Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors

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1 0 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors Report December 14 th, 2012 author and coauthors institution address phone, , webpage SFOE Project-No duration of project 01/03/ /12/2014 Antonio Paone, Dr. Andreas Schüler Ecole Polytechnique de Lausanne EPFL Laboratoire d Energie Solaire et de Physique du Bâtiment LESO-PB Bâtiment GC, 1015 Lausanne (021) , antonio.paone@epfl.ch (021) , andreas.schueler@epfl.ch SUMMARY Overheating and the resulting stagnation of solar thermal collectors is a common problem even in central European latitudes. The high temperatures occurring during stagnation lead to water evaporation, glycol degradation and stresses in the collector with increasing vapour pressure. Special precautions are necessary to release this pressure; only mechanical solutions exist nowadays. Additionally, the elevated temperatures lead to degradation of the materials that compose the collector, such as sealing, thermal insulation and the selective absorber coating. The goal of this project is to find a new way of protecting solar thermal systems without any mechanical device (e.g. for shading or for pressure release). Novel thermochromic coatings have been developed, which exhibit a change in optical properties at a critical temperature T c. Undoped, tungsten and aluminum doped samples of inorganic coatings showing thermochromic switching behaviour have been produced by magnetron sputtering. A dynamical switching of the thermal emittance ε can be achieved by thermochromic transition metal oxides. The structural and optical properties of thermochromic transition metal oxide thin films have been characterized in great detail by methods such as X-ray diffraction for phase identification (XRD), infrared temperature-dependent spectrophotometry and temperaturedependent ellipsometry in the visible and in the infrared wavelength range by Fourier 1 Project Goals transform infrared spectroscopy (FTIR). The In general fascinating optical properties of these switchable films elucidate the way towards novel intelligent thermal solar collector materials.

2 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 1 Project Goals The main objective of this project is to limit the stagnation temperature of solar collectors to a value below the boiling point of the heat transfer liquid without degrading the optical performance of the selective coating during normal operation. Advantages: - Evaporation of the heat transfer liquid due to overheating will be avoided and the hydraulic system can be simplified. - The lifetime of the collector materials used for thermal insulation, the joints and the selective coating itself will increase. - The glycol component of the heat transfer liquid will be protected from degradation. We target an optical switching behaviour for selective coatings. This change in the optical properties will occur at a critical temperature T c. For solar thermal collectors this transition temperature corresponds to 90 C. The temperature range under the critical temperature defines the standard working condition for the collector. In this range, the solar absorptance α should correspond to 95% and the thermal emittance ε to maximum 5%. The solar collector suffers overheating connected problems and resulting stagnation above the transition temperature. In this temperature range, the input of solar energy has to be reduced. Therefore, the solar absorptance α should correspond to 35% and the thermal emittance ε to above 40%. Since we have already shown that the durability of organic thermochromic paints is not high enough for the considered application [Huo08], we focus on inorganic materials in this project: thermochromic transition metal oxides. Project Goals Temperature-dependent ellipsometric measurements in the infrared wavelength range by Fourier transform infrared spectroscopy (FTIR) should be performed in order to precisely determine the optical properties of undoped and doped thermochromic films. These optical properties shall be used to determine the thermal emitttance for both switching states. - The effect of aluminium doping should be verified if the decrease of the transition temperature is confirmed, it would be necessary to search in the literature for alternative dopants. - A structural investigation by X-ray diffraction (XRD) of undoped and doped thermochromic films should be envisaged.

