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Drastically Enhancing Moduli of Graphene-Coated Carbon Nanotube Aerogels via Densification While Retaining Temperature Invariant Superelasticity and Ultrahigh Efficiency Michelle N. Tsui, Kyu Hun Kim, and Mohammad F. Islam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12243 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Drastically Enhancing Moduli of Graphene-Coated Carbon Nanotube Aerogels via Densification While Retaining Temperature Invariant Superelasticity and Ultrahigh Efficiency Michelle N. Tsui, Kyu Hun Kim, and Mohammad F. Islam* Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213-3815, USA KEYWORDS: Densified aerogels, energy damping, fatigue resistance, creep resistance, temperature invariant

ABSTRACT: 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 last 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

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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 mg/mL to ρ ≈ 400 mg/mL dramatically enhances Y from 0.9 MPa 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.


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INTRODUCTION Open-cell foams or aerogels that are simultaneously lightweight, superelastic with high Young’s modulus (Y), and efficient but resist fatigue and creep are highly desirable for structural applications.1-2 Foam properties are shaped by the intrinsic material characteristics of the cell walls, cell geometry, and solid volume fraction (φ).3 The storage modulus (E′) and Y of foams with open-cell geometry typically vary as ρ2 compared with ρ2–3 and ρ3 for foams with closedcell geometry and for fully solid materials, respectively; here, ρ is the average mass density.3 Consequently, open-cell foams are strongly preferred in applications with weight limitations because Y and E′ decrease comparatively minimally with a decrease in ρ.4 Unfortunately, foams fabricated from conventional materials such as metals, ceramics and polymers do not capture all the desirable properties concurrently because many of these properties are orthogonal. For example, typical metal and ceramic foams can achieve high Y ≈ 1.5–10 GPa and very high efficiency, represented by low loss ≈ 10-3–10-2, albeit utilizing closed-cell microarchitecture with correspondingly high ρ ≈ 200–1500 mg/mL and negligible superelasticity.3 Note, efficiency in foams is quantified conventionally by the loss coefficient, tan δ = E″/E′, where E″ is the loss modulus, with low tan δ values indicate high efficiencies and vice versa.4-5 Manipulating microarchitectures by incorporating open-cell instead of closed-cell geometries reduces the ρ of metal foams to 1.0–43 mg/mL, but also leads to drastic reduction in Y to 0.1 kPa–10 MPa along with a decrease in efficiency with tan δ ≈ 0.2–0.8 at compressive strain (ε) ≥ 5% due to crack formation or buckling of struts.6 Similarly, open-cell ceramic foams of low ρ ≈ 6–30 mg/mL show slightly larger Y ≈ 4–40 MPa, but are highly brittle, fracturing unpredictably and catastrophically for small ε ≥ 2%.7-8 On the other hand, open-cell polymer foams display superelasticity with low ρ ≈ 20 mg/mL, but have small Y ≈ 300 kPa and low efficiency with tan δ


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≈ 0.4, limiting them to damping rather than structural applications.3 Furthermore, in contrast to metal and ceramic foams, polymer foam properties can change drastically with temperature (T), particularly near glass transition (Tg) and melting (Tm) temperatures, and frequency (ω), making them impractical in environments with large T variations. Recently, carbon nanotubes (CNTs) have been utilized as struts to engineer lightweight opencell foams because of the attractive intrinsic properties of CNTs.9-11 These CNT foams have the potential to overcome many of the shortcomings of metal, ceramic and polymer foams. An overwhelming majority of these foams are CNT forests fabricated via chemical vapor deposition processes, which allow them to be ultralight with ρ ≈ 3.3 mg/mL although with low Y ≈ 0.3 MPa, no superelasticity, and moderate efficiency (tan δ ≈ 0.3).12 Interestingly, densification of these CNT forest foams through various processes including capillary tension and mechanical compression show an increase in Y that remains constant over T = -140–600 °C and develop superelasticity for ε ≤ 60% when ρ ≥ 26 mg/mL.12-15 Disappointingly, these foams can only be densified to ρ ≈ 54 mg/mL with Y and E′ scaling as ρ2 only for ρ < 36 mg/mL due to drastic bundling mediated microstructural changes at higher ρ. While greater ρ and Y can be achieved with foams of CNT bundles fabricated at ρ = 80 mg/mL, superelasticity drastically degrades for ε ≥ 20%.16 We have recently assembled purified, solution processed single-walled CNTs (SWCNTs) into lightweight (ρ ≈ 7–18 mg/mL; φ = 0.004–0.01) open-cell wetgels using a sol-gel approach followed by critical point drying that converts the wetgels to aerogels.17-18 These aerogels, comprised of three-dimensional (3D) networks of isotropically oriented SWCNTs that act as struts, display reasonable Y ≈ 0.1–0.5 MPa, ultrahigh efficiency with tan δ ≈ 0.02, high porosity, ultrahigh specific surface area, and large electrical conductivity.18-19 Disappointingly, Y is too


