemicallv corn,erred textured black copper ~'elecm,e sur/ac~

strong oxidizing agent [13-15]. The PH of the solution being one of the importan~ factors, was controlled by the addition of N a O H . The (99.9%) pure copper substraw was chemically etched (polished) in 20% H N O 3 at a temperature - 5 0 o ( ` for ~ period of 1 - 2 minutes. This was followed by rinsing in distilled water prior tt~ chemical conversion. The conversion of the Cu substrate was carried out in alkaline baths of potassium persulphate in the temperature range between 40 and 85 ° C and for times varying between 5 and 12 rain. During optimization the concentrations of the salts were kept in the range given as follows: N a O H : 40-65 g / #

and

K2S20~: 5-15 g / ( .

For an optimized deposit the solution temperature has been controlled within + 2 ° C. The growth kinetics of the film has been studied by preparing the films with varied dipping time for a typical optimized condition. The data reported in this paper are for the films prepared under the optimized conditions. The optimized conditions are as given below: K2S2Os : 14 g / ( ,

N a O H : 60 g / ( ,

temperature: 53 ± 2 ° C, time: 11 min.

2.2. Characterization techniques The total reflectance measurements were made on a Hitachi 330 double beam spectrophotometer equipped with an integrating sphere, in the range 0.3-2.5 #m. The infrared specular reflectance in the range 2.5-50.0 t~m has been measured using a Perkin Elmer double beam spectrophotometer model 683. The integrated solar absorptance ( a ) has been computed from the total reflectance for air mass 1.5 condition, while the emittance has been measured using a radiometric technique setup in our laboratory [16]. Microstructural studies were carried out using a Cambridge stereo-10 scanning electron microscope operating at an accelerating voltage of 20 keV while the transmission electron microscopy was carried out using a JEM-200CX transmission electron microscope. The sample for T E M analysis was prepared by chemically etching (in - 50% HNO3) the surface for a few seconds. The film was then floated in distilled water and lifted on a 200 mesh grid. This was then viewed in the T E M microscope. Glancing incidence diffraction ( G I D ) was performed on a Rigaku X-ray diffraction system Geigerflex D / M A X - R B equipped with a 12 kW rotating anode C u K a source model RU-200B and a thin film attachment with sample rotation facility. N o special sample preparation was required. The samples prepared at oxidation time t = 3 min and t = 11 rain have been examined at glancing incidence angle a = 3 °. A Nicolet 5DX F T I R spectrophotometer was employed to study the absorption bands present in the film. The transmittance spectra has been recorded using a KBr pellet. The copper oxide powder was mechanically removed from the surface. The oxide powder was mixed in KBr and a pellet was prepared.

P. Richharia / Chemically converted textured black copper selective surface

201

The chemical compositions of the films were studied by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). These studies were made with a PH1 model 590. A super S A M / E S C A instrument having a double-pass cylindrical mirror analyser with a coaxial electron gun and M g K a X-rays. AES depth profiling was done using a differentially pumped ion gun with Ar ÷ ions at a constant current density of approximately 600/tA/cm 2 at an energy of 5 keV, while XPS has been recorded using M g K a X-rays with energy 10 keV. Depth profiling has been done relative to Ta205 at a sputter rate of - 100 A/min. Both XPS and AES analyses were carried out at a pressure of 0.8 × 10 -9 Torr to minimize the background gas contamination during chemical analyses.

3. Results and discussion

3.1. Optical properties Potassium persulfate being a powerful oxidizer rapidly oxidizes the polished (etched) copper substrate and forms an oxide layer which becomes an integrated part of the underlying metal substrate. The process parameters, such as temperature, concentration of the alkaline bath and time of deposition, control the thickness of the oxide layer. Fig. 1 shows typical reflectance curves for films of various thickness prepared under identical conditions but for increasing oxidation times. Table 1 lists

1.0 .

t =0t

0.8

0.6 / c t= 3'

0.4

0

d

I

0.3

0.7

1.1

13

1.9

,

2.3

2.5

(;~) WAVELENGTH ( p r o ) Fig. 1. Reflectance versus wavelength of the black copper selective surface at optimized parameters with increasing oxidation time (corresponding to table 1).

