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Could transparent wood soon bring more light into our homes?

Posted by Alex Barrett on 9 December 2016 at 3:15 pm

Transparent wood could soon provide a new building material which will allow us to brighten up our homes by making wooden parts of the structure translucent. It could have applications for building design and as a transparent coating for solar cells.

Wood remains one of the most commonly used building materials, and when grown sustainably its production can have a reasonably low impact on the environment. Now new innovations in biological engineering could change the way in which we use this material. Researchers from the KTH Royal Institute of Technology in Stockholm have created prototype transparent wood with an optical transmittance of 85%.

Getting the refraction right

So how do you make transparent wood? And how sustainable is it?

Wood is primarily composed of Cellulose, hemicellulose and lignin. It is the lignin that provides the majority of the brown colour we are familiar with. Striping this out produces wood with a very white colour, but it isn’t transparent. This is because of the refraction of light as it passes through the cellulose structure and the air spaces within.

Refraction is the process whereby light changes direction slightly while moving from one material to another. This is controlled by a property called the refractive index. If the refractive indices of two materials are similar then light passing between them won’t change direction very much at all. It will pass through in a straight line, and come out the other side. This results in transparency, when we look through we can clearly see what is on the other side, because the path the light takes is largely uninterrupted. Glass is very good in this respect. There isn’t much refraction when light moves between glass and air, and the refractive index is constant through a piece of glass, so there is little internal scattering.

Our piece of wood is very different. It is made up of lots of small cells of cellulose, with air spaces in between. Light is constantly moving between these two materials, and their refractive indices are very different. The result is that light changes direction whenever it moved from the cell wall to the air space, and so is scattered in all directions as it tries to pass through the wood. It doesn’t make it out on the other side, and so we can’t see through the wood. Instead it gets bounced around inside the wood, which ends up looking white as a result.  

Air and cellulose have very different refractive indices, so the researchers had to fill the air spaces with a different material. They added a type of acrylic called poly(methyl methacrylate). This can be made to have a very similar refractive index to the cellulose and hemicellulose making up the wood structure. The result is that it regulates the amount of light scattering that occurs within the wood.

Extra light with privacy

The result is a transparent material, but one which has a reasonably high amount of haze. This means that it might not be the best candidate for windows, but the team behind the development argue that there are circumstances where haziness could be an advantage. It would be possible to build structures with large sections of transparent wood in order to let in light, but this would not impact on privacy due to the haziness of the material.

It also makes it a good candidate for use in solar cells. The team state that “Light will be trapped in the solar cell for longer time due to light scattering caused by the wood tissue. Longer trapping time means better interaction between light and active medium, which can lead to better solar cell efficiency.”

Making transparent wood is clearly more energy intensive than conventional wood production. After all it does require changing the wood on a fairly fundamental level, both to remove the lignin and to add additional polymers. Nonetheless the researchers are confident that their technology will have benefits for sustainable engineering. They point out that unlike most nanomaterials wood is “synthesized at ambient temperature and pressure, by the plant itself, using carbon dioxide and water.”

They hope that it will provide a good material for “lightweight and low-cost structures in light transmitting buildings” and its potential for use in solar cells make it an environmentally friendly material.

The research was published in the open access journal biomacromolecules

References

Li, Y., Fu, Q., Yu, S., Yan, M., & Berglund, L. (2016). Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance. Biomacromolecules, acs.biomac.6b00145. http://doi.org/10.1021/acs.biomac.6b00145

Image credit: KTH Royal Institute of Technology

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