3 Aerogel research in aerospace field

3.1 High temperature resistant aerogel

In recent years, with the rapid development of aerospace technology, the flight speed of aircraft is getting faster and faster. Because the aircraft travels in the atmosphere at a high speed for a long time, its windward surface and wing leading edge and other parts are subject to severe aerodynamic heating, and the thermal environment of these parts is particularly harsh. It is reported that when the aircraft flies in the atmosphere at the speed of Ma=8, the maximum temperature at the nose cone and the leading edge of the wing can reach 1793 ℃ and 1455 ℃ respectively. In order to make the instruments and equipment inside the aircraft work in the normal temperature range, it is urgent to adopt an efficient thermal protection system, which also puts forward higher requirements for the high temperature resistance of thermal insulation materials.

Silica aerogel is one of the most mature and widely used high-performance aerogel thermal insulation materials at present. However, the short-time use temperature of silica aerogel will not exceed 700~800 ℃. This is because the size range of the particles and pore structures that make up silica aerogels is in the nanometer level. Under high temperatures, its surface activity is high, and it is easy to sinter. The particles will become larger, and the pore structure will disappear, which will lead to the destruction of the aerogel microstructure, severe shrinkage of the macro size, and greatly reduce its thermal insulation performance. The insufficient temperature resistance of aerogels has become one of the main bottlenecks hindering their practical application in the field of high temperature insulation. The Institute of Aerospace Special Materials and Technology has developed a high-temperature aerogel material with a maximum service temperature of 1200 ℃ from the perspective of nanostructure regulation, based on the medium temperature silica aerogel with a temperature resistance of 650 ℃. On the one hand, by selecting appropriate silicon source precursors, microstructure control is carried out in the sol-gel stage to reduce the surface energy of silica particles and initially improve the temperature resistance of materials; On the other hand, the strength of the gel skeleton network is enhanced through post-treatment operations such as improving the aging process and hydrophobic process to further improve the temperature resistance of the material (Figure 3). The prepared silica aerogel has a linear shrinkage of only 3% after being tested at 1200 ℃/0.5h, which greatly improves the temperature resistance of silica aerogel to 1200 ℃. At present, this type of aerogel has achieved mass production and engineering application. As a high temperature resistant and high-performance thermal insulation material, it has been widely used in the external thermal insulation layer of many types of aircraft and the thermal insulation layer of carrier rocket engine in China.

Due to the limitation of temperature resistance of silica aerogels, the amorphous form of silica aerogels will change to quartz form at 1200~1300 ℃, so it is difficult to further improve the temperature resistance of silica aerogels. Compared with silica aerogels, alumina aerogels exhibit higher thermal stability and chemical stability at high temperatures due to their good crystallization properties and unique fiber network structure, which is expected to solve the problem of higher temperature insulation of aircraft in aerobic environment. However, at present, the initial state of alumina aerogels prepared by sol-gel method is generally boehmite phase, and multiple crystal transformations will occur with the increase of temperature. How to further improve the high-temperature stability of alumina aerogels has become a research hotspot. Different from the way in which researchers used alumina nanoparticles as the building unit of aerogel materials in the past, the Institute of Aerospace Special Materials and Technology has designed and prepared an alumina nanorod, and controlled the assembly and annealing process of alumina nanorods and silica nanoparticles to achieve the preparation of 1400 ℃ resistant aerogel materials, as shown in Figure 4. On the one hand, the nanorod one-dimensional unit overcomes the weakness of the traditional pearl necklace like aerogel framework and solves the sintering problem caused by high surface energy; On the other hand, due to the self-supporting effect of nanorods, the heat treatment process enables the appropriate silicon aluminum components to generate mullite phase with higher temperature resistance at high temperatures. The temperature resistance of the fiber reinforced composite can reach 1500 ℃, and the linear shrinkage in Z direction is only 2.33% when the quartz lamp is tested at 1500 ℃ for 1800s on one side. This work provides a new perspective for the development of high-performance heat insulation aerogel materials used at ultra-high temperatures in the aerospace field.

3.2 Ultra low density aerogel

Aerogel, as a kind of unique porous material, has a very high porosity (up to 99.8%), and its interior is almost completely occupied by air. It is a solid material with the lowest known density. The low density of aerogel has laid a great potential for its application in the aerospace field. When the future space probe moves to a deeper space, it will be particularly important to reduce the weight of the detector by reducing the mass of thermal insulation materials. In addition, the solid phase volume ratio of low-density aerogels is very low, which greatly reduces the solid phase thermal conductivity, thus improving the thermal insulation performance of materials. Therefore, low density control technology has become one of the key technologies of aerogels in the aerospace field.

