5. Building insulation and home life

Under the strategic background of carbon neutralization, building thermal insulation materials are also developing towards energy conservation, environmental protection and high efficiency. At present, the main building thermal insulation materials on the market, such as rock wool, glass wool and other inorganic fiber cotton, have problems such as loose fiber structure, easy moisture absorption, etc., and the thermal insulation performance will decline significantly during the service life. Organic thermal insulation materials such as polystyrene (PS) and polyurethane (PU) foam have fire risk.

Silica aerogel is light, low in heat conduction, long in life and good in hydrophobic performance, which can meet the needs of thermal insulation, fire prevention, sound insulation and waterproofing in the construction field. At present, the application forms of silica aerogel mainly include aerogel energy-saving glass, aerogel coating, aerogel felt pad, aerogel gel plate, aerogel gel concrete and mortar, roof solar collector, etc.

5.1 Silica aerogel energy-saving glass

Transparent envelope is the weak link of building energy conservation, in which glass is the main material of transparent envelope, and its energy conservation performance is crucial. Silica aerogel has obvious advantages in the application of building, especially building glass, due to its good light transmission, thermal insulation and noise reduction capabilities.

Application of aerogel glass in civil buildings

The application of aerogel to glass can not only reduce the heat dissipation of glass, but also meet the lighting needs. On the basis of ensuring the appearance and daylighting, silica aerogel glass has better heat resistance, stronger radiation resistance, and can also adjust color and absorb sound, which has significant application advantages. At present, the application of silica materials in architectural glass mainly includes aerogel coated glass, block aerogel glass and particle aerogel gel filled glass.

Aerogel glass is still in the stage of industrial research and development, and the related technical barriers are high, so there are only a few engineering applications in practice. At present, the existing manufacturers of granular gel filled glass are mainly concentrated in developed countries in Europe and the United States. In 2015, the first time in China to achieve mass production in Changsha, but the aerogel gel glass is still in its infancy, and there is still a long way to go before its practical application.

5.2 Silica aerogel coating

Aerogel thermal insulation coating is an important branch of the application of silica aerogel. The preparation of aerogel thermal insulation coating includes the following steps: ① silica aerogel particles, stabilizer (or defoamer) and water are mixed and ground to form uniform aerogel slurry; ② Then add resin and dispersant to further stir and disperse; ③ According to actual needs, various additives (such as titanium dioxide, far-infrared ceramic powder, hollow glass beads, etc.) and colorants are mixed to obtain silica aerogel coating.

Flow Chart of Preparation of Silica Aerogel Thermal Insulation Coating

The research on aerogel thermal insulation coating started early abroad. Schmidt et al. prepared aerogel thermal insulation coating by adding silica aerogel to polyurethane film-forming agent in 1998. Its thermal conductivity is only 0.015W/(m · K), which has excellent thermal insulation performance.

Aerogel coating has low thermal conductivity, simple construction and great application potential, but there is still no good method to solve the problem of high thermal conductivity of coating caused by poor dispersion of silica aerogel in slurry and easy agglomeration.

5.3 Silica aerogel felt

Silica aerogel blanket refers to the thermal insulation blanket made by compounding silica aerogel with fiber reinforcement in the sol stage and then preparing it through gel, aging, drying and other processes. The typical production process is shown in the figure.

Industrial production flow chart of silica aerogel felt cushion

On the one hand, the silica aerogel felt pad better retains the excellent thermal insulation performance of the aerogel, and the thermal conductivity can be as low as 0.0142W/(m · K). On the other hand, the aerogel felt pad effectively solves the difficult application problem caused by the low mechanical strength of silica aerogel.

At present, the fiber matrix of aerogel felt pad mainly includes inorganic fiber and organic fiber. Inorganic fiber matrix mainly includes glass fiber, alumina fiber and quartz fiber. Inorganic fiber has high thermal stability and low coefficient of thermal expansion, but its flexibility is poor and its binding force with aerogel is weak, which is easy to cause "powder falling". Organic fibers, such as polypropylene fiber, polyester fiber, aramid fiber, cellulose fiber, etc., can give aerogel felt pads better flexibility and aerogel bonding strength. However, organic fibers have poor thermal stability and are not suitable for actual thermal insulation applications.

At present, glass fiber needle felt is mainly used as the fiber reinforcement of aerogel felt pad in the market, and the service temperature can generally reach 550 ℃. This kind of product has been successfully applied to oil pipelines and urban heat pipe networks.

5.4 Silica aerogel concrete mortar

Cement and concrete are the most common building engineering materials. When silica aerogel is mixed with concrete mortar, it can improve the porosity of concrete mortar and optimize the internal heat transfer path, thus improving the thermal insulation performance of concrete mortar.

SEM images of concrete mortar samples containing 40% aerogel (left) and the change of concrete mechanical strength with the doping content of aerogel

Tao et al. of Norwegian University of Science and Technology mixed silica aerogel powder into concrete and successfully prepared a silica aerogel concrete mortar. When the volume fraction of aerogel is 60%, the density of the aerogel concrete obtained is 1g/cm3, and the thermal conductivity is reduced from 1.86W/(m · K) to 0.26W/(m · K). The thermal conductivity of the concrete mortar obtained by this method is greatly reduced, but its compressive strength will also be reduced. As shown in Figure 10, when the content of aerogel reaches 60%, the compressive strength of concrete decreases from 150MPa to 20MPa, down 86.7%.

