Mechanical properties of helically twisted carbyne fibers

Hightlights

• Thick beam is considered in this work and Poisson's ratio is analytical calculated.

• The tensile behavior of CTFs under different twist laps are predicted using continuum method.

• Based on molecular dynamic simulation results, mechanical properties of P-CTFs and C-CTFs are enhanced as pitch angle increases.

• Three chains carbyne fibers always possess higher tensile strength and Young's modulus than other carbyne fibers under high pitch angle.

• The phase transition of carbyne phenomenon make CTFs strongest nanostructure.

Abstract

Carbyne composed of sp-hybridized carbon atoms is perfectly one-dimensional material, showing superior mechanical properties as a promising building material for nanodevices. Such nanomaterials as carbon nanotube ropes with hierarchical helical structures hold a promise for potential applications. Here, a bottom-up theoretical model is established to investigate the mechanical properties of this kind of novel nanomaterials. The effect of helical structures is revealed by comparing the mechanical properties of carbyne ropes. The dependence of the mechanical properties of materials on the initial helical angles and fiber numbers at different structural levels are examined. Carbyne ropes are found with higher deformation ability and elastic property which can be easily tuned via their microstructural parameters. This work provides inspirations for optimal design of advanced nanomaterials with helical structures.

Introduction

Carbon is one of the unique elements in the periodic table that has attracted continuous interest because of its chemical versatility.[1] In natural settings, typically elemental carbon exists in two crystalline allotropes, namely diamond and graphite, which are structurally featured by unique networks of sp³- and sp²-hybridized carbon atoms, respectively. Both forms show unique physical properties. For example, diamond is one of the hardest natural occurring materials and also a good electrical insulator. In contrast, graphite is a soft material but shows high conductivity.[2] In laboratory settings, a variety of sp²-hybridized carbon allotropes, including zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs) and two-dimensional (2D) graphene, have been discovered/synthesized during the last three decades. Those carbon nanostructures also show unique properties differing from those of either diamond or graphite, and they offer great promise of nanotechnological breakthroughs.

Beyond sp³- and sp²-hybridized carbon materials, a stable form of sp-hybridized carbon allotrope, namely carbyne, has been gaining attention as a promising carbon structure due to its interesting properties. In natural settings, it was demonstrated that it exists in meteorites, interstellar dust and terrestrial plant, fungal, and marine sources.[3,4] In laboratory, a number of routines, including gas-phase deposition, epitaxial growth, electrochemical synthesis, dehydrohalogenation of polymers, and stretching the atomic chains from sp²-hybridized carbon nanostructures, have been developed to achieve finite-length carbon chains in the past decades.[5], [6], [7], [8], [9] Extensive investigations have also been performed to characterize the properties of carbyne both theoretically and experimentally.

Using STM-break junction method, the conduction of single carbyne and three different conduction group in carbyne series show a remarkably low β values of (0.06 ± 0.03) Å⁻¹ which means the conductance is almost independent of molecular length.[10] Besides, using in situ Raman spectroscopy and electrical conductivity measurements, Ravagnan et al. studied vibrational and electronic properties of cumulene. Cumulenes are stable up to a temperature of roughly 250 K and they influence the electrical transport properties of the films acting as metallic doping species.[11] Using the quantitative Raman spectra and the Raman scattering intensities, the non-resonant molecular second hyperpolarizabilities increases as a function of length and the nature of the endgroups is negligible for longer chains which can be concluded that the origin of the nonlinear optical properties is derived primarily from the sp-hybridized carbyne.[12,13]

Using molecular simulation, the thermal conductivity of cumulene (83W/m•K at 480 K) and polyyne chain (42 W/m K at 500 K) were predicted[14], comparable to that of the widely used thermal and electric conductor copper nanowire (75 W/m K).[15] Also, for the phase transition from cumulene to polyyne, thermal conductivity is reduced from 83 to 42 W/m K, which gives rise to a tunability of 200%.[16] Using density functional theory (DFT) and non-equilibrium Green's function, the calculated peak quantum conductance of carbyne was found to be around five-folds smaller than that of CNTs. Based on the unique electrical and optical nature of carbyne, carbyne is a promising material in a variety of applications, such as nanoelectronic or spintronic devices, nonlinear optical materials, molecular wires, and so on.