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small for any structural applications due to the ultralow ρ and because the SWCNTs are only connected via van der Waals interactions at the junctions between SWCNTs. Further, these SWCNT aerogels plastically deform for ε > 10% likely due to formation of additional irreversible junctions during compression.18 Interestingly, conformally coating the junctions with 1–5 layers of ≈ 3 nm long graphene nanoplatelets, which presumably adhere to the SWCNTs via π–π and van der Waals interactions, imparts emergent features such as superelasticity at least up to ε = 80% as well as fatigue and creep resistance over broad T = -100–500 °C and strain rate (εሶ ) = 0.01–0.16 1/s ranges to the fragile SWCNT aerogels.11,20-21 The graphene coating possibly suppresses SWCNTs from irreversibly sliding and freely rotating at the junctions, limiting bundling and plastic buckling of SWCNTs while maintaining the open-cell structure and ultrahigh efficiency of the underlying SWCNT aerogels. Unfortunately, the coating minimally increases ρ by 30–60% and consequently enhances Y to only ≈ 0.75 MPa, undermining their use as structural foams.20-21 Here we report the fabrication of graphene-coated SWCNT aerogels with a ρ-range of more than a decade (16–400 mg/mL; φ = 0.01–0.26) while maintaining an open-cell microarchitecture and nearly constant ultrahigh efficiency with tan δ ≈ 0.01–0.04, yielding Y = 0.9–400 MPa with superelasticity at least up to ε ≈ 10% even at the highest ρ. We produce these aerogels by first coating struts and junctions of SWCNT wetgels with graphene-precursor polymers and then isostatically compressing the polymer-coated SWCNT wetgels followed by converting the polymers to graphene. The polymer coating prevents changes in the microstructure during densification that we affirm through imaging with scanning and transmission electron microscopy (SEM and TEM, respectively). Aerogels of both low and high ρ exhibit fatigue resistance for at least 106 loading-unloading cycles at 100 Hz to ε ≈ 2% as well as creep


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resistance for at least 30 min under ε ≈ 2% (corresponding compressive stress, σ ≈ 0.02 and 2.5 MPa for ρ = 16 and 400 mg/mL, respectively). Finally, we demonstrate that these attractive mechanical properties remain unchanged at any ρ over extreme T (-100–500 °C) and ω (0.628– 628 rad/s) variations. Our facile densification and graphene coating approach, which imparts exceptional thermomechanical properties to otherwise fragile SWCNT aerogels without altering microstructure, could be readily applied to other CNT foams to integrate similar properties. Furthermore, the ability to densify these aerogels to achieve unparallel values of Y and E′, fatigue and creep resistances, and efficiency without microstructural alterations demonstrate unique verstality of graphene-coated SWCNT aerogels. MATERIALS AND METHODS Fabrication of Graphene-Coated SWCNT Aerogels with Low and High ρ. CoMoCAT SWCNTs with diameters = 0.7–1.4 nm and lengths ≈ 1 um were purchased from Chasm Technologies Inc. (batch CG300) and used as received. We fabricated pristine SWCNT wetgels using a previously reported approach.17-20,22-25 The SWCNT wetgels were coated with dextran by immersing them in a 0.25 mM aqueous solution of dextran with an average molecular weight (MW) = 60,000–90,000 kDa (J. T. Baker) for 24 hours in a ventilated oven at 60 °C. To coat SWCNT wetgels with polyacrylonitrile (PAN), they were first soaked in 70% ethanol followed by gradual solvent replacement with 99.8% dimethyl formamide (DMF; VWR). The DMF-filled SWCNT wetgels were then soaked in 0.5 wt.% PAN (MW = 10,000 g·mol-1 with polydispersity index 1.10) dissolved in DMF at 50 °C for 8 hrs. The dextran- and PAN-coated SWCNT wetgels were subjected to isostatic compression through uniform evaporation. For all samples, evaporation was promptly stopped when the sample reached appropriate dimensions that provided the desired ρ. For instance, to fabricate


