Ab initio predictions of graphite-like phase with anomalous grain boundaries and flexoelectricity from collapsed carbon nanotubes

Abstract

Although large-radius carbon nanotubes (CNTs) are now available in macroscopic quantities, little is known about their condensed phase. Large-scale density functional theory calculations predict a low energy phase in which the same-diameter “dog-bone” collapsed CNTs form a graphite-like phase with complex, anomalous grain boundaries (GBs). The excess GB volume does not prevent the strong van der Waals coupling of the flattened CNT sides into AB stacking. The associated GB energetics is dominated by the van der Waals energy penalty and high curvature bending of the loop CNT edges, which exhibit reactivity and flexoelectricity. The large density and superior mechanical rigidity of the proposed microstructural organization as well as the GB flexoelectricity are desirable properties for developing ultra-strong composites based on large-radius CNTs.

Computationally Guided Design of Large-Diameter Carbon Nanotube Bundles for High-Strength Materials

“…While the hexagonal CNT assembly, Figure 1a, is a direct modification of the organization observed in the bundles of HiPco-produced CNTs, the assembly of the "dog-bone"collapsed CNTs, Figure 1b, relies on the ability of CNTs 20 to organize into a graphitelike phase characterized by large mass density (ρ) and tensile Young's modulus (Y). With an increase in the CNT diameter, ρ and Y of the "dog-bone" collapsed phase 18,19,21 converge rapidly to those of the ideal graphite. 18 Moreover, the AB stacking at flattened faces, most commonly found in atomistic models where CNTs have the same chiral angle, 22 and the flexoelectricity present at the collapsed edges 19 T h i s c o n t e n t i s can be exploited to enable load transfer by shear within a bundle, 23 thus increasing the material strength.…”

“…With an increase in the CNT diameter, ρ and Y of the "dog-bone" collapsed phase 18,19,21 converge rapidly to those of the ideal graphite. 18 Moreover, the AB stacking at flattened faces, most commonly found in atomistic models where CNTs have the same chiral angle, 22 and the flexoelectricity present at the collapsed edges 19 T h i s c o n t e n t i s can be exploited to enable load transfer by shear within a bundle, 23 thus increasing the material strength. Notably, we have identified 18,19 an energetic crossover between these two crystalline phases.…”

“…40 This interatomic potential describes well the morphologies of interacting collapsed CNTs (see the Supporting Information); it gives a 1020 GPa value for the in-plane Young's modulus of AB graphite, in good agreement with the 1083 GPa predicted by density functional theory. 19 3.2. MD of CNT Assembly: The Yarn's Bundle Structure.…”

“…14 They can naturally adopt polygonal crosssections, a change in shape driven by the intertube van der Waals (vdW) forces 12 or even more dramatic shape changes, 2,15,16 including circumferential collapse into a "dog-bone" shape, 13,14 which are also known to impact electrical properties. 17 Based on atomistic calculations and theoretical analysis, we have recently compared 18,19 the energetic stability of the two primary crystalline organizations of monodispersed largediameter double-walled CNTs presented in Figure 1. While the hexagonal CNT assembly, Figure 1a, is a direct modification of the organization observed in the bundles of HiPco-produced CNTs, the assembly of the "dog-bone"collapsed CNTs, Figure 1b, relies on the ability of CNTs 20 to organize into a graphitelike phase characterized by large mass density (ρ) and tensile Young's modulus (Y).…”

“…18 Moreover, the AB stacking at flattened faces, most commonly found in atomistic models where CNTs have the same chiral angle, 22 and the flexoelectricity present at the collapsed edges 19 T h i s c o n t e n t i s can be exploited to enable load transfer by shear within a bundle, 23 thus increasing the material strength. Notably, we have identified 18,19 an energetic crossover between these two crystalline phases. For double-walled CNTs with an outer diameter (2R) above 4.8 nm, the collapsed phase becomes increasingly favorable.…”