The applications of carbyne-constructed structures have been also widely studied. For example, Ca-decorated carbyne networks can be utilized as a hydrogen storage media because a Ca-decorated carbyne could absorb up to 6 H₂ molecules per Ca atom with a binding energy of about 0.2 eV and the hydrogen storage capacity exceeds 8 wt%.[17] As a cathode material in lithium/sulfur batteries, carbyne polysulfide holds a high reversible capacity of 960 mAh g_sulfur⁻¹ after 200 cycles at 0.1 C (168 mA g_sulfur⁻¹) current rate in carbonic ester electrolyte.[18] Moreover, Li-intercalated carbyne bundles (C₆Li₁₃) acting as a lithium-ion-battery anode possesses specific and volumetric capacities of 4840 mAh g⁻¹ and 2038 mAh cm⁻³, respectively, far exceeding those of graphite-based lithium-ion-battery (370 mAh g⁻¹ and 820 mAh cm⁻³).[19] Besides, carbyne chains possess particular electromagnetic properties. It was found that the spin-filtering efficiencies are highly independent of the number of carbyne chains in the parallel magnetic configuration, but have a dramatic odd-even oscillating behavior in the antiparallel magnetic configuration.[20] Furthermore, when carbyne chain is connected by two ZGNR electrodes, current could be efficiently tuned via twisting right ZGNR electrode.[21]

So far, only few studies have reported on the experimental mechanical properties of carbyne. The reason can be mainly due to the difficulty to obtain it. Only recently, the first experimental data on in situ determination of the tensile strength of carbyne (exceed 270 GPa at 5 K) by high-field method have been published.[22,23] This value is more than twice higher than the experimental strength of graphene which is equal to 130 GPa.[24] On the other hand, using molecular dynamics and density functional theory, huge amount of work have been done on mechanical properties of carbyne. The strength of carbyne is up to 7.5 × 10⁷ N m/kg and bending stiffness is 3.56 eV Å.[25] Similarly, Liu et al. also get the strength of polyyne and cumulene up to 67.1 GPa and ~590 GP with the diameter of cross-sectional area 0.335 nm, respectively.[14] Besides, the stretchability of polyyne and cumulene is 7.3% and 10.8%, respectively. Furthermore, mechanical properties of carbyne are also influenced by the length and even-odd.[26]

It has to be pointed out a fact that the nanoscale size of carbyne chains always limits their practical applications. In order to fully utilize their excellent performances, it's necessary to explore an available route to assemble carbyne chains into macroscale structures. It has been proved that twisting operation is an efficient way to assemble nanoscale fibers to macro-scale continuous fibers.[27], [28], [29] In this article, we employed this mechanical twisting operation to obtain a helical carbyne fibers, and the tensile characteristics of helical carbyne fibers assembled by twisting monoatomic carbon chains are investigated by molecular dynamics (MD) and continuum mechanics (CM) approaches. In MD, novel concepts of helical structures to build atomic carbon chain-twisted fibers (CTFs) are used. In CM, a new helical model will be developed and explicit expressions of elastic constants for covalent bonds are analytically derived. To investigate mechanical properties of carbyne-twisted fibers (CTFs), CTFs are hypothetically engineered by twisting different numbers of carbon chains. Finally, the numerical results predicted by MD and CM are presented and compared. Despite the considerable interest in the carbyne, information about its mechanical properties is scattered in specific works that does not allow obtaining complete representation about these properties. This work is addressed to a brief description of the experimental data obtained by measuring of strength of carbyne, as well as to the results of the molecular dynamics and ab initio modeling of tension of monatomic carbon chains of finite length.