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Chromium-based thin sputtered composite coatings for solar thermal collectors
Vacuum 64 (2002) 299–305
Chromium-based thin sputtered composite coatings for solar thermal collectors V. Teixeiraa,*, E. Sousaa, M.F. Costaa, C. Nunesb, L. Rosab, M.J. Carvalhob, M. Collares-Pereirab, E. Romanc, J. Gagoc a
Departamento de F!ısica, IMAT-Instituto de Materiais, Universidade do Minho, Campus de Gualtar, P-4710-057 Braga, Portugal b INETI-Instituto Nacional de Engenharia e Tecnologia Industrial, ITE, Lisboa, Portugal c ICMM-Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain
Abstract One of the fundamental components of a solar thermal collector is the absorber surface that should be spectrally selective for solar radiation. Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. Most solar selective coatings use metal-dielectric composites, known as cermets, as the absorber of solar energy. In this study we will present a numerical model that allow us to correlate the selectivity of the produced absorbers to the collector efficiency. The cathodic magnetron sputtering being a promising method to produce thin solar selective films, a study of some cermet Cr–Cr2O3–CrO3 coatings obtained by this technique in reactive atmosphere and using a DC power unit will be presented. The multilayered composites produced were based in metallic chromium in a matrix of a chromium oxide with a gradient in oxygen composition. The selective graded films were produced by a reactive DC magnetron sputtering of pure chromium target in a plasma of argon–oxygen at different sputtering pressures and substrate temperatures. The microstructure, surface roughness, crystallographic phases, composition and chemical analysis by X-ray photoelectron spectroscopy and reflectivity spectra in vis-NIR were analysed. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction A large number of solar selective surfaces for thermal collectors have been studied [1–3]. Most solar selective coatings use metal–dielectric composites, known as cermets, as the absorber of solar energy. Different methods for producing solar absorber coatings are available: electroplated black chromium, which employs Cr–Cr2O3 cermet *Corresponding author. Tel.: +351-53-604334; fax: +35153-678981. E-mail address: vasco@fisica.uminho.pt (V. Teixeira).
material, is the most widely used solar absorber [3,4]; nickel-pigmented anodic Al2O3, which is produced by electrochemical treatment of an aluminium sheet is another solar selective absorber usually employed in industry [1–5]. In recent years, there has been a renewed interest in cermet coatings because of their potential application in a wide variety of technological applications, such as the production of selective absorbing surfaces to enhance the efficiency of the photothermal conversion process. Electrochemical and electroplating coatings are most widely used in thermal collectors mainly due
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 7 2 - 4
300
V. Teixeira et al. / Vacuum 64 (2002) 299–305
to low cost production and large area processing. However, the optical properties of these coatings are not suitable for very efficient photothermal conversion (e.g. emissivity is high). Sputtering technique is a clean coating technology without the pollution problems found in electroplating processes. Thus, in recent years, the focus of the study of solar selective surfaces has been on the production of coatings with spectral selectivity by sputtering techniques [3–6]. In the first part of this work we present a numerical model that allows us to correlate the selectivity of the produced absorbers to the collector efficiency. In this contribution, we use a suitable method for the deposition of composite cermet coatings of Cr–Cr2O3. The thin coatings were produced by reactive DC magnetron sputtering of chromium in an argon and oxygen atmosphere. The oxygen flow, during film deposition, was periodically switched on and off, leading to an alternating deposition of metallic chromium and chromium oxide resulting in a multilayered film. The deposition parameters, structural and optical properties of the multilayers films are presented and described in this paper.
