Advancements in Optical Properties of Thermochromic VO2 Films through Experimental and Numerical Investigations

Thesis (Ph.D.)--University of Washington, 2016-03The Department of Energy reports that buildings consume more than 40% of primary energy in the U.S. and that this trend will continue for the foreseeable future. Furthermore, windows constitute a major path for energy losses from buildings and therefore also present a significant opportunity for efficiency improvement and waste reduction. With this in mind, the work in this dissertation is focused on improving the control of solar and thermal radiation through windows. These radiation spectra can be controlled independently because they peak at different wavelengths due to the much higher temperature (5500 °C) of the Sun compared to objects on Earth (25 °C). In this work, a thermochromic material is utilized to control solar irradiance and a low-emissive (low-E) material is used to control thermal radiation. Thermochromic materials possess optical properties that change in response to temperature and low-E coatings are reflective in the mid-infrared (thermal) region. VO2 is an exciting candidate for thermochromic coatings because its transmittance in the visible region is relatively constant, but its reflectance in the IR increases significantly with temperature. The main technical issues limiting VO2 are luminous transmittance (Tlum) that is too low and a transition temperature (τc) that is too high. For the low-E coating, (SnO2)x(In2O3)1-x (ITO glass) was chosen because it has both high luminous transmittance and low emissivity. In this dissertation it is shown that τc of VO2 can be lowered from 50 to 45 °C by reducing the grain size of the film from 70 to 31 nm. In the area of luminous transmittance, TiO2 is investigated as an anti-reflective coating which can be used to increase Tlum. Later in this work, it is demonstrated that the energy efficiency gained through VO2 can be further improved by combining it with a low-E coating. The multilayer design combines anti-reflection in the visible region, thermochromism the near-IR and low emissivity in the mid-IR for an outstanding energy-efficient coating across the solar and thermal spectra. This dissertation contributes knowledge that helps reduce the barriers which currently limit thermochromic materials from being utilized for energy efficiency in built environments