DURINT Review

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DURINT Review
Processing and Behavior of Nanoenergetic
Materials November 17, 2005 Aberdeen, Maryland
MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF
ENERGETIC MATERIALS
Donald L. Thompson Department of
Chemistry University of Missouri-Columbia
Final Review
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Collaborators
Saman Alavi (Now NRC-Ottawa) Jerry Boatz
(AFRL-Edwards) Don Brenner (NCSU) John Mintmire
(OSU) Ali Siavosh-Haghighi (MU) Dan Sorescu
(NETL-Pittsburgh) Gustavo Velardez (MU)
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Focus
Simulations of Nanoparticles of Energetic
Materials
Model physical and chemical properties of
energetic nanoparticles

• Systems
• Al and Al2O3
• Nitro and Nitramine compounds

Understanding the properties of nanoparticles
how they relate to bulk materials

• Processes
• ? Structure
• ? Melting
• Chemistry

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Overview
Reaction of HCl on Al2O3 (validation
study) Reactions of energetic molecules on Al
and Al2O3 surfaces Oxidation of Al
nanoparticles Structures and properties of
nanoparticles Al, NM, RDX,
CL-20 Melting of Al, nitromethane, and
CL-20 Past year Completed Melting of Al
study Current Shapes of nanoparticles
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RDX crystals grown in various solvents
Acetone
Cyclohexanone (with water)
g Butyrolactone
Original dissolved crystal
Cyclohexanone (without water)
E. D. M. van der Heijden and R. H. B. Bouma,
Cryst. Growth Des. 4, 999 (2004)
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Theoretical Predictions of Shapes
The equilibrium shapes of crystals are the result
of the dependence of the interface free energy
per unit area on the orientation of the interface
relative to the crystallographic axes of the bulk
solid, and the microscopic properties of solids
and interfaces determine the details of this
dependence. The shapes can be predicted, given
an accurate potential, by using Wulff
construction G. Wulff, Z. Kristallogr. 34, 449
(1901).

Some preliminary studies of predictions of the
shapes of RDX nanoparticles
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Wulff Construction
The interfacial free energy per unit area fi(m)
is plotted in a polar frame. A radius vector is
drawn in each direction m and a plane is
drawn perpendicular to it where it intersects the
Wulff plot.
The envelop of the family of Wulff planes is the
shape of the crystal.
A cusp in the Wulff plot occurs for a facet of
the corresponding orientation of the crystal
shape.
m
M. Wortis, Chemistry and Physics of Solid
surfaces VII Vol 10(7), 367-405, 1998.
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Generating Initial Conditions
9x9x9
5x5x5
Begin with a 9x9x9 supercell
Rotate by angles of ? and f, then cut from the
core a 5x5x5 simulation supercell with various
crystallographic surfaces
A 5x5x5 supercell contains 1000 RDX
molecules.
Simulations DL-POLY-2.15 10000 time steps of NVT
simulation which of 7000 steps are equilibration.
(time steps 0.1 fs)
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Simulations
5x5x5
A series of crystals, with various surfaces, were
equilibrated in a vacuum (no boundary conditions.
Actually, 1x10-8 K
We take T 0 K so that we need only compute the
interaction energy (avoiding the difficulty of
computing the entropy). 10,000 time steps of
NVT simulation of which 7,000 steps are
equilibration. (time steps 0.1 fs)
Force Field SRT (intermolecular)
AMBER (intramolecular) Approximate, but satisfies
basic requirements for our purpose Accurate
description of solid-phase properties flexible
to qualitatively account for molecular behavior
in response to surface tension. vdw cutoff
radius 11Å
Sorescu, Rice, and Thompson, J. Phys. Chem. B
101, 798, 1997.
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Surface free energy
To avoid the complexity of calculating DS, we
determine the equilibrium shape of the crystal at
a temperature very close to 0 K (T1x10-8K). So
that the problem is reduced to calculating the
surface enthalpy of the crystal at various angles.
Surface
Free energy at 0 K
core
The interaction energy is calculated for the
molecules in the bins
Repeat for different values of ? and f.
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Crystallographic orientations of Wulff planes
calculated
Wulff Plane ? f
200 0 0
002 90 0
102 70 0
210 0 30
111 40 49
110 0 49
332 150 49
020 0 90
021 30 90
Blue numbered Wulff planes are not reported in
Bouma and van der Heijden study. Cryst. Growth
Des. 4, 999 (2004)
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For example, results for f30
Cusps in a Wulff plot indicate surfaces with low
surface energy. The line that is perpendicular
to the vector from the center represents an
equilibrium plane a Wulff plane.
Cusps
Wulff plane Of a cusp
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f49
Black labels Seen in lab-grown RDX crystal
002
111
332
Blue labels Not seen in lab-grown RDX crystal
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Interaction energy (kJ/mol)
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f0
200
102
002
g(q) plot
Interaction energy (kJ/mol)
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f30
002
210
Interaction energy (kJ/mol)
16
f49
002
332
111
110
Interaction energy (kJ/mol)
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f90
002
021
020
Interaction energy (kJ/mol)
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Shape
002
102
021
111
200
210
020
332
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Shape
002
102
332
020
200

