Oliva Bouvard obtains PhD for research in thin film coatings
Low-e insulation glazing has ceased to generate Faraday cages and novel electrochromic coatings were developed thanks to Olivia Bouvard's research in thin film coatings. On Friday 18 January she obtained her PhD for the two research projects, one of which has already found practical application in an award winning project on railway energy efficiency.
Modern envelope glazing relies on nanoscale coatings to meet a range of specific needs. Low emissivity coatings can improve the thermal performance of glazing thanks to a thin metallic film. However, the conductive nature of this layer leads to an attenuation of the microwaves used in telecommunications. Electrochromic coatings on the other hand can modulate the solar heat gains of a building while preserving the view toward the exterior. Yet, the switching speed and contrast as well as the durability of commercial products are unsatisfactory. This thesis focuses on the improvement of the electronic and optical properties of such coatings to address the shortcomings listed above. A novel approach to increase the transmission of microwaves through transparent conductive coatings is presented. Experiments are performed to validate the proof of concept, a variety of patterns obtained by laser ablation are then determined. These allow a transmission of microwaves close to the one of uncoated glass while preserving the thermal and visual properties of the glass. Glazing with these novel coatings is now used by a Swiss railway company for the windows of its wagons to avoid the use of signal amplifiers. Transparent conductive coatings are also used as electrode in electrochromic devices; in order not to damage the underlying layers, deposition of this electrode without heating is preferable. Deposition parameters giving suitable optical and electrical properties are presented. Since gel or polymer electrolytes limit the durability of electrochromic glazing, all-solid state, inorganic devices are considered. The optical contrast and switching dynamics of tungsten trioxide can be improved by increasing the nanoporosity of the layer. Furthermore, the use of a nickel-tantalum oxide nanocomposite for the counter electrode yields a better color rendering. In combination with tantalum pentoxide as a solid electrolyte, the migration of lithium ions should be facilitated. Other solid electrolytes such as lithium phosphate oxynitride (LiPON) and lanthanum lithium titanium oxide (LLTO) are envisioned. It is shown that the stability of LiPON in ambient air can be enhanced by increasing the amount of argon in an argon-nitrogen plasma during deposition. In addition, it is shown that LLTO films have a high solar transmittance, allowing its use in electrochromic devices. Lithium insertion during a vacuum process would facilitate the production of electrochromic devices. A dry lithiation method, which does not require the use of elements sensitive to humidity, is presented. Used for the lithiation of a tungsten trioxide, it makes it possible to reduce its light transmittance from 83 to 2%. All-solid-states devices, lithiated in vacuum and produced using a nanoporous tantalum pentoxide as a lithium ion conductor, exhibit a fast switching time and a good contrast. Except for the top electrode, they are manufactured using metallic sputtering targets, which allow faster deposition rates compared to ceramic targets. Additionally, the study of lithiated films and interfaces by photoelectron spectroscopy provides information on the modification of the work function and oxidation levels of the host material, leading to a better understanding of the semiconductor heterostructures constituting all-solid-state-devices. These insights are expected to help to develop optimized electrochromic glazing for a better management of solar heat gains and daylight in buildings.
This thesis was supervised by Dr Andreas Schueler and Prof. Jean-Louis Scartezzini.
Bouvard Olivia, Scartezzini Jean-Louis and Schueler Andreas (Dirs.). Coatings with tailored electronic and optical properties for advanced glazing. EPFL Thesis n° 9199 (2019)