Silica aerogel is a high porosity inorganic lightweight nano material with gas as the dispersion medium. Its own structure determines that it has a variety of unique excellent properties at the same time, such as low density, high porosity, high specific surface area, low refractive index, low thermal conductivity and low dielectric constant. In practical application, silica aerogels show the defects of brittleness and poor mechanical properties, and silica aerogels are hydrophilic due to the large amount of Si-OH on their surface, which can easily cause expansion and fracture of the skeleton structure after moisture absorption or water absorption, so they cannot be used alone. In order to make better use of the excellent characteristics of aerogels, silica aerogels can be processed into composite products by a variety of processes (immersion, hybrid compression molding, deposition, etc.). These composites have become new stars in the field of materials due to their ultra-low thermal conductivity, safety performance of inorganic materials, and excellent hydrophobicity and durability. With the rapid development of high-speed trains in China in recent years, the use of aerogel has provided new material support for breakthroughs in vehicle lightweight, noise control, comprehensive energy saving and other technologies, and has been initially applied to high-speed trains.

At present, silica aerogels are the most studied in the field of aerogels, and their production and manufacturing processes are relatively mature, which is also the most widely used oxide aerogels. The researchers have made constant attempts on the research and selection of precursors, catalysts, aging process, surface modification and drying process, and finally obtained the open-hole nano-solid materials with three-dimensional skeleton structure in the gas medium, which are highly cross-linked by Si-O-Si chain. Table 1 shows the typical performance parameters and applications of silica aerogel.

Table 1 Typical performance parameters and application of silica aerogel

Silica aerogel

Thermal insulation, sound insulation and flame retardancy

Due to the high specific surface area of silica aerogel, nearly infinite solid/gas interfaces are formed inside the material, and heat will reflect, absorb, transmit and re radiate at each layer of interface, thus reducing the thermal radiation. Table 2 shows the comparison of heat transfer modes of silica aerogel and ordinary porous materials. Figure 1 shows the comparison of thermal insulation performance between traditional porous thermal insulation materials and silica aerogel. It can be seen from the figure that the thermal insulation performance of aerogel is significantly better than that of traditional thermal insulation materials. Compared with other insulation materials, silica aerogel has the lowest thermal conductivity, lightest weight and higher heat resistance temperature. To achieve the same thermal insulation effect, the thickness of silica gas gel is less than 1/3 of that of traditional materials, with thinner thickness and smaller heat dissipation area, saving space to the maximum extent.

Table 2 Comparison of heat transfer modes between silica aerogel and ordinary porous materials

Fig. 1 Comparison of thermal insulation performance between silica aerogel and traditional porous thermal insulation materials

Silica aerogel can be used as a lightweight sound insulation material for high-speed trains. When sound waves pass through the pores, the sound energy is converted into heat energy due to the friction between the air and the skeleton. At the same time, sound waves reflect back and forth in the pores, causing string vibration of flexible silica aerogel framework, resulting in internal energy consumption, reducing sound pressure intensity, and achieving the effect of noise reduction. The extremely high porosity of silica aerogel makes it have ten times higher sound absorption effect than ordinary porous materials. At the same time, hydrophobic silica aerogel has good thermal stability and corrosion resistance, and still has good sound absorption performance in high temperature and harsh environment. At present, commonly used sound-absorbing materials such as asbestos and felt have limitations in environmental protection, waterproof, fireproof and lightweight. Silica aerogel gel can be used as a lightweight sound insulation material with both vibration and noise reduction and sound absorption capabilities.

The framework of silica aerogel is composed of three-dimensional O-Si network, which is bonded by Si and O elements, and has good flame retardancy. However, silica aerogels are hydrophilic, and their comprehensive performance declines after absorbing water in the air. Therefore, hydrophobic aerogels with surface modification are used in engineering. In order to ensure the use reliability of silica aerogels, the flame retardancy of hydrophobic silica aerogels is generally improved by changing the precursors and adding flame retardants. Combined with thermal stability analysis and flammability study, the following conclusions can be drawn: the flame retardancy of hydrophobic silica aerogels can be improved by using sodium silicate as the precursor and introducing phosphorus compounds such as phosphoric acid.

Silica aerogel

Type of composite material

At present, silica aerogel gel is used as nanopore carrier in the world's reported nano mesoporous thermal insulation materials. However, the ultra light aerogels prepared by the existing process all have the disadvantages of low strength and poor toughness. At present, aerogel composites with high practical value are reinforced and toughened by various methods, such as precursor modification copolymerization, fiber reinforced materials and polymer cross-linking reinforcement.

Silica aerogel composites can be divided into four categories: doped silica based aerogel composites, coated silica based aerogel composites, interpenetrating network silica based aerogel composites and combined silica based aerogel gel composites. The microstructure is shown in Figure 2. The combined silicon based aerogel thermal insulation composite is to combine the second phase materials in the form of doping, coating or interpenetrating network to give play to the advantages of the three composite modified materials, overcome the disadvantages, and form a composite with better comprehensive performance and better applicability.

Fig. 2 Microstructure of silica aerogel composite

Silica aerogel composite coating

Application on high-speed trains

High speed trains are designed with high performance materials (such as thermal insulation, fire protection and explosion protection, vibration and energy absorption, sound insulation and noise reduction, etc.) due to large changes in the operating environment, high speed per hour, and small compartment interlayer space. Silica aerogel composites have excellent properties such as light weight, heat insulation, sound insulation, flame retardant, hydrophobic and environmental protection, and have extensive potential application value in the optimization design of high-speed train body in the future. At present, the main silica aerogel composite products include thermal insulation board, thermal insulation felt, coating, film, particle, paper and profiled parts. Some composite products are shown in Figure 3.

