Nanofiber insulation: great for the aerospace industry!

Aerospace vehicles are subject to prolonged aerodynamic heating during flight, and the surface of the substrate will generate high temperatures. In order to ensure the safety of the main structure of the aircraft and the internal equipment, high-efficiency insulation materials must be used to prevent external heat flow from spreading to the inside. At the same time, the lightweight and efficient heat insulation protection system is of great significance for reducing the load of the aircraft and extending the flight distance. The nanofiber material has the advantages of small pore size and high porosity, and is an ideal lightweight and efficient heat insulating material. This paper mainly introduces the latest research progress of two-dimensional nanofiber membranes and three-dimensional nanofiber aerogel insulation materials.

Two-dimensional nanofiber membrane insulation material

Missile battery insulation sleeves, engines and other small spaces require materials with small thickness but excellent thermal insulation performance. Two-dimensional nanofiber membrane materials can be used for narrow size due to small fiber diameter, controllable stack thickness (generally less than 100 μm), and high porosity. Space insulation. The nanofiber membrane heat insulating material can be divided into a polymer nanofiber membrane, a carbon nanofiber membrane and a ceramic nanofiber membrane according to the composition.

Polymer nanofibers, such as polyvinylidene fluoride (PVDF) nanofiber fiber membranes, have higher porosity and tortuous mesh channels, which make the air molecules have a longer transmission path inside the material, and the heat is lost during propagation. Therefore, the thermal conductivity of the material can be lowered. In order to further reduce the thermal conductivity of the material, some scholars have applied SiO2 nanoparticles to the surface of PVDF nanofibers by immersion modification technology to further reduce the pore size of the fiber membrane and reduce thermal convection. However, the structure is easily destroyed in a high temperature environment, and it is difficult to meet the application requirements.

Carbon nanofibers have the advantages of large specific surface area, high porosity, good chemical stability and high specific strength. They have broad application prospects in the fields of electronics, energy, aerospace and other fields. As the degree of graphitization increases, the carbon nanofiber membrane material gradually increases in high temperature resistance, but its thermal insulation performance will also drop drastically, so it is difficult to meet the demand for simultaneous improvement of high temperature resistance and heat insulation performance.

Ceramic materials have the advantages of high temperature resistance, corrosion resistance, good insulation, etc., and have wide applications in the fields of high temperature insulation, sound absorption, and catalysis. However, most of the existing ceramic nanofibers have defects such as large brittleness, poor mechanical properties, and resistance to bending, which limits their practical use. In order to overcome this shortcoming, some scholars have prepared SiO2 nanofiber membranes with amorphous structure and good flexibility by adjusting the properties of spinning solutions and process parameters. At the same time, SiO2 aerogel nanoparticles can be introduced into the fibers by immersion modification method to construct SiO2 nanoparticles/nanofiber composites , and the thermal insulation properties of SiO2 nanofiber membranes can be improved.

Three-dimensional nanofiber aerogel insulation material

Although the two-dimensional nanofiber has good thermal insulation performance, it is difficult to achieve effective increase (>1 cm) in the thickness direction, which severely limits its application in high-power engine insulation, bulkhead fire insulation and other fields. Compared with the two-dimensional nanofiber membrane, the three-dimensional nanofiber aerogel material has the advantages of controllable size, high porosity and high degree of tortuosity, and thus has broad application prospects in the fields of heat insulation, warmth preservation and sound absorption. At present, common nanofiber aerogel insulation materials mainly include two kinds of polymer nanofiber aerogels and ceramic nanofiber aerogels.

Ceramic nanofiber aerogel

Ceramic aerogel materials have excellent high temperature resistance, corrosion resistance and thermal insulation properties, and are one of the main materials for thermal protection of aerospace vehicles. The aerogel insulation materials currently used are mainly ceramic fiber reinforced SiO2 nanoparticle aerogels. Due to the weak interaction between the nanoparticles and the ceramic fibers, the nanoparticles tend to fall off during use, thereby stabilizing the structure of the material. Sex and thermal insulation performance has dropped significantly. In order to solve the above problems, some scholars have constructed ultra-lightweight and superelastic ceramic nanofiber aerogel materials by using flexible ceramic nanofibers as the building elements and using the original three-dimensional fiber network reconstruction method.

Schematic diagram of the preparation process of ceramic nanofiber aerogel

The aerogel material has a mesh structure similar to a honeycomb, and the fibers in each mesh are entangled and bonded to each other to form a stable fiber network, which gives the aerogel good structural stability. It can rebound quickly under large deformation (80% strain) compression, and its plastic deformation is only 12% after 500 compression cycles, which is superior to the existing ceramic aerogel materials. At the same time, the material can still recover after 50% compression under an alcohol lamp flame (about 600 ° C) and a butane torch (about 1100 ° C), showing excellent high temperature compression resilience.

Compressed photo of ceramic nanofiber aerogel under alcohol lamp

Compressed photo of ceramic nanofiber aerogel under butane torch flame

Photographs of different materials with petals on a hot plate at 350 ° C for 10 min

Polymer nanofiber aerogel

Aiming at the problems of poor mechanical properties and large brittleness of existing aerogel materials. Some scholars have used cellulose nanocrystals with high elastic modulus, high strength and low density as the building blocks, and prepared cellulose nanocrystalline aerogels with good transparency and mechanical properties by gel and supercritical drying methods. It can be bent to 180° without damage, while it recovers after compression under large deformation (80%) and the maximum stress is greater than 200 kPa. In addition, cellulose nanocrystals also exhibit excellent thermal insulation properties.