Thanks to many excellent characteristics 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, showing a huge application prospect of aerogel. With the continuous improvement of energy density of new energy vehicles, especially lithium ion batteries, the heat insulation and fire protection of battery cells has become one of the most important topics.

Aspen said that from 2021 to 2030, the market space of aerogel products in the thermal insulation field of electric vehicles will reach 30 billion dollars.

Lithium ion battery core fire protection scheme

This paper mainly introduces the research progress of silica aerogel in the field of thermal insulation, analyzes its common technical difficulties and development trends in the application process, with a view to expanding the research and application fields of gel.

1. Aerospace

High performance thermal insulation material is one of the key components of thermal protection for aerospace vehicles. For hypersonic vehicles, when subjected to aerodynamic heating for a long time, the body surface will generate extremely high temperature. In order to avoid the damage of thermal erosion to the main structure and internal instruments and equipment of the aircraft, it is important to select thermal insulation materials with excellent comprehensive performance.

On the one hand, thermal insulation materials need to effectively block the external heat flow to the interior of the machine body, so as not to affect the normal operation of the machine body related equipment; On the other hand, the thermal insulation material needs to have good thermal stability and lightweight characteristics, which is of great significance for improving the payload of missiles and spacecraft and increasing the flight distance. The density of silica aerogel is only about 0.08g/cm3, and the room temperature thermal conductivity is as low as 0.016W/(m · K), which can meet the needs of aerospace for lightweight and efficient thermal insulation materials.

At present, fiber reinforced silica aerogel composites prepared by Aspen Systems have been applied to relevant projects of NASA. For example, composite functional materials are obtained by compounding aerogel with ceramic fibers, organic fibers, etc., which have been successfully applied to the external protection of the temperature sensitive probe of the Mars probe and the nebula trap. At the same time, silica aerogel composites are used in hypersonic vehicles The internal thermal protection of supersonic cruise missiles has been successfully applied.

The ceramic fiber based silica aerogel gel composite developed by NASA has been successfully used as thermal insulation material for spacecraft end faces and fuel tanks. It can not only insulate heat, but also prevent cryogenic fuel tanks from freezing before launch. The thermal insulation performance of this material is 10~100 times higher than that of existing spacecraft thermal insulation pads, and it has significant advantages of high temperature resistance. Ceramic fiber SiO2 aerogel composites are also used in aircraft black boxes and the cabin insulation of British Jaguar fighter.

Aerogel thermal insulation material is used in the cockpit of British "Jaguar" fighter

The lightweight and low thermal conductivity of silica aerogels make them one of the most concerned materials in aerospace thermal insulation materials. However, silica aerogels still have the following two problems in aerospace applications: ① The mechanical strength of gel is low, so it is usually necessary to composite aerogels with fiber materials in aerospace applications. ② The limiting operating temperature of silica aerogel is usually less than 600 ℃, which is not suitable for the end face insulation of supersonic or supersonic aircraft that is developing rapidly. In the future, it is also necessary to consider expanding the application temperature range of silica aerogel to higher temperatures through multiphase fusion and microstructure design.

2. Military industry

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

Aspen Corporation of the United States earlier carried out research on the application of silica aerogel in new destroyer structural firewall insulation system, launch vehicle fuel cryogenic tank and valve pipe insulation system. The cavity of the Indian Navy's "INSArihant" strategic missile nuclear submarine uses aerogel for thermal insulation, thus reducing NOX emissions from traditional thermal insulation materials.

The cavity of India's "INS Arihant" strategic missile nuclear submarine is made of aerogel thermal insulation material

In addition, the US NASA Ames Research Center uses aluminum silicate fiber as the support skeleton, and fills the pores in the refractory fiber skeleton with silica aerogel to prepare aluminum silicate fiber reinforced silica aerogel insulation tile, which has been used in nuclear reactors of nuclear submarines and steam powered missile destroyers. 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 use space in the cabin, while maintaining the temperature in the cabin and improving the working environment in the cabin. The heat insulation tile is also applied to the power device of the weapon to block the thermal radiation, which is conducive to the anti infrared reconnaissance of the weapon 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 R&D in the military industry. Therefore, how to realize the multifunctional design of silica aerogel is an important issue to be considered in its application in the military industry.

3. Industrial storage tank and pipeline

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

The research shows that the thermal conductivity of the material covered with aerogel thermal insulation composite film on the surface of the metal pipe can be reduced to 0.084W/(m · K). In addition, the fire resistance limit time of pipeline materials covered with aerogel composite film can reach 70min, effectively improving the pipeline safety. In the thermal insulation application of chemical pipeline, silica aerogel composite felt is mainly used, without special waterproof measures (hydrophobic ratio ≥ 99%), and can still be constructed in rainy days or wet environments.

Application of aerogel felt pad in chemical pipeline

In addition, the aerogel composite felt has good anti-seismic tensile property, no particle accumulation, sedimentation and other phenomena in the use process, and has a long service life. In the application of thermal insulation layer for directly buried steam pipeline, on the premise of meeting the maximum allowable heat loss, the thickness of thermal insulation layer required by silica aerogel composite felt can be saved by 40%~54% compared with glass fiber felt, thus reducing the space occupied by directly buried pipeline. Silica aerogel felt has excellent thermal insulation performance, and has better room to play under the conditions of high steam temperature and narrow and harsh site space.

In addition, silica aerogel felt pad 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.

4. Boiler

During normal operation of steam injection boiler, its heat loss mainly includes three aspects: first, heat loss of exhaust gas, second, heat loss of furnace body, and third, heat loss of incomplete combustion. When the boiler runs for a long time, the insulation layer in the boiler will age, the insulation effect will be poor, the heat loss will increase, and the insulation performance of the boiler will decline. The rise of surface temperature outside the furnace not only increases the heat loss, but also has certain potential safety hazards.

Using the excellent thermal insulation performance of aerogel, its application in the thermal insulation of boiler surface can significantly reduce the boiler surface temperature and boiler heat loss. In practical use, fiber matrix and silica aerogel gel are often combined to form aerogel felt pad, which is then applied to the boiler body. After the use of aerogel composite materials in the boiler, the surface temperature of the furnace body can be reduced by about 39 ℃, the thermal efficiency can be increased from 79.7% to 81.9%, and the energy can be saved by 2.2%.

The aerogel composite developed by KLAY EnerSol has been successfully applied to boiler body, steam pipeline, interface and other areas, effectively reducing heat loss.

Application of aerogel composites in boiler system

The boiler temperature is generally high, so the fiber in the silica aerogel composite material needs to have high temperature resistance. The use of high-temperature resistant polycrystalline mullite fiber and silica aerogel gel composite is the preferred solution.

At present, silica aerogel is seldom used in boilers, which is mainly related to its manufacturing cost. On the other hand, the temperature of industrial boilers is high, and the temperature tolerance limit of aerogel for long time operation is generally lower than 600 ℃. Improving the high temperature resistance of silica aerogel is the future development trend.