Aerogels

Aerogels are gels that have had their liquid content replaced with air to form a lightweight substance that is solid to the touch. Highly porous, aerogels have numerous unique physical and chemical properties including extremely low rates of heat transfer, which is referred to as thermal conductivity. Because of this, the National Aeronautics and Space Administration (NASA) began using aerogels in the 1960s to insulate the spacesuits worn by astronauts.

In contemporary industry, aerogels are mainly used as insulating and soundproofing materials. Certain aerogels have strong resonance absorption characteristics, making them effective acoustic insulators. The fragile nature of aerogel solids has historically limited their commercial applications, but researchers have developed a new generation of aerogels with improved fragility and brittleness characteristics. This breakthrough has created many future possibilities for aerogel usage. Aerogels have also been identified to help combat climate change, as their superior home and building insulation characteristics enable occupants to drastically reduce their fossil fuel usage for home heating purposes.

Brief History

The American chemical engineer and research scientist Samuel Stephens Kistler (1900–1975), commonly known as Steven Kistler, is credited with inventing aerogels. Kistler was educated in sciences such as physics, chemistry, geology, botany, and astronomy at the College of the Pacific and Stanford University. After a brief stint as an engineer for the Standard Oil Company, Kistler accepted a faculty position at the College of the Pacific in the early 1920s. Later, while teaching undergraduate courses at the College of the Pacific, Kistler began pursuing a doctoral degree at Stanford. Biographers are unsure of precisely when Kistler invented aerogels, or whether the research that led to their discovery was performed as part of his faculty duties at the College of the Pacific or while conducting doctoral research at Stanford.

It is known that Kistler’s research interest in the physical and chemical processes used to produce aerogels dates to at least 1922, when Kistler was pursuing a master’s degree at Stanford. Kistler introduced the scientific community to aerogels via a paper published in a 1931 edition of the peer-reviewed academic journal Nature that described the technical aspects of the process Kistler used to replace the liquid content of a jelly substance with pockets of air to produce a solid material of extremely low density. Observers believe Kistler succeeded in creating the first aerogels in either 1929 or 1930.

Kistler transferred to a faculty position at the University of Illinois in 1931, remaining at the institution until 1935. During this period, Kistler continued his research into aerogels and published a series of papers detailing their unique chemical and physical properties, which fascinated scientists of the era. Kistler began working to commercialize aerogels in the early 1940s, licensing his process for making silica aerogels to Monsanto Corporation. Monsanto made various attempts to bring aerogel-based products to market, but it was not until NASA began using aerogels as spacesuit insulation in the 1960s that the material made a major commercial breakthrough.

As space exploration activity declined during the 1970s, aerogels languished without obvious applications until they were rediscovered by researchers seeking exotic methods for storing rocket fuels and oxygen in solid materials. In the late 1970s and 1980s, resurgent research activity yielded a new class of aerogels with improved performance characteristics and stronger safety profiles. By the 1990s, researchers had discovered ways to produce aerogels using a vastly expanded group of metal oxides, which led to their commercial reintroduction as insulating and soundproofing materials.

Topic Today

Contemporary aerogel production techniques involve drying standard gels at very high temperatures using a process known as supercritical drying, which removes liquid from gels at a very slow rate, allowing engineers to replace the lost liquid with pockets of air without altering the structural integrity of the gel. The process yields a solid material of extremely low density, which gives aerogels a super-lightweight profile. Aerogel densities range from approximately 0.0011–0.5 grams per cubic centimeter, with most aerogels having a density of about 0.02 grams per cubic centimeter. The lightest aerogels ever created have densities only three times that of air, and aerogels were long recognized as the lowest-density synthetic solids ever invented until they were supplanted by metallic microlattice and aerographite.

In addition to extremely low rates of thermal conductivity, aerogels display several other defining properties and characteristics. They have very low mean-free paths of diffusion, sound speeds, dielectric constants, and refractive indices while also displaying very high surface areas relative to their mass. Their superior ability to inhibit heat transfer has been famously depicted through a demonstration in which wax coloring crayons are placed on top of a thin aerogel surface while an open flame applies heat to the bottom side of the aerogel. The aerogel is so effective at inhibiting heat transfer that the flame is unable to melt the wax crayons.

Silica aerogels, which are created by replacing the liquid components of silica gels with air, are both the most studied and widely used type of aerogel. However, there are many other types such as metal oxide aerogels, chalcogenide aerogels, organic aerogels, and carbon-based aerogels. In addition to being widely used in insulation and soundproofing, aerogels are also commonly used in nanotechnology research. This is an emerging field of science that seeks to create products and materials to work on the nanoscopic scale (nanoscale), which is defined as operating at sizes ranging from 1–100 nanometers (nm), with 1 nm being about 1,000 times smaller than the diameter of a human hair.

The well-established insulating properties of aerogels has led to their dramatically increased use in insulation products in the twenty-first century. Aerogels are commonly found in board insulation, cavity-injected insulation, and specialized forms of plaster that have been used by engineers in Switzerland to restore and protect historic buildings. New production techniques have significantly improved the fragility and brittleness characteristics of aerogels, with experts describing the breakthroughs as potentially leading to new applications in fields such as apparel. Aerogel insulation also allows building occupants to use much less energy to maintain comfortable indoor temperatures during winter, making it an important candidate in ongoing efforts to find sustainable solutions to high rates of fossil fuel consumption and climate change.

Bibliography

Adnan, Sayed Mohammed and Tariq Altalhi (eds.). Aerospace Polymeric Materials. Wiley, 2022.

Erkey, Can and Michael Turk. Synthesis of Nanostructured Materials in Near and/or Supercritical Fluids. Elsevier, 2021, pp. 19–30.

Khan, Aftab Aslam Parwaz, et. al. (eds.). Advances in Aerogel Composites for Environmental Remediation. 2021, pp. 109–124.

“Origins of Aerogel.” Aerogel.org, www.aerogel.org/?cat=45. Accessed 31 Mar. 2023.

Quinlan, Heather. "How Aerogels Work." How Stuff Works, 27 Jul. 2010, science.howstuffworks.com/aerogel.htm. Accessed 31 Mar. 2023.

Ratke, Lorenz and Pavel Gurikov. The Chemistry and Physics of Aerogels. Cambridge University Press, 2021.

“The History of Aerogel.” Aerogel.org, www.aerogel.org/?cat=38. Accessed 31 Mar. 2023.

Thomas, G.P. “What Is Aerogel? Theory, Properties, and Applications.” AZO Materials, 22 Aug. 2012, www.azom.com/article.aspx?ArticleID=6499. Accessed 31 Mar. 2023.

Woods, Tori. “Aerogels: Thinner, Lighter, Stronger.” NASA, 28 July 2011, www.nasa.gov/topics/technology/features/aerogels.html. Accessed 31 Mar. 2023.