This material, described online Wednesday in the journal Nature, could one day help slash energy costs and reduce pollution from buildings like the glass monsters that tower out of urban cityscapes.
Windows allow warmth to escape on a cold day or let unwanted heat to filter in on a hot one – which can seriously hike up a business’s power bills.
"It’s become very in vogue to have a whole lot of glass on your facade, and that creates a problem from an efficiency perspective – you can’t control the heat," said Delia Milliron, a materials chemist at Lawrence Berkeley National Laboratory and lead author of the study.
Milliron and her colleagues designed a window that uses nanocrystals loaded with free electrons to help them absorb certain wavelengths of light. The more free electrons carried by the tiny crystals, the higher the frequency of light absorbed.
To block heat, the researchers focused on absorbing infrared light. Because infrared is lower on the electromagnetic spectrum than visible light, their crystals had to pick up a 'Goldilocks' number of electrons. Too few, and the crystals wouldn't block the infrared. Too many, and they could interfere with the visible light.
To do this, they used indium oxide that had roughly a tenth of its indium replaced with tin. With an electric current sent through the nanocrystals, this small amount of tin let them carry just enough electrons to block the infrared radiation. A window with this coating would keep the heat out, without marring the view.
The researchers took these heat-blocking nanocrystals and embedded them in niobium oxide glass, which can darken to block visible light. By itself, this tintable glass was a so-so performer. But with the nanocrystals embedded in it, it was a whopping five times darker than it would have been otherwise.
That’s because the nanocrystals, like a rude, suitcase-laden passenger on a crowded subway, force the glass particles around them into a new formation – one with a lot of empty spaces that make it easier for electrical charge to pass through.
It was an almost-accidental discovery: The combined power of the two materials was greater than the sum of their parts.
“We’ve been jumping up and down" in excitement, Milliron said. “It’s not often that things work even better than you could have hoped.”
By running a very low electrical current, the researchers say they can make a clear window block some of the sun’s heat and still stay clear. Then, applying a little more voltage, they can also tint the windows and block the light on an unpleasantly bright day. Flip the switch, and they can turn it back to a normal, unassuming window, letting all heat and light through. (This process is kind of a one-way street: If the glass blocks the light, it also blocks the heat.)
Most of these kinds of materials degrade fairly quickly, because moving a charge also comes with moving charged particles -- in this case, lithium ions. It's similar to why a battery goes bad over time. But this smart glass retained 96% of its charge capacity even after being switched on and off 2,000 times, they said. That’s probably because all the empty spaces created in the reorganized glass allowed the traveling ions a way to easily slip back into place.
Such a smart window would have to withstand something like 50,000 or 100,000 on-off cycles without falling apart, Milliron said, but the smart glass’s resilience was a promising start.
This smart glass coating would be a coating sandwiched between two panes of normal glass – one that could potentially be cheaper than the other 'smart' options because this layer could potentially be sprayed on, Milliron said. (Other methods require using expensive equipment, like a vacuum chamber.)
Such 'tunable' windows could help reduce lighting, heating and air-conditioning costs across cities – no small feat, given that buildings account for about 40% of energy use in the United States, Brian Korgel of the University of Texas at Austin wrote in a commentary on the study. In doing so, it could indirectly lessen environmental pollution too.
Korgel noted a few caveats with the budding technology. The researchers have yet to create the entire system out of transparent parts and safer materials. They’ll also need to make sure that the technology is cheap enough that it’s financially worth it for people to install, he added.
"Having the ability to take the glass of an entire office building and make it so one side might be blocking the light in the morning, and the other side is totally transparent – that seems like that would be a great application for it," Korgel said in an interview.