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Thermally and air–plasma-oxidized titanium and stainless steel plates as solar selective absorbers
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Solar Energy Materials & Solar Cells 90 (2006) 2556–2568 www.elsevier.com/locate/solmat
Thermally and air–plasma-oxidized titanium and stainless steel plates as solar selective absorbers Alejandro Avila-Garcı´ aa,, Ulises Morales-Ortizb a
Seccio´n de Electro´nica del Estado So´lido, Departamento de Ingenierı´a Ele´ctrica, CINVESTAV del IPN, Ap. Postal 14-740, Me´xico DF, 07360, Me´xico b Departamento Quı´mica, Area Electroquı´mica, UAM Iztapalapa, AP 55-534, 09340 Me´xico DF, Mexico Available online 2 May 2006
Abstract Solar selective surfaces can be constructed in many ways. In particular, low emissivity (e) is currently sought for by starting with surfaces bearing high infrared reflectance. It is well known that metallic surfaces behave in this way. Then, high solar absorptance (a) can be achieved by adding an appropriate thin layer through a wide variety of possibilities. In this work, a simple direct method to produce such type of coatings on titanium and 304 stainless steel plates is assessed: thermal oxidation. Good selectivities S( ¼ a/e) 10 or higher in some cases were obtained mainly on steel substrates. An alternative is to expose these metallic surfaces to ionized oxygen species, rather than to neutral oxygen molecules. This was accomplished by oxidizing some plates in a typical glow discharge capacitive system. In this case, acceptable but not so high selectivities, as in the first case, were obtained. Some comments on the metallic surface morphology and the stability of the oxidized surfaces are also presented. r 2006 Elsevier B.V. All rights reserved. Keywords: Selective coatings; Thermal oxidation; Plasma oxidation; Titanium; Stainless steel
1. Introduction Selective coatings have been extensively discussed in the literature. Six main categories for such coatings are currently accepted [1,2]: (a) intrinsic, (b) semiconductor–metal tandems, (c) multilayer absorbers, (d) multidielectric composite coatings, (e) textured Corresponding author. Tel.: +52 50616259; fax: +52 50613978.
E-mail address: [email protected] (A. Avila-Garcı´ a). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.03.032
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surfaces, and (f) selectively solar-transmitting coatings. Each type has name-related specific features defining it. Varied examples of coatings, falling in one of the above types or combining the principles of several of them, have been fabricated by different methods. Each one performs well in a certain temperature interval, depending on its stability under ambient conditions and high temperature. Stability is a key factor for practical applications [3], but the simplicity of the structure and the fabrication method are also important. Metal oxides are expected to be more immune to thermal and environmental conditions. Several of them have been already assessed as active materials for selective coatings. Some of the most widely known are copper oxide [4–6], chromium oxide [7–10], nickel oxide [11,12], tungsten oxide [13,14], aluminum oxide [15,16], titanium oxide [17] and cerium oxide [17]. Reports on these oxides mention their usage in both simple and complex structures. They have been produced by using more or less complex methods like electrodeposition, CVD, vacuum evaporation, plasma spray or sputtering, but some other were produced by simpler methods like chemical conversion, dipping, spray pyrolysis or thermal annealing. Some have been classified as adequate for mid- to high-temperature applications [2]. Cobalt oxide has raised some interest due to its potential for high-temperature applications [9,18–22]. In some of these reports, emphasis has been made on non-expensive methods of fabrication, like dipping or spray pyrolysis. More recently, ruthenium oxide films deposited on titanium substrates have also been assessed as selective coatings [23]. Acceptable a and e values were obtained [24–27] for colored stainless steel (SS) selective surfaces, obtained by immersion in a hot solution made of chromic and sulfuric acid. The films so formed were too soft, making applications difficult. Nevertheless, this disadvantage was overcame [28] by cathodic treatment in a similar bath. The best parameters corresponded to blue films (a/e ¼ 0.90/0.1). Evidently, colored SS consists of the substrate plus a chemically produced oxide on the surface, the colors depending on the oxide thickness and composition. Contrary to these chemically grown colored oxides, reactively sputtered SS oxides are considered good prospects for high-temperature applications [2]. This is one of the reasons to propose additional studies on SS oxides, obtained directly from the substrates. In this work, as in a previous one [23], titanium substrates are also used, due to their good corrosion resistance. Two straightforward methods are proposed to get directly the oxide films from the substrates: thermal and air–plasma oxidation. A second reason is that metallic substrates insure an infrared high reflectance; this is equivalent to a low emittance. Then, a film that is thin enough to keep a small increase of emittance and to enhance absorption in the solar range can be grown upon them. It is desired to take advantage of two well-known additional factors, the substrate surface morphology and the film thickness, to improve absorptance without largely increasing emittance. The optical properties of these films were evaluated. Finally, the effect of a thermal treatment on the properties of the films is presented. 2. Experimental details 2.1. Sample preparation 304 and 430 SS besides ASTM grade 2 titanium (Ti) plates about 2.5 2.5 cm2 were used as substrates. A mechanical polishing process was applied to some titanium and 304 SS substrates to obtain some texture on the surface. 430 SS is acquired with a uniform surface
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texture, which was left unchanged. The mechanical polishing procedure consisted of rubbing the metallic plates first with 600 grade sandpaper, and then with 1 mm alumina powder. A cleaning process including ultrasonic rinses in extran, 10% NaOH aqueous solution, 50%–50% alcohol–acetone mixture, and de-ionized water was applied to the substrates previously polished. After drying, the substrates were kept in alcohol prior to the oxide preparation. Blue-colored coatings on SS were formerly reported to have the best a and e values [24]. It was found that thermal oxidation of Ti plates in air at 500 1C for 30 min was enough to get blue-colored coatings. SS plates needed a temperature of 700 1C for 2 h to yield blue thermally grown coatings. Also, SS and titanium oxides were produced by air–plasma oxidation. The metallic plates were attached to the vertical copper cathode of a typical capacitive glow discharge system. Air was allowed to flow in the process chamber, keeping a constant 0.15–0.23 Torr pressure. The substrate temperature was raised from room temperature up to 350 1C along 1 h, hereafter keeping this temperature constant until the process ended 2 h later. A 13.56 MHz radio frequency (RF) signal was applied between anode and cathode to produce air ionization since the temperature started rising. The RF signal power used was about 10 W, equivalent to a power density of 0.1 W/cm2. After one of these cycles, a blue titanium oxide film was obtained upon the corresponding substrates. In the case of SS, a second almost identical cycle excepting a lower pressure (5 mTorr) was applied to try obtaining blue-colored films. 2.2. Sample characterization The main characterization technique was of optical type. It consisted of spectral nearnormal specular reflectance measurements, carried out with a Fourier transform spectrometer (Nexus 670) in the 0.4–25 mm wavelength range. The calculation of absorptance and emittance was done as outlined by Duffie and Beckman [29]. Also, atomic force microscopy measurements helped to determine the surface morphology. These were made with a 250 Quesant system in the usual contact mode. An additional technique was ellipsometry to determine the film thickness. Ellipsometry measurements were carried out with a 0.633 mm wavelength Gaertner LSE model Stokes ellipsometer. 3. Results and discussion 3.1. The metallic substrates As it has been formerly said, it is desired to obtain appropriate absorptance values by means of controlling two factors: one of them is the surface morphology of the substrate, and the other is the film thickness upon it. Polishing the substrates allows to get a specific size of the surface features. In principle, a mechanical process to make this surface treatment possible can be done. Here, It has been applied to grade 2 titanium (Ti) and 304 SS plates (SS3). 430 SS plates (SS4) bear a rather uniform surface, so no polishing process was applied to them. SS3 substrates polished with sandpaper (SS3s), with 1 mm alumina powder (SS3m) and no-polish (SS3n) were studied. Also, Ti substrates polished with sandpaper (Tis), with 1 mm alumina powder (Tim), and no-polish (Tin) were prepared for this study.