The preparation methods of low density silica aerogels mainly include one-step method and two-step method. In the one-step method, silica sources including water glass, organic silane and silicone ester are used to generate sol-gel reaction under the catalysis of acid or alkali, and are prepared by combining supercritical drying. The density of aerogels is usually only 0.06g/cm3 at the lowest. In view of the problem that the density of silica aerogels cannot be further reduced, Lawrence Livermore National Laboratory of the United States has developed a two-step method to prepare ultralight silica aerogels with a density of 0.003~0.08g/cm3. They use tetraethyl orthosilicate to partially hydrolyze under strong acid conditions to generate highly reactive semi hydrolyzed semi condensed silica precursors, In a weak alkaline environment, the fibrous porous structure formed by the interconnection of silica nanoparticles is rapidly formed. The density of silica aerogels prepared after supercritical drying is only 0.003g/cm3, which is the record of the lowest density of silica aerogels at present.

Based on the strategy of two-step preparation of low-density aerogels, the Institute of Aerospace Special Materials and Technology systematically studied the influence of raw material ratio of reaction solution on the performance of final low-density aerogels by optimizing reaction parameters and exploring the relationship between structure and performance. It was found that controlling the size shrinkage during aging and supercritical drying was the key to preparing ultra-low density aerogels, and the density of the final prepared silica aerogels was less than 0.005g/cm3, The pore size is about 7nm and the particle size is about 6nm, which greatly improves the particle size, pore size and uniformity of aerogels, as shown in Figure 5. In addition, in order to solve the problem that low density aerogels are too weak to be used, ultra-low density silica aerogels reinforced by organic foam composites were prepared. The density is only 0.022g/cm3. The thermal conductivity of this material under vacuum and room temperature conditions is only 0.0067W/m · K-1. This cold proof and thermal insulation structural material has effectively guaranteed a deep space exploration mission in China.

Compared with the brittleness of inorganic aerogels, organic aerogels and carbon based aerogels have relatively good mechanical properties. The Institute of Aerospace Special Materials and Technology has prepared a variety of other types of ultra-low density aerogels through structural design and performance optimization. A low-density flexible polyimide aerogel was prepared based on the sol gel reaction of dianhydride and diamine with biphenyl structure. The density can be as low as 0.033g/cm3, the thermal conductivity at room temperature is 0.026W/m · K-1, and the fracture strain can reach 21.7%. This greatly improves the flexibility of the aerogel material, which can realize large angle bending. The material can be used in the thermal insulation system of space vehicles, the thermal protection system of supersonic inflatable pneumatic reducer Space suit insulation and other fields have broad application prospects. The highly conductive graphene doped carbon aerogel powder material with the lowest density of 0.0093g/cm3 was successfully prepared by the technological process of sol-gel, supercritical drying and pyrolysis of phenol formaldehyde reaction. Recently, the Institute of Aerospace Special Materials and Technology has prepared a highly elastic ultralight graphene aerogel with a minimum density of 0.0012g/cm3 by using large diameter single-layer graphene oxide as the building unit and using the bubble template method in combination with freeze drying and high temperature annealing strategies. The resilience of the aerogel can still reach 100% under 90% compression deformation, As a super elastic and super light aerogel thermal insulation material, it has a good application prospect in aerospace vehicles.

3.3 Wave transmitting aerogel

Radome and antenna window are important functional components of spacecraft, which require effective transmission of 0.3~300GHz broadband electromagnetic wave to ensure that the aircraft can normally carry out key functions such as communication, guidance, telemetry and detonation under harsh working conditions. Therefore, they are widely used in missiles, carrier rockets, spacecraft, recoverable satellites and other aerospace vehicles. The radome of an aircraft navigating at high speed in the atmosphere will face a worse environment. The surface temperature is often above 1000 ℃, and the time to withstand ultra-high temperature is getting longer and longer. To this end, the U.S. Army Strategic Defense Command launched the hypersonic missile radome research program, focusing on finding radome materials that have many functions, such as high temperature resistance, high efficiency heat insulation, high temperature wave transmission, and ultra lightweight.