202

P. Richharia / ('hemically cone,erred textured black copper selectit,e .~urli~ c'

Table 1 Optical properties of black copper selective surface at an optimized composition NaOH: 60 g / c . K2S2Os: 14 g/g', temperature = 53_+2°C with increasing immersion time t (corresponding to fig. t l a = solar absorptance, ~ = emittance Curve No.

t (min)

a

qoo ( -+0.02)

a b c d e f g h

0 l 3 5 7 9 10 11

0.16 0.31 - 0.86 0.91 0.94 - 0.96 0.96 0.97

a/q(,)

- 0.04 0.07 0.11 0.13 0.165 0.18 0.19 0.2

4.03 4.44 7.81 7.04 5.71 5.33 5.05 4.85

1.0 Q

0.8

- -

SPECULAR REFLECTANCE VE

~

~

v"

CU

o0.6 Z

f.<9

~,o.4 h hl nr

0.2

ol 1.'0

0.3

1.5

2,0 2.5 6.0 WAVELENGTH ( ~ m )

10.0

20.0 3().0' ;0.O

TEXTURED R E M O V E D WITH TEXTURED

0.3 uJ L)

,~ 0.2

....

F'¢J Itl ,,.J

- A - - , -~'4,

u. 0.1

ILl rr"

-

0.3

-

~ "

|

1-0

I

1

2.0

2.5

WAVELENGTH (iJm) Fig. 2. (a) Specular reflectance versus wavelength of the as-deposited black copper selective surface. (b) Total reflectance profile versus wavelength of the black copper selective surface. ( ) Reflectance profile of textured surface. ( - - - - - ) Reflectance spectra after removing the texture by dry rubbing (mechanically).

P. Richharia / Chemically converted textured black copper selective surface

203

the corresponding optical properties of the reflectance spectra shown in fig. 1. At t -~ 3 minutes the film appears black in colour possessing a = 0.85 as indicated in table 1 (curve c). As the oxidation time and therefore the thickness of the films is increased, the high reflectance at short wavelengths shifts towards longer wavelengths. This reduces the reflectance profile in the shorter wavelength region and hence increases the solar absorptance [17]. The reflectance is - 0.01 between 0.35 and 0.7/~m as seen from curves (e-g) in fig. 1; consequently, these films have black velvety appearance. Further on looking at fig. 1, it is evident that a certain oxide thickness is necessary to obtain high a and relatively low emittance which is achieved by adjusting the three aforesaid parameters. On further increasing the immersion time it has been observed that the growth of the film stops, indicating that the oxide layers now start acting as a protective layer for the underlying copper substrate and hence stops further oxidation. It has also been noticed that a ratio of oxidizer to N a O H concentration of 0.2 + 0.04 can produce a stable and good quality black copper films. The total and specular reflectance curves of the as-deposited film before and after removing (displacing) the texture by dry rubbing (mechanically) the surface are presented in figs. 2a and 2b. Curve 2a represents the specular reflectance R(X) of the textured surface. It may be observed that there is a very low reflectance profile for wavelength X < 3.0/xm, while there is a rapid increase in the reflectance profile for X > 3.0 /~m. The solid curve shown in fig. 2b represents the total reflectance profile of the textured surface; for this curve a ~ 0.97-0.98 and q00 --- 0.18. The dashed curve in fig. 2b displays total reflectance R(X) for the texture (displaced) removed films for which we computed a < 0.9 and q00 --- 0.14. The comparison of the two spectra in fig. 2b reveals that the texturing improves the absorptance by almost 0.08. This reduces the front surface reflection losses to - 1% between the wavelengths 0.3 and 0.7 /~m, which probably is caused by antireflecting and texturing effects.

3.2. Microstructural analyses 3.2.1. Scanning electron microscopy Fig. 3 depicts the SEM micrographs of the films corresponding to the reflectance spectra in fig. 1. It has been noticed that the film starts growing after about 20 seconds. In the initial stages, the film is brown in colour. At this stage, a rough surface is seen as indicated in figs. 3b and 3c. It has been noticed that only after an oxidation time t ~ 2 min the colour starts changing dramatically to deep brown (blackish) and then to black, indicating a change in the oxide stoichiometry. This is the transition region and at this stage texturing also begins to appear. Fig. 4a illustrates a typical SEM micrograph of the optimized black copper films, It may be observed that the predominant morphology consists of small-scale roughness. It is made of a combination of needle-like long and fine grains. The long grains are 0.1-0.5/tin in length and 0.04-0.1 /~m in diameter, while the mean diameter of the fine grains are - 0.08-0.1/~m. The separation between these grains as observed are of the order of 0.3 /~m. The loose packing of the grains on the top regions

