Aerogel is a kind of lightweight solid material that aggregates nano colloidal particles to form a nano framework and a nano porous network structure, and is filled with gaseous dispersion media in the pores. It has high void ratio (up to 99.8%), very low density (as low as 3mg/cm3), very high specific surface area (up to 2000m2/g), ultra-high pore volume ratio, high temperature resistance, high elasticity, strong adsorption, catalysis and other characteristics. Aerogels can be divided into three categories: inorganic gel, organic gel and organic-inorganic hybrid gel. Among them, inorganic aerogels are based on inorganic materials, including simple gel (such as carbon, graphene, metal gold, etc.), oxide aerogels (such as SiO2, Al2O3, TiO2, SiO2-Al2O3, TiO2, B2O3-SiO2, CuO-ZnO-ZrO2, CuO-ZnO-Al2O3, MgO-SiO2-Al2O3, etc.) and sulfide aerogels. Organic aerogels are based on organic substances, mainly including phenolic gel, cellulose aerogels, polyimide aerogels, polyurethane (polyurea) aerogels, chitosan aerogels and chitosan cellulose aerogels. The organic-inorganic hybrid aerogels make use of the advantages of organic and inorganic materials to realize the special functionalization of aerogel materials.

1.  Research Status of Aerogel Materials

In 1931, American scholar Kistler successfully prepared SiO2 aerogel materials by using supercritical ethanol fluid drying method and sodium silicate as raw material, while maintaining the gel structure, replacing the ethanol liquid in the network structure with gas. Later, he successively prepared inorganic aerogels such as Al2O3, W2O3, Fe2O3, NiO3, and organic aeronenenebd gels such as cellulose, gelatin, agar. Major countries have paid great attention to the research of aerogel materials, developed a variety of new aerogel materials, and expanded the application range of aerogel.

（1） Major countries have developed a variety of new aerogel materials

The preparation process of aerogel can be divided into two steps: one is to prepare gel through the sol gel process; The other is to use a certain drying method to replace the liquid substances in the gel with gas, so as to prepare the aerogel. The drying process can be divided into supercritical drying, subcritical drying, freeze drying, atmospheric drying and other methods. Among them, the sol gel process is the core process for the preparation of aerogels, which directly determines the various microstructures and properties of aerogels, including hydrolysis and polycondensation.

In recent years, researchers from China, the United States and Europe have developed a variety of new aerogel materials such as bio based aeronenenebb gel, graphene aeronenenebc gel and polymer aeronenenebd gel by improving the preparation process of aerogels. Researchers at the University of Colorado in the United States used the waste from the beer brewing industry as the culture medium, used bacterial cellulose prepared by Acetobacter, and prepared a bacterial cellulose aerogel material through supercritical drying and other methods, which has the characteristics of low thermal conductivity. Researchers from the French National Science Research Center used hydrothermal treatment method different from the traditional preparation process to prepare tannin based carbon aerogels with high specific surface area and specific electric capacity. Researchers from Shandong University have successfully prepared a high-performance amidoxime modified cyclodextrin/graphene aerogel, which shows strong affinity and selectivity for uranium in seawater. It has excellent uranium extraction ability in natural seawater, and 19.7mg/g of uranium can be adsorbed in 21 days. Researchers from Sichuan University, China, used bi-directional oriented carbon aerogels to composite multi walled carbon nanotubes, and developed a new polymer aerogel material that can maintain functionality and hyperelasticity at extreme temperatures. It can play a role in the temperature range of - 196 ℃ to 500 ℃.

（2） Biomass based aerogel material has become a major national research hotspot

Carbon aerogel (CA) is a new type of nano porous carbon material obtained from organic gel as the precursor after pyrolysis in inert gas atmosphere. At the same time, it has the characteristics of high porosity, high specific surface area and low density of aerogel, as well as the characteristics of carbon materials such as heat resistance, acid and alkali resistance and high conductivity. However, due to its complex process, long production cycle, small production scale and expensive raw materials, it is easy to cause environmental pollution, The industrial production and application of carbon aerogel are limited. However, biomass raw materials have a wide range of sources, low cost and rich carbon sources, so it is an economic and sustainable production method to prepare environmentally friendly porous carbon fiber aerogels from biomass raw materials.

Chinese, American and European researchers have carried out research on the preparation and application of biomass based aerogel materials, and achieved a series of research results. Researchers from the French National Science Research Center dissolved the cellulose material in sodium hydroxide solution to prepare a new highly porous pure cellulose aerogel material, whose internal specific surface area is 200-300m2/g and density is only 0.06-0.3g/cm3. Researchers at the University of Colorado in the United States used the waste of the beer brewing industry as the culture medium to prepare bacterial cellulose using Acetobacter, and then prepared bacterial cellulose aerogel materials with low thermal conductivity by supercritical drying and other methods. Researchers from the China Australia Advanced Materials and Manufacturing Research Institute (IAMM) of Jiaxing University in China developed cellulose nanofiber based gel with underwater mechanical tenacity, high elasticity and super hydrophilicity, which can be used for water in oil lotion separation and solar steam power generation. It solved the problem of poor water resistance and low underwater mechanical tenacity of cellulose aerogel, and broke the barrier of its application.