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aerogels with ρ = 400 mg/mL, cylindrical samples with starting dimensions of 11.6 mm (both diameter and height) underwent evaporation until the dimensions reached 3.82 mm (diameter and height). Note that in these samples, evaporation generates uniform compression in all dimensions. Densified polymer-coated SWCNT wetgels were then gradually exchanged to anhydrous ethanol and were dried using an Autosamdri®-815, Series A Automatic Critical Point Dryer (tousimis research corporation) followed by pyrolysis to convert polymer to graphene according to a previously published scheme.11,20-21,26 The mass ratio of graphene:SWCNT was kept constant at 3:5 for all graphene-coated SWCNT aerogels. The final ρ of each graphenecoated SWCNT aerogel after densification and critical point drying was determined by dividing the measured mass of the aerogel by its volume, which was determined by measuring the dimensions (i.e., diameter and height). The aerogels had ρ = 16, 80, 200, and 400 mg/mL. Characterization of Microstructure and Mechanical Properties. SEM images were taken using a spherical aberration corrected FEI Quanta 600 at 30 kV. High-resolution TEM images were taken with an FEI Titan 83 at 300 kV. The σ versus ε, viscoelastic (E', E", and tan δ), fatigue, and creep measurements were carried out using a RSA-G2 DMA (TA Instruments) equipped with a forced convection oven to maintain sample T between T = -100–500 °C. For all measurements, cylindrical aerogels were loaded between two vertical, parallel plate compression heads. To maintain complete contact between the compression heads and the samples, and to prevent the samples from sliding horizontally during the measurements, aerogels with ρ = 16 mg/mL and 80 mg/mL were prestrained by ≈ 3% while aerogels with ρ = 200 mg/mL and 400 mg/mL were prestrained by ≈ 0.05%. All T-dependent measurements were carried out in an inert nitrogen gas environment to avoid oxidation of SWCNTs and graphene at high T. Before beginning data collection, the samples were held for at least 10 min to equilibrate to the set T.


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Measurements of σ versus ε during loading-unloading cycles at T = -100, 25, and 500 °C were made using a constant εሶ = 0.01 1/s to ε = 10%. E' and E" of the aerogels were measured as a function of ω under an oscillatory ε = 1%. Fatigue measurements were executed under oscillatory ε = 1% at ω = 100 Hz, corresponding to εሶ = 4 1/s. Creep tests were carried out by subjecting aerogels with ρ = 16 and 400 mg/mL to a constant step σ of 20 kPa and 2.5 MPa, respectively, corresponding to an initial ε ≈ 2%, for 1 min and 30 min. RESULTS AND DISCUSSION Aerogel Fabrication and Characterizations. The key fabrication steps of graphene-coated SWCNT aerogels with large ρ-range (16–400 mg/mL) are schematically shown in Figure 1. We began by creating 3.5 mg/mL aqueous suspensions of SWCNTs stabilized by the surfactant, sodium dodecylbenzene sulfonate (SDBS), with a SWCNTs:SDBS weight ratio of 1:10.27 At this concentration, SWCNTs-SDBS typically form wetgels within a few hours; here, wetgels refer to SWCNT networks filled with water or any solvent.17,19,22,24,28 We loaded the SWCNTs-SDBS suspensions into cylindrical molds and allowed the suspensions to gel. We note that the wetgels can be molded into any shapes and sizes because the gel formation is a sol-gel process, which is a unique advantage of our fabrication approach. We chose cylindrical molds because we needed cylindrical samples for mechanical characterization. The SDBS surfactant molecules were then rinsed-off of the wetgels that were subsequently removed from the molds. The surfactant removal step typically induces some shrinkage of the wetgels, leading to an increase in the SWCNT concentration from 3.5 mg/mL to 7.0–8.8 mg/mL.18 Next, we coated the junctions and the struts of SWCNT wetgels with graphene-precursor polymers: polyacrylonitrile (PAN) or dextran. The polymer-coated SWCNT wetgels were isostatically compressed through uniform evaporation to final dimensions needed to reach desired ρ (Figure 1a). Finally, to fabricate


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graphene-coated SWCNT aerogels, the compressed polymer-coated SWCNT wetgels were critical-point-dried and subsequently pyrolyzed to convert the polymer into graphene.11,20-21,26 Note that many different carbon sources such as polydopamine, glucose and other polysaccharides can be used as graphene precursors to fabricate graphene-coated SWCNT aerogels without altering aerogel microstructure and mechanical responses.11,20-21,23,26,29 The dimensions (diameter × height) of the graphene-coated SWCNT aerogels with ρ = 16 mg/mL were 6 mm × 6 mm (Figure 1b, left) and ρ = 400 mg/mL were 4 mm × 4 mm (Figure 1b, right).

