Direct detonation initiation with thermal deposition due to pore collapse in energetic materials – towards the coupling between micro- and macroscale

In this work we investigate the initiation of detonations in energetic materials through thermal power deposition due to pore collapse. We solve the reactive Euler equations, with the energy equation augmented by a power deposition term. The deposition term is partially based on previous results of simulations of pore collapse at the microscale, modelled at the macroscale as hotspots. It is found that a critical size of the hotspots exists. If the hotspots exceed the critical size, direct initiation of detonation can be achieved even with a low power input, in contrast to the common assumption that a sufficient power is necessary to initiate detonation. We show that sufficient power is necessary only when the size of the hotspots is below the critical size. In this scenario, the so-called ‘explosion in the explosion’, the initial ignition does not lead to a detonation directly, but detonation occurs later as a result of shock-to-detonation transition in the region processed by the shock wave generated by the initial ignition.     

Density-based kinetics for mesoscale simulations of detonation initiation in energetic materials

In this work we present one- and two-dimensional mesoscale simulations of detonation initiation in energetic materials. We solve the reactive Euler equations, with the energy equation augmented by a power deposition term. The reaction rate at the mesoscale is modelled using a density-based kinetics scheme, adapted from standard ‘Ignition and Growth’ models. The deposition term is based on previous results of simulations of void collapse at the microscale, modelled at the mesoscale as hot spots. For an isolated hot spot in a homogeneous medium, it is found that a critical size of the hot spots exists. If the hot spots exceed the critical size, initiation of detonation can be achieved. For sub-critical hot-spot sizes, we show that it takes a collection of hot spots to achieve detonation. We also carry out two-dimensional mesoscale simulations of random packs of HMX crystals in a binder, and show that the transition between no detonation and detonation depends on the number density of the hot spots, the initial radius of the hot spot, the post-shock pressure of an imposed shock, and the amplitude of the power deposition term.     

N-Nitrosopiperazines form at high pH in post-combustion capture solutions containing piperazine: a low-energy collisional behaviour study

During the process of exploring aqueous piperazine chemistry under simulated flue‐gas scrubbing conditions, positive‐ion electrospray ionisation mass spectrometric (ESI‐MS) analyses of the resulting reaction mixtures in a triple quadrupole system revealed the presence of peaks at m/z 116 and 145, the putative N‐nitroso derivatives of piperazine. Confirmation of the presence of these species in the reaction mixtures was achieved using collision‐induced dissociation experiments. A purchased standard, together with in‐house synthesised N‐nitrosopiperazine standards (including N‐nitroso derivatives derived from deuterium‐labelled precursor materials), were used for this purpose. Across a small range of collision energies, large fluctuations in the abundance of the two major product ions of protonated N‐nitrosopiperazine, m/z 86 and 85, were observed. Using B3LYP/6‐311 + +G(d,p) computations, the potential energy surface was determined for loss of NO and [H,N,O]. At an activation energy slightly in excess of 1 eV, intramolecular isomerisation precedes loss of NO (m/z 86) via a 4,1 H‐shift, and at activation energies between 2.1–2.3 eV, consecutive loss of NO and atomic hydrogen competes with the direct loss of nitrosyl hydride (m/z 85). It is recommended that any multiple reaction monitoring method for quantifying N‐nitrosopiperazines at low collision energies use the sum of both transitions (m/z 116 ← 85, m/z 116 ← 86) to avoid errors that could be introduced by subtle changes in ES source conditions or collision voltages. This approach is adopted in an HPLC/MS/MS method used to monitor the degradation of N‐nitrosopiperazine exposed to (i) broad‐band UV light and (ii) heat typical of an amine regeneration (stripper) tower. The results reveal that aqueous N‐nitrosopiperazine is thermally stable at 150°C but will degrade slowly upon exposure to UV light. Copyright © 2010 John Wiley & Sons, Ltd.     

