- Email: [email protected]
Preparation and characterization of low-emitting black zinc pigment for spectrally selective paints
Solar Energy Materials 21 (1991) 267-281 North-Holland
267
Preparation and characterization of low-emitting black zinc pigment for spectrally selective paints Z. Crnjak Orel and B. Orel Boris Kidrig Institute of Chemistry, P.O. Box 30, 61115 Ljubljana, Yugoslavia Received 2 July 1990; in revised form 25 July 1990
In this paper we describe the preparation of black zincated steel plates and zinc powders. Infrared reflection/absorption (6 °, 78 ° ), reflection (6 ° ) and transmission spectroscopy have been used for identification of layers formed on the corresponding substrates. It was found that modification of the existing method proposed by Telkes is necessary in order to obtain zinc powder with sufficient solar absorptance. Colour strengths of the layers and powders formed have been correlated with the existence of various copper(I) and copper(II) oxides in the layers. A negative reflectance peak at 595 c m - 1 has been detected in the reflectance spectrum of black zinc powder and its origine was attributed to the coupled 1,cu_o phonon mode with free electron oscillations of metallic zinc.
1. Introduction Spectrally selective black copper surfaces for solar energy utilization, produced by various chemical oxidation processes, have been extensively studied and investigated in the past [1,2]. The ease of application of the chemical oxidation process, connected with low cost of the raw materials, makes it possible to utilize black copper surfaces for solar collector panels. Moderate to high spectrally selective surfaces are known which are reasonably resistant to wear, weathering and thermal cycles [3]. Paint coatings are generally accepted as cheap alternatives to electroplated coatings when spectrally non-selective surfaces are sufficient for solar energy application [4]. Nevertheless, paint coatings with reasonable spectral selectivity are much more difficult to prepare [5], and besides require careful formulation of the paint and high-technology application procedures. This is particularly true for thickness sensitive spectrally selective paint coatings (TSSS), which are applied in layers a few micrometers thick on highly reflecting substrates by the coil-coating process [6]. Among the spectrally selective paint coatings, thickness insensitive (TISS) paint coatings, which even in thick layers (30-50/~m) exhibit spectral selectivity, occupy an exceptional position. They could be utilized not only as spectrally selective paint 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
268
/. ('rtzjak ()t'¢4. B. Or~,l
Bhzck zlm pignle#ll /)~r V~eclra//l' ~c/e,l~r~'pamt,.
coating [7] for solar energy conversion but could also serve for compensation ~imi equalization of emitted and reflected radiation, particularly in the thermovisiuN (8-13 /~m) region. They are appropriate for covering larger objects smcc their application does not require special painting techniques. Preparation of TISS paint coatings requires a low-emitting vehicle (pol 5butadiene-polystyrene co-polymer) and a low-emitting pigment such as various bronzes [8]. In order to keep the solar absorptance high, admixing a black pigment with the paint dispersion is necessary. There were attempts to formulate TISS paints which would contain only one type of pigment with the combined optical characteristics of b r o n z e / b l a c k pigment mixtures. Impact ball milling [8] of spinel-type black pigments with various bronzes requires careful preparation, otherwise the achieved spectral selectivity is low. From these reasons black zinc and black cobalt pigments [9] represent promising alternatives, since their preparation could be performed easily and without special precautions. The spectral selectivity of these kinds of pigments is achieved by the mutual action of highly reflecting metallic substrate particles coated by a thin layer of the appropriate black oxide. In such a way each particle of the pigment forms an absorber/reflector tandem. In this paper we describe the preparation and the optical properties of black pigments based on oxidized zinc pigment. Telkes [9] and Gupta [10] reported it.~ preparation and properties. According to these authors a, = 0.9 and e r = 0.4 of black zinc pigment could be achieved. We have modified this preparation method in order to obtain better efficiency and reproducibility of the process of oxidizing zinc powder. Infrared spectroscopy was used for structural identification of oxide layers formed on the surface of the zinc powder particles. Reflectance infrared spectra of pressed black zinc powders in pellets were measured in order to evaluate their use as spectrally selective pigments for TISS. Since mixed copper(I, II) oxides are formed on metallic particulates or solid copper substrates [11 14], investigation of the corresponding infrared spectra is reduced to the identification of various C u - O phonon modes appearing in the region ~ < 1000 cm ~ of the infrared spectrum. The infrared spectra of copper oxides formed on various metallic substrates are still not fully investigated by infrared spectroscopy. This is particularly true for other metals such as zinc or the corresponding powders and bronzes. Part of this work stems from the need to obtain more insight into the band assignment of the C u - O phonon modes of different cuprate compounds, which seems to be of importance for understanding the structural features concerning the oxidation state of copper in high-T~, superconducting ceramics of 1-2-3 type [15]. In such sintered materials, the structure of the interfaces, their composition and the oxidation state of copper are still not fully explained. Without this knowledge it would be impossible to correlate the phonon frequencies with the bond lengths, which is of prerequisite importance when lattice dynamics need to be constructed for 1-2-3 compounds. Therefore, the aim of this paper is also to show to what extent the appearance of copper oxide phonon modes are changed due to the interphase interactions which take place among mixed copper oxides and metallic substrates.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectivepaints
269
2. Experimental procedure Black zinc was prepared from zinc powder obtained from Cinkarna - Celje (Yugoslavia). After degreasing in Na2CO 3, any oxides possibly present were removed using diluted HC1 (5%) several times. The sample was reground and used for black zinc preparation. Black zinc was prepared according to Telkes [9]. Cu20 is readily formed, resulting in the dark brown colour of the prepared zinc powder. The spectral selectivity of the sample was classified according to its infrared reflectance spectra. The colour of the pigment was estimated visually. FTIR spectra were recorded on a FTS-80 Digilab spectrometer using a hemispherical reflection cell (DRIFT), with a collecting solid angle of 30 ° and a near-normal reflection cell (6 ° ). The reflection-absorption spectra of thin black zinc samples were recorded with a near-grazing incidence angle attachment (78 ° ). No polarizer (Rp component) was used throughout this work.
3. Results First attempts to prepare black zinc powder after Telkes' method (hereafter referred to as method T) [9] showed that by variation of temperature, dipping time and changing the content of CuSO 4 in the NaOH solution it was not possible to obtain a zinc powder with black colour. Nevertheless, the infrared reflectance of the powder pressed into a pellet was quite high ( - 60% at 10 btm), which promised the preparation of spectrally selective zinc powder. With regard to the dark brown colour of the powder, we considered that the reaction is proceeding in the direction of increased reduction of copper up to its elementary state. In order to prevent reduction of Cu 2+, the solution was prepared from [Cu(NH3)4]SO4. H20. This compound acts as a buffer in which the pH was kept high by addition of a certain amount of NaOH. A hot solution of buffer and NaOH was poured into the boiling zinc powder slurry which turned black in a few seconds (method M). Samples were filtered and washed with an abundance of water in order to remove SO2- and N H 3 residues and then used for spectroscopic investigations. Optimization of solar apsorptance (a~) and thermal emittance (ex) values was obtained by changing the temperature, the concentration of buffer in solution and the concentration of NaOH. As a result, different black and brown shades of Zn powder were obtained. The efficiency of the oxidation process was estimated visually. The solar absorptances of powders were not measured spectrometrically because of the problems caused by the different particle sizes and consequent scattering, which makes the reproducibility of the determination questionable. For quick estimation of the thermal emittance achieved, the reflectance values at 10/~m were used. Tables 1-3 show the results. The optimum conditions are indicated in these tables.