Figure 1. Fabrication of open-cell graphene-coated SWCNT aerogels with readily tunable density. a) The wetgels coated with polymer (polyacrylonitrile or dextran) before (left) and after (right) isostatic compression by partial solvent evaporation to desired dimensions along with schematics of the corresponding microstructure. b) Images of aerogels, obtained by critical point drying of polymer-coated SWCNT wetgels followed by pyrolysis, with schematic of microstructure. Aerogel density can be varied from 16 mg/mL (no compression) to 400 mg/mL (with compression). We evaluated the retention of open-cell microstructure, network morphology, and the graphene coating through our fabrication process by comparing SEM and high-resolution TEM images of cross-sections of the graphene-coated SWCNT aerogels with the lowest (16 mg/mL)


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and the highest (400 mg/mL) ρ (Figure 2). The SEM images show that aerogels of both ρ possess an isotropic, open-cell network of graphene-coated SWCNTs (cf. Figure 2a with Figure 2b). As expected, the number density of junctions increases while the separation between the junctions and the overall porosity decrease with an increase in ρ. Notice that the SEM images do not display significant bundling of graphene-coated SWCNTs, which is a typical outcome of anisotropic evaporation. Further, the uniformity and homogeneity of the network at length scales ≥ 100 nm in these densified aerogels are an outstanding improvement compared to those in other densified CNT aerogels such as dense CNT forest foams generated through capillary compression, in which extensive bundling is facilitated by strong van der Waals interactions between bare CNTs.15 The high-resolution TEM images from aerogels with the lowest ρ = 16 mg/mL show that nearly all the junctions and ≈ 25–40% of the struts are physically wrapped with 1–5 layers of ≈ 3 nm long graphene nanoplatelets (Figure 2c).11,20-21,26 Note that we have previously shown that the graphene coating process in similar aerogels with low ρ does not damage SWCNTs, and the graphene coating does not degrade over T = 25−800 °C in nitrogen atmosphere, although it completely burns off at ≈ 600 °C in atmospheric air due to the oxidative decomposition of graphene and SWCNTs.21 The graphene coating in aerogels with the highest ρ = 400 mg/mL is nearly identical (cf. Figure 2c with Figure 2d), likely retaining the characteristics of the graphene coating of aerogels with lower ρ and demonstrating the robustness of our fabrication method.


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Figure 2. Comparison of the open-cell microstructures of graphene-coated SWCNT aerogels at low and high density. a–b) SEM images show the open-cell microstructure of aerogels with densities of 16 mg/mL and 400 mg/mL. Scale bar represents 500 nm. c–d) High-resolution TEM images of graphene-coated SWCNT struts show the conformal, multilayered graphenecoating lying parallel against the walls of individual SWCNTs, and is similar between aerogels with density of 16 mg/mL and 400 mg/mL. Scale bar represents 5 nm. Superelasticity Preservation with Aerogel Density. We determined the preservation of superelasticity of graphene-coated SWCNT aerogels across ρ = 16–400 mg/mL and T = -100– 500 °C by measuring σ versus ε curves during static, uniaxial loading-unloading cycles up to ε = 10% using a constant εሶ = 0.01 1/s. We have previously shown that the shape of σ versus ε responses of graphene-coated SWCNT aerogels with low ρ ≈ 14 mg/mL during compressive loading-unloading cycles over ε = 5–80% are analogous to those of open-cell viscoelastic foams comprising of a linear or Hookean regime for ε ≤ 7%, a plateau regime for 7% < ε < 65%, and a densification regime at ε ≥ 65%.3,11,21,23,26 The responses also display hysteresis between loading and unloading.11,20-21,26 Representative σ versus ε curves for the lowest (16 mg/mL) and highest


