Molecular Dynamics Studies of Nanoparticles of Energetic Materials
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MOLECULAR DYNAMICS STUDIES OF NANOPARTICLES OF ENERGETIC MATERIALS Saman Alavi, Gustavo F. Velardez, and Donald L. Thompson Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 USA ABSTRACT The structural properties of several nanoparticles of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane, HNIW or CL-20, are studied by using molecular dynamics simulations. The internal structure of the CL-20 molecule is held rigid and the intermolecular interactions in the nanoparticles are taken from a previously developed force field. [Sorescu et al., J. Phys. Chem. B, 102, 948 (1998)] Molecular dynamics simulations of solid-like and annealed nanoparticles with 48 and 88 CL-20 molecules have been carried out in the solid-state range of temperatures from 50 to 500 K. The center-of-mass to center-of-mass radial distribution functions, dipole-dipole correlation function, the orientations of the surface dipoles, and the density of the nanoparticles were calculated at fixed temperatures for the nanoparticles.
INTRODUCTION Highly nitrated polycyclic compounds constitute a class of energetic materials that have received a substantial amount of interest in the past ten years. The relatively new energetic material 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), also known as CL-20, has attracted considerable attention as a possible replacement for high-energy compounds such as 1,3,5,7-tetraazocyclooctane (HMX) and hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX), both of which are widely used in explosives and propellants.[1,2] CL-20 is a very high energy density material that exists in at least five polymorphic solid-state forms, four of which are stable at ambient conditions (α-hydrate, β, ε, and γ) and have been characterized by X-ray diffraction. The ε form is preferred for general applications since it has the highest crystal density and the highest stability at ambient conditions.[3] The ε phase has monoclinic space group P21/n with four molecules per unit cell (Z = 4).[4] The density of the ε phase is reported to be between 1.96 and 2.044 g/cm3, the melting temperature is 523 K, and the decomposition temperature is 533 K.[5] Nanoparticles have been the subject of much study in recent years.[6,7] Theoretical and experimental studies of atomic, semiconductor, metal oxide, and molecular nanoparticles have appeared in the literature.[8-26] Nanoparticles have high surface-to-volume ratios which can affect their physical and chemical behavior. A size dependence of many physical and chemical properties of nanoparticles such as electron delocalization, band gap,[27] specific heat,[28] and melting point[29-32] is characteristic of nanoparticles. We have previously reported molecular dynamics (MD) simulations of the structure and melting of nanoparticles of nitromethane.[33] Solid nanoparticles of nitromethane have an ordered “crystalline” core with an amorphous surface shell. Unlike the bulk phase, a solid nanoparticle melts over a temperature range, which becomes broader as the size of the nanoparticle d
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