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dielectric multilayers for high-temperature solar selective absorbers
Sollar ~ MaMtmials and Solar C~ls
ELSEVIER
Solar Energy Materials and Solar Cells 43 (1996) 59-65
Computer modeling of the performance of some metal/dielectric multilayers for high-temperature solar selective absorbers J.H. Sch~Sn *, E. Bucher Unit,ersitiit Konstanz, FakultiJtfiir Physik, D-78434 Konstanz, Germany
Abstract Solar thermal performance of metal/dielectric multilayer coatings of various material combinations was calculated using a computer program. The simulated material combinations were selected with respect to their use in high-temperature (T _> 500°C) solar thermal energy conversion. Maximum solar absorptances et = 0.94 with low emittances e = 0.16 (T = l l00 K) were calculated for A1zOa/noble metal (Rh, Ir) coatings. Simulations on multilayers of noble metal oxides, like RuO 2 and [rO2, were carried out to optimize solar absorption a using optical data in the region from 300 nm to 2.3 ~m. Absorptances as high as o~ = 0.96 were obtained for both systems. Keywords: Metal/dielectric multilayers; Solar thermal energy conversion; Solar absorptance; Noble metal oxides
I. Introduction Several selective coatings for photothermal conversion have been described in literature. Many of them are stable in vacuum up to 800°C or even more, as for example the well known A1203/Mo-multilayers [1,2], but only few of them show good thermal stability in air above 400°C [2-5]. However, higher temperatures would be favourable to enhance system operating efficiency. Besides optical performance, thermal stability and oxidation resistance of the used materials is crucial for possible high-temperature solar selective absorber systems based on metal/dielectric multilayers. Therefore noble metals like Pt, Rh, Ir, and their oxides, like RuO 2 and IrO2, could be a proper choice for the metal layer, as well as Pt/A1203 cermets [6,7] and multilayer coatings [8] showed
* Corresponding author. 0927-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 7 - 0 2 4 8 ( 9 5 ) 0 0 1 6 4 - 6
60
J.H. Schan, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
good optical properties and thermal stability. Furthermore, it was reported that IrO 2 and RuO 2 should show very good optical properties in metal-based tandem absorbers [9]. Using the data for the complex refractive index solar thermal performance of P t / , Rh/, RuO2/, and IrOz/A1203-multilayers was studied. Solar absorptances up to ct = 0 . 9 4 and emittances as low as e = 0 . 1 6 resulting in a conversion efficiency "q = 0.86 at an operating temperature T O = 1100 K and a 100-fold sunlight concentration (C = 100) were obtained (definition of "q, see below).
2. Calculations Optical properties of multilayer-coatings were evaluated from optical bulk properties by means of the matrix method described by Yeh [10]. Calculations were carried out using a computer simulation program presented earlier [11]. In order to classify optical properties of various multilayers with respect to high temperature solar thermal applications we determine quality criterions such as the solar absorptance oL, the total thermal emittance e, and the figure of merit f [1]. They are defined as follows:
f a,(~). ~(~) d,
a=
"A
,
fb(,.r). ~(,) d, "A
s=
£4,( ,) d,
,
f=
OL --,
f b(,,r) d,
where c~00 = spectral absorptance, q~(X) = intensity of solar radiation, e 0 0 = spectral emittance, and b(A,T) = intensity of black body radiation. As solar distribution function the AM 1.5 spectrum was used, which was measured in the NASA-Lewis Research Center, Cleveland, OH 44134 [12]. The values for a , s, and f can easily be evaluated from the calculated reflection spectra of the multilayers ( r ( * ) ) using the following formula:
~ ( a ) = ~(a)
=
1
-
r(a).
In general large values of f indicate good spectral selectivity; however the figure of merit is not always wellsuited to estimate solar thermal power conversion efficiency of optical absorbers [13]. The conversion efficiency aq compares the heat flux into the working fluid with the incident total solar flux [14]. It takes the operating temperature of the absorber and the concentration factor for the incident sunlight C into account. T o -- 1100 K and C = 100 was chosen as parameter set throughout the calculations. This quality criterion is defined as:
f ~(,). c. ~ ( , ) . d , - f?(*,to). ~(*) d, 7 1=
A
A
f~(A).
