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"Weakness" hard carbon aerogels seem to have a lot of power

2019-06-14


Recently, a research group led by Professor Shuhong Yu from the University of Science and Technology of China, inspired by the high strength and elasticity of natural spider webs, has skillfully prepared a series of hard carbon aerogels with a network structure of nanofibers through the template method. This series of aerogels has the advantages of super elasticity, fatigue resistance and good stability. The research results were selected as the back cover of the paper published in Advanced Materials.

Aerogel is sometimes referred to as "solid smoke" or "frozen smoke" because of its translucent color and ultra-light weight. Aerogel looks fragile, but it's actually very strong and durable. It can withstand thousands of times its own mass and does not melt until temperatures reach 1,200 degrees Celsius. It also has low thermal conductivity and refractive index, and insulation that is 39 times stronger than the best glass fiber. Because of these properties, aerogel has become an irreplaceable material for space exploration. Both the Russian Mir space station and the American Mars Pathfinder spacecraft have used it for thermal insulation.

Carbon materials can be roughly divided into graphite carbon, soft carbon and hard carbon according to the different hybrid orbitals of carbon atoms. Soft carbon and hard carbon are mainly used to describe carbon materials prepared by polymer pyrolysis. During the pyrolysis process, some carbon atoms reconstitute two-dimensional aromatic graphene sheets. If these graphene sheets are roughly parallel, they are prone to graphitization at high temperatures. If these graphene sheets are stacked randomly and cross-linked by edge carbon atoms that cannot be graphitized at high temperatures, the carbon is called hard carbon.

Generally speaking, graphite carbon and soft carbon have high elasticity, easy to deformation, but low strength; Due to the existence of the "house of cards" structure on the microstructure of hard carbon, the hard carbon material shows great advantages in mechanical strength and structural stability, but the intrinsic nature is brittle and fragile. How to prepare hard carbon material into super - elastic block is a challenge at present.

By using resorcine-formaldehyde (RF) resin as the hard carbon source and using a variety of one-dimensional nanofibers as the structural template, the researchers prepared RF nanofiber aerogels, which were superelastic hard carbon aerogels obtained by carbonization at high temperature. This hard carbon aerogel has a fine microstructure and is made up of a large number of nanofibers and the weld joints between them. This method is simple, efficient and easy to scale production. By adjusting the addition amount of template and resin monomer, the diameter of nanofibers, the density of aerogel and the mechanical properties of the nanofibers can be easily controlled.

Unlike traditional hard carbon blocks, which are hard and brittle, this kind of hard carbon aerogel shows excellent elastic properties, such as stable structure, microstructure can still recover after compression by 50%. The rebound speed is higher than that of many graphite-carbon-based elastic materials; Low energy loss coefficient, general graphite and soft carbon materials in the existence of intermolecular forces, will cause adhesion and friction force and 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% initial stress.

The researchers have also explored the application of this hard carbon aerogel in elastic conductors where resistance is almost constant after multiple compression cycles at 50% strain, demonstrating stable mechanical-electrical properties while maintaining hyperelasticity and resistance stability under harsh conditions, such as in liquid nitrogen.

Because of its excellent mechanical properties, this kind of hard carbon aerogel is expected to be used in stress sensors with high stability, large range, stretchability or bendability. In addition, this method can be extended to prepare other non-carbon based composite nanofiber aerogels, providing a new way to convert rigid materials into elastic or flexible materials by designing the microstructure of nanofibers in the future.



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