Z, Crnjak Oret. B. Orel / Black zinc pigment l~r spectrallv ,~ele~m,e pamt~
270
Table 1 Spectral reflectivity (in percent) of differently prepared black zinc powders; concentration of inte~ mediate compound has been varied at constant weight of NaOH (0,62 g/ Weight
Reflectivity at
[Cu(NH3)4]SO4 . H : O (g)
4000 cm
A
0.9
B
0.27
C
0.32
D
0.36
E
0.4
F
0.45
30.6 27.5 15.4 1.4 17.2 13.4 11.5 13.95 13.95 21.6 17.9 17 1(/.95 8.39
i
Visually' estimated 2000 cm 44.8 43.8 28.2 24.8 32 18.9 19.2 21.43 26.25 35.7 32.3 32 21.49 16.12
t
1000 cm i
~hade a~
57.4 57.7 41.9 36.6 43 25.6 28.7 30.70 39.09 49.7 35.4 45.6 32.8 24.4
Gray Gray black
Gray-black Dark gray-black Black
Brown-black
Similar results were also obtained when zincated steel plates were dipped into the solution with the buffer compound (M), or without (T) it. They always appeared black when the former method was used, but a dark brown shade was obtained after the latter. The adhesion of the layers formed was better for the M samples (black) than for the T samples (brown). Nevertheless, the quality was not sufficient to promise the efficient use of these plates as spectrally selective coatings. Therefore
Table 2 Spectral reflectivity (in percent) of differently prepared black zinc powders: concentration of NaOH has been varied, keeping the weight of the intermediate compound at 0.27 g Weight
Reflectivity at
NaOH
4000 cm 1
2000 c m - 1
1000 cm - i
A
0.432
B
0.62
C
0.8
D
1.2
17.76 20.43 13.03 15.4 17.26 14 12.65 13.69 17.65 16.8 19.1
30.4 33.7 26.2 28.2 32.1 24.8 20.27 21.41 27.9 23.73 26.79
42.2 46.7 39.6 41.9 43.6 36.6 30.27 30.15 37.4 30.97 33.58
(g)
Visually estimated shade
a~
Grey
Black
Black
Black
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectioe paints
271
Table 3 Spectral reflectivity (in percent) of differently prepared black zinc powders; concentration of N a O H has been varied, keeping the concentration of intermediate c o m p o u n d at 0.4 g Weight
Reflectivity at
NaOH
4000 c m - 1
2000 c m - 1
1000 cm - l
A
0.48
B
0.62
C
0.80
D
0.88
E
1.2
1.4 11.5 17.9 14.9 17.9 13.7 9.28 15.7 15.26 10.96 10.80 18.2 18 16.7
24 18.5 32.3 27.4 32.3 20.9 16.34 26.8 26.06 16.62 17.32 21.45 22.36 21.6
37 30 45.4 38.4 45.6 27 23.8 38.1 36.2 22.1 23.4 22.78 24.4 25.0
(g)
Visually estimated shade
a~
Black Black
Black
Black
Black
these samples were mainly used for systematic study of the structure and composition of the layers formed on plates and on zinc powder.
3.1. IR spectra of black zincated steel plates In order to identify the dark brown layers of black zinc formed on zincated steel after methods M and T, infrared reflection/absorption ( R / A ) (6 ° and 78 ° incidence angle) and transmission spectra were recorded and the bands assigned. The R / A (6 °) spectrum of the thick black layer (method M) (figs. l a - l c ) exhibits, besides the bands characteristic for SO42- and NH 3 groups (at 1432, 1124, 880, 701 and 615 cm -1) also a band at 500 cm -I which could not be attributed to SO42- or NH 3 groups. A part of the layer was removed from the plate, washed in water, dried and the transmission spectrum was recorded. As was expected all the bands disappeared except the bands at 500 and 601 cm -1 (fig. lc). According to the chemical reactions which take place in solution, it was expected that the layers formed contain copper(l) and copper(II) oxides as well as ZnO, various cuprates and also elementary copper. The reflection and transmission spectra of these compounds are reproduced in figs. 2-5. After comparing the bands appearing in the transmission and reflection spectra, and bands from reflection spectra calculated from the K spectra of CuO and Cu 2O, we concluded that the black layer consists of both types of copper oxides, the former (CuO) being formed in large excess (90%) with respect to the latter (Cu 20). Detailed examination of the band positions revealed that there is no one-to-one correspondence between the Cu-O mode position (fig. lb) in the spectra of black layers and CuO and/or Cu20 stoichiometric compounds [15,16]. For example, the
272
Z. CrnjaA Ord, B. Ore/ / Blucl, zinc ptgnlen£/hr spectr~d!r .~dcrHl'e l*eem~
1.=i
• !,
:£ J
/i ~ i; ,; !! i , i
i~3J
,' iii i
R%j
.
n A
-
. . . . . . . . . . .
40~0
352@
23@@ 2@@@ W~.ve n utah e ~,9, °
300@
15~@
1@@@~ cm'
/
i
!
S@@
532 ~501
z'•I042"
J
R% [ 1418
i
i
ag@O
i
36@@
. . . . . r ....... -[ . . . . . . . . . . . . . . . T 95@@ 2@@@ 1S0@ 3@@@ W~venumber s
....
T _~ 1@@~ cm
. 5@@
-/-
e
I
'i
T,:
/
'i,P\
• 13[~,M • ~d
\ $0@•~ c m
i f3fdM ~ f3
3!)~, 3
~A VENUMf3~[~
Fig. 1. Vibrational spectra of black layers (method M) on zincated steel plate: (a) r e f l e c t i o n / a b s o r p t i o n (6 o) s p e c t r u m of thick layer; (b) r e f l e c t i o n / a b s o r p t i o n (78 ° ) s p e c t r u m of thin layer; (c) t r a n s m i s s i o n s p e c t r u m of removed layer•
273
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectiue paints
0,70.5-
,
/ \
',
\&&3
W Fig. 2. Transmission spectra of stoichiometric compounds: (a) CuO; (b) Cu20; (c) ZnO.
~'cuo mode of the stoichiometric CuO compounds which is split at 595, 526 and 488 cm -~ in the K spectrum, appears as a single band at 495 c m - t in the R/A ( 6 ° ) spectrum of the thick black layer (figs. 4-6). Obviously, the influence of the too great a thickness of the layer smears out the detailed structure of the R/A (6 °) band at 495 cm-1 (fig. la). For this reason the R/A (78 °) spectrum of the thin ( < 1 /~m) black layer was measured and the corresponding bands detected (fig. lb). It was found that the single band at 495 cm -a (R/A at 6 °) split into 627, 532 and 501 cm -~, while the band at 615 cm-~ is shifted to 627 cm-1 with decreased intensity. The general appearance of the bands in the R/A (78 ° ) spectrum of the thin black layer is very similar to the ucu-o mode of the K spectrum of the CuO
Io"
618
25-
-X
2{]--
15-R%
10-
5b
0150B
I 12gO
I
1100
iZgg
1
1
990 800 W~venumbers
I
780
BBB
5~cm ~
40~
Fig. 3. Kramers-Kronig analysis of polycrystalline Cu20: (a) reflectivity spectrum; (b) k spectrum; (c) n spectrum.
