Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session B2: Energetic Materials I |
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Chair: Von Whitley, Naval Surface Warfare Center, Indian Head Room: Hyatt Regency Constellation C |
Monday, August 1, 2005 9:00AM - 9:15AM |
B2.00001: Links between crystal morphology and shock sensitivity in granular RDX Helen Czerski, Martin Greenaway, William Proud It has been known for several years that equivalent batches of the secondary explosive RDX can show significant variations in shock sensitivity. In particular, the relatively recent emergence of reduced sensitivity (RS-) RDX has focused attention to grain-level defects and heterogeneities. Although the role of particle size is well-known, there have been some papers reporting shock sensitivity correlating with marginal changes in grain roughness or density and the presence of internal voids or trapped gas. The aim of this research is correlate shock sensitivity with crystal morphology and defects for a large number of classes, types and manufacturers of RDX. A number of techniques have been employed, including volumetric adsoption (BET method), environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM) and optical microscopy. This range of methods aims to yield quantitative data on internal voids, surface roughness and surface area to volume ratio. Shock sensitivity is quantified using small-scale gap tests. [Preview Abstract] |
Monday, August 1, 2005 9:15AM - 9:30AM |
B2.00002: First-principles studies of PETN molecular crystal vibrational frequencies under high pressure Warren Perger, Jijun Zhao, J.M. Winey, Y.M. Gupta The vibrational frequencies of the PETN molecular crystal were calculated using the first-principles CRYSTAL03 program which employs an all-electron LCAO approach and calculates analytic first derivatives of the total energy with respect to atomic displacements. Numerical second derivatives were used to enable calculation of the vibrational frequencies at ambient pressure and under various states of compression. Three different density functionals, B3LYP, PW91, and X3LYP were used to examine the effect of the exchange-correlation functional on the vibrational frequencies. The pressure-induced shift of the vibrational frequencies will be presented and compared with experiment. The average deviation with experimental results is shown to be on the order of 2-3{\%}, depending on the functional used. [Preview Abstract] |
Monday, August 1, 2005 9:30AM - 9:45AM |
B2.00003: First-principles studies of RDX crystals under compression Jijun Zhao, J.M. Winey, Y.M. Gupta, Warren Perger Using the plane-wave pseudopotential technique (CASTEP program) within the generalized gradient approximation (GGA), we have performed first principles calculations to examine RDX crystals under hydrostatic and uniaxial strain compression. Crystal structures were calculated at ambient conditions for the orthorhombic structure and a vibrational analysis was performed. Second order elastic constants such as C$_{11}$, C$_{22}$, and C$_{33}$ were calculated by applying very small uniaxial strains along [100], [010], and [001] orientations. Unit cell dimensions and fractional coordinates of the RDX crystal were determined under hydrostatic and uniaxial compressions up to about 4 GPa. Compression-induced changes in the RDX molecular geometry for these two loading conditions are discussed. [Preview Abstract] |
Monday, August 1, 2005 9:45AM - 10:00AM |
B2.00004: Vibrational Spectroscopic Studies of Reduced-Sensitivity RDX under Static Compression Chak Wong, Jared Gump Explosives formulations with Reduced- Sensitivity RDX showed reduced shock sensitivity using NOL Large Scale Gap Test, compared with similar formulations using normal RDX. Molecular processes responsible for the reduction of sensitivity are unknown and are crucial for formulation development. Vibrational spectroscopy at static high pressure may shed light to the mechanisms responsible for the reduced shock sensitivity as shown by the NOL Large Scale Gap Test. SIRDX, a form of Reduced- Sensitivity RDX, was subjected to static compression at ambient temperature in a Merrill-Bassett sapphire cell from ambient to about 6 GPa. The spectroscopic techniques used were Raman and Fourier-Transformed IR (FTIR). The pressure dependence of the Raman mode frequencies of SIRDX was determined and compared with that of normal RDX. The behavior of SIRDX near the pressure at which normal RDX, at ambient temperature, undergoes a phase transition from the $\alpha $ to the $\gamma $ polymorph will be presented. Implications to the reduction in sensitivity will be discussed. [Preview Abstract] |
Monday, August 1, 2005 10:00AM - 10:15AM |
B2.00005: Time-Resolved, Optical Spectroscopy Measurements of RDX Single Crystals Shocked along Different Orientations James Patterson, Naoki Hemmi, Yogendra Gupta Optical transmission measurements were obtained on RDX crystals shocked along three different orientations with stepwise loading up to 5.5 GPa. These orientations were chosen to activate one, both, or none of the known slip systems of RDX. All orientations exhibited broadband visible extinction as the shock front propagated through the RDX crystal. Crystals were shocked to peak stresses both above and below 3.8 GPa, the pressure for the $\alpha $ to $\gamma $ phase transition obtained in static compression studies. For some orientations, the extinction did not increase smoothly, but exhibited transient changes before approaching a final level. The nature of this transient varied depending on the crystalline orientation and the peak stress reached during the experiment. Three general transient behaviors have been observed: a smooth increase to a final extinction level, a step-wise increase to a final level, or an overshoot in the absorption before reducing to a final level. Work supported by DOE and ONR. [Preview Abstract] |
Monday, August 1, 2005 10:15AM - 10:30AM |
B2.00006: Comparison of Reaction Kinetics of iRDX vs. RDX at High Pressure Jared Gump, Suhithi Peiris Due to the close proximity of personnel and ammunition on military ships, the Navy is very interested in decreasing the sensitivity of its energetic ingredients. RDX is a common ingredient in many munitions, and there is now a less sensitive version of RDX, known as iRDX developed by SNPE. The testing of this new material is of prime importance, especially at high pressure. Samples of iRDX (from SNPE) and standard RDX were loaded into gem anvil cells (GACs) with cubic zirconia gems. Laser initiation experiments were performed with a 5ns pulsed ND:YAG laser set at 532 nm wavelength. The samples were exposed to various pressures and laser fluences. A pulsed UV/Vis arc lamp was used as a light source and changes in transmittance through the sample were monitored during initiation. Both samples were initially transparent under pressure. Upon initiation samples become opaque due to dark intermediate products, then clear as the final gaseous products are formed. A comparison of the reaction times for iRDX and RDX samples will be presented under various pressure and fluence conditions. [Preview Abstract] |
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