In this regard, aerogel materials are widely known for their advantages of low thermal conductivity, low dielectric constant, low density, high wave transmission and high temperature resistance, which make them a new type of high temperature antenna wave transmission materials and have broad application prospects in the aerospace field. Wave transmitting materials refer to materials that can penetrate electromagnetic waves without changing the energy of electromagnetic waves. The most important indicators to measure the wave transmitting performance of materials are dielectric constant and loss tangent. In general, the smaller the dielectric constant and loss tangent, the better the dielectric property of the material, and the higher the wave transmission rate can be obtained. A number of research teams at home and abroad have carried out research on the wave transmission performance of silica aerogels. It is found that the dielectric properties of silica aerogels are greatly affected by the apparent density, surface chemical properties and impurity content of aerogels. The dielectric constant of the prepared aerogel materials is generally 1.2~1.6, and the tangent value of the loss angle is 0.0027~0.0048. However, the high porosity and low density of silica aerogels make the mechanical properties of the materials poor, which seriously restricts the practical application of aerogel wave transmitting materials in the field of wave transmission. In order to solve this problem, some researchers used tetraethyl orthosilicate as the silicon source to prepare fiber reinforced aerogel composites to improve the mechanical strength, but often caused the high temperature stability, thermal insulation and wave transmission may not meet the application requirements.

The Institute of Aerospace Special Materials and Technology compounded the wave transmitting quartz fiber reinforcement with high-performance silica sol. After going through the sol gel, supercritical drying and moisture-proof treatment processes, it successfully prepared the high temperature wave transmitting aerogel thermal insulation composite, with a density of about 0.3g/cm3, a temperature resistance of ≥ 1100 ℃, a room temperature thermal conductivity of ≤ 0.02W/m · K-1, an adjustable dielectric constant of 1.28-1.39, and a loss tangent of ≤ 0.005, Its dielectric property hardly changes with temperature when it is below 1000 ℃, as shown in Fig. 6, the linear expansion coefficient at 1000 ℃ is -11.13 × 10-6/K, tensile strength at 1000 ℃ is 1.82MPa. The radome heat shield made of this aerogel material with high temperature resistance and excellent high temperature wave transmission performance has excellent comprehensive performance, which can meet the requirements of heat insulation and wave transmission for high Mach number aircraft flying in harsh thermal environment.

4 Overview of aerogel application in aerospace field

4.1 Thermal insulation application

Thermal insulation is the most prominent functional characteristic and the most extensive application scenario demand of aerogel materials. Compared with general thermal insulation materials, aerogel materials are generally applicable to the thermal insulation of aircraft in multiple airspace, such as aviation, near space, aerospace, deep space, etc. First of all, aerogel is the solid material with the lowest thermal conductivity at present. The thermal conductivity can be as low as 0.01W/m · K-1 at normal temperature and pressure, 0.004W/m · K-1 in vacuum environment, or even lower; Secondly, aerogel materials are not only suitable for thermal insulation in high vacuum environments such as the moon, near Earth orbit, and near space stratosphere, but also suitable for thermal insulation in low vacuum environments such as Mars, Saturn, and near space mesosphere. The thermal insulation of conventional multilayer thermal insulation materials in the latter environment will fail; In addition, the temperature resistance range of aerogel materials is very wide (40~2100K), which can meet the requirements of detectors and aircraft for ultra-high temperature, ultra-low temperature, high alternating temperature and wide temperature range insulation in different complex airspace environments, as shown in Figure 7; Finally, the low density characteristics of aerogel materials make it the best lightweight thermal insulation material scheme for the new generation of missile weapons, future deep/ultra deep space exploration and high load human exploration missions.

NASA of the United States took the lead in using aerogel as a highly efficient thermal insulation material on multiple parts of a series of Mars probes. In 1996, the low density silica aerogel was used as the thermal insulation material of the electronic thermostat on the Sojaina Mars Rover to protect the α The particle X-ray spectrometer is protected from the extremely cold environment (- 120 ℃) on Mars at night, as shown in Fig. 8 (a) - (b). In 2003, 0.4% graphite was doped into the transparent silica aerogel on Opportunity and Spirit rovers to reduce the thermal radiation and further improve the thermal insulation performance. The Curiosity rover launched by NASA in 2011 uses radioisotope thermoelectric generator (commonly known as nuclear battery) to provide enough power instead of solar energy due to the substantial increase in power required for operation. The heat end temperature of its heat exchanger can reach up to 1000 ℃, and conventional thermal insulation materials will fail. Silicon dioxide aerogel material doped with graphite is used as an efficient thermal barrier to isolate the hot side and cold side of the heat exchanger, The utility model greatly improves the heat energy utilization efficiency and energy supply stability of the nuclear battery. The above thermal and thermal insulation technology based on aerogel has also been applied to the nuclear battery of the Perseverance rover launched in 2020.

