Inspired by the flexibility and rigidity of natural spider silks webs, a research team led by Prof. YU Shuhong from the University of Science and Technology of China (USTC) developed a simple and general method to fabricate superelastic and fatigue resistant hard carbon aerogels with nanofibrous network structure by using resorcinol-formaldehyde resin as a hard carbon source. This work is published on Advanced Materials entitled as “Superelastic hard carbon nanofiber aerogels” on April 15th as a back cover (Advanced Materials 2019, 1900651).
Conductive and compressible carbon aerogels are admirable in a variety of applications. 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.
Fabrication of hard carbon aerogels (Image by YU Zhilong)
This work was supported by National Natural Science Foundation of China, Foundation for Innovative Research Groups of the National Natural Science Foundation of China, Key Research Program of Frontier Sciences, Strategic Priority Research Program, National Basic Research Program of China, Fundamental Research Funds for the Central Universities, Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS, National Postdoctoral Program for Innovative Talents, China Postdoctoral Science Foundation.