01 Introduction to aerogel

Since its invention in the 1930s, aerogel has set 15 "Guinness World Records", and is known as one of the "top ten magic materials that change the world" by virtue of its extreme light, heat insulation, minimum dielectric constant and other hard strength. Aerogel is a kind of highly dispersed solid material, which is composed of colloidal particles or polymer molecules to form a nano porous network structure and filled with gaseous dispersion medium in the pores. Aerogel is the lightest solid in the world, also known as "solid smoke". In the early 1930s, Samuel S. Kistler of Stanford University had prepared SiO2 aerogel by hydrolyzing water glass.

1.1 Classification of aerogels

According to the skeleton composition of aerogels, they can be divided into three categories: inorganic aerogels, the main types are silicon aerogels and metal oxide aerogels gel; Organic aerogel, the precursor used in this type is mostly resorcinol formaldehyde; Under the conditions of inert atmosphere and high temperature, carbon gel retain only the carbon skeleton structure. It is easy to understand that the aerogel gel composite is an aerogel+composite structural material that becomes an aerogel gel composite through process. According to the dimensions of the material, it can be divided into one-dimensional, two-dimensional, three-dimensional, etc., as shown in the following figure.

1.2 Characteristics of aerogel

(1) Amazing surface area: Although aerogel is a solid, 99% of this material is composed of gas, which makes it look like a cloud. Scientists say that because it has millions of holes and folds, if we count the surface area of 1 cubic centimeter of aerogel, it is as big as a football field;

(2) High specific strength: 2.801 g aerogel bears 10000 g pressure, without any damage after several hours;

(3) With extremely low thermal conductivity, the performance comparison between aerogel and traditional thermal insulation materials is as follows:

(4) The porosity is high, ranging from 80% to 99.8%;

In addition, aerogel also has its own shortcomings, such as low strength, high energy consumption and time consumption, and poor elasticity.

1.3 Physical properties of aerogel

Aerogels exhibit unique properties in mechanics, acoustics, thermodynamics, optics, etc. Extremely low thermal conductivity; Having a large specific surface area; The scattering of light and sound is much smaller than that of traditional porous materials; Its small pores not only adsorb pollutants like a sponge, but also serve as air pockets. It is found that some forms of aerogels made of platinum can be used to accelerate hydrogen production. Therefore, aerogels can also be used to produce hydrogen based fuels. Its related physical properties are as follows:

02 Preparation method of aerogel

The preparation of aerogels usually consists of two processes, namely the sol gel process and drying. There are dozens of aerogels that have been developed so far. They are divided into single component aerogels such as SiO2, Al2O3, V2O5, TiO2, etc., multi-component aerogels such as Al2O3/SiO2, TiO2/SiO2, Fe/SiO2, Pt/TiO2, (C60/C70) - SiO2, CaO/MgO/SiO2, etc., organic aerogels such as RF, MF, etc., and carbon aeronenenebe gels.

2.1 Sol gel method

(1) The porous network structure of aerogel first forms alcohol gel through hydrolysis condensation reaction under certain conditions. For example, preparation of SiO2 alcohol gel

Si (OH) 4 generated by hydrolysis is further dehydrated and condensed:

Finally, the polymer with siloxane bonds and silicon oxygen bonds as the main body is formed, and the alcohol gel with spatial network structure is formed

2.2 Drying

When drying wet gel, the solvent should be removed under the condition of keeping the original nano network structure unchanged. The commonly used drying methods include supercritical drying, atmospheric drying, and freeze-drying.

In the conventional drying process, the presence of surface tension between gas and liquid can cause material shrinkage and fragmentation. In order to keep the gelled structure while drying, the supercritical drying process is used in the preparation of gel, that is, alcohol gel is placed in a high-pressure container, and the solvent in the container is replaced by a drying medium. Then the pressure and temperature of the container exceed the critical point of the drying medium, the gas-liquid interface disappears, and the surface tension no longer exists. At this time, the drying medium is released through the drain valve of the container, The aerogel material with nanoscale fine network structure is obtained by cooling down. The schematic diagram of the supercritical drying device is shown in the following figure:

03 Thermal insulation principle of aerogel

Silica aerogel is by far the best material for thermal insulation. There are three ways of heat transfer: thermal convection, thermal conduction, and thermal radiation. The best insulation material needs to be a wooden barrel type. Although the thermal conductivity of air is very low, we can feel the temperature of air conditioners and furnaces within a certain distance because of the presence of air thermal conduction and radiation. The gel, with its extremely low bulk density and the curved path of the nano grid structure, prevents the heat transfer in the gaseous and solid state. At the same time, because its pore size is lower than the average free path of air molecules under normal pressure, the air molecules in the pores of the gel are nearly static, so the convection and heat transfer of the air are limited, and the gap walls tend to be "infinite" to minimize the heat radiation. Aerogel perfectly resists three heat transfer modes, and its thermal conductivity even reaches below 0.013W/m.K, which is lower than 0.025W/m.K of static air at room temperature, so it is the most perfect thermal insulation material so far.