In order to meet the new demand for thermal insulation in deep space exploration missions represented by manned landing on Mars in the near future, NASA also tried to propose solutions using aerogels. During the EDL phase, the manned Mars probe needs to quickly complete high-speed aerodynamic deceleration for safe landing. The outer surface of the inflatable deployment flexible structure of the reducer needs to be protected by a flexible thermal protection system, as shown in Figure 8 (c) - (d). They made use of the combination of flexible thermal insulation materials composed of fiber composite silica aerogels that can withstand up to 1100 ℃ and oligomeric silsesquioxane cross-linked polyimide aerogels that have a decomposition temperature greater than 560 ℃, so that the aircraft could withstand the extreme aerodynamic heating environment assessment with a reentry speed of up to 3km/s, a maximum heat flow density close to 20W/cm2, a maximum heat flow duration of up to 90s, and a maximum stagnation point temperature of 1260 ℃ in the validation experiment. In addition, in order for astronauts to safely perform extravehicular activities on Mars, the spacesuits used should have excellent thermal protection effect in the Martian space environment. With the support of NASA Johnson Space Center, Aspen Company has developed a fiber reinforced silica aerogel flexible composite fiber material, which has a thermal conductivity of 0.005W/m · K-1 in the Martian low vacuum environment, which is only one fifth of the multi-layer thermal insulation structure, The aerogel material with high efficiency and heat insulation is expected to be used in the flexible thermal protection structure of future space suits.

In addition to using aerogels for thermal insulation of deep space probes, NASA also uses aerogels for thermal insulation of multiple parts of space shuttles and launch vehicles. NASA Kennedy Space Center used Cabot's commercialized aerogel particle materials in the liquid hydrogen cryogenic propellant tanks of space shuttles, launch vehicles, and aerospace vehicles, demonstrating the excellent thermal insulation performance of aerogels in the ultra-low temperature environment of - 147 ℃, which not only solved the problem of launch safety risks caused by condensed ambient air, but also reduced the weight of the space shuttle by up to 230 kg. NASA Ames Research Center also infiltrated the aerogel precursor into the gap of ceramic fiber tile to prepare composite rigid thermal insulation materials for aerogel thermal insulation tiles, which improved the thermal insulation performance of conventional thermal insulation tiles used in space shuttles by 1-2 orders of magnitude. This new thermal insulation material can be used in the fuel tank thermal insulation layer of future reusable spacecraft.

As one of the first major units to develop and apply functional aerogels in China, the Institute of Aerospace Special Materials and Technology has developed a series of silica aerogel materials, including medium temperature type, high temperature type, high temperature wave transmission type and ultra-high temperature type. This series of high-performance aerogel insulation materials have been used in dozens of aircraft in China. In the field of space thermal insulation, the Institute of Aerospace Special Materials and Technology has developed a series of lightweight high-performance thermal insulation materials to meet the needs of aircraft and detectors for lightweight and harsh space environment in China's space exploration, and has undertaken multiple supporting tasks for China's space exploration projects; Fiber reinforced aerogel and aerogel vacuum heat insulation panel are developed to meet the requirements of precise temperature control for low temperature orbit of cargo spaceship, as shown in Figure 9 (a); In order to meet the high temperature thermal insulation requirements of the gas system, isolation cylinder and oxygen turbine of the launch vehicle engine, a high-performance nano aerogel thermal insulation composite was developed; To meet the long-term thermal insulation requirements of a key electronic device of the lunar probe, an aerogel thermal insulation module with long service life is developed; The ultra-low density aerogel composite insulation panel was developed to meet the thermal insulation requirements of Mars exploration and landing rover in the high and low temperature alternating environment, as shown in Figure 9 (b). The quality of relevant aerogel products has been stable and reliable in previous missions, which has successfully guaranteed the success of a series of national major space missions, such as manned spaceflight, lunar exploration project and fire exploration project, and will continue to make greater contributions to the construction of China's space station and future deep space exploration missions.

