Charles R. Welch

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Youngs modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Youngs modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Youngs modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Youngs modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Youngs modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.

Very-high-strength (60-GPa) carbon nanotube fiber design based on molecular dynamics simulations

The mechanical properties of carbon nanotubes such as low density, high stiffness, and exceptional strength make them ideal candidates for reinforcement material in a wide range of high-performance composites. Molecular dynamics simulations are used to predict the tensile response of fibers composed of aligned carbon nanotubes with intermolecular bonds of interstitial carbon atoms. The effects of bond density and carbon nanotube length distribution on fiber strength and stiffness are investigated. The interstitial carbon bonds significantly increase load transfer between the carbon nanotubes over that obtained with van der Waals forces. The simulation results indicate that fibers with tensile strengths to 60 GPa could be produced by employing interstitial cross-link atoms. The elastic modulus of the fibers is also increased by the bonds.

Processing, Microstructure, and Properties of Carbon Nanotube Reinforced Silicon Carbide

The addition of multi-walled carbon nanotube reinforcements to a ceramic matrix has been suggested to improve the fracture toughness. The hypothesized improvement is thought to be the result of crack bridging and other toughening mechanisms. However, no such improvement in toughness has not been achieved to date for a multi-walled carbon nanotube and silicon carbide composite. However, there are several processing techniques, compositions, and methods for producing said composite, which may inhibit or foster success. Here, we report the processing, microstructure, and properties of a multi-walled carbon nanotube and silicon carbide composite material. The processing required careful mixing of the carbon nanotubes within the matrix in order to maximize dispersion and minimize carbon nanotube damage. The sintering required careful control of specific parameters to produce the desired microstructure and maximum density. The spark plasma sintering technique used was. These processing methods resulted in unique microstructures which in turn affected the material properties. The effect on the mechanical strength was evaluated using three-point flexural testing.

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-toweight 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 singlechirality 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 1,600Å. A series of simulations were performed on filament with lengths ranging from 400 to 6,400Å. 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.

Brittle ductile transition in carbon nanotube bundles

The superior strength and stiffness of carbon nanotubes (CNTs) make them attractive for many structural applications. Although the strength and stiffness of CNTs are extremely high, fibres of aligned CNTs have been found to date to be far weaker than the constituent CNTs. The intermolecular interactions between the CNTs in the fibres are governed by weak van der Waals forces, resulting in slippage between CNTs which occurs at tensions well below the breaking strength of the CNTs. Both theoretical and experimental studies show that by introducing chemical bonds between the CNTs increases load transfer and prevents the CNTs from slipping.

Design of Very High-Strength Aligned and Interconnected Carbon Nanotube Fibers Based on Molecular Dynamics Simulations

The principal objective of this work is to implement a new material development paradigm using atomistic simulations to guide the molecular design of materials. Traditional empirical macroscopic material development studies omit the fundamental insight needed to understand material behavior at the atomic and molecular levels where material response begins. The new paradigm relies heavily on a tight integration between simulation and experimental efforts to design and process new materials with nanometer-scale precision. Exploiting nanotechnology requires atomic-molecular-level material design and the ability to process these materials with atomic-molecular-level precision. Processing materials with nanoscale precision poses formidable theoretical, computational, and experimental challenges to developing advanced materials. High performance computers and advanced physics-based simulations can complement experimental efforts to design, test, synthesize, and analyze novel materials and innovative structural designs. This method can be applied to a wide range of material designs. As a proof of concept, we began our work on the design of novel carbon nanotube-based materials. The mechanical properties of carbon nanotubes such as low-density, high-stiffness, and exceptional strength make them ideal candidates for reinforcement material in a wide range of high performance composites. Molecular dynamics simulations are used to predict the tensile response of fibers composed of aligned carbon nanotubes with intermolecular bonds of interstitial carbon atoms. The effects of bond density and carbon nanotube length distribution on fiber strength and stiffness are investigated. Results indicate that including cross link atoms between the carbon nanotubes in the strands significantly increases the load transfer between the carbon nanotubes and prevents them from slipping. This increases the elastic modulus and yield strength of the fibers by an order-of-magnitude. Carbon nanotube-based materials appear poised to affect civil and military engineering significantly over the next two decades by providing materials with an order-of- magnitude improvement in strength-to-weight and stiffness-to-weight ratios over existing materials.

Atomistic Simulations of Tribological Properties of Ultra-Thin Carbon Nanotube Films on Silicon

Molecular dynamics simulations are used to study relationships between material morphology, adhesion, and sliding friction in carbon nanotube (CNT) coatings at the nanoscale. Two controlled quantities, CNT chirality and vacancy defects, are found to have significant effects on CNT coating adhesion to Si surfaces and sliding friction in turn. For example, using free energy calculations, a CNT of chirality (10,0) with a corresponding diameter of 7.777Å was observed to have an adhesion energy-per-unit-length of approximately three-times that of a CNT with (5,5) chirality and corresponding diameter of 6.732Å. Simulations of aligned carbon nanotube arrays containing various vacancy defect densities in sliding contact with Si substrates were also performed. Friction and wear were shown to increase with defect density. Similar studies are underway to investigate how other characteristics of CNTs in addition to chirality, such as CNT length distribution and defect concentration, affect adhesion and friction in CNT-Si coatings. Outcomes may shed light on fundamental principles governing, for example, sliding interfaces in micro- and nano-electro-mechanical and other tribological systems.