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Optical design and co-sputtering preparation of high performance Mo–SiO2 cermet solar selective absorbing coating
Applied Surface Science 280 (2013) 240–246
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Optical design and co-sputtering preparation of high performance Mo–SiO2 cermet solar selective absorbing coating Liqing Zheng a,c , Fangyuan Gao a,∗ , Shuxi Zhao b , Fuyun Zhou c , Jean Pierre Nshimiyimana c , Xungang Diao a a
Solar Film Laboratory (SFL), School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China Division of Solid State Physics, Ångström Laboratory, Uppsala University, Sweden c CAMDA New Energy Equipment Co., Ltd, Dongguan, Guangdong 523407, China b
a r t i c l e
i n f o
Article history: Received 23 February 2013 Received in revised form 26 April 2013 Accepted 27 April 2013 Available online 7 May 2013 Keywords: Solar selective coating Mo–SiO2 Cermet film Optical performance
a b s t r a c t In order to optimize and prepare high performance Mo–SiO2 cermet solar selective absorbing coating, a series of Mo–SiO2 cermet films with different metal volume fraction were deposited on optical glass using mid-frequency (MF) and radio frequency (RF) co-sputtering. The reflectance (R) and transmittance (T) in the wavelength range of 250–2500 nm have been simulated using SCOUT software with different dielectric function models. The optical constants, film thickness, metal volume fraction and other parameters have been deduced from the modeling. The fitted optical constants were then used to simulate and optimize the Mo–SiO2 solar selective coating and samples were prepared based on the optimized parameters. The Maxwell Garnett (MG) and Bruggeman (BR) effective-medium theory have been added in the dielectric function models to describe low metal volume fraction cermet layer (LMVF) and high metal volume fraction cermet layer (HMVF), separately. The optical spectra (R and T) of all single films were in a good agreement with the fitted spectra by dielectric function models. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. The solar absorptance of theoretically optimized selective coating was 0.945, while the absorptance of the experimental coating was 0.95. The thermal emittance of 0.15 (at 400 ◦ C) was obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Improving the properties of solar selective coating can enhance the photo-thermal conversion efficiency, and adding the operating temperature of the coating can improve the solar thermal to electric efficiency [1]. Therefore, the high performance of solar selective coating for high temperature applications is necessary to reduce the cost of producing electricity from solar thermal conversion. Metal-dielectric composite coatings, also known as cermet films, can be easily prepared by magnetron co-sputtering. Due to their high absorption in solar spectral region and the metal volumetunability, the cermet films have been widely used in the solar energy conversion coatings [2], such as C–NiO [3–5], Mo–Al2 O3 [6], Ag–Al2 O3 [7], Cr/␣–Cr2 O3 [8], and Al–AlN [9]. The optical performance of solar selective coating is strongly dependent on the optical constants of the constituents. Computer simulation and optimization based on optical constants of
∗ Corresponding author. Tel.: +86 010 82313931. E-mail addresses: [email protected], [email protected] (F. Gao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.142
cermet films have been successfully used as the design guide to prepare solar selective coatings [10–12]. The optical constants of cermet films can be deduced in different ways. Usually, the Maxwell Garnett [13] and Bruggeman [14] effective-medium theory are used to theoretically calculate the dielectric function of cermet films. In many cases, the results are not accurate enough to describe the actual experimental cermet films. Ellipsometry measurements have been used to evaluate the optical constants of cermet films [15,16]. Another rapid and accurate method based on the reflectance (R) and transmittance (T) has been used to evaluate the optical constants; this method uses the software to simulate R and T with suitable dielectric function models, and the optical constants are deduced from these models [17–21]. The authors in papers [9,22,23] added the Bruggeman approximation to form the dielectric function model to estimate the optical constants of cermet films. In this paper, Mo–SiO2 cermet films were chosen to prepare the high performance and high temperature solar selective coating, because Mo is a refractory metal and SiO2 is a high temperature durability dielectric. In order to obtain accurate optical constants of Mo–SiO2 cermet films, we added Maxwell Garnett theory to the dielectric function model for LMVF and Bruggeman theory for
L. Zheng et al. / Applied Surface Science 280 (2013) 240–246
241
Fig. 1. Schematic diagram of magnetron sputtering system.