3.1 "Infinite Path" Effect

The almost infinite number of nanopores allows heat flow to only transfer along the pore wall in a solid, and the nearly infinite number of pore walls constitute an "infinite path" effect, reducing the ability of solid heat conduction to the lowest limit.

3.2 "Zero Convection" Effect

When the pore diameter in the aerogel material is less than 70nm, the air molecules in the pores will lose the ability of free flow and relatively attach to the pore wall. At this time, the material is in an approximate vacuum state.

3.3 "Infinite number of heat shields" effect

The number of internal pore walls in the material tends to be "infinite," and for each pore wall, it serves as a heat shield, thereby reducing radiation heat transfer to almost the lowest limit. In addition, when used above 400 ℃, a sunscreen is needed to enhance the resistance of aerogel to high-temperature infrared radiation.

04 Application of aerogel

Aerogels have excellent properties and have been widely used in petrochemical, military, aerospace, battery, environmental protection, construction, transportation and other fields. Aerogels have obvious advantages over raw materials in these fields, so there is a huge space for replacement.

4.1 Petrochemical field

In the petrochemical field, gas gel can be used as external thermal insulation materials with excellent thermal insulation performance, such as distillation tower, reaction pipeline, storage tank, pump, valve, natural gas and LNG liquefied gas pipeline, deep-sea pipeline, etc. Wrapping the high-temperature steam, heat transfer oil or fluid medium pipeline with aerogel, on the one hand, reduces the heat loss of pipeline exposure; on the other hand, these areas are often limited by weight and space, requiring lightweight and thin insulation materials. Aerogel is the only perfect material. At the same time, in the treatment of offshore oil spill accidents, aerogel is recognized for its light weight and strong adsorption capacity.

4.2 Environmental protection field

In the field of environmental protection, cellulose aerogel can be used as an adsorbent to adsorb oil and other toxic organics from water, and is widely used to adsorb dye wastewater. In addition, biomass carbon aerogel can remove a variety of heavy metal ions in water, such as Co (II), Cd (II), Pb (II) and Sr (II).

4.3 Construction field

In the field of construction, the insulation and insulation of doors, windows, and walls in houses are increasingly being valued. The existing insulation materials or insulation capabilities are not ideal, or the thickness is too thick or heavy to achieve the desired effect. There are also some materials with good insulation capabilities but poor flame retardancy, which can easily cause house fires. While aerogel can be used as an upgrade and replacement of existing thermal insulation materials, while taking into account fire prevention, sound insulation and other functions, it is expected to overturn the existing pattern of building thermal insulation materials.

4.4 Military Industry

In the field of military industry, the performance of aerogel has been fully verified. The 6mm bulletproof aerogel covered on the military vehicle can withstand the destructive force of explosives. Dongfeng-17 uses aerogel thermal insulation material as its coat, so that Dongfeng-17 will not be damaged by the high temperature generated by air friction while accelerating extremely fast, and the good wave transmission performance of aerogel material will not block the guidance device inside Dongfeng-17. In addition, aerogel can be used as the outer material of aircraft, ships/boats, tanks, missiles, etc. to prevent radiation, absorb infrared and diffuse reflection waves to achieve stealth functions, and shield its own electronic signals to achieve anti reconnaissance functions. The low sound velocity and high porosity of aerogel make it an ideal acoustic resistance coupling material and the best underwater acoustic reflection material for ultrasonic detectors. Of course, aerogel is also used in military thermal tents and other fields.

4.5 Aerospace field

In the aerospace field, the requirements for thermal insulation, lightweight, and compressive strength of materials are the most stringent. The Russian "Peace" space station, the American "Mars Pathfinder" probe and the "Mars rover" probe, China's "Long March 5" carrier rocket and the "Zhurong" Mars rover have all used aerogel materials for thermal insulation, and the "Star" spacecraft has also used aerogel collection "gloves" to collect cometary dust. In August 2016, the 306 Institute of the Chinese Academy of Aerospace Science and Technology, in collaboration with Huaxing Meike New Materials Technology (Jiangsu) Co., Ltd., held a signing ceremony to jointly establish Gasification

The international R&D center of gel technology strives to build an internationally advanced and domestically leading aerogel technology R&D base.

4.6 Battery field

In the field of batteries, thermal runaway accidents often occur in lithium-ion power battery packs, and preventing thermal runaway cells from transferring heat to other battery systems is the main solution. Aerogel felt has the characteristics of fire prevention, heat insulation and flame retardancy, and is soft and easy to process. It is an ideal prevention material; In addition, in the field of thermal battery application, aerogel, as the thermal insulation material of thermal battery insulation cylinder, can meet the requirements of high performance and long life of thermal batteries in many fields. At present, the newly developed aerogel glass fiber felt can improve the high temperature resistance of the battery pack to more than 800 ℃, greatly improving the heat resistance of the battery.

