Transparent wood opens a window to cooler homes

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The researchers constructed a rectangular model home to put the wooden windows to the test(Credit: Tian Li/University of Maryland)

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A few months ago, we looked at the work of material scientists who had managed to make a block of linden wood see-through. Altering the molecular makeup to turn timber transparent is an impressive bit of science to be sure, but what kind of benefits could this new material offer over its comparably opaque counterpart? The same scientists are now finding some conclusive answers to this question, using their transparent wood as a window for a model home and finding that not only does it let in a similar amount of light to glass, but it is much better at keeping the interior cool.

The research team, from the University of Maryland (UMD), create their transparent wood by bleaching it in a beaker to remove a molecule called lignin. This is the molecule that makes wood brown, but critically, also gives it its rigidity, so removing it is kind of like robbing Peter to build a see-through wall.

One research team from Sweden was able to find a way around this problem earlier in the year, removing lignin from wood and then mixing the material with acrylic glass to restore its rigidity. But the Maryland researchers took a different route, instead filling in the gaps with epoxy. Their early testing indicated that the resulting material was actually four to six times stronger than the original untreated version.

But the wood exhibited some other interesting properties too, particularly when it came to light. When the wood is treated in this way, the channels within that carry water and nutrients up and down the tree are turned into passages for light. The team reported that the material had high transparency and also high haze, a property that pertains to its ability to scatter light.

Further down the track, these characteristics could be valuable in creating advanced solar panels, with the light entering through the transparent wood and then being held in as a result of the high haze factor, allowing the panel to soak up more solar energy. But the team's latest research suggests that it could find some uses around the home, too.

The researchers constructed a rectangular model home, measuring 13.5 by 7.5 cm (5.3 x 2.95 in) and 25 cm tall (9.84 in). The ceiling consisted of the transparent wooden panel to serve as a window. The team then applied a solar simulator, which offers the same spectrum and intensity of the sun, to the top of the home to measure how the transparent wood performs as a window material.

It was found that the transparent wooden window let in nearly as much light as glass, but at the same time was much better at eliminating glare and more evenly distributing the light. This more consistent lighting is a result of the haze and is not only easier on the eyes, but also means that the direction of the incoming light doesn't change as the sun moves across the sky, as it would with regular glass.

"This means your cat would not have to get up out of its nice patch of sunlight every few minutes and move over," says Tian Li, a postdoctoral researcher at UMD and lead author of the study. "The sunlight would stay in the same place. Also, the room would be more equally lighted at all times."

The team also discovered that the channels in the wood allow for the wavelengths of visible light to pass, while blocking most of the wavelengths that carry heat, making for thermal insulation that is superior to traditional glass windows. They arrived at these conclusions by using a photo-detector to measure the amount of light being transmitted, and by monitoring the temperature inside the home to gauge the heat passing through.

Further adding to the material's credentials is the fact that it is waterpoof, a result of its polymer content, and much less likely to break or shatter than glass. Li tells us there is no reason the approach couldn't be scaled up to create a proper-sized window, as long as the new-age glazier has a big enough container to treat the material.

"The scalability of making the transparent wood window is only limited by how large the beaker is," she explains. "If someone were to make a real wood window, they just need to get a large enough container for the process. The chemicals to remove lignin and the polymer for filling are just the same."

The research was published in the journal Advanced Energy Materials.

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