The technology to make glass become darker when the light gets brighter has been around for a while. The most common example would be sunglasses. Photochromic materials are used for these and they tend to have slow response times and undergo a small change in their levels of opacity. Photochromic materials react to a change in light intensity. In contrast, electrochromic materials change their color and transparency in response to an applied voltage.

Existing electrochromic materials do however suffer from the same limitations as photochromic materials. They are therefore only used in niche applications such as the Boeing 787 aircraft that 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. It does however take a few minutes for the window to turn dark.

A new method of making windows that can switch from transparent to opaque has been developed by a team of researchers at MIT. The new technology has significant advantages in that it combines rapid response times and low power needs. This has the potential for saving energy by blocking sunlight on hot days, thereby reducing air-conditioning costs. Once the glass is switched from dark to clear, or vice versa, the new system requires virtually no power to maintain its new state. Electricity is only needed when it’s time to switch back again.

Current technology has a slow response time because the changes within the material rely on the movement of electrons. Once the electrons give the whole window a negative charge, positive ions move through the material to restore the electrical balance. This creates the color-changing effect. Ions move through materials much more slowly than electrons do, and this limits the overall reaction speed.

By using sponge-like materials called metal-organic frameworks (MOFs), the MIT team overcame this problem. MOFs conduct both electrons and ions at very high speeds. Although these materials have been used for their ability to store gases within their structure for about 20 years, the MIT team was however the first to utilize them for their electrical and optical properties.

Existing versions of self-shading materials can also not change from completely transparent to completely black. Even the windows in the 787 does not become opaque, but only changes to a dark shade of green. The MIT team has previously made material that could turn from clear to shades of blue or green. With the new inventions however, they can produce a coating that can go all the way from perfectly clear to nearly black. The black is produced by blending two complementary colors – green and red. Two chemical compounds, an organic material and a metal salt, are combined to self-assemble into a thin film of the switchable material.

The new windows could potentially do much more than just prevent glare. By drastically reducing the need for air conditioning in buildings with many windows in hot climates, they could lead to significant energy savings. Simply by flipping a switch when the sun shines through the window, it will turn dark. The whole side of a building could even be made to go dark automatically and all at once.

The team’s next step is to make a small-scale model for further testing, as the properties of the material have only been demonstrated under laboratory conditions. To demonstrate the principle in action for potential investors, they plan to build a one-inch square sample. This will also help in determining what the manufacturing costs for such windows would likely be.

The team does have to do further testing to demonstrate that once the switch is flipped and the material changes color, no further power is required to maintain its new state. Only when the switch is flipped to turn the material back to its former state, whether opaque or clear, will power be needed again. By contrast, many existing electrochromic materials require a continuous voltage input.

Smart windows is only one of the possible applications of the material. The team plans to utilize their knowledge for some types of low-power displays, similar to displays used in devices such as the Kindle. Their approach will however be completely different to what is currently being done.

The results were published by MIT professor of chemistry Mircea Dincă, doctoral student Khalid Al-Kaabi, and former postdoc Casey Wade in the journal Chem.

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