CdA
The following notation was chosen to describe the composition of the multilayers: S/(lJMy)
* n.
J.H. Schgn, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
61
S represents the substrate material, I the insulating component, and M the metallic component of the multilayer. The substrate material S was chosen as the metal M for all our calculations. The indices x and y correspond to the thicknesses of the dielectric, and metallic layer respectively. The modulation number n describes the number of periods and is normally halfvalued. A value of n = 2.5 means two I / M layers with an additional insulating top-layer. AI203 was selected as insulator throughout the whole work. A is used as abbreviation for A1203.
3. R e s u l t s 3.1. N o b l e m e t a l s
An optimization of layer thicknesses with respect to maximum conversion efficiency "q was carried out for A 1 2 0 3 / I r and A 1 2 0 3 / R h multilayers at fixed modulation numbers from n = 0.5 (antireflection coating) to n = 6.5 using the optical data for bulk material of AI203, Rh, and Ir [15]. The calculated reflection spectra can be seen in Fig. 1. A comparison with the results of calculations for A / P t absorber coatings [8,11] is shown in Table 1 and Fig. 2. The best optical properties of both systems are for high modulation numbers (n = 6.5). Solar absorption ~ as well as emittances e, rise with higher modulation number, while the emittances grow more slowly, resulting in an increasing conversion efficiency "q. This behaviour is in contrast to that of A / P t multilayers. For this system the emittance grows much faster than the solar absorptance, so that the maximum conversion efficiency r I is reached for n = 2.5. A reason for this slower increasing reflection in the infrared region is the lower refractive index, the real as well as the imaginary part, of Pt compared to Ir and Rh [15].
1.0
0.8
0.6
0.4
17Ill, t '
0.2
0.0 300
Wavelength [nm] Fig. 1. Reflection spectra of multilayers: ( Pt/(A65/Pt 5) * 6.5.
) Rh/(A 7o/Rh 3) * 6.5, (- - -) Ir(A70fir4) * 6.5, (. • - )
62
J.H. Sch6n, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
Table 1 Comparison of the spectral selectivity of optimized A/noble metal multilayer coatings (x-y [nm]) n
0.5 1.5 2.5 3.5 4.5 5.5 6.5
P~-AI203
Rh-A1203 Ir-Al203
x-y
~
a
•
f
x-y
~
a
•
f
x-y
~
a
e
f
70-0 80-8 65-5 55-4 60-4 65-5 70-5
0.50 0.82 0.84 0.84 0.83 0.83 0.82
0.53 0.90 0.93 0.93 0.94 0.94 0.94
0.06 0.09 0.12 0.14 0.16 0.18 0.20
8.8 10.0 7.8 6.7 5.9 5.2 4.8
70-0 80-5 70-4 60-3 65-3 70-3 70-3
0.35 0.74 0.82 0.84 0.84 0.84 0.85
0.38 0.80 0.90 0.91 0.92 0.93 0.94
0.04 0.08 0.11 0.12 0.13 0.14 0.15
9.5 9.9 8.2 7.6 7.1 6.6 6.3
70-0 80-7 70-5 75-5 65-4 70-4 70-4
0.45 0.80 0.84 0.85 0.86 0.86 0.86
0.47 0.85 0.91 0.93 0.93 0.94 0.94
0.04 0.09 0.12 0.13 0.14 0.15 0.16
11.8 9.4 7.6 7.2 6.6 6.3 5.9
H o w e v e r , this high m o d u l a t i o n n u m b e r for the o p t i m i z e d A / R h and A / I r multilayers c o u l d be a d r a w b a c k for application because o f higher e x p e n s e s during fabrication o f the selective absorbers, although their predicted optical properties are better than for A / P t coatings. T h e r m a l stability o f Ir- and R h - c o a t i n g s should be c o m p a r a b l e to A / P t - m u l t i layers. T h e s e absorbers s h o w e d no degradation in air at temperatures up to 600°C for up to 400 h [8]. Simulations and e x p e r i m e n t s w e r e found to be in g o o d a g r e e m e n t for these absorbers. 3.2. N o b l e m e t a l o x i d e s
Calculations for A / R u O 2 and A / I r O 2 multilayers were carried out using the optical data o f R u O 2 and IrO 2 reported in literature [16,17]. Optimization was restricted to solar absorption because o f a lack o f data in the infrared region. The m a x i m u m absorption for
I
I
l
I
I
I
0.9
A
A
A •
• 0.8
O
0.5
A O 0.4
i
0,3
•
&
0
I
I
i
I
2
t
I
3
,
I
•
PffA120 3
•
RNbJ20 3
A
Ir/Al20 3
,
4
I
5
i
I
,
8
Modulation Number n Fig. 2. Conversion efficiency "q of optimized multilayers vs. modulation number n.
