The performance of gel is mainly determined by its nano pore structure. Generally, the required nano pores and the corresponding gel skeleton are obtained through the sol gel process. Because the solvent inside the gel skeleton has surface tension, the skeleton will collapse under ordinary drying conditions. The core of aerogel preparation technology is to avoid the collapse of nano pore structure due to capillary force during drying.
The preparation of gel generally goes through two processes, namely, sol gel method to prepare wet gel and drying of gel.
The sol gel process can be described as follows: the first particles are generated in the reactant solution, the particles grow up to form a sol, and the particles continue to gather and cross link to form a three-dimensional network structure, which is the gel.

1. The sol-gel process for preparing aerogels generally has three methods:
Precursors such as metal alkoxides or metal nitrates and hydrochlorides are hydrolyzed and polycondensated to form gel;
Gel is formed by polymerization of single polymer monomer or copolymerization of several polymer monomers in solution;  
Gel of colloidal powder sol.
Synthesis of inorganic aerogels:
Inorganic aerogels generally use organometallic compounds or metal salts as raw materials. Sol particles are first formed in the solution by the sol gel process, and the particles are agglomerated and cross-linked to form a three-dimensional disordered, dendritic continuous network skeleton structure. The solvent is wrapped between the skeletons, and then the supercritical drying process is used to remove the residual solvent in the gel body to prepare low-density amorphous solid materials with a nano scale continuous porous disordered network structure.
At present, there are dozens of inorganic aerogels, including:
Single oxide aerogels include SiO2, Al2O3, TiO2, MgO, Cr2O3, etc;  
The binary oxide aerogels include Al2O3/SiO2, B2O3/SiO2, Fe2O3/SiO2, Lu2O3/Al2O3, CuO/Al2O3, etc;
Ternary oxide aerogels include CuO/ZnO/ZrO2, MgO/Al2O3/SiO2, etc;
Metal aerogels include Cu/Al2O3, Ni/Al2O3, Pd/Al2O3, etc.
Synthesis of organic aerogel and carbon aerogel gel:
The preparation process of organic gel is similar to that of inorganic gel. Organic monomers or oligomers are dissolved in the solvent and undergo chemical reaction to generate chain or disordered dendritic network structure. Finally, gel is achieved through the sol gel process. After solvent replacement, the solvent is removed by supercritical drying to obtain the dried aerogels. At present, MF and RF aerogels are studied by many people.
Preparation of carbon aerogel:
The dried organic aerogel gel is carbonized to obtain carbon aerogel gel.
Its preparation generally consists of three steps:
Preparation, supercritical drying and carbonization of organic gel wet gel. The formation of organic gel wet gel can produce gel with three-dimensional space network structure;
Supercritical drying can maintain the structure of gel while removing solvents in pores;
The carbonization makes the dried gel maintain the structure of the organic gel, and make the carbon element become the main component of the skeleton structure of the aerogel, and has certain electrical conductivity, which increases the application range of the aerogel.

2. Drying of gel:
The gel has extremely low density and high porosity, and its system itself has not very strong structural strength. Therefore, when ordinary drying methods such as normal temperature drying and baking drying are used, the gel skeleton will collapse and shrink easily due to the existence of gas and liquid two-phase interface and surface tension. In order to minimize the collapse and shrinkage of the basic network structure of gel in the drying process, so as to obtain an aerogel similar to the wet gel skeleton structure, scientists have proposed several feasible measures: freeze drying, conventional drying after gel modification, supercritical drying, etc.
(1) Supercritical fluid drying technology:
Supercritical drying technology is the first technology to realize batch preparation of aerogel, and it is also the technology adopted by many aerogel enterprises at home and abroad at present. Through pressure and temperature control, the solvent can reach its own critical point in the drying process. The solvent in the supercritical state has no obvious surface tension, so that the gel can maintain a good skeleton structure in the drying process, and the aerogel can be prepared by removing a large amount of liquid in the coagulation on the premise of maintaining the original structure.
The key to this technology lies in temperature and pressure control, as well as controlling the appropriate drying rate. Common drying media include methanol, ethanol, carbon dioxide, etc. Supercritical carbon dioxide drying is usually used.
(2) Normal pressure drying:
A new type of aerogel preparation process is currently the most active research and development potential of aerogel mass production technology.
The principle is to select a solvent with low surface tension to replace water and alcohol with high surface tension in the pores of wet gel, and then hydrophobic modify the surface of gel to minimize the shrinkage of gel; In addition, the influence of capillary pressure can be reduced by adjusting the uniformity of gel pores and enhancing the strength of the network skeleton, so that the structure and properties of the aerogels prepared under normal pressure are similar to those of the aerogels prepared by supercritical drying.
The difference between the two:
Supercritical drying uses high-pressure equipment, with a general working pressure of up to 7MPa~20MPa. The initial investment is high, and the operation and maintenance costs are also high; The atmospheric pressure drying technology adopts conventional atmospheric pressure equipment, which does not require high pressure conditions, has low initial investment, but has a high technical threshold, requiring high requirements for formula design and process combination optimization.

Structure control and characterization of aerogels
1. Structure control of inorganic aerogels:
Taking SiO2 aerogel as the representative of inorganic aerogel gel, the preparation of its wet gel is mainly achieved through the sol gel process, in which the concentration of catalyst is an important factor affecting the structure of the aerogel gel.
2. Organic aerogel and carbon aerogel gel:
The unique properties of organic aerogels and carbon aerogels in mechanics, acoustics, electricity, optics, and heat, in the final analysis, lie in their unique nano network structures. The catalyst concentration, reactant concentration and ratio, solvent selection, reaction temperature control, and aging effect are all factors that affect its structure