Thermochromic VO2-based materials for smart windows : Progress towards applications in buildings

Vanadium dioxide is a well-known thermochromic material, whose optical properties can be varied reversibly in response to fluctuation of temperature. It is attractive in various fields due to its unique properties as well as its prospective applications. Especially, it is the most favorite candidate for smart window applications which can significantly lower energy consumption. Ideally, VO2-based thermochromic windows can regulate solar radiation gain dynamically according to the exterior environment conditions. However, commercial utilization of VO2 thermochromic windows is still on the way. There are still a few issues needed to be overcome, such as high phase transition temperature, the unfavorable yellow-brown color, low luminous transmittance and weak solar energy modulation ability. Vanadium oxides are known to have rich polymorphs and devices using thermochromic effect often require absence of impurities, so that stringent process control is crucial for practical manufacturing of VO2 materials. In this work, a novel route for fabrication of VO2 thin films was developed thermodynamically and verified experimentally. Another concern related to VO2-based materials is the relatively short durability of their desirable properties as VO2 is not the most thermodynamic stable species. Hence the lifetimes of the thermochromic VO2 films under various environmental conditions were evaluated. Furthermore, studies have been made to investigate the impacts of substrates on VO2 film growth. For window coating applications, the light scattering is of importance. Therefore, the light scattering effect for particulate VO2 film was studied. Additionally, the low luminous transmittance of VO2 film can be substantially increased by use of a top coating with suitable refractive index. In our study a TiO2 top layer was used, which leads to improved thermochromic behavior. Moreover, incorporation of VO2 plasmonic pigments into a matrix is a useful way to overcome the unsatisfied thermochromic performance of conventional VO2 films. A composite film of VO2-SiO2 was fabricated and its optical properties were studied. Besides, phase-pure VO2 nanospheres were synthesized via chemical route and their thermochromic properties were investigated. In general, these studies promote development and progress of VO2-based material further to be used in heat and light regulation applications.