3 2 Approach: 1 DEPOSITION PROCESS Thermochromic metal oxide films were deposited. These coatings switch from semiconducting to metallic state at critical temperatures around 69 C, implying a resistance change of typically three orders of magnitude. Dynamical switching of the thermal emittance ε is achieved by undoped and tungsten doped thermochromic films. The transition temperature depends on the doping concentration. The films are doped by reactive magnetron co-sputtering of metallic targets in argon/oxygen atmosphere. Substrate heating is necessary and mandatory for good crystallization of thermochromic films. We therefore heated the substrate to around 500 C during deposition. 2 FILM CHARACTERIZATION OF INDIVIDUAL LAYERS At a critical temperature T c, thermochromic metal oxide films undergo a reversible phase transition from a metallic to a semiconducting state, resulting in a sudden change in thermal emissivity and in a resistance change of typically three orders of magnitude. In order to verify whether the desired switching films are obtained, the electrical properties of thermochromic films are investigated with a temperature-dependent measurement of conductivity. Measurements of the temperature-dependent electrical resistivity allow us to identify rapidly thermochromic samples exhibiting the characteristic metal-to-semiconductor phase transition, and to determine the transition temperature as a function of dopant concentration. Optical properties have been determined for thermochromic films by VIS-NIR ellipsometry and NIR-MIR (FTIR) ellipsometry above and below the transition temperature. Subsequently, the ellipsometric data were point by point fitted for first evaluation of the optical constants and verified by Kramers-Kronig approach based on Lorentz Drude and Lorentz Cauchy formula. For validation, the solar reflectance is measured by FTIR spectrophotometry in the near infrared and mid infrared range.

4 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 3 Description of Work and Results: 1 DEPOSITION PROCESS During the depositions of thermochromic metal oxide films, the oxygen partial pressure has to be controlled very precisely. A thermochromic metal oxide is formed only in a narrow stoichiometric range. Therefore a fine adjustment of the evaporation deposition pressure is mandatory. During the depositions of thermochromic metal oxide films in our chamber for reactive magnetron co-sputtering, the oxygen partial pressure has been controlled very precisely by a PID (proportional-integral-derivative) control circuit combined with a special sensor for the measurement of the oxygen partial pressure (a lambda-probe). In general, undoped thermochromic films switch from semiconducting to metallic state at transition temperatures around 69 C, but doping can raise or lower the transition temperature [Goo71]. By magnetron co-sputtering controlled doping by a suitable element can be achieved. Furthermore, very constant deposition rates are achieved by this technique. This makes magnetron co-sputtering the method of choice for the deposition of doped transition metal oxide and optically switchable metal alloy thin films. Our vacuum chamber for reactive magnetron co-sputtering is tailored to the needs of a development of doped, alloyed up to penternary nanocomposite films. Varying the power on the magnetron sources, it is possible to tune the chemical composition (stoichiometric ratio) of the alloy constituents. The power applied to the magnetron sources was changed in order to vary the chemical composition. The main procedure and parameters for the deposition of thermochromic transition metal oxides by magnetron sputtering were determined. Undoped and aluminium doped thermochromic films were deposited on silicon wafers by magnetron sputtering. The undoped coatings switch from semiconducting to metallic state at critical temperatures around 69 C, implying a resistively change of typically three orders of magnitude. Therefore, a measurement of the specific resistivity, which is temperature dependent, is a quick, precise and reliable method to identify a thermochromic film and its transition temperature. Literature about aluminium doped thermochromic films is far from complete and in some cases even controversial. Papers [Kit67, Ghe77, Goo71] have shown that Al-doped thermochromic films are characterized by a higher transition temperature, in contradiction to some other publications [Che09, Gen08, Lu 99], which have shown a lower transition temperature due to this doping. These contradictions can be explained because thermochromic films are deposited by different methods and following different procedures for each publication. We chose aluminium as dopant of our thermochromic films in order to rise the transition temperature appreciating the scientific quality of the corresponding papers [Goo71, Kit67]. Preliminary experiments [Pao11] have shown that the transition temperature dropped from 68,4 C for an undoped thermochromic film on silicon wafer to 57,3 C for an Al-doped thermochromic film on silicon wafer. Our findings are in contradiction with the considered publications, but it encourages us to dope these films with some other elements. Literature [Goo71] suggests other suitable dopants to raise the transition temperature. In order to confirm these preliminary results we adopted the following strategy. First, we deposited undoped thermochromic films. Then, thermochromic films doped by tungsten were co-sputtered. The RF power (W) applied to the tungsten target was low and the magnet field of this magnetron reduced by removing several magnets in order to introduce very small perturbation in the switching coating and to deposit a solid substitution solution of a dopant in the oxide matrix.

