Besides being the lightest solid known to man, the aerogel has another remarkable property: it’s thermal insulation.With other words its ability to act as a barrier against heat. Aerogels are so good at this that you can put the flame of a Bunsen burner on one side of a piece of aerogel and a flower on the other and still have a flower to sniff a few minutes later. That makes aerogel to be not only the lightest solid but also the best thermal insulator ever known.
WHY AEROGEL IS THE BEST THERMAL INSULATOR?
Like for all materials, of course internal structure is responsible for any property. That’s exactly the case here too. The nano-scale pores of the aerogel are what makes it such a good thermal insulator. If you heat aerogel at red hot state does that look hot? of course It’s definitely hot. You might think that because aerogel is largely comprised of air, like 99,8% air, that has the same thermal properties as air, but that is not correct. It’s actually a better insulator than air is. The reason behind is because the width of the pores is smaller than the distance air molecules travel in average before colliding with something. They’re so called mean free path (λ).
In physics ”mean free path” is the average distance over which a moving particle (such as a photon, an atom, a ion or a molecule) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a result of one or more successive collisions with other particles.
Silica aerogel it looks odd too, placed against a dark background it looks bluish, yet if places again light background it’s almost invisible. Often called blue stuff, it is made of the same material as sand or glass, but if you zoom in into the nanoscale you’d see it has a sponge-like structure with tiny pores just tens of nanometers across. Aerogel can be to up to 99,98% air but it’s a better thermal insulator than air because those posers are so tiny that hot air struggles to diffuse through them. Plus the nanoscale structure itself is a poor conductor of heat. Hence it’s really difficult for the hot, fast-moving air molecules below the aerogel to diffuse through it and transfer heat to the top of the aerogel. This is called Knudsen Effect.
This behavior can be characterized by the The Knudsen number which is a dimensionless number defined as: Kn = λ/L, where
- λ = mean free path
- L =representative physical length scale.
The representative length scale considered, L may correspond to various physical traits of a system, but most commonly relates to a gap length over which thermal transport or mass transport occurs through a gas phase. This is the case in porous and granular materials, where the thermal transport through a gas phase depends highly on its pressure and the consequent mean free path of molecules in this phase.
It is so weird because you don’t expect something that’s transparent to block the heat that well but the aerogel really does. That is why NASA used aerogel insulation on the Sojourner rover, Spirit and Opportunity, the Curiosity Rover or on Parker Solar Probe and they plan to use it on future missions to Mars and other planets. Aerogel can successfully protect electronic equipment on board of each of these rovers and spacecrafts both against hot conditions or against cold conditions.
There is a joke running among urban architects saying that when they will build skyscrapers in Antarctica, they will use aerogel as thermal insulation. Why? Well… because then they will really care about how to make a building thermal efficient. It would be so cold there. So instead of having 3 meters thick of fiberglass insulation, you could have just 15cm or something of aerogel. 🙂
The drawback is that aerogel is pretty fragile in its basic form and so it’s not really practical for most uses. However instead what you can do is actually take tiny particles of aerogel and embed them in a composite material such as thermal protection blankets. In this way it can be largelly used.
The thermal insulation properties of aerogel can be easily tested. For comparison let’s take any commonly used material as thermal insulator, for instance something made of fiberglass in the form of a blanket of 1cm thick. On one side put a chunk of chocolate and on the other side give it heat with a flamethrower at full bore. Within seconds the temperature is hundreds of °C, and around 30 seconds the whole thing starts to collapse, and even if the flamethrower is removed parts of the chocolate are still over 600°C.
Now let’s do the same with aerogel. For this you can use a thing called Pyrogel XTE a product of Aspen Aerogel company. This is currently one of the most efficient thermal insulation material commercially available. That too is 1cm thick, which it doesn’t look like the blue stuff because it’s actually a Fiberglas blanket that’s infused with aerogel. But it has the additional component iron oxide just basically rust, that makes it opaque to infrared radiation, so it’s good at stifling conduction, convection and radiation. At full bore with the flame thrower, you can even replace the chunk of chocolate and put yourself behind the Pyrogel blanket, you won’t feel any significant heat. Propane burns at about 2000°C, you will barely feel anything. On the other side even if the material was heated at 600°C you can still touch it, you won’t get burned. In fact you don’t even need a big piece of aerogel to protect something from heat. It’s sufficient to have silica aerogel particles immersed on your protective shield like the Pyrogel blanket and that already does a good job.
