1. Overview of aerogel
Aerogel is a kind of inorganic amorphous porous material whose solid phase and pore structure are nanometer. It has a continuous and irregular open nanonetwork structure, a porosity of up to 80% to 99.8%, and a density of up to 3kg/mP P, which is a relatively light solid material. The porous nanostructure of aerogel makes it exhibit the interface effect and small size effect of nanomaterials on the macro level, and has excellent properties such as low refractive index, low dielectric constant, low sound transmission velocity and low heat transfer coefficient. Aerogel materials with its excellent structural properties have been widely used in thermal insulation materials, catalyst and catalyst carrier materials, waste gas adsorption materials, optical materials and many other fields.
The preparation process of aerogel is mainly divided into three steps: sol-gel process, gel aging process and drying process. The schematic diagram of the gel process is shown in Figure 1.

FIG. 1 Schematic diagram of silicon aerogel preparation process

First, the precursor solution forms colloidal particles dispersed in the solvent under the action of the catalyst, that is, the so-called sol. The colloidal particles in the sol form a disordered crosslinked wet gel with spatial three-dimensional network structure through the gel process of aggregation and condensation. Due to the insufficient strength of the three-dimensional structure of the newly formed wet gel, it is easy to break and fracture, so it needs to age in the parent solution for a period of time. During the aging process, the unreacted functional groups inside and on the surface of the gel will further condense, which will increase the strength of the prepared gel. After aging, the gel, through a specific drying process to ensure that its structure is not destroyed, removes a large amount of solvent in its nanoscale pore structure, and obtains a porous solid material with high porosity and low density - aerogel.

2. Drying method of aerogel
The drying process is very important in the preparation of aerogel. In the process of drying the wet gel into aerogel, the gel structure has to withstand huge drying stress, up to 100MPa-200MPa, which will make the gel structure continue to shrink and crack, and eventually lead to structural collapse. Drying stress mainly comes from capillary stress, osmotic pressure, separation pressure and so on. One of the most important is capillary pressure. During the drying process of wet gel, the volatilization of solvent causes the transition of the solid and liquid phase boundary to the high-energy solid and gas phase interface in the pore, forming a meniscus, and the capillary pressure is generated. During the drying process, the gel structure will shrink or even crack greatly under the action of drying stress, so that the aerogel material with ideal structure cannot be obtained. The main factors affecting the drying stress include: the strength of the gel structure, the size and homogeneity of the gel aperture, the surface tension of the solvent in the gel, the contact Angle between the solvent and the surface of the gel structure, and so on. It is the most important step in various aerogel drying methods to control the damage degree of drying stress to gel structure by adjusting these factors. At present, the most commonly used drying methods are divided into the following.

2.1 Supercritical drying
The supercritical drying method refers to that under the condition of higher than the critical temperature and pressure, the solvent in the gel is replaced by a specific supercritical fluid, and then the supercritical fluid in the gel aperture is converted into a gas by the way of first depressurizing and then cooling, and the dried aerogel is obtained. This method replaces the liquid-gas phase transition in the conventional method with liquid-supercritical phase transition and supercritical phase transition, and effectively avoids the drying stress generated in the liquid-gas phase transition. Currently commonly used supercritical drying methods are divided into two types:
(1) High-temperature supercritical drying: high-temperature supercritical drying is the earliest silicon aerogel drying method. In 1931, Kistler prepared the first silicon aerogel using this method. In high temperature supercritical drying, organic solvents such as methanol are usually used as supercritical fluids. This leads to the secondary esterification of the reactive -Oh group on the surface of the silica gel structure with an organic solvent (such as methanol) at a higher temperature under supercritical conditions, and the hydrophilic -Oh group is replaced by a hydrophobic alkyl group. The aerogel dried by this method will not cause structural cracking due to the absorption of water in the air, and its stability is stronger. However, high-temperature supercritical drying also has its drawbacks. Under the condition of high temperature and high pressure, flammable organic solvent is used as supercritical fluid, which increases the risk factor of the experiment. Therefore, researchers are trying to find a better drying method, low temperature supercritical drying method came into being.
(2) Low temperature supercritical drying: In 1985, Tewari used carbon dioxide as a supercritical fluid to prepare silicon aerogel through low temperature supercritical drying. Since then, carbon dioxide with a critical temperature close to room temperature has become the most commonly used optimal fluid in low-temperature supercritical drying, and its lower critical temperature (31℃) and critical pressure (7.39MPa) as well as the non-toxic and non-flammable characteristics of carbon dioxide make low-temperature supercritical drying technology safer. However, due to the poor compatibility between carbon dioxide and water, it is necessary to replace the wet gel with water-ethanol solvent first, and then replace the ethanol in the gel with carbon dioxide, and then get the aerogel after drying. The silicone aerogel obtained by supercritical drying at low temperature with carbon dioxide has no hydrophobicity, and the surface of the obtained aerogel has hydrophilic OH groups. In 1995, Ehrburg-Dolle et al. compared and analyzed the structure of silicon aerogels obtained by two supercritical drying techniques, and found that the silicon aerogels obtained by supercritical drying at low temperature in carbon dioxide had higher microporosity than those obtained by supercritical drying at high temperature in methanol. This may be due to the high critical temperature and pressure of methanol, which speeds up the aging of the gel, making the gel structure coarser and the porosity reduced.

