Drastically Enhancing Moduli of Graphene-Coated Carbon Nanotube Aerogels via Densification while Retaining Temperature-Invariant Superelasticity and Ultrahigh Efficiency.

Lightweight open-cell foams that are simultaneously superelastic, possess exceptionally high Young's moduli (Y), exhibit ultrahigh efficiency, and resist fatigue as well as creep are particularly desirable as structural frameworks. Unfortunately, many of these features are orthogonal in foams of metals, ceramics, and polymers, particularly under large temperature variations. In contrast, foams of carbon allotropes including carbon nanotubes and graphene developed over the past few years exhibit these desired properties but have low Y due to low density, ρ = 0.5-10 mg/mL. Densification of these foams enhances Y although below expectation and also dramatically degrades other properties because of drastic changes in microstructure. We have recently developed size- and shape-tunable graphene-coated single-walled carbon nanotube (SWCNT) aerogels that display superelasticity at least up to a compressive strain (ε) = 80%, fatigue and creep resistance, and ultrahigh efficiency over -100-500 °C. Unfortunately, Y of these aerogels is only ∼0.75 MPa due to low ρ ≈ 14 mg/mL, limiting their competitiveness as structural foams. We report fabrication of similar aerogels but with ρ spanning more than an order of magnitude from 16-400 mg/mL through controlled isostatic compression in the presence of a polymer coating circumventing any microstructural changes in stark contrast to other foams of carbon allotropes. The compressive stress (σ) versus ε measurements show that the densification of aerogels from ρ ≈ 16 to 400 mg/mL dramatically enhances Y from 0.9 to 400 MPa while maintaining superelasticity at least up to ε = 10% even at the highest ρ. The storage (E') and loss (E″) moduli measured in the linear regime show ultralow loss coefficient, tan δ = E″/E' ≈ 0.02, that remains constant over three decades of frequencies (0.628-628 rad/s), suggesting unusually high frequency-invariant efficiency. Furthermore, these aerogels retain exceptional fatigue resistance for 106 loading-unloading cycles to ε = 2% and creep resistance for at least 30 min under σ = 0.02 MPa with ρ = 16 mg/mL and σ = 2.5 MPa with higher ρ = 400 mg/mL. Lastly, these robust mechanical properties are stable over a broad temperature range of -100-500 °C, motivating their use as highly efficient structural components in environments with extreme temperature variations.