What Is Aerogel?

Aerogels are synthetic, nanoporous solids derived from gels in which the liquid component has been replaced with a gas, without significant collapse of the gel structure. This process results in a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.

How Is Aerogel Made?

The creation of aerogels involves a two-step process: gelation and drying.

1. Gelation

The first step of the sol-gel process is the creation of a colloidal suspension of solid particles known as a "sol". The precursors are a liquid alcohol such as ethanol which is mixed with a silicon alkoxide, such as tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and polyethoxydisiloxane (PEDS). The solution of silica is mixed with a catalyst and allowed to gel during a hydrolysis reaction which forms particles of silicon dioxide. The oxide suspension begins to undergo condensation reactions which result in the creation of metal oxide bridges linking the dispersed colloidal particles.

2. Drying

Once the gelation is completed, the liquid surrounding the silica network is carefully removed and replaced with air, while keeping the aerogel intact. It is crucial that the gel is dried in such a way as to minimize the surface tension within the pores of the solid network. This is typically accomplished through supercritical fluid extraction using supercritical carbon dioxide (scCO₂) or freeze-drying.

Gels where the liquid is allowed to evaporate at a natural rate are known as xerogels and are not considered aerogels. As the liquid evaporates in such a manner, forces caused by surface tensions of the liquid-solid interfaces are enough to destroy the fragile gel network. As a result, xerogels cannot achieve the high porosities and instead peak at lower porosities and exhibit large amounts of shrinkage after drying.

How Strong Is Aerogel?

Aerogels are known for their impressive load-bearing abilities due to their dendritic microstructure, in which spherical particles of average size 2–5nm are fused together into clusters. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores just under 100nm. The average size and density of the pores can be controlled during the manufacturing process.

Despite their low density, aerogels can support significant weight. For instance, a 1 cm³ piece of silica aerogel can support up to 2,000 times its weight. However, they are prone to shattering under high stress due to their brittleness. Recent advancements have led to the development of more flexible aerogels that can bend without breaking, unlike traditional brittle aerogels.

Applications of Aerogel

Aerogels have found applications across various fields due to their unique properties:

• Thermal Insulation: Aerogels are among the best insulating materials, with thermal conductivity lower than still air.

• Space Exploration: NASA used aerogels to trap cosmic dust particles in the Stardust mission.

• Environmental Cleanup: Silica aerogels have high surface area and porosity, making them suitable for removing heavy metals in wastewater treatment.

• Catalysis: Aerogels can serve as catalysts or catalyst carriers in chemical reactions.

• Drug Delivery: Aerogels can adsorb drugs from supercritical fluids, with release rates controlled by varying the aerogel properties.

• Electrochemical Supercapacitors: Aerogels' high surface area allows for capacitors that are much smaller than similarly rated electrolytic capacitors.