Energetic Materials Combustion

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Theoretical Chemistry Applications in Energetic
Materials Research Betsy M. Rice U. S. Army
Research Laboratory Aberdeen Proving Ground,
Maryland 21005-5066
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Acknowledgements

• Donald L. Thompson, Oklahoma State University
• Samuel F. Trevino, ARL
• William Mattson, U. Illinois Urbana Champaign and
  ARL
• Dan C. Sorescu, National Energy Technology
  Laboratory
• John Grosh and Jen Hare, formerly of ARL
• Herman Ammon, University of Maryland

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OBJECTIVE

• Use standard theoretical chemical approaches to
• Screen proposed materialseliminate poor
  candidates before expending resources on
  synthesis, formulation and tests
• Identify and understand the individual
  fundamental chemical and physical steps that
  control the conversion of the material to final
  products

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METHODS

• Quantum Mechanics First principles
• Solution of HYEY for collection of atoms -
  characterizes system
• Provides information for parameterization of
  classical models
• Molecular Dynamics A classical simulation
  method
• Integration in time of Fma for every atom
  requires model
• Provides molecular-level details of chemical and
  physical processes through computer simulation of
  dynamic events
• How a material responds to set of initial
  conditions at the atomic level.
• What mechanisms control the responsewill provide
  guidance on how to manipulate system such that
  desired response obtained.
• Molecular Packing A classical simulation method
  for ab initio crystal prediction
• Evaluates lattice energy of molecule in a variety
  of possible crystalline environments requires
  model
• Ranks possible crystal structures (usually in
  order of increasing energy)
• Provides density and details of structure of
  crystal (size, shape and position of atoms in it)
  

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Prediction of Energetic Materials Properties from
correlations with charge distribution
Electrostatic Potential
Mapping out e- Density

1 Nanometer
(electron poor)
(electron rich)
CL20
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Correlations of Quantum mechanical predictions
with bulk properties
B. M. Rice, S. V. Pai and Jennifer Hare,
Predicting Heats of Formation of Energetic
Materials Using Quantum Mechanical Calculations,
Combustion and Flame, Vol. 118, p. 445 (1999).
Condensed Phase Heats of Formation

• DHGas from quantum mechanics
• DHSub and DH Vap estimated from correlation
  between bulk properties and electrostatic
  potential of a molecule.

J. S. Murray and P. Politzer, A General
Interaction Property Function (GIPF) An
Approach to Understanding and Predicting
Molecular Interactions in Quantitative
Treatments of Solute/Solvent Interactions, ed.
P. Politzer and J. S. Murray, (Elsevier Pub. Co.,
New York, 1994).
B. M. Rice, S. V. Pai and J. Hare, Predicting
Heats of Detonation Using Quantum Mechanical
Calculations, Thermochemica Acta, Vol. 38, p.
377 (2002).
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Impact machine

• Explosives (in mg) placed in between on flat tool
  steel anvil and flat surface of tool striker.
• 2.5 kg drop weight is dropped from predetermined
  height onto the striker plate.
• Result of the event (explosion or otherwise) is
  determined by sound, smell and visual inspection
  of the sample.
• Drop height is varied, with height increased or
  decreased depending on result of previous event.
  
• Sequence of tests carried out, with result quoted
  at h50, the height at which 50 of tests result
  in explosions.

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B. M. Rice and J. J. Hare, A Quantum Mechanical
Investigation of the Relation Between Impact
Sensitivity and the Charge Distribution in
Energetic Molecules, B. M. Rice and J. J. Hare,
Journal of Physical Chemistry, Vol. 106, 1770
(2002).
11 cm
71 cm
28 cm
11 cm
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Evaluating the Model Predicting Crystal
Structures using molecular packing

• Place single molecule in variety of crystalline
  environments
• Using classical force field, minimize energy with
  respect to crystal parameters
• Rank various crystal structures (usually lattice
  energy)

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Potential Energy Functions for classical
molecular simulation of energetic molecular
crystals

• 8 Papers published in J. Physical Chemistry
• Transferability (4)
• Limitations of Rigid Body Approximation (1)
• Inclusion of Flexible Motion (1)
• Behavior in Liquid State(1)
• Current investigation prediction of crystal
  structure using molecule packing

D. C. Sorescu, B. M. Rice and D. L. Thompson,
Intermolecular Potential for the
Hexahydro-1,3,5-trinitro-1,3,5-triazine Crystal
(RDX) A Crystal Packing, Monte Carlo and
Molecular Dynamics Study, the Journal of
Physical Chemistry B, vol. 101, pp-798-808, 1997.
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MOLPAK (MOLecular PAcKing)J. R. Holden, Z. Du
and H. L. Ammon, J. Comp. Chem. 14, 422 (1993)