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(400 mg/mL) ρ aerogels are shown in Figure 3a and 3b, respectively. Aerogels of any ρ completely recover their original shapes after compression to at least ε = 10% at any T, demonstrating their ρ- and T-invariant superelasticity at least over the tested ranges. Particularly, even with an increase in ρ by a factor of 25 from 16 mg/mL to 400 mg/mL and at extreme T = -100 °C and 500 °C, the aerogels do not undergo brittle fracture or plastic deformation (Figure 3b). In contrast, metal and ceramic foams show negligible superelasticity.6-7 Further, Tg and Tm of polymers typically exist between -100–300 °C, and consequently, polymer foams, which are often superelastic for ρ = 20–100 mg/mL near room T, undergo severe plastic deformation over the tested ρ- and T-range.3,30-32 On the other hand, CNT forest foams, which can only be densified up to ρ = 54 mg/mL, show superelasticity over ρ = 26–54 mg/mL and at room T, while foams of CNT bundles synthesized at higher ρ ≈ 80 mg/mL do not show superelasticity for ε ≥ 20%, instead plastically deforming by 10–30% that is also probed only at room T.13,16


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Figure 3. Superelasticity and power-law dependence of Y on ρ as well as efficiency of graphene-coated SWCNT aerogels over T = -100–500 °C. σ versus ε curves from loadingunloading cycles to ε = 10% for aerogels with a) ρ = 16 mg/mL and b) ρ = 400 mg/mL. c) Log-log Ashby plot of Y versus ρ of representative foams to compare and contrast with graphene-coated SWCNT aerogels.3-4,6,12,16,33-41 Each parallel dashed line represent a constant magnitude of sound velocity (vs) with larger vs signifying higher efficiency.

Young’s Modulus Enhancement with Aerogel Density. We calculated Y of aerogels at different ρ and T from the slopes of σ versus ε curves within the linear regime and during loading. We also compare Y of aerogels and representative foams of natural materials (e.g., wood and cork),3 rigid and flexible polymers,3 ceramics (e.g., silica, alumina),6,39-40,42 metals (e.g., aluminum, Al, nickel, Ni),6 boron nitride nanosheets (BNNS),37 and carbon allotropes including carbon (C),38 graphene,33 other CNTs and C-CNT composites,12-13,35,43 on a log-log Ashby plot of Y versus ρ (Figure 3c). With an increase in aerogel ρ from 16 mg/mL to 400 mg/mL, Y increases from 0.90 ± 0.02 MPa to 400 ± 40 MPa. Interestingly, Y remains nearly independent of T from 100 °C to 500 °C. In contrast, Y of crosslinked (uncrosslinked) polymer foams can change by at


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least 1 (5) order of magnitude due to Tg and Tm.31 Additionally, Y of graphene-coated SWCNT aerogels scales as ρ2 over more than an order of magnitude in ρ (16–400 mg/mL), corroborating the retention of the open-cell microarchitecture in these aerogels through the fabrication scheme including the densification step. Guidelines for Y scaling with ρn with n = 1, 2 and 3 are provided at the bottom right corner in Figure 3c. We point out that the Y of these aerogels extend into the upper right region generally occupied by foams of high modulus such as wood and Al honeycombs, and has never been observed for foams of carbon allotropes.3,6 For example, graphene foams can only be densified by 9× from 0.5–6.6 mg/mL, consequently limiting Y to low values of 0.1–30 kPa.33 Meanwhile, densification of CNT forest foams via mechanical compression from ρ = 3.3 mg/mL to 36 mg/mL, increases Y from 0.3 MPa to 3.2 MPa, but further densification to ρ ≈ 54 mg/mL only raises Y to 3.6 MPa due to CNT bundling induced microstructural changes.12 Alternatively, foams of CNT bundles prepared at ρ ≈ 80 mg/mL display larger Y ≈ 180 MPa but a ρ-dependence of Y was not explored.16 Efficiency and Viscoelastic Properties versus Aerogel Density. We began determination of efficiency by estimating the magnitude of sound velocity, vs = (Y/ρ)1/2, in the graphene-coated SWCNT aerogels. Note that vs characterizes mechanical stress transmission through a material with larger vs indicating higher efficiency, and is a figure of merit to identify optimal Y versus efficiency. To highlight and ease the comparison of vs between different foams, guidelines representing vs values are included in Figure 3c, while direct comparisons of vs versus ρ against the same representative foams are compiled in an Ashby plot in Figure S1. Open-cell graphenecoated SWCNT aerogels display vs = 0.24–1.0×103 m/s that are significantly larger compared to that of foams with similar microstructure and ρ-range (Figure 3c and S3). Furthermore, the preserved microarchitecture of these aerogels over more than a decade in ρ combined with Y


