Doctor of Philosophy
Chemical and Biochemical Engineering
Energy-efficient or smart windows are an important part of net zero energy housing as excellent candidates for energy conservation. They can control the amount of light and heat passing through them in response to an external stimulus such as heat (thermochromic) or electricity (electrochromic). Vanadium Oxide (VO2) is a potential thermochromic material to provide an automatic reversible semiconductor-metal phase transition from the monoclinic structure to tetragonal at 68 °C. However, there remain major challenges in fabricating desirable VO2-based films to meet the requirements for large-scale implementation. Mainly, the phase transition temperature (Tc) should be close to the ambient temperature (25 ºC), while the integrated luminous transparency (Tlum), and solar modulation ability should be high. Another obstacle is the preparation of pure and highly crystalline VO2 (M) with a small hysteresis width for fast switching ability while using green solvents and milder conditions for large-scale production. The selection of a suitable polymeric matrix for long-term stabilization of VO2 nanostructure is another crucial concern for industrial goals. This doctoral research project aimed to develop new approaches to the synthesis and fabrication of VO2 thermochromic films using imidazolium ionic liquids (ILs) or imidazolium poly-ionic liquids (PIL). In “Chapter 2”, Vanadium oxide VO2 (M) synthesis is studied using supercritical CO2 (scCO2), imidazolium ionic liquids (ILs), and their biphasic combination as environmentally benign solvents under mild condition. The role of ILs and scCO2 on the gelation yield as well as the crystallinity, morphology, and optical properties of final VO2 (M) is investigated. In “Chapter 3”, the role of different imidazolium ILs on stabilization of VO (acac)2 precursor is studied, and a new fabrication method is suggested to prepare pure and well crystalline VO2 (M) using a fast processing time. In Chapter 4, two complementary methods for the fabrication of VO2 and VO2/ SO2 nanostructures are suggested using PIL as a dielectric host. In this doctoral thesis, we have tried to respond to the main challenges regarding the synthesis and fabrication of VO2 (M) thermochromic films with improved optical properties by using simple synthetic methods and green solvents.
Summary for Lay Audience
Energy consumption is continuously increasing worldwide, and energy conversion has been a great challenge to sustainable development. A substantial component of primary energy (~40%) is used in form of air conditioning and it is estimated that energy exchange through the windows is responsible for more than 50% of a building’s energy loss. Other than high energy costs, this energy loss can also lead to a higher carbon footprint through increased CO2 emissions. Applying self-controlled coatings to glass windows or using so-called “energy-efficient windows” can be one effective method to reduce energy loss. The energy-efficient windows or smart windows typically function by an external stimulus such as electricity (electrochromic windows) or heat (thermochromic windows). Vanadium oxide (VO2) as a typical thermochromic material has been receiving extensive attention as it undergoes a reversible semiconductor-to-metal phase transition which is accompanied by remarkable changes in electrical and optical properties. However, there are remaining challenges in the synthesis and fabrication of VO2. It is important to prepare pure and highly crystalline VO2 with fast switching ability using green solvents and milder conditions. The selection of a suitable polymeric matrix for long-term stabilization of VO2 nanostructure is also another crucial concern for industrial goals. This doctoral research project is aimed to use ionic liquids (ILs) or poly-ionic liquids (PIL) to develop new approaches in the synthesis and fabrication of VO2 thermochromic films.
Nazari, Shidokht, "Using Ionic Liquids in Fabrication of VO2 for Thermochromic Coatings" (2021). Electronic Thesis and Dissertation Repository. 7755.