r~

P. Richharia / Chemically converted textured black copper selective surface

205

Fig. 4. (a) Scanning electron micrograph of the as-deposited black copper selective surface (corresponding to the solid curve in fig. 2b). (b) SEM micrograph after displacing the texture by dry rubbing (mechanically) (corresponding to dashed curve in fig. 2b). produces large void fractions which result from the high degree of texturing. This probably causes refractive-index grading to occur. Another scanning electron micrograph presented in fig. 4b has been taken by removing the texture of the film by dry rubbing (mechanically) the surface. Visually this film looks like a polished black surface with the collapse of the texture. These SEM micrographs correspond to the curves shown in fig. 2b. Fig. 5a shows a typical cross sectional view of the same coating. The thickness estimated is, however, large. It is of the order of 2.6-3.0/xm. The micrograph clearly exhibits a layered-type deposit with porous cavities. The cross sectional view depicts a sponge-like appearance. Clearly no large differences amongst different parts of the film are noticed while the m e t a l - o x i d e interface appears to be rough. These porous cavities being of the order of a few microns can trap the incoming solar radiation by probably resonance scattering and geometric maze effects [18]. Fig. 5b presents another cross sectional (CS) view along with the edge (E) and the surface (S) of the film. Clearly the surface reveals a small-scale rough morphology. The inhomogeneities are less than 1/~m. The presence of rough morphology may be due to voids present throughout the depth of the film, thus making an inhomogeneous, porous, layered structure along with rough a i r - o x i d e and oxide-metal interfaces [18,19]. 3.2.2. Transmission electron microscopy

The transmission electron micrscopy (bright field) of the internal layers of the as-deposited selective surface prepared at oxidation time t -- 11 min presented in fig.

Fig. 3. SEM micrographs of black copper taken at various stages during growth. (a) t = 0 min; (b) t -- 20 s; (c) t -- 1/2 min; (d) t = 1 rain; (e) t = 2 rain; (f) t = 5 n'fin; (g) t = 9 min.

206

P. Richharia / (heroically converted textured black copper selectme suriac¢ •

!

!

Fig. 5. (a) Cross section of the as-deposited black copper film prepared at oxidation time t = 11 rain as observed by SEM micrography. (b) SEM micrograph with the cross section (CS), edge (E) and surface (S) in a single view.

6a reveals two distinct regions, i.e., (a) needle-like long grains and (b) fine grain deposit. As observed by TEM, needles have length -0.09-0.5 /~m and diameter - 0.01-0.06/~m. These are again of the same order as seen above. The grains are oriented parallel (along) to the substrate and appear randomly distributed. These long grains appear to be made up of finer grains. The electron diffraction pattern presented in fig. 6b shows the presence of predominantly CuO with strong ring pattern of Cu2S and weak ring patterns of Cu20, Cu(OH)2 and CuCO3. The presence of the 6CuOCu20 phase indicates the presence of a mixed oxide. The corresponding values for the spacing of the planes (d) and the Miller indices (hkl) are listed in table 2.

Fig. 6. (a) Transmission electron micrograph of the internal layers of the as-deposited black copper selective coating. (b) Diffraction pattern corresponding to fig. 6a.

P. Richharia / Chemically converted textured black copper selective surface

207

Table 2 D (diameter of the rings), d (spacing between the planes) and hkl (Miller indices) values corresponding to the electron diffraction pattern given in fig. 6b Ring

D (ram)

Intensity

d

Phases present

hkl

1 2 3

22.5 24.5 27.5

w s s

3.71 3.41 3.09

4 5

32 33.5

VW M

2.61 2.49

6 7

39 42

W W

2.14 1.99

8 9 10

45 49 54

M W W

1.856 1.71 1.55

Cu2S Cu 2S 6CuOCu 20 CuS Cu(OH) 2 6CuOCu 20 Cu(OH) 2 Cu20 CuCO3Cu(OH) 2 CuO CuO CuO CuO Cu 20