HMVF. SCOUT software was used to deduce the optical constants of a number of single layer Mo–SiO2 cermet films in the wavelength range of 250–2500 nm. Meanwhile, the thickness and the metal volume fraction (f) had been deduced. The fitted optical constants and deduced thicknesses were then used to optimize and prepare the Mo–SiO2 solar selective coating. The optimized reflectance was compared with the experimental measurement. 2. Experimental details Samples of Mo–SiO2 cermet films with different metal volume fraction were deposited in a self-designed magnetron co-sputtering system shown in Fig. 1. The sputtering system is equipped with a 99.99% pure Mo cathode with MF power and a 99.99% pure quartz glass cathode with RF power. Both cathodes have a diameter of 60 mm and thickness of 5 mm. The substrate rotation rate is 20 r/min. The films were deposited on the Schott BK7 optical glass for R and T measurement. The whole solar selective coating was deposited on the polished stainless steel sheets for R measurement. The substrates were cleaned with a 5% HF solution for 3 min, followed by rinsing in acetone and distilled water in an ultrasonic bath. High-purity Ar gas was introduced into the chamber after the chamber was evacuated to below 5 × 10−3 Pa. Mo and SiO2 films were deposited by MF and RF sputtering, respectively. Mo–SiO2 cermet films with different metal volume fraction were deposited by co-sputtering through adjustment of the MF and RF power. The processing conditions are listed in Table 1. The X’ Pert Pro MPD XRD instrument with Cu K␣ radiation was used for XRD measurement. A Perkin-Elmer Lambda750 spectrometer and a Perkin-Elmer S2000 Fourier Transform Infrared Spectrometer were used to measure R and T of the single films and the whole selective coating between 0.25–2.5 m and 2.5–20 m, respectively. For all single layer films, the optical spectra were used to obtain the optical constants by fitting of the R and T using the SCOUT software program [24].
Fig. 2. Measured transmittance and reflectance (a) and deduced dielectric function (b) of Schott BK7 optical glass substrate.
3. Theoretical models 3.1. Molybdenum film Molybdenum layer was deposited as an infrared reflector as well as a barrier layer. It is a transition metal with d-electrons. There are interband transitions at the longer wavelengths that would affect the fit to a free-electron model. Based on the sputtering condition, the dielectric function of molybdenum consists of three parts.
Table 1 Processing conditions and estimated values of thickness, metal volume fraction and deviation value of fitting procedures. Samples
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
Processing conditions
Time (min)
MF power (W)
RF power (W)
114 0 14.5 21 21 24 28 41.5 54 65.5
0 120 180 180 120 120 120 120 120 120
8 70 40 40 40 40 40 30 20 20
Fitting results Thickness (nm)
Metal volume fraction (f)
Deviation value
95.7 177.5 239.7 250.5 170.4 174.9 176.7 149.8 116.3 138.5
1 0 0.159 0.201 0.265 0.301 0.327 0.393 0.47 0.554
254 994 788 839 716 883 792 407 588 384
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Fig. 4. Measured and fitted transmittance and reflectance (a) and deduced optical constants (b) of Mo thin film.
Fig. 3. XRD pattern of Mo thin film (a) and SiO2 thin film (b).
• Drude model describes free-electron contributions to the optical properties. There are two parameters: plasma frequency ˝P and damping constant . • OJL model for interband transitions is due to O’Leary [25] based on the joint density of states, which are given for the optical transition from the valence band to the conduction band. The parameters are the gap energy Eo , the damping constant of the valence band v , the ratio v / c and the overall strengths of the transition expressed as mass m and a decay factor. • Kim oscillator model is an extension of the simple harmonic oscillator model for vibrational modes suggested by Kim [26], which allows a continuous shift of the line shape between a Gaussian and a Lorentzian profile. The four parameters are the ˝TO resonance position, ˝P oscillator strength, ˝ damping constant, and Gauss–Lorentz-switch constant . may vary between 0 and infinity. For = 0 a Gaussian line shape is achieved, large values of (larger than 5) lead to a Lorentzian. This makes a total of 11 parameters to describe the optical properties of the molybdenum film. 3.2. Silicon dioxide film In the infrared and vacuum ultraviolet spectral regions, silicon dioxide has strong intrinsic absorption [27]. The dielectric function model of SiO2 in SCOUT consists of one oscillator suggested
by Brendel [28]. For modeling of the optical properties in the range of 250–2500 nm, a Kim oscillator is added. The model for silicon dioxide film consists of two parts. • Brendel oscillator model is an extension of the simple harmonic oscillator model, which accounts for local variations in disordered systems by a Gauss-distribution of resonance frequencies. The four parameters are the ˝TO resonance position, ˝P oscillator strength, ˝ damping constant, and distribution width. • Kim oscillator model includes 4 parameters as described in Section 3.1. The parameters for these two models (total of eight) determine the optical constants in the wavelength range of 250–2500 nm. 3.3. Mo–SiO2 cermet film For heterogeneous materials that contain more than one phase, effective-medium theory has been developed. The most classical models used for effective dielectric function of a composite have been proposed by Maxwell–Garnett [13] and Bruggeman [14]. For the identical spherical grains with the size much less than the wavelength of light, the average dielectric function of Mo–SiO2 cermet in the Maxwell–Garnett (MG), Bruggeman (BR), is calculated by the following formulae: εeff = εSiO2
εMo + 2εSiO2 + 2f εMo − εSiO2 εMo + 2εSiO2 − f εMo − εSiO2
(1)
L. Zheng et al. / Applied Surface Science 280 (2013) 240–246
Fig. 5. Measured and fitted transmittance and reflectance (a) and deduced optical constants (b) of SiO2 thin film.
243
Fig. 6. Measured and fitted transmittance and reflectance of LMVF Mo–SiO2 cermet films.
