New Insights into Current Issues in Nanoindentation from Finite
Element/Atomistic Multiscale Modeling
National Science Foundation
Atomistic simulations are being used to determine whether atomic-force
microscopy can be used as a nondestructive, nanoscale probe of surface
stress distributions, and the production of local work-hardened regions
using multiple indentation. Local moduli obtained from load curves during
shallow indentation around a nanoscale “trench” in a gold surface were
compared to the surface stress distribution calculated directly from the
atomistic potential used in the simulation. The calculations predict a
strong quantitative relationship between the indentation results and
surface stress. Other nanostructures, as well as the influence of the
substrate material and tip radius, are currently being explored.
Modeling Studies of the Deposition of AlN
Office of Naval Research
Ab initio calculations together with a free energy model for growth
environments are being used to predict the gas-phase composition and
optimal temperature for the vapor deposition of AlN crystals. Free
energies are calculated for a large number of potential gas-phase
species (over 40 for the AlN system) using statistical mechanics with
input from quantum chemical calculations on small molecules and clusters.
From these calculations, the predominant gas-phase species are identified,
and optimal growth temperatures for given experimental conditions are
predicted from the minimum in the chemical potential of the vapor.
Agreement with experimental growth temperatures is excellent.
More information:
Y. Li and D.W. Brenner, “First Principles Prediction of the Gas-Phase
Precursors for AlN Sublimation Growth”, Phys. Rev. Lett. 92,
75503 (2004).link
Theory and Simulation of Nanotube Composites
Combined Experimental and Theoretical Study of Tribochemistry at the
Nanometer Scale with Application to Nano-Mechanical Devices
Reduced Degree of Freedom Predictive Methods for Control and
Design of Interfaces in Nanofeatured Systems: Nanocrystalline Materials,
Sensors and Composites
Quantum-Based, Reactive Potentials for Simulating Shock Dynamics of
Condensed-Phase Energetic Materials: A Bridge between ab initio
Calculations and Experimental Shock Dynamics
Atomic Level Modeling of Energy Transfer, Friction, Wear, and
Microstructure Evolution at the Rail-Projectile Interface
Multifunctional Extreme Environment Surfaces: Nanotribology for Air
and Space
Characterizing Friction and Wear Properties of Engineering Materials
from Atomic Simulations
Surfactant Self-Assembly on Nano-Structured Surfaces: Multi-Scale
Computational Prediction and Design
Nanofluidics and Smart Materials
Multiscale Modeling of Polycrystalline Ceramics
Modeling of Nanotube-Based Materials and Devices
NASA
Mechanisms of load and heat transfer in nanotubule-polymer composites are
being studied as a function of matrix-fiber strength, nanotubule buckling,
and functionalization of the nanotubules. Current calculations predict that
a small density of crosslinks between a (10,10) nanotube and a polyethylene
matrix can increase the efficiency of load transfer by over an order of
magnitude without compromising the tensile modulus of the nanotube.
Simulations aimed at understanding the thermal properties of these systems
for thermal management applications are currently underway.
More information:
S. J. V. Frankland, A. Caglar, D. W.
Brenner, and M. Griebel, "Molecular Simulation of the Influence of
Chemical Cross-Links on the Shear Strength of Carbon Nanotube-Polymer
Interfaces", J. Phys. Chem. B 106, 3046 (2002).
link
S.J.V. Frankland. V.M. Harik, G.M. Odegard, D.W. Brenner and
T.S. Gates, "The Stress-Strain Behavior of Polymer-Nanotube
Composites from Molecular Dynamics Simulation", Composites
Sci. and Tech. 63, 1655 (2003).
link
Department of Energy
This joint experimental and modeling effort is aimed at understanding
tribochemical reactions at the nanoscale, specifically the competing roles
of enhanced equilibrium rates via surface heating versus non-equilibrium
mechano-chemistry under ultra-fast sliding conditions. The experimental
effort uses the unique capabilities developed in Professor Krim's laboratory.
In the modeling effort, specific reaction mechanisms are being determined
for tip-surface sliding conditions using molecular dynamics simulations.
This joint effort, the first in which tribochemical modeling and experiment
occur on comparable time scales and contact areas, is producing powerful
new insights into detailed tribochemistry at the extremely high rates for
next-generation nanoscale devices.
National Science Foundation (NIRT)
New hierarchical materials modeling approaches are being developed that
span multiple length and time scales and that couple quantum mechanical
methods at the atomic scale to continuum defect modeling at the micron
scale and above. These approaches allow for the accurate prediction of
processing-structure-property relations from both the atomic to macroscale,
and from the macroscale down to the atomic scale. Applications of these
methods that are being explored include quantum dots embedded in solids
to be used as sensors, and nanostructured materials and nanocomposites
for structural applications.
Army Research Office (MURI)
A transferable and robust analytic reactive potential for C, H, O and N
containing species is being developed to model large-scale atomic-level
chemical reactivity in molecular solids. The potential function is based
on a bond order formalism that couples bond lengths, energies and force
constants. The potentials will be used to help develop safer and more
efficient energetic materials.
Office of Naval Research (MURI)
One of the limiting phenomena for the development of railgun technology
is wear at the armature/rail interface. We are using a combination of
atomic and continuum-level modeling techniques to charactize wear
mechanisms at sliding metallic interfaces, in particular the interplay of
frictional heating, heat conduction, plasma formation and electromagnetic
migration, to explore coatings that can potentially inhibit wear.