133, 320, 062 433 200 102 040 004 111 200 060 112 202 020 202 220

3.3. Glancing incidence diffraction

Thin films of the black copper were also characterized using the glancing i n c i d e n c e d i f f r a c t i o n ( G I D ) t e c h n i q u e ) . T h e G I D s c a n h a s b e e n d o n e at a = 3 °. G I D scans o f a f i l m p r e p a r e d at t = 3 m i n i n d i c a t e t h e p r e s e n c e o f m a i n l y C u O a n d Cu along with Cu20, Cu(OH)2, Cu2S and CuCO 3 phases. The top surface of the a s - d e p o s i t e d s u r f a c e p r e p a r e d at o x i d a t i o n t i m e t = 11 r a i n h a s a l s o b e e n a n a l y z e d

Table 3 d (spacing between the planes) I / I o (ratio of the intensity transmitted to that of the original beam intensity) and hkl (Miller indices) obtained by glancing incidence diffraction taken at a glancing angle a = 3 ° of the (top surface) as-deposited surface d

1 /I o

2.52

22

2.479 2.460 2.348 2.331 2.308

16 17 17 20 16

2.079 1.802

100 12

1.278 1.273 1.093

12 10 10

Phases

hkl

Cu(OH) 2 CuO Cu 2S CuCO3Cu(OH) 2 Cu(OH) 2 Cu(OH) 2(CO3) 2

111 111 460 330 022 104 231 200 111 200 060 220 004 131

CuCO3Cu(OH) 2

CuO Cu Cu Cu(OH) 2 Cu CuO CuO

208

P. Richharia / Chemically com, erted textured black copper ~electwe surla~ c

Table 4 Auger survey (data) of the top surface of the as-deposited black copper selective surface prepared ~l oxidation time t = 11 rain Element

Atomic concentration (%)

Copper Oxygen Sulphur Carbon

64.44 19.4 1.3 14.76

where the phases present are Cu, CuO, C u C O 3 , Cu(OH)2 and Cu2S as shown in table 3. The film consists of mainly CuO along with a significant amount of Cu and a small amount of CuCO 3, Cu(OH) 2 and Cu2S on the top region. The lower region consists of mainly CuO, Cu and Cu20. However, traces of Cu(OH)2, CuCO 3 and Cu2S are also present. From this analysis it can be inferred that the film is absorbing due to the presence of predominantly CuO and possibly also due to the presence of small amounts of Cu2S phases. The resonance scattering effect is expected in the film, which is perhaps due to interactions amongst different phases. The above analysis also supports the idea that Cu grains are dispersed in an oxide (CuO) matrix.

3. 4. Surface and compositional analysis 3.4.1. Auger electron spectroscopy Table 4 lists the percentage of the elements present on the surface as observed by Auger survey. The depth profile of a sample with oxidation time t --. 3 min shows the emergence of the uniform region consisting of copper and oxygen, after the graded transition region. The graded region is copper rich and oxygen deficient. This indicates that with the appearance of texturing the uniform region emerges. Fig. 7 presents the Auger depth profile of an optimized film. Impurities such as C and S which occur in traces are not shown in the profile. From this figure it can be

I00 8o

Cu

¢5 ~

0

~ X._..

20 0

0

~ :

,'0

!

,~

i0

2's

....

3;

3'5

,0

SPUTTER TIME (MIN) Fig. 7. Auger depth profile of the as-deposited black copper selective surface showing the variation of the atomic concentration (%) versus sputtering time (rain).

209

P. Richharia / Chemically conoerted textured black copperselectioesurface

93/..3 eV 954.2 eV

"E 0 Ill W Z

I - 970

1

I -

1

960

I

-950 BINDING

|

I

-9t.0 ENERGY

I

I

I,

-930

(eV)

Fig. 8. XPS spectrum of the (top surface) as-deposited black copper selective surface prepared at oxidation time t = 11 min. The spectrum shows N ( E ) / E versusbinding energy(eV) profile, where N(E) is the intensity of peak in counts per second and E is the kinetic energyof the photo electron.

inferred that the film consists of two approximately equal regions. The top region is an uniform extended region of copper and oxygen mostly consisting of mainly CuO along with a significant amount of Cu. This region probably is - 1.3-1.5 /~m in length. Beyond this zone, a gradual transition region is observed which grades off to Cu rich and oxygen deficient region as the substrate/metal-oxide interface is approached. 3.4.2. X-ray photoelectron spectroscopy