4.2. Single layer Mo and SiO2 films εSiO2 − εeff εMo − εeff + (1 − f ) =0 εMo + 2εeff εSiO2 + 2εeff
4. Results and discussion
Fig. 3(a) shows the measured XRD pattern of Mo thin film (Sample 1). Mo thin film with body centered cubic structure grew along the (1 1 0) surface. XRD pattern of SiO2 thin film (Sample 2) is shown in Fig. 3(b). There is no clear peak for SiO2 , which means the SiO2 film is amorphous. Fig. 4(a) shows the measured and fitted R and T of Mo thin film. The fitting procedure was performed following the methods described in [20]. Both spectra could be very well fitted. The Mo dielectric function model parameters plus the film thickness were set as the fit parameters. The optical constants, thickness of Mo film could be deduced, the fitting results are listed in Table 1. In the SCOUT software, the value of the deviation can evaluate the fitting result. It means a good fitting when the value of deviation is below 1000 [24]. Fig. 4(b) shows the deduced optical constants of SiO2 thin film in the wavelength range of 250–2500 nm. The measured and fitted R and T are shown in Fig. 5(a). The estimated optical constants are shown in Fig. 5(b).
4.1. Dielectric function of optical glass
4.3. LMVF Mo–SiO2 cermet layer
The measured R and T of the Schott BK7 optical glass substrate are shown in Fig. 2(a). The substrate thickness is 1.5 mm. The dielectric function of the glass substrate (Fig. 2(b)) was deduced from Direct DF software based on the optical spectra and thickness. It was then used to describe the substrate’s parameters in the fit of the optical data of the thin films (air/thin film/glass geometry).
LMVF cermet films (metal volume fraction f below 0.3) were deposited through the decrease of MF power density and increase of the RF power density. Fig. 6 shows the measured and fitted R and T of LMVF cermet films. The cermet film (f = 0.16) with low reflectance and high transmittance is similar to SiO2 ; low Mo volume fraction has limited effect on optical property of cermet film. The reflectance
f
(2)
The f represents the volume fraction occupied by molybdenum. MG theory is used to calculate the dielectric constants of cermet films, when f is less than 0.3, otherwise BR theory is used. The effective dielectric function of a composite depends on the dielectric function of the constituents and their volume fraction. Hence, a total number of 20 parameters (including 11 parameters describing Mo, eight parameters describing SiO2 and the metal volume fraction f) are used to determine the optical constants of Mo–SiO2 cermet films.
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Fig. 7. Deduced optical constants of LMVF Mo–SiO2 cermet films.
increases as Mo volume fraction increases, while the transmittance decreases with the increase of the volume fraction. The reflectance spectra of those samples are below 0.3 within the wavelength range of 250–1600 nm and the transmittance spectra are greater than 0.3 at the wavelength range of 1500–2500 nm. Those LMVF cermet films exhibit the dielectric-like behavior. The optical properties of the cermet films depend on the distribution of the metal particles inside the films: if the metal inclusions are isolated, the films exhibit an insulator-like behavior: high transmittance and low reflectance [29]. Maxwell Garnett theory is expected to be valid at low volume fractions since it is assumed that the metal particles are spatially separated [30]. Hence, the MG theory was suggested to fit R and T of LMVF cermet films. The evaluated optical constants of LMVF films are shown in Fig. 7. Both refractive index (N) and extinction coefficient (K) increase with the Mo volume fraction. When the cermet film f = 0.16, both N and K are almost stable along the wavelength range; when the cermet film f is between 0.2 and 0.3, the refractive index N increases with the increase of the wavelength and the extinction coefficient K decreases with wavelength. The trend of optical constants of the LMVF Mo–SiO2 cermet films is in good agreement with the results in [15]. 4.4. HMVF Mo–SiO2 cermet layer HMVF cermet films (metal volume fraction f great than 0.3) were deposited through the increase of MF power density. Fig. 8
Fig. 8. Measured and fitted reflectance (a) and transmittance (b) of HMVF Mo–SiO2 cermet films.
