Search

We present a combined experimental and theoretical study on carbon nanodiamonds (NDs) using Raman and DAC experimentation and ab initio calculations. Our calculations confirm the surface reconstruction to a fullerene-like structure, and indicate compression of the diamond core, producing an estimated internal pressure of 50 GPa. Quantum molecular dynamics simulations of hypervelocity collisions of NDs show that upon collision, shock-induced amorphization first occurs, followed by complete disruption of the ND surface and ejection of reactive particles into the vacuum. Raman spectra of oxidized ND samples at increasing pressures showed a subtle increase in the vibrational intensity of the Raman feature centered near 1335 cm-1 near 18 GPa and continued under subsequent pressure increases. The intensification of this vibrational feature is consistent with a thinning of the amorphous carbon outer shell, which results in greater exposure to the diamond core with increasing pressure and may be a precursor to Structural Bond Energy Release (SBER) initiation. Additionally, a nearly twofold increase in the vibrational intensity of the sp2 graphite peak centered near 1630 cm-1 in the spectra of the oxidized nanodiamond sample suggests the possibility of a sluggish partial phase transition from sp3 hybridized diamond to sp2 hybridized graphite., The original document contains color images.

Quantum mechanical calculations based on density functional theory (DFT) are used to study dynamic behavior of shocked energetic materials (EM). In this work, we present results of quantum molecular dynamics (QMD) simulations of shocked pentaerythritol tetranitrate (PETN), a conventional high explosive, and the polymeric cubic gauche phase of nitrogen (cg-N), proposed as an environmentally acceptable energetic alternative to conventional explosive formulations. These simulations, made possible through a Challenge grant awarded by the DoD High Performance Computing Modernization Program (DoD HPCMP), represent the leading edge of DFT simulations in both system size and simulation time with over 4000 atoms and up to ten thousand time steps utilizing as many as 512 processors per run., Presented at the Army Science Conference (26th) in Orlando, FL on 1-4 Dec 2008 and published in proceedings of the same. See also ADM002187. The original document contains color images.

Molecular dynamics (MD) simulations using a first principles-based Embedded-atom-Method (EAM) potential are used to simulate the exothermic alloying reactions of a NiAl bilayer at 1500 K and 1100 K. Both microcanonical (NVE) and isoenthalpic (NPH) MD simulations are used to demonstrate the influence of pressure on atomic mixing and subsequent alloying reaction. The NVE simulations, in which the volume is fixed and in which pressure increases as the exothermic reactions cause an increase in the system temperature, have a much slower reaction rate than those of the NPHMD simulations in which the pressure is maintained at 1 bar. The mechanism of the mixing is the same for all simulations: As mixing and reaction occurs at the interface, the heat generated first melts the Al layer, and subsequent mixing leads to further heat generation after which the Ni layer melts, leading to additional mixing until the alloying reactions are completed. The results indicate that pressure has a significant influence on the rates of atomic mixing and alloying reactions., Prepared in collaboration with High Performance Technologies, Inc. and with The Johns Hopkins University. Presented at the Army Science Conference (26th) in Orlando, FL on 1-4 Dec 2008 and published in proceedings of the same. See also ADM002187. The original document contains color images.

We present a combined experimental and theoretical study on carbon nanodiamonds using Raman and DAC experimentation and ab initio calculations. Our calculations confirm the surface reconstruction to a fullerene-like structure, and indicate compression of the diamond core, producing an estimated internal pressure of 50 GPa. Quantum molecular dynamics simulations of hypervelocity collisions of NDs show that upon collision shock-induced amorphization first occurs, followed by complete disruption of the ND surface and ejection of reactive particles into the vacuum. Raman spectra of oxidized ND samples at increasing pressures showed a subtle increase in the vibrational intensity of the Raman feature centered near 1335 cm-1 near 18 GPa and continued under subsequent pressure increases. The intensification of this vibrational feature is consistent with a thinning of the amorphous carbon outer shell, which results in greater exposure to the diamond core with increasing pressure and may be a precursor to SBER initiation. Additionally, a nearly two-fold increase in the vibrational intensity of the sp2 graphite peak centered near 1630 cm-1 in the spectra of the oxidized nanodiamond sample suggests the possibility of a sluggish partial phase transition from sp3 hybridized diamond to sp2 hybridized graphite., See also ADM002187. Presented at the Army Science Conference (26th) held in Orlando, FL on 1-4 December 2008. Published in the Proceedings of the Army Science Conference (26th), December 2008. The original document contains color images.

We describe a new extensible software system that performs multi-million atom molecular dynamics simulations. This new software system is based on a program environment and a set of components stored in shared object libraries that are executed by the program environment as specified by an XML description of the simulation. Fixed and scaled speedups for this software are demonstrated in molecular dynamics simulations of +/- quartz, using up 256 processors and up to 4.4 million atoms. Scaled speed up is over 80% of optimal on 256 processors and fixed speed up is also over 80% on 64 processors for system sizes as small as 1,000 atoms per processor. The outstanding scaling for both small and large systems is somewhat unusual, since other multimillion atom simulation codes tend to perform well only for very large systems., See also ADM002075., The original document contains color images. Presented at the Army Science Conference (25th) held in Orlando, FL on 27-30 November 2006. Sponsored in part by ONR.

We describe a new extensible software system to perform molecular simulations of energetic materials. A new approach for extensible software development based on an XML description of a program structure and a set of components stored in shared object libraries is described first. A specific example of molecular dynamics simulations for energetic materials is given next and this is finally expanded with the capability to perform molecular packing calculations to show the extensibility and applicability of the system., See also ADM001736, Proceedings of the Army Science Conference (24th) held in Orlando, FL on 29 Nov-2 Dec 2004.