Aerogels are a diverse class of ultralow density solids that combine multiple disparate and extreme materials properties into a single material envelope. Aerogel materials generally exhibit a high degree of porosity, high specific surface area, and superlative energy damping (thermal, acoustic, and impact) properties. The term aerogel refers to the sparse, porous solid backbone of a gel isolated from the liquid component of the gel and similar porous solid materials with mesoporosity, that is, primarily containing pores ranging from about 2-50 nm. The name aerogel may be misleading at first, as aerogels are dry, rigid or elastic foam-like materials and are not wet or wobbly. The name originates from the fact that aerogels are typically made by replacing the liquid component of a gel—think something physically similar in consistency to edible gelatin—with a gas or a vacuum in a way that preserves the structural integrity of gel’s sparse solid, porous backbone.
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Historically, the most commonly researched and commercially available type of aerogel has been the holographic-looking “blue” silica type. Today, aerogels of numerous different substances can be prepared, including:
Each of these different types of aerogels provides unique properties, which can include electrical conductivity (carbon and metal aerogels), extreme (up to 80%) elastic return (nanotube and graphene aerogels), catalytic functions (various oxide and metal aerogels), photoluminescence (quantum dot and metal chalcogenide aerogels), water repulsion and oil sorption (hydrophobic silica and polymer aerogels), and more.
The insulating ability (or thermal conductivity) of an aerogel material depends on its composition, form factor, and density, as well as the temperature of its environment. Silica aerogel-based materials are typically used for insulating applications, although Airloy® and other polymer aerogels such as BASF’s Slentite® are beginning to be used as well. For a typical silica aerogel monolith with a density of ~100 mg/cc, the thermal conductivity is usually between 10-20 mW/m-K, or about 2-3 times more insulating per unit thickness than polyurethane foam (PUF) or Styrofoam® (expanded polystyrene), which have thermal conductivities of 29-35 mW/m-K typically. Composite aerogel blankets such as Aspen Aerogels’ Spaceloft® typically have a thermal conductivity of ~14-21 mW/m-K. Cabot Aerogel’s Lumira® aerogel particles used for daylighting applications similarly have a thermal conductivity of ~9-12 mW/m-K. Cabot Aerogel’s Thermal Wrap™ blankets, used in construction, daylighting, and low-dust insulating applications, has a thermal conductivity of ~23 mW/m-K. In general, the thermal conductivity of an aerogel decreases (that is, its insulating ability increases) as its density decreases. While aerogels are available in a wide range of densities, from as low as 0.001 g/cc up to ~0.55 g/cc, in general only materials with densities in the range of 0.06-0.55 are practical for industrial applications. Thermal conductivity of non-silica aerogels depends on composition and density, with some materials being equal to or better than silica aerogels at the same density, and others exhibiting higher thermal conductivity at the same density.
Aerogels are extremely good thermal insulators for several reasons. First, it is important to understand how heat is transported through materials. Heat is transported through a material three different ways: through conductive transport, that is, through the solid part of the material; through convective transport, that is, by being carried by gas diffusing through a material; and through radiative transport, that is, by electromagnetic energy like infrared energy penetrating through the material.
Aerogels are extremely low-density materials, typically 50-99.98% air by volume. This means aerogels have very little mass through which heat can conduct. Additionally, the solid part of an aerogel is highly disordered and thus makes conduction of heat through the little solid that is there inefficient.
Additionally, aerogels have extremely tiny pores, typically between 2-50 nm in diameter. These pores are actually so tiny that they are smaller than the mean free path of air at room temperature and pressure, that is, the average distance a molecule of air can travel before hitting another air molecule is greater than the width of the pores in a typical aerogel. As a result, air has an extremely difficult time diffusing through and thus carrying heat energy through an aerogel by convection. This phenomenon, called the Knudsen effect, differentiates aerogels from traditional foams, which typically have pore sizes of tens to thousands of microns in diameter and thus allow more heat through by convection.
Aerogels are not necessarily good at stifling radiative transport, however, and so at high temperatures, heat can pass through aerogels in the form of infrared energy. As a result, commercial aerogel insulation products include additional materials called IR opacifiers embedded in the aerogel to reflect and/or absorb infrared energy. This helps limit radiative transport, making aerogel insulators excellent insulators at high temperatures as well as room temperature.
First, not all aerogels are easy to break! Classic (or “legacy”) aerogels exhibit extremely high strength-to-weight ratios and are able (in principle) to hold thousands of times their weight in applied force, however also typically exhibit extremely low fracture toughness, that is, the ability to resist propagation of flaws in the material. As a result, it is possible for a classic aerogel block that is 96% air by volume to hold a brick thousands of times its own weight, but only if the weight is placed on the monolith gently and there are no major cracks in the aerogel.
New mechanically strong and machinable aerogels such as Airloy® strong aerogels made by Aerogel Technologies fix this problem. Airloy aerogels are hundreds of times stronger and stiffer than classic aerogels and simultaneously durable and fracture tough. Unlike legacy aerogels, Airloy aerogels can be machined (drilled, tapped, turned, milled) and bent without breaking. The strength, stiffness, thermal conductivity, and other properties of Airloy aerogels depend on the product series. See our page about Airloy materials properties for specific information about the mechanical properties of different Airloy products.
Aerogel materials vary in price depending on form factor and composition. Once very costly due to specialty manufacturing processes and lack of commercial availability, today aerogel materials of various types are produced commercially on massive scales at prices that are in many instances competitive with traditional materials technologies. Aerogel particles such as Cabot Aerogel’s Lumira® aerogel, used in the daylighting panels in office buildings, gyms, and sports arenas around the world, while composite aerogel blankets such as Aspen Aerogels Spaceloft® and Cabot Aerogel’s Thermal Wrap™ insulate subsea oil pipelines, refineries, and residential apartments. Strong aerogel panels such as Airloy® strong aerogels from Aerogel Technologies are making planes, cars, and rockets lighter, more energy efficient, and cheaper to operate. Sub-bulk pricing for these and other aerogel products is available at BuyAerogel.com. Please contact us for bulk pricing requests.
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.
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!
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.
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.
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 °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 °F (>°C)—that’s 50% hotter than Space Shuttle tile!
Something all aerogels have in common is extremely low density, typically ranging from ~0.6 grams per cubic centimeter to as low as 0. 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. g/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. 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!
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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.
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!
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 times better sound insulation than even polyurethane foam.
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!
Believe it or not, aerogels were first invented some time around by Prof. Samuel Stephens Kistler at the College of the Pacific. They were first described in the journal Nature in . In fact, powdered silica aerogel was first commercialized by Monsanto (yes, that Monsanto) in the ’s and was used even used as insulation in a line of freezers, although was discontinued in the ’s when foams like Styrofoam first became commercially viable. But with the ’s also came the dawn of the nanotechnology age, and scientists began to appreciate aerogels as the nanostructured, nanoporous marvels they are. Throughout the ’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.
Aerogels of various types are used in many applications today. Here are some applications of different aerogel materials:
Aerogel Composite Blankets
Hydrophobic Silica Aerogel Particles
Monolithic Silica Aerogel Panels
Carbon Aerogels
Airloy Strong Aerogels
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 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.
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:
One of the most fascinating aspects of silica aerogels is their amazing optical properties. Here are some cool things to try:
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!
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!
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