2. Numerical modelling of thermal collector efficiency Thermal solar collector efficiency can be defined as the rate between the collector energy transferred to the fluid (water or oil) and solar incident energy over the collector: Z¼
mcp ðTs Te Þ ; Ac Icol
ð1Þ
where m is the massic flux of transfer fluid, cp the fluid specific heat, Ac the collector area, Icol the incident radiative power per unit collector area and Ts and Te the output and input fluid temperature. Collector efficiency is a function of the average temperature of the fluid, Tf ¼ ðTe þ Ts Þ=2 and can be given by a linear model of the type: Z ¼ F 0 Z0 F 0 UL
ðTf Tamb Þ ; Icol
ð2Þ
F 0 Z0 characterises the optical behaviour of the collector. In the case of a flat collector Z0 ¼ at; where t is the transmissivity of the transparent cover (glass) and a the absorptivity of the absorber plate. F 0 UL characterises the collector thermal losses. These are losses by convection, conduction and radiation. Tamb is the ambient temperature. One fundamental component of the collector is the solar radiation absorber surface, which must have high absorptivity (a) in the wavelength band of visible radiation and near infrared (300– 3000 nm) to which corresponds almost all solar energetic flux (at least 99%). The use of an absorber surface with selective coating also permits the reduction of losses by radiation, i.e., thermal losses reduction in the collector. One selective absorber surface is characterised by low emissivity (e) for wavelengths belonging to the thermal infrared (6000–15 000 nm). The application of solar collector heat production for high temperature (typically industrial applications), as well as its application in systems for heat and cold production, imposes the necessity to use collectors which show good efficiency for high temperatures. Thus it becomes important to produce good selective absorber surfaces. In order to prove the influence of surfaces optical characteristics on the behaviour of the collector efficiency, we applied a theoretical collector model [7–9] which, based on optical and thermal properties of the components, enables the calculation of the collector efficiency parameters. As part of a research project to develop spectrally selective surfaces for high efficiency thermal collectors, a model was applied to study the influence of optical absorber surface properties in the collector (a typical flat collector was considered). Collector efficiency values were determined considering only changes in the values of a and e of the absorber surface. Table 1 shows the efficiency values for an average fluid temperature of 901C (with Tamb ¼ 201C and Icol ¼ 1000 W m2). The values in Table 1 are obtained as a result of the efficiency curves generated by our model. Comparing a very good selective surface (case A) with a non-selective surface (case B) it can be seen that the efficiency of A is double that of B for
V. Teixeira et al. / Vacuum 64 (2002) 299–305 Table 1 Influence of optical surface properties in flat collector yield Sample
a
e
Z (Tf ¼ 901C)
Case Case Case Case Case Case
0.95 0.96 0.93 0.75 0.95 0.75
0.05 0.86 0.26 0.41 0.60 0.05
0.46 0.24 0.37 0.17 0.29 0.28
A B C D E F
the temperature considered. Considering the surface case C which is also a selective surface (that we can find frequently in the market), but not as good as A, it can be seen that its efficiency is only 0.9, lower than A. This shows that for high temperatures the increase in emissivity has a strong influence on efficiency reduction. The influence of low absorptivity values on efficiency reduction is shown when case A is compared with F or when B is compared with D. The cases F and D are only referred to show the influence of absorptivity efficiency. It is not usual that the surfaces in the market show these values.
3. Experimental details In this study Cr–Cr2O3 cermet solar thin coatings were deposited by DC reactive magnetron sputtering in an argon and oxygen atmosphere, from a 99.99% pure Cr target, which was cleaned by pre-sputtering in an argon atmosphere at 0.5 Pa during 10 min, prior to each deposition. The sputtering chamber was evacuated with a turbomolecular pump down to a base vacuum pressure of less than 2 10–4 Pa. Copper and glass substrates were used for the deposition of the absorber films. These substrates were cleaned by presputtering with Ar+ to remove any oxide surface layers from the surface. The distance between the substrate and the target was kept constant, at 60 mm and the substrate temperature was constant too, at 1501C. The sputtering working pressure was varied from 0.5 to 1.2 Pa and the negative DC substrate bias was varied from 50 to 65 V, depending on
301
the experiment. The thickness of the produced films was about 300 nm. During the deposition of composite coating of Cr–Cr2O3 the argon flow was kept constant at 100 or 160 sccm, while the oxygen flow was varied (from 0 to 15 sccm, depending on the experiment) in order to obtain a multilayer coating consisting of alternated metallic and oxide layers. A gradient coating was also produced by changing gradually the oxygen flow during reactive sputtering of chromium. After coating deposition, all coating systems were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), compositional analysis by energy dispersive X-ray microanalysis (EDS), chemical analysis by X-ray photoelectron spectroscopy (XPS) and coating reflectivity by optical reflectance spectrometry.