shows the measured and fitted R and T of HMVF. The reflectance of HMVF increases as Mo volume fraction increases, the transmittance decreases with the increase of the volume fraction. The reflectance spectra of those samples are greater than 0.3 within the wavelength range of 750–2500 nm and the transmittance spectra are below 0.2 at the wavelength range of 250–2500 nm. These films show the metallic behavior, high reflectance and low transmittance. Compared with Maxwell Garnett theory, Bruggeman theory is expected to be valid at high volume fractions since it is assumed that the metal grains are spatially interconnected. Hence, the Bruggeman theory was found appropriate to calculate the dielectric function of HMVF cermet films. The predicted optical constants of HMVF films are shown in Fig. 9. Both refractive index and extinction coefficient increase with the Mo volume fraction, also both N and K increase along the wavelength range of 250–2500 nm. This trend of HMVF cermet films is also in accordance with literature [15]. 4.5. Solar selective coating In order to design and prepare the solar selective coating, the deduced optical constants were used as the database to simulate and optimize the solar selective coating through Essential Macleod software [31]. The structure of solar selective coating was from
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Table 2 Optimized solar selective coating structure and its sputtering process. Layer
Thickness (nm)
Metal volume fraction (f)
MF power (W)
RF power (W)
Sputtering time (min)
Antireflection LMVF cermet HMVF cermet Metal reflector
69.8 29.6 40.1 175.5
0 0.16 0.33 1
0 14.5 28 0.3
120 180 120 0
27.4 4.9 9.0 14.7
bottom to top: Mo (metal reflector layer), LMVF and HMVF (double absorber layers) and SiO2 (antireflection layer). The optimized reflectance for the best selective coating is shown in Fig. 10. The calculated solar absorptance of the coating was 0.945. The optimized coating was then deposited on the polished stainless steel sheets; the deposition rate was calculated by the evaluated thickness and sputtering time. Table 2 shows the optimized solar selective coating structure and its sputtering process. The measured reflectance was compared to the optimized result. The properties of deposited selective coating are in good agreement with the optimized result, which confirms that the method to deduce the optical constants of cermet films from fitting the reflectance and transmittance is valid. The reflectance in the visible and infrared region is shown in Fig. 11. The curve shows the low reflectance in solar irradiation region and the high reflectance in thermal infrared region. There is a sharp increase of the reflectance at the wavelength range of
Fig. 10. Optimized and experimental reflectance spectra of the solar selective coating.
Fig. 11. Experimental reflectance curve and corresponding photo-thermal parameters. In the picture, the direct AM 1.5 solar spectrum and the ideal blackbody emission (400 ◦ C) are also shown.
1500–3000 nm. Based on spectra of AM 1.5 in the wavelength range from 250 to 2500 nm, the experimental solar absorptance reached 0.95, which is better than the value (0.94) obtained in [15]. The thermal emittance of 0.15 was calculated based on spectra of ideal black-body emission (at 400 ◦ C) in the wavelength range from 1 m to 2.5 m, which is higher than the value (0.072 at 400 ◦ C) from [15]. 5. Summaries
Fig. 9. Deduced optical constants of HMVF Mo–SiO2 cermet films.
The paper presents the optimization and preparation process of Mo–SiO2 cermet solar selective coating. The optical data (R and
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T) measured in the wavelength range 250–2500 nm of a series of Mo–SiO2 cermet films have been simulated by the SCOUT software using suitable dielectric function models. The Maxwell Garnett and Bruggeman effective-medium theory have been added in the dielectric function models to describe LMVF and HMVF, separately. The optical spectra (R and T) of all single films were in good agreement with the fitted spectra. The optical constants deduced from the fitting have been used as the database to optimize the solar selective coating; the optimized reflectance for the best selective coating (solar absorptance of 0.945) was obtained. The selective coating has been prepared based on sputtering rate evaluated from the fitting. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. After these procedures, the Mo–SiO2 cermet solar selective coating with solar absorptance of 0.95 and thermal emittance of 0.15 (at 400 ◦ C) was achieved. Acknowledgment This work was financially supported by the International Science & Technology Cooperation Program of China (No. 2012DFG61930). References [1] C.E. Kennedy, Progress to develop an advanced solar-selective coating, in: 14th Biennial CSP Solar PACES Symposium, NREL/CD-550-52709, 2008. [2] N. Selvakumar, H.C. Barshilia, Solar Energy Material and Solar Cells 98 (2012) 1. [3] K.T. Roro, N. Tile, A. Forbes, Applied Surface Science 258 (2012) 7174.
[4] K.T. Roro, N. Tile, B.W. Mwakikunga, et al., Materials Science and Engineering B 177 (2012) 581. [5] K.T. Roro, B. Mwakikunga, N. Tile, et al., International Journal of Photo Energy 2012 (2012), Article ID 678394. [6] X. Du, C. Wang, T. Wang, et al., Thin Solid Films 516 (2008) 3971. [7] H.C. Barshilia, P. Kumar, et al., Solar Energy Material and Solar Cells 95 (2011) 1707. [8] S. Khamlich, O. Nemraoui, et al., Physica B – Condensed Matter 407 (2012) 1509. [9] S. Zhao, E. Wäckelgård, Solar Energy Material and Solar Cells 90 (2006) 1861. [10] J.H. Schdön, E. Bucher, Solar Energy Material and Solar Cells 43 (1996) 56. [11] M. Farooq, Z.H. Lee, Renewable Energy 28 (2003) 1421. [12] M.R. Nejati, V. Fathollahi, M.K. Asadi, Solar Energy 78 (2005) 235. [13] J.C.M. Garnett, Philosophical Transactions 203 (1904) 885. [14] D.A.G. Brugeman, Annalen der Physik 24 (1935) 636. [15] S. Esposito, A. Antonaia, M.L. Addonizio, et al., Thin Solid Films 517 (2009) 6000. [16] A. Biswas, D. Bhattacharyya, H.C. Barshilia, et al., Applied