5.5 Silica aerogel for solar collectors

The aerogel can be used in the collector plate, water storage tank, pipeline and collector insulation system of the water heater, so as to improve the heat collection efficiency of the existing solar water heater and reduce its heat loss.

Foreign research shows that solar collectors equipped with 20mm thick aerogel have excellent thermal insulation performance. Compared with the traditional receiver, when the inlet heat flow temperature is in the range of 583~823K, and the vertical irradiance is in the range of 400~1000W · m, the aerogel can reduce the heat loss of the collector by 7.3%~10.1%, and the collector efficiency can be improved by 0.01%~2.92%.

Schematic diagram of application of aerogel in solar collector

6. Refrigerated container

Refrigerated containers need to have good thermal insulation performance, can maintain a low temperature environment, and can be used for the transportation of various perishable goods. The thermal insulation materials of traditional refrigerated containers generally use glass fiber, asbestos, rock wool, polystyrene foam block, foamed polyurethane and other materials. The thermal insulation effect of organic materials is excellent but not environmental friendly. The thermal insulation performance of traditional inorganic materials is general although non-toxic and harmless.

Using silica aerogel instead of traditional materials as thermal insulation material for cryogenic systems such as refrigerated containers can meet the requirements of environmental protection and thermal insulation performance. Herchester Company in Germany and Cabot Company in the United States have carried out a lot of research work on SiO2 aerogel composites, and their products have been successfully applied to the thermal insulation system of refrigerators.

Luo Jialian of Guangzhou University and others obtained the cold insulation plate of the refrigerator by compounding glass fiber, water washing cotton and silica aerogel, and compared the thermal conductivity, mechanical properties and hydrophobic properties of aerogel composite cold insulation plate and traditional polyurethane foam plate. It was found that the cold insulation effect of aerogel composite cold insulation plate was 36% higher than that of traditional polyurethane foam plate, showing good application potential in the field of refrigerators.

7. New energy vehicles

With the reform of energy structure, new energy vehicles such as pure electric vehicles, solar vehicles and hybrid electric vehicles have entered the stage of commercialization. As the main energy storage component of new energy vehicles, lithium-ion batteries have developed rapidly, but the thermal safety of lithium-ion batteries cannot be ignored. High temperature, overcharge, internal short circuit and mechanical damage may cause thermal runaway of lithium-ion battery pack in new energy vehicles, causing fire or even explosion, which seriously threatens the life safety of drivers and passengers.

In May 2020, the Safety Requirements for Power Batteries for Electric Vehicles issued by the Ministry of Industry and Information Technology clearly stated that the battery system would not fire or explode within 5 minutes after the thermal runaway of the single battery, providing time for passengers to escape safely. The battery factory or the main engine factory generally uses fireproof and thermal insulation materials between the battery cells and the upper cover of the modules and PACK, so as to delay or prevent the thermal diffusion of the battery pack and the spread of the flame, leaving enough time for passengers to leave the accident site.

At present, the commonly used thermal insulation materials include glass fiber wool, aluminum silicate wool, composite thermal insulation board, etc. However, the above thermal insulation materials have high thermal conductivity, large thickness, take up space, general fireproof and waterproof performance, and fast thermal insulation performance degradation, so it is urgent to find a kind of fireproof thermal insulation material with high temperature resistance, good thermal insulation performance, and long service life.

Silica gas gel has significant advantages in thermal insulation performance. Compared with traditional thermal insulation materials, it only needs 1/5~1/3 of the thickness to achieve the same thermal insulation effect, saving more space for power batteries. At present, it has been tested and partially applied in large lithium ion battery manufacturers such as Ningde Times and Guoxuan High tech.

The aerogel felt acts as a fireproof and thermal insulation layer in the lithium-ion battery pack

The application of silica aerogel thermal insulation composites in new energy vehicles also needs to pay attention to the following issues:

① The existing silica aerogels have a heat-resistant temperature ≤ 550 ℃, but the peak temperature of lithium-ion battery thermal runaway exceeds 600 ℃, so the development of aerogels with higher heat-resistant temperature is one of the research trends;

② The cost of preparing aerogel composites by supercritical drying process is high, so the development of atmospheric drying process with relatively low cost is an important direction for large-scale application in the future;

③ How to balance the contradiction between thermal insulation of aerogel and heat release of battery under high load is a hot problem to be studied.

In the future, the application of silica aerogel in the field of thermal insulation can focus on the following aspects:

(1) The use temperature of silica aerogel is limited, and it cannot meet the growing demand for heat insulation in high temperature areas. It is important to study and improve the thermal stability of silica aerogel at high temperatures.

(2) Silica aerogel is mainly applied in the form of composite felt pad, which has the problem of "powder shedding". Therefore, it is necessary to explore the use of surface modification, fiber arrangement optimization and other methods to enhance the adhesion between aerogel particles and fibers.

(3) When air gel powder is doped in thermal insulation coatings, composite plates and other systems, it is easy to appear phase stratification and lead to the decline of thermal insulation material performance. Research on improving the dispersion uniformity and stability of air gel powder in composite materials is one of the key problems to be solved in its application.

(4) The existing silica aerogel gel adopts the supercritical drying process with high cost, which limits its large-scale popularization and application. One of the development trends in the future is to study low-cost preparation methods such as atmospheric drying process to reduce its production cost.