274
Z. Crnlak Ore/. B. ()re/ / Black zim pigment/or
.~psclra//l'
~C/t'ctit'd [~al#il~
h'
2.04 ¸
C
n,k
R°,,, !
3
b
0.0-
120{3
1300
1 100
1000
900
600
] 200
F 600
r S00 c~'
I- ~ 40l~
W~venL~mbe~s
Fig. 4. Kramers-Kronig analysis of polycrystalline CuO: (a) reflectivity spectrum: (b) k spectrum: (c) n spectrum.
compound. All the bands appear, except the band at 596 cm- J which could not be detected in the R/A (78 °) spectrum of the thin black layer. Similarly, the band at 627 c m - 1 could not be assigned to the P('uO mode of the CuO compound but to v(.uo of Cu 20 (figs. 2 and 3). The differences in the band positions appearing in the R/A (78 °) spectra and transmission or R/A (0 o) spectra deserve a comment. It is generally accepted that the maximum of extinction (a = 4eK/X), i.e. phonon frequency, corresponds well
411
50-7
529
~ !
• L',
3~
i
R°/o r
/
/
//
4 1,V
: .,k
/
i
I !
a
b t,
]300
! 200
I 100
1000
9{30
800
?{30
G00
50~ cn~~ 400
W~ven L~mber s
Fig. 5. Kramers-Kronig analysis of polycrystalline ZnO: (a) reflectivity spectrum; (b) k spectrum: (c) n spectrum.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints
275
55.0 50.0
G
~
~(10-
,
1300 1200
O
-
,
,
I000
BOO II
, BOO
~00(cn~I)
Fig. 6. Transmission spectrum of the layer formed on zincated steel of method T.
to the minimum of transmission [17-19]. The refractive index n which varies across the band does not shift the position of the minimum away from the frequency at which the absorption coefficient ( K ) reaches maximum values. In contrast, in the R/A spectra the bands are split a n d / o r shifted to higher wavenumbers, i.e. to that region where the refractive index of the particular phonon mode reaches the minimal values. Their appearance depends upon the thickness of the layer and the incidence angle, and can be calculated when K(~) and n(~) for a certain band are known. For example, the Vcuo modes of Cu 20 and CuO in the R/A (78 o) spectra of thin ( - 200 ,~) layers appear at 650 and 540 cm -1, respectively [16,19], which is higher with respect to the bands of bulk samples (620 and 510 cm-~). A detailed assignment of the CuO modes appearing in the R / A (78 o) and R/A (0 °) spectra is not possible without previous calculations of n and K. It is impossible to estimate the blue shifts of the bands, appearing in the R / A (78 ° ) spectrum of black zinc layer in order to check unambiguously the one-to-one correspondence of these modes with vcu_o modes of CuO and C u 2 0 compounds. Additionally, it was found that the Uz,_ o band of the ZnO compound appears at 529 cm -1, which corresponds fairly well to the 532 cm-1 band detected in the R/A (78 ° ) spectrum of the thin layer. Since ZnO is white, and the layers were not completely black, we could not eliminate the possibility of the formation of ZnO. Analogously, an identification of the brown layer formed on zincated steel plate according to method T was made. The infrared transmission spectrum of a removed layer (fig. 6) clearly shows that Cu 20 is formed in excess of CuO, as demonstrated by the stronger VCuo mode of Cu20 (603 cm -1) compared to the same mode of the CuO compound (520 cm-1). Similarly, the influence of the R/A (78 o ) measuring technique was demonstrated by the shifts of the corresponding bands to 609 and 528 cm-~ (fig. 7). Identification of compounds which are present in the layer seems not to be straightforward for Cu 2° and CuO. Again the band positions do not exactly correspond to the position
276
Z. [ rnjak OreL B. Ore/ ,/ Black zm~ ptA,men: l~,r V~e<'tral]l ~e/e
%7.~ I
~uO
/
509
L,O.O-
-~C~O
I
36.0
13oo
12oo
looo w
Fig. 7. Reflection/absorption (78 ° ) spectrum of the layer formed on zincated steel by method 7.
of the bands which are characteristic of P(:u-o modes of pure stoichiometric CuO and Cu20 compounds. Both types of layers (T and M), black and dark brown, could be considered as mixtures of various oxides of copper and zinc. Nevertheless, they are not simple mixtures of pure stoichiomeric oxides with the corresponding structures C6h (CuO [20]) and 0 4 (Cu20 [21]), since the one-to-one correspondence between the infrared bands of layers and pure oxides was never obtained. 3.2. IR spectra of black zinc powder
The most obvious fact which results from the comparison of different black zinc powders is the striking difference in their reflectance spectra. While the reflectance spectrum of the brown sample (method T) (fig. 8) shows the presence of C u 2 0 oxide, the corresponding reflectance spectra of the black sample (method M) (fig. 9) could not be identified as a simple sum of IR reflectance spectra of either of the copper a n d / o r Zn oxides . This feature was confirmed by the corresponding K spectra (fig. 8c) which clearly showed that in the brown sample the Ucuo mode appears at 619 cm -1 indicating the presence of C u 2 0 (see also fig. 3b), while no distinct band could be detected in the K spectrum in the region 600-400 cm 1. In spite of this a deep negative reflectance peak has been found in the spectrum at 594 cm 1. Such a negative band is not present in the brown sample (method T). Transmission spectra of the brown sample (T) (fig. 10a) confirms the presence of C u 2 0 from the band at 630 cm-~. This band fairly well coincides with the UCuo mode determined from the reflectance (fig. 3b) and transmission spectra (fig. 2 curve b) of the C u 2 0 compound. The difference (10 cm -1) between band positions observed in reflectance and transmission or absorbance spectra could be attributed to the decreasing (or increasing, in the absorptance spectra) background onto which the band is superimposed (fig. 10a).
277
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectivepaints 7-]
,
5
- 66
SI~
-
1500
1201~
1100
1000
91~0 800 W~,vanttmbo~'s
200
600
500 ca' 400
Fig. 8. Kramers-Kronig analysis of the black zinc powder (sample $1, method T) pressed into a pallet: (a) reflectivity spectrum; (b) n spectrum; (c) k spectrum.
The transmission spectrum of the black sample (M) (fig. 10b) exhibits bands at 529 and 472 cm -1, which could both correspond to ZnO or CuO compounds (fig. 2), or to some kind of new oxides which are formed o n the surface of the zinc particle. Hence the black zinc powder could not be considered to be a simple mixture of pure oxides and zinc powder.
3,5-
-45
3.0-
~
2.5
b
n,k
35
2.[~-
R%
1.5-
3~
1.0-
25 c
0.5-
1500
a I
I
1200
1100
~ - ~ T
I000
F
900 800 Waven~mber~
[-
I
71~0
800
20
I
500 e~
400
Fig. 9. l~amers-I~onig analysis of the black zinc powder (sample $2, method M) pressed into a pellet: (a) reflectivity spectrum; (b) n spectrum; (c) k spectrum.
