The Wonderful World of Aerogels

Congratulations! You are now the proud owner of an aerogel—an amazing example of what chemistry and materials science can do! Learn all about aerogels and their incredible properties below.

What is aerogel?

Aerogels are the world’s lightest solid materials. Now before you get all Big Bang Theory on us, that doesn’t mean every aerogel is the world’s lightest solid. Aerogels are a diverse class of materials, and come in a range of densities, compositions, sizes, and properties. The aerogel you have is what we here at Aerogel Technologies call a Classic Silica™ aerogel and is an incredible 96% air by volume, the rest being made out of the same stuff glass is made of—silicon dioxide (SiO₂). But just like plastics, ceramics, and metals, there are lots of different types of aerogels, which you can find out more about below.

But wait, we didn’t really answer what an aerogel is now, did we?

Essentially, an aerogel is a nanoporous sponge—structured very much like a kitchen sponge, except for with pores that are literally a million times smaller than in a sponge. We say that aerogels are a form of nanotechnology because they are riddled with zillions of nanometer-scale nooks and crannies, most of which are about 10 nm across (only about 100 atoms wide!). For perspective, that’s about 10,000 times smaller than the diameter of a human hair. Accordingly, the branch-like struts that make up the solid part of the aerogel are also nanometer-sized.

So what’s the big deal about aerogels? Read on!

Special properties of aerogels

Because the pores and struts that make up an aerogel are so small, lots of weird physics happens inside an aerogel—weird physics that gives aerogels apparent superpowers, or to be technical, extreme materials properties. And which “superpowers” an aerogel has depends on what the aerogel is made of and its density.

Superinsulation

The best known superpower possessed by aerogel materials is their superior thermal insulating abilities. In fact, the world’s best insulating solids are all aerogels, which usually means low-density silica aerogels or polymer aerogels. In fact, an inch-thick (2.5-cm-thick) aerogel tile can have the same insulating ability of 15 panes of windowglass, or the equivalent of three inches (7.5 cm) of Styrofoam. And as gorgeous as aerogel tiles may be, for real-world applications flexible fiber-reinforced superinsulating aerogel blankets such as Cabot’s Thermal Wrap and Aspen Aerogels’ Spaceloft are typically used instead and today insulate subsea oil pipelines, energy-efficient buildings, and winter apparel around the world.

High-Temperature Resistance

Some aerogels, including silica, are not only thermally superinsulating but are superinsulating at high temperatures. This property has been used not only to save things such as flowers, crayons, and Hershey’s Kisses^® (see below) from the inferno of a blowtorch…

…but also save a scientist from the blast of the flamethrower from Terminator 2 (see below)!

That’s flame temperatures of up to 1700°F(>700°C)! Now, a blowtorch can easily the melting point of most aerogels if it’s turned up all the way (as seen at the end of the first video above), but never fear! Aerogel Tech has an experimental aerogel material called Galacticlad™ that can survive temperatures up to 4800°F (>2700°C)—that’s 50% hotter than Space Shuttle tile!

Ultralow Density

Something all aerogels have in common is extremely low density, typically ranging from ~0.6 grams per cubic centimeter to as low as 0.00016 g/cc (yup, that’s 0.16 milligrams per cubic centimeter)! That lower value represents the world record for the world’s lightest solid material, which is a formulation of aerogel made of graphene. For comparison, the density of water is 1 g/cc where most plastics are about 1.2 g/cc and air is about 0.00123 g/cc.

Graphene aerogel with a density of 0.16 mg/cc

Wait a second aerogel gurus… how is it possible that a solid that is filled with air can weigh less than air?

Okay you got us. That number is the density of the aerogel structure minus air, meaning the actual density of that ultralight graphene aerogel is 0.16 mg/cc plus the density of air, which is about 0.00139 g/cc (1.39 mg/cc). But if you sucked all of the air out or replaced it with helium, in fact an aerogel that light could float in air—until air diffuses back into its pores.

We know what you’re thinking now. Could one somehow seal the outside of an evacuated graphene aerogel and make a flying device of some sort? Not really. Aside from it being very difficult to make ultralight aerogels and having generally low mechanical strength, if you do the math, adding a sealant to the outside of the aerogel would add so much weight that you really wouldn’t have much of a floating object anymore. But maybe. Prove us wrong!

Ultrahigh Strength-to-Weight Ratio

One remarkable property of aerogels is their incredible high strength-to-weight ratios. Most aerogels can hold thousands of times their weight in applied force. Unfortunately, the blue silica aerogels of NASA lore tend to have low fracture toughness, meaning that even though they can hold thousands of times their weight, they tend to fracture easily. As a result, pinching or poking can break Classic Silica™ aerogels very easily.

