627 research outputs found
Surface Effects on the Piezoelectricity of ZnO Nanowires
We utilize classical molecular dynamics to study surface effects on the
piezoelectric properties of ZnO nanowires as calculated under uniaxial loading.
An important point to our work is that we have utilized two types of surface
treatments, those of charge compensation and surface passivation, to eliminate
the polarization divergence that otherwise occurs due to the polar (0001)
surfaces of ZnO. In doing so, we find that if appropriate surface treatments
are utilized, the elastic modulus and the piezoelectric properties for ZnO
nanowires having a variety of axial and surface orientations are all reduced as
compared to the bulk value as a result of polarization-reduction in the polar
[0001] direction. The reduction in effective piezoelectric constant is found to
be independent of the expansion or contraction of the polar (0001) surface in
response to surface stresses. Instead, the surface polarization and thus
effective piezoelectric constant is substantially reduced due to a reduction in
the bond length of the Zn-O dimer closest to the polar (0001) surface.
Furthermore, depending on the nanowire axial orientation, we find in the
absence of surface treatment that the piezoelectric properties of ZnO are
either effectively lost due to unphysical transformations from the wurtzite to
non-piezoelectric d-BCT phases, or also become smaller with decreasing nanowire
size. The overall implication of this study is that if enhancement of the
piezoelectric properties of ZnO is desired, then continued miniaturization of
square or nearly square cross section ZnO wires to the nanometer scale is not
likely to achieve this result
Computational Modeling of Electro-Elasto-Capillary Phenomena in Dielectric Elastomers
We present a new finite deformation, dynamic finite element model that
incorporates surface tension to capture elastocapillary effects on the
electromechanical deformation of dielectric elastomers. We demonstrate the
significant effect that surface tension can have on the deformation of
dielectric elastomers through three numerical examples: (1) surface tension
effects on the deformation of single finite elements with homogeneous and
inhomogeneous boundary conditions; (2) surface tension effects on instabilities
in constrained dielectric elastomer films, and (3) surface tension effects on
bursting drops in solid dielectrics. Generally, we find that surface tension
creates a barrier to instability nucleation. Specifically, we find in agreement
with recent experimental studies of constrained dielectric elastomer films a
transition in the surface instability mechanism depending on the
elastocapillary length. The present results indicate that the proposed
methodology may be beneficial in studying the electromechanical deformation and
instabilities for dielectric elastomers in the presence of surface tension
A Staggered Explicit-Implicit Finite Element Formulation for Electroactive Polymers
Electroactive polymers such as dielectric elastomers (DEs) have attracted
significant attention in recent years. Computational techniques to solve the
coupled electromechanical system of equations for this class of materials have
universally centered around fully coupled monolithic formulations, which while
generating good accuracy requires significant computational expense. However,
this has significantly hindered the ability to solve large scale, fully
three-dimensional problems involving complex deformations and electromechanical
instabilities of DEs. In this work, we provide theoretical basis for the
effectiveness and accuracy of staggered explicit-implicit finite element
formulations for this class of electromechanically coupled materials, and
elicit the simplicity of the resulting staggered formulation. We demonstrate
the stability and accuracy of the staggered approach by solving complex
electromechanically coupled problems involving electroactive polymers, where we
focus on problems involving electromechanical instabilities such as creasing,
wrinkling, and bursting drops. In all examples, essentially identical results
to the fully monolithic solution are obtained, showing the accuracy of the
staggered approach at a significantly reduced computational cost
Mechanical Properties of MoS2/Graphene Heterostructures
We perform classic molecular dynamics simulations to comparatively
investigate the mechanical properties of single-layer MoS2 and a
graphene/MoS2/graphene heterostructure under uniaxial tension. We show that the
lattice mismatch between MoS2 and graphene will lead to an spontaneous strain
energy in the interface. The Young's modulus of MoS2 can be enhanced by a
factor of five by sandwiching it between two graphene layers. While the
stiffness is enhanced, the yield strain of the MoS2 is reduced due to lateral
buckling of the outer graphene layers due to the applied mechanical tension.Comment: Appl. Phys. Lett., publishe
Mechanical Properties of Single-Layer Black Phosphorus
The mechanical properties of single-layer black phosphrous under uniaxial
deformation are investigated using first-principles calculations. Both Young's
modulus and the ultimate strain are found to be highly anisotropic and
nonlinear as a result of its quasi-two-dimensional puckered structure.
