Aerogel, often nicknamed “frozen smoke,” is a synthetic, ultralight porous material renowned for its exceptional properties, including extremely low density and low thermal conductivity. It is essentially the solid framework of a gel where the liquid component has been replaced by a gas without causing the structure to collapse. The resulting material is up to 99% air by volume, featuring a delicate three-dimensional network of nanoscale pores. The manufacturing process is designed to preserve this fine nanoporous structure, which enables the material’s performance.
The Sol-Gel Method
The creation of aerogel begins with the sol-gel process, the foundational chemical method used to establish the initial solid network. For silica aerogel, the process starts with a silicon alkoxide precursor, such as tetraethoxysilane (TEOS), mixed with a solvent, water, and a catalyst.
The first reaction is hydrolysis, where water breaks the alkoxide bonds, forming silanol groups (Si-OH). These groups then undergo condensation, linking together to form siloxane bonds (Si-O-Si), which create the solid network backbone.
This initial stage produces a colloidal suspension known as a “sol.” As condensation continues, the solid particles crosslink and agglomerate, forming a continuous, three-dimensional network throughout the liquid. When this network solidifies and entraps the solvent, the solution transitions from a liquid sol to a semi-rigid “wet gel.”
Structural Stabilization Through Aging
The wet gel formed during the sol-gel reaction is often too fragile to withstand the stresses of the final drying phase, requiring an intermediate step called aging. Aging is a controlled process where the gel is allowed to sit in its mother liquor or a fresh solution for a period, which can range from hours to days. This allows the chemical reactions to continue, strengthening the gel’s structure.
During this time, a process similar to Ostwald ripening occurs, where silica molecules dissolve and reprecipitate onto the necks connecting the nanoparticles. This dissolution and reprecipitation effectively thickens the inter-particle bonds and reinforces the entire skeletal structure. A stronger network is better equipped to resist the intense capillary forces that will try to collapse the pores during the liquid removal stage.
After aging, the initial solvent, such as ethanol, is often exchanged for a liquid compatible with the subsequent drying process, typically liquid carbon dioxide. This solvent exchange is necessary because many organic solvents have critical points that are too high in temperature and pressure to be practical for industrial-scale drying. The wet gel is repeatedly soaked in the new liquid to ensure the complete replacement of the original solvent within the nanopores.
The Critical Drying Phase
The drying phase is the most technically complex step in aerogel manufacturing, determining whether the final product will be a high-porosity aerogel or a dense, collapsed material called a xerogel. If the liquid were simply evaporated under normal conditions, the surface tension of the receding liquid menisci would create immense capillary pressures. These forces, which can exceed 200 megapascals (MPa), are strong enough to crush the gel’s delicate, nanometer-scale pore structure, causing significant shrinkage.
The standard commercial method to circumvent this destructive force is Supercritical Fluid Drying (SCD), which uses a fluid in its supercritical state. A substance becomes supercritical when it is simultaneously heated and pressurized above its critical point, where the liquid and gas phases become indistinguishable. This state entirely eliminates the liquid-gas interface and, consequently, the surface tension responsible for capillary stress.
In the most common form of SCD, the solvent-exchanged wet gel is placed in a pressure vessel with liquid carbon dioxide (\(CO_2\)). The vessel is then heated and pressurized above the \(CO_2\)‘s critical point. The \(CO_2\) transitions into a supercritical fluid, which has liquid-like density for efficient solvent extraction but gas-like viscosity for easy penetration and removal from the pores.
Once the supercritical \(CO_2\) has fully replaced the pore liquid, the pressure is slowly released while the temperature is maintained above the critical point. This controlled depressurization allows the supercritical fluid to transition directly into a gas without crossing the liquid-gas boundary. The absence of a meniscus during this transition prevents structural collapse, preserving the high porosity and low density that define the final aerogel product. While SCD yields the highest quality aerogels, research into ambient pressure drying (APD) is ongoing, aiming to use matrix-strengthening chemicals to allow the gel to be dried at room temperature and pressure, which would dramatically reduce production costs.
Variations in Aerogel Composition
While silica aerogels are the most widely recognized, the manufacturing process can be adapted to produce aerogels from a variety of materials by changing the initial precursors in the sol-gel step. This allows for the creation of materials with different properties, such as electrical conductivity or enhanced flexibility. The fundamental requirement remains the initial formation of a stable wet gel, followed by the specialized drying phase.
Carbon Aerogels
Carbon aerogels are typically made by first creating an organic aerogel using precursors like resorcinol and formaldehyde. The resulting organic gel is then subjected to pyrolysis, a process of high-temperature heating in an inert atmosphere, which carbonizes the polymer network. This results in an electrically conductive aerogel with high surface area, useful for applications like supercapacitors.
Polymer Aerogels
Polymer aerogels, made from synthetic polymers like polyimide or natural polymers like cellulose, offer greater flexibility and mechanical strength than their silica counterparts. In these cases, the polymer chains are induced to crosslink within a solvent, forming the gel structure. Although the starting chemistry differs from silica, the subsequent steps of aging and supercritical drying remain the standard method to achieve the characteristic ultralow density structure.