J.H. Schrn, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65 1.0
i
i
J
63
i
0.8 (•o
0.6
~
t" .'J
0.4
,•1 •
."
0.2
• .•*
." / i
/
/¢ '
0.0 500
1000
1500
2000
Wavelength [nm] Fig. 3. Reflection spectra of multilayers: ( - IrO 2/(A70/1IO2/70)* 6.5, (. • .) Ir/(A7o/Ir4)* 6.5.
) RuO 2 / ( A 6 5 / R U O 2 / 6 5 ) , 6 . 5 ,
(---)
both systems is ~t = 0.96, but is reached for the R u O 2 system for n = 2.5 to 6.5, while for IrO 2 only for n = 6.5. Comparing the reflection spectra of optimized 6.5-multilayers of Ir, IrO 2, and RuO 2 (Fig. 3), we notice a better solar absorption for both oxides than for Ir. Furthermore, the metal layers of IrO 2 and RuO 2 are about one order in magnitude thicker than the Pt-, Ir-, and Rh-layers in the optimized coatings (see Tables 1 and 2). This is due to the higher refractive index of the noble metals [ 15,16] resulting in a nearly equal optical pathlength for all 5 materials. In addition, the absorption in these thin layers is higher for noble metals, so that a thicker noble metal oxide layer is required to absorb the same amount of radiation. While RuO 2 exhibits very low infrared reflectance (up to 2.3 txm), the reflection spectra of A / I r O 2 increase even faster than A / I r , so that very low emittances are expected• This can also be seen from the reflection spectra of the bulk materials• The reflection of IrO 2 reaches more than 95% in the IR (above 2 I~m), while 70% for RuO 2 [16], so one has to expect higher emission losses for this material.
Table 2 Comparison of the spectral selectivity of optimized A/noble metal oxide multilayer coatings (x-y [nm]) n
0.5 1.5 2.5 3.5 4.5 5.5 6.5
A1203/RuO 2
A1203/IrO e
x-y
a
x-y
a
55-0 60-33 65-25 60-21 60-20 65-21 65-20
0.84 0.93 0.96 0.96 0.96 0.96 0.96
60-0 70-38 70-28 70-24 70-21 70-20 70-18
0.82 0.88 0.92 0.94 0.95 0.95 0.96
64
J.H. Sch6n, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
4. Discussion Simulations predict very good spectral selectivity for all four systems, especially for Ir and IrO 2 in combination with alumina. Furthermore, there should be good agreement of the optical properties with if-sputtered samples as seen in earlier investigations for A / P t and A / M o S i z selective coatings [8]. Additionally, all four materials, as well as AI20 3, show very good thermal stability and corrosion resistance [17,18]. As a result of these multilayer systems, all of these material combinations may be expected to be stable in air at temperatures above 500°C. RuO 2 also seems to be a good diffusion barrier [19], so that thermal stability might compensate for the expected emission losses. It must be mentioned, that all calculations were carried out using the optical data of the bulk-materials at room temperature. Hence higher emission losses can be expected. For example, the emittance of copper at 1100 K is doubled from 0.015 at room temperature to 0.03 [20]. Another point is an increased emittance for selective surfaces due to lattice resonances at infrared frequencies. However, as an interesting result, the emittance losses are lower at very high temperatures as at temperatures of about 300°C [21], so that high temperature solar selective absorbers are less likely troubled by this problem. Furthermore, optical constants of ultrathin metal films may differ from bulk data [22]; this may cause differences between the calculated and experimental reflection spectra of the multilayers, although this is not seen for Pt- and MoSi2-absorber coatings, as stated below [8].