• Uses rigid-body molecular structure to provide
  packing arrangements in 13 space groups.
• Triniclinic P1, P-1
• Monoclinic P21, P21/c, Cc, C2, C2/c
• Z4 Orthorhombic P21212, P212121, Pca21, Pna21
• Z8 Orthorhombic Pbcn, Pbca
• MOLPAK search produces initial guesses ---
  needed to energy refinement. For each space group
  7000 Possible structures are generated.
• 25 most dense structures are further refined
  using WMIN

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How good is the force field?
Applied to 39 nitramine and non-nitramines From
Nitramine and non-Nitramine paper, nitrocubane
series Predicted experimental structure for 38
of 39 (1 catastrosphic failure, believed numeric)
max. deviation no more than 4 in edge length,
largest deviation of cell angle is
7º. Low-energy structure is experimental
structure for 28 For remaining 10 cases, all
within 1.5 kcal/mol of low-energy structure 7
were within 0.4 kcal/mol.
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Modeling Results for candidate materials from
ARDEC
Rapid Assessment
Heat of Formation (solid) 113.1 kcal/mol Heat of
Detonation 1.41 (kcal/g) h50 9 cm Density of
low-energy structure 1.77 g/cc
Tetradecanitrobicubane Heat of Formation
(solid) 242 kcal/mol Heat of Detonation 1.81
(kcal/g) h50 68 cm Density of low-energy
structure 1.81 g/cc
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What now?

• Ab initio crystal prediction of chemical families
  of explosives
• Nitrocubane series (6 have been resolved)
• Nitramines (71)
• Nitrate Esters (32)
• Notified October 4 that team consisting of Rice,
  Mattson, (ARL), Ammon (U MD), Singh (NRL) and Kim
  (U Miss) awarded 2003 DOD High Performance
  Computing Modernization Plan CHSSI grant to
  parallelize MOLPAK and incorporate DOD Planewave

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MOLECULAR DYNAMICS SIMULATION OF DETONATION
Model Explosive A-B. Reactions that can occur
2 A-B ? A2 B2 A-B ? A B Initial crystal
at 10 K, molecules arranged in equilibrium
configuration Left side of plate hit with flyer
plate of molecules moving at a very high speed.
The impact compresses the quiescent crystal, and
a shock wave propagates through the
material. Reactions begin, and heat released
from the reaction drives the shock-wave,
resulting in a self-sustained detonation
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MOLECULAR SIMULATION OF DETONATION
B. M. Rice, W. Mattson, J. Grosh and S. F.
Trevino, A Molecular Dynamics Study of
Detonation II. The Reaction Mechanism,
Physical Review E, Vol. 53, 623 (1996). B. M.
Rice, W. Mattson, J. Grosh and S. F. Trevino, A
Molecular Dynamics Study of Detonation I. A
Comparison with Hydrodynamic Predictions,
Physical Review E, Vol. 53, 611 (1996).
Reaction Mechanism Pressure-induced
atomization, little thermal excitation
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REBO Potentials
Intramolecular bonds (covalent)
Intermolecular bonds
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DESENSITIZATION OF DETONABLE MATERIAL
B. M. Rice, W. Mattson and S. F. Trevino,
Molecular Dynamics Investigation of the
Desensitization of Detonable Material, Physical
Review E, Vol. 57, 5106 (1998).
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Reactive Potentials

• Reactive Force Fields (ReaxFF) (Goddard et al.,
  Center for Simulation of Dynamic Response of
  Materials, California Institute of Technology)

• Uses QM calculations to parameterize a function
• Applied to RDX and HMX
• Flyer-plate shock simulations show
• Initiation threshold exists
• Large fraction of products have been observed in
  experiment
• Some unlikely fragments
• Improvements will include products of secondary
  reaction channels

Chakraborty, D. Muller, R. P. Dasgupta, S.
Goddard, W. A., J. Phys. Chem. A 2000, 104, 226.
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SUMMARY

• Theoretical chemistry calculations will provide
  information necessary to tailor explosives BUT
  OFTEN RESULTS ARE COMPLETELY DEPENDENT ON QUALITY
  OF THE MODEL
• Realistic classical molecules exist for
  non-reactive events for CHNO explosives -- are
  not bad
• Reactive potentialsbasic concepts there, but
  need additional and better information for
  parameterization
• Direct Ab initio MD simulations progressing, not
  quite there, but extremely promising