Alexander S. Tappan

Characterization of pore morphology in molecular crystal explosives by focused ion-beam nanotomography

The initiation and detonation properties of explosives are often empirically correlated to density, surface area, and particle size. Although these correlations are sometimes used successfully to predict the performance of bulk samples, the data are spatially averaged, which unfortunately muddles information critical to understanding fundamental processes. Density and surface area are essentially an indirect measure of porosity, which is arguably a more appropriate metric in many applications. We report the direct characterization of porosity in polycrystalline molecular crystal explosives by focused ion beam nanotomography, a technique that is typically reserved for robust materials such as ceramics and metals. The resulting three-dimensional microstructural data are incredibly rich, promising a substantial advance in our ability to unravel the processes governing initiation and detonation of molecular crystal explosives. In a larger context, this work demonstrates that focused ion beam nanotomography may be successfully extended to the investigation of nanoscale porosity in other molecular crystal or polymer materials.

MESOSCALE SIMULATIONS OF SHOCK INITIATION IN ENERGETIC MATERIALS CHARACTERIZED BY THREE‐DIMENSIONAL NANOTOMOGRAPHY

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.

MICROENERGETICS: COMBUSTION AND DETONATION AT SUB‐MILLIMETER SCALES

At Sandia National Laboratories, we have coined the term “microenergetics” to describe sub‐millimeter energetic material studies aimed at gaining knowledge of combustion and detonation behavior at the mesoscale. Our approach is to apply technologies developed by the microelectronics industry to fabricate test samples with well‐defined geometries. Substrates have been fabricated from materials such as silicon and ceramics, with channels to contain the energetic material. Energetic materials have been loaded into the channels, either as powders, femtosecond laser‐micromachined pellets, or as vapor‐deposited films. Ignition of the samples has been achieved by simple hotwires, integrated semiconductor bridges, and also by lasers. Additionally, grain‐scale patterning has been performed on explosive films using both oxygen plasma etching and femtosecond laser micromachining. We have demonstrated simple work functions in microenergetic devices, such as piston motion, which is also a relevant diagnostic to examin...

Energy Transfer Between Coherently Delocalized States in Thin Films of the Explosive Pentaerythritol Tetranitrate (PETN) Revealed by Two-Dimensional Infrared Spectroscopy

Pentaerythritol tetranitrate (PETN) is a common secondary explosive and has been used extensively to study shock initiation and energy propagation in energetic materials. We report 2D IR measurements of PETN thin films that resolve vibrational energy transfer and relaxation mechanisms. Ultrafast anisotropy measurements reveal a sub-500 fs reorientation of transition dipoles in thin films of vapor-deposited PETN that is absent in solution measurements, consistent with intermolecular energy transfer. The anisotropy is frequency dependent, suggesting spectrally heterogeneous vibrational relaxation. Cross peaks are observed in 2D IR spectra that resolve a specific energy transfer pathway with a 2 ps time scale. Transition dipole coupling calculations of the nitrate ester groups in the crystal lattice predict that the intermolecular couplings are as large or larger than the intramolecular couplings. The calculations match well with the experimental frequencies and the anisotropy, leading us to conclude that the observed cross peak is measuring energy transfer between two eigenstates that are extended over multiple PETN molecules. Measurements of the transition dipole strength indicate that these vibrational modes are coherently delocalized over at least 15-30 molecules. We discuss the implications of vibrational relaxation between coherently delocalized eigenstates for mechanisms relevant to explosives.

An evaluation of complementary approaches to elucidate fundamental interfacial phenomena driving adhesion of energetic materials.

Cohesive Hamaker constants of solid materials are measured via optical and dielectric properties (i.e., Lifshitz theory), inverse gas chromatography (IGC), and contact angle measurements. To date, however, a comparison across these measurement techniques for common energetic materials has not been reported. This has been due to the inability of the community to produce samples of energetic materials that are readily compatible with contact angle measurements. Here we overcome this limitation by using physical vapor deposition to produce thin films of five common energetic materials, and the contact angle measurement approach is applied to estimate the cohesive Hamaker constants and surface energy components of the materials. The cohesive Hamaker constants range from 85zJ to 135zJ across the different films. When these Hamaker constants are compared to prior work using Lifshitz theory and nonpolar probe IGC, the relative magnitudes can be ordered as follows: contact angle>Lifshitz>IGC. Furthermore, the dispersive surface energy components estimated here are in good agreement with those estimated by IGC. Due to these results, researchers and technologists will now have access to a comprehensive database of adhesion constants which describe the behavior of these energetic materials over a range of settings.

