New experiments towards understanding shock sensitivity of energetic materials
Abstract
There has been much debate over the origin of shock sensitivity or the mechanism of shock initiation in energetic materials. The directional dependence of shock initiation was described by Dick (Appl. Phys. Lett., 44, 859-861, 1984) for PETN, and related to the anisotropy of the crystal properties. This was later related to differential heating due to shock compression via the anharmonic crystal potential by Jindal and Dlott (J. Appl. Phys., 83, 5203-5211, 1998). More recently McGrane et al. (J. Phys. Chem. A, 109, 9919-9927, 2005) used variable temperature Raman spectroscopy to determine anharmonic vibrational properties of energetic materials. This was then translated into energy transfer rates and related to shock sensitivity by Ye and Koshi (J. Phys. Chem. B, 110, 18515-18520, 2006). Because of their typically high densities, energetic materials have particularly strong intermolecular interaction energies. It has been postulated by Zhurova et al. (J. Am. Chem. Soc., 128, 14728-14734, 2006) that rupture of such interactions during impact deformation may contribute to the destabilization of the crystal leading to shock initiation. Using advanced X-ray crystallographic techniques, we will experimentally determine the intermolecular interaction energies, and obtain the anharmonic contributions to vibrational atomic motion for a series of solid energetic materials of varying impact sensitivities. Thus we propose a new protocol to add complementary information to previous attempts to understand shock initiation. The interaction energies will be obtained from non-standard X-ray crystallographic techniques to collect highly redundant, extremely accurate diffraction data at various temperatures down to 20K using a prototype helium cooling system and in-house developed data integration software. The diffractometer used for these measurements is based on a Rigaku Mo rotating anode generator with a large cylindrical image plate detector, 2?(max) = 144¡. Analysis of the diffraction data uses an atom-centered multipole expansion to accurately describe the electron density distribution. The topology of the electron density is then used to obtain the intermolecular interaction energies. There is typically minimal anharmonic motion at 20K, although it would be included in the refinement model. To obtain information on anharmonicity, highly redundant data sets will be obtained at several temperatures between 20K and room temperature. As the temperature increases, the effects of thermal motion (both harmonic and anharmonic) become more important and can be successfully included with the multipole parameters in refinement of the overall structural model. This methodology will be applied to crystals of energetic materials with a range of shock sensitivities. This proposal addresses topics of interest to ONR to Òi) establish the connectivity between molecular structure, crystal morphology prediction and synthesis chemistry to provide IM compliant energetic ingredients shock and thermal sensitivity, ii) focus modeling and simulation to predict stable crystal structures/crystal morphology, iii) establish methodologies to model, measure and predict molecular and crystal energetic material response to external shock and thermal modeling, iv) validate design criteria for molecular stability as a function of insensitivity.Ó
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 10, 2016
- Source ID
- N000141612058
Entities
People
- Alan Pinkerton
Organizations
- Office of Naval Research
- United States Navy
- University of Toledo