A 2-g silica aerogel block supports the weight of a 2-kg brick

But never fear! A new class of mechanically strong aerogels called Airloys^® have recently become available and exhibit the ultralow density and superior thermal insulating properties inherent to aerogels while being strong, stiff, tough, and machinable at the same time! And not only are Airloys more robust than classic aerogels, but they can hold hundreds of thousands of times their weight in compression—lots more than even ordinary aerogels!

Acoustic Superinsulation

Besides keeping things toasty (or cool, depending what you’re trying to do), aerogels are excellent acoustic dampers—that is, they work as phenomenal sound proofing. Airloy aerogels, for example, are 10 to 1000 times better sound insulation than even polyurethane foam.

Ultrahigh Surface Area

A typical piece of aerogel (say a silica aerogel or a carbon aerogel) has about 700 square meters of surface area per gram—that means an aerogel the size of ice cube has wrapped up inside all its nooks and crannies about half a football field’s worth of surface area!

What’s especially cool is that scientists can put stuff on all that surface area to make aerogels do bizarre things. One example is by reacting waterproofing agents with the skeletal surfaces of the aerogel to make waterproof, or hydrophobic, aerogels. In fact, with hydrophobic aerogel particles, you can even make yourself waterproof (don’t worry, it’s just temporary).

And if an aerogel is made of an electrically conductive substance, like carbon aerogels are, you now have an awesome electrode material for supercapacitors, batteries, and desalination filters!

Where did aerogels come from?

Believe it or not, aerogels were first invented some time around 1929 by Prof. Samuel Stephens Kistler at the College of the Pacific. They were first described in the journal Nature in 1931. In fact, powdered silica aerogel was first commercialized by Monsanto (yes, that Monsanto) in the 1940′s and was used even used as insulation in a line of freezers, although was discontinued in the 1970′s when foams like Styrofoam first became commercially viable. But with the 1970′s also came the dawn of the nanotechnology age, and scientists began to appreciate aerogels as the nanostructured, nanoporous marvels they are. Throughout the 1980′s, more and more scientists around the world became interested, and significant research efforts led by Lawrence Livermore National Laboratory, Lawrence Berkeley National Laboratory, and NASA helped push the frontiers of what aerogels could do.

Today there are tens of thousands of scientific papers about aerogels, numerous books, and three major commercial manufacturers, Aspen Aerogels, Cabot Aerogel, and Aerogel Technologies.

Some people have speculated that aerogels are actually some form of alien technology that was reverse engineered by scientists after Roswell. On episode #207 of Brad Meltzer’s Decoded, “UFO”, for example, Brad tries to connect aerogels to alien technology by noting that Bob Lazar (of Area 51 fame) has a company that has sold aerogel in the past, and notes “… the people that claim to have had contact with aliens, also keep suddenly coming up with materials and technologies that the Earth has never seen [like aerogels].” Brad you had us up until aerogel, because we happen to know where United Nuclear’s aerogel comes from, and trust us, it isn’t an alien spaceship.

What are aerogels used for?

Aerogels of various types are used in many applications today. Here are some applications of different aerogel materials:

Aerogel Composite Blankets

Insulating subsea oil pipelines
Building insulation, especially where space is limited
Light-transmissive insulating fabric roofing

Hydrophobic Silica Aerogel Particles

Energy-efficient skylights
Safety coatings that prevent burns when contacting hot steam pipes
Matting agents for paints and cosmetics

Monolithic Silica Aerogel Panels

Chemical sensors
Cherenkov particle detectors in particle acccelerators including the Large Hadron Collider
Visual arts and sculpture
Insulating electronics on spacecraft including the Mars Exploration Rovers (MER)
Capturing comet dust (Stardust mission)

Carbon Aerogels

Electrodes for supercapacitors
Electrodes for batteries
Electrodes for desalination systems
Substrates for growing carbon nanotubes

Airloy Strong Aerogels

Ultralight plastics replacements for tablets and smartphones
Ultralight panels for aircraft and refrigerated trucks
Bulletproof vests

How are aerogels made?

Aerogels start their lives out as gels, physical similar in consistency to edible gelatin. Like how Michelangelo’s David was already in the marble and all Michelangelo had to do was reveal him, the material that will be an aerogel is already inside a gel, it just needs to be isolated. You see, gels have two components—a nanoporous, spongelike solid framework that gives the gel its solid-like cohesiveness, and a liquid that permeates the pores of that framework. The material that will be the aerogel is that first part, we just have to replace the liquid in its pores with air. But we need to be careful about how we do that, because if we just evaporate the liquid out of the gel, the gel’s solid skeleton will collapse in on itself resulting in a dense solid that is not an aerogel. This is due to capillary stresses that evolve when liquid evaporates from the nanosized pores of the gels that cause the struts of the gel’s skeleton to be drawn into each other. These struts are lined with sticky chemical groups such as hydroxyl groups (-OH) that then stick to each other, causing the struts to stay stuck to each other after collapsing which results in a densified solid. To circumvent this, a process called supercritical drying is typically used (although there are other methods). In this process, the liquid in the gel is heated and pressurized past a characteristic temperature and pressure specific to the liquid in the gel’s pores, that is, its critical point. At these conditions, the liquid transforms into a semi-liquid/semi-gas called a supercritical fluid that can gently diffuse out of the pores of the gel like a gas and be depressurized without causing capillary stress. Upon reaching ambient conditions, the dry, ultralow-density, nanoporous solid skeleton of the gel remains isolated from its liquid component. This material is aerogel.

