COLLEGE PARK, MD. - New developments have been made in the study of transparent wood, which first made waves in March 2016 when Swedish researchers at KTH Royal Institute of Technology discovered that they were able to remove the lignin in wood, the molecule that makes it rigid and dark in color, and replace it with epoxy.
 
Engineers at the University of Maryland demonstrated later that year that windows made of transparent wood could provide more even and consistent natural lighting and better energy efficiency than glass. This study was published in the Journal of Advanced Materials.
 

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Transparent wood could replace glass in solar cells, and buildings

A transparent wood material claimed to be suitable for mass production could replace windows and solar panels in the future, say researchers at Stockholm's KTH Royal Institute of Technology.

UMD researchers patented their process, which, similar to the Swedish study, begins with bleaching the lignin from the wood, then soaking the wood in epoxy, adding strength back in and making the wood clearer. While the Swedish study used Balsa wood in 10- by 10-centimeter pieces, the UMD team initially used tiny squares of linden wood about two by two centimeters. Researchers from both studies say there is potential for the process to be used on larger pieces of wood.

Transparent wood is also sturdier than traditional wood, researchers say, and can be used in place of less environmentally friendly materials, such as plastics. It could potentially transform architecture by enabling novel structures such as load-bearing windows. Such elements could also yield improvements in energy efficiency over glass or other traditional materials.

In Feb. 2019, a study published in the Journal of Materials Research revealed that a research team in China developed a new process for removing the wood’s lignin on larger panels. The goal of their study was to leave less lignin in the wood to leave more room for the epoxy, which would result in samples that are more optically transparent and stronger.
 
To achieve this, researchers steamed pieces of natural pine and basswood by placing them on a grid above a boiling aqueous solution of hydrogen peroxide. When the yellow color of the samples disappeared, which occurred anywhere from two to 12 hours, researchers rinsed the samples with cold water and ethanol.
 
Researchers say the steam penetrates wood samples better than bleaching solutions and leaves transparent wood composites with half the lignin content of previously made transparent wood. Succeeding with 21- by 19-centimeter samples, the study revealed that this technique retains the structural integrity of the wood by keeping the cellulose in the cell walls intact.
 
The success of this steam-modification technique is believed to be attributed to how the wood is cut. While researchers in the 2016 studies cut against the grain to ensure that the channels that drew water and nutrients up from the roots lie along the shortest dimension of the window, the China team cut across the fibers.
 
Lars Berglund of the Wallenberg Wood Science Center at KTH Royal Institute of Technology notes that when the wood is cut across the fibers, the mechanical properties are not very good, so this study may be less interesting from an application point of view. The steam-modification technique would need to be further studied using various cuts of wood to further explore the mechanical properties and scalability of the technique.
 

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This advancement in the study of transparent wood brings us closer to the possibility of more environmentally-friendly and energy-efficient windows and greenhouses, which has been the goal of prior research.

For instance, the 2016 UMD study, led by Liangbing Hu of UMD’s Department of Materials Science and Engineering and the Energy Research Center, showed that transparent wood provides better thermal insulation and lets in nearly as much light as glass, while eliminating glare and providing uniform and consistent indoor lighting.
 
The channels in the wood direct visible light straight through the material, but transparent wood still has all the cell structures that comprised the original piece of wood. Through a property called haze, the cell structures bounce around the light that hits the surface, which means light won’t shine directly into someone’s eyes when looking at a window made of this material.
 
The team’s findings were derived, in part, from tests on tiny model houses with a transparent wood panel in the ceiling that the team built. The tests showed that the light was more evenly distributed around a space with a transparent wood roof than a glass roof. According to Tian Li, the lead author of the Aug. 2016 study, the transparent wood lets through just a little bit less light than glass, but a lot less heat.
 
 
“It is very transparent, but still allows for a little bit of privacy because it is not completely see-through,” said Li. “We learned that the channels in the wood transmit light with wavelengths around the range of the wavelengths of visible light, but that it blocks the wavelengths that carry mostly heat.”
 
Additionally, during the Department of Energy’s 2017 ARPA-E Innovation Summit, Shell GameChanger hosted a pitching competition entitled “Energy Ideas to Change the World,” where Hu pitched to a panel of judges his idea for energy-efficient homes through the use of transparent wood windows as thermal insulators.
 
During his pitch, Hu noted that when the transparent wood is installed as a daylight-harvesting roof, the natural aligned wood channels inside help guide the sunlight into the house without relying on the sun’s angle. This is good news for all those indolent kitties out there who are frustrated with having to interrupt their daytime naps by constantly following the ray of sun coming through the window.  
 
"[Not relying on the sun's angle] means your cat would not have to get up out of its nice patch of sunlight every few minutes and move over," says Li. "The sunlight would stay in the same place. Also, the room would be more equally lighted at all times."
 

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