The XPS spectra of the film prepared at oxidation time t = 3 min are indicative of the presence of CuO along with the presence of Cu 20 a n d / o r elemental Cu. The Cu2p XPS spectra obtained match very well with CuO, while relatively small characteristic satellite peaks are indicative of the presence of elemental Cu metal or Cu20. The X-ray photoemission of the Cu2p level of the top surface of the as-deposited film prepared at oxidation time t = 11 min is presented in fig. 8. The peak position at 2p3/2 has shifts of - 0.5 eV towards the higher side of the standard peak position of cupric oxide (933.8 eV) as indicated in table 5. The reported peak position 2p3/2 of CuCO 3 is 934.8 eV, while the peak position of observed 2p3/2 peak is 934.3 eV. This is a situation between CuO and CuCO3. However, the prominent satellite peaks observed adjacent to Cu2pl/2 and Cu2p3/2 at the higher binding energy side at 963 and 944 eV are strong and a typical characteristic of (CuO) Cu II compound [20-22]. From table 5, it is observed that F W H M values are closer to that of standard CuO for the top surface. However, table 4 and fig. 7 show the presence of excess of Cu which indicates that a significant amount of Cu is also present along with CuO. Hence it may be concluded that the top surface consists of mainly CuO along with a significant amount of Cu and traces of CuCO 3. Their presence have also been supported by G I D studies as shown in table 3. Further more, chemical etching has been employed instead of sputtering to see the internal

P. Richharia / ('heroically converted textured black copper selectwe surface

210

Table 5 Cu 2p peak position and the full width at half maximum value (FWHM) in the observed XPS spectra ¢~f the as-deposited surface taken at different depths

2pt/2 peak position (eV) - FWHM (eV) 2p3/2 peak position (eV) - FWHM (eV)

Top surface

Intermediate region

Near the oxidemetal interface

Cu

Cu ~0

CuO

954.2

953.5

953.2

952.2

952.3

953.6

2.0

2.0

932.4

932.5

1.6

1.6

4.04

934.3 3.3

2.72

933.7 2.95

2.72

933.6 2.95

Standard values ~)

4.48

933.8 4.16

CuC03

-

934.8 -

~) Refs. [20-22].

composition [21]. The etching was done in a dilute H N O 3 bath for a few seconds to see the intermediate layers and for - 50 seconds to see the lower base layers, i.e., the layers nearer to the oxide-metal interface. The comparison of peak positions and full widths at half maximum (FWHM) values of the observed peaks along with the standard values are presented in tables 5 and 6. The peak position and shape of Cu2p peaks of the intermediate layers and the layer nearer to the oxide-metal interface have shown peak positions matching well with CuO. However, decreases in satellite height at 963 and 944 eV have been observed when compared to that of the top surface. From table 5 also, it may be observed that their F W H M values lie between that of CuO and Cu 2° or elemental Cu. The above observation indicates that with depth the film is becoming metallic-like. However, a significant amount of CuO is also evident at these depths. On comparing the F W H M value of the O Is level with that of the standard O ls value (table 6) broader oxygen peaks are indicated. These broader oxygen peaks are indicative of the presence of more than one type of oxygen in the film [23].

Table 6 Oxygen O ls spectra of the as-deposited black copper selective surface taken at different depths

O(ls) peak position (eV) FWHM (eV) a) Ref. [23].

Standard oxygen a)

Top surface

Intermediate region

Near the oxidemetal interface

530.2

530.2

530.1

530.4

2.3

3.8

3.2

3.52

P. Richharia / Chemically converted textured black copper selective surface

211

3.5. Fourier transform infra-red spectroscopy Fig. 9 illustrates the FT infrared spectra of the pellet made of copper oxide powder dispersed in KBr. The inspection of the FT infrared spectra reveals significant absorption due to OH (absorption band at 3460 cm -1) and organic impurities such as C=O, Sz--O S=O (absorption bands in the range 1650 to 650 cm -1) [24-26]. These absorption bands support the presence of Cu(OH) 2, CuCO 3 and Cu 2S phases. The inspection of the C u - O absorption bands in the range of 615 to 410 cm-1 are indicative of stretching vibration band of predominantly CuO, the vibration band of Cu20 at 615 cm -1 is weak and close to that of CuO at 610 cm -1 and differentiation between these two frequencies is difficult. However, other peaks at 500 and - 410 cm- ~ are indicative of the presence of CuO predominantly. The presence of Cu 2° and CuO have been indicated by TEM (diffraction pattern) and GID (glancing incidence diffraction) analyses as shown in tables 2 and 3. The structural picture which emerges from this study reveals that the black copper films are inhomogeneous both in the plane of the film and through the film thickness. It has been observed that the air-oxide and oxide-metal interfaces appear to be rough. The film has small-scale rough morphology. The typical layered structure is built up of needle-like long and fine grains. The films are divided into two approximately equal regions. The top one is uniform - 1.3-1.5 /~m in length and consists mainly of CuO along with a significant amount of Cu and small amounts of CuCO 3, Cu2 S and Cu(OH)2. This uniform region is highly porous (low