278
Z. Crnjak Ore/, H. O r e / / Black :im pigment/i~r .v?e('tra:6' se/ectire paint,
2S--
/
/
J TO/o
I
I d
f I
/
E
J
l....... P00
630 F- .......... ] 6S0 60fl
[ SSO
] $00
l 1 4S0C111 4 ~
~ven~mbo~o
T
'
!
~0
T-
T
GSO
G00
T
T
T
550 W~von~mbo~G
SB~
4S~
-1 CrI'l 1
~00
Fig. 10. Transmission spectra of brown and black ~nc powders:(a) sample St, method T;(b)sample S2, method M.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints
279
4. Discussion and conclusions
Firstly, we like to point out that throughout our experiments method T produced black layers or powders of insufficient quality. Using [Cu(NH 3)4]SO4 • H 20 (method M) improves the solar absorptance of the samples. The common feature of both methods is poor adherence of the layers onto zincated steel substrates. The resulting spectrally selective coatings could not be considered sufficiently good for utilization in solar collector panels. Since the main purpose of our work was to prepare a spectrally selective pigment for TISS paint coatings, the potential use of the prepared black zinc powders was estimated from the reflectance they exhibit in pressed form. Pressed pellets could be considered as ultimate coatings where the pigment-to-volume (PVC) ratio is at its maximum. Therefore only worse spectrally selective properties could be expected when the corresponding paint would be prepared. One feature is worth mentioning, i.e. the higher reflectance of brown zinc powders compared to the black ones (figs. l l a and llb). This could be expected on account of smoother surface of the pressed pellet of brown samples, but it also reflects the higher transparency of the oxide layer formed on the zinc powder. The layer screens only slightly the infrared reflectance of the zinc powder itself.
100 617
80 a
60 ¸ 576
f R (%) 40-
b
2O
/
/
J
t
4000
3500
f
/
- - F
3000
t
r
2500 2000 Wavenumbers
T
1500
~
E
1000
500 cm- 1
Fig. 11. (a) Reflectivity spectrum of brown zinc powder: samples $1, method T. (b) Reflectivity spectrum of black zinc powder: sample $2, method M.
280
z. ('r&ak Ore/, B Ore/
Blacl~ :me pt~ment lor Vwctra/Iv ~eh'vti~,e paml~
D e t a i l e d classification of oxide layers f o r m e d on zincated steel plates a n d zin~ p o w d e r particles is not possible without a p p l i c a t i o n of a d d i t i o n a l and a p p r o p r i a t e methods, i.e. A u g e r spectroscopy, p o w d e r X - r a y diffraction, etc. The latter m e t h o d was used for i d e n t i f i c a t i o n of our samples, recently. Results o b t a i n e d c o n f i r m e d the function of C u O ( m e t h o d M), C u 2 0 ( m e t h o d T) c o m p o u n d s i n d i c a t i n g that C u ( ) which is f o r m e d on zinc p o w d e r particles is highly a m o r p h o u s . C o r r e l a t i o n between the infrared s p e c t r a and the crystallinity of s t o i c h i o m e t r i c C u O c o m p o u n d was also established. The most striking features o b s e r v e d in the reflectance s p e c t r u m of the black p o w d e r (M) is the existence of negative reflectance peaks, which c o r r e s p o n d s to the c o u p l e d m o d e s between p l a s m a oscillations a n d the one p a r t i c u l a r p h o n o n mode. Such negative p e a k s are also k n o w n from m o l e c u l a r t r a n s m i s s i o n a n d reflectance s p e c t r a as Evans b a n d s [22-24], a n d a p p e a r to be d u e to i n t e r a c t i o n s between a weak p h o n o n m o d e and closely s p a c e d p h o n o n levels. T h e shape a n d a p p e a r a n c e of the negative reflectance p e a k in black s a m p l e s indicates that vc~o mode(s) (for e x a m p l e the b a n d at 594 cm ~, see text) of the c o r r e s p o n d i n g oxide(s) f o r m e d in the presence of zinc p o w d e r are able to interact with the metal p l a s m a oscillations, thus c h a n g i n g their p o s i t i o n a n d the reflectance of the metal in the vicinity of the p h o n o n modes. Such an interaction requires i n t i m a t e c o n t a c t between the oxides a n d the substrate. I d e n t i f i c a t i o n of the n a t u r e a n d c o m p o s i t i o n of the oxides f o r m e d on the surface of the zinc particles will be p u b l i s h e d separately.
Acknowledgement A u t h o r s are i n d e b t e d to T h e R e s e a r c h C o m m u n i t y of Slovenia for financial support.
References [1] H. Tabor, Bull. Res. Counc. Isr. 5 A (2) (1956), [2] J. Vuletin, C.M. Lampert, P. Kuli~i~, M. Bosanac and J. Masteli6, Sol. Energy Mater. 19 (1989) 249. [3] Sold by Enthone Inc., New Haven, CT, USA, trade name Ebonol C. [4] C.M. Lampert, Sol. Wind Technol. 4 (1987) 347. [5] S.W. Moore, Sol. Energy Mater. 12 (1985) 435. [6] B. Orel, Z. Crnjak Orel and I. Radoczy, Sol. Energy Mater. 18 (1989) 97. [7] B.K. Gupta, F.K. Tiwari, O.P. Agnihotri and S,S. Mather, Int. J. Energy Res. 3 (1979) 371. [8] W.D. McKelvey and P.B. Zimmer, in: Proc. llth Natl. Technical Conf., 13-15 November 1979. [9] M. Telkes, US Patent 4011 (8 March 1977). [10] O.P. Agnihotri and B.K. Gupta, in: Solar Selective Surfaces (Wiley, New York, 1981) p. 146. [11] T. Karlsson and A. Roos, Sol. Energy Mater. 10 (1984) 105. [12] A. Roos, T. Chibuye and B. Karlsson, Sol. Energy Mater. 7 (1983) 453. [13] A. Aveline and I.R. Bonilla, Sol. Energy Mater. 5 (1981) 24. [14] A. Scherer, O.T. Ianl and A.J. Signh, Sol. Energy Mater. 9 (1983) 139. [15] A. Ishltani, H. Ishida, F. Socda and Y. Nagasawa, Anal. Chem. 54 (1982) 682. [16] G.W. Poling, J. Coll. Interface Sci. 34 (1970) 365.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints [17] [18] [19] [20] [21] [22] [23]
281
R.G. Greenler, J. Chem. Phys. 44 (1966) 310. T. Nguyen and E. Byrd, J. Coating Technol, 59 (1987) 39. R.G. Greenler, R.R. Rahn and J.P. Schwartz, J. Catal. 23 (1971) 42. S. Abrink and L.J. Norrby, Acta Cryst. B 26 (1970) 8. R.J. Elliot, Phys. Rev. 24 (1961) 340. J. Evans, Spectrochim. Acta 16 (1960) 994. R. Sudharsanan, S. Perkovitz, B. Lon, B.R. Caldwell and G.L. Carr, in: High Temperature Superconducting Materials, Eds. W.E. Hatfield and T.H. Miller (Dekker, New York, 1981) pp. 283-288. [24] Z. Crnjak Orel, B. Orel, M. Hodo~Eek and V. KauEiE, J. Mater. Sci. 25 (1990).
