Application of silica aerogel in the field of thermal insulation

Thanks to the many excellent properties of silica aerogel, it has been successfully applied in many fields, such as catalyst carrier materials, sound insulation materials, thermal insulation materials, toxic gas adsorption materials and cosmic dust collection materials, etc. Huge application prospects. With the continuous improvement of the energy density of new energy vehicles, especially lithium-ion batteries, the heat insulation and fire prevention of batteries has become one of the most important topics.

01 Aerospace

High-performance thermal insulation materials are one of the key components of thermal protection for aerospace vehicles. For hypersonic vehicles, under the condition of long-term aerodynamic heating, the surface of the body will generate extremely high temperatures. In order to prevent the main structure of the aircraft and internal instruments The equipment is harmed by thermal erosion, so it is very important to choose a thermal insulation material with excellent comprehensive performance.

On the one hand, the heat insulation material needs to effectively block the flow of external heat to the inside of the body, so as not to affect the normal operation of the related equipment of the body; It is of great significance to increase the payload and increase the flight distance. The density of silica aerogel is only about 0.08g/cm3, and the thermal conductivity at room temperature is as low as 0.016W/(m·K), which can meet the needs of aerospace for lightweight and efficient thermal insulation materials.

The light weight and low thermal conductivity of silica aerogel make it one of the most concerned materials in aerospace insulation materials, but there are still two problems in the application of silica aerogel in aerospace: 

① The mechanical strength of aerogel itself is low, so it is usually necessary to combine aerogel with fiber materials in aerospace applications. 

②The limit working temperature of silica aerogel is usually less than 600°C, which cannot be applied to the heat insulation of the end faces of supersonic or hypersonic aircraft that are developing rapidly. In the future, multi-phase fusion and microstructure design should be considered to integrate The application temperature range of silica aerogel is extended to higher temperature.

02 Military Industry Field

The demand for high-performance technical products in the military industry is stronger than that in the civilian field. As an important member of new high-performance thermal insulation materials, silica aerogel has been favored by the military industry.

In addition, the NASA Ames Research Center of the United States used aluminum silicate fibers as the supporting framework, and filled the pores in the refractory fiber framework with silica aerogel to prepare SiO2 aerogel insulation tiles reinforced with aluminum silicate fibers, which have been applied to nuclear submarines , The nuclear reactor of a steam-powered missile destroyer. The thermal conductivity of this material is lower than that of ordinary refractory fiber materials, which can effectively reduce the amount of thermal insulation materials and increase the usable space in the cabin. At the same time, it can maintain the temperature in the cabin and improve the working environment in the cabin. The thermal insulation tile is also used in weapon power devices to block heat radiation, which is beneficial to the anti-infrared reconnaissance of weapons and equipment; in addition, aerogel is also used in military thermal batteries, which can improve the thermal life of military thermal batteries.

Giving silica aerogel more functions is one of the main directions of its application and research and development in the military field. For example, military protective clothing not only needs to have thermal insulation function, but also needs to have infrared shielding function (stealth), so as to better adapt to modern war needs. Therefore, how to realize the multifunctional design of silica aerogel is an important issue that needs to be considered in its application in the military field.

03 Industrial Storage Tanks and Piping

At present, the common insulation materials for HVAC and other living pipes on the market are mainly organic polymer foams, such as polyurethane foam, phenolic foam, polystyrene foam, etc. However, these materials are flammable and have a high fire risk. Silica aerogel is safe, light in weight, good in heat insulation performance, and has great advantages in comprehensive performance.

Studies have shown that the thermal conductivity of the covered material can be reduced to 0.084W/(m·K) when the aerogel thermal insulation composite film material is covered on the surface of the metal pipe. In addition, the fire resistance limit time of the pipeline material covered with the aerogel composite film can reach 70 minutes, which effectively improves the safety of the pipeline. In the application of chemical pipeline insulation, silica aerogel composite felt is mainly used, without special waterproof measures (hydrophobic rate ≥ 99%), and it can still be constructed in rainy or humid environments.