Surface Science 254 (2008) 1694. [17] A. Solieman, A.A. Abu-Sehly, Physica B – Condensed Matter 405 (2010) 1101. [18] A. Solieman, A.A. Abu-Sehly, Materials Chemistry and Physics 129 (2011) 1000. [19] C.M. Maghanga, G.A. Niklasson, C.G. Granqvist, Thin Solid Films 518 (2009) 1254. [20] A. Solieman, M.A. Aegerter, Thin Solid Films 502 (2006) 205. [21] T. Schuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125. [22] S. Zhao, E. Wäckelgård, Solar Energy Material and Solar Cells 90 (2006) 243. [23] S. Zhao, et al., Solar Energy Material and Solar Cells 90 (2006) 308. [24] M. Theiss, Hard and Software for Optical Spectroscopy, 2013, Dr. BernhardKlein-Str. 110, 52078 Aachen, Germany, www.mtheiss.com [25] P. Gravier, G. Chassaing, M. Sigrist, Thin solid Films 57 (1979) 93. [26] S.K. O’Leary, S.R. Johnson, P.K. Lim, Journal of Applied Physics 82 (1997) 3334. [27] H.R. Philipp, Silicon Dioxide (SiO2 ) (Glass), Handbook of Optical Constants of Solids, vol. I, Academic Press, San Diego, CA, USA, 1997, pp. 749–763. [28] R. Brendel, Applied Physics A 50 (1990) 587. [29] M. Gajdardziska-Josifovska, R.C. McPhedran, D.R. McKenzie, et al., Applied Optics 28 (1989) 2744. [30] P.U. Jepsen, B.M. Fischer, A. Thoman, et al., Physic Review B 20 (2006) 2051. [31] L. Zheng, F. Zhou, X. Diao, S. Zhao, Chinese Optics Letters 11 (s2) (2013) S10501.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Optical design and co-sputtering preparation of high performance Mo–SiO2 cermet solar selective absorbing coating Liqing Zheng a,c , Fangyuan Gao a,∗ , Shuxi Zhao b , Fuyun Zhou c , Jean Pierre Nshimiyimana c , Xungang Diao a a
Solar Film Laboratory (SFL), School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China Division of Solid State Physics, Ångström Laboratory, Uppsala University, Sweden c CAMDA New Energy Equipment Co., Ltd, Dongguan, Guangdong 523407, China b
a r t i c l e
i n f o
Article history: Received 23 February 2013 Received in revised form 26 April 2013 Accepted 27 April 2013 Available online 7 May 2013 Keywords: Solar selective coating Mo–SiO2 Cermet film Optical performance
a b s t r a c t In order to optimize and prepare high performance Mo–SiO2 cermet solar selective absorbing coating, a series of Mo–SiO2 cermet films with different metal volume fraction were deposited on optical glass using mid-frequency (MF) and radio frequency (RF) co-sputtering. The reflectance (R) and transmittance (T) in the wavelength range of 250–2500 nm have been simulated using SCOUT software with different dielectric function models. The optical constants, film thickness, metal volume fraction and other parameters have been deduced from the modeling. The fitted optical constants were then used to simulate and optimize the Mo–SiO2 solar selective coating and samples were prepared based on the optimized parameters. The Maxwell Garnett (MG) and Bruggeman (BR) effective-medium theory have been added in the dielectric function models to describe low metal volume fraction cermet layer (LMVF) and high metal volume fraction cermet layer (HMVF), separately. The optical spectra (R and T) of all single films were in a good agreement with the fitted spectra by dielectric function models. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. The solar absorptance of theoretically optimized selective coating was 0.945, while the absorptance of the experimental coating was 0.95. The thermal emittance of 0.15 (at 400 ◦ C) was obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Improving the properties of solar selective coating can enhance the photo-thermal conversion efficiency, and adding the operating temperature of the coating can improve the solar thermal to electric efficiency [1]. Therefore, the high performance of solar selective coating for high temperature applications is necessary to reduce the cost of producing electricity from solar thermal conversion. Metal-dielectric composite coatings, also known as cermet films, can be easily prepared by magnetron co-sputtering. Due to their high absorption in solar spectral region and the metal volumetunability, the cermet films have been widely used in the solar energy conversion coatings [2], such as C–NiO [3–5], Mo–Al2 O3 [6], Ag–Al2 O3 [7], Cr/␣–Cr2 O3 [8], and Al–AlN [9]. The optical performance of solar selective coating is strongly dependent on the optical constants of the constituents. Computer simulation and optimization based on optical constants of
∗ Corresponding author. Tel.: +86 010 82313931. E-mail addresses: [email protected], [email protected] (F. Gao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.142
cermet films have been successfully used as the design guide to prepare solar selective coatings [10–12]. The optical constants of cermet films can be deduced in different ways. Usually, the Maxwell Garnett [13] and Bruggeman [14] effective-medium theory are used to theoretically calculate the dielectric function of cermet films. In many cases, the results are not accurate enough to describe the actual experimental cermet films. Ellipsometry measurements have been used to evaluate the optical constants of cermet films [15,16]. Another rapid and accurate method based on the reflectance (R) and transmittance (T) has been used to evaluate the optical constants; this method uses the software to simulate R and T with suitable dielectric function models, and the optical constants are deduced from these models [17–21]. The authors in papers [9,22,23] added the Bruggeman approximation to form the dielectric function model to estimate the optical constants of cermet films. In this paper, Mo–SiO2 cermet films were chosen to prepare the high performance and high temperature solar selective coating, because Mo is a refractory metal and SiO2 is a high temperature durability dielectric. In order to obtain accurate optical constants of Mo–SiO2 cermet films, we added Maxwell Garnett theory to the dielectric function model for LMVF and Bruggeman theory for
L. Zheng et al. / Applied Surface Science 280 (2013) 240–246
241
Fig. 1. Schematic diagram of magnetron sputtering system.