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scaling as ρ2 suggest that the efficiency in the aerogels must be ultrahigh to be able to reach exceptionally large values of vs. To directly probe the efficiency across broad ranges of ρ, T and relaxation timescales, we measured viscoelastic properties (E', E", and tan δ) versus ω of graphene-coated SWCNT aerogels for ρ = 16–400 mg/mL in the linear deformation regime under oscillatory ε = 1% at T = -100, 25, and 500 °C using dynamic mechanical analysis (DMA). E' (Figure 4a–c), E" (Figure 4d–f), and tan δ (Figure 4g–i) for any ρ are nearly independent of T and over 3 decades of accessible ω = 0.628–628 rad/s, indicating that the timescales associated with energy dissipative microstructural relaxation and reorganization for graphene-coated SWCNT aerogels and the graphene coating are beyond the physical timescales probed in our measurements. We note that the small variations in E″ with ω are likely an artifact of the measurement system, since E″ of similar aerogels with ρ = 14 mg/mL measured using an alternate DMA system (Bose Enduratec ELF3200) is independent of ω over the same ω-range.20 We point out that tan δ values remain extremely small (≈ 0.01–0.04) versus ω at any of the tested T and ρ, further corroborating ultrahigh efficiency of these aerogels.


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Figure 4. Viscoelastic properties (a-c) E', (d-f), E", and (g-i) tan δ of graphene-coated SWCNT aerogels with ρ = 16–400 mg/mL versus ω under oscillatory ε = 1% at (a, d, g) T = 100 °C, (b, e, h) T = 25 °C, and (c, f, i) T = 500 °C. We compile E', E", and tan δ at low ω = 1 Hz against ρ in Figure 5a to highlight their T and ρ dependence. At any given T, E' (E") increases by 3 decades from ≈ 1.0 MPa (≈ 10 kPa) at ρ = 16 mg/mL to 1.2 GPa (≈ 20 MPa) with densification by a factor of 25 to ρ = 400 mg/mL. Additionally, E' and E″ remain nearly constant with changes in T from -100 °C to 500 °C at any fixed ρ, consistent with Y variations with T and ρ (cf. Figure 3b with Figure 5a). In addition, Eʹ and E" scale as ρ2, supporting the Y dependence on ρ and open-cell microstructure of graphenecoated SWCNT aerogels. The striking stability of E' and E″ for aerogels with any ρ over T = 100–500 °C (Figure 4 and 5a) is highly unusual for lightweight, superelastic, and compressible foams. For example, viscoelastic polymer foams are highly sensitive to T, becoming drastically


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brittle or ductile near Tg or Tm, with >5 orders of magnitude changes in E' and E″ due to the strong T dependence of polymer chain mobility.30-32 Interestingly, the identical power-law dependence of E' and E" on ρ across T translates to a relatively unchanged tan δ versus ρ over the same T-range, indicating ρ- and T-invariant efficiency. Furthermore, the magnitude of tan δ, which ranges from 0.01–0.04, is ultralow and invariant for exceptionally large ranges of E' and Y values, in stark contrast to the common behavior of alternative foams presented in Ashby plots in Figure S2 and Figure S3, suggesting exceptionally high efficiency. For instance, the efficiency of viscoelastic foams dramatically varies with T, particularly near Tg and Tm, also because of Tdependent polymer mobility. Furthermore, the efficiency of rigid polymer foams that display similarly large Y values from 0.6 GPa to 0.02 GPa is low and varies by more than an order of magnitude with tan δ being ≈ 0.04–0.5.4 Since both tan δ and Y often strongly depend on ρ for many alternative representative foams that have shown rather limited success in achieving high ρ, we plot tan δ as a function of specific modulus (Y/ρ) for graphene-coated SWCNT aerogels with all tested ρ and T along with other representative foams in an Ashby plot (Figure 5b) to facilitate comparison. Note that foams with properties near the lower right corner are highly desirable in structural applications because they possess high efficiency and Y/ρ.3 CNT forest foams display drastic decrease in efficiency (tan δ increases from 0.008 to 0.32) for only a factor of 2 decrease in Y/ρ from 0.03 to 0.015 MPa·mL/g, primarily because Y in these foams is ∝ ρ2–3.12 Graphene foams similarly suffer from extremely low Y/ρ ≈ 0.2–5 MPa·mL/g with very low albeit nearly constant efficiency (tan δ ≈ 0.67–0.87).33 In striking contrast to other foams of carbon allotropes, open-cell graphene-coated SWCNT aerogels occupy a distinct region with ultrahigh efficiency, characterized by extremely small tan δ ≈ 0.01–0.04 that is 10 times lower than that of most CNT forest foams and 30 times