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3.1.1. Surface morphology Atomic force microscope (AFM) surface images of two different substrates are shown in Fig. 1. According to the geometrical point of view, where a specific texture is used to achieve selectivity [30], surface features with sizes between 400 nm and 5 mm must be sought for. In order to observe such feature sizes, squares of 5 mm side were scanned in the
Fig. 1. Atomic force microscope images of the (a) SS3m and (b) SS3n surfaces. The sizes of the surface features are distinct for different surface treatment. These differences are expected to produce differences also in the optical response of such surfaces, which can be seen in the spectral reflectance measurements.
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case of the polished substrates. However, the larger size of features on no-polish surfaces makes 40 40 mm2 size images better to describe them. Fig. 1a (SS3m) exhibits grooves less than a micrometer (0.25 mm) wide and with vertical variations less than 37 nm.In Fig. 1b, the 40 40 mm2 image of the SS3n substrate makes evident the existence of random-direction grooves up to 4 mm wide, which separate large size (up to 10 mm) flattened regions. Vertical variations reach up to 853 nm. As it will be shown in the next section, all the substrates used can be grouped into three sets of surfaces on the basis of their optical parameters, which can be associated to the size of their surface features. 3.1.2. Optical properties of the surfaces The optical responses of the surfaces used are shown in Fig. 2, as the spectral reflectance in the 0.4–25 mm wavelength range. Two plates (SS4 and SS3m) have high reflectance in the whole range, leading to a low absorptance and low emittance parameters (group 1). Another group is formed by the samples SS3s and Tim, bearing midreflectance in the short-wavelength region (group 2). The SS3n, Tin and Tis plates form a third group (group 3). They resemble intrinsic selective surfaces, with a higher absorptance, but emittance is enlarged due to a long decay of reflectance around 10 mm wavelength for increasing frequencies. The results of the numerical calculations for the absorptance and the emittance referred to a blackbody spectrum at 200 1C, are contained in Table 1, and also in 100
Reflectance (%)
80
60 SS3n SS3m
40
SS4 Tin Tim
20
SS3s Tis
0 1
10 Wavelength (µm)
Fig. 2. Curves of the spectral reflectance for the metallic substrates used, showing noticeable differences depending on the substrate morphology. Three groups can be noticed. Group 1, including SS3m and SS4, has high reflectance for all wavelengths. Group 2 is formed by Tim and SS3s, having mid-reflectance in the shortwavelength region. Finally, group 3, including SS3n, Tin and Tis, which have the lowest reflectance for short wavelengths. Group 3 shows the best intrinsic selective properties.
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Table 1 Solar parameters of the substrates, obtained as outlined in the introduction Substrate
SS4 SS3m Tim SS3s Tis SS3n Tin
a
0.247 0.266 0.565 0.591 0.804 0.736 0.825
e(200 1C)
0.088 0.095 0.208 0.188 0.27 0.219 0.282
S
2.81 2.80 2.72 3.14 2.98 3.36 2.93
Approx. feature size (mm) Vertically
Horizontally
0.121 0.037 0.110 0.063 1.350 0.854 2
1 0.250 2 2 p10 p10 p10
The last columns show approximate vertical and horizontal feature size for the surfaces of the substrates.
Fig. 3. The three groups defined above can be clearly distinguished in both, the table and the figure. In the first group, emittance is less than 0.1 and absorptance is also low, around 0.25. Substrates in group 2 have higher absorptances between 0.55 and 0.6, but emittance also raises up to near 0.2. Finally, for the substrates within group 3, absorptance is much larger, about 0.8, while emittance almost reaches 0.3. It should be noticed that, as expected, grouping the substrates according to these optical parameters yield the same result than that obtained by grouping according to the feature size of the surfaces. This is shown in Table 1, where the feature size is also included in the last column. In accordance with these results, the effect of polishing SS3 plates with 1 mm powder turns out to be smoothening for large (41 mm) surface features. Then, absorption of electromagnetic radiation mainly in the near- and mid-infrared ranges, and also in the visible region, is eliminated, increasing consequently the reflectance in the whole range. We were not able to achieve an identical result for Ti plates. Due to the particular mechanical properties of such material, large voids upon polished areas are produced as a result of the polishing process. Instead, a midreflectance value was obtained, similar to that of SS3 plates polished with sandpaper. On the other hand, polishing the Ti plates with sandpaper does not change advantageously their reflectance, as it is quite similar to that of no-polish Ti and no-polish SS3 plates. As noted in Fig. 3, the emittance value is more critical than absorptance to produce large increases of selectivity. For the structures we planned to build, oxides are less conductive than metal. Hence, it is not possible to increase the mid-infrared reflectance of the film–substrate structure above that of the metallic substrate alone, preventing then emittance lowering. Instead, absorptance is feasible to be raised. Hence, further than improving emittance, we shall look for absorptance improvements. Hence, we should start from low-emittance substrates. This means that better results can be expected from substrates in groups 1 and 2, as compared to those in group 3. Nevertheless, results about substrates within all three groups are reported in the following sections. 3.2. Results of coatings 3.2.1. Ellipsometry measurements This type of measurement was done only on samples whose substrates were not too rough, due to the difficulties appearing when the narrow laser beam is dispersed by the
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α ε
S
3.4
0.8
3.3
0.7
3.2
0.6 3.1 0.5 3.0 0.4 0.3
2.9
0.2
2.8
Selectivity
Absorptance, Emittance
0.9
0.1 2.7 0.0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate Fig. 3. Optical Solar parameters of the distinct substrate surfaces used in this work. Group 1 has the first two samples from left to right, group 2 the next two, and group 3 the remaining three. Both, absorptance and emittance can be seen to change value in blocks. Even small variations of the emittance value strongly change selectivity, since it is the divisor in the equation defining S.
