Aerogels are a diverse class of porous, solid materials that exhibit an uncanny array of extreme materials properties. Most
notably aerogels are known for their extreme low densities (which range from 0.0011 to ~0.5 g cm-3). In fact, the lowest density solid materials that have ever been produced are all aerogels, including a silica aerogel that as produced was only three times heavier than air, and could be made lighter than air by evacuating the air out of its pores. That said, aerogels usually have densities of 0.020 g cm-3 or higher (about 15 times heavier than air). But even at those densities, it would take 150 brick-sized pieces of aerogel to weigh as much as a single gallon of water! And if Michaelangelo’s David were made out of an aerogel with a density of 0.020 g cm-3, it would only weigh about 4 pounds (2 kg)! Typically aerogels are 95-99% air (or other gas) in volume, with the lowest-density aerogel ever produced being 99.98% air in volume.
Essentially an aerogel is the dry, low-density, porous, solid framework of a gel (the part of a gel that gives the gel its solid-like cohesiveness) isolated in-tact from the gel’s liquid component (the part that makes up most of the volume of the gel). Aerogels are open-porous (that is, the gas in the aerogel is not trapped inside solid pockets) and have pores in the range of <1 to 100 nanometers (billionths of a meter) in diameter and usually <20 nm.
Aerogels are dry materials (unlike “regular” gels you might think of, which are usually wet like gelatin dessert). The word aerogel refers to the fact that aerogels are derived from gels–effectively the solid structure of a wet gel, only with a gas or vacuum in its pores instead of liquid. Learn more about gels,aerogels, and how aerogels are made.
How is Aerogel Made?
Aerogels start their life out as a gel, physically similar to Jell-O®. A gel is a colloidal system in which a nanostructured network of interconnected particles spans the volume of a liquid medium. Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. In the case of Jell-O, this network of particles is composed of proteins and spans the volume of some sort of fruit juice. A gel is structurally similar to a wet kitchen sponge, only with pores a thousand to a million times smaller. Because a gel’s pores are so small, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out. It’s important to remember that gelatin isn’t the only type of gel–in fact, chemists can prepare gels with backbones composed of many organic and inorganic substances and many liquid interiors.
Once a gel is prepared, it must be purified prior to further processing. This is because the chemical reactions that result in the formation of a gel leave behind impurities throughout the gel’s liquid interior that interfere with the drying processes used to prepare aerogel (as described below). Purification is done by simply soaking the gel under a pure solvent (depending on the gel this could be acetone, ethanol, acetonitrile, etc.), allowing impurities to diffuse out and pure solvent to diffuse in. The solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days. Depending on the volume and geometry of the gel, diffusive processes can take any where from hours to weeks. A ice-cube size sample can usually be purified in 1 or 2 days.
Aerogels start their life out as a gel, physically similar to Jell-O®. A gel is a colloidal system in which a nanostructured network of interconnected particles spans the volume of a liquid medium. Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. In the case of Jell-O, this network of particles is composed of proteins and spans the volume of some sort of fruit juice. A gel is structurally similar to a wet kitchen sponge, only with pores a thousand to a million times smaller. Because a gel’s pores are so small, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out. It’s important to remember that gelatin isn’t the only type of gel–in fact, chemists can prepare gels with backbones composed of many organic and inorganic substances and many liquid interiors.
Once a gel is prepared, it must be purified prior to further processing. This is because the chemical reactions that result in the formation of a gel leave behind impurities throughout the gel’s liquid interior that interfere with the drying processes used to prepare aerogel (as described below). Purification is done by simply soaking the gel under a pure solvent (depending on the gel this could be acetone, ethanol, acetonitrile, etc.), allowing impurities to diffuse out and pure solvent to diffuse in. The solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days. Depending on the volume and geometry of the gel, diffusive processes can take any where from hours to weeks. A ice-cube size sample can usually be purified in 1 or 2 days.
For more information about gels and gel preparation, see The Sol-Gel Process under The Science of Aerogel.
What are the properties of Silica Aerogel?
Values | Units | Silica Aerogel |
| Class | Silica | |
| Composition | SiO2 | |
| Density Range | g cm-3 | 0.0011-0.650 |
| Surface Area | 500-950 | |
| Pore Volume | cm g-1 | |
| Primary Particle Size | nm | 2.0-3.0 |
| Average Pore Size | nm | 20 |
| Transparency | Clear to foggy | |
| Appearance | Transparent or white with blue cast from Rayleigh scattering | |
| Monolithicity | Monolithic | |
| Flexibility | Rigid, friable | |
| Gel Synthesis | Hydrolysis of silicon alkoxide or acid-driven condensation of waterglass | |
| Drying Method | Supercritical CO2 or high-temperature drying from organic solvent | |
| Thermal Conductivity at Room Temperature | W m-1 K-1 | 0.016-0.03 |
| Electrical Conductivity | S cm-1 | 1×10-18 |
| Index of Refraction | Dimensionless | 1.002-1.046 |
| Dielectric Constant (DC) | Dimensionless | 1.008-2.27 |
| Young’s Modulus | MPa | 0.05-400 |
| Coefficient of Thermal Expansion | microstrain °C-1 | 2 |
| Compressive Strength | MPa | |
| Speed of Sound | m s-1 | 70-1300 |
| Invented By | Samuel Kistler | |
| Major Players | Arlon Hunt, Mike Ayers, Tom Tillotson, C. Jeff Brinker, Debra Rolison, Peter Tsou | |
| Special Properties | ||
| Note 1 | ||
| Note 2 | ||
| Reference 1 | T.M. Tillotson, L.W. Hrubesh, “Transparent Ultralow-Density Silica Aerogels Prepared by a Two-Step Sol-Gel Process”, 1991, Lawrence-Livermore National Laboratory Lab-Authored Report 218496. | |
| Reference 2 | ||
| Reference 3 | ||
| Reference 4 | ||
| Reference 5 |
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