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While the large windows and glass that often surround modern buildings allow for the immense amount of lighting that enters these areas during the day. However, the glazings that are placed as a film over these windows and glass often limit the energy efficiency of the building as well.
To compensate for this loss or increased absorption of energy, modern buildings are equipped with indoor cooling and heating platforms, however, this causes the energy consumption of these buildings to skyrocket, thereby simultaneously increasing the costs associated with maintaining such an area.
Smart windows have offered a revolutionary response to this challenge, as they are often covered by either electrochromic or thermochromic films to regulate the inflow of visible light and solar energy to acquire better energy efficiency throughout the building. Electrochromic smart windows sandwich certain materials between two panes of glass that energize and darken the window following the application of an electrical current to the material.
Thermochromic windows, which are becoming some of the most advanced window technologies available, despite their current limited availability on the market, function by taking the heat acquired by direct sunlight to change the color of the window film. As sunlight becomes more direct and intense onto the glass, the color will drastically darken, thereby allowing the windows to limit the heat load that enters the building.
Thermochromic windows can incorporate both inorganic, which are commonly transition metal compounds, or organic materials. Vanadium dioxide (VO2), whose innate potential to affect infared properties, enables this material to be an ideal candidate for the application of smart window devices. In a recent joint collaboration effort between Sandia National Laboratories and Santa Fe, New Mexico-based IR Dynamics, Researchers looked to VO2 as a prospective nanoparticle to be incorporated into the polymer films typically used in smart window technologies.
VO2 is a thermochromic material that exhibits a clear switch in temperature at the near infared (NIR) and infared wavelength regions from low temperature transmitting states to a high temperature reflecting state, both of which are not far from average room temperature at approximately 68 °C. At this temperature VO2 is present in its monoclinic phase, which is characterized by alternative V-V distances within the lattice. In contrast to this phase, the tetragonal form of VO2 exhibits a uniform bond distance between V-V, resembling a Rutile-type structure.
In this state, tetragonal VO2 displays a low electrical resistivity, as well as a significant decrease in its transmission of near-IR wavelengths. The group of Researchers in this study successfully transformed monoclinic VO2 to tetragonal VO2 through a multistage process conducted at the nanolevel.
Despite the frequent challenges associated with the synthesis of VO2, particularly when present a the nanoparticle level, the Researchers converted Vanadium Oxide Precursor (VOP) powder to VO2 by careful monitoring through High Temperature X-Ray Diffraction (HTXRD) methods, which confirmed the transformation of these nanoparticles to the desired tetragonal phase. By developing a method to disperse the VO2 nanoparticles onto the window through a spray-paint-like mixture, the Researchers also successfully adjusted the “switching” temperature of VO2 to an impressive 78 °C, which drastically improves the level of efficacy of this material.
The successful adjustment in this temperature, as well as the incorporation of several miniscule amounts of different types of metals, allows the nanoparticles to switch, and thereby convert the absorbed heat energy into a darkened window pane, up to 200 °F.
Future applications of the VO2 nanoparticle film expands far beyond that in smart window applications, as the Researchers believe that its use for architectural plastics, such as those used in the Water Cube of the 2008 Beijing Olympics, is promising.
Image Credit:
zhu difeng/ Shutterstock.com
References:
- “In-situ monitoring of vanadium dioxide formation using high-temperature XRD” M. Rodriguez, N. Bell, et al. International Centre for Diffraction Data. (2017). DOI: 10.1017/S0885715614000311.
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