surface. The thickness of coatings on SS3m, SS4 and Tim substrates are presented in Table 2. As shown, all the coatings have thickness between 145 and 260 nm. Most of them are about 150 nm thick and a few pass beyond 200 nm. The two thickest coatings were grown on SS substrates, one from plasma oxidation and thermally the second. Also, the refractive index obtained from these measurements is listed in Table 2. As seen, the refractive indices of titanium oxide, expected for both plasma and thermal oxidation of Tim substrates, are not far from the reported value [31], about 2.85. One sample was found to have a high refractive index, the thermal oxide on SS4. As said above, on titanium substrates, thermal or plasma oxidation will produce mostly titanium oxide, but further chemical characterization is needed to determine the precise composition and structure of the film. On SS plates, the situation is more complicated, since the substrates contain chromium, which bears a high affinity for oxygen, but a larger amount of iron is also present. Hence, chromium and iron oxides might be embedded in the coating as a result of thermal or air–plasma oxidation. Further chemical and structural studies are needed to provide such information. 3.2.2. Coatings made by thermal oxidation The temperature and time of oxidation were different for titanium and SS substrates. In the first case, oxidation was carried out at 500 1C for 30 min. This procedure yielded blue coatings on the three Ti substrate types (Tin, Tis and Tim). In the case of the SS, the
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Table 2 Thickness and refractive index of some coatings fabricated in this work, deposited on the smoothest substrates, as obtained from ellipsometric measurements at 633 nm wavelength Substrate type
SS3m SS4 Tim
Plasma oxide
Thermal oxide
Thickness (nm)
Refractive index
Thickness (nm)
Refractive Index
260.5714.2 175.4735.1 147.374.7
2.7370.13 2.8070.51 2.9570.06
157.472.5 208.575.8 164.579.5
2.6370.03 3.5470.09 2.6370.14
oxidation was carried out at 700 1C for 2 h. A blue coating on SS4 and SS3s was obtained, but it was gray on SS3n, and reddish on SS3m. The absorptance, emittance and selectivity values obtained from the corresponding reflectance spectra are shown in Fig. 4. As expected, absorptance of the substrate alone was increased in all cases, but in coatings on substrates of group 1, this is more noticeable. The best selectivity values correspond to SS3s, SS3m and SS4 substrates, but the first bears a higher absorptance than the others. These substrates belong to the above-mentioned groups 1 and 2. Clearly, the high selectivities of the SS3s and SS3m samples, are due to the good values of emittance in such cases, which are even better than those of the substrates in Fig. 3. This means that there is certainly some lack of uniformity in the surface properties of substrates assumed to be identical. The samples in group 3 have the highest absorptances, but emittance is also too high. 3.2.3. Coatings made by air– plasma oxidation Oxidation in air–plasma as outlined before yielded blue coatings on all the titanium substrates, but blue on SS3n, gold–red on SS3s, golden on SS3m, and blue on SS4. The optical parameters from these coating–substrate systems are shown in Fig. 5. The coatings produced by air–plasma oxidation increase absorptance in all cases, those in group 3 being the best in this respect. This increase of absorptance must be improved in the case of the samples in groups 2 and 3, in order to get better selectivities. Improvement could be achieved by searching new convenient conditions for the plasma process, leading to higher oxidation rates or increasing the process time. Since the chemical reactivity of ionized oxygen is expected to be higher than that of neutral oxygen, the substrate temperature used here is much lower than that of the thermal processes. Once more, the highest selectivities, which are appreciably smaller than those of thermal oxides, correspond to the coatings on SS3s, SS3m and SS4 substrates. By comparing the emittances of the SS3m substrates in Figs. 3 and 5, a certain lack of reproducibility in surface finishing of the substrates is again evidenced. 3.3. Thermally treated samples The plasma-produced coatings on SS3m and Tim along with the thermally grown film on Tim substrates were selected to assess the effect of a long-term thermal process. This process was applied to the coatings for 120 h at atmospheric pressure and 450 1C, which
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40
α ε
0.9
S
0.8
0.6 20
0.5 0.4
Selectivity
Absorptance, Emittance
30 0.7
0.3 10 0.2 0.1 0.0
0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate Type Fig. 4. Optical parameters of thermally grown oxides on the different substrates. 1.0 0.9
S
α ε
6
0.7 0.6 4
0.5 0.4
Selectivity
Absorptance, Emittance
0.8
0.3 0.2
2
0.1 0.0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate type Fig. 5. Optical solar parameters of the oxides produced by air–plasma oxidation.
corresponds to a mid-range work temperature. The thermally oxidized SS samples were not included, since they were fabricated at 700 1C and no change is expected under the conditions of the process described.
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1.0 0.9 0.8
α, before α, after ε, before
Absorptance, Emittance
ε, after 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Pl ox / SS3m
Pl ox / Tim
Th ox / Tim
Sample
(a)
26
7 S decrease 6
24
Selectivity
20 5 18 16 4
S decrease (%)
22
14 S before 3
12
S, after
10 Pl ox / SS3m (b)
Pl ox / Tim
Th ox / Tim
Sample
Fig. 6. (a) Comparison of absorptance and emittance before and after the long-term thermal treatment of some samples fabricated at medium temperatures. In all the samples, the main effect is upon the emittance; (b) the selectivity values and their corresponding decrease.
