All Cleared Up

As clear as wood. Photo courtesy of Liangbing Hu.

A team of engineers at the University of Maryland has created transparent wood windows that are waterproof and less breakable than glass. What’s more, the windows provide more even and consistent natural lighting and better insulating properties than glass.

According to Professor Liangbing Hu and postdoctoral researcher Tian Li, who published their research in the journal Advanced Energy Materials, the development of transparent wood takes advantage of the vertically aligned channels that trees use to draw water and nutrients up from their roots. These channels help guide sunlight through the wood windows.

To make transparent wooden windows, the researchers cut a thin slice of wood against the grain so the channels lie along the shortest dimension of the window. They then put the wood slice in a chemical bath to bleach out the lignin, the component in wood that makes it both brown and strong. The wood is then soaked in epoxy to fill in the channels, which adds strength and makes the wood clearer.

“The result is a window with unique optical properties,” said Li.

Visible light is transmitted through glass and transparent wood at nearly the same rate, though ultraviolet light does not travel through wooden windows nearly as well. But because light bounces around the cell structure of the wood as it travels through the channels, the light doesn’t shine directly into your eyes, making transparent-wood windows more comfortable to look at.

“Glass windows produce shadows inside the room, especially at sunrise and sunset, but with transparent wood, the channels function to guide sunlight into the house, providing uniform lighting throughout the day,” Li said.

To determine the usefulness of transparent wood as a building material, the researchers immersed it in water for three days and found that its mechanical and optical properties did not change. “So you won’t have to worry about rainy days,” Li said. They also conducted tests of its strength and found it very similar to that of wood in board form. And unlike glass, which is brittle and can shatter easily, transparent wood does not.

Li and Hu also found that transparent-wood windows are far superior to glass ones in their energy efficiency. “Glass is not a good thermal insulating material,” Li said, “but transparent wood has intrinsically low thermal conductivity, just like wood does, so it keeps your house at a more constant temperature.”

The only downside to the wooden windows, according to Li, is that the epoxy they contain absorbs ultraviolet light and eventually turns yellow.

The researchers used basswood for their studies, but they have made transparent wood from oak and other species, as well. They claim that transparent windows could be made from any species of tree, and that transparency can be maintained up to a thickness of at least 1.4 centimeters, thicker than typical window glass. Their findings were derived, in part, from tests on a small model house they constructed with a transparent-wood panel in the ceiling.

Their next step is to increase the size of the wood windows they can create and to reduce the cost of production. The researchers are also seeking industry partners to commercialize the product.

 
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