Publications

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The use of physical vapor deposition is an attractive technique to produce microenergetic samples to study sub-millimeter explosive behavior. Films of the high explosive PETN (pentaerythritol tetranitrate) were deposited through vacuum thermal sublimation. Deposition conditions were varied to understand the effect of substrate cooling capacity and substrate temperature during deposition. PETN films were characterized with surface profilometry and scanning electron microscopy. Detonation velocity versus PETN film thickness was analyzed using a variation of the standard form for analysis of the diameter effect. Results were compared with previous work conducted on PETN films deposited with lower substrate cooling capacity. Seemingly subtle variations in PETN deposition conditions led to differences in detonation behaviors such as critical thickness for detonation, detonation velocity at "infinite" thickness, and the shape of the critical thickness curves. © 2012 American Institute of Physics.

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We have demonstrated the ability to control the microstructure of PETN films deposited using physical vapor deposition by altering the interface between the film and substrate. Evolution of surface morphology, average density, and surface roughness with film thickness were characterized using surface profilometry and scanning electron microscopy. While films on all of the substrates investigated showed a trend toward a lower average density with increasing film thickness, there were significant variations in density, pore size, and surface morphology in films deposited on different substrates.

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In an effort to better understand the structural changes occurring during hydrogen loading of erbium target materials, we have performed in situ D{sub 2} loading of erbium metal (powder) at temperature (450 C) with simultaneous neutron diffraction analysis. This experiment tracked the conversion of Er metal to the {alpha} erbium deuteride (solid-solution) phase and then into the {beta} (fluorite) phase. Complete conversion to ErD{sub 2.0} was accomplished at 10 Torr D{sub 2} pressure with deuterium fully occupying the tetrahedral sites in the fluorite lattice.

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Density Functional Theory (DFT) has over the last few years emerged as an indispensable tool for understanding the behavior of matter under extreme conditions. DFT based molecular dynamics simulations (MD) have for example confirmed experimental findings for shocked deuterium, enabled the first experimental evidence for a triple point in carbon above 850 GPa, and amended experimental data for constructing a global equation of state (EOS) for water, carrying implications for planetary physics. The ability to perform high-fidelity calculations is even more important for cases where experiments are impossible to perform, dangerous, and/or prohibitively expensive. For solid explosives, and other molecular crystals, similar success has been severely hampered by an inability of describing the materials at equilibrium. The binding mechanism of molecular crystals (van der Waals forces) is not well described within traditional DFT. Among widely used exchange-correlation functionals, neither LDA nor PBE balances the strong intra-molecular chemical bonding and the weak inter-molecular attraction, resulting in incorrect equilibrium density, negatively affecting the construction of EOS for undetonated high explosives. We are exploring a way of bypassing this problem by using the new Armiento-Mattsson 2005 (AM05) exchange-correlation functional. The AM05 functional is highly accurate for a wide range of solids, in particular in compression. In addition, AM05 does not include any van der Waals attraction, which can be advantageous compared to other functionals: Correcting for a fictitious van der Waals like attraction with unknown origin can be harder than correcting for a complete absence of all types of van der Waals attraction. We will show examples from other materials systems where van der Waals attraction plays a key role, where this scheme has worked well, and discuss preliminary results for molecular crystals and explosives.

The difficulty of calculating the ambient properties of molecular crystals, such as the explosive PETN, has long hampered much needed computational investigations of these materials. One reason for the shortcomings is that the exchange-correlation functionals available for Density Functional Theory (DFT) based calculations do not correctly describe the weak intermolecular van der Waals' forces present in molecular crystals. However, this weak interaction also poses other challenges for the computational schemes used. We will discuss these issues in the context of calculations of lattice constants and structure of PETN with a number of different functionals, and also discuss if these limitations can be circumvented for studies at non-ambient conditions.

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Films of the high explosive PETN (pentaerythritol tetranitrate) up to 500-μm thick have been deposited through physical vapor deposition, with the intent of creating well-defined samples for shock-initiation studies. PETN films were characterized with microscopy, x-ray diffraction, and focused ion beam nanotomography. These high-density films were subjected to strong shocks in both the out-of-plane and in-plane orientations. Initiation behavior was monitored with high-speed framing and streak camera photography. Direct initiation with a donor explosive (either RDX with binder, or CL-20 with binder) was possible in both orientations, but with the addition of a thin aluminum buffer plate (in-plane configuration only), initiation proved to be difficult. Initiation was possible with an explosively-driven 0.13-mm thick Kapton flyer and direct observation of initiation behavior was examined using streak camera photography at different flyer velocities. Models of this configuration were created using the shock physics code CTH. © 2009 American Institute of Physics.

Three-dimensional shock simulations of energetic materials have been conducted to improve our understanding of initiation at the mesoscale. Vapor-deposited films of PETN and pressed powders of HNS were characterized with a novel three-dimensional nanotomographic technique. Detailed microstructures were constructed experimentally from a stack of serial electron micrographs obtained by successive milling and imaging in a dual-beam FIB/SEM. These microstructures were digitized and imported into a multidimensional, multimaterial Eulerian shock physics code. The simulations provided insight into the mechanisms of pore collapse in PETN and HNS samples with distinctly different three-dimensional pore morphology and distribution. This modeling effort supports investigations of microscale explosive phenomenology and elucidates mechanisms governing initiation of secondary explosives. © 2009 American Institute of Physics.