4. Results and discussion Our first approach to deposit chromium cermet coatings was to use low oxygen pressure to produce substoichiometric oxides. However, it is well documented that a monotonous increase of the oxygen flow results in an unsteady change of the deposition process. At a critical value of oxygen flow the deposition rate shows a sharp drop [10] and the coating passes abruptly from a metal to a dielectric. This can be explained by the poisoning of the target (oxidation of target) with a subsequent change in the sputtering yield. This transition was observed in the oxygen–chromium system. We observed a switching of the plasma colour when increasing the oxygen flow from below to above critical oxygen flow. At low oxygen flow the coatings consisted of mainly metallic chromium and the plasma colour during sputtering was blue. At higher oxygen pressures the coatings were almost completely oxidised and the plasma colour was reddish. Similar results were reported by other authors [11]. In our work a novel DC magnetron sputtering method for the deposition of composite layered coatings based in cermet of Cr–Cr2O3 was studied. By switching on and off the oxygen flow during sputtering of chromium, Cr and Cr2O3 monolayers
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V. Teixeira et al. / Vacuum 64 (2002) 299–305
were deposited alternatively and a multilayered coating results. Furthermore, for a better optical performance of the selective absorbers we developed graded coatings by changing gradually the oxygen flow, which results in a gradient coating. The surface microstructure of two layered coatings deposited at different sputtering pressures, by the method described above, is shown in the micrograph in Fig. 1. It is clearly shown that the surface roughness and the porosity of the film are affected by the total sputtering pressure (Ar+O2 pressure) during deposition. The coating CR1 was deposited at higher pressure PArþO2 ¼ 1:2 Pa : The coating CR2 was produced at lower pressure PArþO2 ¼ 0:5 Pa and presents a more compact structure. As expected, due to the higher energy of the particles arriving the growing coating, a more dense microstructure will develop [12–15]. A detailed surface micro-topography study of the selective chromium based absorbers (not presented in the present work) by atomic force microscopy
(AFM) and non-contact laser topography inspection is described elsewhere [3,16]. The chemical analysis of the coatings have been realised by EDS and XPS. Prior to the XPS analysis the coating surface has been in-situ eroded during a few minutes by argon ions. The deconvolution of peaks corresponding to the Cr 2p3/2 core levels and O 1s allows us to distinguish from the metallic Cr phase (energy 2p3/2 centred at B574.3 eV) to the Cr2O3 and CrO3 phases. A mixture of CrO3 (energy 2p3/2 centred at B576.9 eV) and Cr2O3 (energy 2p3/2 centred at B579.3 eV) compounds were identified near the coating surface (see Table 2 with the main XPS parameters used for analysis and Fig. 2). It is well known that sputter cleaning using Ar+ may lead to compositional changes at the surface of binary compounds. When Ar+ is used to clean oxide surfaces a reduction of the oxide is generally observed so the different oxides presented near the surface can be an artefact from the cleaning process. To verify this we also studied by XPS the as-deposited surface (prior to sputter cleaning), although it was somewhat dirty with carbon, and confirmed the presence of a mixture of Cr2O3 and CrO3. The subsurface compositional analysis using sputtering erosion may be affected and so a quantitative analysis of the ratio of the Cr2O3 and CrO3 phases was not performed. In the present study only qualitative and semi-quantitative analysis was taken into consideration. Other authors have done XPS experiments to analyse Cr–Cr2O3 and found similar results [11]. At and near the surface it was not possible to identify any metallic chromium. The metallic chromium was
Table 2 Parameters of the semi-quantitative analysis of the graded coating system for X-ray photoelectron spectroscopy (XPS) using Mg Ka radiation Peak Element/core level Sensitivity factor Binding energy (eV) Fig. 1. SEM micrographs of two solar absorbers Cr–Cr2O3 produced by DC reactive magnetron sputtering at different pressures. The micrograph of the left correspond to a higher magnification. The coating CR1 was deposited at higher pressure PArþO2 ¼ 1:2 Pa : The coating CR2 was produced at lower pressure PArþO2 ¼ 0:5 Pa :
1 2 3 4 5 6
O1s O 1 s (CrO3) O1 s (Cr2O3) Cr 2p3/2 (Cr2O3) Cr 2p3/2 (CrO3) C1s
2.85 2.85 2.85 7.6 4.6 1
530.99 533.09 529.84 576.90 579.28 284.5
V. Teixeira et al. / Vacuum 64 (2002) 299–305
303
Fig. 2. X-ray photoelectron spectroscopy analysis of the Cr 2p3/2 core levels for a graded Cr–Cr2O3 coating. The peaks for the Cr2O3 and CrO3 phases are shown.
present much more deeper than the few nanometers below the surface that were analysed by XPS, was identified only by the XRD analysis. The XRD patterns show only pure chromium peak centred at about 2y ¼ 44:71 corresponding to the crystallographic plane (1 1 0) parallel to the surface (see Fig. 3). The peak is generally broad and slightly intense. We suggest that this broadening of diffraction peaks is probably due to a trapping of oxygen atoms into the chromium cell leading to a dispersion of inter-planar spacing. It can also be due to oxygen atoms located at grain boundaries, limiting the grain growth and leading to a small grain structure [10–13,17]. In fact the measurement of average grain size in this composite coatings have shown that they present values of about 70 nm. The presence of peaks from Cr2O3 and CrO3 phases are not evident from the XRD analysis both for multilayered and graded films; thus the cermet is characterised as an XRD amorphous dielectric matrix in a crystalline metallic chromium dispersed phase. A more detailed quantitative in depth coating analysis by XPS and ellipsometry is being considered to study the through thickness compositional profile in order to develop a numerical model of the optical behaviour. The spectral reflectance in the visible and near infra-red of the deposited cermet Cr–Cr2O3 films
Fig. 3. XRD spectrum of a typical composite Cr–Cr2O3 coating produced on a glass substrate by DC reactive magnetron sputtering. The crystallographic plane Cr(1 1 1) for the metallic phase is the only one that can be revealed by the XRD analysis. The dielectric matrix (Cr2O3 and CrO3 phases) has an XRD amorphous structure.
