Available online at ScienceDirect. Energy Procedia 69 (2015 )

Transcription

1 Available online at ScienceDirect Energy Procedia 69 (2015 ) International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014 Self-Cleaning and antireflective films for all-glass evacuated tube solar collectors H. L. Yang*, L. Hao, J. N. Wang, Z. N. Zhang, X. P. Liu and L. J. Jiang Department of Energy Material & Technology, General Research Institute for Non-ferrous Metals, Beijing , China Abstract SiO 2 antireflective nanoporous films were prepared by sol-gel process, combined with dip coating technology. The highest values of solar transmittance (96% in the broader spectrum range of 250 nm-2500 nm) are obtained. The static contact angle values increase after the treatment with 100% ethylchlorosilane and hexamethyldisilazane solutions. The antireflective films has to display long term stability as the operation conditions in solar collectors include low temperatures, high temperatures, and humid environment, all of which can degrade its optical properties The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Keywords: sol-gel; antireflective film; SiO 2; hydrophobicity 1. Introduction Because of their ability to enhance the transmittance of light and remove ghost images, antireflective (AR) coatings have recently attracted much interest for their applications in photovoltaic and display devices, and all kinds of optical lenses. It has been reported that where the temperature of the solar collector fluid is 100 o C, the absorbed thermal energy in solar thermal collectors can be increased by about 20% [1]. Generally, when the refractive index (nc) for an ideal homogeneous AR coating meets the condition of n c = (n 1 n 2 ) 1/2, reflection will be suppressed at the wavelengths near the quarter-wavelength optical thickness, where n 1 and n 2 are the refractive indices of the air and the substrate, respectively [2]. For a borosilicate glass substrate, the refractive index of AR material should be However, nature materials with such low refractive index are either rare or expensive to * Corresponding author. Tel.: ; fax: address: yhailing0430@163.com The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi: /j.egypro

2 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) obtain in thin film form. An effective method to obtain coatings with reduced refractive index is to introduce nanopores during the coating fabrication. The refractive index of porous coatings can be well tailored by controlling the percentage of pores introduced, in which larger percentage of pores leads to lower refractive index. Up to now, compared with the plasma enhanced chemical vapor deposition [3], nanophase separation [4], UV or thermaldecomposition of porogen [5], sol gel processes [6] holds a great potential in the fabrication of AR coatings because of its simplicity in preparation of films with large area and better adhesion strength. On the other hand, the Acid-catalyzed sol-gel nanoporous silica films are usually rich in hydroxy Si OH, which does not participate in the condensation reaction. These Si OH groups are very reactive and induce the adsorption of water vapour and contaminants under humid conditions, resulting in a deterioration of the optical properties of the AR films, the refractive index of the films increases and the antireflective property disappears. In recent decades, the fabrication of superhydrophobic surface, has attracted much attention of researchers [7]. The charming wettability of such superhydrophobic surfaces made them useful in numerous fields, such as self-cleaning surface, marine coating, anti-adhesive coatings and so forth [8]. It is expected that the combination of superhydrophobic surface with AR coating will endow the AR coating with the water-repellent ability. In our previous work, the antireflctive films has to display a longterm stability as the operation conditions in solar collectors include high temperatures, UV exposure, and outdoor humidity, all of which can degrade its optical properties. 2. Experiment Tetraethyl silicate (TEOS) was first diluted in ethanol and it was partially hydrolyzed with water under Acidic conditions, molar ratios TEOS:ethanol:water:HCl=1:20:5:0.5. After stirring for 24 h, Triton X-100 was added in increasing the porosity of the films. This polymer is regularly used at lower concentrations 40 mg/ l as surfactant. Methyltriethoxysilane (MTES) was added too, being the proportion TEOS:MTES 80%:20%.After the deposition on the borosilicate glasses by dip coating, the samples were heated at 500 C for 1 h. The thicknesses and refractive indices of nanoporous organosilicate films on silicon wafers were measured by an ellipsometer and scanning electron microscope. UV-vis transmittance was collected at normal incidence with a UV- VIS-NIR spectrophotometer at room temperature. 3. Results and discussions Refractive indices of prepared films were measured with an ellipsometer and plotted against porogen content, as shown in figure 1 and figure 2. As the TEOS and porogen content is increased, the refractive index of the nanoporous SiO 2 films decreases. This effect is due to a decrease in the film refractive index, as the coating porosity is increased. During the film heat treatment, the Triton polymer is decomposed and removed, leaving voids in the polymeric matrix. It can be seen from figure 2, the porosity of the films increased with the Triton content increasing mol/l 0.25 mol/l 0.30 mol/l Refractive Index Content of Triton Fig. 1. The refractive index of SiO 2 films (TEOS:0.20,0.25, 0.30mol/L).

