Nanostructured vanadium dioxide thermochromic smart windows
Date of Issue2017-05
School of Materials Science and Engineering
In the recent years, thermochromic smart windows, as a category of solar control windows, provide the solutions for saving the energy usage in the buildings or vehicles. Vanadium dioxide (VO2) is considered as a promising smart window material because of its infrared-switchable optical properties, which is led by the intrinsic phase transition at a critical temperature of 68 C. The temperature stimulated thermochromic effects make VO2 as the superior candidate of energy-efficient smart windows, but the real application of VO2 continuous thin films is hindered mainly due to the low luminous transmittance (Tlum) and the insufficient solar modulation ability (ΔTsol), which are largely caused by the intrinsically high optical absorption and the unwanted high refractive index (RI). To tackle these issues, the work covered by this thesis utilizes three attempts, namely VO2 nanocomposites, grid-patterning and RI tunable anti-reflection coatings (ARC) on the basis of various VO2 substrates, to achieve largely enhanced both Tlum and ΔTsol. The VO2 nanoparticles (VO2-NP) prepared by mechanical attrition are encapsulated in the polymer matrix, Si-Al gel, to form nanocomposites, which embody the concept of “nanothermochromism”. This method provides a industrial scalable way of fabriating nanoparticles and offer the comparable thermochromic performances of Tlum(avg) = ca. 59% and ΔTsol = ca. 12%. The simulation work using parameter search algorithm illustrates the excellence of VO2 nanogrids. The “touching-hole” design demonstrates the largest possible Tlum of ca. 80% with the remarkable ΔTsol of 14%. The proposed VO2 nanogrids overcome the thickness constraint of continuous thin films from ca. 200 nm up to 400 nm theoretically. In practice, VO2 NP are electrochemically deposited (from their suspensions) at the conductive copper grids with controllable line-width, thickness and fill factor to implement the simulated VO2 grid patterning concept. The computed relationship between the energy modulation abilities and fill factor is experimental proved by the experimental results. Furthermore, this work demonstrates a new irreversible “red-ox free” electrodeposition approach, which is induced by altering the ionic strength in the vicinity of the electrode surface. The new deposition method also provides a universal one-step solution for the formation of patterns with other nanomaterials (e.g., gold nanoparticles, carbon nanotubes and graphene oxides). Therefore, besides achieving excellent thermochromic performance, this method provides a versatile approach for the guided growth on a patterned conducting substrate which can be used in different devices. Based on the understanding of the intrinsic problems of the RI of VO2, the thermochromic performances of VO2 nanocomposites and gridded structures can be further improved by applying the RI-tunable ARC. The sol-gel based ARC with tunable RI (from 1.47 to 1.92 at the wavelength of 550 nm) offer more anti reflection at a lower temperature, thereby maximizing ΔTsol for various VO2 based films with largely enhanced Tlum simultaneously.