was measured using a double-beam spectrophotometer equipped with an integrating sphere. Fig. 4 shows the measured reflectivities for Cr–Cr2O3 coatings (cermet and graded coatings deposited on Cu substrates). The reflectivity spectra of the coatings deposited on glass were also analysed and showed similar behaviour, presenting reflectivities lower than 6% in the spectral region studied (the solar absorptance of these coatings was measured and ranged from 0.90 to 0.95). The reflectivity spectra for coatings deposited on aluminium and copper substrates showed significantly different optical responses presenting very
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low reflectivity at the visible and an increase of reflectivity in the near IR spectral region of about 2 mm depending on the sputtering conditions. The coating deposited on Cu substrate (sample T21Cu) was deposited as a pure cermet and the optical performance was not so good as the graded layers, although it was possible to change the metallic Cr content and/or increase the total sputtering pressure to enhance the spectral selectivity of the cermet coating. The best approach was to deposit a graded structure which enable us to obtain high performance solar absorber surfaces with absorptivity (a) ranging from 0.91 to 0.96 and emissivity (e) ranging from 0.1 to 0.05. Samples T23Cu and T24Cu were deposited with sputtering conditions such that the resulting coating had a graded structure with a better optical selectivity (see Fig. 4). It should be noted that the T24Cu differs from the T23Cu only in the sputtering pressure. T24Cu was deposited at a higher pressure (1.2 Pa) compared to T23Cu (0.5 Pa). The resulting effect is evident from the reflectance spectrum since a higher pressure leads to a rougher surface [10,14] which enhances the absorption of the solar radiation [1]. The use of a metallic substrate and/ or a metallic layer at the interface will act as an IR reflector which will improve the performance due to reduction of emittance [1,4]. Parallel studies are being carried out on cermets based on Mo–Al2O3, NiCr–Cr2O3 and Mo–AlN [3,6,16] to analyse the effect of the metallic fraction in the ceramic coating, anti-reflection top layer and compositional gradient profile on the optical selectivity behaviour (very low reflectance at wavelengths below 2 mm and then showing an higher reflectivity in the IR region).
5. Conclusions Black solar absorber based in layered and graded coating of Cr–Cr2O3–CrO3 was deposited by DC reactive magnetron sputtering. By switching on and off the oxygen flow during sputtering of Cr, Cr and Cr2O3 were deposited alternatively and a multilayered coating results. By changing gradually the oxygen flow a gradient coating results. The magnetron sputtering technique has demon-
Fig. 4. Measured near normal reflectance spectra for the sputtered composite Cr–Cr2O3 coatings produced on a Cu substrate. T21Cu was deposited as a pure cermet. Samples T23Cu and T24Cu were deposited as graded coating. T24Cu was deposited at a higher pressure (1.2 Pa) compared to T23Cu (0.5 Pa).
strated that it is a potential method to produce high reproducible spectrally selective coatings with graded structure, since it is possible to control in an atomic level the addition of metal phases to the dielectric medium and produce layered and composite coatings as well as enable us to control the microstructure of the coatings. The controlled variation of the process parameters during deposition makes a controlled composite graded index layer feasible. However, some intermixing of adjacent layers was found by photoelectron spectroscopy analysis as well a mixture of Cr2O3 and CrOx compounds.
Acknowledgements This work was partially financially supported by Fundac*ao para a Ci#encia e Tecnologia under the - programme, research project ‘‘Spectrally PRAXIS Selective Coatings for Solar Applications’’, PRAXIS/P/CTM/11235/98. The exchange programme between University of Minho (Portugal) and CSIC (Spain) is financially supported by CRUP-CSIC Acco* es Integradas Luso-Espanholas/2000. The authors wish to thank Mr. A. Azevedo and Dr. F. Guimar*aes from University of Minho for XRD and SEM-EDX measurements and Dr. Carlos S!a from CEMUP (Univ. Porto) for the XPS analysis.
V. Teixeira et al. / Vacuum 64 (2002) 299–305
References [1] Seraphin BO. In: Seraphin BO, editor. Solar energy conversionFsolid state physics aspects. Topics in Applied Physics, vol. 31. Springer: Berlin, 1979. [2] Sella C, Lafait J. Appl Surf Sci 1988;33/34:942. [3] Costa MFM, Teixeira V, Nunes C. Microtopographic inspection of Cr–Cr2O3 cermet solar absorbers Proceedings of SPIEs 44th Annual Meeting and Exhibition Optical Science Engineering, and Instrumentation Symposium, Solar Optical Materials XVI, SPIE-vol. 3789, 1999. p. 140–8. [4] Zhang Q, Mills D. Sol Energy Mater Sol Cells 1992;27:273–90. [5] Suzer S, Kadirgan F, Sohmen HM, Wetherilt, AJ, , Ture IE. Sol Energy Mater Sol Cells 1998;52:55–60. [6] Teixeira V, Sousa E, Monteiro A, Costa MF, Roman E, Gago J. Deposition of Mo–Al2O3 cermet spectrally selective films by magnetron sputtering, presented in RIVA IV. Fourth Iberian Meeting of Vacuum and Applications, Avila, Spain, 13–15 July, 2000, Vacuum 2001, in press. [7] Ari Rabl. Active solar collectors and their applications. Oxford: University Press, 1985.
