Ultrahigh strength, modulus, and conductivity of graphitic fibers by macromolecular coalescence

Abstract

Theoretical considerations suggest that the strength of carbon nanotube (CNT) fibers be exceptional; however, their mechanical performance values are much lower than the theoretical values. To achieve macroscopic fibers with ultrahigh performance, we developed a method to form multidimensional nanostructures by coalescence of individual nanotubes. The highly aligned wet-spun fibers of single- or double-walled nanotube bundles were graphitized to induce nanotube collapse and multi-inner walled structures. These advanced nanostructures formed a network of interconnected, close-packed graphitic domains. Their near-perfect alignment and high longitudinal crystallinity that increased the shear strength between CNTs while retaining notable flexibility. The resulting fibers have an exceptional combination of high tensile strength (6.57 GPa), modulus (629 GPa), thermal conductivity (482 W/m·K), and electrical conductivity (2.2 MS/m), thereby overcoming the limits associated with conventional synthetic fibers.

Fig. 1.. Development of CNT fibers through the coalescence of nanotubes.

(A) The schematic illustration of CNT coalescence via annealing temperature. (B) Cross-sectional transmission electron microscopy (TEM) image of as-prepared CNT fibers with S·DWNT bundles. Cross-sectional TEM images of CNT fibers with MIW-CNT and C-CNT hybrid structures after annealing at (C) 1400, (D) 1700, (E) 2000, and (F) 2400°C. (G) TEM image of CNT fibers with a highly aligned GNR structure after annealing at 2700°C. High-resolution TEM images of (H) collapsed DWNT, (I) MIW-CNT, and (J) DWNT bundles wrapped with a collapsed SWNT.

Fig. 2.. Structural evolution of CNT fibers after annealing.

(A) 2D WAXS patterns of representative S·DWNT fibers. (B) Equatorial and (C) meridional radial profiles showing the transition from bundles to highly crystalline graphitic domains upon high-temperature annealing. (D) Plot of crystal sizes transversal (L_c) and parallel (L_a) to the fiber axis for CNT fibers as well as conventional graphitic CFs from different precursors (48). (E) Orientation parameters ⟨cos²(ϕ)⟩ calculated from the interlayer reflections (Eq. 1).

Fig. 3.. Properties of high-performance CNT fibers.

(A) Stress-strain curves of S·DWNT fibers after annealing at various temperatures. (B) Comparison of mechanical properties with other CNT fibers and CFs (7, 49). (C) Relationship between longitudinal compliance and the orientation parameter 〈cos²(ϕ)〉, and comparison with other CNT fibers and CFs, showing an apparent internal crystal shear modulus that is substantially lower that than for CFs. (D) Comparison of electrical conductivity as a function of tensile strength with other CNT fibers and CFs (, , –51). (E) Comparison of thermal conductivity as a function of tensile modulus with other CFs, CNT, and graphene fibers (7, 10, 52). (F) A radar chart of properties for CNT fiber and CFs. Each property is normalized to the highest value within this set of fibers.