 Application of aerogel felt pads on chemical pipelines

In addition, the aerogel composite felt has good anti-seismic and tensile properties, no particle accumulation and settlement during use, and a long service life. In the thermal insulation layer application of directly buried steam pipelines, under the premise of meeting the maximum allowable heat loss, the thickness of the thermal insulation layer required by the silica aerogel composite felt can be saved by 40% to 54% compared with the glass fiber felt. Thereby reducing the space occupied by direct burial of pipelines. Silica aerogel felt has excellent thermal insulation performance, and has a better space to play under the conditions of high steam temperature and narrow and harsh site space. The silica aerogel felt mat has also been successfully applied to oil pipeline insulation and CNOOC Hainan LNG transmission pipeline. The long-term stable operation of the pipeline has verified its excellent thermal insulation and safety and stability performance.

04 Boiler

Taking advantage of the excellent thermal insulation performance of aerogel, applying it to boiler surface thermal insulation can greatly reduce the boiler surface temperature and boiler heat loss. In actual use, the fiber matrix and silica aerogel are often combined to form an aerogel felt mat, which is then applied to the boiler body. After the boiler uses aerogel composite materials, the surface temperature of the furnace body can be reduced by about 39°C, the thermal efficiency is increased from 79.7% to 81.9%, and the energy saving is 2.2%.

Application of aerogel composites in boiler systems

The temperature of the boiler is generally high, so the fibers in the silica aerogel composite material must have high temperature resistance. It is a more preferred solution to use high temperature resistant polycrystalline mullite fibers and silica aerogel composites.

Silica aerogel is currently less used in boilers, mainly related to its manufacturing cost. On the other hand, the temperature of industrial boilers is relatively high, and the long-term working temperature limit of aerogel is generally lower than 600°C. Improving the high temperature resistance of silica aerogel is the future development trend.

05 Building insulation and home life

Silica aerogel is lightweight, low thermal conductivity, long life and good hydrophobicity, which can meet the needs of thermal insulation, fireproofing, soundproofing 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 sheet, aerogel concrete and mortar, and roof solar collectors.

5.1 Silica aerogel energy-saving glass

The transparent envelope structure is the weak link of building energy conservation, among which glass is the main material of the transparent envelope structure, and its energy-saving performance is very important. The good light transmission, heat insulation and noise reduction capabilities of silica aerogel make it have obvious advantages in the application of architectural field, especially architectural glass.

Application of aerogel glass in civil buildings

Applying aerogel to glass can not only reduce the heat dissipation of the glass, but also meet the lighting requirements. On the basis of ensuring the appearance and lighting, silica aerogel glass has better heat resistance, stronger radiation resistance, and can also be used for color adjustment and sound absorption, which has significant application advantages. At present, the application of silica materials in architectural glass mainly includes aerogel coated glass, bulk aerogel glass and granular aerogel filled glass.

Aerogel glass is still in the stage of industrial research and development, the relevant technical barriers are high, and there are only a small number of engineering applications in practice. At present, the existing manufacturers of particle aerogel-filled glass are mainly concentrated in developed countries in Europe and the United States. In 2015, China achieved mass production in Changsha for the first time. However, aerogel glass is still in its infancy, and there is still a long way to go before practical application.

5.2 Silica aerogel coating

Aerogel thermal insulation coating is an important branch of silica aerogel application. The preparation of aerogel thermal insulation coating comprises the following steps: 1. silica aerogel particles, stabilizer (or defoamer) and water are mixed and ground to form a 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 and hollow glass beads, etc.) and dyeing agents are mixed to obtain silica aerogel coatings.

Aerogel coatings have low thermal conductivity, simple construction, and great application potential. However, there is still no good method to solve the problems of poor dispersion and easy agglomeration of silica aerogel in slurry, which lead to high thermal conductivity of coatings. problem.

5.3 Silica aerogel felt

Silica aerogel felt refers to the thermal insulation felt mat prepared by compounding silica aerogel with fiber reinforcement in the sol stage, followed by gelation, aging, drying and other processes.

