Aerogel: The Amazing Material You Need to Know About

Aerogel is a remarkable synthetic material with a highly porous structure that yields exceptional physical and thermal properties. First synthesised in the early 20th century, it has since become a cornerstone of extreme thermal insulation and advanced scientific research.

Many people first discover this material searching for the term air gel, which is an understandable description given that the substance is essentially a gel scaffold filled with air. Whether you call it aerogel or air gel, this substance remains one of the most technologically advanced materials available today.

What is aerogel?

Aerogel is a type of solid that is made up of 99.8% air. It is created by removing the liquid from a gel, leaving behind a solid with a porous structure. The result is a material that is incredibly lightweight, yet incredibly strong.

Aerogel is so light that it feels as though one is holding nothing at all, but it is actually very strong. It is like holding a piece of smoke. Aerogel can also withstand high temperatures and pressures, making it suitable for extreme environments.

The Invention of Aerogels

Historically, aerogels were invented in 1931 by Samuel Stephens Kistler. Kistler set out to prove that a gel contained a continuous solid network of the same size and shape as the wet gel itself. He succeeded by replacing the liquid in the gel with gas without causing the structure to shrink or collapse.

To understand how aerogel is made, it is helpful to look at the transition from a liquid-filled gel to a dry, porous nanostructure via the sol-gel process and supercritical drying:

By utilising supercritical drying, the liquid inside the gel is brought to a "supercritical" state where liquid and gas phases coexist. This eliminates capillary pressure, allowing the liquid to be vented away while keeping the intricate solid framework completely intact.

What are the properties of aerogel?

One of the most unique properties of aerogel is its hydrophobicity. This means that it repels water and other liquids. In fact, it is so hydrophobic that it is often referred to as "frozen smoke." When water is placed on a highly hydrophobic aerogel surface, it forms near-perfect spherical droplets that roll off effortlessly without wetting or degrading the underlying solid framework.

Note: While untreated silica aerogel is hydrophilic and can degrade in water, commercial aerogels are chemically treated to make them highly hydrophobic, ensuring they retain their structural integrity in humid or wet conditions.

Another key property of aerogel is its ability to be used as a thermal insulation material. Aerogel is an excellent insulator, which means that it can help to keep things warm or cool depending on the application. In fact, aerogel is so effective at insulating that it is often used in space exploration. The material is used to insulate spacecraft and space suits, helping to protect astronauts from the extreme temperatures of space.

Why is Aerogel Such an Outstanding Insulator?

Aerogel's exceptional insulating capability is due to its nanostructured pores, which average between 20 and 100 nm in size. This pore size is smaller than the mean free path of air molecules (which is approximately 70 nm at atmospheric pressure). As a result, air molecules trapped inside the pores cannot collide with each other to transfer heat—a phenomenon known as the Knudsen effect.

The total thermal conductivity ($k_{\text{total}}$) of aerogel can be mathematically expressed as:

where:

• $k_{\text{total}}$ is the overall thermal conductivity of the aerogel,
• $k_{\text{solid}}$ is the thermal conductivity through the solid nanostructured network,
• $k_{\text{gas}}$ is the thermal conductivity of the gas trapped within the microscopic pores,
• $k_{\text{radiation}}$ is the radiative heat transfer through the material.

Because both $k_{\text{solid}}$ and $k_{\text{gas}}$ are minimised to extreme degrees, silica aerogels can achieve an incredibly low thermal conductivity of just 0.015 to 0.020 W/(m·K) in ambient conditions.

What are the types of aerogels?

While silica aerogels are the most common, aerogels actually represent a broad class of materials:

• Silica Aerogels: The most widely researched type. They are silica-based, highly insulating, and appear translucent blue due to Rayleigh scattering of light.
• Carbon Aerogels: Composed of covalent carbon networks. They are black, electrically conductive, have high surface areas (often exceeding 1000 m²/g), and are highly chemically stable.
• Metal Oxide Aerogels: Made from metal oxides such as alumina, titania, or zirconia. These are heavily utilised as catalysts in industrial chemical reactions.
• Polymer and Biopolymer Aerogels: Created using polymers like polyimide or biopolymers like cellulose and pectin. They offer greater mechanical flexibility and are less brittle than silica versions.

What are the applications of aerogel?

Aerogel has a wide range of applications in various fields, such as energy, environment, defence, aerospace, medicine, and more. Here are some examples of how aerogel can be used:

• Energy conversion and storage: Aerogels can be used for energy conversion and storage devices, such as solar cells, fuel cells, batteries, and supercapacitors. Aerogels can enhance the performance of these devices by providing high surface area, low density, and good electrical conductivity. For example, carbon aerogels make excellent electrode materials for high-capacity supercapacitors.
• Air purification and filtration: Aerogels can be used for air purification and filtration. Aerogels can absorb harmful gases and pollutants from the air due to their high porosity and large internal capacity. Aerogels can also act as catalysts for chemical reactions that degrade or remove contaminants from the air.
• Fire retardation and protection: Aerogels can be used for fire retardation and protection. Aerogels can resist flames and prevent heat transfer due to their low thermal conductivity and high-temperature stability. Aerogels can also act as barriers or coatings for flammable materials or structures, such as in aerogel-infused insulation blankets used in high-risk industrial facilities.
• Optical applications: Aerogels can be used for optical applications, such as light guides, lenses, mirrors, and displays. Aerogels can manipulate light due to their low refractive index and high transparency. Aerogels can also emit light when exposed to certain wavelengths or electric fields.
• Biomedical applications: Aerogels can be used for biomedical applications, such as drug delivery, wound healing, tissue engineering, and biosensors. Aerogels can deliver drugs or nutrients to specific targets in the body due to their biocompatibility and controlled release properties. Aerogels can also promote wound healing and tissue regeneration by providing scaffolds or matrices for cell growth and differentiation.
• Aerospace exploration: Beyond insulating planetary rovers and space suits, silica aerogel was famously used in NASA's Stardust mission. The spacecraft used a lightweight collector grid filled with aerogel to capture high-velocity comet dust and interstellar dust particles without melting or damaging them on impact.

Conclusion

Aerogel is a highly versatile material with significant potential across numerous industries, from aerospace engineering to environmental cleanup. To explore the scientific research and engineering principles behind aerogels in greater depth, the following academic and industrial resources are recommended:

Historical Foundations: Kistler, S. S. (1931). "Coherent Expanded Aerogels and Jellies." Nature*, 127, 741. DOI: 10.1038/127741a0. Synthesis and Overview: Gurav, J. L., Jung, I. K., Park, H. H., Kang, E. S., & Nadargi, D. Y. (2010). "Silica Aerogel: Synthesis and Applications." Journal of Nanomaterials*, 2010, Article ID 409310. DOI: 10.1155/2010/409310.

• Space Exploration Applications: NASA Jet Propulsion Laboratory. "Stardust: Catching Comet Dust." NASA's Comet Sample Return Mission. Available at: stardust.jpl.nasa.gov.