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lower than that of graphene foams, while Y/ρ reaches ≈ 1.2 GPa·mL/g. In fact, these graphenecoated SWCNT aerogels with high modulus and ultrahigh efficiency overlap regions of rigid polymer foams and are comparable to lightweight wood such as balsa, pine, ash, and oak, which are considered to have evolved over millions of years to possess mechanically optimized microarchitectures.3

Figure 5. T- and ρ-invariant ultrahigh efficiency of graphene-coated SWCNT aerogels captured through loss coefficient. a) tan δ against ρ at T = -100, 25, and 500 °C (left axis); tan δ values are calculated from E' and E" values at ω = 1 Hz of aerogels with ρ = 16–400 mg/mL (right axis). b) Comparison of tan δ versus Y/ρ with common alternative foams.34,6,12,16,33-37

Fatigue and Creep Resistances with Aerogel Density. We examined the fatigue and creep resistance of graphene-coated SWCNT aerogels, because such highly desired material properties are often profoundly dependent on efficiency. To evaluate the ρ- and T-dependent fatigue response of graphene-coated SWCNT aerogels, we measured E′ for aerogels with ρ = 16 mg/mL


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and 400 mg/mL under oscillatory ε = 1% at ω = 100 Hz for 106 cycles at T = -100, 25, and 500 °C (Figure 6). Despite the broad T-range and exceptionally large number of cycling, E′ remains nearly constant with values ≈ 1.0 MPa and ≈ 350–760 MPa for aerogels with ρ = 16 mg/mL and 400 mg/mL, respectively, confirming the structural robustness that makes these aerogels superelastic with ultrahigh efficiency. Unfortunately, there have been very limited studies on fatigue resistance of foams with similar ρ- and T-ranges. For example, densified CNT forest foams with ρ = 36 mg/mL show reasonable fatigue resistance under similar shear σ and number of cycles from -140 to 600 °C, but such foams could not be densified to ρ > 54 mg/mL.12,14 On the other hand, polymer foams with small Y and low efficiency show drastic fatigue with large T variations due to changes in viscoelastic properties originating from polymer hardening, melting, and decomposition.30-32

Figure 6. The ρ- and T-invariant fatigue resistance of graphene-coated SWCNT aerogels. Fatigue resistance is demonstrated through constant E' over 106 compressive loadingunloading cycles under oscillatory ε = 1%, ω = 100 Hz, and at T = -100, 25, and 500 °C for aerogels with ρ = 16 mg/mL and 400 mg/mL. Error bars were determined from measurements using multiple samples.


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To probe creep responses, we used standard creep tests on graphene-coated SWCNT aerogels with ρ = 16 mg/mL and 400 mg/mL at T = -100, 25, and 500 °C. In one set of experiments, a step constant σ of 20 kPa (2.5 MPa) was applied to aerogels with ρ = 16 mg/mL (400 mg/mL) for 1 min followed by creep recovery for at least 10 s at T = -100, 25, and 500 °C. The aerogels instantaneously respond by reaching corresponding ε = 2% for all T. The aerogels with either ρ stay at this ε until the applied σ is removed, prompting immediate recovery with no resolvable residual ε and indicating complete creep resistance. The creep profiles of the aerogels under these creep tests are shown in Figure S4. To further challenge the creep resistance, the same creep tests were performed for a significantly longer duration of 30 min under the same applied σ followed by 5 min creep recovery; the creep profiles are shown in Figure 7. The aerogels again deform instantly to corresponding ε ≈ 2% for all T. At T = -100 °C and 25 °C, aerogels with either ρ do not exhibit any resolvable creep ε over the duration of applied σ and residual ε after recovery. However, at T = 500 °C, aerogels with ρ = 16 mg/mL display creep ε ≈ 0.3% over the duration of applied σ with a small creep εሶ of 1.7 × 10-6 1/s. The aerogels recover rapidly after relieving the σ with residual ε ≈ 0.5% monitored over 5 min. Meanwhile, at T = 500 °C, the aerogels with ρ = 400 mg/mL exhibit reduced creep ε ≈ 0.15% and creep εሶ of 8.3 × 10-7 1/s as well as rapid recovery with residual ε ≈ 0.25% (Figure 7). The small creep at 500 °C is likely due to partial delamination of the graphene-coating, which is exacerbated by the applied σ.21