267
Preparation and characterization of low-emitting black zinc pigment for spectrally selective paints Z. Crnjak Orel and B. Orel Boris Kidrig Institute of Chemistry, P.O. Box 30, 61115 Ljubljana, Yugoslavia Received 2 July 1990; in revised form 25 July 1990
In this paper we describe the preparation of black zincated steel plates and zinc powders. Infrared reflection/absorption (6 °, 78 ° ), reflection (6 ° ) and transmission spectroscopy have been used for identification of layers formed on the corresponding substrates. It was found that modification of the existing method proposed by Telkes is necessary in order to obtain zinc powder with sufficient solar absorptance. Colour strengths of the layers and powders formed have been correlated with the existence of various copper(I) and copper(II) oxides in the layers. A negative reflectance peak at 595 c m - 1 has been detected in the reflectance spectrum of black zinc powder and its origine was attributed to the coupled 1,cu_o phonon mode with free electron oscillations of metallic zinc.
1. Introduction Spectrally selective black copper surfaces for solar energy utilization, produced by various chemical oxidation processes, have been extensively studied and investigated in the past [1,2]. The ease of application of the chemical oxidation process, connected with low cost of the raw materials, makes it possible to utilize black copper surfaces for solar collector panels. Moderate to high spectrally selective surfaces are known which are reasonably resistant to wear, weathering and thermal cycles [3]. Paint coatings are generally accepted as cheap alternatives to electroplated coatings when spectrally non-selective surfaces are sufficient for solar energy application [4]. Nevertheless, paint coatings with reasonable spectral selectivity are much more difficult to prepare [5], and besides require careful formulation of the paint and high-technology application procedures. This is particularly true for thickness sensitive spectrally selective paint coatings (TSSS), which are applied in layers a few micrometers thick on highly reflecting substrates by the coil-coating process [6]. Among the spectrally selective paint coatings, thickness insensitive (TISS) paint coatings, which even in thick layers (30-50/~m) exhibit spectral selectivity, occupy an exceptional position. They could be utilized not only as spectrally selective paint 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
268
/. ('rtzjak ()t'¢4. B. Or~,l
Bhzck zlm pignle#ll /)~r V~eclra//l' ~c/e,l~r~'pamt,.
coating [7] for solar energy conversion but could also serve for compensation ~imi equalization of emitted and reflected radiation, particularly in the thermovisiuN (8-13 /~m) region. They are appropriate for covering larger objects smcc their application does not require special painting techniques. Preparation of TISS paint coatings requires a low-emitting vehicle (pol 5butadiene-polystyrene co-polymer) and a low-emitting pigment such as various bronzes [8]. In order to keep the solar absorptance high, admixing a black pigment with the paint dispersion is necessary. There were attempts to formulate TISS paints which would contain only one type of pigment with the combined optical characteristics of b r o n z e / b l a c k pigment mixtures. Impact ball milling [8] of spinel-type black pigments with various bronzes requires careful preparation, otherwise the achieved spectral selectivity is low. From these reasons black zinc and black cobalt pigments [9] represent promising alternatives, since their preparation could be performed easily and without special precautions. The spectral selectivity of these kinds of pigments is achieved by the mutual action of highly reflecting metallic substrate particles coated by a thin layer of the appropriate black oxide. In such a way each particle of the pigment forms an absorber/reflector tandem. In this paper we describe the preparation and the optical properties of black pigments based on oxidized zinc pigment. Telkes [9] and Gupta [10] reported it.~ preparation and properties. According to these authors a, = 0.9 and e r = 0.4 of black zinc pigment could be achieved. We have modified this preparation method in order to obtain better efficiency and reproducibility of the process of oxidizing zinc powder. Infrared spectroscopy was used for structural identification of oxide layers formed on the surface of the zinc powder particles. Reflectance infrared spectra of pressed black zinc powders in pellets were measured in order to evaluate their use as spectrally selective pigments for TISS. Since mixed copper(I, II) oxides are formed on metallic particulates or solid copper substrates [11 14], investigation of the corresponding infrared spectra is reduced to the identification of various C u - O phonon modes appearing in the region ~ < 1000 cm ~ of the infrared spectrum. The infrared spectra of copper oxides formed on various metallic substrates are still not fully investigated by infrared spectroscopy. This is particularly true for other metals such as zinc or the corresponding powders and bronzes. Part of this work stems from the need to obtain more insight into the band assignment of the C u - O phonon modes of different cuprate compounds, which seems to be of importance for understanding the structural features concerning the oxidation state of copper in high-T~, superconducting ceramics of 1-2-3 type [15]. In such sintered materials, the structure of the interfaces, their composition and the oxidation state of copper are still not fully explained. Without this knowledge it would be impossible to correlate the phonon frequencies with the bond lengths, which is of prerequisite importance when lattice dynamics need to be constructed for 1-2-3 compounds. Therefore, the aim of this paper is also to show to what extent the appearance of copper oxide phonon modes are changed due to the interphase interactions which take place among mixed copper oxides and metallic substrates.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectivepaints
269
2. Experimental procedure Black zinc was prepared from zinc powder obtained from Cinkarna - Celje (Yugoslavia). After degreasing in Na2CO 3, any oxides possibly present were removed using diluted HC1 (5%) several times. The sample was reground and used for black zinc preparation. Black zinc was prepared according to Telkes [9]. Cu20 is readily formed, resulting in the dark brown colour of the prepared zinc powder. The spectral selectivity of the sample was classified according to its infrared reflectance spectra. The colour of the pigment was estimated visually. FTIR spectra were recorded on a FTS-80 Digilab spectrometer using a hemispherical reflection cell (DRIFT), with a collecting solid angle of 30 ° and a near-normal reflection cell (6 ° ). The reflection-absorption spectra of thin black zinc samples were recorded with a near-grazing incidence angle attachment (78 ° ). No polarizer (Rp component) was used throughout this work.
3. Results First attempts to prepare black zinc powder after Telkes' method (hereafter referred to as method T) [9] showed that by variation of temperature, dipping time and changing the content of CuSO 4 in the NaOH solution it was not possible to obtain a zinc powder with black colour. Nevertheless, the infrared reflectance of the powder pressed into a pellet was quite high ( - 60% at 10 btm), which promised the preparation of spectrally selective zinc powder. With regard to the dark brown colour of the powder, we considered that the reaction is proceeding in the direction of increased reduction of copper up to its elementary state. In order to prevent reduction of Cu 2+, the solution was prepared from [Cu(NH3)4]SO4. H20. This compound acts as a buffer in which the pH was kept high by addition of a certain amount of NaOH. A hot solution of buffer and NaOH was poured into the boiling zinc powder slurry which turned black in a few seconds (method M). Samples were filtered and washed with an abundance of water in order to remove SO2- and N H 3 residues and then used for spectroscopic investigations. Optimization of solar apsorptance (a~) and thermal emittance (ex) values was obtained by changing the temperature, the concentration of buffer in solution and the concentration of NaOH. As a result, different black and brown shades of Zn powder were obtained. The efficiency of the oxidation process was estimated visually. The solar absorptances of powders were not measured spectrometrically because of the problems caused by the different particle sizes and consequent scattering, which makes the reproducibility of the determination questionable. For quick estimation of the thermal emittance achieved, the reflectance values at 10/~m were used. Tables 1-3 show the results. The optimum conditions are indicated in these tables.
