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 × 10(6) 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.

Flexible conductors of various shapes and sizes with high electrical stability under large elastic stretching and bending are of significant importance in diverse fields ranging from microelectronics to biological implants. Stretchable conductors are fabricated by completely backfilling single‐walled carbon nanotube aerogels with elastomeric polymers. The resistance of the stretchable conductors remains nearly unchanged under tensile strain and high bending strain.

Lightweight aerogels with large specific surface area (SSA) have numerous applications. Free‐standing aerogels are created from single‐walled carbon nanotubes (SWCNTs), and their SSA and pore characteristics, electrical conductivity, mechanical properties, and thermal management attributes are determined. The SSA of the aerogels is extraordinarily high and approaches 1291 m2 g−1 at a density of 7.3 mg mL−1, which is close to the theoretical limit (≈1315 m2 g−1). Mechanical characterization shows that these aerogels have open‐cell structures and their Young's moduli are higher than other aerogels at comparable density. The aerogels also enhance heat transfer in a forced convective process by ≈85%, presumably due to their large porosity and surface area.

The thermal conductivity of gas-permeated single-walled carbon nanotube (SWCNT) aerogel (8 kg m − 3 density, 0.0061 volume fraction) is measured experimentally and modeled using mesoscale and atomistic simulations. Despite the high thermal conductivity of isolated SWCNTs, the thermal conductivity of the evacuated aerogel is 0.025 ± 0.010 W m − 1 K − 1 at a temperature of 300 K. This very low value is a result of the high porosity and the low interface thermal conductance at the tube-tube junctions (estimated as 12 pW K − 1 ). Thermal conductivity measurements and analysis of the gas-permeated aerogel (H 2 , He, Ne, N 2 , and Ar) show that gas molecules transport energy over length scales hundreds of times larger than the diameters of the pores in the aerogel. It is hypothesized that ineffi cient energy exchange between gas molecules and SWCNTs gives the permeating molecules a memory of their prior collisions. Low gas-SWCNT accommodation coeffi cients predicted by molecular dynamics simulations support this hypothesis. Amplifi ed energy transport length scales resulting from low gas accommodation are a general feature of CNT-based nanoporous materials.

Lightweight, superelastic foams that resist creep and fatigue over a broad temperature range are being developed as structural and functional materials for use in numerous diverse applications. Unfortunately, conventional foams display superelasticity degradation, undergo considerable creep, show fatigue under repeated usage, or fracture over large strains, particularly under significant temperature variations. We report that graphene-coated single-walled carbon nanotube (SWCNT) aerogels remain superelastic, and resist fatigue and creep over a broad temperature range of −100−500 °C. The microstructure of these ultralow density (≈14 mg/mL; corresponding volume fraction ≈9 × 10 −3 ) aerogels is composed of a three-dimensional network of randomly oriented SWCNTs with junctions between SWCNTs coated with 2−5 layers of ≈3 nm long graphene nanoplatelets. Compressive stress (σ) versus compressive strain (ε) curves show that the aerogels fully recover their shapes even when strained by at least 80% over −100−300 °C and 20% at 500 °C, whereas the Young's modulus remains similar over the temperature (−100−500 °C) and strain rate ε̇(0.01−0.16 1/s) ranges. We suggest that under compression, the graphene layers hinder free rotation and irreversible sliding of the SWCNTS about the junctions, leading to bending of the graphene layers, while the struts form new junctions stabilized via van der Waals interactions. When the compressive load is removed, the bent graphene layers provide a restoring force that breaks the junctions created during compression, accounting for full aerogel shape recovery, albeit with hysteresis. The storage (E′) and loss (E″) moduli measured in the linear regime show ultralow damping ratio (tan δ = E″/E′) ≈0.02, and these viscoelastic properties remain constant over three decades of frequencies (0.628−628 rad/s) and across −100−500 °C. The low loss in these aerogels is corroborated by exceptional fatigue resistance for 2000 (5 × 10 5 ) cycles at ε = 60% (1%) from −100−300 °C (−100−500 °C) and creep resistance at least under σ = 20 kPa, for a minimum of 1 min from −100−500 °C. Furthermore, these aerogels retain their exceptional creep resistance under the same creep test conditions but for much longer time of 30 min at all tested temperatures except at 500 °C where they show small creep ε of ≈0.7% with ≈0.8% residual ε. The emergent thermomechanical stability of these aerogels that arises from, in part, microscopic deformations of the graphene-coated junctions, motivate junction modification as a means to control the mechanical properties of CNT foams in general. Furthermore, the temperature-invariant mechanical properties of these aerogels combined with their facile fabrication method, which is readily applicable to other nanotube foams, make this class of aerogels a strong alternative to conventional foams, particularly in environments with large temperature variations.

The thermal, electrical, and thermoelectric properties of aerogels of single-walled carbon nanotubes are characterized. Their ultralow density enables the transport properties of the junctions to be distinguished from those of the nanotubes themselves. Junction thermal and electrical conductances are found to be orders of magnitude larger than those found in typical dense SWCNT networks. In particular, the average junction thermal conductance is close to the theoretical maximum for a van der Waals bonded SWCNT junction.

We report on measurements of thermal conductivity in single-walled carbon nanotube (SWCNT) aerogels in vacuum, and as infiltrated by different gases. The remarkable thermal, mechanical and electrical properties of single CNTs have led to great interest in bulk carbon nanotube materials, including the CNT aerogels. Carbon nanotube aerogels are light-weight (7–8kg/m3) and porous, which means that heat will be conducted in parallel through the SWCNT matrix and the filling gas. The overall thermal conductivity of the aerogel was measured with helium, and argon filling gases, using a modified 3ω method designed to interrogate low thermal effusivity materials. Measurements of thermal conductivity at vacuum are 0.023 W/m-K and at atmospheric pressure infiltrated SWCNT aerogels have thermal conductivities in helium of 0.19 W/m-K and in argon of 0.039 W/m-K. Our vacuum measurement suggests that transport within the aerogel is limited by the thermal interface resistance between SWCNTs, rather than by phonon transport within the SWCNT itself. We have also extracted the mean distance traveled by gas molecules between collisions with SWCNT aerogel by fitting the gas contribution to thermal conductivity using a kinetic theory based model.

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 10 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.