AEROGEL INSULATION BENEFITS
Except of being fragile another drawback of more commercial application for aerogel is the manufacturing price. Although aerogels are the best insulators we currently have, they are very expensive and it is not clear that even now we care about energy conservation enough to value aerogels economically. Currently there are several companies already selling aerogel for such thermal insulation applications, but at the moment the main ones are for extreme environments such as drilling operations. Alright, but you now might ask Why would they insulate subsea oil pipelines with this material? That’s a good question. This was what would call the killer app for aerogel.
The oil that comes out of wells in the deep ocean is very viscous and slaggy and so if you just had a pipe with that oil it would basically gum up and so what you have to do is put another pipe around it and fill that gap with insulation. That’t called a pipe-and-pipe configuration and so they heat the oil they have to keep it flowing but you need to insulate that so that it doesn’t lose all of its heat to the cold temperatures of the surrounding ocean. So if you think about laying pipes like that from a ship you have these long segments of pipe that the ship has to basically pick it off. Put it over the side of the boat and drop it down the ocean so at some point the pipe becomes so big and so heavy that it will capsize the boat. There are really only 3 vessels on the planet that are big enough to lay that really large diameter pipe for subsoil pipelines.
Therefore the company Aspen Aerogel came along and said “hey guys we’ve got this really great new insulation”. It’s 3 times better than polyurethane so you can take this much polyurethane foam and shrink it into an insulation that’s that much aerogel. Now what it did was allowed you to shrink the outer diameter pipe of this pipe-and-pipe configuration substantially and because of that mass reduction from the smaller diameter pipe all of the sudden 250 ships around the world call that smaller diameter pipe without any loss of performance in the oil pipeline. So that resulted in alleviating years back logs of these pipe lines that need to get laid and saved billions of billions of dollars.
In the near future It’s possible that, because of environmental considerations, our energy costs will get higher and higher and in a sufficiently high-cost energy future, it is conceivable that the monolithic double glazing we are all used to may be replaced with a much more sophisticated glass material based on aerogel technology. So we have good reasons to be optimistic. Research on developing new aerogels has been taking place at an increasingly rapid pace in recent years.
However the overall potential benefits are huge. Although Aerogel is typically more expensive than other insulating materials, its enhanced thermal insulating properties with thinner layers make it uniquely suited for confined spaces. In insulation, Aerogel easily outperforms traditional fillers such as wool and fiberglass. In fact, Aerogel offers the same quality insulation with 1/3 the thickness of other insulating materials. Aerogel normally withstands temperatures up to 600 °C on constant exposure, but is not suitable for extremely high-temperature applications that permanently operate above that level. In addition, it’s extremely lightweight characteristics make it ideal for use on light and breakable components that could be damaged by the weight of more traditional insulating materials.
Using aerogel in hot applications but it works equally well at the other end of temperature spectrum at cryogenic temperatures. And this comes in handy for things like liquefied natural gas plants or by NASA when they’re using liquid helium (He). You need really good insulation to keep the heat out. I mean those cold pipes if they’re not insulated property can end up with huge ice falls on them which not only is inefficient it’s also incredibly dangerous. If you take a piece of cryogel and dip it in liquid nitrogen for a good while it is still flexible when you bring it out and that’s kind of essential when you’re working with materials that needs to function at ultracold temperatures. A carbon aerogel that has been submerged in liquid nitrogen and as that liquid nitrogen turns back into the gas state, it functions like its own air hockey puck, except instead of the air coming from the table it comes from the puck itself.
You can even buy ski jackets these days that have special aerogel lined pockets that stay significantly warmer than standard jacket pockets and they’re specially made for your cell phone so that it doesn’t freeze up in the cold weather.Although it is more expensive than other insulating materials, Aerogel makes up for its cost by the benefits it offers. Some of these include:
- Achieving greater temperature reduction with less insulation volume.
- Thinner blankets mean easier installation and removal for maintenance and repairs, thereby reducing time and expenses.
- When used as a thermal insulation material, Aerogel helps to enhance equipment energy efficiency, thereby reducing energy costs and emissions.
- Aerogel is especially valued for its extremely low thermal conductivity, which can be as low as 0.015 W m-1 K-1.
- Aerogel is extremely water repellent. Its hydrophobic nature allows Aerogel to protect insulating and operating components from moisture damage and corrosion.
- Aerogel retains its shape in high-temperature settings and does not crack, clump, or sag like other insulating materials. It withstands vibration, impacts, and frequent traffic well, and can be reused after a thorough inspection.
- Exceptional durability gives Aerogel a longer service life, which makes it an increasingly cost-effective option over time.
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