On the one hand, the silica aerogel felt mat well retains the excellent thermal insulation performance of 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 mats mainly includes inorganic fibers and organic fibers. The inorganic fiber matrix mainly includes glass fiber, alumina fiber and quartz fiber. Inorganic fibers have high thermal stability and low thermal expansion coefficient, but their flexibility is poor and the binding force with aerogel is weak, which is easy to cause "powder dropping". Organic fibers, such as polypropylene fibers, polyester fibers, aramid fibers, cellulose fibers, etc., can give aerogel mats better flexibility and aerogel bonding strength, but organic fibers have poor thermal stability and are not suitable For practical thermal insulation applications.

At present, the fiber reinforcement part of the aerogel mat on the market is mainly made of glass fiber needle-punched felt, and the service temperature can generally reach 550°C. 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 construction engineering materials. Compounding silica aerogel with concrete mortar can increase the porosity of concrete mortar and optimize the internal heat transfer path, thereby improving the thermal insulation performance of concrete mortar.

5.5 Silica aerogel for solar collectors

Aerogel can be applied in heat collecting plates, water storage tanks, pipes and heat collector insulation systems of water heaters, so as to improve the heat collection efficiency and reduce heat loss of existing solar water heaters.

Solar collectors equipped with aerogel with a thickness of 20mm have excellent thermal insulation properties. Compared with traditional receivers, 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%. The device efficiency can be increased by 0.01%~2.92%.

 06 Refrigerated Container

Refrigerated containers need to have good thermal insulation performance, can maintain a low temperature environment, and are used for the transportation of various perishable items. The heat insulation materials of traditional refrigerated containers generally use materials such as glass fiber, asbestos, rock wool, polystyrene foam blocks, and foamed polyurethane. Organic materials have excellent heat insulation effects but are not environmentally friendly. Although traditional inorganic materials are non-toxic and harmless But the insulation performance is more general.

The use of silica aerogel to replace traditional materials as insulation materials for low-temperature systems such as refrigerated containers can take into account the needs of environmental protection and thermal insulation performance. Herchester Company of Germany and Cabot Company of the United States have carried out a lot of research work on SiO2 aerogel composite materials, and the products developed by them have been successfully applied to the insulation system of refrigerators.

 07 New Energy Vehicles

At present, the commonly used thermal insulation materials include glass fiber cotton, aluminum silicate cotton, and composite heat insulation boards. There is an urgent need to find a fire-proof and thermal insulation material with high temperature resistance, good thermal insulation performance and long life.

Silica aerogel has significant advantages in thermal insulation performance. Compared with traditional thermal insulation materials, only 1/5~1/3 of the thickness can 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 Hi-Tech.

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

①The heat-resistant temperature of the existing silica aerogel is ≤550°C, but the peak thermal runaway temperature of lithium-ion batteries exceeds 600°C, so the development of aerogel materials with higher heat-resistant temperatures is one of the research trends;

②Using supercritical drying process to prepare aerogel composite materials is costly, so the development of relatively low-cost atmospheric drying process is an important direction for large-scale application in the future;

③ How to balance the contradiction between aerogel thermal insulation and battery heat release under high load is a hot problem that needs 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 aerogels at high temperatures.

(2) Silica aerogels are mainly applied in the form of composite mats, and there is a problem of "powder dropping". Therefore, it is necessary to explore methods such as surface modification and fiber arrangement optimization to enhance the bonding force between aerogel particles and fibers.

(3) When aerogel powder is mixed in thermal insulation coatings, composite panels, etc., it is prone to phase delamination and leads to a decrease in the performance of thermal insulation materials. Research on improving the uniform dispersion of aerogel powder in composite materials Safety and stability are one of the key issues to be solved in its application.

(4) The supercritical drying process with high cost is used in the existing silica aerogel, which limits its large-scale application. It is one of the future development trends to study the use of low-cost preparation methods such as atmospheric pressure drying process to reduce its production cost. one.