Development and characterization of niobium-doped vanadium dioxide based thermochromic thin films for energy saving in buildings

The energy requirement of the world is increasing day by day. Accordingly, the need for efficient use of energy can be achieved by reducing the losses of the energy used in the environment in which it is used. Worldwide, buildings consume 40% of the total primary energy used. In the buildings, the windows are where the heat is lost and the gains are greatest. For this reason, energy-saving glasses have been started to be used in window systems. Glasses that automatically react to changing ambient conditions within these energy-saving glasses, in other words smart glasses, have gained considerable importance in recent years. Thereby, studies on thermochromic glasses that change their optical properties depending on temperature have been increasing dramatically. Vanadium dioxide (VO2) is an inorganic material that has a thermochromic transition temperature at 68 °C and it shows the nearest thermochromic transition temperature to room temperature among all other inorganic materials. Therefore, VO2 has a great potential to be used in smart window systems. As a result of an increase in temperature, a phase transition (from monoclinic to tetragonal) occurs, so the transmittance values of VO2 in the infrared region are decreased enabling the temperature control. However, the thermochromic transition temperature of VO2 is still quite high for their applicability in smart window systems. Therefore, studies are being carried out to reduce the thermochromic conversion temperature to the room temperature. Studies have shown that the transition temperature can be reduced by metal ion doping such as W, Nb and Mo. The aim of this thesis work is to develop and characterize niobium-doped VO2-based thermochromic thin films for energy saving in buildings. In order to reduce the thermochromic transition temperature of the VO2 thin films, VO2 based solutions were prepared by doping with Nb metal ions in different molar ratios (1%, 2.5%, 5% and 7.5%). Afterwards, these solutions were coated on soda lime glass substrates which had been already coated with SiO2 intermediate layer to prevent sodium diffusion from the glass substrate. Two different precursors, vanadium triisopropoxide (Vtip) and vanadium oxyacetylacetone (VO(acac)2), were used as the vanadium source for sol preparations. The solutions were coated onto the substrates using dip-coating method. Samples prepared with Vtip: 5+ valence precursor were heat treated at 525 °C for 2.5 hours under Ar/H2 (5% by volume) reductive gas atmosphere. Thin film samples prepared using VO(acac)2 :+4 valence precursor, were heat treated at 550° C for 1 hour by controlled heating and cooling under inert gas atmosphere. A UV-Vis spectrophotometer with custom made heating unit is used to characterize the temperature dependent optical properties of thin films. The thermochromic transition temperatures of the solutions were determined by DSC technique. Phase analysis of thin films and powder samples obtained by drying solutions was performed using XRD technique. Microstructure analysis of thin film samples was performed using SEM/EDX technique. Thickness of thin films were determined by SEM and ellipsometric analysis. In this study, thin film samples prepared using Vtip and VO(acac)2 precursors showed different thermochromic characteristics. In terms of optical characterization, the visible transmittance values of samples obtained using VO(acac)2 were determined to be higher than those of obtained using Vtip. The optical characterizations showed that thermochromic transition efficiency of samples prepared with Vtip was higher than samples prepared with VO(acac)2 for studied spectral wavelength. During VO2 synthesis, sodium diffusion was not completely prevented by SiO2 interlayer and sodium vanadate phase formed in the structure together with vanadium dioxide phase. As a result of XRD analysis, different oxide phases of vanadium (V6O13 and V2O3) were detected with the VO2 phase during heat treatment process in the powder samples. Thermochromic transition efficiencies of Vtip samples reduced with increasing Nb content and thermochromic transition efficiencies of VO(acac)2 samples increased depending on Nb content. It was determined that thermochromic transition temperatures decreased with increasing Nb content for the samples prepared with Vtip precursors. The Vtip-5Nb sample was found to demonstrate the optimum properties with a transition temperature of about 41 °C, 40% visible transmittance and 5.3% transition efficiency at 1100 nm. Transmittance values of undoped Vtip sample, which shows optimum thermochromic properties, were measured up to 1600 nm and it was proven that thermochromic efficiency increases with increasing wavelength in IR region.