Figure 3 Silica aerogel composite products

Silica aerogel composite coatings have different functions by using different characteristics of silica aerogel, and adding pigments, fillers, additives, etc. Figure 4 (a) is a damping adhesive compounded by silica aerogel powder with nano pore structure and high temperature resistant binder system, which is applied to the surface of metal plate of high-speed train body structure, as shown in Figure 4 (b). When the metal plate is excited to produce bending vibration, its vibration can be quickly transferred to the tightly fitting damping coating. The damping coating changes the vibration energy into heat energy through internal corresponding dislocation and friction, which plays a role in vibration reduction and noise reduction. Waterborne coatings can be prepared by dispersing silica aerogel in special high-performance resin lotion. Its performance parameters are as follows: thermal conductivity 0.031W/(m · K), density 120kg/m ³， Flame retardant grade V0, adhesion 1.2MPa, hydrophobicity not less than 98%. The coating is effectively applied to the high-speed train body, which can achieve the purpose of structural vibration reduction, noise reduction, heat insulation and surface protection, achieve thermal insulation in winter and summer, and reduce energy consumption of air conditioning. Figure 5 shows the comparison before and after combustion of aerogel fire retardant coating for steel structure. When the thickness of this coating is 3.0mm, the fire endurance time reaches 2h (generally, the thickness of fire retardant coating needs 3.5mm); Strong adhesion with steel plate, aluminum plate and other metal materials, and the bonding strength is greater than 1.0MPa; After combustion, the metal plate will not deform and ablate. It is applied to protect the load-bearing steel structure of high-speed trains, which can effectively prevent the uncontrolled spread of fire.

Fig. 4 Silica gas gel heat insulation damping adhesive and its application

Figure 5 Comparison of morphology of thermal insulation and flame retardant coating before and after combustion

Silica aerogel composite coiled material

Application on high-speed trains

The continuous speed increase of the train makes the noise in the train significantly increase. At present, the interior wall panel and the body of the high-speed train are generally filled with porous materials to form a composite sound insulation structure. The designer designs and optimizes the combination of porous materials, air layer thickness and layered composite materials based on the transfer matrix method. The sound wave propagates in the framework network of aerogels. The energy of the sound wave is transferred and attenuated, and the amplitude and speed are reduced, making aerogels have low sound speed characteristics. Therefore, aerogels are ideal acoustic delay and high temperature sound insulation materials. By using silica aerogel composite coiled material, designers can reduce the overall mass of layered composite materials, reduce the use of porous materials, and achieve the design goal of low noise, lightweight, and low energy consumption.

The thermal conductivity of the existing silica aerogel composite coiled materials on the market is 1/5~1/3 of that of traditional materials, which effectively reduces the thickness of the insulation layer. The thermal conductivity increases slowly with temperature.. The manufacturer can generally customize the product specifications according to customer needs. The technical parameters of silica aerogel insulation blanket are as follows: thermal conductivity 0.017W/(m · K), density 160kg/m ³， The tensile strength is 1.409MPa, and the hydrophobicity is 99.9%. Figure 6 shows the application of 5mm thick silica aerogel thermal insulation felt to the cold proof material structure in the flat roof area of a high-speed train. The closed coating treatment process not only reduces the volume and quality of the structure, but also improves the cold proof and noise reduction effect.

Silica aerogel is compounded with glass fiber, ceramic fiber or carbon fiber to obtain felt products. Although the heat insulation effect is good, the surface fiber is easy to break and pulverize, causing floating fiber or powder pollution. To solve this problem, the outer part of the aerogel composite layer can be covered with a layer of materials with higher strength and toughness, such as the composite layer of expanded polytetrafluoroethylene and flame retardant polyethylene terephthalate fiber, as shown in Figure 7.

Figure 6 Application of silica aerogel thermal insulation felt in high-speed trains

Figure 7 New silica aerogel insulation blanket

Silica aerogel composites

Other applications on high-speed trains

At present, the application of silica aerogel composites in high-speed trains has just started, and most of them are used as small auxiliary parts to meet the corresponding technical requirements. As the most promising new energy-saving and environmental protection material in the 21st century, silica aerogel composite is enriching the market with more products. In addition to the plates and coiled materials described above, silica aerogel glass, as a new energy-saving glass, also has many advantages, such as good heat insulation, sound insulation and noise reduction, wind pressure resistance, no hidden danger of cold and hot self explosion, good light transmittance, anti condensation, and service life of more than 30 years. The window area of the high-speed train (taking Fuxing as an example) accounts for about 5.8% of the surface area of the car body. Due to the environmental changes generated during the driving process, these open areas have very high requirements for window glass. The silica aerogel glass has good comprehensive performance and has potential development value. In addition, as a new type of kinetic energy absorption material, silica aerogel has shown a broad application prospect in the field of energy absorption and buffer. The head of 350km/h standard EMUs and intercity railway passenger cars are made of special explosion-proof and vibration damping materials. According to the existing test and research, silica aerogel composite materials also have development and application value in the special position of the head of high-speed trains.

conclusion

This paper analyzes and introduces the structural and performance advantages of silica aerogel composites, focusing on the requirements of high-speed train body lightweight, noise control, comprehensive energy conservation and safety fire prevention, and comprehensively considering the characteristics of silica aerogel composites such as light weight, heat insulation, sound insulation, flame retardant, anti-corrosion, vibration reduction, etc, As an important green and environmental protection new material, silica aerogel composite can provide new material support for the future development of high-speed trains. At present, the application of silica aerogel composites on train bodies is still in its infancy, and the comprehensive application of different performance advantages still needs continuous exploration.