By Patricia McNeil
Heat loss is a cunning villain, and probably one of the most vastly accepted forms of wasted energy. Now energy is something you as humans take very seriously, especially electrical energy. With this you can do all sorts of useful things, like heat up your house in the winter or cool your car in the summer. Despite its usefulness, generating electrical energy has a high impact on the environment, so we all must do what we can to help save it. But heat transport, this is a hidden terror, lurking in the shadow of useful processes and lowering efficiency, wasting energy. I cannot stand by and let this happen, not when I have the power to help. And I’m hoping to make my mark- as an insulating window.
What does that mean, you ask? Well first off, I need to be transparent. If I’m not transparent I’m not a good window, plain and simple, and you as humans don’t get to enjoy the natural light, especially in places like sunny Southern California. I also need to be insulating; this means I’m good at fighting off heat transport and stopping energy loss before it infiltrates your homes. And finally, I need to be cheap and scalable; this is how I get mass produced, and I can’t help you if I can’t get to you.
Transparent. Insulating. Scalable. If I’m going to combat energy loss, I need to be all three… but these properties don’t necessarily mix very well, especially in window technology. Take your single pane windows, for example. Transparent, but not the most insulating (1.4 W/mK), and the Department of Energy estimates that you lose about 25% of your household heat energy with single pane windows in cold climates. Double panes are a little more insulating because of the air gap between the two pieces of glass. But these are a little on the bulky side and require you to completely remake your window frame, plaguing you with a costly inconvenience. The silica aerogel, though, is a porous glass material. With porosities of more than 95%, aerogels are made almost entirely of empty space, basically air, and have some of the lowest known thermal conductivities (0.01W/mK, #insolationgoals). The major issue is that their pores are big enough to act as light scattering centers, and they end up with this hazy white color you can’t see through; so, they can’t serve as a window. I included the following family portrait to give you a better idea of what I mean (the red arrows are our personal struggles with heat transport):
As you can see, I’m clearly the most awesome one here.
I just realized, I never told you my name- hi! I’m Ambigel. In many ways I’m a lot like Aerogel in that I’m a porous material, 90% porosity to be exact. (And I don’t want to brag… but yeah, my current TC record is 0.02W/mK.) But there are a few key differences between me and my predecessor, but in order to explain myself you need a little background on the silica sol-gel family.
In many ways, we are very similar to the polymer family. You take your precursor, TEOS (tetra-ethyl-chlorosilane) for example, and you mix it with some other chemicals and catalysts to help the TEOS to form Si-O chains and cross links:
This structure grows into little particles (sols) which then aggregate to form a porous gel-like network (hence the term sol-gel) filled with the remaining liquid.
Once you have a gel, there are a few different drying methods that will affect the porosity of the gel. If you allow the gel to dry as made, there will be a large surface tension which pulls in along the curvature of the pores, causing most of the pores to collapse. This provides more pathways for heat transport; energy loss strikes again. Aerogel is made from supercritical drying, meaning you raise the temperature and pressure of the drying solvent (usually CO2) to eliminate the surface tension during evaporation. However, since supercritical drying is done in a high-pressure chamber (picture given below for your edification), your ability to mass produce large aerogels is limited. As awesome cutting-edge technology, scalability is truly the bane of our existence. I am made using a non-polar drying solvent, which still has significant surface tension but allows me to dry at room temperature and pressure without too much pore collapse and theoretically no scaling problems.
Pretty cool, right? However, this alone can’t get me up to 90% porosity. Luckily, over the years, people have developed all sorts of ways to control this process so that I could have a particular microstructure (a fancy word for how I look on the micrometer scale). This means, my engineer can specifically design me with the tools I need to effectively combat energy loss while still looking good. There was some trial and error involved, but what my engineer decided was coating me with a surface modifying agent, which provides a springy force against capillary pressure and prevents pore collapse without my pores being too big. Altogether, I’m an ambiently dried highly porous structure, a transparent insulator, great for combating heat loss and excited to take my place as an energy saving window.
Except, remember when I implied I had no scaling problems? Well, I lied. See, in theory there shouldn’t be a problem, but when you scale things, cost becomes a major consideration. Your typical single pane window will run around $3 per square foot whereas, I cost about $9 per square foot. Not a big deal to my engineer because this is still the same order of magnitude, but to you, the consumer, this is three times higher! So that means a window that originally cost $30 to manufacture now costs $90!!! This is especially terrible especially for people who need me most. I was meant to be an affordable and easy solution; a better and cheaper alternative to retrofitting your house for costly double pane windows. But if only a few people will be able to afford me, my usefulness as an energy saving product goes down. I’m not okay with this; I can’t let heat loss win, so my engineer and I are going to have to work on a solution.
I want to be helpful. From what I know of human culture, energy generation and conservation is a big deal. You need to convert energy into useful work to function in the modern world and any amount of energy that is wasted translates into a larger impact on the environment. But I can help, at least in my own little niche application. I just need to trim down the cost first.
Patricia is part of the 2018-2019 INFEWS program cohort and a PhD Student in the Department of Materials Science and Engineering at UCLA. Her research focuses on creating an insulating silica ambigel with the optical transparency required for application onto a window. See her article about her 2020 internship in Japan in our News section.
This is a pop science article produced in the Science Communication course. The blog is part of the INFEWS Social Media Series.