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.
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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.
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