MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF ENERGETIC MATERIALS Donald L. Thompson Department of Chemistry University of Missouri-Columbia Processing and Behavior of Nanoenergetic Materials November 17, 2005 Aberdeen, 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 Al 2 O 3  Nitro and Nitramine compounds Simulations of Nanoparticles of Energetic Materials Understanding the properties of nanoparticles & how they relate to bulk materials

Overview Reaction of HCl on Al 2 O 3 (validation study) Reactions of energetic molecules on Al and Al 2 O 3 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

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 RDX nanoparticles…

The interfacial free energy per unit area f i (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. Wulff Construction m M. Wortis, Chemistry and Physics of Solid surfaces VII Vol 10(7), , 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.

Begin with a 9 x 9 x 9 supercell Rotate by angles of θ and φ, then cut from the core a 5 x 5 x 5* simulation supercell with various crystallographic surfaces Simulations: DL-POLY 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 boundary conditions. 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, * Actually, 1 x 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=1 x K). So that the problem is reduced to calculating the surface enthalpy of the crystal at various angles. Surface free energy The interaction energy is calculated for the molecules in the bins Free energy at 0 K Repeat for different values of θ and φ. core Surface

Crystallographic orientations of Wulff planes calculated Wulff Planeθφ 2000°0°0°0° 00290°0°0° 10270°0°0° 2100°0°30° 11140°49° 110*0°49° °49° 0200°0°90° 02130°90° * Blue numbered Wulff planes are not reported in Bouma and van der Heijden study. [Cryst. Growth Des. 4, 999 (2004)]

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 For example, results for φ=30°

Area enveloped by Equilibrium surfaces Black labels: Seen in lab-grown RDX crystal Blue labels: Not seen in lab-grown RDX crystal Interaction energy (kJ/mol) φ=49°

 (  ) plot Interaction energy (kJ/mol) φ=0°

Interaction energy (kJ/mol) φ=30°

Interaction energy (kJ/mol) φ=49°

Interaction energy (kJ/mol) φ=90°

Oxygen Nitrogen Carbon Hydrogen Shape

Oxygen Nitrogen Carbon Hydrogen 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 K Other materials, e.g., CL-20 Effects of binders

Very Brief Review Reaction of HCl on Al 2 O 3 (validation study) Reactions of energetic molecules on Al and Al 2 O 3 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  -Al 2 O 3 (0001) Surface,” J. Phys. Chem. B 107, (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, (2003).  S. Alavi and D. L. Thompson, “A Molecular Dynamics Study of Structural and Physical Properties of Nitromethane Nanoparticles,” J. Chem. Phys. 120, (2004).  S. Alavi, G. F. Velardez, and D. L. Thompson, “Molecular Dynamics Studies of Nanoparticles of Energetic Materials,” Materials Research Society Symposium Proceedings 800, (2004).  S. Alavi, J. W. Mintmire, and D. L. Thompson, “Molecular Dynamics Simulations of the Oxidation of Aluminum Nanoparticles,” J. Phys. Chem. B 109, (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 Al 2 O 3 (0001) Surface,” J. Phys. Chem. B 109, (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 Al 2 O 3 Minimum energy reaction pathway for dissociation NM leading to adsorbed OH and CH 2 NO 2 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 Adsorption of Nitromethane and 1,1-Diamino-2,2-dinitroethylene (FOX-7) Molecules on the Al (111) Surface,” J. Phys. Chem. 107, (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

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

  (2 nd shell, corners)  (2 nd shell) (core atom)  (1 st 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 molecules240 molecules96 molecules 170 K 230 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 show magic 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, (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, (2004). Nanoparticles of CL-20 or HNIW

48-molecule non-annealed 48-molecule annealed 88-molecule annealed bulk solid CL-20 Open 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