HMVF. SCOUT software was used to deduce the optical constants of a number of single layer Mo–SiO2 cermet films in the wavelength range of 250–2500 nm. Meanwhile, the thickness and the metal volume fraction (f) had been deduced. The fitted optical constants and deduced thicknesses were then used to optimize and prepare the Mo–SiO2 solar selective coating. The optimized reflectance was compared with the experimental measurement. 2. Experimental details Samples of Mo–SiO2 cermet films with different metal volume fraction were deposited in a self-designed magnetron co-sputtering system shown in Fig. 1. The sputtering system is equipped with a 99.99% pure Mo cathode with MF power and a 99.99% pure quartz glass cathode with RF power. Both cathodes have a diameter of 60 mm and thickness of 5 mm. The substrate rotation rate is 20 r/min. The films were deposited on the Schott BK7 optical glass for R and T measurement. The whole solar selective coating was deposited on the polished stainless steel sheets for R measurement. The substrates were cleaned with a 5% HF solution for 3 min, followed by rinsing in acetone and distilled water in an ultrasonic bath. High-purity Ar gas was introduced into the chamber after the chamber was evacuated to below 5 × 10−3 Pa. Mo and SiO2 films were deposited by MF and RF sputtering, respectively. Mo–SiO2 cermet films with different metal volume fraction were deposited by co-sputtering through adjustment of the MF and RF power. The processing conditions are listed in Table 1. The X’ Pert Pro MPD XRD instrument with Cu K␣ radiation was used for XRD measurement. A Perkin-Elmer Lambda750 spectrometer and a Perkin-Elmer S2000 Fourier Transform Infrared Spectrometer were used to measure R and T of the single films and the whole selective coating between 0.25–2.5 m and 2.5–20 m, respectively. For all single layer films, the optical spectra were used to obtain the optical constants by fitting of the R and T using the SCOUT software program [24].
Fig. 2. Measured transmittance and reflectance (a) and deduced dielectric function (b) of Schott BK7 optical glass substrate.
3. Theoretical models 3.1. Molybdenum film Molybdenum layer was deposited as an infrared reflector as well as a barrier layer. It is a transition metal with d-electrons. There are interband transitions at the longer wavelengths that would affect the fit to a free-electron model. Based on the sputtering condition, the dielectric function of molybdenum consists of three parts.
Table 1 Processing conditions and estimated values of thickness, metal volume fraction and deviation value of fitting procedures. Samples
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
Processing conditions
Time (min)
MF power (W)
RF power (W)
114 0 14.5 21 21 24 28 41.5 54 65.5
0 120 180 180 120 120 120 120 120 120
8 70 40 40 40 40 40 30 20 20
Fitting results Thickness (nm)
Metal volume fraction (f)
Deviation value
95.7 177.5 239.7 250.5 170.4 174.9 176.7 149.8 116.3 138.5
1 0 0.159 0.201 0.265 0.301 0.327 0.393 0.47 0.554
254 994 788 839 716 883 792 407 588 384
242
L. Zheng et al. / Applied Surface Science 280 (2013) 240–246
Fig. 4. Measured and fitted transmittance and reflectance (a) and deduced optical constants (b) of Mo thin film.
Fig. 3. XRD pattern of Mo thin film (a) and SiO2 thin film (b).
