Million-Atom Count Simulations of the Effects of Carbon Nanotube Length Distributions on Fiber Mechanical Properties

The extraordinary mechanical properties of carbon nanotubes (CNTs) make them prime candidates as a basis for super infrastructure materials. Ab initio, tight binding, and molecular dynamics simulations and recent experiments have shown that CNTs have tensile strengths up to about 15.5 million psi (110 GPa), Young's modulus of 150 million psi (1 TPa), and density of about 80 lbs/ft3 (1.3 g/cm3). These qualities provide tensile strength-to-weight and stiffness-to-weight ratios about 900 times and 30 times, respectively, those of high-strength (100,000- psi) steel. Building macromaterials that maintain these properties is challenging. Molecular defects, voids, foreign inclusions, and, in particular, weak intermolecular bonds have, to date, prevented macromaterials formed from CNTs from having the remarkable strength and stiffness characteristics of CNTs. The van der Waals forces associated with CNTsrepresent a force per unit length between CNTs. Accordingly, one would expect the bond strength between aligned CNTs to increase with overlap length. Real filaments are likely composed of CNTs with some distribution of lengths. To understand the effects that CNT length distributions have on the tensile strength of neat filaments of aligned CNTs, we performed a series of quenched molecular dynamics simulations on high performance computers using Sandia Laboratory's Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code. The cross-section of each filament was composed of hexagonal closest-packed (HCP) array CNT strands that formed two HCP rings. The filaments were constructed by placing (5,5) chirality CNTs end to end. While the choice of a single-chirality CNT fiber is currently unrealizable, the use of a single chirality fiber allowed us to focus only on the effects of CNT lengths on filament response. The lengths of the CNTs were randomly selected to have Gaussian distribution with the average length ranging from 100 to 1,600 Å. A series of simulations were performed on filament with lengths ranging from 400 to 6,400 Å. For each filament, the strain was increased in small increments and quenched between strain increments. The total tensile force on the filament was recorded and used to determine the uniaxial stress-strain response of the filaments. The results of the simulations quantified the improvements in Young's modulus, tensile strength, and critical strain as a function of the increase in the average component CNT lengths. These are the first molecular dynamics simulations that the authors are aware of that treat statistical qualities of realistic CNT structures. The simulation results are being used to guide the molecular design of CNT filaments to achieve super (1 million psi) strength. The simulations would be impractical, and perhaps impossible, without massively parallel, highperformance computational platforms and molecular dynamics simulation tools optimized to run on such platforms.