Sol-gel deposition and characterization of low refractive index nanocomposite Mg:F:Si:O thin films for solar and protective coatings

Coatings for thermal solar energy conversion have to be abrasion-resistant, stable at elevated temperatures, and often have to protect the substrate against corrosion. Due to the conditions of refractive index matching between the substrate and the ambient air, low refractive index materials are desired regarding a large variety of optical applications. Novel nanocomposite thin films based on the elements Mg, F, Si and O have been deposited by a sol-gel dip-coating process starting from trifluoroacetic acid, magnesium acetate and alcoxide precursors. Optical properties of this new material have been characterised by spectrophotometry and spectral ellipsometry, and show surprisingly low refractive index values. The experimental data obtained for varying chemical composition have been compared to the optical properties as predicted by Bruggeman and Ping-Sheng effective medium theories. Strong deviations from both theories are reported, suggesting modifications of the involved crystalline phases and/or formation of nanopores. Evidence for embedded crystalline nanoparticles has been found by Transmission Electron Microscopy (TEM). Compared to well known existing low refractive index materials such as porous SiO2 and MgF2, the quaternary Mg:F:Si:O thin films can be expected to show higher hardness and improved aging stability with respect to pore filling by hydrocarbons. The novel thin film material opens promising perspectives for new solar and protective coatings.

Nanostructured Coatings on Glazing for Active Solar Facades

Estelle De Chambrier, Dipanwita Dutta, Maria Cristina Munari Probst, Christian Roecker, Jean-Louis Scartezzini, Josef Andreas Schuler

Thin films on glazing play an important role in the design of solar energy conversion systems as thermal collectors or photovoltaic panels. Antireflective coatings based on porous silicon dioxide are well established in the field of solar collector glazing. Regarding architectural integration of thermal collectors, colored glass panes based on multilayer interference stacks of dielectric thin films offer a smart alternative to the coloration of the absorber sheets. Materials with tunable refractive index are highly desirable in such coatings in order to give access to optimised designs. Thin films of silicon titanium mixed oxides TixSi1-xO2 have been deposited in a particle free atmosphere by a sol-gel dip-coating process. Optical properties of thin films have been characterized by spectrophotometry and spectral ellipsometry. A nanocomposite structure has been evidenced by Transmission Electron Microscopy (TEM). As low tunable refractive index material, novel nanocomposite thin films based on the elements Mg, F, Si and O have been deposited. Transparent and homogenous thin layers were achieved with variable Mg:Si molar ratios. Surprisingly low refractive index values have been found, reaching desired values for anti- reflective applications. Broad spectral transmittance maxima are observed with values up to 99.8 %. The nanostructure of quaternary Mg-F-Si-O thin films has been investigated by TEM, showing evidence of embedded crystalline nanoparticles. In comparison to existing anti- reflective materials, the novel quaternary thin films might be highly interesting regarding improvement of hardness and improved aging stability with respect to pores filling by hydrocarbons. Intermediate size A4 colored glazing based on TixSi1-xO2 films have been produced exhibiting 14.0 and 12.6 % visible reflectance and only small energy losses (1.4 and 3.4 % with respect to the uncoated substrate) showing the high potential of such materials for colored solar facades. A real-sized prototype of colored thermal solar collector has been realized, providing a convincing demonstration device.

EPFL2007

Optical properties of epitaxial SrHfO3 thin films grown on Si

Jin Won Seo

The perovskite SrHfO3 can be a potential candidate among the high-permittivity materials for gate oxide replacement in future metal-oxide semiconductor field-effect transistor technology. Thin films of SrHfO3 were grown by molecular beam epitaxy and compared with SrTiO3 films. Their optical properties were investigated using spectroscopic ellipsometry and analyzed with respect to their structural properties characterized by x-ray diffractometry, atomic force microscopy, and transmission electron microscopy. A band gap of E-g=6.1 +/- 0.1 eV is measured optically, which renders this material better suited for gate dielectric applications than SrTiO3 with E-g similar to 3.4 eV. At similar equivalent oxide thickness, SrHfO3 also exhibits lower gate leakage current than SrTiO3 does. (c) 2007 American Institute of Physics.

2007

Selective solar absorber coatings on receiver tubes for CSP - energy-efficient production process by sol-gel dip-coating and subsequent induction heating

Yannik Antonetti, Thomas Gascou, Aïcha Hessler-Wyser, Martin Joly, Martin Python, Jean-Louis Scartezzini, Andreas Schueler

The energy efficiency of production processes for components of solar energy systems is an important issue. Other factors, which are important for the production of black selective solar coatings include production speed, cycle time and homogeneity of the coating, as well as the minimization of the quantity of the needed chemical precursors. In this paper a new energy-efficient low-cost production process is presented for production of optically selective coatings for solar thermal absorbers. The used method to produce such coatings is sol-gel dip-coating for which all the solutions have been synthetized at the Solar Energy and Building Physics Laboratory of EPFL. The used precursors are tetraethyl orthosilicate, manganese acetate tetrahydrate, copper chloride dihydrate and cobalt chloride hexahydrate. Solutions were obtained by dissolving these precursors in a solution based on a mixture of absolute ethanol, nitric acid and demineralized water. The layers deposited on sheetlike substrates were annealed in a benchtop furnace. For 2 meter long austenitic stainless steel tubes, a novel, fast and energy-efficient process based on induction heating was developed for the thermal annealing. An induction coil passes along the tube. An alternating current flowing in the coil induces a current in the tube. By resistive heating, the temperature of both the metallic tube wall and the deposited film increases. The homogeneity of the temperature distribution of the tube after a single passage of the coil was monitored by infrared imaging. The optical and morphological properties of the Cu-Co-Mn-Si-0 based triple layer have been characterized by spectrophotometry, transmission electron microscopy (TEM), time-of-flight secondary ion mass spectroscopy (ToFSIMS) and X-ray photoelectron spectroscopy (XPS). After optimization of the multilayer design, a solar absorptance of 0.95 and a thermal emissivity of 0.12 at 100 C have been achieved. The intermediate Cu-Co-Mn-Si-0 layer was analyzed by transmission electron microscopy. The likewise obtained images show an agglomeration of crystalline grains with 5-20 nm in diameter. Therefore, we can consider that the Cu-Co-Mn-Si-0 phase is nanocrystalline. In order to roughly estimate the corrosion resistance of the coating in an acidic environment, a simple corrosion test in harsh conditions confirmed the durability of the novel sol-gel coating. Moreover, the excellent stability at elevated temperatures in ambient air makes the coating an interesting candidate for solar applications involving concentrated solar radiation, such as the generation of solar electricity (concentrated solar power), industrial process heating and solar cooling. For that reason, prototype coatings consisting of stacks of three individual layers were deposited on 2 meter long stainless steel tubes. (C) 2014 Published by Elsevier Ltd.