5. Conclusion Simulation of reflection spectra of metal/dielectric multilayer coatings using the material combinations of AI203 with Ir, Rh, RuO2, and IrO 2 show very good optical properties. Solar absorptances as high as e~ = 0.94 can be expected combined with emittances lower than e = 0.16 resulting in a maximum conversion efficiency "q = 0.86 ( C = 100, T O= 1100 K). Multilayers, as well as cermets, may be prepared using (reactive) rf-magnetron sputtering techniques. All material combinations promise good thermal stability and corrosion resistance, so that they seem to be promising candidates for high temperature (T > 500°C), high concentrating solarthermal power production in air. However, experimental sample preparation will be necessary for a definitive evaluation.
Acknowledgements We would like to thank Mr. R. Kirchner for the development of the computer program for the simulation of optical properties of multilayer coatings.
References [1] R.E. Peterson and J.W. Ramsey,J. Vac. Sci. Technol. 12 (1975) 174. [2] R.E. Hahn and B.O. Seraphin, Phys. Thin Films 10 (1978) 1.
J.H. Sch6n, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
65
D.M. Mattox and R.R. Sowell, J. de Phys. Colloq 1 42 (1981) C1. S.A. Herzenberg and R. Silberglitt, Proc. of SPIE 324 (1982) 92. T.S. Sathiaraj, R. Thangaraj and O.P. Aghnihotri, Solar Energy Mat. 18 (1989) 343. G.A. Nyberg, H.G. Craighead and R.A. Buhrman, Thin Solid Films 96 (1982) 185. T.K. Vien, C. Siella, J. Lafait and S. Berthier, Thin Solid Films 126 (1985) 17. J.H. Sch~Sn, G. Binder and E. Bucher, Solar Energy Mat. 33 (1994) 403. C.G. Ribbing and B. Karlsson, Thin Solid Films 116 (1984) 341. P. Yeh, Optical Waves in Layered Media (Wiley-Interscience Publication, 1988). R. Kirchner, M.Ch. Lux-Steiner, E. Liebemann and E. Bucher, Solar Energy Mat. 33 (1994) 453. A.L. Fahrenbruch, R.H. Bube, Fundamentals of Solar Cells (Academic Press, New York, 1983). B.O. Seraphin (Editor), Solar Energy Conversion, Solid-State Physics Aspects, Topics in Applied Physics, Vol. 31, (Springer, Berlin, 1979). J.I. Gittleman, B. Abeles, P. Zanzucchi and Y. Arie, Thin Solid Films 45 (1977) 9. E.D. Palik, Handbook of Optical Constants of Solids (Academic Press, New York, 1985). A.K. Goel, G. Skorinko and F.H. Pollak, Phys. Rev. B 24 (1981) 7342. A. Belkind, Z. Orban, J.L. Vossen and J.A. Woollam, Thin Solid Films 207 (1992) 242. R.G. Vadimsky, R.P. Frankenthal and D.E. Thompson, J. Electrochem. Soc. 126 (1979) 2017. E. Kolawa, F.C.T. So, E.T.-S. Pan and M.-A. Nicolet, Appl. Phys. Lett. 50 (1987) 854. R. Smalley and A.J. Sievers, J. Opt. Soc. Am. 68 (1978) 1516. D.R. Mills and L.C. Botten, Appl. Optics 22 (1983) 3182. M. Yamamoto and T. Namioka, Appl. Optics 31 (1992) 1612.