4.2 Particle capture applications

Thermal insulation system is the most mature application field of aerogel in aircraft, and space particle capture is another widely known application of aerogel in detectors. Cosmic dust provides the most original samples for the study of the formation and evolution of the solar system and planets, and even the origin of life. It is of great astronomical value to capture and sample these particles for analysis without damage. The high-speed moving cosmic dust (5~80km/s) will be extinguished after hard collision with conventional materials. On the one hand, the low-density transparent aerogel material is conducive to complete capture of high-speed particles due to its low-density porous characteristics, and on the other hand, its transparent characteristics help to locate and remove captured particles for research and analysis. It has been proved to be the best medium material for cosmic dust capture, The low density and transparency of aerogels are indispensable in this field.

On the basis of multiple STS space shuttle carrying experiments in the 1980s, NASA carried a transparent aerogel with a density of 0.02g/cm3 on the Mir space station in 1995 to capture high-speed orbital debris particles in low earth orbit. Since then, NASA launched the famous Stardust Program in 1999, using 0.005-0.05g/cm3 gradient density transparent aerogel to capture a large number of comets and planetary dust, as shown in Figure 10. Based on the analysis of dust samples, NASA has obtained a large number of subversive astronomical new understanding, for which the international authoritative magazine Science published an album for reporting. Since 2011, with the support of NASA, the Jet Propulsion Laboratory of California Institute of Technology has begun to study the use of low-density alumina, phenolic, polyimide and other non silicon transparent aerogels to capture Mars or other planetary dust samples. The United States launched the Mars survey sample collection plan at the beginning of 2019, and the White House approved the budget of the plan for 2020 to be US $109 million.

In addition to the United States, the European Space Agency once deployed a silica aerogel capture device with a density of 0.05g/cm3 in the EuReCa recoverable satellite, and collected 12 particles from micrometeoroids. In 2001, the Japan Aerospace Development Agency (JAXA) carried the dust collector composed of silicon dioxide aerogel blocks with a density of 0.03g/cm3 on the International Space Station for particle capture test. In 2015, the Japanese Aerospace Development Agency launched the dandelion project, which is based on the preparation of double-layer silica aerogel plates with an inner layer density of 0.01g/cm3 and an outer layer density of 0.03g/cm3 to collect dust particles floating in space and explore whether there are living substances in outer planets. In 2013, the French Academy of Development and the National Academy of Sciences used 0.09g/cm3 aerogel traps to collect space particle fragments on the International Space Station. The higher density significantly enhanced the mechanical strength of the traps. Although China has not yet carried out space exploration experiments using aerogel materials to capture cosmic dust, with the construction of space station platforms that can carry capture devices, relevant research has ushered in significant development opportunities.

5 Conclusion

With the rapid development of aerogel preparation technology and application technology, the application of various new high-performance aerogels in multi airspace aircraft has been the focus and hotspot of research at home and abroad, and has achieved great success.

(1) The excellent properties of aerogels, such as light weight and heat insulation, are closely related to the unique microstructure of aerogels. The regulation of aerogel structure mainly depends on the key preparation processes, such as sol-gel, aging and drying. On the basis of the existing research work, closely focusing on the relationship between the preparation, structure, performance and application of aerogels, aerogels with higher performance will be developed, which will further promote the research and application of aerogel materials in the aerospace field.

(2) The rapid development of aerospace technology has put forward new requirements for thermal protection systems with high performance such as high temperature resistance, lightweight, high wave transmission, etc. Through structural design and performance optimization of various aerogels, key technologies such as high-temperature resistant aerogel preparation, ultra-low density aerogel preparation and wave transmissive aerogel preparation have been broken through, initially meeting the use needs of various aircraft. The service environment in the future will be more complex and harsh, and the comprehensive performance of aerogel materials, such as temperature resistance, heat insulation, load bearing, wave transmission and stealth, need to be further improved to meet the requirements of use.

(3) The nano porous structure of aerogels and their high thermal insulation, high temperature resistance, low density and other properties make them applied in many aerospace missions, such as aircraft thermal insulation and particle capture. In addition, aerogels also have many excellent properties, such as low dielectric, high wave absorption, high sound insulation, high transparency, low refraction, etc. It is expected to further develop and expand new applications of aerogels in other aerospace fields besides thermal insulation.