5 4 Once the best deposition parameters were found the doping target was changed from tungsten to aluminium. The best deposition parameters for such coating were found as well. 2 FILM CHARACTERIZATION OF INDIVIDUAL LAYERS 2.1 Electrical properties of thermochromic films The MF (Medium Frequency) power applied to the principal target was kept constant while the RF-power applied to the doping target, tungsten or aluminium, was varied. The transition temperature is reduced proportionally with increasing power on the tungsten target. Figure 1: Temperature dependent electrical resistance of undoped and tungsten doped thermochromic films obtained by a four probe measurements. During the semiconductor-to-metal transition, the electrical resistance of undoped films undergoes a change about two orders of magnitude (logarithmic scale). The transition temperature is progressively reduced with increasing power on the tungsten target. This result confirms undoped thermochromic films switch from semiconducting to metallic state at transition temperatures around 69 C, but tungsten doping lower the transition temperature [Goo71]. Aluminium doped thermochromic films were deposited once experience on co-sputtering was acquired and the best deposition parameters for doping thermochromic films were achieved.

6 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 5 Figure 2: Temperature dependent electrical resistance of undoped and aluminium doped thermochromic films obtained by a four probe measurements. During the semiconductor-to-metal transition, the electrical resistance of undoped films undergoes a change about two orders of magnitude (logarithmic scale). This change is progressively reduced with increasing power on the aluminium target. Aluminium doping seems to make thermochromic films loosing their special peculiarity of the semiconductorto-metal transition. It could be explained by a progressive amorphization of the film with increasing the aluminium doping. This should be confirmed by XRD analyses which are currently in progress. Alternative approaches to deposit aluminium doped thermochromic films exist. In order to improve the film crystallization and to avoid amorphization the deposition temperature could be increased. The mechanical properties of a coating are greatly influenced by substrate bias and can be correlated to microstructure variations. It has been shown [Che04] as positive bias results in the increase in both the hardness and elastic modulus, while negative bias enhances the hardness and toughness. Ion bombardment due to substrate bias is an effective approach to enhance the rate of surface diffusion. In addition, increased nucleation density, reduced grain size, and porosity are the well-known related effects. Therefore, a promising approach is to bias the substrate for increasing the density of the film. Anyway, literature [Goo71] suggests other suitable dopants to raise the transition temperature such as chrome and germanium. 2.2 Determination of optical constants by spectroscopic ellipsometry Ellipsometry is a highly accurate method for investigating the optical properties of thin films. To our knowledge, only little information is available about ellipsometrically determined n and k of doped thermochromic films. For high accuracy and reliability, five different angles of reflection were used for the measurements of the ellipsometric quantities ψ and Δ in the visible and in the near infrared wavelength

7 6 range, from 350 nm to 2000 nm. Four different angles were used for the measurements in the near infrared wavelength range and in the middle infrared range, from 2000 nm to nm. A point-by-point fitting algorithm was applied in order to infer the refractive index n and the extinction coefficient k for all wavelength points λ. Spectroscopic ellipsometry is a precise optical measurement technique working with polarized light. The method compares the ratio of the complex Fresnel reflection coefficients for parallel and perpendicular polarized light R p and R s. A scheme of this principle is shown in figure. 3 (a). Figure 3 (b) shows our FTIR ellipsometer modified with a heating stage able to stabilize the temperature of a sample during a measurement at 90 C. This relative measurement makes the technique independent from intensity fluctuations of the source, thus favouring a high precision. Point by point fitting for determination of the optical constants and verification by Kramers-Kronig consistent approach based on Lorentz-Drude and Lorentz-Cauchy model n and k of a thermochromic transition metal oxide coating in the semiconducting and metallic state can be determined by fitting the measured curves point by point while changing the assumed film thickness every time. We looked for a thickness value which would minimize the root mean square error (RMSE). The initial thickness test value was found by alpha step measurements. A fine adjustment of this value was obtained fitting the measurement and looking for the thickness which best minimized the fitting error. We adopted the Lorentz-Drude model to verify Kramers-Kronig relations in the visible and near infrared wavelength range. Lorentz-Cauchy model was chosen to verify Kramers-Kronig relations in the near infrared and middle infrared range. These relations are fundamental to confirm n and k determined by point-by-point interpolation and to physically interpret the optical properties by the band structures theory. In fact, the energy values associates to the Lorentz oscillators describe the interband and intraband optical transitions. In Figure 4 we report our results which are in good agreement between the n and k determined by point-bypoint interpolation and those determined by the Lorentz Drude and Lorentz Couchy model. According our knowledge it is the first time in literature that the optical constants of a quaternary thermochromic transition metal oxide film have been inferred in this wavelength range.

