MMaterialsgateNEWS 2016/08/15

Related MaterialsgateCARDS

Self-shading windows switch from clear to opaque

New system can rapidly switch glass from transparent to dark - and keep it that way without power

A team of researchers at MIT has developed a new way of making windows that can switch from transparent to opaque, potentially saving energy by blocking sunlight on hot days and thus reducing air-conditioning costs. While other systems for causing glass to darken do exist, the new method offers significant advantages by combining rapid response times and low power needs.

Once the glass is switched from clear to dark, or vice versa, the new system requires little to no power to maintain its new state; unlike other materials, it only needs electricity when it's time to switch back again.

The results are reported this week in the online journal Chem, in a paper by MIT professor of chemistry Mircea Dinca, doctoral student Khalid Al-Kaabi, and former postdoc Casey Wade, now an assistant professor at Brandeis University.

The new discovery uses electrochromic materials, which change their color and transparency in response to an applied voltage, Dinca explains. These are quite different from photochromic materials, such as those found in some eyeglasses that become darker when the light gets brighter. Such materials tend to have much slower response times and to undergo a smaller change in their levels of opacity.

Existing electrochromic materials suffer from similar limitations and have found only niche applications. For example, Boeing 787 aircraft have electrochromic windows that get darker to prevent bright sunlight from glaring through the cabin. The windows can be darkened by turning on the voltage, Dinca says, but "when you flip the switch, it actually takes a few minutes for the window to turn dark. Obviously, you want that to be faster."

The reason for that slowness is that the changes within the material rely on a movement of electrons -- an electric current -- that gives the whole window a negative charge. Positive ions then move through the material to restore the electrical balance, creating the color-changing effect. But while electrons flow rapidly through materials, ions move much more slowly, limiting the overall reaction speed.

The MIT team overcame that by using sponge-like materials called metal-organic frameworks (MOFs), which can conduct both electrons and ions at very high speeds. Such materials have been used for about 20 years for their ability to store gases within their structure, but the MIT team was the first to harness them for their electrical and optical properties.

The other problem with existing versions of self-shading materials, Dinca says, is that "it's hard to get a material that changes from completely transparent to, let's say, completely black." Even the windows in the 787 can only change to a dark shade of green, rather than becoming opaque.

In previous research on MOFs, Dinca and his students had made material that could turn from clear to shades of blue or green, but in this newly reported work they have achieved the long-sought goal of producing a coating that can go all the way from perfectly clear to nearly black (achieved by blending two complementary colors, green and red). The new material is made by combining two chemical compounds, an organic material and a metal salt. Once mixed, these self-assemble into a thin film of the switchable material.

"It's this combination of these two, of a relatively fast switching time and a nearly black color, that has really got people excited," Dinca says.

The new windows have the potential, he says, to do much more than just preventing glare. "These could lead to pretty significant energy savings," he says, by drastically reducing the need for air conditioning in buildings with many windows in hot climates. "You could just flip a switch when the sun shines through the window, and turn it dark," or even automatically make that whole side of the building go dark all at once, he says.

While the properties of the material have now been demonstrated in a laboratory setting, the team's next step is to make a small-scale device for further testing: a 1-inch-square sample, to demonstrate the principle in action for potential investors in the technology, and to help determine what the manufacturing costs for such windows would be.

Further testing is also needed, Dinca says, to demonstrate what they have determined from preliminary testing: that once the switch is flipped and the material changes color, it requires no further power to maintain its new state. No extra power is needed until the switch is flipped to turn the material back to its former state, whether clear or opaque. Many existing electrochromic materials, by contrast, require a continuous voltage input.

In addition to smart windows, Dinca says, the material could also be used for some kinds of low-power displays, similar to displays like electronic ink (used in devices such as the Kindle and based on MIT-developed technology) but based on a completely different approach.

Not surprisingly perhaps, the research was partly funded by an organization in a region where such light-blocking windows would be particularly useful: The Masdar Institute, based in the United Arab Emirates, through a cooperative agreement with MIT. The research also received support from the U.S. Department of Energy, through the Center for Excitonics, an Energy Frontier Center.

Source: Massachusetts Institute of Technology – 11.08.2016.

Investigated and edited by:

Dr.-Ing. Christoph Konetschny, Inhaber und Gründer von Materialsgate
Büro für Material- und Technologieberatung
The investigation and editing of this document was performed with best care and attention.
For the accuracy, validity, availability and applicability of the given information, we take no liability.
Please discuss the suitability concerning your specific application with the experts of the named company or organization.

You want additional material or technology investigations concerning this subject?

Materialsgate is leading in material consulting and material investigation.
Feel free to use our established consulting services

MMore on this topic

Smart windows clean themselves, save energy and mimic moth eyes to cut glare

A revolutionary new type of smart window could cut window-cleaning costs in tall buildings while reducing heating bills and boosting worker productivity. Developed by University College London (UCL) with support from EPSRC, prototype samples confirm that the glass can deliver three key benefits: Self-cleaning: The window is ultra-resistant to water, so rain hitting the outside forms spherical droplets that roll easily over the surface – picking up dirt, dust and other contaminants and carrying them away. This is due to the pencil-like, conical design of nanostructures engraved onto the glass, trapping air and ensuring only a tiny amount of water comes into contact with the surface. This... more read more

Researchers in the Cockrell School of Engineering at The University of Texas at Austin are one step closer to delivering smart windows with a new level of energy efficiency, engineering materials that allow windows to reveal light without transferring heat and, conversely, to block light while allowing heat transmission, as described in two new research papers.

By allowing indoor occupants to more precisely control the energy and sunlight passing through a window, the new materials could significantly reduce costs for heating and cooling buildings. In 2013, chemical engineering professor Delia Milliron and her team became the first to develop dual-band electrochromic materials that blend two materials with distinct optical properties for selective control of visible and heat-producing near-infrared light (NIR). In a 2013 issue of Nature, Milliron's research group demonstrated how, using a small jolt of electricity, a nanocrystal material could be switched back and forth, enabling independent control of light and energy. The team now has... more read more

Technology developed by the University of Cincinnati and industry partners can do something that neither blinds nor existing smart windows can do.

This patent-pending research, supported by the National Science Foundation, will lead to low-cost window tinting which dynamically adapts for brightness, color temperatures and opacity (to provide for privacy while allowing light in). A partnership between the University of Cincinnati, Hewlett Packard, and EMD/Merck Research Labs has resulted in a patent-pending breakthrough in 'tunable' window tintings. The breakthrough means traditional window shades could soon be replaced by a low-cost tinting where the brightness, color temperature (warm or cool just like incandescent light bulbs) and opacity (privacy) are adjustable by the user. Details on this research, partly funded by... more read more

A newly developed light shutter may pave the way for see-through displays and smart windows.

The secret desire of urban daydreamers staring out their office windows at the sad brick walls of the building opposite them may soon be answered thanks to transparent light shutters developed by a group of researchers at Pusan National University in South Korea. A novel liquid crystal technology allows displays to flip between transparent and opaque states -- hypothetically letting you switch your view in less than a millisecond from urban decay to the Chesapeake Bay. Their work appears this week in the journal AIP Advances, from AIP Publishing. The idea of transparent displays has been around for a few years, but actually creating them from conventional organic light-emitting diodes... more read more


Partner of the Week

Search in MaterialsgateNEWS

Books and products