Atomistic Modeling of Nanowires
This project focuses on studying the deformation mechanisms and assessing strength
of single crystal gold nanowires using molecular dynamic simulations.
In this project, metalic nanowires with circular cross section are modeled in order
to investigate the effect of wire diameter and its crystallographic orientation on
its strength under tensile/compressive loading. Single crystal wires with surface
and volume defects are also studied in order to understand various deformation
mechanisms and issues related to size scale plasticity.
LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator), a MD code
developed by Sandia National Labs is being used for these atomistic simulations.
In the code, equations of motion are solved using velocity-verlet algorithm to
keep track of atomic positions. Thermal stabilization is achieved by means of the
Nose Hoover thermostat. The EAM (Embedded Atoms Method) potential developed by
Baskes and co-workers is used for modeling the interaction between atoms. These
potentials consist of two terms: one constituting the pairwise interaction and
second accounts for embedding energy, which is the energy required for embedding
an atom in the electron cloud generated by surrounding atoms.
Diameters of the wires studied vary between 5.0nm and 20.0nm with an aspect ratio of 3.
Two crystallographic orientations of the wire axis: <100> and <111> are studied.
It was been found that all the wires undergo compression, when subjected to an
energy minimization routine as a result of surface effects. The initial equilibrium
strain is a function of wire diameter, being highest for the smallest wire diameter
with maximum surface-to-volume ratio. Non-linear elastic response in different
crystallographic orientations is also being studied.
Our current efforts also include:
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Modeling "realistic" nanowires and studying the effects of surface defects
like vacancy clusters and twin boundaries, and
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investigating the propagation of existing dislocations and their interactions
with the surface when subjected to an external stress.
We are currently studying the effect of surface defects in the form of vacancy
clusters or waviness due to twin boundaries (bamboo type structures) present on the
surface of wires. This effort envisions the modeling of surface defects which are
found in real nanowires fabricated using template-synthesis methods. The effect of
these defects on the strength of nanowires is investigated.
In combination with Discrete Dislocation Dynamic (DDD) modeling of nanopillars with
pre-defined dislocation density, our efforts also include building similar models
with atomistic details. The idea is to capture the effects of dislocation motion
and escape when constrained to small sizes. Wires with pre-defined volume defects
are modeled and then simulated under compression.
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H.D. Espinosa (PI)
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R. Agrawal (Graduate Student)
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Brian Hyde, H.D. Espinosa and Diana Farkas.
"An Atomistic Investigation of Elastic and Plastic properties
of Au Nanowires"
Journal of Minerals, Metals and Materials, Vol. 57, No. 9, p. 62-66, 2005.
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H. Tang, K. Schwarz and H.D. Espinosa.
"Dislocation Escape-Related Size Effects in
Single-Crystal Micropillars under Uniaxial Compression,"
Acta Materialia, Vol. 55, No. 5, p. 1607-1616, 2007.
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H.D. Espinosa, M. Panico, S. Berbenni and K.W. Schwarz.
"Discrete dislocation dynamics simulations to interpret plasticity
size and surface effects in freestanding FCC thin films."
International Journal of Plasticity, Vol. 22, No. 11, p. 2091-2117, 2006.
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