Z, Crnjak Oret. B. Orel / Black zinc pigment l~r spectrallv ,~ele~m,e pamt~
270
Table 1 Spectral reflectivity (in percent) of differently prepared black zinc powders; concentration of inte~ mediate compound has been varied at constant weight of NaOH (0,62 g/ Weight
Reflectivity at
[Cu(NH3)4]SO4 . H : O (g)
4000 cm
A
0.9
B
0.27
C
0.32
D
0.36
E
0.4
F
0.45
30.6 27.5 15.4 1.4 17.2 13.4 11.5 13.95 13.95 21.6 17.9 17 1(/.95 8.39
i
Visually' estimated 2000 cm 44.8 43.8 28.2 24.8 32 18.9 19.2 21.43 26.25 35.7 32.3 32 21.49 16.12
t
1000 cm i
~hade a~
57.4 57.7 41.9 36.6 43 25.6 28.7 30.70 39.09 49.7 35.4 45.6 32.8 24.4
Gray Gray black
Gray-black Dark gray-black Black
Brown-black
Similar results were also obtained when zincated steel plates were dipped into the solution with the buffer compound (M), or without (T) it. They always appeared black when the former method was used, but a dark brown shade was obtained after the latter. The adhesion of the layers formed was better for the M samples (black) than for the T samples (brown). Nevertheless, the quality was not sufficient to promise the efficient use of these plates as spectrally selective coatings. Therefore
Table 2 Spectral reflectivity (in percent) of differently prepared black zinc powders: concentration of NaOH has been varied, keeping the weight of the intermediate compound at 0.27 g Weight
Reflectivity at
NaOH
4000 cm 1
2000 c m - 1
1000 cm - i
A
0.432
B
0.62
C
0.8
D
1.2
17.76 20.43 13.03 15.4 17.26 14 12.65 13.69 17.65 16.8 19.1
30.4 33.7 26.2 28.2 32.1 24.8 20.27 21.41 27.9 23.73 26.79
42.2 46.7 39.6 41.9 43.6 36.6 30.27 30.15 37.4 30.97 33.58
(g)
Visually estimated shade
a~
Grey
Black
Black
Black
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectioe paints
271
Table 3 Spectral reflectivity (in percent) of differently prepared black zinc powders; concentration of N a O H has been varied, keeping the concentration of intermediate c o m p o u n d at 0.4 g Weight
Reflectivity at
NaOH
4000 c m - 1
2000 c m - 1
1000 cm - l
A
0.48
B
0.62
C
0.80
D
0.88
E
1.2
1.4 11.5 17.9 14.9 17.9 13.7 9.28 15.7 15.26 10.96 10.80 18.2 18 16.7
24 18.5 32.3 27.4 32.3 20.9 16.34 26.8 26.06 16.62 17.32 21.45 22.36 21.6
37 30 45.4 38.4 45.6 27 23.8 38.1 36.2 22.1 23.4 22.78 24.4 25.0
(g)
Visually estimated shade
a~
Black Black
Black
Black
Black
these samples were mainly used for systematic study of the structure and composition of the layers formed on plates and on zinc powder.
3.1. IR spectra of black zincated steel plates In order to identify the dark brown layers of black zinc formed on zincated steel after methods M and T, infrared reflection/absorption ( R / A ) (6 ° and 78 ° incidence angle) and transmission spectra were recorded and the bands assigned. The R / A (6 °) spectrum of the thick black layer (method M) (figs. l a - l c ) exhibits, besides the bands characteristic for SO42- and NH 3 groups (at 1432, 1124, 880, 701 and 615 cm -1) also a band at 500 cm -I which could not be attributed to SO42- or NH 3 groups. A part of the layer was removed from the plate, washed in water, dried and the transmission spectrum was recorded. As was expected all the bands disappeared except the bands at 500 and 601 cm -1 (fig. lc). According to the chemical reactions which take place in solution, it was expected that the layers formed contain copper(l) and copper(II) oxides as well as ZnO, various cuprates and also elementary copper. The reflection and transmission spectra of these compounds are reproduced in figs. 2-5. After comparing the bands appearing in the transmission and reflection spectra, and bands from reflection spectra calculated from the K spectra of CuO and Cu 2O, we concluded that the black layer consists of both types of copper oxides, the former (CuO) being formed in large excess (90%) with respect to the latter (Cu 20). Detailed examination of the band positions revealed that there is no one-to-one correspondence between the Cu-O mode position (fig. lb) in the spectra of black layers and CuO and/or Cu20 stoichiometric compounds [15,16]. For example, the
272
Z. CrnjaA Ord, B. Ore/ / Blucl, zinc ptgnlen£/hr spectr~d!r .~dcrHl'e l*eem~
1.=i
• !,
:£ J
/i ~ i; ,; !! i , i
i~3J
,' iii i
R%j
.
n A
-
. . . . . . . . . . .
40~0
352@
23@@ 2@@@ W~.ve n utah e ~,9, °
300@
15~@
1@@@~ cm'
/
i
!
S@@
532 ~501
z'•I042"
J
R% [ 1418
i
i
ag@O
i
36@@
. . . . . r ....... -[ . . . . . . . . . . . . . . . T 95@@ 2@@@ 1S0@ 3@@@ W~venumber s
....
T _~ 1@@~ cm
. 5@@
-/-
e
I
'i
T,:
/
'i,P\
• 13[~,M • ~d
\ $0@•~ c m
i f3fdM ~ f3
3!)~, 3
~A VENUMf3~[~
Fig. 1. Vibrational spectra of black layers (method M) on zincated steel plate: (a) r e f l e c t i o n / a b s o r p t i o n (6 o) s p e c t r u m of thick layer; (b) r e f l e c t i o n / a b s o r p t i o n (78 ° ) s p e c t r u m of thin layer; (c) t r a n s m i s s i o n s p e c t r u m of removed layer•
273
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectiue paints
0,70.5-
,
/ \
',
\&&3
W Fig. 2. Transmission spectra of stoichiometric compounds: (a) CuO; (b) Cu20; (c) ZnO.
~'cuo mode of the stoichiometric CuO compounds which is split at 595, 526 and 488 cm -~ in the K spectrum, appears as a single band at 495 c m - t in the R/A ( 6 ° ) spectrum of the thick black layer (figs. 4-6). Obviously, the influence of the too great a thickness of the layer smears out the detailed structure of the R/A (6 °) band at 495 cm-1 (fig. la). For this reason the R/A (78 °) spectrum of the thin ( < 1 /~m) black layer was measured and the corresponding bands detected (fig. lb). It was found that the single band at 495 cm -a (R/A at 6 °) split into 627, 532 and 501 cm -~, while the band at 615 cm-~ is shifted to 627 cm-1 with decreased intensity. The general appearance of the bands in the R/A (78 ° ) spectrum of the thin black layer is very similar to the ucu-o mode of the K spectrum of the CuO
Io"
618
25-
-X
2{]--
15-R%
10-
5b
0150B
I 12gO
I
1100
iZgg
1
1
990 800 W~venumbers
I
780
BBB
5~cm ~
40~
Fig. 3. Kramers-Kronig analysis of polycrystalline Cu20: (a) reflectivity spectrum; (b) k spectrum; (c) n spectrum.
