T.
Belytschko, S. P. Xiao, G. C. Schatz and R. S. Ruoff
Physical Review B, Vol 65, 2003
The fracture of carbon nanotubes is studied by molecular mechanics simulations. The fracture behavior is found to be almost independent of the separation energy and to depend primarily on the inflection point in the interatomic potential. The fracture strain of a Zigzag nanotube is predicted to be between 10% and 15%, which compares reasonably well with experimental results. The predicted range of fracture stresses is 65-93 GPa and is markedly higher than observed. The computed fracture strengthes of chiral and armchair nanotubes are above these values. Various plausible small-scale defects do not suffice to bring the failure stresses into agreement with available experimental results. As in the experiments, the fracture of carbon nanotubes is predicted to be brittle.
This paper is available in PDF form .
S. L. Mielke,
D. Troya, S. L. Zhang, J. L. Li, S. P. Xiao, R. Car,
R. S. Ruoff, G. C. Schatz and T. Belytschko
Chemical Physics Letter, Vol 390, 413-420, 2004
We present quantum mechanical calculations using density functional theory and semiempirical methods, and molecular mechanics (MM) calculations with a Tersoff-Brenner potential that explore the role of vacancy defects in the fracture of carbon nanotubes under axial tension. These methods show resonable agreement, although the MM scheme systematically underestimates fracture strength. One- and two-atom vacancy defects are observed to reduce failure stresses by as much as ~26% and markedly reduce failure strains. Large holes -- such as might be introduced via oxidative purification processes -- greatly reduce strength, and this provides an explanation for the extant theoretical-experimental discrepancies.
This paper is available in PDF form .
S.
P. Xiao and W. Y. Hou
Fullerenes, Nanotubes, and Carbon Nanostructures, 14: 9-16, 2006
We use molecular
mechanics calculations to study size effects on mechanical properties of carbon
nanotubes. Both single-walled nanotubes (SWNTs) and multi-walled nanotubes
(MWNTs) are considered. The size-dependent Young's modulus decreases with the
increasing tube diameter for a reactive empirical bond order (REBO) potential
function. However, we observe a contrary trend if we use other potential
functions such as the modified Morse potential and the universal force field
(UFF). This confliction is only obtained for small tubes within cutoff
diameters (3$nm$ for REBO and 1.5$nm$ for others). In light of these
predictions, Young's moduli of large nanotubes concur
with experimental results for all the potential functions. No matter which
potential function is used, the Poisson's ratio decreases with the increasing
tube diameter. We also study the chirality effects on mechanical properties of
SWNTs. We find that the Young's moduli are
insensitive to the chirality of nanotubes. The chirality effect on Poisson's
ratio is significant for the UFF but not the REBO or modified Morse potential
functions.
This paper is available in PDF form .
.
S.
P. Xiao, D. R. Andersen, R. Han and W. Y. Hou
Journal of Computational and Theoretical Nanoscience,
Vol 3, 143-147, 2006
In this study, we
investigate some oscillation mechanisms of double-walled carbon nanotube-based
oscillators using molecular dynamics. If an oscillator is an isolated system,
stable oscillations of the inner tube inside of the outer tube can be observed
with oscillatory frequencies as high as 72GHz. If the same inner tube is used
in designing nano-oscillators, molecular dynamics simulations illustrate that
the length of the outer tube is the only consideration necessary for achieving
various oscillatory frequencies. We also study the interlayer friction between
the outer tube and the inner tube when nano-oscillators are at finite
temperatures. A larger interlayer friction is observed in an oscillator when it
is at a higher temperature, and the oscillation will stop quickly. The interlayer
friction is also related to the chirality and defects in the outer tube. Based
on the above studies, we propose to design a nanoelectromechanical oscillator
system, which can provide stable oscillation.
This paper is available in PDF form .
S.