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[8] Guillett BW. The equivalence of outdoor mixed indoor/ outdoor solar collector testing. Sol Energy 1980;25: 543–8. [9] John A, Duffie W, Beckman A. Solar engineering of thermal processes. New York: Wiley, 1980. [10] Ohring M. The materials science of thin films. London: Academic Press Inc., 1992. [11] Gall D, Gampp R, Lang HP, Oelhfen P. J Vac Sci Technol A 1996;14(2):374–9. [12] Thornton JA. Ann Rev Mater Sci 1977;20:239. [13] Gao P, Meng LJ, dos Santos MP, Teixeira V, Andritschky M. Vacuum 2000;56:143. [14] Teixeira V, Andritschky M. Multicomponents multilyered thin films for advanced microtechnologies: techniques fundamentals devices. NATO Series EFApplied Sciences, vol. 234. Dordrecht: Kluwer Academic Publication, 1992. p. 121–7. [15] Andritschky M, Teixeira V. Vacuum, 1992;43:455–60. [16] V. Teixeira V, et al., Spectrally selective composite coatings for solar energy applications. Proceedings of the third ICCG-International Conference on Coatings on Glass, TNO-TPD, Maastricht, Holland, 2000. p. 647–57. [17] Teixeira V, Andritschky M. High Temperatures–High Pressures 1993;25:213–9.
Chromium-based thin sputtered composite coatings for solar thermal collectors V. Teixeiraa,*, E. Sousaa, M.F. Costaa, C. Nunesb, L. Rosab, M.J. Carvalhob, M. Collares-Pereirab, E. Romanc, J. Gagoc a
Departamento de F!ısica, IMAT-Instituto de Materiais, Universidade do Minho, Campus de Gualtar, P-4710-057 Braga, Portugal b INETI-Instituto Nacional de Engenharia e Tecnologia Industrial, ITE, Lisboa, Portugal c ICMM-Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain
Abstract One of the fundamental components of a solar thermal collector is the absorber surface that should be spectrally selective for solar radiation. Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. Most solar selective coatings use metal-dielectric composites, known as cermets, as the absorber of solar energy. In this study we will present a numerical model that allow us to correlate the selectivity of the produced absorbers to the collector efficiency. The cathodic magnetron sputtering being a promising method to produce thin solar selective films, a study of some cermet Cr–Cr2O3–CrO3 coatings obtained by this technique in reactive atmosphere and using a DC power unit will be presented. The multilayered composites produced were based in metallic chromium in a matrix of a chromium oxide with a gradient in oxygen composition. The selective graded films were produced by a reactive DC magnetron sputtering of pure chromium target in a plasma of argon–oxygen at different sputtering pressures and substrate temperatures. The microstructure, surface roughness, crystallographic phases, composition and chemical analysis by X-ray photoelectron spectroscopy and reflectivity spectra in vis-NIR were analysed. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction A large number of solar selective surfaces for thermal collectors have been studied [1–3]. Most solar selective coatings use metal–dielectric composites, known as cermets, as the absorber of solar energy. Different methods for producing solar absorber coatings are available: electroplated black chromium, which employs Cr–Cr2O3 cermet *Corresponding author. Tel.: +351-53-604334; fax: +35153-678981. E-mail address: vasco@fisica.uminho.pt (V. Teixeira).
material, is the most widely used solar absorber [3,4]; nickel-pigmented anodic Al2O3, which is produced by electrochemical treatment of an aluminium sheet is another solar selective absorber usually employed in industry [1–5]. In recent years, there has been a renewed interest in cermet coatings because of their potential application in a wide variety of technological applications, such as the production of selective absorbing surfaces to enhance the efficiency of the photothermal conversion process. Electrochemical and electroplating coatings are most widely used in thermal collectors mainly due
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 7 2 - 4
300
V. Teixeira et al. / Vacuum 64 (2002) 299–305
to low cost production and large area processing. However, the optical properties of these coatings are not suitable for very efficient photothermal conversion (e.g. emissivity is high). Sputtering technique is a clean coating technology without the pollution problems found in electroplating processes. Thus, in recent years, the focus of the study of solar selective surfaces has been on the production of coatings with spectral selectivity by sputtering techniques [3–6]. In the first part of this work we present a numerical model that allows us to correlate the selectivity of the produced absorbers to the collector efficiency. In this contribution, we use a suitable method for the deposition of composite cermet coatings of Cr–Cr2O3. The thin coatings were produced by reactive DC magnetron sputtering of chromium in an argon and oxygen atmosphere. The oxygen flow, during film deposition, was periodically switched on and off, leading to an alternating deposition of metallic chromium and chromium oxide resulting in a multilayered film. The deposition parameters, structural and optical properties of the multilayers films are presented and described in this paper.