274
Z. Crnlak Ore/. B. ()re/ / Black zim pigment/or
.~psclra//l'
~C/t'ctit'd [~al#il~
h'
2.04 ¸
C
n,k
R°,,, !
3
b
0.0-
120{3
1300
1 100
1000
900
600
] 200
F 600
r S00 c~'
I- ~ 40l~
W~venL~mbe~s
Fig. 4. Kramers-Kronig analysis of polycrystalline CuO: (a) reflectivity spectrum: (b) k spectrum: (c) n spectrum.
compound. All the bands appear, except the band at 596 cm- J which could not be detected in the R/A (78 °) spectrum of the thin black layer. Similarly, the band at 627 c m - 1 could not be assigned to the P('uO mode of the CuO compound but to v(.uo of Cu 20 (figs. 2 and 3). The differences in the band positions appearing in the R/A (78 °) spectra and transmission or R/A (0 o) spectra deserve a comment. It is generally accepted that the maximum of extinction (a = 4eK/X), i.e. phonon frequency, corresponds well
411
50-7
529
~ !
• L',
3~
i
R°/o r
/
/
//
4 1,V
: .,k
/
i
I !
a
b t,
]300
! 200
I 100
1000
9{30
800
?{30
G00
50~ cn~~ 400
W~ven L~mber s
Fig. 5. Kramers-Kronig analysis of polycrystalline ZnO: (a) reflectivity spectrum; (b) k spectrum: (c) n spectrum.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints
275
55.0 50.0
G
~
~(10-
,
1300 1200
O
-
,
,
I000
BOO II
, BOO
~00(cn~I)
Fig. 6. Transmission spectrum of the layer formed on zincated steel of method T.
to the minimum of transmission [17-19]. The refractive index n which varies across the band does not shift the position of the minimum away from the frequency at which the absorption coefficient ( K ) reaches maximum values. In contrast, in the R/A spectra the bands are split a n d / o r shifted to higher wavenumbers, i.e. to that region where the refractive index of the particular phonon mode reaches the minimal values. Their appearance depends upon the thickness of the layer and the incidence angle, and can be calculated when K(~) and n(~) for a certain band are known. For example, the Vcuo modes of Cu 20 and CuO in the R/A (78 o) spectra of thin ( - 200 ,~) layers appear at 650 and 540 cm -1, respectively [16,19], which is higher with respect to the bands of bulk samples (620 and 510 cm-~). A detailed assignment of the CuO modes appearing in the R / A (78 o) and R/A (0 °) spectra is not possible without previous calculations of n and K. It is impossible to estimate the blue shifts of the bands, appearing in the R / A (78 ° ) spectrum of black zinc layer in order to check unambiguously the one-to-one correspondence of these modes with vcu_o modes of CuO and C u 2 0 compounds. Additionally, it was found that the Uz,_ o band of the ZnO compound appears at 529 cm -1, which corresponds fairly well to the 532 cm-1 band detected in the R/A (78 ° ) spectrum of the thin layer. Since ZnO is white, and the layers were not completely black, we could not eliminate the possibility of the formation of ZnO. Analogously, an identification of the brown layer formed on zincated steel plate according to method T was made. The infrared transmission spectrum of a removed layer (fig. 6) clearly shows that Cu 20 is formed in excess of CuO, as demonstrated by the stronger VCuo mode of Cu20 (603 cm -1) compared to the same mode of the CuO compound (520 cm-1). Similarly, the influence of the R/A (78 o ) measuring technique was demonstrated by the shifts of the corresponding bands to 609 and 528 cm-~ (fig. 7). Identification of compounds which are present in the layer seems not to be straightforward for Cu 2° and CuO. Again the band positions do not exactly correspond to the position
276
Z. [ rnjak OreL B. Ore/ ,/ Black zm~ ptA,men: l~,r V~e<'tral]l ~e/e
%7.~ I
~uO
/
509
L,O.O-
-~C~O
I
36.0
13oo
12oo
looo w
Fig. 7. Reflection/absorption (78 ° ) spectrum of the layer formed on zincated steel by method 7.
of the bands which are characteristic of P(:u-o modes of pure stoichiometric CuO and Cu20 compounds. Both types of layers (T and M), black and dark brown, could be considered as mixtures of various oxides of copper and zinc. Nevertheless, they are not simple mixtures of pure stoichiomeric oxides with the corresponding structures C6h (CuO [20]) and 0 4 (Cu20 [21]), since the one-to-one correspondence between the infrared bands of layers and pure oxides was never obtained. 3.2. IR spectra of black zinc powder
The most obvious fact which results from the comparison of different black zinc powders is the striking difference in their reflectance spectra. While the reflectance spectrum of the brown sample (method T) (fig. 8) shows the presence of C u 2 0 oxide, the corresponding reflectance spectra of the black sample (method M) (fig. 9) could not be identified as a simple sum of IR reflectance spectra of either of the copper a n d / o r Zn oxides . This feature was confirmed by the corresponding K spectra (fig. 8c) which clearly showed that in the brown sample the Ucuo mode appears at 619 cm -1 indicating the presence of C u 2 0 (see also fig. 3b), while no distinct band could be detected in the K spectrum in the region 600-400 cm 1. In spite of this a deep negative reflectance peak has been found in the spectrum at 594 cm 1. Such a negative band is not present in the brown sample (method T). Transmission spectra of the brown sample (T) (fig. 10a) confirms the presence of C u 2 0 from the band at 630 cm-~. This band fairly well coincides with the UCuo mode determined from the reflectance (fig. 3b) and transmission spectra (fig. 2 curve b) of the C u 2 0 compound. The difference (10 cm -1) between band positions observed in reflectance and transmission or absorbance spectra could be attributed to the decreasing (or increasing, in the absorptance spectra) background onto which the band is superimposed (fig. 10a).
277
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selectivepaints 7-]
,
5
- 66
SI~
-
1500
1201~
1100
1000
91~0 800 W~,vanttmbo~'s
200
600
500 ca' 400
Fig. 8. Kramers-Kronig analysis of the black zinc powder (sample $1, method T) pressed into a pallet: (a) reflectivity spectrum; (b) n spectrum; (c) k spectrum.
The transmission spectrum of the black sample (M) (fig. 10b) exhibits bands at 529 and 472 cm -1, which could both correspond to ZnO or CuO compounds (fig. 2), or to some kind of new oxides which are formed o n the surface of the zinc particle. Hence the black zinc powder could not be considered to be a simple mixture of pure oxides and zinc powder.
3,5-
-45
3.0-
~
2.5
b
n,k
35
2.[~-
R%
1.5-
3~
1.0-
25 c
0.5-
1500
a I
I
1200
1100
~ - ~ T
I000
F
900 800 Waven~mber~
[-
I
71~0
800
20
I
500 e~
400
Fig. 9. l~amers-I~onig analysis of the black zinc powder (sample $2, method M) pressed into a pellet: (a) reflectivity spectrum; (b) n spectrum; (c) k spectrum.
