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Recognition of helical structural motifs in the experimentally observed cubic gauche ͑CG͒ crystal lattice has led to the discovery of a single-bonded nonlayered nitrogen structure that we have named chaired web ͑CW͒. First-principles density functional theory calculations reveal that CW, which was originally identified at high pressures, possesses metastability at ambient conditions as well. The metastability is demonstrated by both high-quality phonon dispersion calculations and finite-temperature first-principles molecular dynamics simulations. In addition, the CW phase is thermodynamically more stable than the CG phase in the ambient pressure regime.

A systematic method to unravel a large class of single-bonded (SB) polymeric phases of nitrogen under high pressure is presented. The approach is based on the combinatorial generation of different Peierls-like distortions of a given reference structure that maintain the threefold connectivity of SB nitrogen, followed by first-principles calculations. Using an eight atom simple cubic reference structure, the approach not only recovers all four SB nitrogen phases reported to date, but eight new metastable structures (confirmed by phonon density of states calculations) are found. Basic properties of the structures are computed and the trends analyzed. Extensions to the method are straightforward and should lead to the discovery of more phases of polynitrogen.

Detonation of a spherical high explosive charge containing solid particles generates a high-speed two-phase flow comprised of a decaying spherical air blast wave together with a rapidly expanding cloud of particles. The particle momentum effects associated with this two-phase flow have been investigated experimentally and numerically for a heterogeneous explosive consisting of a packed bed of inert particles saturated with a liquid explosive. Experimentally, the dispersion of the particles was tracked using flash radiography and high-speed photography. A particle streak gauge was developed to measure the rate of arrival of the particles at various locations. Using a cantilever gauge and a free-piston impulse gauge, it was found that the particle momentum flux provided the primary contribution of the multiphase flow to the near-field impulse applied to a nearby small structure. The qualitative features of the interaction between a particle and the flow field are illustrated using simple models for the particle motion and blast wave dynamics. A more realistic Eulerian two-fluid model for the gas-particle flow and a finite-element model for the structural response of the cantilever gauge are then used to determine the relative contributions of the gas and particles to the loading.

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