ELSEVIER
Solar Energy Materials and Solar Cells 43 (1996) 59-65
Computer modeling of the performance of some metal/dielectric multilayers for high-temperature solar selective absorbers J.H. Sch~Sn *, E. Bucher Unit,ersitiit Konstanz, FakultiJtfiir Physik, D-78434 Konstanz, Germany
Abstract Solar thermal performance of metal/dielectric multilayer coatings of various material combinations was calculated using a computer program. The simulated material combinations were selected with respect to their use in high-temperature (T _> 500°C) solar thermal energy conversion. Maximum solar absorptances et = 0.94 with low emittances e = 0.16 (T = l l00 K) were calculated for A1zOa/noble metal (Rh, Ir) coatings. Simulations on multilayers of noble metal oxides, like RuO 2 and [rO2, were carried out to optimize solar absorption a using optical data in the region from 300 nm to 2.3 ~m. Absorptances as high as o~ = 0.96 were obtained for both systems. Keywords: Metal/dielectric multilayers; Solar thermal energy conversion; Solar absorptance; Noble metal oxides
I. Introduction Several selective coatings for photothermal conversion have been described in literature. Many of them are stable in vacuum up to 800°C or even more, as for example the well known A1203/Mo-multilayers [1,2], but only few of them show good thermal stability in air above 400°C [2-5]. However, higher temperatures would be favourable to enhance system operating efficiency. Besides optical performance, thermal stability and oxidation resistance of the used materials is crucial for possible high-temperature solar selective absorber systems based on metal/dielectric multilayers. Therefore noble metals like Pt, Rh, Ir, and their oxides, like RuO 2 and IrO2, could be a proper choice for the metal layer, as well as Pt/A1203 cermets [6,7] and multilayer coatings [8] showed
* Corresponding author. 0927-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 7 - 0 2 4 8 ( 9 5 ) 0 0 1 6 4 - 6
60
J.H. Schan, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
good optical properties and thermal stability. Furthermore, it was reported that IrO 2 and RuO 2 should show very good optical properties in metal-based tandem absorbers [9]. Using the data for the complex refractive index solar thermal performance of P t / , Rh/, RuO2/, and IrOz/A1203-multilayers was studied. Solar absorptances up to ct = 0 . 9 4 and emittances as low as e = 0 . 1 6 resulting in a conversion efficiency "q = 0.86 at an operating temperature T O = 1100 K and a 100-fold sunlight concentration (C = 100) were obtained (definition of "q, see below).
2. Calculations Optical properties of multilayer-coatings were evaluated from optical bulk properties by means of the matrix method described by Yeh [10]. Calculations were carried out using a computer simulation program presented earlier [11]. In order to classify optical properties of various multilayers with respect to high temperature solar thermal applications we determine quality criterions such as the solar absorptance oL, the total thermal emittance e, and the figure of merit f [1]. They are defined as follows:
f a,(~). ~(~) d,
a=
"A
,
fb(,.r). ~(,) d, "A
s=
£4,( ,) d,
,
f=
OL --,
f b(,,r) d,
where c~00 = spectral absorptance, q~(X) = intensity of solar radiation, e 0 0 = spectral emittance, and b(A,T) = intensity of black body radiation. As solar distribution function the AM 1.5 spectrum was used, which was measured in the NASA-Lewis Research Center, Cleveland, OH 44134 [12]. The values for a , s, and f can easily be evaluated from the calculated reflection spectra of the multilayers ( r ( * ) ) using the following formula:
~ ( a ) = ~(a)
=
1
-
r(a).
In general large values of f indicate good spectral selectivity; however the figure of merit is not always wellsuited to estimate solar thermal power conversion efficiency of optical absorbers [13]. The conversion efficiency aq compares the heat flux into the working fluid with the incident total solar flux [14]. It takes the operating temperature of the absorber and the concentration factor for the incident sunlight C into account. T o -- 1100 K and C = 100 was chosen as parameter set throughout the calculations. This quality criterion is defined as:
f ~(,). c. ~ ( , ) . d , - f?(*,to). ~(*) d, 7 1=
A
A
f~(A).
CdA
The following notation was chosen to describe the composition of the multilayers: S/(lJMy)
* n.
J.H. Schgn, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
61
S represents the substrate material, I the insulating component, and M the metallic component of the multilayer. The substrate material S was chosen as the metal M for all our calculations. The indices x and y correspond to the thicknesses of the dielectric, and metallic layer respectively. The modulation number n describes the number of periods and is normally halfvalued. A value of n = 2.5 means two I / M layers with an additional insulating top-layer. AI203 was selected as insulator throughout the whole work. A is used as abbreviation for A1203.
