David Oschwald

Thermal decomposition of energetic materials viewed via dynamic x-ray radiography

We describe the evolution of solid density, leading up to ignition in the slow thermal decomposition of the solid organic secondary explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. This work describes an x-ray radiographic diagnostic, allowing the study of solid density in a fully encased explosive heated to thermal explosion. The result of this study is the ability to observe and manipulate the ignition volume in a thermal explosion.

X-ray transmission movies of spontaneous dynamic events

We describe a new x-ray radiographic imaging system which allows for continuous x-ray transmission imaging of spontaneous dynamic events. We demonstrate this method on thermal explosions in three plastic bonded formulations of the energetic material octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. We describe the x-ray imaging system and triggering developed to enable the continuous imaging of a thermal explosion.

Fast Internal Temperature Measurements in PBX9501 Thermal Explosions

We have made spatially and temporally resolved temperature measurements internal to a thermal explosion in PBX9501, which is a plastic bonded explosive composed of 95% HMX and 2.5% estane mixed with 2.5% nitroplasticizer (BDNPA/F). In order to study the evolution of ignition in a thermally treated piece of explosive, we have pushed the time resolution of several different temperature diagnostics. In this paper, we will discuss the details of the time response of these diagnostics including temperature uncertainties. The temperature measurements are made both by thermocouples with corrections applied to compensate for the thermocouple response time and with optical pyrometry. An additional goal of adding high energy radiography diagnostics to future experiments has motivated an effort to synchronize thermal explosions to an external clock. In this paper, I discuss our current capabilities for controlling and measuring the development of an ignition within a piece of heated PBX9501.

Observations on the Mechanism of Reaction Propagation in Metastable Intermolecular Composites

Metastable Intermolecular Composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. They are a subclass of materials known as thermites. The mechanism responsible for the propagation of reaction in loose compacts is not well understood. We have conducted a series of experiments using high‐speed photography and pressure transducers in an attempt to identify the dominant mechanism. We studied a mixture of aluminum and molybdenum trioxide. Of the four possible candidates (radiation, convection, conduction, and acoustic/compaction), these preliminary studies identify convection as the most likely. However, the extent of contribution of the other modes is not yet known and this will receive further study.

Measurement of the Specific Area of HMX and PBX 9501 by Physical Adsorption

We present physical adsorption data on HMX and PBX 9501. Adsorption isotherms were obtained by pressure measurements using Krypton at liquid nitrogen temperatures. The data were analyzed to obtain the specific area of the samples. We observed a particle size dependence in the HMX crystals with small crystals having specific areas consistent with a solid geometric particle with the aspect ratio of an oblate spheroid. The larger particles, however, had specific areas above those predicted by the simple geometric surface area. This implies that the surface area of the larger particles is dominated by internal porosity and is relatively independent of particle size above approximately 100 micron diameter. The specific area of PBX 9501 was measured and agreed with that predicted by a calculation based on the particle size distribution and measured specific area of the component crystals.

Morphology Changes during Thermal Decomposition of PBX9501

Our goal is to be able to predict the morphology of PBX9501 as a function of time at temperature. This is necessary in order to be able to predict the behavior of material that has been subjected to a known temperature trajectory. We have begun by studying the mechanism of the initial solid state phase transition between the beta and delta phases of HMX. This leads to a volume expansion and a large degree of mechanical damage in the material. On continued heating above the solid‐solid phase transition, a solid to gas transformation occurs. We have monitored the solid to gas transition as a function of isothermal temperature and particle size in order to address mechanistic questions concerning chemical production of gas phase species by surface regression or internal pore formation. We have also performed experiments to characterize the morphology changes in the material caused by the loss of solid mass.

Internal sub-sonic burning during an explosion viewed via dynamic X-ray radiography

We observe internal convective and conductive burn front propagation and solid consumption subsequent to thermal ignition for plastic bonded formulations of the solid organic secondary explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-triamino-2,4,6-trinitrobenzene. This work describes x-ray radiographic diagnostics enabling the study of solid density in a fully encased explosive during internal burning subsequent to ignition. The result of this study is the ability to directly observe and measure rates of energy release during a thermal explosion.

Initial results and designs of dual-filter and plenoptic imaging for high-temperature plasmas

Mass injection has found new applications in magnetic fusion including edge-localized-mode control. Better understanding of injected-mass-plasma interactions requires spatially and temporally resolved diagnostics that can characterize the dynamics of the mass interactions with plasmas. Fast imaging can be used to characterize the ionization dynamics such as the propagation of the ionization front, which moves at the thermal sound or higher speed, and mixing of the neutral atoms with the ambient plasma. Multi-wavelength spectral imaging is promising since different parts of the plasma give different spectral signatures. Here we describe a dual-spectral imaging technique based on a monochromatic camera sensor and filters with two passing optical wavelengths. The method is shown to improve image contrast, and it compares favorably with alternatives such as color cameras and methods using a filter wheel. Further improvements through relative filter area ratios and plenoptic imaging are possible. The initial results from EAST and plenoptic imaging are also included.