To make supercritical drying easier, the liquid in the gel can be exchanged for liquid carbon dioxide prior to supercritical drying. This is because most liquids, such as alcohols, have critical points of several hundred degrees and nearly 100 atmospheres of pressure, and are dangerous flammable and explosive at those conditions. Carbon dioxide, on the other hand, has a critical point of only 87.7°F (31.1°C) at 1071 psi (73.4 atm) and is non-flammable, making it easier and safer to supercritically dry aerogels with.

But supercritical drying isn’t the only way to make an aerogel. Some gels, particularly if they are treated so that their solid skeletons are hydrophobic (water-repelling), can be dried by soaking in a series of solvents until the liquid in the gel is nearly 100% a solvent with a very low surface tension, such as pentane, hexane, or heptane. The solvent can then be slowly dried evaporatively or under vacuum with gentle heating. During this process, the gel’s skeleton will shrink a bit, however because its surface has been rendered hydrophobic, its struts cannot stick to each other and the skeleton can “spring back” once dried, affording an aerogel.

Information about your aerogel

Now that you’ve read about aerogels, how they’re made, and all the amazing things they can do, here are some specifications about your aerogel:

Density: 0.1 g/cc (~96%)
Composition: Hydrophilic (water-absorbing) silica (SiO₂)
Surface Area: ~700 m²/g
Skeletal Density: 1.9 g/cc
Drying Method: Supercritical drying from CO₂
Manufactured By: Aerogel Technologies, LLC
Place of Manufacture: Boston, Massachusetts, United States of America

How to handle your aerogel

Rule number one, don’t pinch or poke your aerogel! While Classic Silica™ aerogels can in principle hold thousands of times their weight in compression, they are very brittle and will easily fracture with even gentle pressure applied from your fingers.
Wipe any moisture off your hands.
Open the plastic sample box and gently dump the aerogel into your hand.
To pass the aerogel to someone else, dump it from your hand into their hand.
Place the aerogel back in its sample box when done.

Light magic with your aerogel

One of the most fascinating aspects of silica aerogels is their amazing optical properties. Here are some cool things to try:

Light beams. Silica aerogels scatter light like fog or smoke (the Tyndall effect). Try shining a laser pointer, LED keychain, smartphone flash, LCD projector, or a flashlight into the aerogel and visualize the light beams in their full photonic glory!
A piece of the sky. Place the aerogel on a white background and the aerogel will virtually disappear. Place the aerogel on a black background and it will appear to be a stunning blue. This effect is due to a phenomenon called Rayleigh scattering. Light passing through the aerogel scatters off the nanosized particles that make up the aerogel’s structure, and the short wavelengths of light (blue and violet) scatter more easily than the longer wavelengths (like red, orange, yellow, and green). This is the same reason the sky is blue—light from the sun scatters off of microscopic dust particles in the upper atmosphere, and blue and violet light are scattered more easily. So you can say aerogels are truly sky blue. Since a white background reflects all colors of light and a black background absorbs all colors, the blue/violet scattering off the aerogel is more appreciable on a black background since blue and violet wavelengths are not drowned out by blue and violet reflecting off the background, giving your aerogel its magic color-shifting properties.
Silica Aerogel on Black Background

Silica Aerogel on White Background
Portable sunset. Hold your aerogel up to the sun (an incandescent light will do as well). Notice the aerogel is now yellow! This is due to Mie scattering—since the aerogel scatters blue and violet light, the rest of the light passing through appears yellow to the eye. Think about an LCD screen, which is made up of tiny red, green, and blue color elements. Turning on all of the red and green elements, but not the blue, will make the screen appear yellow to the eye. Same thing with aerogels—filter out the blue and the remaining light appears an lovely orange-yellow hue.
Aerogel Sculpture exhibiting Mie scattering by Ioannis Michalou(di)s. Copyright © Ioannis Michalou(di)s.

Learn more!

There’s lots more information about aerogels available on Aerogel.org, an open-source nanotechnology initiative. Here are some resources on Aerogel.org to get you started!

Not just for NASA anymore

Thanks for your interest in aerogels. We welcome you to the wonderful world of aerogels and remind you that aerogel’s are not just for NASA anymore—log on and get involved!