Lightweight materials that are both highly compressible and resilient under large cyclic strains can be used in a variety of applications. Carbon nanotubes offer a combination of elasticity, mechanical resilience and low density, and these properties have been exploited in nanotube-based foams and aerogels. However, all nanotube-based foams and aerogels developed so far undergo structural collapse or significant plastic deformation with a reduction in compressive strength when they are subjected to cyclic strain. Here, we show that an inelastic aerogel made of single-walled carbon nanotubes can be transformed into a superelastic material by coating it with between one and five layers of graphene nanoplates. The graphene-coated aerogel exhibits no change in mechanical properties after more than 1 million compressive cycles, and its original shape can be recovered quickly after compression release. Moreover, the coating does not affect the structural integrity of the nanotubes or the compressibility and porosity of the nanotube network. The coating also increases Young's modulus and energy storage modulus by a factor of ~6, and the loss modulus by a factor of ~3. We attribute the superelasticity and complete fatigue resistance to the graphene coating strengthening the existing crosslinking points or ‘nodes’ in the aerogel.
Mechanical properties of graphene-coated aerogels. a, Macroscopic visualization, showing that nanotube aerogels collapse and graphene-coated aerogels recover their original shape after compression by over 90%. b, s versus 1 curves for nanotube aerogels along the loading direction and for graphene-coated aerogels during loading–unloading cycles. The hysteresis increases at larger 1 for the graphene-coated aerogels. Insets: photographs of aerogels after graphene coating at 1 ¼ 0% (left) and 60% (right). c,d, Fatigue resistance of graphene-coated nanotube aerogel at 60% strain, 1Hz, for the 1st and 2,000th cycles (c) and at 2% strain, 100 Hz, for the 1st and 106th cycles (d).
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In conclusion, we have shown that a graphene coating transforms mechanically fragile nanotube networks into superelastic materials while maintaining the shape, strength, ultracompressibility, high porosity and conductivity of the networks. The graphene-coated nanotube aerogels resist fatigue and completely recover their original shape very quickly after removal of load over very large strain rates. Our methodology of applying a nanographitic coating to introduce superelasticity and fatigue resistance to nanotube networks, coupled with the inherent flexibility of aerogel synthesis in manipulating material shapes and sizes, will make these materials attractive for dampers, electrodes, sieves, artificial muscles, scaffolds for composites and complex mechanical structures.
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