Charles F. Cornwell

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.

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.

Visualizations of molecular dynamics simulations of high-performance polycrystalline structural ceramics

The implications of improving the strength of synthetic ceramics are described.HPC simulations are having a significant impact on engineered materials.Visualizing the data from LAMMPS required the development of a custom program.VTK was used to connect the atoms. 3D Studio Max was used to create the images.VisIt accomplishes the feat of drawing more than 70 million atoms at once. Initiated by the Department of Defense (DOD) High Performance Computing Modernization Program (HPCMP), the Data Analysis and Assessment Center (DAAC), serves the needs of DOD HPCMP scientists by facilitating the analysis of an ever-increasing volume and complexity of data 1. A research scientist and HPCMP user ran nanoscale molecular dynamics simulations using Large-scale Atomic/Molecular Massively Parallel Simulator code (LAMMPS) from Sandia National Labs. The largest simulation contained over 70 million atoms (Fig. 7). Data sets this large are required to study crack propagation and failure mechanisms that span multiple length scales with atomic resolution. The DAAC developed new methods to visualize the time evolution of data sets this large. The size and complexity of the molecular dynamics simulations and the analytics required the use of DOD HPCMP High Performance Computing (HPC) resources.

Numerical investigation of FAST powder consolidation of Al2O3 and additive free β-SiC

In this work we examine ceramic synthesis through powder consolidation and the field assisted sintering technique. In particular, we investigate the sintering of Al2O3 and additive free from both an experimental and numerical perspective. For the numerical model, the continuum theory of sintering model is employed, and the densification mechanisms corresponding to power law creep and grain boundary diffusion are considered. Experiments are used for comparison and validation purposes. The results indicate that in general, the densification kinetics simulated by the numerical model compare favorably with the experimental results. Parametric studies involving initial grain size, heating rate, and applied stress are also examined using the numerical model, and confirm many of the expected results from previous research, including increased densification due to higher heating rates, smaller grain sizes, and increased applied loading conditions.

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.