05 Industry Analysis

Since its discovery, aerogel has gone through three times of industrialization, and is now in the fourth wave of industrialization led by Chinese enterprises. In 1931, Steven S. Kistler published Copolymerization Diffusion Aerogel and Jelly in Nature magazine, marking the discovery of aerogel, and then went through three industrialization successively. Since the establishment of Aspen in 2001 and the third industrialization of aerogel, the global aerogel industry has gone through the R&D and introduction stages in the past 20 years, and is now in the early stage of growth. Since 2010, the first batch of domestic aerogel production enterprises have successfully developed the application market of industrial equipment and pipeline energy conservation, new energy vehicle safety protection, rail transit car and hull fire protection, thermal insulation and heat preservation, and the aerogel market has become increasingly mature.

5.1 The supply and demand of aerogel are booming, and the aerogel is in a rapid growth period

Due to its excellent thermal insulation performance, gel materials have blossomed at many points in the downstream of gel under the background of "double carbon", which is an industry with both high growth and scale. At present, relevant domestic enterprises include Morninglight New Material, Hongbai New Material, Jianghan New Material, Pan Asia Microporous with mature technology, which have the largest capacity planning and integrated capacity, as well as ChemChina, which has the leading capacity of aerogel, and Sanfu, which has the advantage of upstream raw material layout.

5.2 Policy support, vast downstream space

With the key support of national policies, China's aerogel market has become the world leader. According to statistics, the market size of aerogel in China is expected to reach 4.86 billion yuan and 12.26 billion yuan respectively in 2023 and 2025, and the compound annual growth rate from 2021-2025 will reach 68.9%. Among them, oil and gas pipelines are the most important downstream application field of aerogels. It is estimated that the market space of aerogels in the oil and gas field will be about 5.44 billion yuan in 2025. The market scale of building thermal insulation materials exceeds 170 billion yuan, while the permeability of aerogel is less than 1%. The future application space is broad. It is estimated that the demand for aerogel in the construction field will reach 2 billion yuan in 2025.

At present, the new energy vehicle field constitutes the fastest growth in the downstream of aerogel. At present, most of the top ten battery manufacturers in China have used aerogel insulation materials. Against the background of automobile manufacturers' pursuit of higher energy density, the permeability of aerogel is expected to accelerate. It is estimated that aerogel will have a space of 3.35 billion yuan in this field in 2025, and the compound growth rate from 2021 to 2025 will be as high as 89%. Since 2020, aerogel heat insulation sheets have gradually been widely used in the field of power batteries, and leading battery manufacturers such as Ningde Times, Fodi Battery, Zhongchuangxin Aviation, Guoxuan High tech, Xinwangda, etc. have started to use them. In addition to battery manufacturers, BYD, Geely, CRRC and other automobile manufacturers have also started to improve the safety performance of vehicles through aerogel.

5.3 Domestic enterprises actively expand production and have more advantages in integrated layout

China's gel industry is in the fourth wave of industrialization. Driven by downstream applications, domestic enterprises have accelerated the expansion of capacity, vigorously promoted the cost reduction, efficiency increase and production expansion of gel. According to statistics, as of March 2023, the capacity of China's gel materials is 273600 m3/year, and the proposed capacity is 1238500 m3/year.

Among them, tetraethyl orthosilicate is one of the main raw materials in the upstream of gel. At present, the domestic production capacity is only about 51500 tons, which is not enough to support the current expansion plan of gel, and the price will rise significantly in 2022. Therefore, integrated enterprises with upstream production capacity of ethyl orthosilicate, or upstream production capacity of silicon tetrachloride and trichlorosilane of ethyl orthosilicate will have more advantages in competition. At the same time, if the silane enterprise can realize the double cycle of hydrogen chloride and ethanol, the cost of the aerogel collagen material can be approximately reduced to the cost of silica fume, which further reduces the cost significantly.

06 Summary

The excellent physical properties of gel make it have broad application prospects in many fields such as thermal insulation, adsorption catalysis, aerospace, microelectronics and so on. However, its weak mechanical properties have always hindered the realization of real daily application of gel. Especially for many inorganic aerogels, the fragile nature and the lack of elastic properties greatly limit their applications in various fields. Although the current methods can significantly improve the mechanical properties of aerogels, these methods also lead to new problems. For example, the sample made by compounding the aerogel with the fiber has a serious problem of "powder falling", and the aerogel is very easy to fall off from the surface of the fiber substrate, so its application scope is very limited; However, the flame retardancy of the sample prepared by compounding the aerogel with organic compound will be greatly reduced, so it cannot be used in the high-temperature field; Although the mechanical properties of aerogels can be improved by introducing organic substituents into the forebody, this method is currently only applicable to the preparation of silica aerogels, and it is not universal for most inorganic aerogels. Therefore, on the premise that other properties of inorganic aerogels will not be affected, finding a way to improve their mechanical properties will still be one of the most important development directions of future aerogels.