Aerospace vehicles need to withstand long-term aerodynamic heating during flight, and the surface of the substrate will generate high temperatures. In order to ensure the safety of the aircraft's main structure and internal equipment, high-efficiency thermal insulation materials must be used to prevent external heat flow from spreading to the interior. At the same time, a lightweight and efficient thermal insulation protection system is of great significance for reducing aircraft loads and extending flight distance. Nanofiber material has the advantages of small pore size and high porosity, and is an ideal lightweight and efficient thermal insulation material. This article mainly introduces the latest research progress of two-dimensional nanofiber membranes and three-dimensional nanofiber aerogel thermal insulation materials.
Two-dimensional nanofiber membrane thermal insulation material
Missile battery insulation jackets, engines, and other small spaces require materials with small thickness but excellent thermal insulation performance. Two-dimensional nanofiber membrane materials can be used in small spaces due to the small fiber diameter, controllable stacking thickness (generally less than 100 μm), and high porosity Space insulation. Nanofiber membrane insulation materials can be divided into polymer nanofiber membrane, carbon nanofiber membrane and ceramic nanofiber membrane according to their composition.
Polymer nanofibers, such as polyvinylidene fluoride (PVDF) nanofiber membranes, have higher porosity and meandering mesh channels, which makes the transmission path of air molecules inside the material longer, and heat is lost during the propagation process. , Which can reduce the thermal conductivity of the material. In order to further reduce the thermal conductivity of the material, some scholars have coated SiO2 nanoparticles on the surface of PVDF nanofibers by dipping modification technology to further reduce the pore diameter of the fiber membrane and reduce thermal convection. However, the structure of the material is easily damaged in a high temperature environment, and it is difficult to meet application requirements.
Carbon nanofibers have the advantages of large specific surface area, high porosity, good chemical stability, high specific strength, etc., and have broad application prospects in electronics, energy, aerospace and other fields. As the degree of graphitization of carbon nanofiber membrane materials increases, the high temperature resistance performance gradually increases, but its thermal insulation performance will also decline significantly, so it is difficult to meet the needs of simultaneous improvement of high temperature resistance and thermal insulation performance.
Ceramic materials have the advantages of high temperature resistance, corrosion resistance, good insulation, etc., and have a wide range of applications in high temperature heat insulation, sound absorption, catalysis and other fields. 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 the SiO2 nanofiber membrane with amorphous structure and good flexibility by adjusting the spinning solution properties and process parameters. At the same time, it is also possible to introduce SiO2 aerogel nanoparticles between the fibers through the impregnation modification method to build a SiO2 nanoparticle / nanofiber composite material and improve the thermal insulation performance of the SiO2 nanofiber membrane.
Three-dimensional nanofiber aerogel insulation material
Although two-dimensional nanofibers have good thermal insulation properties, it is difficult to effectively increase them in the thickness direction (> 1 cm), which seriously limits their applications in high-power engine insulation, bulkhead fire insulation and other fields. Compared with two-dimensional nanofiber membranes, three-dimensional nanofiber aerogel materials have the advantages of controllable size, high porosity, and high degree of pore tortuosity, so they have broad application prospects in the fields of heat insulation, warmth, and sound absorption. At present, common nanofiber aerogel thermal insulation materials mainly include polymer nanofiber aerogel and ceramic nanofiber aerogel.
Ceramic nanofiber aerogel
Ceramic aerogel material has excellent high temperature resistance, corrosion resistance and thermal insulation properties, and is 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 materials tend to fall off during use, thereby stabilizing the structure of the material The performance and thermal insulation performance are greatly reduced. In order to solve the above problems, some scholars use flexible ceramic nanofibers as a building element and use original three-dimensional fiber network reconstruction methods to construct ultra-lightweight and super-elastic ceramic nanofiber aerogel materials.
Schematic diagram of ceramic nanofiber aerogel preparation process
The aerogel material has a honeycomb-like mesh structure, and fibers in each mesh are entangled with each other to form a stable fiber network, which gives the aerogel good structural stability. It can quickly rebound under large deformation (80% strain) compression. After 500 compression cycles, its plastic deformation is only 12%, which is better than existing ceramic aerogel materials. At the same time, the material can recover after being compressed by 50% under the flame of an alcohol lamp (about 600 ° C) and a butane torch (about 1100 ° C), showing excellent high-temperature compression resilience.
Photograph of different materials with petals on a 350 ° C hot plate 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 building elements, and prepared cellulose nanocrystal aerogels with good transparency and mechanical properties by gel and supercritical drying methods. It can be bent to 180 ° without damage. At the same time, it can recover 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.