Progress Report: Optical Properties of Thermochromic VO2 Coatings for Windows

The properties of VO₂, thermochromic coatings were investigated for application to building windows. The solar transmittance of VO₂ decreases above a certain temperature, blocking excess solar gain. Films were deposited on glass by reactive magnetron sputtering, but stoichiometric compositions have not yet been achieved. The composition and structure of these films have been determined by Auger spectroscopy and x-ray diffraction. Optical properties were measured by photometric and ellipsometric methods. Using this optical data, composite coatings were designed having improved optical characteristics. The high-temperature metallic state of VO₂ was not found to be suitable as a static low-emissivity coating. When this VO₂, is used as a thermochromic coating, however, its moderately low-emissivity would contribute to energy savings. The ratio of solar transmittance between the high-temperature and low-temperature states increases with decreasing average transmittance. Higher values of this ratio might be obtained by proper choice of additional coating layers.