• Drude model describes free-electron contributions to the optical properties. There are two parameters: plasma frequency ˝P and damping constant . • OJL model for interband transitions is due to O’Leary [25] based on the joint density of states, which are given for the optical transition from the valence band to the conduction band. The parameters are the gap energy Eo , the damping constant of the valence band v , the ratio v / c and the overall strengths of the transition expressed as mass m and a decay factor. • Kim oscillator model is an extension of the simple harmonic oscillator model for vibrational modes suggested by Kim [26], which allows a continuous shift of the line shape between a Gaussian and a Lorentzian profile. The four parameters are the ˝TO resonance position, ˝P oscillator strength, ˝ damping constant, and Gauss–Lorentz-switch constant . may vary between 0 and infinity. For = 0 a Gaussian line shape is achieved, large values of (larger than 5) lead to a Lorentzian. This makes a total of 11 parameters to describe the optical properties of the molybdenum film. 3.2. Silicon dioxide film In the infrared and vacuum ultraviolet spectral regions, silicon dioxide has strong intrinsic absorption [27]. The dielectric function model of SiO2 in SCOUT consists of one oscillator suggested
by Brendel [28]. For modeling of the optical properties in the range of 250–2500 nm, a Kim oscillator is added. The model for silicon dioxide film consists of two parts. • Brendel oscillator model is an extension of the simple harmonic oscillator model, which accounts for local variations in disordered systems by a Gauss-distribution of resonance frequencies. The four parameters are the ˝TO resonance position, ˝P oscillator strength, ˝ damping constant, and distribution width. • Kim oscillator model includes 4 parameters as described in Section 3.1. The parameters for these two models (total of eight) determine the optical constants in the wavelength range of 250–2500 nm. 3.3. Mo–SiO2 cermet film For heterogeneous materials that contain more than one phase, effective-medium theory has been developed. The most classical models used for effective dielectric function of a composite have been proposed by Maxwell–Garnett [13] and Bruggeman [14]. For the identical spherical grains with the size much less than the wavelength of light, the average dielectric function of Mo–SiO2 cermet in the Maxwell–Garnett (MG), Bruggeman (BR), is calculated by the following formulae: εeff = εSiO2
εMo + 2εSiO2 + 2f εMo − εSiO2 εMo + 2εSiO2 − f εMo − εSiO2
(1)
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Fig. 5. Measured and fitted transmittance and reflectance (a) and deduced optical constants (b) of SiO2 thin film.
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Fig. 6. Measured and fitted transmittance and reflectance of LMVF Mo–SiO2 cermet films.
4.2. Single layer Mo and SiO2 films εSiO2 − εeff εMo − εeff + (1 − f ) =0 εMo + 2εeff εSiO2 + 2εeff
4. Results and discussion
Fig. 3(a) shows the measured XRD pattern of Mo thin film (Sample 1). Mo thin film with body centered cubic structure grew along the (1 1 0) surface. XRD pattern of SiO2 thin film (Sample 2) is shown in Fig. 3(b). There is no clear peak for SiO2 , which means the SiO2 film is amorphous. Fig. 4(a) shows the measured and fitted R and T of Mo thin film. The fitting procedure was performed following the methods described in [20]. Both spectra could be very well fitted. The Mo dielectric function model parameters plus the film thickness were set as the fit parameters. The optical constants, thickness of Mo film could be deduced, the fitting results are listed in Table 1. In the SCOUT software, the value of the deviation can evaluate the fitting result. It means a good fitting when the value of deviation is below 1000 [24]. Fig. 4(b) shows the deduced optical constants of SiO2 thin film in the wavelength range of 250–2500 nm. The measured and fitted R and T are shown in Fig. 5(a). The estimated optical constants are shown in Fig. 5(b).
4.1. Dielectric function of optical glass
4.3. LMVF Mo–SiO2 cermet layer
The measured R and T of the Schott BK7 optical glass substrate are shown in Fig. 2(a). The substrate thickness is 1.5 mm. The dielectric function of the glass substrate (Fig. 2(b)) was deduced from Direct DF software based on the optical spectra and thickness. It was then used to describe the substrate’s parameters in the fit of the optical data of the thin films (air/thin film/glass geometry).
LMVF cermet films (metal volume fraction f below 0.3) were deposited through the decrease of MF power density and increase of the RF power density. Fig. 6 shows the measured and fitted R and T of LMVF cermet films. The cermet film (f = 0.16) with low reflectance and high transmittance is similar to SiO2 ; low Mo volume fraction has limited effect on optical property of cermet film. The reflectance
f
(2)
The f represents the volume fraction occupied by molybdenum. MG theory is used to calculate the dielectric constants of cermet films, when f is less than 0.3, otherwise BR theory is used. The effective dielectric function of a composite depends on the dielectric function of the constituents and their volume fraction. Hence, a total number of 20 parameters (including 11 parameters describing Mo, eight parameters describing SiO2 and the metal volume fraction f) are used to determine the optical constants of Mo–SiO2 cermet films.
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Fig. 7. Deduced optical constants of LMVF Mo–SiO2 cermet films.
increases as Mo volume fraction increases, while the transmittance decreases with the increase of the volume fraction. The reflectance spectra of those samples are below 0.3 within the wavelength range of 250–1600 nm and the transmittance spectra are greater than 0.3 at the wavelength range of 1500–2500 nm. Those LMVF cermet films exhibit the dielectric-like behavior. The optical properties of the cermet films depend on the distribution of the metal particles inside the films: if the metal inclusions are isolated, the films exhibit an insulator-like behavior: high transmittance and low reflectance [29]. Maxwell Garnett theory is expected to be valid at low volume fractions since it is assumed that the metal particles are spatially separated [30]. Hence, the MG theory was suggested to fit R and T of LMVF cermet films. The evaluated optical constants of LMVF films are shown in Fig. 7. Both refractive index (N) and extinction coefficient (K) increase with the Mo volume fraction. When the cermet film f = 0.16, both N and K are almost stable along the wavelength range; when the cermet film f is between 0.2 and 0.3, the refractive index N increases with the increase of the wavelength and the extinction coefficient K decreases with wavelength. The trend of optical constants of the LMVF Mo–SiO2 cermet films is in good agreement with the results in [15]. 4.4. HMVF Mo–SiO2 cermet layer HMVF cermet films (metal volume fraction f great than 0.3) were deposited through the increase of MF power density. Fig. 8
Fig. 8. Measured and fitted reflectance (a) and transmittance (b) of HMVF Mo–SiO2 cermet films.