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3.3.1. Ellipsometric measurements Several of those samples under treatment could not be analyzed by ellipsometry. From AFM observations, it was found that the thermal treatment produced larger surface roughness by means of structural changes which formed particles of different heights in the film surface. This larger vertical non-uniformity of the surface produced dispersion effects on the narrow measurement beam of the ellipsometer. Only the plasma-oxidized SS3m substrate could be measured with the same equipment used formerly. After the long-term thermal treatment, its thickness increased from 260.5 up to 382.4 nm and its refractive index decreased from 2.73 down to 1.98. The thickness increase indicates further oxidation due to long-time heating. Meanwhile, the decrease of refractive index points to a possible change in the structure of the coating. This can occur since at least chromium, nickel and iron oxides might be forming the original coating, and the relative content and structure of them could be changed by different oxidation conditions, as is the case. To quantitatively establish this assertion, further chemical characterization to study these samples and their evolution, is needed. 3.3.2. Optical parameter comparison The optical parameters before and after the thermal treatment can be compared as shown in Figs. 6a and b. In Fig. 6a, the absorptance does not change appreciably in most samples, as indicated by the solid and empty squares. Instead, emittance increases slightly, as shown by the circles, producing a decrease of selectivity. This decrease and its equivalent percentage are shown in Fig. 6b. The solid squares correspond to the selectivity before the thermal treatment and the empty ones to selectivity after this process. According to these results, the selectivity degradation, comprised between 10% and 25% for these samples, is entirely due to the emittance increase. The degradation is lower in the case of both thermally and plasma-oxidized titanium substrates. Instead, almost 25% degradation occurs for the SS substrate. Again, the precise mechanism responsible of such emittance degradation deserves further detailed morphological and chemical study on each specific case. 4. Conclusion In order to benefit from their high electronic conductivity, which leads to low thermal emittance, metallic (titanium, SS3 and SS4) substrates with different surface morphologies were used to fabricate several types of selective surfaces. Three types of surface finishing were used: no-polish, polished with sandpaper, and polished with 1 mm alumina powder. Two types of coatings were fabricated upon the metallic surfaces: films obtained by either thermal- or plasma oxidation. Some lack of surface roughness reproducibility was observed under the mechanical polishing method that was used. This lack of reproducibility strongly affects the results on Ti substrates. Substrates could be divided into three groups, according to their surface morphology, which is in turn related to their solar optical parameters. Substrates with initial high or mid-reflectance in the whole wavelength range give the best results for the selectivity value. This means that better results were obtained from metallic surfaces bearing initially low emittance and also low absorptance, depending on the surface morphology, as expected. Thermal oxides on SS polished substrates yielded the best selectivity values, which were higher than 10. The maximum selectivity values of plasma oxides were also obtained on SS substrates and were
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between 4 and 7. In general, absorptance was not very high, being at most near 0.8 in the best cases. Despite coatings on polished Ti substrates yielding moderate selectivity values, they turned out to be the most stable under long-term thermal treatment, with about 10% of degradation. On the other hand, the SS plasma oxide coating degraded about 25%. The expected advantage of SS plates that were oxidized at 700 1C is their thermal stability near this temperature. Then our best results in terms of solar performance, work temperature and thermal stability, corresponded to thermally oxidized SS substrates, processed with any polishing method of those used here. Further structural and chemical studies of the coatings and their evolution after thermal treatment will provide a close understanding of their degradation mechanisms. Undoubtedly, among these mechanisms one can certainly mention further oxidation of the substrate, as seen in this work after the long-term thermal treatment. Acknowledgments A. Avila-Garcia gratefully thanks CONACYT, Me´xico, for supporting the development of this work, through project 39116A. Also, help in ellipsometry measurements of Ing. Juan Garcı´ a and Drs. Miguel A. Aguilar F. and Ciro Falcony G. of the Physics Department, CINVESTAV, is acknowledged. References [1] C.G. Granqvist, Spectrally selective surfaces for heating and cooling applications, Tutorial Texts in Optical Engineering, vol. TT1, SPIE Optical Engineering Press, Bellingham, WA, 1989, p. 57. [2] C.E. Kennedy, Technical Report No. NREL/TP- 520-31267 National Renewable Energy Laboratory, Golden, CO, 2002, p. 4. [3] W.F. Bogaerts, C.M. Lampert, J. Mater. Sci. 18 (1983) 2847. [4] R.E. Peterson, J.W. Ramsey, J. Vac. Sci. Technol. 12 (1975) 174. [5] H. Tabor, in: Proceedings of the International Conference on Solar Building Technology, 1977, p. 204. [6] C.M. Lampert, US-DOE Report No. W-7405-ENG-48, 1979. [7] G.B. Smith, A. Ignatiev, Sol. Energy Mater. 4 (1981) 119. [8] J.V. Iyer, S.B. Gadgil, A.K. Sharma, B.K. Gupta, O.P. Agnihotri, Sol. Energy Mater. 6 (1981) 113. [9] D.M. Mattox, G.J. Kominiak, R.R. Sowell, R.B. Pettit, SAND 75-0361, Sandia Laboratory, Albuquerque, NM, 1975. [10] B.O. Seraphin, in: Proceedings Symposium on Materials Science Aspects of Thin Film System Solar Energy Conversion, 1974, p. 7. [11] A.M. Schneiders, P. Beucherie, J. Phys. Colloq. CI. 42 (Suppl. 1) (1981) C1. [12] S. Craig, G.L. Harding, Sol. Energy Mater. 4 (1981) 245. [13] G.D. Pettit, IBM J. Res. Dev. 22 (1978) 372. [14] M. Van Der Leij, Proc. Int. ISES Meet. (1978) 837. [15] H.G. Craighead, R.A. Buhrman, J. Vac. Sci. Technol. 15 (1978) 269. [16] J.A. Thornton, DOE Report No. DE/AC04/78CS35306, 1979. [17] J.A. Thornton, J.A. Lamb, SERI Final Subcontract Report No. SERI/STR-255-3040, Solar Energy Research Institute, Golden, CO, 1987. [18] P. Kokoropoulos, E. Salam, F. Daniels, Sol. Energy 3 (1959) 19. [19] R.B. Gillete, Sol. Energy 4 (1960) 24. [20] E. Barrera, T. Viveros, A. Avila, P. Quintana, M. Morales, N. Batina, Thin Solid Films 346 (1999) 138. [21] E. Barrera, A. Avila, J. Mena, V.H. Lara, M. Ruiz, J. Me´ndez-Vivar, Sol. Energy Mater. Sol. Cells 76 (2003) 387. [22] A. Avila G., E. Barrera C., L. Huerta A., S. Muhl, Sol. Energy Mater. Sol. Cells 82 (2004) 269.
