A material for next-generation dynamic windows has been illustrated by scientists, which would enable building occupants to change their windows between three modes.
These windows have three modes: transparent, or “normal” windows, windows that block infrared light to keep buildings cool, and tinted windows that reduce glare while still allowing one to see outside.
Dynamic windows rely on electrochromic – implying their opacity changes in reaction to an electric stimulus – and are not a new vision. However, to this point, the majority of the dynamic windows were either clear or dark.
Our work demonstrates that there are more options available. Specifically, we’ve shown that you can allow light to pass through the windows while still helping to keep buildings cooler and thus more energy efficient.
Veronica Augustyn, Study Co-Corresponding Author and the Jake and Jennifer Hooks Distinguished Scholar, Materials Science and Engineering, North Carolina State University
The key to highly dynamic window materials is water.
The scientists discovered that when water has been bound inside the crystalline structure of a tungsten oxide–developing tungsten oxide hydrate–the material displays an earlier unknown behavior.
For a long time, tungsten oxides have been utilized in dynamic windows. Generally, that is because tungsten oxide is transparent. However, when one applies an electrical signal and injects lithium ions and electrons into the material, the material turns dark and hinders light.
Now, scientists have shown that one could efficiently tune the wavelengths of light that are blocked when one injects lithium ions and electrons into a related material known as tungsten oxide hydrate.
When lithium ions and electrons have been injected into the hydrate material, it first transitions into a so-called “heat blocking” phase, thereby enabling visible wavelengths of light to pass through but hindering infrared light.
If a greater number of lithium ions and electrons are injected, the material further transitions into a dark phase, obstructing both infrared and visible wavelengths of light.
The presence of water in the crystalline structure makes the structure less dense, so the structure is more resistant to deformation when lithium ions and electrons are injected into the material. Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes.
Jenelle Fortunato, Study First Author and Postdoctoral Fellow, North Carolina State University
Fortunato added, “There’s a ‘cool’ mode – when injection of lithium ions and electrons affects the optical properties, but structural change hasn’t occurred yet – which absorbs infrared light. And then, after the structural change occurs, there’s a ‘dark’ mode that blocks both visible and infrared light.”
“The discovery of dual-band (infrared and visible) light control in a single material that’s already well-known to the smart windows community may accelerate development of commercial products with enhanced features,” says Delia Milliron, co-corresponding author of the paper and the Ernest Cockrell, Sr. Chair #1 in Engineering at the University of Texas at Austin.
Milliron added, “More broadly speaking, the unforeseen role of structural water in producing distinctive electrochemical properties may inspire the research community beyond smart window developers, leading to innovation in energy storage and conversion materials.”
The study was co-authored by Noah Holzapfel, a Postdoctoral Researcher at NC State; Matthew Chagnot, a Ph.D. Student at NC State; James Mitchell, a recent Ph.D. Graduate of NC State; Benjamin Zydlewski and Hsin-Che Lu of the University of Texas at Austin; and Ming Lei and De-en Jiang of Vanderbilt University.
The study was carried out with financial support from the National Science Foundation, under grant 1653827; the US Department of Energy’s Office of Science, under grant DE-SC0023408; and the Welch Foundation, under grant F-1848.
Fortunato, J., et al. (2023) Dual-Band Electrochromism in Hydrous Tungsten Oxide. ACS Photonics. doi.org/10.1021/acsphotonics.3c00921.