COLLEGE PARK -- Need light but want privacy? A new type of wood that’s transparent, tough, and beautiful could be the solution. This nature-inspired building material allows light to come through (at about 80%) to fill the room but the material itself is naturally hazy (93%), preventing others from seeing inside.
Materials engineers at the University of Maryland have transformed wood into a transparent building material that directs light for a diffused effect, is tougher and insulates better than glass, and has a natural wood-grain pattern. They published their results last week in the journal Nature Communications.
"In this patented research, we demonstrate the first aesthetic wood with patterns following the density variation in natural wood. Such patterned, transparent wood can also block UV and heat, is mechanically strong, which could find many applications in buildings where sustainability and energy efficiency are desired," said Liangbing Hu, Herbert Rabin Distinguished Professor and Director of the Center for Materials Innovation at the University of Maryland, College Park.
Tiny nanostructures made of cells pass water and nutrients up and down the tree, but they hold importance here as vertically aligned channels in the wood, a naturally-grown structure that can be used to pass light along.
The team started with a small block of Douglas fir and used chemicals to selectively remove its lignin, the part that makes wood brown and strong. This makes the wood transparent to light. They left behind all the other cell structures in the wood, which provide a natural pattern.
The remaining wood cell structures also direct light straight through the plank. The wood is cut against the grain, so that the channels that drew water and nutrients up from the roots lie along the shortest dimension. The cell structure that still remains bounces the light around just a little bit, a property called haze, but in general the light travels directly through microchannels in the wood. At no matter what angle the sun strikes, the light travels perpendicularly through the wood and consistently falls on the same diffuse area all day.
The researchers simulated a room with a ceiling made of the new 'aesthetic wood', as the researchers have dubbed it, and showed that it insulates better than glass. Aesthetic wood inherits characteristics from wood and epoxy, another major ingredient in the process. This also allows it to be tougher than glass. Aesthetic wood proved to be two and a half times more effective at blocking heat than glass, and three and a half times as tough as untreated wood, or almost 15 times as tough if the wood is sliced with the grain. It also effectively blocks UV light, but allows visible light through.
The researchers liked the pattern made by the lighter brown early wood, so named because it is produced early in the growth phase of the wood, and darker latewood. Early wood has less lignin because it has less material altogether, with cell walls about a quarter as thick. The researchers also tried basswood and pine, which can be used physically similarly and have lighter wood grain patterns.
Inventwood, a spin off company from UMD, is commercializing this type of transparent wood technology.
The team photographed the wood’s cell structure in UMD’s Advanced Imaging and Microscopy (AIM) Lab. Researchers at the Swiss university ETH Zurich, and EPMA, Switzerland’s federal laboratory for materials science, conducted Raman spectroscopy of the wood to determine its chemical makeup. The co-first authors of the research are Chaoji Chen, a postdoctoral scholar in the department of Materials Science and Engineering at the University of Maryland, and Ruiyu Mi, a visiting graduate student at China University of Geosciences in Beijing.
This is just the latest development in Hu’s research on the macro and nanoscale capabilities of wood: His group has created a battery made of wood, a transparent wood that is the precursor to the research above, super strong wood that would replace steel, a bio-compatible battery made of grass, a cooling wood and many more.
Scalable aesthetic transparent wood for energy efficient buildings
Mi and Chen et al, Nature Communications, 2020
Note to Editors: Pictures available at https://drive.google.com/drive/folders/1tUlQXwRDnD-WwVFjFMtLbmpiNiRhbALp?usp=sharing
August 5, 2020