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[23] U. Morales-Ortiz, A. Avila-Garcı´ a, C.V. Lara, Sol. Energy Mater. Sol. Cells, IMRC 2004, Symposium 4 (Available online since May 2005). [24] B. Karlsson, C.G. Ribbing, Proc. Soc. Photo-Opt. Eng. 161 (4) (1978) 161/09. [25] B. Karlsson, C.G. Ribbing, Technical Report NSF Contract 3879-1, National Science Foundation, 1977. [26] G.B. Smith, Metal Aust. (1977) 204. [27] G. Granziera, Met. Aust. (1977) 211. [28] T.E. Evans, A.C. Hart, H. James, V.A. Smith, Trans. Inst. Met. Finish. 50 (1972) 77. [29] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, Wiley, New York, NY, 1991. [30] C.M. Horwitz, Opt. Commun. 11 (1974) 210. [31] http://www.ioffe.ru/SVA/NSM/nk/Oxides/Gif/tio2.gif.
Solar Energy Materials & Solar Cells 90 (2006) 2556–2568 www.elsevier.com/locate/solmat
Thermally and air–plasma-oxidized titanium and stainless steel plates as solar selective absorbers Alejandro Avila-Garcı´ aa,, Ulises Morales-Ortizb a
Seccio´n de Electro´nica del Estado So´lido, Departamento de Ingenierı´a Ele´ctrica, CINVESTAV del IPN, Ap. Postal 14-740, Me´xico DF, 07360, Me´xico b Departamento Quı´mica, Area Electroquı´mica, UAM Iztapalapa, AP 55-534, 09340 Me´xico DF, Mexico Available online 2 May 2006
Abstract Solar selective surfaces can be constructed in many ways. In particular, low emissivity (e) is currently sought for by starting with surfaces bearing high infrared reflectance. It is well known that metallic surfaces behave in this way. Then, high solar absorptance (a) can be achieved by adding an appropriate thin layer through a wide variety of possibilities. In this work, a simple direct method to produce such type of coatings on titanium and 304 stainless steel plates is assessed: thermal oxidation. Good selectivities S( ¼ a/e) 10 or higher in some cases were obtained mainly on steel substrates. An alternative is to expose these metallic surfaces to ionized oxygen species, rather than to neutral oxygen molecules. This was accomplished by oxidizing some plates in a typical glow discharge capacitive system. In this case, acceptable but not so high selectivities, as in the first case, were obtained. Some comments on the metallic surface morphology and the stability of the oxidized surfaces are also presented. r 2006 Elsevier B.V. All rights reserved. Keywords: Selective coatings; Thermal oxidation; Plasma oxidation; Titanium; Stainless steel
1. Introduction Selective coatings have been extensively discussed in the literature. Six main categories for such coatings are currently accepted [1,2]: (a) intrinsic, (b) semiconductor–metal tandems, (c) multilayer absorbers, (d) multidielectric composite coatings, (e) textured Corresponding author. Tel.: +52 50616259; fax: +52 50613978.
E-mail address: [email protected] (A. Avila-Garcı´ a). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.03.032
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surfaces, and (f) selectively solar-transmitting coatings. Each type has name-related specific features defining it. Varied examples of coatings, falling in one of the above types or combining the principles of several of them, have been fabricated by different methods. Each one performs well in a certain temperature interval, depending on its stability under ambient conditions and high temperature. Stability is a key factor for practical applications [3], but the simplicity of the structure and the fabrication method are also important. Metal oxides are expected to be more immune to thermal and environmental conditions. Several of them have been already assessed as active materials for selective coatings. Some of the most widely known are copper oxide [4–6], chromium oxide [7–10], nickel oxide [11,12], tungsten oxide [13,14], aluminum oxide [15,16], titanium oxide [17] and cerium oxide [17]. Reports on these oxides mention their usage in both simple and complex structures. They have been produced by using more or less complex methods like electrodeposition, CVD, vacuum evaporation, plasma spray or sputtering, but some other were produced by simpler methods like chemical conversion, dipping, spray pyrolysis or thermal annealing. Some have been classified as adequate for mid- to high-temperature applications [2]. Cobalt oxide has raised some interest due to its potential for high-temperature applications [9,18–22]. In some of these reports, emphasis has been made on non-expensive methods of fabrication, like dipping or spray pyrolysis. More recently, ruthenium oxide films deposited on titanium substrates have also been assessed as selective coatings [23]. Acceptable a and e values were obtained [24–27] for colored stainless steel (SS) selective surfaces, obtained by immersion in a hot solution made of chromic and sulfuric acid. The films so formed were too soft, making applications difficult. Nevertheless, this disadvantage was overcame [28] by cathodic treatment in a similar bath. The best parameters corresponded to blue films (a/e ¼ 0.90/0.1). Evidently, colored SS consists of the substrate plus a chemically produced oxide on the surface, the colors depending on the oxide thickness and composition. Contrary to these chemically grown colored oxides, reactively sputtered SS oxides are considered good prospects for high-temperature applications [2]. This is one of the reasons to propose additional studies on SS oxides, obtained directly from the substrates. In this work, as in a previous one [23], titanium substrates are also used, due to their good corrosion resistance. Two straightforward methods are proposed to get directly the oxide films from the substrates: thermal and air–plasma oxidation. A second reason is that metallic substrates insure an infrared high reflectance; this is equivalent to a low emittance. Then, a film that is thin enough to keep a small increase of emittance and to enhance absorption in the solar range can be grown upon them. It is desired to take advantage of two well-known additional factors, the substrate surface morphology and the film thickness, to improve absorptance without largely increasing emittance. The optical properties of these films were evaluated. Finally, the effect of a thermal treatment on the properties of the films is presented. 2. Experimental details 2.1. Sample preparation 304 and 430 SS besides ASTM grade 2 titanium (Ti) plates about 2.5 2.5 cm2 were used as substrates. A mechanical polishing process was applied to some titanium and 304 SS substrates to obtain some texture on the surface. 430 SS is acquired with a uniform surface
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texture, which was left unchanged. The mechanical polishing procedure consisted of rubbing the metallic plates first with 600 grade sandpaper, and then with 1 mm alumina powder. A cleaning process including ultrasonic rinses in extran, 10% NaOH aqueous solution, 50%–50% alcohol–acetone mixture, and de-ionized water was applied to the substrates previously polished. After drying, the substrates were kept in alcohol prior to the oxide preparation. Blue-colored coatings on SS were formerly reported to have the best a and e values [24]. It was found that thermal oxidation of Ti plates in air at 500 1C for 30 min was enough to get blue-colored coatings. SS plates needed a temperature of 700 1C for 2 h to yield blue thermally grown coatings. Also, SS and titanium oxides were produced by air–plasma oxidation. The metallic plates were attached to the vertical copper cathode of a typical capacitive glow discharge system. Air was allowed to flow in the process chamber, keeping a constant 0.15–0.23 Torr pressure. The substrate temperature was raised from room temperature up to 350 1C along 1 h, hereafter keeping this temperature constant until the process ended 2 h later. A 13.56 MHz radio frequency (RF) signal was applied between anode and cathode to produce air ionization since the temperature started rising. The RF signal power used was about 10 W, equivalent to a power density of 0.1 W/cm2. After one of these cycles, a blue titanium oxide film was obtained upon the corresponding substrates. In the case of SS, a second almost identical cycle excepting a lower pressure (5 mTorr) was applied to try obtaining blue-colored films. 2.2. Sample characterization The main characterization technique was of optical type. It consisted of spectral nearnormal specular reflectance measurements, carried out with a Fourier transform spectrometer (Nexus 670) in the 0.4–25 mm wavelength range. The calculation of absorptance and emittance was done as outlined by Duffie and Beckman [29]. Also, atomic force microscopy measurements helped to determine the surface morphology. These were made with a 250 Quesant system in the usual contact mode. An additional technique was ellipsometry to determine the film thickness. Ellipsometry measurements were carried out with a 0.633 mm wavelength Gaertner LSE model Stokes ellipsometer. 3. Results and discussion 3.1. The metallic substrates As it has been formerly said, it is desired to obtain appropriate absorptance values by means of controlling two factors: one of them is the surface morphology of the substrate, and the other is the film thickness upon it. Polishing the substrates allows to get a specific size of the surface features. In principle, a mechanical process to make this surface treatment possible can be done. Here, It has been applied to grade 2 titanium (Ti) and 304 SS plates (SS3). 430 SS plates (SS4) bear a rather uniform surface, so no polishing process was applied to them. SS3 substrates polished with sandpaper (SS3s), with 1 mm alumina powder (SS3m) and no-polish (SS3n) were studied. Also, Ti substrates polished with sandpaper (Tis), with 1 mm alumina powder (Tim), and no-polish (Tin) were prepared for this study.
