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Until now, materials with high elasticity at deep cryogenic temperatures have not been observed. Previous reports indicated that graphene and carbon nanotube–based porous materials can exhibit reversible mechano-elastic behavior from liquid nitrogen temperature up to nearly a thousand degrees Celsius.

Here, Yongsheng Chen’s research team from Nankai University reported wide temperature–invariant large-strain super-elastic behavior in three-dimensionally cross-linked graphene materials that persists even to a liquid helium temperature of 4 K, a property not previously observed for any other material. To understand the mechanical properties of these graphene materials, they showed by in situ experiments and modeling results that these remarkable properties are the synergetic results of the unique architecture and intrinsic elastic/flexibility properties of individual graphene sheets and the covalent junctions between the sheets that persist even at harsh temperatures. These results suggest possible applications for such materials at extremely low temperature environments such as those in outer space.

In this work, they systematically and quantitatively investigated the mechanical properties of the 3DGraphene foam using a homemade in situ large-strain mechanical analysis system. This system simultaneously realizes real-time recording of the deformations for the materials without resetting the tested sample and breaking the vacuum in a wide temperature range, continuously starting from deep cryogenic temperature at 4 K to a high temperature of 1273 K. They find that even at liquid helium temperature, the 3DGraphene foam, in which randomly oriented graphene sheets are chemically cross-linked through covalent bonds mainly at the edges (Fig. 1A), exhibits the same mechanical properties as those at RT, including nearly fully reversible super-elastic behavior of up to 90% strain, unchanged Young’s modulus, near-zero Poisson’s ratio, and great cycle stability. Furthermore, these mechanical properties are also quantitatively proved to be temperature invariant over a wide temperature range from deep cryogenic temperature (liquid helium, 4 K) to 1273 K. These remarkable and macroscopic temperature-invariant mechanical (super-elastic) properties even down to deep cryogenic temperatures have not been observed/reported for any bulk material before. These unique mechanical behaviors arise from the remarkable temperature-invariant elasticity and flexibility of the individual graphene sheets and covalent junctions between the sheets in the bulk material. The temperature-invariant elasticity and flexibility of graphene is the result of the unique bonding of carbon in the sp2-hybridized planar graphene sheets with soft out-of-plane bending modes and strong in-plane stretching modes with very high energy for defect formation. The intrinsic temperature-invariant mechanical properties of graphene, combined with the unique cross-linked structure and high porosity of the 3DGraphene material, makes the bulk material fully elastic and other temperature-invariant mechanical properties down to liquid helium temperature. These results also imply that using the same strategy (cross-linking in the 3D manner) and other 2D structure units as the building blocks, some other unprecedented properties might be achieved for the bulk assembly of recently emerging 2D materials.

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https://advances.sciencemag.org/content/5/4/eaav2589