2. Numerical modelling of thermal collector efficiency Thermal solar collector efficiency can be defined as the rate between the collector energy transferred to the fluid (water or oil) and solar incident energy over the collector: Z¼
mcp ðTs Te Þ ; Ac Icol
ð1Þ
where m is the massic flux of transfer fluid, cp the fluid specific heat, Ac the collector area, Icol the incident radiative power per unit collector area and Ts and Te the output and input fluid temperature. Collector efficiency is a function of the average temperature of the fluid, Tf ¼ ðTe þ Ts Þ=2 and can be given by a linear model of the type: Z ¼ F 0 Z0 F 0 UL
ðTf Tamb Þ ; Icol
ð2Þ
F 0 Z0 characterises the optical behaviour of the collector. In the case of a flat collector Z0 ¼ at; where t is the transmissivity of the transparent cover (glass) and a the absorptivity of the absorber plate. F 0 UL characterises the collector thermal losses. These are losses by convection, conduction and radiation. Tamb is the ambient temperature. One fundamental component of the collector is the solar radiation absorber surface, which must have high absorptivity (a) in the wavelength band of visible radiation and near infrared (300– 3000 nm) to which corresponds almost all solar energetic flux (at least 99%). The use of an absorber surface with selective coating also permits the reduction of losses by radiation, i.e., thermal losses reduction in the collector. One selective absorber surface is characterised by low emissivity (e) for wavelengths belonging to the thermal infrared (6000–15 000 nm). The application of solar collector heat production for high temperature (typically industrial applications), as well as its application in systems for heat and cold production, imposes the necessity to use collectors which show good efficiency for high temperatures. Thus it becomes important to produce good selective absorber surfaces. In order to prove the influence of surfaces optical characteristics on the behaviour of the collector efficiency, we applied a theoretical collector model [7–9] which, based on optical and thermal properties of the components, enables the calculation of the collector efficiency parameters. As part of a research project to develop spectrally selective surfaces for high efficiency thermal collectors, a model was applied to study the influence of optical absorber surface properties in the collector (a typical flat collector was considered). Collector efficiency values were determined considering only changes in the values of a and e of the absorber surface. Table 1 shows the efficiency values for an average fluid temperature of 901C (with Tamb ¼ 201C and Icol ¼ 1000 W m2). The values in Table 1 are obtained as a result of the efficiency curves generated by our model. Comparing a very good selective surface (case A) with a non-selective surface (case B) it can be seen that the efficiency of A is double that of B for
V. Teixeira et al. / Vacuum 64 (2002) 299–305 Table 1 Influence of optical surface properties in flat collector yield Sample
a
e
Z (Tf ¼ 901C)
Case Case Case Case Case Case
0.95 0.96 0.93 0.75 0.95 0.75
0.05 0.86 0.26 0.41 0.60 0.05
0.46 0.24 0.37 0.17 0.29 0.28
A B C D E F
the temperature considered. Considering the surface case C which is also a selective surface (that we can find frequently in the market), but not as good as A, it can be seen that its efficiency is only 0.9, lower than A. This shows that for high temperatures the increase in emissivity has a strong influence on efficiency reduction. The influence of low absorptivity values on efficiency reduction is shown when case A is compared with F or when B is compared with D. The cases F and D are only referred to show the influence of absorptivity efficiency. It is not usual that the surfaces in the market show these values.
3. Experimental details In this study Cr–Cr2O3 cermet solar thin coatings were deposited by DC reactive magnetron sputtering in an argon and oxygen atmosphere, from a 99.99% pure Cr target, which was cleaned by pre-sputtering in an argon atmosphere at 0.5 Pa during 10 min, prior to each deposition. The sputtering chamber was evacuated with a turbomolecular pump down to a base vacuum pressure of less than 2 10–4 Pa. Copper and glass substrates were used for the deposition of the absorber films. These substrates were cleaned by presputtering with Ar+ to remove any oxide surface layers from the surface. The distance between the substrate and the target was kept constant, at 60 mm and the substrate temperature was constant too, at 1501C. The sputtering working pressure was varied from 0.5 to 1.2 Pa and the negative DC substrate bias was varied from 50 to 65 V, depending on
301
the experiment. The thickness of the produced films was about 300 nm. During the deposition of composite coating of Cr–Cr2O3 the argon flow was kept constant at 100 or 160 sccm, while the oxygen flow was varied (from 0 to 15 sccm, depending on the experiment) in order to obtain a multilayer coating consisting of alternated metallic and oxide layers. A gradient coating was also produced by changing gradually the oxygen flow during reactive sputtering of chromium. After coating deposition, all coating systems were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), compositional analysis by energy dispersive X-ray microanalysis (EDS), chemical analysis by X-ray photoelectron spectroscopy (XPS) and coating reflectivity by optical reflectance spectrometry.
