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DURINT Review

Processing and Behavior of Nanoenergetic Materials November 17, 2005 Aberdeen, Maryland. DURINT Review. MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF ENERGETIC MATERIALS. Donald L. Thompson Department of Chemistry University of Missouri-Columbia. Final Review. Collaborators.

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DURINT Review

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  1. Processing and Behavior of Nanoenergetic Materials November 17, 2005 Aberdeen, Maryland DURINT Review MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF ENERGETIC MATERIALS Donald L. Thompson Department of Chemistry University of Missouri-Columbia Final Review

  2. 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)

  3. Simulations of Nanoparticles of Energetic Materials Focus 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

  4. 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

  5. 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)

  6. 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…

  7. 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. Wulff Construction 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.

  8. Generating Initial Conditions 9x9x9 5x5x5 Begin with a 9x9x9 supercell Rotate by anglesof θand φ, then cut from the core a 5x5x5* simulation supercell with various crystallographic surfaces * A5x5x5 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)

  9. 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.

  10. . . . To avoid the complexity of calculating DS, 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 Surface Free energy at 0 K core The interaction energy is calculated for the molecules in the bins Repeat for different values ofθ and φ.

  11. Crystallographic orientations of Wulff planes calculated * Blue numbered Wulff planes are not reported in Bouma and van der Heijden study. [Cryst. Growth Des. 4, 999 (2004)]

  12. For example, results for φ=30° 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

  13. Area enveloped by Equilibrium surfaces. φ=49° Black labels: Seen in lab-grown RDX crystal 002 111 332 Blue labels: Not seen in lab-grown RDX crystal 110 Interaction energy (kJ/mol)

  14. φ=0° 200 102 002 g(q) plot Interaction energy (kJ/mol)

  15. 002 φ=30° 210 Interaction energy (kJ/mol)

  16. 002 φ=49° 332 111 110 Interaction energy (kJ/mol)

  17. φ=90° 002 021 020 Interaction energy (kJ/mol)

  18. Oxygen Nitrogen Carbon Hydrogen Shape 002 102 021 111 200 210 020 332

  19. Oxygen Nitrogen Carbon Hydrogen Shape 002 102 332 020 200 111 021

  20. 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 > 0 K Other materials, e.g., CL-20 Effects of binders

  21. 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

  22. 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

  23. Nitromethane on Al2O3 Minimum energy reaction pathway for dissociation NM leading to adsorbed OH and CH2NO2 Calculations performed using VASP

  24. 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, 8953-8964 (2003). Calculations performed using VASP

  25. 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

  26. Melting of “Non-Magic Number” Aluminum Nanoparticles

  27. Melting of “Magic Number” Aluminum Nanoparticles Bistable regions

  28. Lindemann Index

  29. Melting point determined from the Lindemann Index • Melting range determined from the potential energy curves Melting Point as a Function of Aluminum Nanoparticle Size Magic number nanoparticles Other nanoparticles

  30. 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

  31. 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

  32. 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

  33. After 50 ps runs Nitromethane nanoparticles 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 didn’t find them. 250 K

  34. 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).

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

  36. 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

  37. (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane) Nanoparticles of CL-20 or HNIW • 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).

  38. bulksolid CL-20 Open Sorescu et al. Solid: present study Densities of CL-20 Nanoparticles 88-molecule annealed 48-molecule non-annealed 48-molecule annealed

  39. Snapshots of CL-20 Nanoparticles 48-molecule non-annealed 48-molecule annealed 88-molecule annealed

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