111
021




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Conclusions/Future Work
Tentative Conclusions based on very approximate
potential
In accord with experiment, we predict that the
surfaces more frequently seen in the lab grown
crystals of RDX are the ones with oxygen atoms
sticking out of the surfaces. We predict the
same large faces as seen experimentally.
Next
Simulations in solvents (e.g., acetone) T gt 0
K Other materials, e.g., CL-20 Effects of binders
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Very Brief Review
Reaction of HCl on Al2O3 (validation
study) Reactions of energetic molecules on Al
and Al2O3 surfaces Oxidation of Al
nanoparticles Structures and properties of
nanoparticles Al, NM, RDX,
CL-20 Melting of Al, nitromethane, and CL-20
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Publications

• ? S. Alavi, D. C. Sorescu, and D. L. Thompson,
  Adsorption of HCl on a Single-Crystal ?-Al2O3
  (0001) Surface, J. Phys. Chem. B 107, 186-195
  (2003).
• ? D. C. Sorescu, J. A. Boatz, and D. L. Thompson,
  First-Principles Calculations of the Adsorption
  of Nitromethane and 1,1-Diamino-2,2-dinitroethylen
  e (FOX-7) Molecules on the Al (111) Surface, J.
  Phys. Chem. 107, 8953-8964 (2003).
• ? S. Alavi and D. L. Thompson, A Molecular
  Dynamics Study of Structural and Physical
  Properties of Nitromethane Nanoparticles, J.
  Chem. Phys. 120, 10231-10238 (2004).
• ? S. Alavi, G. F. Velardez, and D. L. Thompson,
  Molecular Dynamics Studies of Nanoparticles of
  Energetic Materials, Materials Research Society
  Symposium Proceedings 800, 329-338 (2004).
• ? S. Alavi, J. W. Mintmire, and D. L. Thompson,
  Molecular Dynamics Simulations of the Oxidation
  of Aluminum Nanoparticles, J. Phys. Chem. B 109,
  209-214 (2005).
• ? D. C. Sorescu, J. A. Boatz, and D. L. Thompson,
  First Principles Calculations of the Adsorption
  of Nitromethane and 1,1-Diamino-2,2-Dinitroethylen
  e (FOX-7) Molecules on Al2O3(0001) Surface, J.
  Phys. Chem. B 109, 1451-1463 (2005).
• ? S. Alavi and D. L. Thompson, Molecular
  Dynamics Simulations of the Melting of Aluminum
  Nanoparticles, J. Phys. Chem. B, in press.

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Nitromethane on Al2O3
Minimum energy reaction pathway for dissociation
NM leading to adsorbed OH and CH2NO2
Calculations performed using VASP
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Nitromethane on Al
N-O bond broken, Al-O and Al-N bonds formed

• D. C. Sorescu, J. A. Boatz, and D. L. Thompson,
  First-Principles Calculations of the Adsorption
• of Nitromethane and 1,1-Diamino-2,2-dinitroethylen
  e (FOX-7) Molecules on the Al (111) Surface,
• J. Phys. Chem. 107, 8953-8964 (2003).

