In recent decades, carbon aerogels have been widely explored by using graphitic carbons and soft carbons, which show advantages in superelasticity. These elastic aerogels usually have delicate microstructures with good fatigue resistance but ultralow strength. Hard carbons show great advantages in mechanical strength and structural stability due to the sp3 C-induced turbostratic “house-of-cards” structure. However, the stiffness and fragility clearly get in the way of achieving superelasticity with hard carbons. Up to now, it is still a challenge to fabricate superelastic hard carbon-based aerogels.
The polymerization of resin monomers was initiated in the presence of nanofibers as structural templates to prepare a hydrogel with nanofibrous networks, followed by the drying and pyrolysis to get hard carbon aerogel. During polymerization, the monomers deposit on templates and weld the fiber-fiber joints, leaving a random network structure with massive robust joints. Moreover, physical properties (such as diameters of nanofiber, densities of aerogels, and mechanical properties) can be controlled by simply tuning templates and the amount of raw materials.
Due to the hard carbon nanofibers and abundant welded joints among the nanofibers, the hard carbon aerogels display robust and stable mechanical performances, including super-elasticity, high strength, extremely fast recovery speed (860 mm s-1) and low energy loss coefficient (<0.16). After tested under 50 % strain for 104 cycles, the carbon aerogel shows only 2 % plastic deformation, and retained 93 % original stress.
The hard carbon aerogel can maintain the super-elasticity in harsh conditions, such as in liquid nitrogen. Based on the fascinating mechanical properties, this hard carbon aerogel has promise in the application of stress sensors with high stability and wide detective range (50 KPa), as well as stretchable or bendable conductors. This approach holds promise to be extended to make other non-carbon based composite nanofibers and provides a promising way of transforming rigid materials into elastic or flexible materials by designing the nanofibrous microstructures.