Carbon materials can be roughly divided into graphitic carbon, soft carbon, and hard carbon according to the different carbon atom hybrid orbits. Soft carbon and hard carbon are mainly used to describe the carbon materials prepared by the pyrolysis of polymers. During the pyrolysis process, some carbon atoms reconstitute two-dimensional aromatic graphene sheets. If these graphene sheets are roughly parallel, it is easy at high temperatures. Graphitized, this kind of carbon is called soft carbon; if these graphene sheets are randomly stacked and crosslinked by edge carbon atoms, they cannot be graphitized at high temperatures, and this carbon is called hard carbon. Generally speaking, graphitic carbon and soft carbon have high elasticity and are easy to deform, but the strength is low; due to the existence of a large number of sp3-C hard carbon micro-layered "card house" structure, hard carbon materials have mechanical strength and structure Shows great advantages in terms of stability, but is inherently brittle and brittle. How to prepare hard carbon materials into super-elastic blocks is currently a challenge.
Recently, the research group led by Professor Yu Shuhong of the University of Science and Technology of China was inspired by the high strength and elasticity of spider webs in nature and constructed a nanofiber network structure cleverly through the template method. A series of hard carbon aerogels with nanofiber network structure were prepared. . This series of aerogels have the advantages of super elasticity, fatigue resistance and good stability. The research paper, entitled "Superelastic hard carbon nanofiber aerogels", was recently published in Advanced Materials (Advanced Materials 2019, 1900651) and was selected as the back cover paper. The co-first authors of the thesis are our postdoc Yu Zhilong and doctoral student Qin Bing.
The researchers prepared RF nanofiber aerogels by using resorcinol-formaldehyde (RF) resin as a hard carbon source and a variety of one-dimensional nanofibers as a structural template. The superelastic hard carbon aerogel can be obtained by high-temperature carbonization gum. This hard carbon aerogel has a fine microstructure and consists of a large number of nanofibers and the welding points between them (Figure 1). This method is simple and efficient, and it is easy to scale up production. By adjusting the amount of template and resin monomer added, the diameter of nanofibers, the density of aerogels, and mechanical properties can be easily adjusted.
Unlike traditional hard and brittle hard carbon blocks, this hard carbon aerogel exhibits excellent elastic properties (Figure 2), mainly including: structural stability (after 50% compression, the microstructure can still recover); High rebound speed (860 mm s-1), higher than many graphite-carbon-based elastic materials; low energy loss coefficient (<0.16), intermolecular forces existing in general graphite and soft carbon materials will cause adhesion And friction to dissipate a lot of energy; fatigue resistance, after 104 cycles of testing at 50% strain, the carbon aerogel showed only 2% plastic deformation and maintained 93% of the initial stress. The researchers also explored the application of this hard carbon aerogel in elastic conductors. After multiple compression cycles at 50% strain, the resistance is almost unchanged, showing stable mechanical-electrical properties, and it can be used in harsh environments. Maintain superelasticity and electrical resistance stability under conditions (such as in liquid nitrogen).
Based on its excellent mechanical properties, this hard carbon aerogel is expected to be applied to stress sensors with high stability, large range (50 KPa), stretchable or bendable. In addition, this method can be extended to prepare other non-carbon-based composite nanofiber aerogels, providing a new way to transform rigid materials into elastic or flexible materials by designing the microstructure of nanofibers in the future.