4. Results and discussion Our first approach to deposit chromium cermet coatings was to use low oxygen pressure to produce substoichiometric oxides. However, it is well documented that a monotonous increase of the oxygen flow results in an unsteady change of the deposition process. At a critical value of oxygen flow the deposition rate shows a sharp drop [10] and the coating passes abruptly from a metal to a dielectric. This can be explained by the poisoning of the target (oxidation of target) with a subsequent change in the sputtering yield. This transition was observed in the oxygen–chromium system. We observed a switching of the plasma colour when increasing the oxygen flow from below to above critical oxygen flow. At low oxygen flow the coatings consisted of mainly metallic chromium and the plasma colour during sputtering was blue. At higher oxygen pressures the coatings were almost completely oxidised and the plasma colour was reddish. Similar results were reported by other authors [11]. In our work a novel DC magnetron sputtering method for the deposition of composite layered coatings based in cermet of Cr–Cr2O3 was studied. By switching on and off the oxygen flow during sputtering of chromium, Cr and Cr2O3 monolayers
302
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were deposited alternatively and a multilayered coating results. Furthermore, for a better optical performance of the selective absorbers we developed graded coatings by changing gradually the oxygen flow, which results in a gradient coating. The surface microstructure of two layered coatings deposited at different sputtering pressures, by the method described above, is shown in the micrograph in Fig. 1. It is clearly shown that the surface roughness and the porosity of the film are affected by the total sputtering pressure (Ar+O2 pressure) during deposition. The coating CR1 was deposited at higher pressure PArþO2 ¼ 1:2 Pa : The coating CR2 was produced at lower pressure PArþO2 ¼ 0:5 Pa and presents a more compact structure. As expected, due to the higher energy of the particles arriving the growing coating, a more dense microstructure will develop [12–15]. A detailed surface micro-topography study of the selective chromium based absorbers (not presented in the present work) by atomic force microscopy
(AFM) and non-contact laser topography inspection is described elsewhere [3,16]. The chemical analysis of the coatings have been realised by EDS and XPS. Prior to the XPS analysis the coating surface has been in-situ eroded during a few minutes by argon ions. The deconvolution of peaks corresponding to the Cr 2p3/2 core levels and O 1s allows us to distinguish from the metallic Cr phase (energy 2p3/2 centred at B574.3 eV) to the Cr2O3 and CrO3 phases. A mixture of CrO3 (energy 2p3/2 centred at B576.9 eV) and Cr2O3 (energy 2p3/2 centred at B579.3 eV) compounds were identified near the coating surface (see Table 2 with the main XPS parameters used for analysis and Fig. 2). It is well known that sputter cleaning using Ar+ may lead to compositional changes at the surface of binary compounds. When Ar+ is used to clean oxide surfaces a reduction of the oxide is generally observed so the different oxides presented near the surface can be an artefact from the cleaning process. To verify this we also studied by XPS the as-deposited surface (prior to sputter cleaning), although it was somewhat dirty with carbon, and confirmed the presence of a mixture of Cr2O3 and CrO3. The subsurface compositional analysis using sputtering erosion may be affected and so a quantitative analysis of the ratio of the Cr2O3 and CrO3 phases was not performed. In the present study only qualitative and semi-quantitative analysis was taken into consideration. Other authors have done XPS experiments to analyse Cr–Cr2O3 and found similar results [11]. At and near the surface it was not possible to identify any metallic chromium. The metallic chromium was
Table 2 Parameters of the semi-quantitative analysis of the graded coating system for X-ray photoelectron spectroscopy (XPS) using Mg Ka radiation Peak Element/core level Sensitivity factor Binding energy (eV) Fig. 1. SEM micrographs of two solar absorbers Cr–Cr2O3 produced by DC reactive magnetron sputtering at different pressures. The micrograph of the left correspond to a higher magnification. The coating CR1 was deposited at higher pressure PArþO2 ¼ 1:2 Pa : The coating CR2 was produced at lower pressure PArþO2 ¼ 0:5 Pa :
1 2 3 4 5 6
O1s O 1 s (CrO3) O1 s (Cr2O3) Cr 2p3/2 (Cr2O3) Cr 2p3/2 (CrO3) C1s
2.85 2.85 2.85 7.6 4.6 1
530.99 533.09 529.84 576.90 579.28 284.5
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Fig. 2. X-ray photoelectron spectroscopy analysis of the Cr 2p3/2 core levels for a graded Cr–Cr2O3 coating. The peaks for the Cr2O3 and CrO3 phases are shown.
