Material would allow users to ‘tune’ windows to block targeted wavelengths of light
Researchers have demonstrated a material for next generation dynamic windows, which would allow building occupants to switch their windows between three modes: transparent, or “normal” windows; windows that block infrared light, helping to keep a building cool; and tinted windows that control glare while maintaining the view.
Dynamic windows based on electrochromism – meaning their opacity changes in response to electric stimulus – are not a new concept. But, to this point, most dynamic windows were either clear or dark.
“Our work demonstrates that there are more options available,” says Veronica Augustyn, co-corresponding author of a paper on the work and the Jake and Jennifer Hooks Distinguished Scholar in Materials Science and Engineering at North Carolina State University. “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.”
The key to more dynamic window materials is water.
Specifically, the researchers found that when water is bound within the crystalline structure of a tungsten oxide – forming tungsten oxide hydrate – the material exhibits a previously unknown behavior.
Tungsten oxides have long been used in dynamic windows. That’s because tungsten oxide is normally transparent. But when you apply an electrical signal, and inject lithium ions and electrons into the material, the material becomes dark and blocks light.
Researchers have now shown that you can effectively tune the wavelengths of light that are blocked when you inject lithium ions and electrons into a related material called tungsten oxide hydrate. When lithium ions and electrons are injected into the hydrate material, it first transitions into a “heat blocking” phase, allowing visible wavelengths of light to pass through, but blocking infrared light. If more lithium ions and electrons are injected, the material then transitions into a dark phase, blocking both visible and infrared 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,” says Jenelle Fortunato, first author of the paper and a postdoctoral fellow at NC State. “Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes. 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. “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 paper, “Dual-Band Electrochromism in Hydrous Tungsten Oxide,” is published in the journal ACS Photonics. First author of the paper is Jenelle Fortunato, a postdoctoral researcher at NC State. The paper 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 research was done with support from the National Science Foundation, under grant 1653827; the U.S. Department of Energy’s Office of Science, under grant DE-SC0023408; and the Welch Foundation, under grant F-1848.
JOURNAL
ACS Photonics
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Dual-Band Electrochromism in Hydrous Tungsten Oxide
Chameleon-inspired coating could cool and warm buildings through the seasons
As summer turns to fall, many people will be turning off the air conditioning and firing up heaters instead. But traditional heating and cooling systems are energy intensive, and because they typically run on fossil fuels, they aren’t sustainable. Now, by mimicking a desert-dwelling chameleon, a team reporting in ACS’ Nano Letters has developed an energy-efficient, cost-effective coating. The material could keep buildings cool in the summers — or warm in the winters — without additional energy.
Many desert creatures have specialized adaptations to allow them to survive in harsh environments with large daily temperature shifts. For example, the Namaqua chameleon of southwestern Africa alters its color to regulate its body temperature as conditions change. The critters appear light grey in hot temperatures to reflect sunlight and keep cool, then turn a dark brown once they cool down to absorb heat instead. This unique ability is a naturally occurring example of passive temperature control — a phenomenon that could be adapted to create more energy-efficient buildings. But many systems, such as cooling paints or colored steel tiles, are only designed to keep buildings either cool or warm, and can’t switch between “modes.” Inspired by the Namaqua chameleon, Fuqiang Wang and colleagues wanted to create a color-shifting coating that adapts as outside temperatures fluctuate.
To make the coating, researchers mixed thermochromic microcapsules, specialized microparticles and binders to form a suspension, which they sprayed or brushed onto a metal surface. When heated to 68 degrees Fahrenheit, the surface began to change from dark to light grey. Once it reached 86 degrees, the light-colored film reflected up to 93% of solar radiation. Even when heated above 175 degrees for an entire day, the material showed no signs of damage. Next, the team tested it alongside three conventional coatings — regular white paint, a passive radiative cooling paint and blue steel tiles — in outdoor tests on miniature, doghouse-sized buildings throughout all four seasons.
- In winter, the new coating was slightly warmer than the passive radiative cooling system, though both maintained similar temperatures in warmer conditions.
- In summer, the new coating was significantly cooler than the white paint and steel tiles.
- During spring and fall, the new coating was the only system that could adapt to the widely fluctuating temperatures changes, switching from heating to cooling throughout the day.
The researchers say that this color-changing system could save a considerable amount of energy for regions that experience multiple seasons, while still being inexpensive and easy to manufacture.
The authors acknowledge funding from the National Natural Science Foundation of China, the Taishan Scholars of Shandong Province, the Royal Society, and the China Scholarship Council.
The American Chemical Society (ACS) is a nonprofit organization chartered by the U.S. Congress. ACS’ mission is to advance the broader chemistry enterprise and its practitioners for the benefit of Earth and all its people. The Society is a global leader in promoting excellence in science education and providing access to chemistry-related information and research through its multiple research solutions, peer-reviewed journals, scientific conferences, eBooks and weekly news periodical Chemical & Engineering News. ACS journals are among the most cited, most trusted and most read within the scientific literature; however, ACS itself does not conduct chemical research. As a leader in scientific information solutions, its CAS division partners with global innovators to accelerate breakthroughs by curating, connecting and analyzing the world’s scientific knowledge. ACS’ main offices are in Washington, D.C., and Columbus, Ohio.
JOURNAL
Nano Letters
ARTICLE TITLE
"Warm in Winter and Cool in Summer" Scalable Biochameleons Inspired Temperature Adaptive Coating with Easy Preparation and Construction
ARTICLE PUBLICATION DATE
20-Sep-2023
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