First-principles study of intermediate size silver clusters: Shape evolution and its impact on cluster properties

Low-energy isomers of Ag(N) clusters are studied within gradient-corrected density functional theory over the size range of N = 9-20. The candidate conformations are drawn from an extensive structural database created in a recent exploration of Cu(N) clusters [M. Yang et al., J. Chem. Phys. 124, 24308 (2006)]. Layered configurations dominate the list of the lowest-energy isomers of Ag(N) for N < 16. The most stable structures for N > 16 are compact with quasispherical shapes. The size-driven shape evolution is similar to that found earlier for Na(N) and Cu(N). The shape change has a pronounced effect on the cluster cohesive energies, ionization potentials, and polarizabilities. The properties computed for the most stable isomers of Ag(N) are in good agreement with the available experimental data.     

Nonclassical symmetries of a class of Burgers' systems

The nonclassical symmetries of a class of Burgers' systems are considered. This study was initialized by Cherniha and Serov with a restriction on the form of the nonclassical symmetry operator. In this paper we remove this restriction and solve the determining equations to show that (1) a new form of a Burgers' system exists that admits a nonclassical symmetry and (2) a Burgers' system exists that is linearizable.     

Comparative study of continuous wave Tm<Superscript>3+</Superscript>-doped silica and fluoride fiber lasers

We compare Tm³⁺-doped silica and fluoride fiber lasers in continuous wave operation using the same experimental setup. For incident pump powers of up to 50 W, the fluoride fiber is shown to have higher output power and efficiency compared to silica and reaches a maximum output power of 20 W at 1.94 μm. A slope efficiency of 49% (41%) and a maximum optical-to-optical efficiency of 45% (35%) at an incident pump power of 25 W (37 W) were determined for the fluoride (silica) fibers, respectively. These optical-to-optical efficiencies are the highest reported for directly diode-pumped Tm³⁺-based fiber lasers. We establish that in free-running mode, fiber lasers using fluoride glass are temporally more stable than fiber lasers using a silica host. PACS 42.55.Wd; 42.55.Xi; 42.60.Mi; 42.70.Hj     

Predicting midsagittal pharyngeal dimensions from measures of anterior tongue position in Swedish vowels: statistical considerations

In a re-analysis of x rays of speakers producing Swedish vowels, midsagittal pharyngeal dimensions were predicted from anterior tongue positions using procedures based on estimated tongue pellet positions. Principal component analysis was used to reduce the number of pellet degrees of freedom from eight to three prior to applying linear regression from these three independent variables to dependent vocal tract midsagittal cross distances. Except for the regions around the laryngopharynx and uvula, the pharynx dimensions were predictable from linear regressions and were significant at the p<0.05 level. Numerical experiments show that it is crucial to reduce the number of independent variables in tests of statistical significance.     

A tandem metathesis–hydrogenation route to dicarba analogues of cystine-containing cyclic peptides

Dicarba cyclic peptide analogues of the cyclic peptide octreotide have been synthesised in good yields using a single pot, on resin, tandem homogeneous metal-catalysed metathesis–hydrogenation sequence.     

γ‐Irradiation of ultrahigh‐molecular‐weight polyethylene: Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation

The shelf aging of irradiated ultrahigh‐molecular‐weight polyethylene (UHMWPE) causes subsurface oxidation, which leads to failure in UHMWPE orthopedic components, yet the mechanisms causing subsurface oxidation remain unclear. The shelf aging of γ‐irradiated UHMWPE bars has been studied with electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) imaging and with microtoming and Fourier transform infrared microscopy. The bars initially contained only allyl radicals, and upon air exposure, a surface layer of peroxyl radicals formed through the reaction of allyl radicals with oxygen. Importantly, a band of low radical intensity just beneath the peroxyl layer became apparent. NMR imaging showed a zone of altered proton relaxation in this zone. With increasing time, surface peroxyl radicals persisted in comparison with the interior allyl radicals, although oxygen did not appear to penetrate any more deeply into the bar. The area of maximal oxidation and mechanical disruption, measured after 3 years, was at the interface between the zone of exterior peroxyl radicals and the zone of low radical intensity. We present a mechanism involving the intermediacy of sterically strained reactive dialkyl peroxides at this interface to explain subsurface oxidation. We also demonstrate that EPR and NMR imaging provides information that could potentially be used to identify subsurface oxidized UHMWPE components before failure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5929–5941, 2004