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3.1.1. Surface morphology Atomic force microscope (AFM) surface images of two different substrates are shown in Fig. 1. According to the geometrical point of view, where a specific texture is used to achieve selectivity [30], surface features with sizes between 400 nm and 5 mm must be sought for. In order to observe such feature sizes, squares of 5 mm side were scanned in the
Fig. 1. Atomic force microscope images of the (a) SS3m and (b) SS3n surfaces. The sizes of the surface features are distinct for different surface treatment. These differences are expected to produce differences also in the optical response of such surfaces, which can be seen in the spectral reflectance measurements.
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case of the polished substrates. However, the larger size of features on no-polish surfaces makes 40 40 mm2 size images better to describe them. Fig. 1a (SS3m) exhibits grooves less than a micrometer (0.25 mm) wide and with vertical variations less than 37 nm.In Fig. 1b, the 40 40 mm2 image of the SS3n substrate makes evident the existence of random-direction grooves up to 4 mm wide, which separate large size (up to 10 mm) flattened regions. Vertical variations reach up to 853 nm. As it will be shown in the next section, all the substrates used can be grouped into three sets of surfaces on the basis of their optical parameters, which can be associated to the size of their surface features. 3.1.2. Optical properties of the surfaces The optical responses of the surfaces used are shown in Fig. 2, as the spectral reflectance in the 0.4–25 mm wavelength range. Two plates (SS4 and SS3m) have high reflectance in the whole range, leading to a low absorptance and low emittance parameters (group 1). Another group is formed by the samples SS3s and Tim, bearing midreflectance in the short-wavelength region (group 2). The SS3n, Tin and Tis plates form a third group (group 3). They resemble intrinsic selective surfaces, with a higher absorptance, but emittance is enlarged due to a long decay of reflectance around 10 mm wavelength for increasing frequencies. The results of the numerical calculations for the absorptance and the emittance referred to a blackbody spectrum at 200 1C, are contained in Table 1, and also in 100
Reflectance (%)
80
60 SS3n SS3m
40
SS4 Tin Tim
20
SS3s Tis
0 1
10 Wavelength (µm)
Fig. 2. Curves of the spectral reflectance for the metallic substrates used, showing noticeable differences depending on the substrate morphology. Three groups can be noticed. Group 1, including SS3m and SS4, has high reflectance for all wavelengths. Group 2 is formed by Tim and SS3s, having mid-reflectance in the shortwavelength region. Finally, group 3, including SS3n, Tin and Tis, which have the lowest reflectance for short wavelengths. Group 3 shows the best intrinsic selective properties.
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Table 1 Solar parameters of the substrates, obtained as outlined in the introduction Substrate
SS4 SS3m Tim SS3s Tis SS3n Tin
a
0.247 0.266 0.565 0.591 0.804 0.736 0.825
e(200 1C)
0.088 0.095 0.208 0.188 0.27 0.219 0.282
S
2.81 2.80 2.72 3.14 2.98 3.36 2.93
Approx. feature size (mm) Vertically
Horizontally
0.121 0.037 0.110 0.063 1.350 0.854 2
1 0.250 2 2 p10 p10 p10
The last columns show approximate vertical and horizontal feature size for the surfaces of the substrates.
Fig. 3. The three groups defined above can be clearly distinguished in both, the table and the figure. In the first group, emittance is less than 0.1 and absorptance is also low, around 0.25. Substrates in group 2 have higher absorptances between 0.55 and 0.6, but emittance also raises up to near 0.2. Finally, for the substrates within group 3, absorptance is much larger, about 0.8, while emittance almost reaches 0.3. It should be noticed that, as expected, grouping the substrates according to these optical parameters yield the same result than that obtained by grouping according to the feature size of the surfaces. This is shown in Table 1, where the feature size is also included in the last column. In accordance with these results, the effect of polishing SS3 plates with 1 mm powder turns out to be smoothening for large (41 mm) surface features. Then, absorption of electromagnetic radiation mainly in the near- and mid-infrared ranges, and also in the visible region, is eliminated, increasing consequently the reflectance in the whole range. We were not able to achieve an identical result for Ti plates. Due to the particular mechanical properties of such material, large voids upon polished areas are produced as a result of the polishing process. Instead, a midreflectance value was obtained, similar to that of SS3 plates polished with sandpaper. On the other hand, polishing the Ti plates with sandpaper does not change advantageously their reflectance, as it is quite similar to that of no-polish Ti and no-polish SS3 plates. As noted in Fig. 3, the emittance value is more critical than absorptance to produce large increases of selectivity. For the structures we planned to build, oxides are less conductive than metal. Hence, it is not possible to increase the mid-infrared reflectance of the film–substrate structure above that of the metallic substrate alone, preventing then emittance lowering. Instead, absorptance is feasible to be raised. Hence, further than improving emittance, we shall look for absorptance improvements. Hence, we should start from low-emittance substrates. This means that better results can be expected from substrates in groups 1 and 2, as compared to those in group 3. Nevertheless, results about substrates within all three groups are reported in the following sections. 3.2. Results of coatings 3.2.1. Ellipsometry measurements This type of measurement was done only on samples whose substrates were not too rough, due to the difficulties appearing when the narrow laser beam is dispersed by the
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α ε
S
3.4
0.8
3.3
0.7
3.2
0.6 3.1 0.5 3.0 0.4 0.3
2.9
0.2
2.8
Selectivity
Absorptance, Emittance
0.9
0.1 2.7 0.0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate Fig. 3. Optical Solar parameters of the distinct substrate surfaces used in this work. Group 1 has the first two samples from left to right, group 2 the next two, and group 3 the remaining three. Both, absorptance and emittance can be seen to change value in blocks. Even small variations of the emittance value strongly change selectivity, since it is the divisor in the equation defining S.
