Silica Aerogel is a kind of nano porous structure material, which has many excellent characteristics such as ultra-low density, ultra-low thermal conductivity, and has been widely used in the field of aerospace vehicles. At present, few literatures summarize the latest research progress of aerogels in the aerospace field in recent years, especially the important breakthroughs in this field in China. In order to solve this problem, the latest research and application prospects of aerogel materials based on aerospace applications at home and abroad are comprehensively introduced in this paper. First, the preparation and performance of aerogels are briefly summarized, focusing on the research of several high-performance aerogels used in the aerospace field, such as high temperature resistance, ultra-low density and wave transmission. The application status and prospects of aerogels in the aerospace field are introduced. Finally, the existing problems and future development direction of aerogels in the aerospace field are reviewed, which has certain significance for promoting the development of aerogel materials in the aerospace field.

1 Introduction

Silica Aerogels were first prepared by Kistler of the United States in 1931 using the sol-gel method and supercritical drying technology. Silica Aerogels are composed of nano colloidal particles or polymer molecules stacked together in three-dimensional space. They have rich porous network structures and are lightweight solid materials. Aerogels have many unique properties, such as ultra-low density, super large specific surface area, ultra-high porosity, ultra-low dielectric constant, ultra-low thermal conductivity, etc. In recent decades, with the rapid and diversified development of preparation technology and application technology, aerogel materials have broad application prospects in many fields such as space exploration, national defense and military affairs, thermal insulation, energy storage, environmental protection, sensing detection, chemical catalysis and high-energy physics. Therefore, it is known as one of the top ten magic materials that change the world. In the mid-1990s, the National Aeronautics and Space Administration (NASA) used aerogel materials in the Mars Pathfinder and the Stardust Program, two far-reaching deep space exploration missions, which greatly stimulated the research interest of countries around the world in using aerogel in various aerospace vehicles, such as space shuttles, rockets, deep space detectors and missiles. Due to its excellent heat insulation performance and lightweight characteristics, aerogel materials have been widely used in aerospace and military industries. For example, aerogel materials are used in the engine room wall heat insulation system and infrared system protection of MKV-22 Osprey tilt rotor aircraft in the United States, and aerogel materials are also used in the cockpit heat insulation wall of the modified British Panther fighter. In recent years, with the rapid development of aerogel preparation and application technology, aerogel materials have been more and more widely used in aerospace vehicles. This paper will introduce the research and application prospects of aerogels in the aerospace field.

2 Structure and properties of aerogels

2.1 Preparation of aerogel

Silica aerogel is the most typical representative of aerogel family, and is also the most mature and widely used type of oxide aerogel. Taking silica aerogel as an example, a deep understanding of the preparation method, structural characteristics and functional characteristics of aerogel materials is important for their application in the aerospace field. Sol gel method is the most common method to prepare aerogel materials at present, which mainly includes three important stages: gel preparation, gel aging and gel drying, as shown in Figure 1.

The preparation of silica wet gel is mainly realized by sol-gel, involving complex reaction processes, including hydrolysis and polycondensation of silicon source. Under the catalysis of catalyst, the silicon source precursor molecule undergoes the hydrolysis reaction of its Si-OR to generate a small molecule containing Si-OH with high activity. Under the action of catalyst, these active small molecules can not only dehydrate and condense each other between Si-OH, but also react with non hydrolyzed Si-OR to form oligomer sol particles. The surface of oligomer sol still contains some unreacted - OH with high activity, which will further condense the sol particles to form a polymer gel with three-dimensional network structure.

After the formation of silica gel, it does not mean that the sol-gel process has been completely completed. When the gel point is reached, the hydrolysis polymerization reaction of precursors or prepolymers in the system is far from stopped. In the early stage of gel formation, the skeleton strength of the gel network is very low, and in the later stage of drying, it is easy to shrink, or even cause cracks, which is not conducive to obtaining complete blocks without shrinkage. Therefore, in order to improve the strength of the gel network structure, it is necessary to continue the condensation reaction of colloidal particles and small clusters in the sol after the formation of the gel to ensure further aggregation and adhesion, so as to extend to the entire gel network. This process is called aging.

Drying is another important step in the preparation of silica aerogels. There are a lot of liquids in the solid network skeleton of the alcohol gel, including organic solvents and water. To obtain aerogels, the solvent in the wet gel must be removed. Supercritical drying is the most effective method to prevent the gel from shrinking and cracking during the drying process. During the drying process of silica aerogels, the cracking of the gels is mainly due to the capillary pressure, which is derived from the surface tension of the liquid and gas phases. If the liquid in the silica gel is pressurized and heated above the critical temperature and pressure, the liquid gas interface in the system will disappear, and the capillary pressure in the gel will no longer exist. The drying method based on the above principle is the supercritical drying method.

2.2 Structure of aerogel

The skeleton particle structure of SiO2 aerogel contains primary particles and secondary particles. The dense amorphous silicon dioxide primary particles (1~2nm) formed by the condensation of silicon sources aggregate into spherical secondary particles (5~10nm). The secondary particles form a pearl necklace like three-dimensional network structure through interconnection, just like a highly branched polymer, as shown in Figure 2. There are a lot of pores between such chaotic nano particles, and the particle size and corresponding pore size of aerogel skeleton particles are basically less than 50nm, which makes aerogels have low density (0.1~0.2 g/cm3) and high porosity (90%~99%). The structure of silica aerogels is closely related to the preparation process, and the sol-gel, aging, drying and other stages have a great impact on the final micro morphology of aerogels.

2.3 Aerogel performance

The nano porous structure of silica aerogels endows them with many excellent functional properties. In the aerospace field, the thermal insulation performance of aerogels is undoubtedly the most interesting. Due to its porous characteristics and nanoscale pore size, silica aerogels are highly insulating materials, and their thermal conductivity is lower than that of static air (0.025W/m · K-1). Kistler first proved that the thermal conductivity of aerogel is about 0.02W/m · K-1 at ambient temperature and 0.005W/m · K-1 at vacuum. At present, the thermal conductivity of silica aerogels can be as low as 0.01 W/m · K-1, which is 0.4 times that of static air. Heat transfer in aerogel materials mainly includes solid phase conduction, gas phase conduction and radiation conduction. Without considering the coupling effect of solid heat transfer and gas heat transfer, the thermal conductivity of aerogel is mainly the sum of the thermal conductivity of three heat transfer modes. From the point of view of solid phase conduction, the solid phase thermal conductivity of ordinary thermal insulation materials is larger due to short heat transfer path and large contact area between particles; However, the heat transfer of aerogel thermal insulation materials is through an infinite path, and the contact area between particles is small, which makes the solid phase thermal conductivity smaller. From the point of view of gas phase conduction, heat transfer is generated by the collision between gas molecules. Since the pore size of aerogel is smaller than the average free path of gas molecules, there is almost no heat transfer between gases, so the gas phase heat transfer coefficient is also significantly lower than that of ordinary macroporous insulation materials. These two factors determine that the thermal insulation ability of aerogel is significantly better than that of ordinary insulation materials. In addition, the radiation conduction mode of aerogel thermal insulation materials will play an important role in heat transfer under high temperature. Adding infrared sunscreen to them can absorb, reflect and scatter infrared radiation, so as to further reduce the thermal conductivity.