Aerogel is a kind of material with high porosity and low density, which is widely used in thermal insulation, adsorption, catalysis and other fields. The preparation process of aerogel usually includes three main steps: gel formation, solvent exchange and drying. Drying is a key step in the preparation of aerogel, as improper drying methods can cause the gel structure to collapse and thus lose its unique physical properties. At present, the commonly used drying methods mainly include atmospheric drying method and supercritical drying method.




Atmospheric drying method
Atmospheric drying is carried out at atmospheric pressure, and the solvent is evaporated from the gel by natural evaporation or heating. This method is simple in operation and low in cost, but it is easy to cause the contraction and collapse of the gel structure.
【 Process 】
1. Pretreatment: The wet gel is placed in an environment of appropriate temperature and humidity, and is slowly dried to reduce shrinkage stress.
2. Drying: gradually increase the temperature to vaporize the solvent, usually between room temperature and 100 ° C.
Advantages: simple operation, no need for complex equipment, low cost, suitable for large-scale industrial production.
Disadvantages: When the solvent evaporates at atmospheric pressure, a large capillary pressure will be generated, resulting in the collapse of the gel network structure, the decrease of the specific surface area and porosity of the aerogel, and the drying time is longer, and the energy consumption is higher.

Supercritical drying process
Supercritical drying method takes advantage of the fact that the solvent has no surface tension in supercritical state, and removes the solvent from the gel, thus avoiding the damage of the gel structure by capillary pressure. The common solvent is carbon dioxide because it has a low critical temperature (31.1 ° C) and critical pressure (7.38 MPa), and is inexpensive and non-toxic.
【 Process 】
1. Solvent exchange: the solvent in the gel (such as ethanol) is gradually replaced with carbon dioxide.
2. Heating and pressurization: the gel containing carbon dioxide is placed in a high-pressure container, heated to a supercritical temperature and increased pressure to a supercritical state.
3. Drying: Slowly reduce pressure, so that carbon dioxide escapes in a supercritical state to complete the drying process.
Advantages: The three-dimensional network structure of the gel can be maintained, and the prepared aerogel has high specific surface area and high porosity, relatively short drying time and high efficiency.
Disadvantages: high equipment requirements, the need to withstand high pressure, high temperature supercritical drying device, high cost, complex operation, the need to strictly control temperature and pressure conditions.

Atmospheric drying method and supercritical drying method are two main drying methods for preparing aerogel, and each has its advantages and disadvantages. The choice of which drying method should be based on the specific application needs, economic costs and technical feasibility of comprehensive consideration.