MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF ENERGETIC MATERIALS

Donald L. ThompsonDepartment of Chemistry

University of Missouri-Columbia

Processing and Behavior of Nanoenergetic MaterialsNovember 17, 2005Aberdeen, Maryland

DURINT Review

Final Review

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)

Focus

Model physical and chemical properties of energetic nanoparticles

Processes:

Structure

Melting Chemistry

Systems:

Al and Al2O3

Nitro and Nitramine compounds

Simulations of Nanoparticles of Energetic Materials

Understanding theproperties of nanoparticles

& how they relate to bulk materials

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 studyCurrent: Shapes of nanoparticles

Acetone Cyclohexanone (with water) –Butyrolactone

Cyclohexanone (without water) Original dissolved crystal

E. D. M. van der Heijden and R. H. B. Bouma, Cryst. Growth Des. 4, 999 (2004)

RDX crystals grown in various solvents

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 RDXnanoparticles…

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 drawnperpendicular to it where it intersects the Wulff plot.

Wulff Construction

m

M. Wortis, Chemistry and Physics of Solid surfaces VII Vol 10(7), 367-405, 1998.

The envelop of the family of Wulff planesis the shape of the crystal.

A cusp in the Wulff plotoccurs for a facet of thecorresponding orientationof the crystal shape.

Begin with a 9x9x9 supercell Rotate by angles of θ and φ,

then cut from the core a 5x5x5* simulation supercell with various crystallographic surfaces

Simulations: DL-POLY-2.1510000 time steps of NVT simulation which of 7000 steps are equilibration. (time steps = 0.1 fs)

Generating Initial Conditions

9x9x9 5x5x5

* A 5x5x5 supercell contains ~1000 RDX molecules.

5x5x5 Simulations

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)

A series of crystals, with various surfaces,were equilibrated in a vacuum (no boundaryconditions.

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.

* Actually, 1x10-8 K

. . .

To avoid the complexity of calculating S, we determine the equilibrium shape of the crystal at a temperature very close to 0 K (T=1x10-8K). So that the problem is reduced to calculating the surface enthalpy of the crystal at various angles.

Surface free energy

The interaction energyis calculated for the molecules in the bins

Free energyat 0 K

Repeat for different values of θ and φ.

core

Surface

Crystallographic orientations of Wulff planes calculated

Wulff Plane θ φ

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)]

0 1e+5 2e+5 3e+5 4e+5 5e+5

0

30

60

90

120

150

180

210

240

270

300

330

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 planeOf a cusp

For example, results for φ=30°

0 1e+5 2e+5 3e+5 4e+5 5e+5

0

30

60

90

120

150

180

210

240

270

300

330

Area enveloped by Equilibrium surfaces.

111

110

332

002Black labels: Seen inlab-grown RDX crystal

Blue labels: Not seen in lab-grown RDX crystal

Interaction energy (kJ/mol)

φ=49°

0 1e+5 2e+5 3e+5 4e+5 5e+5

0

30

60

90

120

150

180

210

240

270

300

330

200102

002

() plot

Interaction energy (kJ/mol)

φ=0°

0 1e+5 2e+5 3e+5 4e+5 5e+5

0

30

60

90

120

150

180

210

240

270

300

330

210

002

Interaction energy (kJ/mol)

φ=30°

0 1e+5 2e+5 3e+5 4e+5 5e+5

0

30

60

90

120

150

180

210

240

270

300

330

111

110

332

002

Interaction energy (kJ/mol)

φ=49°

0 1e+5 2e+5 3e+5 4e+5

0

30

60

90

120

150

180

210

240

270

300

330

020

002021

Interaction energy (kJ/mol)

φ=90°

002

021102

111

210

200

020

332

OxygenNitrogenCarbonHydrogen

Shape

102

200 020

002

332

111 021

Oxygen NitrogenCarbonHydrogen

Shape

Conclusions/Future Work

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.

Tentative Conclusions based on very approximate potential

Next:Simulations in solvents (e.g., acetone)T > 0 KOther materials, e.g., CL-20Effects of binders

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

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-dinitroethylene (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-Dinitroethylene (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.

Publications

Nitromethane on Al2O3

Minimum energy reaction pathway for dissociation NM leading to adsorbed OH and CH2NO2

Calculations performed using VASP

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 Adsorptionof Nitromethane and 1,1-Diamino-2,2-dinitroethylene (FOX-7) Molecules on the Al (111) Surface,” J. Phys. Chem. 107, 8953-8964 (2003).

Calculations performed using VASP

• Streitz-Mintmire potential. More flexible than other model potentials used in metal nanoparticle simulations * Simulated annealing * NVT simulation * T = 250 K * Δt = 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

Aluminum Nanoparticles

Melting of “Non-Magic Number” Aluminum Nanoparticles

Bistable regions

Melting of “Magic Number” Aluminum Nanoparticles

∑<

−

−=

jitij

tijtij

r

rr

NN

22

)1(

2δ

Lindemann Index

Magic number nanoparticles

Other nanoparticles

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

Melting Point as a Function of Aluminum Nanoparticle Size

0.29

+0.025

0.22+0.031

+0.018

+0.017

+0.051 (2nd shell, corners)

0.004 (2nd

shell)

+0.038(core atom)

0.065(1st shell)

13 atoms

19 atoms

55 atoms

Average Charge Distribution in 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

Conclusions: Al 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

Nitromethane nanoparticles

480 molecules 240 molecules 96 molecules

170 K230 K

250 K

115 K

After 50 ps runs

“solid”

“liquid”

In solid nanoparticles, dipolar forces maintain the ordered structure in the core

Do not appear to showmagic number structures,or we didn’t find them.

Nitromethane nanoparticles

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).

• The structure is dominated by dipole forces

• We did not discover magic number clusters

• Melting point varies smoothly with nanoparticle size

Nitromethane 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

Simulation of CL-20 nanoparticles

(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).

Nanoparticles of CL-20 or HNIW

48-moleculenon-annealed

48-molecule annealed

88-molecule annealed

bulk solid CL-20Open Sorescu et al.Solid: present study

Densities of CL-20 Nanoparticles

48-molecule non-annealed 48-molecule

annealed 88-molecule annealed

Snapshots of CL-20 Nanoparticles