Thursday, 15 February 2018


The gift material that keeps on giving

Stronger than steel? Transparent? Carbon sequestering? Positive embodied energy?  Remediative waste stream?  Sounds like a material from Marvel Comics.  But it's very likely all true.

I have written specifically about modified timber before.  In Designer materials: Helping nature? I summarized the history of treated timber, culminating in acetylated wood modification. That process protects wood from rot by making it "inedible" to most micro-organisms and fungi, without making it toxic. It also greatly reduces the wood's tendency to swell and shrink, making it less prone to cracking and ensuring that it requires dramatically reduced maintenance.  But the most surprising sustainability bonus of the product is that one of the waste products of the acetylation process is acetic acid, which is a valuable feedstock in other industries.  You can see where this is going.......

Engineered timbers are a whole other field of radical advances, including Glue Laminated Timber (glulam), Laminated Veneer Lumber (LVL) and at least another dozen products which allow designers to consider how they may substitute a renewable resource for other structural systems. But what would you be able to do if the timber itself were stronger than steel? 

That is now a fair question.  Judging by the announcement from University of Maryland, where scientists have demonstrated a wood densification technique, described in Nature, which has led to the creation of a material that is 12 times stronger than natural wood, as well as 10 times tougher.
According to Dr Liangbing Hu the timber material could be a competitor to steel or even titanium alloys, and could be used in cars, airplanes, buildings – any application where steel is used. “It’s also comparable to carbon fibre, but much less expensive.”
Earlier, Swedish researchers had already demonstrated a related technique for removing lignin from wood, to produce a a transparent material which they say could be used as windows, facade elements and even in solar panels.

“When the lignin is removed, the wood becomes beautifully white,” Professor Berglund said. “But because wood isn’t not naturally transparent, we achieve that effect with some nanoscale tailoring.”
This is done by impregnating the white porous veneer substrate with a transparent polymer.  The wood sample had a transmittance up to 85 per cent – comparable to glass.  A haze of 71 per cent is claimed to make the material attractive for solar cell applications, as light would be “trapped in the solar cell for longer”.

The researchers suggest that the modified wood could also be used for semitransparent facades, where both light and privacy are needed.  For these applications, the material "offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity.”  One on the note: given recent bad experiences with flammable facade materials, it is curious that no mention is made of flammability.

The University of Maryland group has also produced transparent – or more properly, translucent timber sheeting.  They report that their transparent wood provides better thermal insulation than glass and lets in almost as much light at glass, though without any glare – providing more uniform and consistent indoor lighting.  But to me, the most exciting news if it's true is the following claim.  Lead author Tian Li reports:
“We also 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.”
Think about it. The reason why glass has been such an almost mystical material is that it lets in short infrared (the heat part of the solar spectrum), but is effectively opaque to long infrared (the heat would normally perceive at earthly temperatures). That is the original 'glasshouse effect' so useful for passive solar heating.

But there has always been a price to pay, where the same effect is the major cause of overheating in summer.  Transparent wood seems to have almost the opposite property of keeping the thermal loads down, while providing lots of daylight. This would be a boon any overheated climate.

The trigger for this post came from three articles in The Fifth Estate:
Transparent wood: the future of windows and solar panels? 
Transparent wood trumps glass on energy efficiency and light.
See ya steel: scientists create wonder material from wood
The research has been published in Advanced Energy Materials.


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