shows the measured and fitted R and T of HMVF. The reflectance of HMVF increases as Mo volume fraction increases, the transmittance decreases with the increase of the volume fraction. The reflectance spectra of those samples are greater than 0.3 within the wavelength range of 750–2500 nm and the transmittance spectra are below 0.2 at the wavelength range of 250–2500 nm. These films show the metallic behavior, high reflectance and low transmittance. Compared with Maxwell Garnett theory, Bruggeman theory is expected to be valid at high volume fractions since it is assumed that the metal grains are spatially interconnected. Hence, the Bruggeman theory was found appropriate to calculate the dielectric function of HMVF cermet films. The predicted optical constants of HMVF films are shown in Fig. 9. Both refractive index and extinction coefficient increase with the Mo volume fraction, also both N and K increase along the wavelength range of 250–2500 nm. This trend of HMVF cermet films is also in accordance with literature [15]. 4.5. Solar selective coating In order to design and prepare the solar selective coating, the deduced optical constants were used as the database to simulate and optimize the solar selective coating through Essential Macleod software [31]. The structure of solar selective coating was from
L. Zheng et al. / Applied Surface Science 280 (2013) 240–246
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Table 2 Optimized solar selective coating structure and its sputtering process. Layer
Thickness (nm)
Metal volume fraction (f)
MF power (W)
RF power (W)
Sputtering time (min)
Antireflection LMVF cermet HMVF cermet Metal reflector
69.8 29.6 40.1 175.5
0 0.16 0.33 1
0 14.5 28 0.3
120 180 120 0
27.4 4.9 9.0 14.7
bottom to top: Mo (metal reflector layer), LMVF and HMVF (double absorber layers) and SiO2 (antireflection layer). The optimized reflectance for the best selective coating is shown in Fig. 10. The calculated solar absorptance of the coating was 0.945. The optimized coating was then deposited on the polished stainless steel sheets; the deposition rate was calculated by the evaluated thickness and sputtering time. Table 2 shows the optimized solar selective coating structure and its sputtering process. The measured reflectance was compared to the optimized result. The properties of deposited selective coating are in good agreement with the optimized result, which confirms that the method to deduce the optical constants of cermet films from fitting the reflectance and transmittance is valid. The reflectance in the visible and infrared region is shown in Fig. 11. The curve shows the low reflectance in solar irradiation region and the high reflectance in thermal infrared region. There is a sharp increase of the reflectance at the wavelength range of
Fig. 10. Optimized and experimental reflectance spectra of the solar selective coating.
Fig. 11. Experimental reflectance curve and corresponding photo-thermal parameters. In the picture, the direct AM 1.5 solar spectrum and the ideal blackbody emission (400 ◦ C) are also shown.
1500–3000 nm. Based on spectra of AM 1.5 in the wavelength range from 250 to 2500 nm, the experimental solar absorptance reached 0.95, which is better than the value (0.94) obtained in [15]. The thermal emittance of 0.15 was calculated based on spectra of ideal black-body emission (at 400 ◦ C) in the wavelength range from 1 m to 2.5 m, which is higher than the value (0.072 at 400 ◦ C) from [15]. 5. Summaries
Fig. 9. Deduced optical constants of HMVF Mo–SiO2 cermet films.
The paper presents the optimization and preparation process of Mo–SiO2 cermet solar selective coating. The optical data (R and
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T) measured in the wavelength range 250–2500 nm of a series of Mo–SiO2 cermet films have been simulated by the SCOUT software using suitable dielectric function models. The Maxwell Garnett and Bruggeman effective-medium theory have been added in the dielectric function models to describe LMVF and HMVF, separately. The optical spectra (R and T) of all single films were in good agreement with the fitted spectra. The optical constants deduced from the fitting have been used as the database to optimize the solar selective coating; the optimized reflectance for the best selective coating (solar absorptance of 0.945) was obtained. The selective coating has been prepared based on sputtering rate evaluated from the fitting. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. After these procedures, the Mo–SiO2 cermet solar selective coating with solar absorptance of 0.95 and thermal emittance of 0.15 (at 400 ◦ C) was achieved. Acknowledgment This work was financially supported by the International Science & Technology Cooperation Program of China (No. 2012DFG61930). References [1] C.E. Kennedy, Progress to develop an advanced solar-selective coating, in: 14th Biennial CSP Solar PACES Symposium, NREL/CD-550-52709, 2008. [2] N. Selvakumar, H.C. Barshilia, Solar Energy Material and Solar Cells 98 (2012) 1. [3] K.T. Roro, N. Tile, A. Forbes, Applied Surface Science 258 (2012) 7174.
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