Specifically, the in-plane Young's modulus is 44.0 GPa in the direction
perpendicular to the pucker, and 92.7 GPa in the parallel direction. The
ultimate strain is 0.48 and 0.20 in the perpendicular and parallel directions,
respectively.Comment: Journal of Physics D: Applied Physics, accepte
Negative Poisson's Ratio in Single-Layer Black Phosphorus
The Poisson's ratio is a fundamental mechanical property that relates the
resulting lateral strain to applied axial strain. While this value can
theoretically be negative, it is positive for nearly all materials, though
negative values have been observed in so-called auxetic structures. However,
nearly all auxetic materials are bulk materials whose microstructure has been
specifically engineered to generate a negative Poisson's ratio. In the present
work, we report using first principles calculations the existence of a negative
Poisson's ratio in a single-layer, two-dimensional material, black phosphorus.
In contrast to engineered bulk auxetics, this behavior is intrinsic for single
layer black phosphorus, and originates from its unique, puckered structure,
where the pucker can be regarded as a re-entrant structure that is comprised of
two coupled orthogonal hinges. As a result of this atomic structure, a negative
Poisson's ratio is observed in the out-of-plane direction under uniaxial
deformation in the direction parallel to the pucker, with the Poisson's ratio
becoming increasingly negative with both increased tension and compression. The
puckered structure also results in highly anisotropic in-plane Poisson's
ratios, which are found to be 0.4 in the direction perpendicular and 1.28 in
the direction parallel to the pucker.Comment: Nature Communications, accepte
An Analytic Study of Strain Engineering the Electronic Bandgap in Single-Layer Black Phosphorus
We present an analytic study, based on the tight-binding approximation, of
strain effects on the electronic bandgap in single-layer black phosphorus. We
obtain an expression for the variation of the bandgap induced by a general
strain type that includes both tension in and out of the plane and shear, and
use this to determine the most efficient strain direction for different strain
types, along which the strongest bandgap manipulation can be achieved. We find
that the strain direction that enables the maximum manipulation of the bandgap
is not necessarily in the armchair or zigzag direction. Instead, to achieve the
strongest bandgap modulation, the direction of the applied mechanical strain is
dependent on the type of applied strain.Comment: Physical Review B, accepte
A Gaussian Treatment for the Friction Issue of Lennard-Jones Potential in Layered Materials: Application to Friction between Graphene, MoS2 and Black Phosphorus
The Lennard-Jones potential is widely used to describe the interlayer
interactions within layered materials like graphene. However, it is also widely
known that this potential strongly underestimates the frictional properties for
layered materials. Here we propose to supplement the Lennard-Jones potential by
a Gaussian-type potential, which enables more accurate calculations of the
frictional properties of two-dimensional layered materials. Furthermore, the
Gaussian potential is computationally simple as it introduces only one
additional potential parameter that is determined by the interlayer shear mode
in the layered structure. The resulting Lennard-Jones-Gaussian potential is
applied to compute the interlayer cohesive energy and frictional energy for
graphene, MoS2, black phosphorus, and their heterostructures.Comment: 9 figures, 3 table
Mechanical properties of carbon nanotube reinforced polymer nanocomposites: A coarse-grained model
In this work, a coarse-grained (CG) model of carbon nanotube (CNT) reinforced
polymer matrix composites is developed. A distinguishing feature of the CG
model is the ability to capture interactions between polymer chains and
nanotubes. The CG potentials for nanotubes and polymer chains are calibrated
using the strain energy conservation between CG models and full atomistic
systems. The applicability and efficiency of the CG model in predicting the
elastic properties of CNT/polymer composites are evaluated through verification
processes with molecular simulations. The simulation results reveal that the CG
model is able to estimate the mechanical properties of the nanocomposites with
high accuracy and low computational cost. The effect of the volume fraction of
CNT reinforcements on the Young's modulus of the nanocomposites is
investigated. The application of the method in the modeling of large unit cells
with randomly distributed CNT reinforcements is examined. The established CG
model will enable the simulation of reinforced polymer matrix composites across
a wide range of length scales from nano to mesoscale
Tensile fracture behavior of short carbon nanotube reinforced polymer composites: A coarse-grained model
Short-fiber-reinforced polymer composites are increasingly used in
engineering applications and industrial products owing to their unique
combination of superior mechanical properties, and relatively easy and low cost
manufacturing process. The mechanical behavior of short carbon nanotube (CNT)
polymer composites, however, remains poorly understood due to size and time
limitations of experiments and atomistic simulations. To address this issue,
the tensile fracture behavior of short CNT reinforced poly (methyl
methacrylate) (PMMA) matrix composites is investigated using a coarse-grained
(CG) model. The reliability of the CG model is demonstrated by reproducing
experimental results on the stress-stain behavior of the polymer material. The
effect of the nanotube weight fraction on the mechanical properties, i.e. the
Young's modulus, yield strength,tensile strength and critical strain, of the
CNT/polymer composites is studied in detail. The dependence of the mechanical
properties of the composites on the orientation and length-to-diameter aspect
ratio of nanotube reinforcements is also examined.Comment: arXiv admin note: text overlap with arXiv:1704.0145
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