Significance Aerogels are synthetic solid nanomaterials formed by the replacement of liquid in a gel with gas. The continuous network structure renders aerogels with extraordinary properties, including ultralow density, super-high porosity, high specific surface area, low thermal conductivity, low optical refractive index and low dielectric constant. They have demonstrated tremendous potentials for various applications, such as thermal insulation, environmental protection and separation. However, in the development process of aerogels, there are still questions to be answered on development of new sol-gel transition principles and strategies, effectively control of macro morphology of aerogels, improvement of service performance, and expansion of the application field. As newly emerging aerogels, aramid aerogels utilized aramid nanofibers as building blocks have the potential to provide answers to the above-mentioned questions. In order to accelerate the development of aramid nanofiber aerogels from laboratory investigation to practical application and thus promote the development of the entire aerogel industry, the research status of aramid nanofiber aerogels are comprehensively reviewed in this paper.
Progress The building blocks of aramid nanofiber aerogels need to be obtained. So far, various methods have been developed to prepare aramid nanofibers, which can be classified into "top-down" and "bottom-up" methods. The rheological behavior of aramid nanofiber dispersion shows significant shear thinning behavior. In addition, the dynamic stress scanning test reveals that the aramid nanofiber dispersion shows elastic behavior in a certain stress range. These distinguishing features indicate that the aramid nanofiber dispersion can be processed through wet spinning, blade coating, 3D printing, and so on. Therefore, aramid aerogel fibers have been prepared via wet spinning or relevant spinning method, such as liquid crystal spinning. During the fabrication process, novel dynamic sol-gel transition principles was proposed. The as-prepared aramid aerogel fibers were shown to possess nano-porous network structure and inherit excellent physical and chemical properties of aramid materials, demonstrating ultra-low thermal conductivity and excellent mechanical properties. Besides, the aramid aerogel fibers were further functionalized to render them with unique functions, such as hydrophobicity, absorbability, and electric conductivity. These aramid aerogel fibers demonstrated wide potential applications in the textile field, which can be woven into thermal insulation fabrics and smart thermal regulation fabrics. In terms of aramid aerogel films, the fabrication process includes blade/spin coating, sol-gel transition, and special drying process. The typical aerogel structure was found to bring about aramid aerogel films with high specific surface area and low thermal conductivity. The post-processing and functionalization were shown to further improve their mechanical properties and electric conductivity. These aramid aerogel films have demonstrated great application prospects in separation and filtration, electromagnetic shielding, infrared stealth, thermal management, and so on. In addition, direct-ink-writing 3D printing and microgel-directed suspended 3D printing strategies were developed to fabricate 3D aramid aerogels. These printing technologies enable aramid aerogels with arbitrary shape realized. Thus, the mechanical or thermal insulation performances can be customized through simple structural design.
Conclusion and Prospect In view of the current urgent demand for high-performance aerogel materials, the research on aramid nanofiber aerogels will remain a research focus. By analyzing the research situation about the preparation, performance and application of aramid aerogel fibers, aramid aerogel films and 3D printed aramid aerogels, it can be confirmed that the development stage of aramid nanofiber aerogels is still in the initial exploration stage. On the one hand, the preparation technology of aramid nanofiber aerogel is not yet mature, and it needs to integrate deeply with traditional fiber, film or 3D printing technologies. There are many key points that need to be improved urgently, such as continuous solvent replacement, drying and other processes. The ultimate performances of aramid nanofiber aerogels have not been reached, and there is still room for improvement in mechanical properties. Therefore, it is necessary to further research on aramid nanofiber aerogels, optimize their preparation technologies, improve their performances, and reduce their production costs, so as to widen applications in thermal management, intelligent protection, separation, filtration and other fields.
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