Air Force Office of Scientific Research (MURI)
This is part of a large experimental-modeling effort that is developing
the scientific basis for tribological properties in terms of scale-dependent
thermal, chemical, and mechanical processes that is critical for the
engineering of advanced materials and coatings with tailor-made properties
for aerospace applications. A hierarchy of continuum and atomic modeling is
used to explore fundamental friction and wear phenomena, and to explore new
materials with unique properties.
Office of Naval Research
We are developing a variety of new modeling tools for linking friction
and wear processes over the disparate time and length scales associated
with molecular modeling and real-world engineering applications. The
modeling tools, which focus on both metals and covalent solids, couple
molecular dynamics simulations with grid-based coarse-grain models for
surface chemical kinetics, heat transfer, wear and microstructure
evolution at sliding interfaces.
National Science Foundation (NIRT)
The self-assembly of surface-active molecules from a bulk phase onto
solid surfaces is of interest in fields ranging from biology to materials
to electronic devices. We are developing coarse-graining procedures to
bridge the electronic-atomistic and atomistic-mesoscale levels of description
of aqueous surfactant solutions, and using these to construct a multi-scale
theory/simulation scheme to predict self-assembly on nano-structured surfaces.
NASA
Atomic simulations and continuum calculations based on the Brinkman
equation are being used to explore fluid flow through smart materials consisting
of polymer chains grafted to the inside of a nanopore. In these sytems
the swelling of the chains, and hence the pore size, is influenced
by effects such as the solvent quality with respect to the chains,
the temperature, and flow rate. Our calculations have shown that the continuum
Brinkman equation for flow through porous media can be used for these
systems provided that the correct correction length ("blob" size) is
taken into account. This model has been able to qualitatively describe
epxerimental measurements of changes in water permeation with pH
through nanoporousglass within which polyglutamic acid chains
are grafted. These calculations are opening new possibilities for optimizing
smart fluid control systems for applications such as drug delivery,
bioremedation,remote analysis and self-assembly.
More information:
S.P. Adiga and D.W. Brenner, "Virtual Molecular Design of an
Environment-Responsive Nanoporous System", Nano-letters 2, 567
(2002).
link
Office of Naval Research
A multiscale modeling approach has been developed in which energies from
first-principles calculations on key structures are combined with a
mesoscopic analytic model based on disclination theory. This is intended
to bridge the gap between atomistic models using quantum-based
first-principle methods and macroscale properties. Calculations on
title grain boundaries in diamond have demonstrated accurate energies
for planar defects can be obtained. This has led to accurate,
first-principles-based predictions of fracture strengths and failure
modes in ceramics with realistic microstructures. These studies are
currently being expanded to include analytic models describing the
mechanical properties of nanocrystalline ceramics.
More information:
O. Shenderova, D.W. Brenner, A. Nazarov, A. Romanov, L. Yang, `
"Multiscale Modeling Approach for Calculating Grain Boundary
Energies from First Principles", Phys. Rev. B. 57, R3181(1998).
link
A Nazarov, O.A. Shenderova, D.W.Brenner, "On the Disclination-Structural
Unit Model of Grain Boundaries’, Mat. Sci. Eng. A 281, 148
(2000). link
Office of Naval Research
The structure and properties of a variety of materials utilizing carbon
nanotubules are being predicted using atomic simulation. Raman shifts
for hydrogen in nanotube bundles have been predicted and are being used
to help identify the position of intercalated hydrogen within these
structures. The influence of local structure on the field emitting
properties of nanotube mats and novel nanotubule-nanodiamond structures
is being evaluated using a novel self-consistent tight binding approach
that allows electric fields to be incorporated into electronic structure
calculations with very modest computing resources.
More information:
D. Srivastava, D.W. Brenner, J.D. Schall, K.D. Ausman, M.F. Yu and
R.S. Ruoff, "Predictions of Enhanced Chemical Reactivity at Regions
of Local Conformation Strain on Carbon Nanotubes: Kinky Chemistry",
J. Phys. Chem. B 103, 4330 (1999).link
S.J.V. Frankland and D.W. Brenner, "Hydrogen Raman Shifts in Carbon
Nanotubes from Molecular Dynamics Simulation", Chem. Phys. Lett.
334, 18 (2001).link
O.A. Shenderova, B.L. Lawson, D. Areshkin and D.W. Brenner,
"Predicted Structure and Electronic Properties of Individual Carbon
Nanocones and Nanostructures Assembled from Nanocones",
Nanotechnology 12, 191 (2001).link
O.A. Shenderova, V. Zhirnov, and D.W. Brenner "Carbon Materials and
Nanostructures", Critical Reviews in Solid State and Materials
Sciences 32, 347 (2002).link
J. Bernholc, D. Brenner, M. Buongiorno Nardelli, V. Meunier and
C. Roland, "Mechanical and Electrical Properties of Nanotubes",
Annual Review of Materials Research 32, 347 (2002).
link
D.W. Brenner, O.A. Shenderova, D.A. Areshkin, J.D. Schall,
"Atomic Modeling of Carbon-Based Nanostructures as a Tool for
Developing New Materials and Technologies", Computer Modeling
in Engineering and Sciences 3, 643 (2002).link
O.A. Shenderova, D. Areshkin, D.W. Brenner, "Bonding and Stability of
Hybrid Diamond/nanotube Structure", Molecular Simulation,
29, 259 (2003).link