3 228 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) Refractive Index :0.05 1:0.1 1:0.15 1: Temperature increasing rate ( o C/min) Fig. 2. The refractive index of SiO 2 films (TEOS:Triton=1:0.05, 1:0.1, 1:0.15 and 1:0.20). (a) (b) (c) (d) (c) Fig. 3. Scanning electron microscopy images of SiO 2 films (a) TEOS:Triton=1:0.05; (b) TEOS:Triton=1:0.1; (c) TEOS:Triton=1:0.15; (d) TEOS:Triton=1:0.2; Among prepared porous films, porous films with 0.05, 0.10, 0.15 and 0.20 mol% porogen loadings were selected to evaluate the refractive index of a film for the antireflective effect. We found that the refractive indices for the chosen porous films were 1.48, 1.38, 1.33 and 1.32, respectively. Although the refractive index decreasing with the Triton increased, but At a higher content of 20% of porogen loading, we obtain a nanoporous organosilicate film with a refractive index of 1.32, which is higher than required for the zero reflection against a glass substrate. 100 nm (d) 100 nm

4 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) One way to solve the problem is to modify the properties of the final materials by changing the initial sol-gel chemistry. In this way, the incorporation of organic compounds in the inorganic matrices of the films has been widely used in this method [9,10]. These compounds have specific functional groups that modify the polymeric network and reduce the internal mechanical stresses within the films, making them more flexible [11]. Moreover, the introduction of the organic groups into the network gives a hydrophobic property to the films, which prevents moisture adsorption. On the other hand, the addition of organic polymers to the precursor solution is a very effective method in increasing the porosity of the films [12,13]. The studies of De Witte et al. [14] determined the distribution of methyl groups from mixtures of TEOS and MTES. When MTES is added to the previous solution, the content of organic materials increased but the amount of silicon atoms with hydrolyzed bonds to form the silicon oxide network increased too. As shown in figure 4, this effect is more remarkable when MTES is added to the solution. In the proportion TEOS:MTES 70%:30%, the refractive index of film is 1.26, close to Refractive Index METS:Trinton=2:8 METS:Trinton=3: Trinton:TEOS Fig. 4. The refractive index of SiO 2 films (MTES:TEOS=2:8, and 3:7). To achieve antireflective properties, the coating thickness of porous films was adjusted to about 135 nm. UV-vis transmittance of the glasses coated with the porous films was measured in the wavelength range from 250 to 2500nm, as shown in figure 5. Glasses coated with porous silicate films show considerable high transmittance compared with the noncoated pristine glass. The nonporous film with a refractive index of 1.28 also yields the high transmittance of 95.57%. As the porogen loading is increased, the maximum transmittance of glasses coated with the porous films with 20 and 40 wt% porogen loadings became 95.68% and 96.23%, respectively. A glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances Investigations have shown that the transmittance of glass can be increased by 4% if the glass is equipped with antireflection surfaces and that the solar collector efficiency can be increased by 4% points if a glass with antireflection surfaces is used instead of a normal glass as the cover plate for the solar collector [15]. For our testing results, the borosilicate glass shows a transmittance of 92.21%, the glasses coated with the porous films with 20 and 40 wt% porogen loadings became 95.68% and 96.23%, respectively. A small increase of transmittance will result in the decrease of the Concentrating Solar Power system costs.

5 230 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) Transmittance (%) Trinton:TEOS = 0.15:1.0 (regractive index=1.29) Trinton:TEOS = 0.20:1.0 (regractive index=1.27) Trinton:TEOS = 0.25:1.0 (regractive index=1.26) nm) Fig. 5. The Transmittance of SiO 2 films (TEOS:Triton =0.15:1, 0.20:1 and 0.25:1). The contact angle measurements allow evaluating the hydrophobicity of the films after the surface treatment and therefore, the capacity of avoiding the adsorption of water in the pores of the coatings. Figure 6 and figure 7 shows the images of water drops on coatings without (d-f) and with different surface modifications (a c). The immersion time was 30 min meanwhile the Hexamethyldisilazane (HMDS) concentration was increased from 100%. It can be clearly seen that the coating surface is modified as a consequence of the HMDS treatment, decreasing the wettability with the increase in the HMDS concentration. These results obtained clearly shows how the surface properties of the coatings are changed from hydrophilic to hydrophobic after the treatment. It could be confirmed that the HMDS treatment has successfully introduced hydrophobic groups onto the porous silica surface. The transmittance measurement demonstrates that when the treatment is made with HMDS, the decrease in the solar transmittance is minor. On the other hand, the film thickness measurements suggest that the HMDS treatment does not affect to the layer thickness. Fig. 6. The contact angle measurements of SiO 2 films with TEOS:Triton=0.20:1.0 (a)-(c) after HMDS treatment; (d)-(f) before HMDS treatment.

