The common preparation methods of aerogels are mainly the following:

Sol-gel method
Principle: This is the most basic method for preparing aerogels. It is based on chemical reactions in the solution. By controlling the hydrolysis and polycondensation reaction conditions of the solution, nanoclusters with different structures are formed in the solution. These clusters adhere to each other to form a gel. Generally, precursors such as metal alkoxides (such as tetraethyl orthosilicate) or inorganic salts are first dissolved in an appropriate solvent (such as ethanol), and hydrolysis reactions occur under the action of catalysts such as acids or alkalis to generate active hydroxyl compounds. Then these hydroxyl compounds undergo polycondensation reactions and gradually form a gel with a three-dimensional network structure.
Features: This method can accurately control the composition and structure of the material and can prepare aerogels with high purity and uniformity. However, the process is relatively complicated, and the reaction conditions (such as temperature, pH value, reactant concentration, etc.) need to be strictly controlled, and the drying process after the gel is formed is also critical.

Supercritical drying technology
Principle: Supercritical drying is to remove the solvent in the gel under a supercritical state. When the liquid in the gel is in a supercritical state, the gas-liquid interface disappears and the surface tension of the liquid no longer exists. The gel is placed in a pressure vessel and heated and pressurized to make the liquid in the gel undergo a phase transition to a supercritical fluid, and then the supercritical fluid is released from the pressure vessel to obtain a porous, disordered, low-density aerogel material with a nanoscale continuous network structure.
Example: For wet silica gel prepared by the sol-gel method, supercritical carbon dioxide drying is used. The wet gel is placed in an autoclave, liquid carbon dioxide is injected, and the carbon dioxide is heated and pressurized to reach a supercritical state (temperature of about 31.1°C, pressure of about 7.38MPa), and then the carbon dioxide is slowly released so that the solvent in the gel is replaced and removed by carbon dioxide, thereby obtaining silica aerogel.
Features: Supercritical drying can effectively prevent the structural collapse of the gel caused by the surface tension of the solvent during the drying process, and can well maintain the porous structure of the aerogel. However, supercritical drying equipment is complex and costly, and the operation process needs to be carried out under high temperature and high pressure, which poses certain safety risks.

Normal pressure drying method
Principle: Unlike supercritical drying, normal pressure drying is carried out under normal pressure. During the preparation of the gel, the gel is treated by chemical modification and other treatments to reduce the capillary force generated by the evaporation of the solvent during the drying process, thereby reducing the structural collapse and maintaining the structure of the aerogel after drying at normal pressure. For example, when preparing silica aerogel, some hydrophobic groups such as trimethylchlorosilane (TMCS) are added to the gel system to make the gel surface hydrophobic, reducing the interaction between water and the gel skeleton during the drying process.
Example: During the preparation process, the wet gel is soaked in an organic solvent containing TMCS, and TMCS reacts with the hydroxyl groups on the surface of the gel to generate hydrophobic silane groups. After such treatment, the gel can be dried at normal pressure to obtain an aerogel.
Features: The normal pressure drying method has simple equipment, convenient operation, low cost, and is suitable for large-scale production. However, in order to obtain high-quality aerogels, it is necessary to finely control the pretreatment process such as chemical modification of the gel, otherwise problems such as structural shrinkage and reduced porosity are likely to occur.