3. R e s u l t s 3.1. N o b l e m e t a l s
An optimization of layer thicknesses with respect to maximum conversion efficiency "q was carried out for A 1 2 0 3 / I r and A 1 2 0 3 / R h multilayers at fixed modulation numbers from n = 0.5 (antireflection coating) to n = 6.5 using the optical data for bulk material of AI203, Rh, and Ir [15]. The calculated reflection spectra can be seen in Fig. 1. A comparison with the results of calculations for A / P t absorber coatings [8,11] is shown in Table 1 and Fig. 2. The best optical properties of both systems are for high modulation numbers (n = 6.5). Solar absorption ~ as well as emittances e, rise with higher modulation number, while the emittances grow more slowly, resulting in an increasing conversion efficiency "q. This behaviour is in contrast to that of A / P t multilayers. For this system the emittance grows much faster than the solar absorptance, so that the maximum conversion efficiency r I is reached for n = 2.5. A reason for this slower increasing reflection in the infrared region is the lower refractive index, the real as well as the imaginary part, of Pt compared to Ir and Rh [15].
1.0
0.8
0.6
0.4
17Ill, t '
0.2
0.0 300
Wavelength [nm] Fig. 1. Reflection spectra of multilayers: ( Pt/(A65/Pt 5) * 6.5.
) Rh/(A 7o/Rh 3) * 6.5, (- - -) Ir(A70fir4) * 6.5, (. • - )
62
J.H. Sch6n, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
Table 1 Comparison of the spectral selectivity of optimized A/noble metal multilayer coatings (x-y [nm]) n
0.5 1.5 2.5 3.5 4.5 5.5 6.5
P~-AI203
Rh-A1203 Ir-Al203
x-y
~
a
•
f
x-y
~
a
•
f
x-y
~
a
e
f
70-0 80-8 65-5 55-4 60-4 65-5 70-5
0.50 0.82 0.84 0.84 0.83 0.83 0.82
0.53 0.90 0.93 0.93 0.94 0.94 0.94
0.06 0.09 0.12 0.14 0.16 0.18 0.20
8.8 10.0 7.8 6.7 5.9 5.2 4.8
70-0 80-5 70-4 60-3 65-3 70-3 70-3
0.35 0.74 0.82 0.84 0.84 0.84 0.85
0.38 0.80 0.90 0.91 0.92 0.93 0.94
0.04 0.08 0.11 0.12 0.13 0.14 0.15
9.5 9.9 8.2 7.6 7.1 6.6 6.3
70-0 80-7 70-5 75-5 65-4 70-4 70-4
0.45 0.80 0.84 0.85 0.86 0.86 0.86
0.47 0.85 0.91 0.93 0.93 0.94 0.94
0.04 0.09 0.12 0.13 0.14 0.15 0.16
11.8 9.4 7.6 7.2 6.6 6.3 5.9
H o w e v e r , this high m o d u l a t i o n n u m b e r for the o p t i m i z e d A / R h and A / I r multilayers c o u l d be a d r a w b a c k for application because o f higher e x p e n s e s during fabrication o f the selective absorbers, although their predicted optical properties are better than for A / P t coatings. T h e r m a l stability o f Ir- and R h - c o a t i n g s should be c o m p a r a b l e to A / P t - m u l t i layers. T h e s e absorbers s h o w e d no degradation in air at temperatures up to 600°C for up to 400 h [8]. Simulations and e x p e r i m e n t s w e r e found to be in g o o d a g r e e m e n t for these absorbers. 3.2. N o b l e m e t a l o x i d e s
Calculations for A / R u O 2 and A / I r O 2 multilayers were carried out using the optical data o f R u O 2 and IrO 2 reported in literature [16,17]. Optimization was restricted to solar absorption because o f a lack o f data in the infrared region. The m a x i m u m absorption for
I
I
l
I
I
I
0.9
A
A
A •
• 0.8
O
0.5
A O 0.4
i
0,3
•
&
0
I
I
i
I
2
t
I
3
,
I
•
PffA120 3
•
RNbJ20 3
A
Ir/Al20 3
,
4
I
5
i
I
,
8
Modulation Number n Fig. 2. Conversion efficiency "q of optimized multilayers vs. modulation number n.
J.H. Schrn, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65 1.0
i
i
J
63
i
0.8 (•o
0.6
~
t" .'J
0.4
,•1 •
."
0.2
• .•*
." / i
/
/¢ '
0.0 500
1000
1500
2000
Wavelength [nm] Fig. 3. Reflection spectra of multilayers: ( - IrO 2/(A70/1IO2/70)* 6.5, (. • .) Ir/(A7o/Ir4)* 6.5.