Calculations performed using VASP
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Aluminum Nanoparticles

• Streitz-Mintmire potential. More flexible than
  other model potentials
• used in metal nanoparticle simulations
• Simulated annealing
• NVT simulation
• T 250 K
• ?t 2 fs
• 400 ps simulation time
• Characterization of structures
• Magic number effects
• Determination of melting points
• Potential energy plots ? bistabilty
• Lindemann Index, ?
• Charge distribution in the nanoparticles
  implications on reactivity

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Melting of Non-Magic Number Aluminum
Nanoparticles
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Melting of Magic Number Aluminum Nanoparticles
Bistable regions
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Lindemann Index
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Melting Point as a Function of Aluminum
Nanoparticle Size

• Melting point determined from the Lindemann
  Index
• Melting range determined from the potential
  energy curves

Magic number nanoparticles
Other nanoparticles
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Average Charge Distribution in Al Nanoparticles
0.025
0.051 (2nd shell, corners)
?0.29
?0.004 (2nd shell)
13 atoms
?0.065 (1st shell)
0.018
0.031
?0.22
0.017
55 atoms
0.038 (core atom)
19 atoms
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Conclusions Al Nanoparticles

• Show magic number behavior
• Some small metallic nanoparticles differ from
  their
• Lennard-Jones analogs
• Small nanoparticles show bistability between
  solid and liquid
• phases at intermediate temperatures
• Atoms in the nanoparticles have non-uniform
  charge distributions
• and may show different reactivities at various
  surface sites
• for different particle sizes

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Nitromethane nanoparticles

• Nanoparticles with 32 to 480 nitromethane
  molecules
• Characterization of structure
• Energetics of the nanoparticle
• ? enthalpy of melting
• ? enthalpy of vaporization
• Determination of melting point
• for different sized nanoparticles
• ? density
• ? diffusion coefficient
• ? Lindemann index

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Nitromethane nanoparticles
After 50 ps runs
480 molecules
240 molecules
96 molecules
solid
170 K
115 K
230 K
In solid nanoparticles, dipolar forces maintain
the ordered structure in the core
liquid
Do not appear to show magic number structures, or
we didnt find them.
250 K
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Melting range and temperature with nanoparticle
size Nitromethane
S. Alavi and D. L. Thompson, A Molecular
Dynamics Study of Structural and Physical
Properties of Nitromethane Nanoparticles, J.
Chem. Phys. 120, 10231-10238 (2004).
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Nitromethane nanoparticles

• The structure is dominated by dipole forces
• We did not discover magic number clusters
• Melting point varies smoothly with nanoparticle
  size

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Simulation of CL-20 nanoparticles

• DL_POLY MD program
• Fixed molecular structures
• Sorescu, Rice, and Thompson potential
• (Buckingham Coulombic)
• Annealed and non-annealed nanoparticles
• Time step 2 fs
• 100 ps equilibration
• 200 ps runs

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Nanoparticles of CL-20 or HNIW
(2,4,6,8,10,12-hexanitrohexaazaisowurtzitane)

• Simulations on CL-20 nanoparticles
• Characterization of structure
• ? density
• ? dipole-dipole correlations
• ? surface dipole alignments
• ? surface functional group alignments
• Energetics of the nanoparticle
• ? enthalpy of vaporization
• Surface coating (next stage)

S. Alavi, G. F. Velardez, and D. L. Thompson,
Molecular Dynamics Studies of Nanoparticles of
Energetic Materials, Materials Research Society
Symposium Proceedings 800, 329-338 (2004).
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Densities of CL-20 Nanoparticles
bulk solid CL-20 Open Sorescu et al. Solid
present study
88-molecule annealed
48-molecule non-annealed
48-molecule annealed
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Snapshots of CL-20 Nanoparticles
48-molecule non-annealed
48-molecule annealed
88-molecule annealed