Spectroscopic analysis of time-resolved emission from detonating thin film explosive samples

We report a series of time-resolved spectroscopic measurements that aim to characterize the reactions that occur during shock initiation of high explosives. The experiments employ time- and wavelength-resolved emission spectroscopy to analyze light emitted from detonating thin explosive films. This paper presents analysis of optical emission spectra from hexanitrostilbene (HNS) and pentaerythritol tetranitrate (PETN) thin film samples. Both vibrationally resolved and broadband emission features are observed in the spectra and area as electronic transitions of intermediate species.

Geometry effects on detonation in vapor-deposited hexanitroazobenzene (HNAB).

Physical vapor deposition is a technique that can be used to produce explosive films with controlled geometry and microstructure. Films of the high explosive hexanitroazobenzene (HNAB) were deposited by vacuum thermal evaporation. HNAB deposits in an amorphous state that crystallizes over time into a polycrystalline material with high density and a consistent porosity distribution. In previous work, we evaluated detonation critical thickness in HNAB films in an effectively infinite slab geometry with insignificant side losses. In this work, the effect of geometry on detonation failure was investigated by performing experiments on films with different thicknesses, while also changing lateral dimensions such that side losses became significant. The experimental failure thickness was determined to be 75.5 µm and 71.6 µm, for 400 µm and 1600 µm wide HNAB lines, respectively. It follows from this that the minimum width to achieve detonation behavior representing an infinite slab configuration is greater than 400 µm.Physical vapor deposition is a technique that can be used to produce explosive films with controlled geometry and microstructure. Films of the high explosive hexanitroazobenzene (HNAB) were deposited by vacuum thermal evaporation. HNAB deposits in an amorphous state that crystallizes over time into a polycrystalline material with high density and a consistent porosity distribution. In previous work, we evaluated detonation critical thickness in HNAB films in an effectively infinite slab geometry with insignificant side losses. In this work, the effect of geometry on detonation failure was investigated by performing experiments on films with different thicknesses, while also changing lateral dimensions such that side losses became significant. The experimental failure thickness was determined to be 75.5 µm and 71.6 µm, for 400 µm and 1600 µm wide HNAB lines, respectively. It follows from this that the minimum width to achieve detonation behavior representing an infinite slab configuration is greater than 4...

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

Hexanitrostilbene (HNS) films were deposited onto polycarbonate substrates using vacuum thermal sublimation. The deposition conditions were varied in order to alter porosity in the films, and the resulting microstructures were 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 were determined. The polycarbonate substrates also acted as recording plates for detonation experiments, and films near the critical thickness displayed distinct patterns in the dent tracks that indicate instabilities in the detonation front when approaching failure conditions.

MICROENERGETIC SHOCK INITIATION STUDIES ON DEPOSITED FILMS OF PETN

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 p...

Ultrafast Shock-Induced Reactions in Pentaerythritol Tetranitrate Thin Films

The chemical and physical processes involved in the shock-to-detonation transition of energetic solids are not fully understood due to difficulties in probing the fast dynamics involved in initiation. Here, we employ shock interferometry experiments with sub-20-ps time resolution to study highly textured (110) pentaerythritol tetranitrate (PETN) thin films during the early stages of shock compression using ultrafast laser-driven shock wave methods. We observe evidence of rapid exothermic chemical reactions in the PETN thin films for interface particle velocities above ∼1.05 km/s as indicated by shock velocities and pressures well above the unreacted Hugoniot. The time scale of our experiment suggests that exothermic reactions begin less than 50 ps behind the shock front for these high-density PETN thin films. Thermochemical calculations for partially reacted Hugoniots also support this interpretation. The experimentally observed time scale of reactivity could be used to narrow possible initiation mechanisms.