P. Xiao, R. Han and W. Y. Hou
International Journal of Nanoscience, 5(1): 47-55,
2006
In
this study, we investigate some oscillation mechanisms of double-walled carbon
nanotube-based oscillators using molecular dynamics. If an oscillator is an
isolated system, stable oscillations of the inner tube inside of the outer tube
can be observed with oscillatory frequencies as high as 72GHz. If the same
inner tube is used in designing nano-oscillators, molecular dynamics
simulations illustrate that the length of the outer tube is the only
consideration necessary for achieving various oscillatory frequencies. We also
study the interlayer friction between the outer tube and the inner tube when
nano-oscillators are at finite temperatures. A larger interlayer friction is
observed in an oscillator when it is at a higher temperature, and the
oscillation will stop quickly. The interlayer friction is also related to the
chirality and defects in the outer tube. Based on the above studies, we propose
to design a nanoelectromechanical oscillator system, which can provide stable
oscillation.
This paper is available in PDF form .
S.
P. Xiao and W. Y. Hou
Physical Review B, Vol 73, 115406, 2006
In
this paper, we investigate effects of vacancy defects on fracture of carbon
nanotubes and carbon nanotube/aluminum composites. Our studies show that even a
one-atom vacancy defect can dramatically reduce the failure stresses and
strains of carbon nanotubes. Consequently, nanocomposites, in which
vacancy-defected nanotubes are embedded, exhibits different characteristics
from those in that pristine nanotubes are embedded. It
has been found that defected nanotubes with a small volume fraction cannot
reinforce but instead weaken nanocomposite materials. Although a large volume
fraction of defected nanotubes can slightly increase the failure stresses of
nanocomposites, the failure strains of nanocomposites are always decreased.
This paper is available in PDF form .
S. P. Xiao and W. Y. Hou
Physical Review B, Vol 75, 125414,
2007
We propose a new multiscale method to study nanotube-based resonant oscillators. In the multiscale model, nanotubes are modeled via molecular dynamics, while the metal paddle is modeled as a rigid body. The molecular and continuum models are attached with each other through the interfaces on which carbon atoms are located. We employ the concepts of “virtual” atoms and bonds to effectively couple the molecular and continuum models. Using the proposed multiscale method, we investigate both linear and nonlinear characteristics of resonant oscillators. Effects of vacancy and temperature on mechanisms of oscillators are discussed.
This paper is available in PDF form .
S. P. Xiao and W. Y. Hou
Journal of Nanoscience and Nanotechnology, Vol 7 (11), 1-8, 2007
In this paper, (10,0) zigzag nanotubes and (6,6) armchair nanotubes are considered to investigate the effects of randomly distributed vacancy defects on mechanical behaviors of single-walled carbon nanotubes. A spatial Poisson point process is employed to randomly locate vacancy defects on nanotubes. Atomistic simulations indicate that the presence of vacancy defects result in reducing nanotube strength but improving nanotube bending stiffness. In addition, the studies of nanotube torsion indicate that vacancy defects prevent nanotubes from being utilized as torsion springs.
This paper is available in PDF form .
S. P. Xiao, S. W. Wang, J. Ni, R.
Briggs, and M. Rysz
Journal of Computational and Theoretical Nanoscience,
Vol 5, 528-534, 2008
The first molecular dynamics simulation-based reliability analysis of carbon nanotubes is conducted in this paper. Instead of uncertainties of loads, we consider uncertainties of defects, especially vacancies, at the nanoscale in our modeling and simulation. A spatial Poisson point process is employed to assist randomly locating vacancies on the surface of nanotubes. With the aid of Grid computing technologies, a large number of molecular dynamic simulations are conducted to obtain statistical properties of nanotube strength and stress for reliability analysis. Reliabilities of nanotubes at various temperatures are also discussed.
This paper is available in PDF form .
S. P. Xiao, and W.
Y. Hou
International Journal for
Multiscale Computational Engineering, 5(6): 447-459, 2007
In this paper, after performing verification of the bridging domain coupling method in a three-dimensional application, we employ this coupling method to study Young’s moduli and failure strengths of nanotube-based aluminum composites. In the multiscale model of nanocomposites, the nanotubes and their surrounding areas, i.e., the interaction zones, are modeled via molecular dynamics while the other regions are modeled via the finite element method. Three types of nanotubes are considered as inclusions: single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and SWNT bundles. Although all types of nanotube inclusions can reinforce nanocomposites, MWNTs play the most significant role compared to the other two inclusions. In addition, SWNT bundles are better inclusions than SWNTs for reinforcing nanocomposites.
This paper is available in PDF form .