surface. The thickness of coatings on SS3m, SS4 and Tim substrates are presented in Table 2. As shown, all the coatings have thickness between 145 and 260 nm. Most of them are about 150 nm thick and a few pass beyond 200 nm. The two thickest coatings were grown on SS substrates, one from plasma oxidation and thermally the second. Also, the refractive index obtained from these measurements is listed in Table 2. As seen, the refractive indices of titanium oxide, expected for both plasma and thermal oxidation of Tim substrates, are not far from the reported value [31], about 2.85. One sample was found to have a high refractive index, the thermal oxide on SS4. As said above, on titanium substrates, thermal or plasma oxidation will produce mostly titanium oxide, but further chemical characterization is needed to determine the precise composition and structure of the film. On SS plates, the situation is more complicated, since the substrates contain chromium, which bears a high affinity for oxygen, but a larger amount of iron is also present. Hence, chromium and iron oxides might be embedded in the coating as a result of thermal or air–plasma oxidation. Further chemical and structural studies are needed to provide such information. 3.2.2. Coatings made by thermal oxidation The temperature and time of oxidation were different for titanium and SS substrates. In the first case, oxidation was carried out at 500 1C for 30 min. This procedure yielded blue coatings on the three Ti substrate types (Tin, Tis and Tim). In the case of the SS, the
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Table 2 Thickness and refractive index of some coatings fabricated in this work, deposited on the smoothest substrates, as obtained from ellipsometric measurements at 633 nm wavelength Substrate type
SS3m SS4 Tim
Plasma oxide
Thermal oxide
Thickness (nm)
Refractive index
Thickness (nm)
Refractive Index
260.5714.2 175.4735.1 147.374.7
2.7370.13 2.8070.51 2.9570.06
157.472.5 208.575.8 164.579.5
2.6370.03 3.5470.09 2.6370.14
oxidation was carried out at 700 1C for 2 h. A blue coating on SS4 and SS3s was obtained, but it was gray on SS3n, and reddish on SS3m. The absorptance, emittance and selectivity values obtained from the corresponding reflectance spectra are shown in Fig. 4. As expected, absorptance of the substrate alone was increased in all cases, but in coatings on substrates of group 1, this is more noticeable. The best selectivity values correspond to SS3s, SS3m and SS4 substrates, but the first bears a higher absorptance than the others. These substrates belong to the above-mentioned groups 1 and 2. Clearly, the high selectivities of the SS3s and SS3m samples, are due to the good values of emittance in such cases, which are even better than those of the substrates in Fig. 3. This means that there is certainly some lack of uniformity in the surface properties of substrates assumed to be identical. The samples in group 3 have the highest absorptances, but emittance is also too high. 3.2.3. Coatings made by air– plasma oxidation Oxidation in air–plasma as outlined before yielded blue coatings on all the titanium substrates, but blue on SS3n, gold–red on SS3s, golden on SS3m, and blue on SS4. The optical parameters from these coating–substrate systems are shown in Fig. 5. The coatings produced by air–plasma oxidation increase absorptance in all cases, those in group 3 being the best in this respect. This increase of absorptance must be improved in the case of the samples in groups 2 and 3, in order to get better selectivities. Improvement could be achieved by searching new convenient conditions for the plasma process, leading to higher oxidation rates or increasing the process time. Since the chemical reactivity of ionized oxygen is expected to be higher than that of neutral oxygen, the substrate temperature used here is much lower than that of the thermal processes. Once more, the highest selectivities, which are appreciably smaller than those of thermal oxides, correspond to the coatings on SS3s, SS3m and SS4 substrates. By comparing the emittances of the SS3m substrates in Figs. 3 and 5, a certain lack of reproducibility in surface finishing of the substrates is again evidenced. 3.3. Thermally treated samples The plasma-produced coatings on SS3m and Tim along with the thermally grown film on Tim substrates were selected to assess the effect of a long-term thermal process. This process was applied to the coatings for 120 h at atmospheric pressure and 450 1C, which
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40
α ε
0.9
S
0.8
0.6 20
0.5 0.4
Selectivity
Absorptance, Emittance
30 0.7
0.3 10 0.2 0.1 0.0
0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate Type Fig. 4. Optical parameters of thermally grown oxides on the different substrates. 1.0 0.9
S
α ε
6
0.7 0.6 4
0.5 0.4
Selectivity
Absorptance, Emittance
0.8
0.3 0.2
2
0.1 0.0 SS4
SS3m
Tim
SS3s
Tis
SS3n
Tin
Substrate type Fig. 5. Optical solar parameters of the oxides produced by air–plasma oxidation.
corresponds to a mid-range work temperature. The thermally oxidized SS samples were not included, since they were fabricated at 700 1C and no change is expected under the conditions of the process described.
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1.0 0.9 0.8
α, before α, after ε, before
Absorptance, Emittance
ε, after 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Pl ox / SS3m
Pl ox / Tim
Th ox / Tim
Sample
(a)
26
7 S decrease 6
24
Selectivity
20 5 18 16 4
S decrease (%)
22
14 S before 3
12
S, after
10 Pl ox / SS3m (b)
Pl ox / Tim
Th ox / Tim
Sample
Fig. 6. (a) Comparison of absorptance and emittance before and after the long-term thermal treatment of some samples fabricated at medium temperatures. In all the samples, the main effect is upon the emittance; (b) the selectivity values and their corresponding decrease.