) RuO 2 / ( A 6 5 / R U O 2 / 6 5 ) , 6 . 5 ,
(---)
both systems is ~t = 0.96, but is reached for the R u O 2 system for n = 2.5 to 6.5, while for IrO 2 only for n = 6.5. Comparing the reflection spectra of optimized 6.5-multilayers of Ir, IrO 2, and RuO 2 (Fig. 3), we notice a better solar absorption for both oxides than for Ir. Furthermore, the metal layers of IrO 2 and RuO 2 are about one order in magnitude thicker than the Pt-, Ir-, and Rh-layers in the optimized coatings (see Tables 1 and 2). This is due to the higher refractive index of the noble metals [ 15,16] resulting in a nearly equal optical pathlength for all 5 materials. In addition, the absorption in these thin layers is higher for noble metals, so that a thicker noble metal oxide layer is required to absorb the same amount of radiation. While RuO 2 exhibits very low infrared reflectance (up to 2.3 txm), the reflection spectra of A / I r O 2 increase even faster than A / I r , so that very low emittances are expected• This can also be seen from the reflection spectra of the bulk materials• The reflection of IrO 2 reaches more than 95% in the IR (above 2 I~m), while 70% for RuO 2 [16], so one has to expect higher emission losses for this material.
Table 2 Comparison of the spectral selectivity of optimized A/noble metal oxide multilayer coatings (x-y [nm]) n
0.5 1.5 2.5 3.5 4.5 5.5 6.5
A1203/RuO 2
A1203/IrO e
x-y
a
x-y
a
55-0 60-33 65-25 60-21 60-20 65-21 65-20
0.84 0.93 0.96 0.96 0.96 0.96 0.96
60-0 70-38 70-28 70-24 70-21 70-20 70-18
0.82 0.88 0.92 0.94 0.95 0.95 0.96
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J.H. Sch6n, E. Bucher / Solar Energy Materials and Solar Cells 43 (1996) 59-65
4. Discussion Simulations predict very good spectral selectivity for all four systems, especially for Ir and IrO 2 in combination with alumina. Furthermore, there should be good agreement of the optical properties with if-sputtered samples as seen in earlier investigations for A / P t and A / M o S i z selective coatings [8]. Additionally, all four materials, as well as AI20 3, show very good thermal stability and corrosion resistance [17,18]. As a result of these multilayer systems, all of these material combinations may be expected to be stable in air at temperatures above 500°C. RuO 2 also seems to be a good diffusion barrier [19], so that thermal stability might compensate for the expected emission losses. It must be mentioned, that all calculations were carried out using the optical data of the bulk-materials at room temperature. Hence higher emission losses can be expected. For example, the emittance of copper at 1100 K is doubled from 0.015 at room temperature to 0.03 [20]. Another point is an increased emittance for selective surfaces due to lattice resonances at infrared frequencies. However, as an interesting result, the emittance losses are lower at very high temperatures as at temperatures of about 300°C [21], so that high temperature solar selective absorbers are less likely troubled by this problem. Furthermore, optical constants of ultrathin metal films may differ from bulk data [22]; this may cause differences between the calculated and experimental reflection spectra of the multilayers, although this is not seen for Pt- and MoSi2-absorber coatings, as stated below [8].
5. Conclusion Simulation of reflection spectra of metal/dielectric multilayer coatings using the material combinations of AI203 with Ir, Rh, RuO2, and IrO 2 show very good optical properties. Solar absorptances as high as e~ = 0.94 can be expected combined with emittances lower than e = 0.16 resulting in a maximum conversion efficiency "q = 0.86 ( C = 100, T O= 1100 K). Multilayers, as well as cermets, may be prepared using (reactive) rf-magnetron sputtering techniques. All material combinations promise good thermal stability and corrosion resistance, so that they seem to be promising candidates for high temperature (T > 500°C), high concentrating solarthermal power production in air. However, experimental sample preparation will be necessary for a definitive evaluation.
Acknowledgements We would like to thank Mr. R. Kirchner for the development of the computer program for the simulation of optical properties of multilayer coatings.
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