8 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 7 Figure 3 (a) shows a scheme of the ellipsometry principle. (b) A picture of our FTIR ellipsometer modified with a heating stage able to warm a sample during a measurement up to 90 C. (a) (b) Figure 4: n (a) and k (b) for thermochromic metal oxide films undoped and doped by tungsten on silicon wafer at around 30 C and at around 90 C determined by point-by-point interpolation. A very reliable and consistent measurement was performed to determine the optical constants of a thermochromic transition metal oxide film. This result is remarkable especially because of the lack of these data in literature. The optical properties of a thermochromic transition metal oxide film for both the low and the high temperature state were inferred by spectroscopic ellipsometry in the VIS-MIR spectral range. These data were used as basis for computer simulations predicting solar absorptance, solar reflectance and the thermal emittance on an aluminium substrate. The thickness of the undoped thermochromic film was varied in order to maximaze the emittance switch between the semiconducting and the metallic state.

9 8 Figure 5: Simulation of emittance switching with thickness increasing for our thermochromic film. See Confidential appendix for further details. Fig. 5 clearly shows the thermochromic undoped film becomes highly emissive in the metallic state. The thermal emittance is depending on the aluminium substrate at low temperature and on the metallic state of the thermochromic film after switching. The optimum layer thickness has been identified. The emittance is evaluated by integration of the absorbance spectra weighted with the thermal spectrum of a black body at 100 C. This integration was performed in the range from 2000 to nm. This result is consistent to a preliminary theoretical prediction of our laboratory [Huo08]. 2.3 Simulations for determining optimized multilayer designs for switching coatings Keeping constant the thickness of the thermochromic film relative to the highest emittance switch, the optical performance of an optimized multilayer composed by the switching coating and an antireflection film deposited on top was simulated in both temperature states. The thickness of the antireflection coating was optimazed in order to minimize the solar reflectance. Simulations were performed for a thermochromic film doped by tungsten as well. Figure 6 shows a simulation of reflectance switching of an optimized multilayer composed by a thermochromic film doped by tungsten and an antireflection coating.

10 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 9 Figure 6: Simulation of reflectance switching of an optimized multilayer deposed on an aluminum substrate composed by a thermochromic film doped by tungsten and an antireflection coating. See Confidential appendix for further details. By means of computer simulations, a promising multilayer design for thermochromic selective absorber coatings was identified. The specular reflectance of doped and undoped thermochromic films was measured by FTIR spectrophotometry in the spectral range from 2000 nm up to nm. We used the optical constants found by ellipsometry to simulate the optical reflectance of these films. A very good agreement between the measured spectra and the simulated ones was found, but it is not showed in this short report. The solar absorptance was inferred from the optical data.

