Aerogel is a synthetic solid known for its extremely low density and high porosity, often nicknamed “frozen smoke” or “solid cloud.” The material can be composed of up to 99.8% air by volume, making it the lightest manufactured solid on Earth. This unique structure provides superior thermal insulation capabilities, surpassing nearly all conventional materials. Aerogel is created through a multi-stage chemical engineering process that replaces the liquid component of a gel with a gas while preserving the gel’s delicate internal network.
The manufacturing process is a controlled sequence designed to maintain a fragile, three-dimensional solid matrix. It begins with liquid precursors that are chemically manipulated to form a wet, jelly-like substance, known as a wet gel. This wet gel must then undergo a specialized drying procedure to remove the internal liquid without collapsing its structure, which is the defining challenge of aerogel production.
Starting the Process: The Sol-Gel Method
The initial phase of aerogel synthesis relies on the sol-gel method, which builds the solid framework from molecular components. This method begins with a liquid mixture called a ‘sol,’ a colloidal suspension where solid nanoparticles are dispersed throughout a liquid solvent. For silica aerogel, precursors are typically silicon alkoxides like tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS).
The process involves two main chemical steps: hydrolysis and condensation. During hydrolysis, the alkoxide precursor reacts with water, often catalyzed by an acid or base, to form reactive hydroxyl groups. Following this, condensation occurs, where these hydroxyl groups react to eliminate water or alcohol molecules. This elimination facilitates the formation of siloxane bonds, linking the precursor molecules together to create small solid particles.
As these particles link and aggregate, they form a continuous, three-dimensional network spanning the liquid volume. The mixture’s viscosity increases until the solid network immobilizes the liquid, marking the transition from a ‘sol’ to a ‘gel,’ a process called gelation. The resulting jelly-like block is termed a hydrogel if the liquid is water, or an alcogel if the liquid is an alcohol.
The conditions of the sol-gel process, including precursor concentration and catalyst type, influence the final properties of the wet gel. After gelation, the wet gel undergoes an aging period where the solid network strengthens through further condensation. This strengthening step is performed by keeping the gel submerged in its solvent, which helps prevent structural collapse during the subsequent drying stage.
The Crucial Step: Controlled Drying Techniques
The most challenging step is removing the liquid from the nanoporous network without destroying the fragile solid structure. Simple drying at ambient pressure causes the liquid-vapor interface to create enormous capillary forces. These forces cause the solid network to shrink severely and collapse, resulting in a dense material called a xerogel instead of the intended low-density aerogel.
To prevent this structural damage, the liquid must be removed in a way that eliminates surface tension. The most effective industrial method is Supercritical Fluid Extraction (SCE), or supercritical drying. This technique uses a fluid heated and pressurized beyond its critical point, where it exists as a supercritical fluid with properties between a liquid and a gas.
Carbon dioxide (CO2) is the most common choice for SCE due to its relatively low critical point: 31.1°C and 7.38 MPa (about 1,070 psi). In this state, supercritical CO2 has gas-like viscosity and zero surface tension, allowing it to penetrate the smallest pores without damaging the gel network.
The SCE process usually involves a solvent exchange step first. If the gel contains water (hydrogel), the water is exchanged for an organic solvent like ethanol. The alcogel is then placed in a high-pressure vessel filled with liquid CO2, which gradually dissolves and replaces the organic solvent.
Once the solvent is fully exchanged, the vessel is sealed, and the temperature and pressure are raised above the critical point of CO2. The CO2 transitions into its supercritical state, and the pressure is slowly released while the temperature is maintained. This controlled depressurization allows the CO2 to escape as a gas without crossing a liquid-vapor phase boundary, preserving the porous structure and yielding the final aerogel.
Material Specific Processing
While silica is the most common type, the sol-gel and drying methods can be adapted to manufacture aerogels from various other substances. Different precursors require specialized chemical and thermal treatments, extending the application range beyond simple thermal insulation.
Carbon Aerogels
Carbon aerogels are a significant class of alternative materials, known for their high electrical conductivity, making them suitable for energy storage. Manufacturing carbon aerogels involves an additional high-temperature step after the initial gelation and drying. The process starts by creating an organic wet gel, typically from precursors like resorcinol and formaldehyde, using the standard sol-gel method.
Once the organic wet gel is dried, the resulting organic aerogel is subjected to pyrolysis. Pyrolysis involves heating the organic aerogel to extremely high temperatures, often between 600°C and 1050°C, in an inert atmosphere like nitrogen or argon. This thermal decomposition drives off all non-carbon elements, leaving behind a porous, electrically conductive carbon network that retains the original aerogel structure.
Other Materials
Other types of aerogels, such as those made from polymers or metal oxides, require specific processing steps tailored to their chemical nature. Polymer aerogels might use different solvents or cross-linking agents during gelation. Metal oxide aerogels require precursors like metal salts or alkoxides that are chemically distinct from silicon alkoxides.
Feasibility and Safety Considerations
For the general public interested in making aerogel, the process presents practical barriers related to specialized equipment and chemical safety. Producing the highest quality aerogel requires Supercritical Fluid Extraction (SCE). SCE requires a specialized pressure vessel capable of safely containing pressures exceeding 20 MPa (nearly 3,000 psi) and maintaining precise temperature control. This high-pressure, laboratory-grade equipment is expensive and inaccessible to individuals.
The chemicals involved in the sol-gel process also pose safety risks. Precursors like tetramethoxysilane (TMOS) are highly reactive and can cause irreversible eye damage. Solvents such as methanol are highly flammable, volatile, and toxic, requiring specialized ventilation and handling protocols. Attempting to synthesize high-quality aerogel without industrial safety measures is unrealistic and dangerous.
For demonstration purposes, simplified methods like ambient pressure drying (APD) can create materials that loosely resemble aerogels. These methods involve chemically modifying the pore walls of the wet gel to repel the liquid, reducing capillary forces. While APD avoids high-pressure equipment, the resulting product is structurally inferior, exhibiting greater density and shrinkage compared to industrially produced aerogel.