present much more deeper than the few nanometers below the surface that were analysed by XPS, was identified only by the XRD analysis. The XRD patterns show only pure chromium peak centred at about 2y ¼ 44:71 corresponding to the crystallographic plane (1 1 0) parallel to the surface (see Fig. 3). The peak is generally broad and slightly intense. We suggest that this broadening of diffraction peaks is probably due to a trapping of oxygen atoms into the chromium cell leading to a dispersion of inter-planar spacing. It can also be due to oxygen atoms located at grain boundaries, limiting the grain growth and leading to a small grain structure [10–13,17]. In fact the measurement of average grain size in this composite coatings have shown that they present values of about 70 nm. The presence of peaks from Cr2O3 and CrO3 phases are not evident from the XRD analysis both for multilayered and graded films; thus the cermet is characterised as an XRD amorphous dielectric matrix in a crystalline metallic chromium dispersed phase. A more detailed quantitative in depth coating analysis by XPS and ellipsometry is being considered to study the through thickness compositional profile in order to develop a numerical model of the optical behaviour. The spectral reflectance in the visible and near infra-red of the deposited cermet Cr–Cr2O3 films
Fig. 3. XRD spectrum of a typical composite Cr–Cr2O3 coating produced on a glass substrate by DC reactive magnetron sputtering. The crystallographic plane Cr(1 1 1) for the metallic phase is the only one that can be revealed by the XRD analysis. The dielectric matrix (Cr2O3 and CrO3 phases) has an XRD amorphous structure.
was measured using a double-beam spectrophotometer equipped with an integrating sphere. Fig. 4 shows the measured reflectivities for Cr–Cr2O3 coatings (cermet and graded coatings deposited on Cu substrates). The reflectivity spectra of the coatings deposited on glass were also analysed and showed similar behaviour, presenting reflectivities lower than 6% in the spectral region studied (the solar absorptance of these coatings was measured and ranged from 0.90 to 0.95). The reflectivity spectra for coatings deposited on aluminium and copper substrates showed significantly different optical responses presenting very
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low reflectivity at the visible and an increase of reflectivity in the near IR spectral region of about 2 mm depending on the sputtering conditions. The coating deposited on Cu substrate (sample T21Cu) was deposited as a pure cermet and the optical performance was not so good as the graded layers, although it was possible to change the metallic Cr content and/or increase the total sputtering pressure to enhance the spectral selectivity of the cermet coating. The best approach was to deposit a graded structure which enable us to obtain high performance solar absorber surfaces with absorptivity (a) ranging from 0.91 to 0.96 and emissivity (e) ranging from 0.1 to 0.05. Samples T23Cu and T24Cu were deposited with sputtering conditions such that the resulting coating had a graded structure with a better optical selectivity (see Fig. 4). It should be noted that the T24Cu differs from the T23Cu only in the sputtering pressure. T24Cu was deposited at a higher pressure (1.2 Pa) compared to T23Cu (0.5 Pa). The resulting effect is evident from the reflectance spectrum since a higher pressure leads to a rougher surface [10,14] which enhances the absorption of the solar radiation [1]. The use of a metallic substrate and/ or a metallic layer at the interface will act as an IR reflector which will improve the performance due to reduction of emittance [1,4]. Parallel studies are being carried out on cermets based on Mo–Al2O3, NiCr–Cr2O3 and Mo–AlN [3,6,16] to analyse the effect of the metallic fraction in the ceramic coating, anti-reflection top layer and compositional gradient profile on the optical selectivity behaviour (very low reflectance at wavelengths below 2 mm and then showing an higher reflectivity in the IR region).
5. Conclusions Black solar absorber based in layered and graded coating of Cr–Cr2O3–CrO3 was deposited by DC reactive magnetron sputtering. By switching on and off the oxygen flow during sputtering of Cr, Cr and Cr2O3 were deposited alternatively and a multilayered coating results. By changing gradually the oxygen flow a gradient coating results. The magnetron sputtering technique has demon-
Fig. 4. Measured near normal reflectance spectra for the sputtered composite Cr–Cr2O3 coatings produced on a Cu substrate. T21Cu was deposited as a pure cermet. Samples T23Cu and T24Cu were deposited as graded coating. T24Cu was deposited at a higher pressure (1.2 Pa) compared to T23Cu (0.5 Pa).
strated that it is a potential method to produce high reproducible spectrally selective coatings with graded structure, since it is possible to control in an atomic level the addition of metal phases to the dielectric medium and produce layered and composite coatings as well as enable us to control the microstructure of the coatings. The controlled variation of the process parameters during deposition makes a controlled composite graded index layer feasible. However, some intermixing of adjacent layers was found by photoelectron spectroscopy analysis as well a mixture of Cr2O3 and CrOx compounds.
Acknowledgements This work was partially financially supported by Fundac*ao para a Ci#encia e Tecnologia under the - programme, research project ‘‘Spectrally PRAXIS Selective Coatings for Solar Applications’’, PRAXIS/P/CTM/11235/98. The exchange programme between University of Minho (Portugal) and CSIC (Spain) is financially supported by CRUP-CSIC Acco* es Integradas Luso-Espanholas/2000. The authors wish to thank Mr. A. Azevedo and Dr. F. Guimar*aes from University of Minho for XRD and SEM-EDX measurements and Dr. Carlos S!a from CEMUP (Univ. Porto) for the XPS analysis.
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