6 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) Fig. 7. The contact angle measurements of SiO 2 films with TEOS:Triton=0.25:1.0 (a)-(c) after HMDS treatment; (d)-(f) before HMDS treatment. In conclusion, the organic compound in the precursor solutions of porous silica AR films has been studied. The use of Surfactant as a porosity maker improves considerably the AR properties of the films, leading to solar transmittance as high as 96% in the broader spectrum range of 250 nm-2500 nm. The MTES can modify the polymeric network and reduce the internal mechanical stresses within the films, making them more flexible. Moreover, the introduction of the organic groups into the network gives a hydrophobic property to the films, which prevents moisture adsorption. The increased hydrophobicity is attributed to HDMS modification method, which will also improve the AR stability by blocking the absorption of contamination from use environment. Besides, this surface modification method might provide a new method for preparation of abrasion resistant sol-gel silica AR coating used in outdoor environment. Acknowledgements This work was supported by High-Tech Research and Development (863) Program of Ministry of Science and Technology of The People s Republic of China. (Grant no.2012aa050601). References [1] Furbo S, Jivan SL. Thermal advantages for solar heating systems with a glass cover with antireflection surfaces. Solar Energy 2003:74: [2] Yoldas BE,Partlow DP. Wide spectrum antiflective coating for fused silica and other glass. AppL. Opt. 1984:23(9):1418. [3] Martinu L, Poitras D. Plasma deposition of optical films and coatings:a review J. Vac. Sci. Technol. A 2000:18:2619. [4] Walheim S, Schäffer E, Mlynek J, Steiner U. Nanophase-Separated Polymer Films as High-Performance Antireflection Coatings. Science 1999:283:520. [5] Fu GD, Yuan Z, Kang ET, Neoh KG, Lai DM, Huan ACH. Nanoporous Ultra-Low-Dielectric-Constant Fluoropolymer Films via Selective UV Decomposition of Poly(pentafluorostyrene)-block-Poly(methyl methacrylate) Copolymers Prepared Using Atom Transfer Radical Polymerization. Adv. Funct. Mater 2005:15:315. [6] D.R. Uhlmann, T. Suratwala, K. Davidson, J.M. Boulton, G. Teowee. Sol-gel derived coatings on glass. J. Non-Cryst. Solids 1997:218:113. [7] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q. Liu, L. Jiang, D.B. Zhu. Super-Hydrophobic Surfaces: From Natural to Artificial. Adv. Mater. 2002:14:1857; [8] P. Gould, Mater. Smart, clean surfacestoday 2003:6:44; [9] Chou TP, Chandrasekaran C, Limmer SJ, Seraji S, Wu Y, Forbess MJ, Nguyen C, and Cao G Z, Organic-Inorganic Hybrid Coatings for Corrosion Protection. J. Non-Cryst. Solids 2001:290: [10] Pellice S, Galliano P, Castro Y, and Durán A, Hybrid Sol-Gel Coatings Produced from TEOS and MPS. J. Sol-Gel Sci. Technol 2003:28:

7 232 H.L. Yang et al. / Energy Procedia 69 ( 2015 ) [11] Innocenzi P, Abdirashid MO, Guglielmi M, Structure and Properties of Sol-Gel Coatings from Methyltriethoxysilane and Tetraethosysilane. J. Sol-Gel Sci. Technol 1994:3(1): [12] Wongcharre K, Brungs M, Chaplin Y, Hong JR, Pillaret R. Sol-Gel Processing by Aging and Pore Creator Addition for Porous Silica Antireflective Coatings. J. Sol-Gel Sci. Technol 2002:25: [13] Baustista MC, Morales A. Silica Antireflective Films on Glass Produced by the Sol-Gel Method. Sol. Energy Mater. Sol. Cells 2003:80: [14] Morales A. Spanish Patent 2005:No. P [15]Nostell, P, Roos, A, Karlsson, B. Antireflection of glazings for solar energy applications. Sol. Energy Mater. Sol.Cells. 1998:54: