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Highly thermally conductive and mechanically strong graphene fibers

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Science  04 Sep 2015:
Vol. 349, Issue 6252, pp. 1083-1087
DOI: 10.1126/science.aaa6502

A superior mix of big and small

Graphene is often described as an unrolled carbon nanotube. However, although nanotubes are known for their exceptional mechanical and conductivity properties, the same is not true of graphene-based fibers. Xin et al. intercalated small fragments of graphene into the gaps formed by larger graphene sheets that had been coiled into fibers. Once annealed, the large sheets provided pathways for conduction, while the smaller fragments helped reinforce the fibers. The result? Superior thermal and electrical conductivity and mechanical strength.

Science, this issue p. 1083

Abstract

Graphene, a single layer of carbon atoms bonded in a hexagonal lattice, is the thinnest, strongest, and stiffest known material and an excellent conductor of heat and electricity. However, these superior properties have yet to be realized for graphene-derived macroscopic structures such as graphene fibers. We report the fabrication of graphene fibers with high thermal and electrical conductivity and enhanced mechanical strength. The inner fiber structure consists of large-sized graphene sheets forming a highly ordered arrangement intercalated with small-sized graphene sheets filling the space and microvoids. The graphene fibers exhibit a submicrometer crystallite domain size through high-temperature treatment, achieving an enhanced thermal conductivity up to 1290 watts per meter per kelvin. The tensile strength of the graphene fiber reaches 1080 megapascals.

As one of carbon’s allotropes, single-layer graphene has the highest thermal conductivity ever reported, up to ∼5000 W m−1 K−1 at room temperature (13); it also has the highest Young’s modulus (~1100 GPa) (4), fracture strength (130 GPa) (4), and mobility of charge carriers (200,000 cm2 V−1 s−1) (5). However, such remarkable properties of single-layer graphene are on a molecular level and have not been achieved when processed into fibers. Macroscopic graphene oxide (GO) fibers can be assembled from a dispersion of GO in aqueous media, with graphene fibers produced upon reduction of the GO fibers (6, 7). The anisotropic liquid crystalline behavior of the GO sheets can lead to a prealigned orientation, which can further be directed under shear flow to form an ordered assembly in a macroscopic fiber structure via a simple and cost-effective wet-spinning process (6, 7). Improvement of the mechanical properties of the GO fibers and graphene fibers is achieved by introducing metal ion cross-linking bonds between graphene or GO sheets (8) or forming graphene or GO–based composite fibers—e.g., by adding carbon nanotubes (9, 10) (see a summary in table S1). A tensile strength of 652 MPa was observed for a GO-hyperbranched polyglycerol-glutaraldehyde composite fiber (11), and for pure graphene fibers fabricated from large-sized GO sheets as the building block, a tensile strength of 501 MPa with 11.2-GPa Young’s modulus was reported (8). GO fibers are typically electrically insulating. Electrical conductivity can be recovered up to 285 S/cm for carbon fiber derived from graphene oxide nanoribbons after thermal reduction (12), or using vitamin C chemical reduction of neat graphene fiber to 8.1 × 103 S/m, with HI-reduced neat fiber to 2.8 × 104 S/m (13). The reported mechanical and electrical properties of graphene fibers are orders magnitudes lower than those of single-layer graphene (4) and substantially inferior to commercialized carbon fibers and carbon nanotube fibers (1417).

It is difficult to realize simultaneously high mechanical and superior thermal and electrical properties. Highly aligned sp2 graphene sheets are required for high thermal and electrical transport in which the mechanical strength is thus primarily limited by the van der Waals interaction between graphene sheets (68, 18). However, heterogeneous structures, including functional groups and sp3 bonds in cross-linked graphene nanosheets needed to improve mechanical strength, behave as effective phonon- and electron-scattering centers, thus reducing the electrical and thermal conductivities (18).

In contrast to the conventional approach, in which only large-sized GO (LGGO) is believed to be favorable for electrical and mechanical properties (because of its greater aspect ratio) (8, 19, 20), we adopt a strategy of using small-sized GO (SMGO) (average size of 0.8 μm, fig. S1A) and LGGO (average size of ~23 μm, fig. S1B) to form an intercalated and compact fiber structure (Fig. 1A-1) (21). LGGO sheets form a highly aligned backbone, whereas SMGO sheets fill the space and voids between LGGO sheets without altering the high degree of sheet orientation and alignment. By varying the content of SMGO sheets occupying the voids between LGGO sheets, an optimal balance between compactness and sheet alignment can be reached. Upon thermal reduction and high-temperature annealing, the as-spun insulating GO fibers transform to ordered, highly thermally and electrically conductive, and mechanically strong graphene fibers (Fig. 1A-2). The optimized graphene fiber with the addition of 30 weight % (wt %) SMGOs, subjected to thermal annealing at 2850°C, achieves thermal and electrical conductivities up to 1290 ± 53 W m−1 K−1 and 2.21 (±0.06) ×105 S m−1, respectively.

Fig. 1 Highly thermally conductive and mechanically strong graphene fibers with an “intercalated” structure of large and small-sized graphene sheets.

(A) Schematics of the “intercalated” structure of the GO fibers and graphene fibers: (1) GO fiber with optimized LGGO and SMGO loadings; (2) optimized graphene fiber with a highly ordered and compact structure with 30 wt % SMGOs filling into the microvoids; (3) graphene fiber from pure LGGOs showing a highly ordered but less dense structure; and (4) graphene fiber from pure SMGOs showing a random sheet alignment. (B) A thermal image showing rapid heat transport on the optimized graphene fiber and comparison with graphene fiber from pure LGGOs and copper wire. All of the fibers and the copper wire have a diameter of 50 μm and are attached vertically on a microheater. The histogram on right indicates the color scale according to temperature (unit: °C). (C) Exceptional mechanical strength as revealed by typical stress-strain curves of graphene fiber from pure LGGO (annealed at 1600°C) and optimized graphene fiber (annealed at 1800°C). The inset shows a highly flexible optimized graphene fiber.

The optimized graphene fiber shows excellent heat transport performance, superior to that of the graphene fiber from pure LGGOs and copper wire (Fig. 1B). The optimized graphene fibers also outperform commercially available thermally conductive mesophase pitch-based carbon fibers (typically thermal conductivities of 600 to 1000 W m−1 K−1) (14, 2224) and are more cost effective owing to low-cost raw materials, simple preparation, and a lower annealing temperature. Mechanically, the graphene fibers from pure LGGOs achieve tensile strength of 940 ± 62 MPa (best value 1005 MPa; Fig. 1C); the optimization of graphene sheet alignment and compactness by intercalating SMGOs further improves the strength of the graphene fiber to 1080 ± 61 MPa (best value 1150 MPa; Fig. 1C).

The alignment of graphene sheets, the compactness of the fiber structure, and the crystalline graphitic domain size ultimately determine the mechanical properties, and the phonon and electron transport behaviors, of the graphene fiber. Liquid crystals with larger domains can yield macroscopic materials with a higher microstructural order (8, 19, 20). Initially, LGGO dispersed in aqueous solution with large liquid crystalline domains is spun into a highly aligned fiber structure, as evidenced by sectional scanning electron microscope (SEM) images and surface morphology of the GO fibers (fig. S2). Upon annealing and reduction at 1800°C, the graphene fibers display well-maintained graphene sheet alignment as determined by small-angle x-ray scattering (SAXS) (2528). The graphene fiber from LGGO shows a strong equatorial streak scattering pattern, indicating slit-shaped microvoids with a high aspect ratio, well-aligned with the fiber axis (Fig. 2A) (2528). The microvoids are primarily induced by graphene sheet restacking and the removal of oxygen functional groups as gaseous H2O, CO, and CO2 during the thermal annealing process (29, 30). Low density and high porosity are observed in the graphene fibers upon thermal reduction of the GO fibers (Fig. 2C), resulting in inferior thermal and mechanical properties. The filling of microvoids with SMGOs leads to a deformation of the equatorial streak from strong anisotropic patterns to more isotropic ellipse patterns, suggesting a random distribution of the slit-shaped microvoids (Fig. 2A) (2528). The misalignment angle (shown in Fig. 2B and the calculation in supplementary materials) increases with the addition of SMGOs, suggesting a reduction in the degree of graphene orientation with respect to the fiber axis (2528). The addition of SMGOs with a low aspect ratio disrupts the large liquid crystalline domains during spinning, resulting in a less oriented microstructure, unfavorable for the physical properties of graphene fibers. By contrast, the addition of SMGO sheets increases the physical density of the graphene fibers and reduces the porosity monotonically (Fig. 2C), and the graphene fiber becomes more compact.

Fig. 2 Graphene sheet alignment and physical, thermal, and electrical properties of the graphene fibers annealed at 1800°C.

(A) Small-angle x-ray scattering patterns of the fibers with a different wt % of SMGO (the fiber axis is vertical in all measurements; scale bar: 1 nm−1); (B) misalignment angle Bg; (C) density and porosity; and (D) thermal and (E) electrical conductivities of the graphene fibers. The uncertainties of the thermal and electrical conductivities are based on a standard deviation of seven to nine measurements.

An optimized balance between graphene sheet alignment and compactness should be achieved to improve thermal and electrical properties. At a low weight percent of SMGOs, the thermal and electrical conductivities increase with the addition of SMGOs until 30 wt %. The measured peak thermal and electrical conductivities are up to 607 ± 25 W m−1 K−1 and 1.11(±0.05) × 105 S m−1, representing improvements of 35.8 and 31.6%, respectively, as compared to those of the fibers from pure LGGOs (Fig. 2, D and E). Further increase in the fraction of SMGOs beyond a 30 wt % threshold leads to a reduction in the thermal and electrical conductivities despite the continuous reduction in the fiber porosity. The graphene sheet alignment is disrupted and the increase in defective boundaries results in a degradation of the graphene fibers' physical properties.

The inner fiber structures of the graphene fibers are revealed by SEM (Fig. 3 and fig. S4). The highly aligned structure in GO fibers from pure LGGO sheets is well maintained in the graphene fibers, and the annealed graphene sheets stack into a layer-by-layer structure throughout the transverse section (Fig. 3, A and B), extending continuously along the longitudinal direction (Fig. 3C). However, microvoids are generated between graphene sheets during thermal annealing, resulting in a higher porosity (Fig. 2C and Fig. 3B). With the addition of 30 wt % SMGOs, the high degree of orientation has been well maintained inside the GO fibers (fig. S2) and the annealed graphene fibers (Fig. 3, D to F). Meanwhile, a more dense structure as compared with the fibers from pure LGGOs has been created inside GO fibers (fig. S2) and annealed graphene fibers (Fig. 3E) by intercalating small-sized graphene sheets into large-sized graphene sheets (fig. S5). The pure SMGO fibers show the most compact structure; however no alignment of graphene sheets can be observed in both transverse and longitudinal sections (Fig. 3, G to I). The small aspect ratio and random distribution of small-sized graphene sheets inside graphene fibers induce high resistance for phonon and electron transport and low thermal and electrical conductivities (Fig. 2, D and E).

Fig. 3 SEM images showing morphology and inner structure of the graphene fibers (annealed at 1800°C).

(A to C) tilted (A), transverse (B), and longitudinal section view (C) of the graphene fibers from pure LGGOs; (D to F) tilted (D), transverse (E), and longitudinal section view (F) of the optimized graphene fiber with 30 wt % SMGOs; (G to I) transverse section (G and H) and longitudinal section view (I) of the graphene fibers from pure SMGOs.

The thermal and electrical properties of the graphene fibers can be markedly improved by eliminating oxygen functional groups and residual defects (Fig. 4 and fig. S6) and continuously increased graphene sheet alignment and compactness (fig. S7) upon thermal annealing. The as-spun GO fibers are essentially poor electrical and thermal conductors (7, 13, 18). For pure LGGO graphene fibers annealed at 2850°C, thermal and electrical conductivities reach 1025 ± 40 W m−1 K−1 and 1.79(±0.06) × 105 S m−1, respectively. The highest thermal and electrical conductivities achieved for the optimized graphene fibers annealed at 2850°C are 1290 ± 53 W m−1 K−1 and 2.21(±0.06) × 105 S m−1. Lower-temperature annealing at 2000° and 2200°C can be applied to improve the cost effectiveness, and the thermal conductivity (800 to 1030 W m−1 K−1) achieved for the optimized graphene fibers is comparable with that of the best mesophase pitch-based carbon fibers (14, 2224).

Fig. 4 Thermal, electrical, and mechanical properties of graphene fibers and the growth of crystallites upon thermal annealing.

(A) Thermal conductivity of graphene fibers; (B) electrical conductivity of graphene fibers; (C and D) tensile strength and Young’s modulus of GO fibers and graphene fibers; (E) polarized Raman spectra from the optimized graphene fibers in different directions; and (F) crystallite sizes in perpendicular and parallel directions to the fiber axis. The uncertainties are evaluated by the standard deviation of seven to nine measurements.

The annealed graphene fibers also show exceptional mechanical properties as measured by tensile testing (Fig. 4, C and D). The tensile strength of the as-spun GO fibers from pure LGGOs is 231 ± 30 MPa (Fig. 4C), comparable with previous reports (8, 19), and the addition of 30 wt % SMGOs increases the tensile strength of the GO fibers to 308 ± 32 MPa. The tensile strength of the graphene fibers from pure LGGOs annealed at 1400° and 1600°C increases to 756 ± 35 and 940 ± 62 Mpa, respectively, and then decreases to the range of 616 to 823 MPa at greater annealing temperatures (above 1800°C). The optimized graphene fibers show an increase in tensile strength from 614 ± 35 MPa at an annealing temperature of 1400°C to 1080 ± 61 MPa at 1800°C, then decreases to 705 to 820 MPa at annealing temperatures above 2000°C. The Young’s modulus of the optimized graphene fibers monotonically increases with annealing temperatures and approaches 135 ± 8 GPa when annealed at 2850°C.

The graphene sheet alignment and compactness (see fig. S7) and the cross-link between adjacent graphene sheets eventually determine the tensile strength of the graphene fibers. The increased tensile strength of the graphene fibers upon lower-temperature annealing can be attributed to the enhancement in the alignment and densification. Substantial oxygen functional cross-links still remain upon annealing below 1800°C (fig. S6), and new C-C cross-links between neighboring graphene sheets may be created during the release of decomposing components of the chemical groups upon thermal reduction of GOs, further opposing the gliding between graphene faces (15). When the annealing temperature exceeds 2000°C, almost all of the cross-links are removed (see Raman and x-ray photoelectron spectroscopy spectra in fig. S6), leading to a reduction of the tensile strength. At higher annealing temperatures, the interlayer graphene sliding is primarily dominated by the van der Waals force interaction between adjacent graphene sheets (15, 22), and no obvious variation in tensile strength is observed. The trend of monotonically increased Young’s modulus of the graphene fibers is consistent with the previously reported polyacrylonitrile (PAN) and mesophase pitch-based carbon fibers (12, 15) and can be primarily attributed to the improvement of the graphene sheet alignment (figs. S6 and S7) and increased dimension of crystallite domains as evidenced by greater Raman graphite band and defect band IG/ID intensity ratios along both transverse and longitudinal directions (fig. S8) (15, 31, 32). The intercalation of the small- and large-sized graphene sheets leads to greater tensile strength and Young’s modulus for the optimized graphene fibers.

The tensile strength (1080 ± 61 MPa) and Young’s modulus (135 ± 8 GPa) of the optimized graphene fibers are still much lower than these of carbon fibers (5.69 GPa and 0.94 TPa) (15, 16) and carbon nanotube fibers (1~4.3 GPa and 120 GPa) (17, 33). The inferior mechanical properties of the graphene fibers can be attributed to the intrinsic limit of the van der Waals interaction between graphene sheets within the macroscopic fiber structure upon reassembly of two-dimensinal individual GO sheets (15, 18). The physical entanglement and strong covalent cross-links between graphitic planes may strongly improve the mechanical properties of conventional PAN-based carbon fibers and carbon nanotube fibers (1517). Additionally, high-performance carbon fibers and carbon nanotube fibers can achieve more compact and more dense structures (e.g., up to a theoretical density of 2.2 g/cm3 for carbon fibers and thus minimized voids and defects) (15, 16, 33).

