We all love the natural light and the views that windows provide. This I declare from the living-room chair in which I often watch the Moon rise among the trees through the picture window facing East. But we wish they wouldn’t allow so much heat to escape outward in the winter and inward in the summer. They can be a weak link in building and home design for that reason.
But what if we had a solid material that was not only a terrific insulator but was also highly transparent, very light, and completely water-resistant? Then we could place that material between two glass windowpanes and allow ourselves much more natural light without having to fret so much about insulation. We could stay comfortable any time of year, yet still benefit from our outdoor environment without wasting energy.
Well, thanks to some innovative thinkers at the University of Colorado, we do have such a material now. They’ve developed an insulating aerogel that checks all those boxes, and even better, it’s also sustainable because it’s made from wood cellulose. You can read their open-access article describing this refreshing new technology in the March 16 issue of Nature Energy.
First let’s check out a couple of key images that illustrate what this material (called SiCellA) looks like and how well it works.
Here’s a 1.5-mm-thick panel of it, adhered onto a clear plastic film. These SiCellA films tend to stick to glass or plastic like a staticky decal, so this is easy to set up:
Now let’s take a thin film like that and laminate it onto a single-pane window at the University of Colorado and see how it does in wintertime! Here we’re using thermal imaging to estimate the surface temperature across the building:
The surface temperature of the treated window is visibly a good bit colder, meaning less heat is escaping through the pane. That film is only 1.5 mm thick, and look how nicely it’s doing!
How does the SiCellA material compare to, say, a double-paned window, where we use a layer of air to insulate? We can use R-value as a measure of that (ft2·°F·h/BTU·in, for those scoring at home), and we find that glass itself gets about a 0.14, a double-pane window gets about a 2, fiberglass insulation gets about 2 to 4, and this material scores between 5 and 9. That’s right up there with polyurethane rigid panel insulation, which scores a 7 or 8 and is considered top-shelf — except, of course, you’re not putting that in your window. People have made other aerogel insulators, too, with similar high scores, but they’re not transparent.
So how do you turn wood or other plant material into a clear aerogel? You start with cellulose:
Cellulose is the main component of plant cell walls. It’s the bulk of the really fibrous stuff you can rip off a celery stalk, for example. It’s also what makes up about 90% of cotton fiber. It’s very strong, not only because the glucose molecules are in long chains, but because the chains themselves are attracted to each other and line up, making something that’s pretty crystalline and tough. When an OH is sticking out, its H will have a partial positive charge, and that will be attracted a little bit to other O’s, which tend to have a partial negative charge. That’s called a hydrogen bond, and you can see that lots of them are able to form within and between cellulose chains, making the chains harder to separate:
This also makes cellulose opaque, and we definitely don’t want that for our windows. But cellulose is tough, cheap, and sustainable, making it a great starting material.
We can use a catalyst called TEMPO that oxidizes all the —CH2OH groups you see in the single-chain figure above into —COOH (organic acid) groups. Those —COOH groups are acidic, which means they like to lose their H’s, leaving behind negatively-charged —COO- groups, and these repel each other. So now we’ll have cellulose that doesn’t form these tight chains anymore but instead expands and gets very loose. Let’s do that reaction in water and see what our cellulose starts looking like:
All right, now we’re getting somewhere! That’s looking much clearer. We can pour this diffuse networkey stuff into a mold of whatever shape we want, and now it’s time to turn it into a gel. A —COOH is a pretty weak acid, so if we add a stronger acid like HCl, the —COO- groups will get their H’s back. Those —COOH’s will be able to hydrogen-bond with other chains again, but now the chains are pretty far apart, so we’ll end up with a loose network, a sort of cotton-flavored Jell-O.
We immerse that Jell-O in a solvent that we’ll easily be able to evaporate and recycle later, like ethanol. The ethanol replaces all the water, and now when we go to dry this Jell-O, the ethanol evaporates from our mold, and we’re left with air in place of all the ethanol. The loose cellulose network holds up, though, and now we have this ultralight and still-transparent solid material. We’re almost there!
One last thing we have to do is make it waterproof. As it is, it’ll drink up water pretty readily. We can do that by treating just the surface with a silanizing agent, adding mini-Teflon-like tails to all the exposed —COOH and —OH groups:
Now let’s see how our material behaves before and after that treatment. The unmodified stuff crinkles up and collapses when a drop of water is added, but the modified material holds up great, with nice, tight droplets beading up on the surface:
Now our material is ready to go! Is it a fragile and crumbly thing we have to walk on eggshells with? You might think so, because some aerogels are indeed like that, but nope. We can just roll up the sheets and take them wherever they need to go:
SiCellA sheets are good up to 200°C (392°F) without notable degradation, so unless you live on Venus, they’re going to last a long time in your window. Standard humidity and UV exposure tests that are used to vet window products showed no loss of clarity or insulation.
SiCellA works a lot better than air as an insulator because air molecules get trapped in little pockets and collide with the fibers more often than they collide with each other, slowing the transfer of heat. It did great when it was used instead of air between panes in a double-paned window, reducing light transmission by less than 1%, and even at high humidity there wasn’t any condensation between the panes.
Now, of course, the key is to manufacture this type of material at low cost, but there’s nothing inherent in the process that would seem to preclude that. It’s just a matter of time now.
As for me, I’m ready to stick a sheet onto the picture window in my living room right now and watch the Moon rise in 99.5% of all its glory.
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