.fi

LLI O Z

U~ Z I--

I

3800

I

2200

I

I

I

WAVENUI,'tBER

I

800

lt, O0

l

I

/.,00

( cm -1 }

Fig. 9. FTIR spectrum showing transmittance (arbitary units) versus wavenumber (cm - l ) of the pellet made of copper oxide powder dispersed in KBr.

212

P. Richharm / ('hemically converted textured black copper selective s'ur!~lcc

density) which results in loose packing of CuO, thus creating a textured surface [27,28]. Below this extended region lies the graded region which appears to be quasimetallic. The grading is however gradual with the backside appearing metallic In the quasimetallic region CuO, Cu 20 and Cu have been observed. In this regime, however, traces of CuCO3, Cu2S and Cu(OH)2 have also been found. The highly absorbing CuO layer on the upper region of the film acts as the antireflecting coating for the transition (graded) region, hence suppressing the resonance scattering effects which probably arise in the compositional graded region i.e. transition region. Texturing of the film plays a significant role in increasing the absorptance (c0 by - 0.08. The increase in thermal emittance ( - 0.04) in the textured surface is because the textured region appears to be of the order of 1.3-1.5/~m. The relatively low emittance is due to the underlying copper substrate at the oxide metal interface which has low emittance in the IR region along with the graded region which appears quasimetallic. Further, these surfaces are found to be stable in both air and vacuum (10 _5 Torr) up to 2 5 0 ° C and are also found to be resistant to external environmental conditions [18,29]. 4. S u m m a r y and c o n c l u s i o n s

In conclusion, this study has established the optical and microstructural properties of a chemically developed black copper selective coating. A synoptic outline of these investigation is presented below: - The optimized black copper surface possesses remarkably high initial solar absorptance a = 0.97-0.98 and relatively low emittance ~100~ 0.2 + 0.02. - The microstructural studies of the film are indicative of an inhomogenous surface with small-scale rough morphology (inhomogeneity is less than 1 ~m). The separation of the grains are of the order of 0.3 ~m. - The cross section reveals a sponge-like porous, layered structure and are built of needle-like long and fine grains. The thickness estimated is however large - 2.6-3.0 /~m with both air-oxide and oxide-metal interfaces appearing rough. - Further the film has two distinct regions almost equal in length. The top region of - 1.3-1.5 /~m is a uniform region and consists of loosely packed CuO mainly of highly absorbing and antireflecting. This uniform region gradually grades off to a Cu rich and oxygen deficient region with the intermediate region appearing quasimetallic. - The high spectral reflectance in the infrared region supports the idea that the top copper oxide layer (uniform region - 1 . 3 - 1 . 5 /~m) is ineffective and appears transparent for the IR radiation. The high IR reflectance is dominated by the quasimetallic region (transition region) and the underlying copper substrate. - Finally, the high absorptance of the film arises as due to the cumulative effect of multiple phenomena occurring simultaneously in the coating. The explanation of the high a is complex. It may be concluded by this study that high absorptance (a) is mainly due to intrinsic absorption by the semiconductor oxide (CuO) and may be also due to Cu 2S phase, an antireflecting effect, a resonant scattering effect and the geometric maze effect which is caused by texturing.

P. Richharia / Chemically converted textured black copper selective surface

213

- I n s u m m a r y , a chemically converted b l a c k copper selective surface possessing o p u l e n t optical properties has b e e n developed which possesses a n e w rnicrostructure. These surfaces are highly r e p r o d u c a b l e a n d gave good stability b o t h i n air a n d v a c u u m (10-5 Torr) u p to 250 ° C. This black copper o n copper appears superior to already existing copper oxide systems reported so far.

Acknowledgements T h e author gratefully acknowledges Professor K.L. C h o p r a for the v a l u a b l e guidance, discussions a n d r e a d i n g this m a n u s c r i p t . Professor L.K. M a l h o t r a is also acknowledged for going t h r o u g h this paper.

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