High-performance carbon fibers are typically categorized into high-strength PAN-based fibers and high-modulus mesophase pitch-based carbon fibers (15, 16, 22). Thermal conductivity is typically lower for PAN-based carbon fibers because cross-linking atoms behave as scattering centers to reduce phonon transport (15). A strong correlation among the tensile strength, Young’s modulus, and thermal and electrical conductivities was identified for mesophase pitch-based carbon fibers (22). High-temperature carbonization allows the development and growth of the crystalline graphitic domains and thus enables simultaneously high modulus and high conductivities for mesophase pitch-based carbon fibers (15, 22, 24). The superior thermal conductivity but lower modulus of the optimized graphene fibers as compared with mesophase pitch-based carbon fibers is unexpected and could be attributed to the unique fiber structure by intercalating large- and small-sized graphene sheets and substantially larger crystalline domain sizes in both transverse and longitudinal directions.

For graphene-based materials, heat conduction is dominated by phonon transport from lattice vibrations of the covalent sp2 bonding network, and the electron transport is largely determined by the delocalized π-bond over the whole graphene sheet (1, 3, 18, 30). The lattice vacancies and the residual functional groups on graphene sheets upon thermal reduction create substantial numbers of phonon- and electron-scattering centers, significantly degrading the thermal and electrical properties (1, 3, 18, 30). High-temperature annealing heals defects in the lattice structure and removes oxygen functional groups and significantly increases the size of the sp2 domains (fig. S6). The crystallite sizes (Fig. 4F) in parallel and perpendicular directions to the fiber axis have been calculated from the integrated intensity ratios of the D-band (1350 cm−1) and the G-band (1581 cm−1) based on polarized Raman spectra of the optimized graphene fibers annealed at different temperatures (Fig. 4E and fig. S8) (31, 32). At lower annealing temperatures (e.g., 1800°C), graphene fibers demonstrate smaller-sized sp2 domains (40 to 50 nm) with residual defects. The domain sizes of the optimized graphene fibers in both longitudinal and transverse directions increase substantially with the annealing temperature (Fig. 4F) and approach 783 and 423 nm, respectively, upon annealing at 2850°C. This is further evidenced by the submicrometer-sized crystalline domains along the fiber axis for the high temperature–treated fibers as observed in the bright-field transmission electron microscope images (fig. S6). These are orders of magnitude larger than the nanocrystalline graphitic domains (several tens of nanometers) inside the mesophase pitch-based and PAN-based carbon fibers (15, 22, 32). Despite the relatively lower density, the reduced phonon scattering from the boundary and interface due to the larger-sized crystalline domains enables more efficient phonon transport and, thus, enhanced thermal conductivity. The highly thermally conductive and mechanically strong graphene fibers with intercalated large- and small-sized graphene sheets have potential for thermal management materials in high-power electronics and reinforcing components for high-performance composite materials.

Correction (14 December 2015): In the sentence “Electrical conductivity can be recovered on the order of 104 S/cm upon thermal and chemical reductions (12, 13) and can be further increased to 9.3 × 104 S/m through doping with silver nanowires (13)” (page 1084, column 1, lines 15 to 19), the values cited have been changed to correspond with those listed in the cited references. The revised sentence reads: “Electrical conductivity can be recovered up to 285 S/cm for carbon fiber derived from graphene oxide nanoribbons after thermal reduction (12), or using vitamin C chemical reduction of neat graphene fiber to 8.1 × 103 S/m, with HI-reduced neat fiber to 2.8 × 104 S/m (13).”

Supplementary Materials

www.sciencemag.org/content/349/6252/1083/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S8

Table S1

References (3440)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
  2. Acknowledgments: This work is financially supported by the U.S. National Science Foundation under awards DMR 1151028 and CMMI 1463083.
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