Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session J5: Spectroscopy and Optical Studies II |
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Chair: David S. Moore, Los Alamos National Laboratory Room: Fairmont Orchid Hotel Plaza III |
Tuesday, June 26, 2007 3:45PM - 4:00PM |
J5.00001: Raman Spectroscopy of RDX Single Crystals under Static Compression Zbigniew Dreger, Yogendra Gupta To gain insight into the high pressure response of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. These results are needed to properly interpret the shock data. Several distinct changes in the spectra were found at 4.0 $\pm $ 0.3 GPa, confirming the $\alpha -\gamma $ phase transition previously observed in polycrystalline samples. Symmetry correlation analyses indicate that the $\gamma $-polymorph may assume a space group isomorphous with a point group $D_{2h}$ with eight molecules occupying the $C_{1}$ symmetry sites, similar to the $\alpha $-phase. It is also proposed that the factor group splitting can account for the observed increase in the number of modes in the $\gamma $-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression. Work supported by DOE and ONR. [Preview Abstract] |
Tuesday, June 26, 2007 4:00PM - 4:15PM |
J5.00002: Raman Spectroscopy Measurements in RDX Single Crystals Shock Compressed Along Different Orientations James Patterson, Zbigniew Dreger, Yogendra Gupta Raman spectroscopy was used to examine the molecular level response of hexahydro-1,3,5-trinitro-$s$-triazine (RDX) single crystals to shock wave compression. Oriented single crystals were shocked under stepwise loading to peak stresses from 3.0 to 5.5 GPa. Changes in the Raman spectra of the CH stretching modes were monitored to determine the stress and orientation dependence of the shock response. Spectral shifts appeared to be similar for three crystal orientations below 3.5 GPa. Significant changes were observed in crystals shocked above 4.5 GPa. These changes were similar to those observed in static pressure measurements, indicating the occurrence of the $\alpha \quad - \quad \gamma $ phase transition in shocked RDX crystals. No measurable orientation dependence in the molecular response of RDX to shock compression was observed up to 5.5 GPa. The phase transition had an incubation time of about 100 ns when RDX was shocked to 5.5 GPa peak stress. The occurrence of the $\alpha \quad - \quad \gamma $ phase transition under shock wave loading has important implications for understanding the onset of chemical decomposition in shocked RDX. Work supported by DOE and ONR. [Preview Abstract] |
Tuesday, June 26, 2007 4:15PM - 4:30PM |
J5.00003: Time-resolved products observed from high pressure deflagrating energetic materials using femtosecond IR spectroscopy J.M. Zaug, E.A. Glascoe, J.C. Crowhurst, L.E. Fried, M.R. Armstrong, C.D. Grant What transient chemical species occur on the nanosecond to microsecond time-scale after an energetic material begins to deflagrate under Chapman-Jouguet conditions? What are the molecular lifetimes of transient species under similar conditions? Using ultrafast infrared spectroscopy to study the transient chemical phenomena of materials encapsulated in high-pressure diamond anvils cells (DACs), these and related questions can be addressed. Here we present a broadband time-resolved IR (TRIR) absorption technique applied to high-pressure deflagrating energetic materials. A 10 nanosecond laser pulse is introduced onto the surface of a high-pressure energetic material. After an induction period of approximately one microsecond the energetic material begins to deflagrate (1500+K) at subsonic velocities radially away from the laser ignited region. A mid-IR femtosecond laser pulse (pulse-gated, 2-10 micron tunable range) is transmitted through the deflagration front. The single-shot mid-IR absorbance is used to detect transient species. Our measurements provide a rigorous test of computational chemistry models. [Preview Abstract] |
Tuesday, June 26, 2007 4:30PM - 4:45PM |
J5.00004: T-Jump/FTIR Studies of Poly-Glycidyl Nitrate (PGN) Pyrolysis. Chad Stoltz, Suhithi Peiris The nitrate-ester binder PGN (poly-glycidyl nitrate) has a high oxygen balance and density, making it one of the most energetic nitrate ester binder systems for potential use. However, when cured using aliphatic isocyanate curing agents it ages poorly, hindering its applicability for use in energetic formulations. Scientists have end-modified the polymer chains with moieities that contain primary --OH groups in an attempt to increase post-curing stability. In an effort to understand the effects of hydroxyl end-modification and isocyanate curing, decomposition of PGN prepolymer has been investigated using T-Jump/FTIR (Fourier Transform Infrared Spectroscopy) of PGN allowing real-time analysis of decomposition gas products under simulated deflagration conditions. Our results identify decomposition products including: CH2O, H2O, CO2, CO, N2O, NO, NO2, HCN and HONO. Kinetic rates relative to CO2 formation lead to calculated activation energies of 22 kcal/mol and 18 kcal/mol. Much higher activation energies (32 kcal/mol) were calculated relative to CH2O formation rates, in agreement with DSC data, indicating that CH2O formation is likely an initial decomposition step while CO2 formation is due to side gas phase reactions. Additional FTIR and optical microscopy studies indicate that condensed phase, backbone scission reactions also occur, causing time delays prior to major gas production. [Preview Abstract] |
Tuesday, June 26, 2007 4:45PM - 5:00PM |
J5.00005: Ultrafast shock wave dynamics at high ambient pressure ($\sim $10 GPa) in a diamond anvil cell Michael Armstrong, Jonathan Crowhurst, Joseph Zaug, Evan Reed, William Howard The measurement and characterization of acoustic phenomena at high pressure is critical to the modelling of planetary dynamics, seismic events, and chemistry in extreme environments. Here we present the results of experiments using ultrafast laser excitation and detection of shock waves in metals in a diamond anvil cell (DAC) at ambient pressure up to at least10 GPa. Using ultrafast interferometry, we directly detect surface motion with less than 1 nm spatial resolution with 100 femtosecond time resolution. Notably, these experiments do not destroy the DAC, allowing multiple shot experiments at multiple pressures for a single DAC load. Such experiments enable examination of acoustic waves with significant strain ($\sim $1{\%}) starting at high ambient static pressure using a convenient, reusable and inexpensive apparatus. [Preview Abstract] |
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