VO2-based Thermochromic and Nanothermochromic Materials for Energy-Efficient Windows Computational and Experimental Studies

Abstract: VO2-based ?lms are thermochromic and exhibit high or low infrared transmittance when the temperature is below or above a critical temperature. The thermochromic switching is passive and reversible, and therefore VO2 based ?lms are promising for energy-ef?cient window appli­cations. However the practicaluse of VO2 for energy-ef?cient windows has long been hampered by low luminous transmittance and low solar energy transmittance modulation. The main goal of this dissertation work is to address these issues.The ?rst half of the work proposes the concept of nanothermochromics for simultaneous improvement of luminous transmittance and modulation of solar energy throughput. nanoth­ermochromics considers VO2 nanoparticle composite layers, whose optical properties were modeled by effective medium theories. Calculations on VO2 spheroids have shown that VO2 nanoparticles, especially nanospheres, can offer dramatically improved luminous transmittance and solar transmittance modulation that are not possible for ?lms. Calculations done on coreshell nanoparticles showed comparable improvements and offer an opportunity to reduce the material costs. It was also found that the composite of In2O3:Sn (ITO) and VO2 can yield moderately high luminous transmittance, solar transmittance modulation and low-emittance properties.In the second half of the dissertation work, Mg-doped VO2 ?lms were sputter deposited. Their band gaps and Mg-content were investigated by means of optical absorption measurement and Rutherford backscattering spectrometry, respectively. The band gaps of VO2 were found to increase by ?3.9±0.5 eV per unit of atom ratio Mg/(Mg+V) for 0<Mg/(Mg+V)<0.21. Computations based on effective medium theory were done to estimate the performance of Mg­-doped VO2 ?lms and nanoparticle composite layers. The results suggest that moderately doped VO2 ?lms with 0<Mg/(Mg+V)<0.06 perform better than un-doped ?lms and that the perfor­mance can be further enhanced with one layer of antire?ection coating. The best results were achieved by un-doped VO2 nanospheres, closely followed by the VO2 nanospheres with low Mg-content.Furthermore, the an experimental study on sputter deposited VO2 nanorods has identi?ed the geometry of the oxygen gas inlet, the type of substrate, the substrate temperature and the layer thickness as important factors that in?uence the growth morphology.Taken as a whole, nanothermochromics offered by VO2 nanoparticles was shown to be the best solution for VO2 based thermochromic energy-ef?cient window coatings.