11 10 Conclusions Considerable progress has been made in the field of ɛ -switching thermochromic films for overheating protection of solar thermal collectors. Controlled doping of thermochromic films is possible using our cosputtering installation. We showed that the transition temperature of thermochromic films can be modified by controlled tungsten doping. In order to extensively characterize the film structure, XRD analyses are currently in progress. Thermochromic coatings on silicon wafers were analyzed by FTIR spectrophotometry in the NIR-MIR spectral range up to nm. A reflectance switch for these films has been shown. A complementary emittance switch has been noticed. The optical properties of thermochromic films were inferred by ellipsometry in the VIS-NIR spectral range and by FTIR ellipsometry in the NIR-MIR spectral range. A validation of our results was obtained comparing the optical constants determined by point-by-point interpolation with those determined by the Lorentz Drude and Lorentz Cauchy model. The switch in the thermal emissivity of these coatings was determined and the result is consistent with our previus estimation [Huo08]. The feasibility of combining ɛ -switching coatings in a multilayer was studied. We simulated an optimized multilayer which shows that the function of overheating protection by a thermochromic coating can be combined with optical selectivity. The absorptance of optimized multilayer deposited on aluminium substrate and containing a thermochromic film was also investigated for temperatures below and above the transition temperature. It has been proven that the thermochromic optical switching and optical selectivity are compatible and can be combined. Our experiments seemed to indicate that the transition temperature is not raised by Al-doping thermochromic films. However, preliminary results showed that another beneficial effect can be achieved by aluminium doping. More details are reported in the confidential appendix. Suitable strategies for raising the transition temperature with other dopants were developed and ideas for promising and alternative approaches exist. Further studies are necessary to understand the influence of doping with respect to T c especially in order to increase the transition temperature to 95 C.

12 Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors 11 Acknowledgements Technical support was kindly provided by Pierre Loesch. We thank Virginie Le Caër, Martin Joly, Stefan Mertin for inspiring suggestions and discussions. Industry Contacts Interaction and discussions with ALCAN that shows a general interest in our research development CTI project on sol-gel deposition of nanostructured selective solar absorber coatings in collaboration with the Swiss solar collector manufacturer ENERGIE SOLAIRE SA ASULAB (SWATCH GROUP) donated equipment for vacuum deposition of thin films, suitable for multilayer deposition Partnership with SWISSINSO: technology transfer of magnetron sputtering and research on novel coatings for innovative solar collector glazing Scientific collaborations - Active participation of Dr. Rosendo Sanjines and Henry Jotterand, Laboratory of Thin Films Physics, Prof. Laszlo Forro, Institute of Complex Matter Physics, EPFL. Experiments on magnetron sputtering and X-ray diffraction analysis - Within EPFL, access to electron microscopes and to the facilities of TEM sample preparation is provided by the Interdepartmental Center of Electron Microscopy CIME. - The research group of Prof. Libero Zuppiroli (LOMM at EPFL) provides access to their new ellipsometer. This contact will be useful for future measurements of the optical properties of thermochromic coatings. - Collaboration with the research group of Prof. Peter Oelhafen, Institute of Physics, University of Basel. - The research group of Prof. S. Mikhailov (IMA-Arc) provided access to their Van der Graaff accelerator for the RBS analyses. - The research group of Prof. F. Bussy at UNIL in the Mineralogy and Geochemical Institute provided access to their EPMA JEOL 8200 super probe equipment for WDS analyses. - An informal scientific collaboration exists with the Polymer Competence Center Leoben (PCCL) in Austria.

13 12 Invited Presentations 2012 A. Schüler, Couches minces optiques composées de matériaux nanocomposites pour applications dans le domaine de l énergie solaire, colloque: Nanotechnologies: une autre vision sur les énergies, entretiens Jacques Cartier, November 19 th, 2012,INSA de Lyon, France Publications A. Paone, M. Joly, G. Guibert, S. Mikhailov, F. Bussy, J.-L. Scartezzini, R. Sanjines, A. Schüler, Temperature dependent spectroscopic multiangle UV-VIS-NIR ellipsometry of thermochromic films, in preparation. References [Huo08] G. Huot, C. Roecker, Dr. A. Schüler, Evaluation of the Potential of Optical Switching Materials for Overheating Protection of Thermal Solar Collectors, SFOE project # (2008). [Pao11] A. Paone, Dr. A. Schüler, Advanced switchable selective absorber coatings for overheating protection of solar thermal collectors, SFOE project # (2011). [Che04] C.-S. Chen, C-P Liu, H-G Yang, and C.-Y. A. Tsao, Influence of substrate bias on practical adhesion, toughness, and roughness of reactive dc-sputtered zirconium nitride films, J. Vac. Sci. Technol. A 22(5), , (2004).