Bulk graphene retains superelastic properties at cryogenic temperatures
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Cross-linking via covalent bonds at edges
Bulk graphene retains superelastic properties at cryogenic temperatures
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foam made from three-dimensional cross-linked graphene sheets (3DGraphene) has become the firstknown bulk material to demonstrate superelasticity at liquid helium temperatures. The unique property was discovered by researchers at Nankai University in China, where 3DGraphene was developed in 2015. As reported in Science Advances (doi:10.1126/sciadv.aav2589), this work suggests a path toward harnessing the exceptional properties of two-dimensional (2D) materials to create bulk materials with new capabilities. Carbon nanotubes and graphene exhibit appealing elastic properties, but these tend to weaken when the 2D materials are assembled in bulk form. Five years ago a team of researchers led by Nankai University’s Yongsheng Chen synthesized a mesh-like material from graphene oxide that demonstrated superelasticity from room temperature down to the lowest temperature they could measure with their equipment at that time, which was 77 K. The material, 3DGraphene, has a structure resembling a honeycomb. The cell walls are micrometer-scale, singlelayer graphene sheets. They are randomly oriented and held together at the nodes by carbon–carbon or carbon–oxygen bonds. The cavities between walls are tens of nanometers in dimension. In this new research, also led by Chen, researchers from Nankai University and Rice University collaborated to measure the elastic properties of 3DGraphene down to deep cryogenic temperatures. Using a customized compression system contained in a vacuum and placed inside a scanning electron microscope, the team flattened samples of 3DGraphene repeatedly over a continuous range of temperatures from 4 K to 1273 K. In situ measurements and scanning electron microscope
Schematic of the structure of 3D cross-linked graphene. Oxygen atoms are represented by red dots and the covalent bonds by arrows. The spatial density of the oxygen atoms has been adjusted for clarity and does not represent the actual ratio in the material. Credit: Zhao et al.
observations revealed that the elasticity was temperature-invariant; the material behaved as if it was at room temperature even when compressed to one-tenth of its original thickness 100 times at 4 K. This result is unprecedented. Until now, bulk materials have always demonstrated temperature-dependent elasticity. “This is the first time the mechanical properties—including Young’s modulus, the mechanical strength, and Poisson’s ratio—have shown temperature-invariant properties in a bulk material,” Chen says. In addition, elasticity is usually thermally activated, causing materials to become brittle and stiff at very low temperatures. To investigate the source of this unusual behavior, the researchers turned to computer modeling and simulations. They modeled 3DGraphene as a periodic, honeycomb-like graphene network. They gave the cell walls the mechanical properties of single-layer graphene sheets and the nodes the mechanical properties of covalent bo
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