（3） 3D printing aerogel materials have made many technical advances

Due to the limited mechanical properties of traditional aerogels, it is difficult to form the required complex shape structure through post processing. Therefore, 3D printing technology for customized preparation of complex shape structure materials is expected to become an advanced manufacturing technology that breaks through the bottleneck of application of aerogel materials. In 2015, researchers at the University of California in the United States prepared graphene aerogels through 3D printing technology for the first time. Since then, 3D printing aerogels have gradually become a research hotspot. At present, there are three main printing methods in the preparation of 3D printing aerogel materials: extrusion, cold field assisted on-demand drop (DOD) and UV curing. According to the main composition of 3D printing aerogels, they can be divided into 3D printing carbon aerogels, 3D printing inorganic aerogels and 3D printing organic aerogels. The researchers of Akron University in the United States first used the stereo light curing molding (SLA) technology to make small bricks similar to Lego bricks, and then used the fused deposition manufacturing (FDM) process to make the forming mold. After injecting aerogel into the mold and bricks, after demoulding, curing and other post-processing, the modular production of aerogel bricks was realized. Researchers in the Swiss Federal Laboratory of Materials Science and Technology directly use the slurry of silica aerogel powder to directly write and print micro silica aerogel objects. This object has high specific surface area and ultra-low thermal conductivity, which can be used as thermal insulator and micro air pump and can degrade volatile organic compounds.

2.  Main applications of aerogel materials

Aerogels have the characteristics of extremely low density, ultra-high porosity, low refractive index, low thermal conductivity, low acoustic impedance, etc., which are not available in general solid materials. These characteristics make it have great application prospects in the fields of thermal insulation, biomedicine, sound insulation, adsorption and so on.

（1） Thermal insulation field

The thermal conductivity of aerogel material is very low, and it is the best known material for heat insulation and heat preservation. NASA has comprehensively compared the thermal insulation effect, quality, material flexibility, mechanical strength, thickness and other properties of porous materials, foam materials, phase change materials, aerogel materials and other materials, and found that aerogel not only has good comprehensive properties, but also has more excellent thermal insulation performance, which can be used as a candidate material for space suits. In addition, aerogel materials are also widely used in civilian fields such as energy-saving windows, roof solar collectors, thermal insulation coatings, as well as military fields such as aircraft black boxes and aircraft cabin insulation.

（2） Biomedical field

Aerogel materials have biocompatibility, appropriate mechanical properties, biodegradability and other characteristics, and have been used in implantable medical devices, non-invasive imaging, bone grafting, biosensors and other fields. In addition, aerogel materials can also be prepared into microspheres for drug release to achieve the function of targeting and controlling drug release to target cells, which has potential application value.

（3） Sound insulation field

Aerogel material not only has a large internal surface area, which can make sound waves attenuate by multiple reflections on its internal surface, but also has a very high porosity. The nano scale pores on its surface make the air viscous flow speed close to the Knudsen diffusion speed of air molecules, so that part of the sound energy transmitted through the air is consumed, achieving good sound insulation effect.

（4） Adsorption field

Aerogel materials have very high porosity, which can effectively adsorb organics and remove metal ions. Compared with traditional methods, the use of carbon gas gel for electro adsorption to remove metal ions from solution has the advantages of reducing secondary pollution, energy saving and renewable. In addition, the modified aerogel material can also selectively adsorb specific substances. For example, tetraethylenepentamine loaded in silica aerogel gel can efficiently adsorb carbon dioxide.

3.  Future research directions of aerogel materials

There are various kinds of aerogel materials, and the synthesis process has gradually formed a system. However, there are still some problems in the research of aerogel at present: the thermal conductivity of aerogel at high temperature increases rapidly; Compared with metal materials, the overall mechanical strength of aerogels is still weak; The adhesion between aerogel and reinforced matrix materials such as fiber is poor; Many organic solvents will be used in the production of aerogel, which will cause environmental pollution and is not conducive to large-scale production; It is difficult to recycle aerogel, which is not conducive to sustainable development.

The future research direction and development trend of aerogels mainly focus on the development of new aerogel materials, such as cellulose aerogels, graphene aerogels, perovskite aerogels, non-metallic elemental aerogels, etc; The structure of aerogels was adjusted by using different precursors, optimizing synthesis methods and changing reinforcements; Expand the application field of aerogel, etc.

4.  Conclusion

The development and application of aerogel materials are still in the process of continuous exploration. Although research progress has been made in some fields, problems such as insufficient resource utilization, high manufacturing costs, and defects in technology and process still need to be solved. Major countries will also develop fierce competition in this field. China's research level in the field of aerogel materials is in the forefront of the world. We should continue to carry out research on the preparation and large-scale production process of aerogel materials, expand the application field of aerogel materials, and maintain a leading position in this field.