Infoscience

Thesis

Reactively Sputtered Nano-Structured Multilayer Coatings on Architectural Glazing for Active Solar Energy Façades

Improving the aesthetic of solar thermal collectors would grant architects a huge potential for a perfect integration of solar panels into building façades: this can be achieved by coloured interference coatings on the cover glass. These coatings also open the possibility to match the colours of the collectors to those of other architectural components and design elements. An important boundary condition for the coloured filter production is however, to provide a sufficiently high solar transmittance. To deal with the latter, sophisticated numerical simulations are needed to improve the coloured filter design. In addition, a large variety of coating materials is important, ideally with huge differences in their refractive indices, while the absorption of all materials needs to be close to zero. This doctoral thesis focuses on the optical film design and the deposition of new coloured thin-film multilayers with an optimised energetic performance and an only little varying coloured reflection at different viewing angles. To access more directly the correlation between solar transmittance, light spectra and visual and energetic performance of the coatings, a thin-film simulation framework has been developed on the basis of Mathematica. New coating designs have reduced the number of parameters necessary to be adjusted and tuned during the coating-development process: this could help to shorten the prototype development phase on industrial coaters significantly as well as the whole coating-development cycle. Based on the new developed coloured filter designs, multilayer stacks were deposited via reactive magnetron sputtering. The deposited coatings exhibit a solar transmittance Tsol > 87%. For greenish colour hues even a Tsol up to 91% was reached, which corresponds to less than 1% loss in Tsol (respectively solar energy performance) in comparison to the uncoated glass substrate. Furthermore, the research deals with the deposition of low-refractive magnesium fluoride (MgF2) and MgF2 containing composites. Hereby, a novel deposition process by reactive magnetron sputtering was developed for MgF2 using a metallic magnesium target and a reactive gas mixture of oxygen (O2) and carbon tetrafluoride (CF4) diluted with argon. The obtained MgF2 films exhibit a very low refractive index of n = 1.382 at 550nm, which is congruent with the polycrystalline bulk, and only a very weak absorption in the whole solar range (300-2500nm). In addition, a novel approach to deposit Mg-F-Si-O composite materials by magnetron co- sputtering from compound MgF2 and silicon dioxide (SiO2) targets was performed. The obtained films show a two-phase system with nanocrystals of MgF2 embedded in a SiO2-rich amorphous matrix. The lowest achieved refractive index amounts to n = 1.424 at 550nm combined with a negligible extinction coefficient (k < 10-9). The investigated sputtered low-index materials open-up manifold possibilities for coloured filters. For instance, by adding a supplementary MgF2 inter-layer, a colour-invariant solar transmittance of Tsol = 85-85.7% could be achieved, as shown by numerical simulations. With this novel optical filter design it will be feasible to offer a whole colour palette for solar thermal collectors exhibiting all the same solar thermal energy performance.

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