278
Z. Crnjak Ore/, H. O r e / / Black :im pigment/i~r .v?e('tra:6' se/ectire paint,
2S--
/
/
J TO/o
I
I d
f I
/
E
J
l....... P00
630 F- .......... ] 6S0 60fl
[ SSO
] $00
l 1 4S0C111 4 ~
~ven~mbo~o
T
'
!
~0
T-
T
GSO
G00
T
T
T
550 W~von~mbo~G
SB~
4S~
-1 CrI'l 1
~00
Fig. 10. Transmission spectra of brown and black ~nc powders:(a) sample St, method T;(b)sample S2, method M.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints
279
4. Discussion and conclusions
Firstly, we like to point out that throughout our experiments method T produced black layers or powders of insufficient quality. Using [Cu(NH 3)4]SO4 • H 20 (method M) improves the solar absorptance of the samples. The common feature of both methods is poor adherence of the layers onto zincated steel substrates. The resulting spectrally selective coatings could not be considered sufficiently good for utilization in solar collector panels. Since the main purpose of our work was to prepare a spectrally selective pigment for TISS paint coatings, the potential use of the prepared black zinc powders was estimated from the reflectance they exhibit in pressed form. Pressed pellets could be considered as ultimate coatings where the pigment-to-volume (PVC) ratio is at its maximum. Therefore only worse spectrally selective properties could be expected when the corresponding paint would be prepared. One feature is worth mentioning, i.e. the higher reflectance of brown zinc powders compared to the black ones (figs. l l a and llb). This could be expected on account of smoother surface of the pressed pellet of brown samples, but it also reflects the higher transparency of the oxide layer formed on the zinc powder. The layer screens only slightly the infrared reflectance of the zinc powder itself.
100 617
80 a
60 ¸ 576
f R (%) 40-
b
2O
/
/
J
t
4000
3500
f
/
- - F
3000
t
r
2500 2000 Wavenumbers
T
1500
~
E
1000
500 cm- 1
Fig. 11. (a) Reflectivity spectrum of brown zinc powder: samples $1, method T. (b) Reflectivity spectrum of black zinc powder: sample $2, method M.
280
z. ('r&ak Ore/, B Ore/
Blacl~ :me pt~ment lor Vwctra/Iv ~eh'vti~,e paml~
D e t a i l e d classification of oxide layers f o r m e d on zincated steel plates a n d zin~ p o w d e r particles is not possible without a p p l i c a t i o n of a d d i t i o n a l and a p p r o p r i a t e methods, i.e. A u g e r spectroscopy, p o w d e r X - r a y diffraction, etc. The latter m e t h o d was used for i d e n t i f i c a t i o n of our samples, recently. Results o b t a i n e d c o n f i r m e d the function of C u O ( m e t h o d M), C u 2 0 ( m e t h o d T) c o m p o u n d s i n d i c a t i n g that C u ( ) which is f o r m e d on zinc p o w d e r particles is highly a m o r p h o u s . C o r r e l a t i o n between the infrared s p e c t r a and the crystallinity of s t o i c h i o m e t r i c C u O c o m p o u n d was also established. The most striking features o b s e r v e d in the reflectance s p e c t r u m of the black p o w d e r (M) is the existence of negative reflectance peaks, which c o r r e s p o n d s to the c o u p l e d m o d e s between p l a s m a oscillations a n d the one p a r t i c u l a r p h o n o n mode. Such negative p e a k s are also k n o w n from m o l e c u l a r t r a n s m i s s i o n a n d reflectance s p e c t r a as Evans b a n d s [22-24], a n d a p p e a r to be d u e to i n t e r a c t i o n s between a weak p h o n o n m o d e and closely s p a c e d p h o n o n levels. T h e shape a n d a p p e a r a n c e of the negative reflectance p e a k in black s a m p l e s indicates that vc~o mode(s) (for e x a m p l e the b a n d at 594 cm ~, see text) of the c o r r e s p o n d i n g oxide(s) f o r m e d in the presence of zinc p o w d e r are able to interact with the metal p l a s m a oscillations, thus c h a n g i n g their p o s i t i o n a n d the reflectance of the metal in the vicinity of the p h o n o n modes. Such an interaction requires i n t i m a t e c o n t a c t between the oxides a n d the substrate. I d e n t i f i c a t i o n of the n a t u r e a n d c o m p o s i t i o n of the oxides f o r m e d on the surface of the zinc particles will be p u b l i s h e d separately.
Acknowledgement A u t h o r s are i n d e b t e d to T h e R e s e a r c h C o m m u n i t y of Slovenia for financial support.
References [1] H. Tabor, Bull. Res. Counc. Isr. 5 A (2) (1956), [2] J. Vuletin, C.M. Lampert, P. Kuli~i~, M. Bosanac and J. Masteli6, Sol. Energy Mater. 19 (1989) 249. [3] Sold by Enthone Inc., New Haven, CT, USA, trade name Ebonol C. [4] C.M. Lampert, Sol. Wind Technol. 4 (1987) 347. [5] S.W. Moore, Sol. Energy Mater. 12 (1985) 435. [6] B. Orel, Z. Crnjak Orel and I. Radoczy, Sol. Energy Mater. 18 (1989) 97. [7] B.K. Gupta, F.K. Tiwari, O.P. Agnihotri and S,S. Mather, Int. J. Energy Res. 3 (1979) 371. [8] W.D. McKelvey and P.B. Zimmer, in: Proc. llth Natl. Technical Conf., 13-15 November 1979. [9] M. Telkes, US Patent 4011 (8 March 1977). [10] O.P. Agnihotri and B.K. Gupta, in: Solar Selective Surfaces (Wiley, New York, 1981) p. 146. [11] T. Karlsson and A. Roos, Sol. Energy Mater. 10 (1984) 105. [12] A. Roos, T. Chibuye and B. Karlsson, Sol. Energy Mater. 7 (1983) 453. [13] A. Aveline and I.R. Bonilla, Sol. Energy Mater. 5 (1981) 24. [14] A. Scherer, O.T. Ianl and A.J. Signh, Sol. Energy Mater. 9 (1983) 139. [15] A. Ishltani, H. Ishida, F. Socda and Y. Nagasawa, Anal. Chem. 54 (1982) 682. [16] G.W. Poling, J. Coll. Interface Sci. 34 (1970) 365.
Z. Crnjak Orel, B. Orel / Black zinc pigment for spectrally selective paints [17] [18] [19] [20] [21] [22] [23]
281
R.G. Greenler, J. Chem. Phys. 44 (1966) 310. T. Nguyen and E. Byrd, J. Coating Technol, 59 (1987) 39. R.G. Greenler, R.R. Rahn and J.P. Schwartz, J. Catal. 23 (1971) 42. S. Abrink and L.J. Norrby, Acta Cryst. B 26 (1970) 8. R.J. Elliot, Phys. Rev. 24 (1961) 340. J. Evans, Spectrochim. Acta 16 (1960) 994. R. Sudharsanan, S. Perkovitz, B. Lon, B.R. Caldwell and G.L. Carr, in: High Temperature Superconducting Materials, Eds. W.E. Hatfield and T.H. Miller (Dekker, New York, 1981) pp. 283-288. [24] Z. Crnjak Orel, B. Orel, M. Hodo~Eek and V. KauEiE, J. Mater. Sci. 25 (1990).