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Figure 7. The ρ- and T-invariant creep resistance of graphene-coated SWCNT aerogels. Creep behavior of these aerogels with ρ = 16 mg/mL (top) and 400 mg/mL (bottom) at T = 100, 25, and 500 °C under a constant σ of 20 kPa and 2.5 MPa for 30 min, respectively, followed by 5 min of creep recovery. We associate the aerogel fabrication scheme with specific interactions between SWCNTs and graphene to elucidate both the ρ-invariant microarchitecture and the origin of the exceptional thermomechanical properties of graphene-coated SWCNT aerogels. During the isostatic compression mediated densification step, the polymer coating stabilizes the junctions between SWCNTs, allowing the number density of junctions to be increased over a large range and avoiding detrimental microstructural rearrangement (e.g., SWCNT bundling, alignment, etc.). This facilitates an exceptional increase in ρ to 400 mg/mL (corresponding φ = 0.26) while simultaneously preserving the open-cell microstructure and network morphology, facilitating the aerogels to reach large Y and maintain ρ2 scaling. On the other hand, the graphene coating hinders SWCNTs from free rotation and sliding during loading-unloading, which reduces frictional loss as well as irreversible bundling and rearrangement of SWCNTs that would otherwise enlarge tan δ and decrease efficiency. Furthermore, the graphene at the junctions likely


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bend during compression and subsequently provide a spring-like restoring force upon load removal, hence imparting superelasticity while keeping efficiency large.11,20-21 The interactions between SWCNTs and graphene that allow for superelasticity and ultrahigh efficiency similarly manifest in the fatigue and creep resistances of these unique graphene-coated SWCNT aerogels.7,11,21 The durability of these various mechanical properties with extreme T and ω variations is likely due to the intrinsic thermomechanical stability of graphene and SWCNTs,12,14 as well as the largely T-invariant van der Waal and π-π interactions that adheres the graphene coating to the SWCNTs.21 CONCLUSIONS In summary, we have fabricated open-cell graphene-coated SWCNT aerogels with a wide ρrange of 16–400 mg/mL through controlled isostatic compression. Our fabrication process preserves the open-cell microstructure even at the highest ρ, resulting in Y between 0.9 MPa–400 MPa with Y scaling as ρ2, consistent with ideal open-cell foams. These aerogels retain superelasticity at least up to ε ≈ 10% even at the highest ρ, while maintaining high efficiencies (tan δ ≈ 0.01–0.04) that distinguishes them from other foams of carbon allotropes as well as common representative foams. Interestingly, these aerogels exhibit both emergent fatigue resistance for 106 loading-unloading cycles to ε = 2% and creep resistance for at least 30 min under an applied σ of 20 kPa (2.5 MPa) for aerogels with at ρ = 16 mg/mL (400 mg/mL). Furthermore, these robust mechanical properties are stable over a large T-range of -100–500 °C. The thermally resilient high modulus, superelasticity, and high efficiency motivate these opencell graphene-coated SWCNT aerogels for use in environments with extreme T variations.


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Finally, our facile densification and graphene coating approach can be readily applied to other fragile CNT foams to integrate similar exceptional thermomechanical properties. ASSOCIATED CONTENT Supporting Information: Supporting figures: (1) Ashby plot of sound velocity (vs) versus density (ρ); (2) Ashby plot of loss coefficient (tan δ) versus Young’s modulus (Y); (3) Ashby plot of tan δ versus vs; (4) ρ- and T-invariant creep resistance under a constant σ of 20 kPa and 2.5 MPa, respectively, for 1 min followed by 30 s of creep recovery. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *M.F.I.: Phone: (412) 268-8999 Fax: (412) 268-7596 E-mail: [email protected] Author Contributions M.F.I. developed and designed the project. M.N.T. and K.H.K. carried out the experiments and collected the data. All authors analyzed the data. M.N.T. and M.F.I. wrote the manuscript. All authors approved the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Science Foundation through grant CMMI-1335417. We acknowledge the use of the Carnegie Mellon Materials Characterization Facility (supported by grant MCF-677785) for TEM and SEM imaging and T. Kowalewski for providing PAN polymer.


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TABLE OF CONTENTS FIGURE


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Drastically Enhancing Moduli of Graphene-Coated ... - ACS Publications

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