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3.3.1. Ellipsometric measurements Several of those samples under treatment could not be analyzed by ellipsometry. From AFM observations, it was found that the thermal treatment produced larger surface roughness by means of structural changes which formed particles of different heights in the film surface. This larger vertical non-uniformity of the surface produced dispersion effects on the narrow measurement beam of the ellipsometer. Only the plasma-oxidized SS3m substrate could be measured with the same equipment used formerly. After the long-term thermal treatment, its thickness increased from 260.5 up to 382.4 nm and its refractive index decreased from 2.73 down to 1.98. The thickness increase indicates further oxidation due to long-time heating. Meanwhile, the decrease of refractive index points to a possible change in the structure of the coating. This can occur since at least chromium, nickel and iron oxides might be forming the original coating, and the relative content and structure of them could be changed by different oxidation conditions, as is the case. To quantitatively establish this assertion, further chemical characterization to study these samples and their evolution, is needed. 3.3.2. Optical parameter comparison The optical parameters before and after the thermal treatment can be compared as shown in Figs. 6a and b. In Fig. 6a, the absorptance does not change appreciably in most samples, as indicated by the solid and empty squares. Instead, emittance increases slightly, as shown by the circles, producing a decrease of selectivity. This decrease and its equivalent percentage are shown in Fig. 6b. The solid squares correspond to the selectivity before the thermal treatment and the empty ones to selectivity after this process. According to these results, the selectivity degradation, comprised between 10% and 25% for these samples, is entirely due to the emittance increase. The degradation is lower in the case of both thermally and plasma-oxidized titanium substrates. Instead, almost 25% degradation occurs for the SS substrate. Again, the precise mechanism responsible of such emittance degradation deserves further detailed morphological and chemical study on each specific case. 4. Conclusion In order to benefit from their high electronic conductivity, which leads to low thermal emittance, metallic (titanium, SS3 and SS4) substrates with different surface morphologies were used to fabricate several types of selective surfaces. Three types of surface finishing were used: no-polish, polished with sandpaper, and polished with 1 mm alumina powder. Two types of coatings were fabricated upon the metallic surfaces: films obtained by either thermal- or plasma oxidation. Some lack of surface roughness reproducibility was observed under the mechanical polishing method that was used. This lack of reproducibility strongly affects the results on Ti substrates. Substrates could be divided into three groups, according to their surface morphology, which is in turn related to their solar optical parameters. Substrates with initial high or mid-reflectance in the whole wavelength range give the best results for the selectivity value. This means that better results were obtained from metallic surfaces bearing initially low emittance and also low absorptance, depending on the surface morphology, as expected. Thermal oxides on SS polished substrates yielded the best selectivity values, which were higher than 10. The maximum selectivity values of plasma oxides were also obtained on SS substrates and were
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between 4 and 7. In general, absorptance was not very high, being at most near 0.8 in the best cases. Despite coatings on polished Ti substrates yielding moderate selectivity values, they turned out to be the most stable under long-term thermal treatment, with about 10% of degradation. On the other hand, the SS plasma oxide coating degraded about 25%. The expected advantage of SS plates that were oxidized at 700 1C is their thermal stability near this temperature. Then our best results in terms of solar performance, work temperature and thermal stability, corresponded to thermally oxidized SS substrates, processed with any polishing method of those used here. Further structural and chemical studies of the coatings and their evolution after thermal treatment will provide a close understanding of their degradation mechanisms. Undoubtedly, among these mechanisms one can certainly mention further oxidation of the substrate, as seen in this work after the long-term thermal treatment. Acknowledgments A. Avila-Garcia gratefully thanks CONACYT, Me´xico, for supporting the development of this work, through project 39116A. Also, help in ellipsometry measurements of Ing. Juan Garcı´ a and Drs. Miguel A. Aguilar F. and Ciro Falcony G. of the Physics Department, CINVESTAV, is acknowledged. References [1] C.G. Granqvist, Spectrally selective surfaces for heating and cooling applications, Tutorial Texts in Optical Engineering, vol. TT1, SPIE Optical Engineering Press, Bellingham, WA, 1989, p. 57. [2] C.E. Kennedy, Technical Report No. NREL/TP- 520-31267 National Renewable Energy Laboratory, Golden, CO, 2002, p. 4. [3] W.F. Bogaerts, C.M. Lampert, J. Mater. Sci. 18 (1983) 2847. [4] R.E. Peterson, J.W. Ramsey, J. Vac. Sci. Technol. 12 (1975) 174. [5] H. Tabor, in: Proceedings of the International Conference on Solar Building Technology, 1977, p. 204. [6] C.M. Lampert, US-DOE Report No. W-7405-ENG-48, 1979. [7] G.B. Smith, A. Ignatiev, Sol. Energy Mater. 4 (1981) 119. [8] J.V. Iyer, S.B. Gadgil, A.K. Sharma, B.K. Gupta, O.P. Agnihotri, Sol. Energy Mater. 6 (1981) 113. [9] D.M. Mattox, G.J. Kominiak, R.R. Sowell, R.B. Pettit, SAND 75-0361, Sandia Laboratory, Albuquerque, NM, 1975. [10] B.O. Seraphin, in: Proceedings Symposium on Materials Science Aspects of Thin Film System Solar Energy Conversion, 1974, p. 7. [11] A.M. Schneiders, P. Beucherie, J. Phys. Colloq. CI. 42 (Suppl. 1) (1981) C1. [12] S. Craig, G.L. Harding, Sol. Energy Mater. 4 (1981) 245. [13] G.D. Pettit, IBM J. Res. Dev. 22 (1978) 372. [14] M. Van Der Leij, Proc. Int. ISES Meet. (1978) 837. [15] H.G. Craighead, R.A. Buhrman, J. Vac. Sci. Technol. 15 (1978) 269. [16] J.A. Thornton, DOE Report No. DE/AC04/78CS35306, 1979. [17] J.A. Thornton, J.A. Lamb, SERI Final Subcontract Report No. SERI/STR-255-3040, Solar Energy Research Institute, Golden, CO, 1987. [18] P. Kokoropoulos, E. Salam, F. Daniels, Sol. Energy 3 (1959) 19. [19] R.B. Gillete, Sol. Energy 4 (1960) 24. [20] E. Barrera, T. Viveros, A. Avila, P. Quintana, M. Morales, N. Batina, Thin Solid Films 346 (1999) 138. [21] E. Barrera, A. Avila, J. Mena, V.H. Lara, M. Ruiz, J. Me´ndez-Vivar, Sol. Energy Mater. Sol. Cells 76 (2003) 387. [22] A. Avila G., E. Barrera C., L. Huerta A., S. Muhl, Sol. Energy Mater. Sol. Cells 82 (2004) 269.
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