2.1 Freeze drying
Freeze drying is another method to achieve gel drying by avoiding the liquid-gas phase interface to effectively avoid capillary pressure during drying. Using this method, the solvent in the gel must have a low diffusion coefficient and a high sublimation pressure. The solvent is first frozen in the gel pore, and then sublimed into a gaseous state under vacuum conditions to obtain a dry aerogel. Due to the high structural strength of the gel required by the freeze-drying method, the gel needs to be aged for a long time to obtain a high enough strength. Even so, the gel pore structure collapse will still occur due to freezing crystallization of solvent in the gel pore. Therefore, the freeze-drying method has not been widely used.

2.2 Drying under atmospheric pressure
Due to the relatively high cost of supercritical drying technology equipment, the industrial production and application of aerogel are limited to a certain extent, and the drying effect of freeze-drying method is not ideal. As a result, researchers are looking for drying methods that are more energy efficient, easy to operate and cost effective. Tyler et al. from Dow Company attempted to prepare silica aerogel materials by drying silica hydrogels under atmospheric pressure.
The success of atmospheric drying mainly depends on the strength of the skeleton structure of the gel, the homogeneity of the gel structure, the surface tension of the solvent in the gel and the contact Angle of the gel surface. Therefore, the drying stress can be effectively reduced by adjusting these factors. The specific control methods include: by controlling the sol-gel process and aging process to improve the gel structure strength and uniformity, by surface modification or selection of suitable precursors to adjust the gel surface contact Angle, select the solvent with low surface tension. Among them, surface modification and replacement of solvent with low surface tension are the most important steps in atmospheric pressure drying. The surface modification methods are divided into two kinds: one is the co-precursor method, that is, the modifier is mixed with silica sol, and the modifier is polymerized with silica sol as a reaction monomer to obtain a hydrophobic gel structure; The other is to modify the gel surface after gel. The first method is usually used to prepare silicone aerogels from silicone. Silica aerogel materials formed from inorganic silicon as silicon source usually adopt the second modification method, that is, the Si-OH group on the surface of silica particles is alkylated into Si-R group, and the gel with surface hydrophobic properties is obtained. Since the alkylation of the gel surface needs to be carried out in an organic solvent, lengthy dialysis and solvent displacement of the gel are also required during surface alkylation modification.

2. Conclusion and outlook
Using a large number of organic solvents and modifiers, as well as time-consuming processing steps, the usual drying of gels to obtain materials, some of its parameters can meet the requirements of aerogel, but it is always impossible to obtain the same properties as the aerogel obtained by supercritical drying materials. Strictly speaking, only the material prepared by supercritical drying method is the true sense of aerogel (aerogel), and the material prepared by atmospheric pressure drying or freeze drying can only be counted as "aerogel-like" materials. Supercritical drying technology is and will be the main drying method for aerogel preparation.With the progress of supercritical carbon dioxide drying process and equipment, this clean and energy-saving special dr