Session U1: Detonation and Shock-induced Chemistry VII: Low Density Explosive Reactivity

Time-Of-Flight Mass Spectrometry of Laser Exploding Foil Initiated PETN Samples

We report the results of time-of-flight mass spectrometry (TOFMS) measurements of the gaseous products of thin film PETN samples reacting \textit{in-vacuo}. The PETN sample spots are produced by masked physical vapor deposition of PETN [A.S. Tappan, \textit{et al}., AIP Conf. Proc. \textbf{1426}, 677 (2012)] onto a first-surface aluminum mirror. A pulsed laser beam imaged through the soda lime glass mirror substrate converts the aluminum layer into a high-temperature high-pressure plasma which initiates chemical reactions in the overlying PETN sample. We had previously proposed [E.C. Fossum, \textit{et al}., AIP Conf. Proc. \textbf{1426}, 235 (2012)] to exploit differences in gaseous product chemical identities and molecular velocities to provide a chemically-based diagnostic for distinguishing between ``detonation-like'' and deflagration responses. Briefly: we expect in-vacuum detonations to produce hyperthermal (v $\sim$ 10 km/s) thermodynamically-stable products such as N$_{2}$, CO$_{2}$, and H$_{2}$O, and for deflagrations to produce mostly reaction intermediates, such as NO and NO$_{2}$, with much slower molecular velocities -- consistent with the expansion-quenched thermal decomposition of PETN. We observe primarily slow reaction intermediates (NO$_{2}$, CH$_{2}$NO$_{3})$ at low laser pulse energies, the appearance of NO at intermediate laser pulse energies, and the appearance of hyperthemal CO/N$_{2}$ at mass 28 amu at the highest laser pulse energies. However, these results are somewhat ambiguous, as the NO, NO$_{2}$, and CH$_{2}$NO$_{3}$ intermediates persist and all species become hyperthermal at the higher laser pulse energies. Also, the purported CO/N$_{2}$ signal at 28 amu may be contaminated by silicon ablated from the glass mirror substrate. We plan to mitigate these problems in future experiments by adopting the ``Buelow'' sample configuration which employs an intermediate foil barrier to shield the energetic material from the laser and the laser driven plasma [S.J. Buelow, \textit{et al}., AIP Conf. Proc. \textbf{706}, 1377 (2003)]. [RW PA{\#}4930]   

Relationship between Exploding Bridgewire {\&} Spark Initiation of Low Density PETN

Recent work has shown that the energy delivered after bridgewire burst affects the function time of an EBW detonator. The spark which is formed post bridgewire burst is the means by which the remaining fireset energy is delivered into the detonator. Therefore, by studying the characteristics of spark-gap detonators insight into the contribution of spark initiation to the functioning of EBW detonators may be achieved. Spark initiation of low density explosives consists of; (i) spark formation, (ii) spark interaction with the bed, and (iii) ignition and growth of reaction. Experiments were performed in which beds of an inert simulant were used to study the formation and propagation of sparks. The effect of the spark on inert porous beds was studied over a limited delivered energy range. The disruption of the bed was found to be dependent on the particle size / pore structure of the bed. The effect of spark initiation on a low density PETN bed was then examined, the relationship between delivered energy and function time was found to be the same as for EBW detonators. This necessitated the development of electrical diagnostic techniques to measure the energy delivered to the spark.   

Near-Failure Detonation Behavior of Vapor-Deposited Hexanitrostilbene (HNS) Films

Physical vapor deposition is an attractive method to produce sub-millimeter explosive samples for studying detonation behavior at near-failure conditions. In this work, we examine hexanitrostilbene (HNS) films deposited onto polycarbonate substrates using vacuum thermal sublimation. Deposition conditions are varied in order to alter porosity in the films, and the resulting microstructures are quantified by analyzing ion-polished cross-sections using scanning electron microscopy. The effects of these changes in microstructure on detonation velocity and the critical thickness needed to sustain detonation are determined. The polycarbonate substrates can act as recording plates for detonation experiments, and films near the critical thickness display distinct patterns in the dent tracks that indicate instabilities in the detonation front when approaching failure conditions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.