Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session Z4: Phase Transitions VII: Low Z Materials |
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Chair: Lawrence Hull, Los Alamos National Laboratory, Jonathan Belof, Lawrence Livermore National Laboratory Room: Grand H |
Friday, June 19, 2015 11:15AM - 11:30AM |
Z4.00001: A new crystal phase of ammonium nitrate: a monoclinic distortion of AN-IV Ivan Oleynik, Brad Steele Ammonium nitrate (AN) is a major component of the energetic material ANFO. It is important to understand the high-pressure crystal phases and corresponding phase transitions of AN as its structural polymorphism might affect the energetic performance, including crystal density, detonation velocity and shock initiation of chemical reactions. A new crystal phase of AN is found using first principles evolutionary crystal structure search. It is a monoclinic distortion of phase IV of AN (AN-IV) in the P2$_{\mathrm{1}}$/m space group (AN-P2$_{\mathrm{1}}$ /m). The calculated Raman spectrum of this new phase is consistent with the recently reported experimental Raman spectrum that contains two peaks at high pressures associated with the phase transition. The new phase is calculated to have lower free energy than AN-IV above 11.2 GPa, a pressure close to the experimentally reported phase transition pressure of 17 GPa. The calculated Raman spectra of both AN-P2$_{\mathrm{1}}$ /m and AN-IV as a function of pressure display good agreement with experiment up to 40 GPa. [Preview Abstract] |
Friday, June 19, 2015 11:30AM - 11:45AM |
Z4.00002: Sound velocities in highly-oriented pyrolytic graphite shocked to 18 GPa: Orientational order dependence Marcel Lucas, J.M. Winey, Y.M. Gupta Previous studies on shocked highly oriented pyrolytic graphite (HOPG) indicated a link between the orientational order and the HOPG response above and below the graphite-to-diamond phase transformation onset ($>$18 GPa) [Erskine and Nellis, Nature 349, 317 (1991); Lucas, et al., JAP 114, 093515 (2013)]. To gain insight into this link, the response of ZYH- and ZYB-grade HOPG shocked to 18 GPa was examined by measuring particle velocity profiles and peak state sound speeds in front surface impact experiments. The measured peak stress-particle velocity states are the same for the two HOPG grades. In contrast, the measured sound speeds and longitudinal moduli in the peak states reveal significant differences: the sound speed and modulus for ZYH-grade HOPG increase smoothly with increasing stress, whereas those for ZYB-grade HOPG exhibit softening for peak stresses of 12-17 GPa and an abrupt increase for 17-18 GPa. Comparison with the calculated moduli reveals an elastic instability for ZYB-grade HOPG shocked above 15 GPa. These findings, together with those from the previous reports, suggest that the elastic instability for shocked ZYB-grade HOPG is likely a precursor to the rapid phase transformation observed for this grade. Work supported by DOE/NNSA. [Preview Abstract] |
Friday, June 19, 2015 11:45AM - 12:00PM |
Z4.00003: Observations on the nucleation of ice VII in shock compressed water Samuel J.P. Stafford, David J. Chapman, Daniel E. Eakins, Simon N. Bland The ability of water to freeze into the ice VII phase under dynamic compression is a good example of a liquid to solid phase change. The ice VII is thought to nucleate and grow from the window surfaces in a relatively slow process (on the order of 100ns) that can be seen in wave profiles and a visible darkening of the sample. On silica windows the process is evident but from sapphire surfaces the heterogeneous nucleation appears to be entirely absent and the water remains metastable. To investigate the influence of silica, and under what conditions sapphire might heterogeneously nucleate ice VII, we present an experimental technique incorporating multiple liquid targets diagnosed with PDV and high speed imaging of the nucleation process from a variety of surfaces and additives. [Preview Abstract] |
Friday, June 19, 2015 12:00PM - 12:15PM |
Z4.00004: A molecular dynamic investigation for shock induced phase transition of water Nilanjan Mitra, Anupam Neogi Atomistic equilibrium molecular dynamics (EMD) was carried out to investigate shock induced phase transition of bulk liquid water. Multi-scale shock technique (MSST) was utilized to investigate low ($U_S=2.5 km/s$) to strong ($U_S=6.5 km/s$) intensity shock response on an extended flexible three point model up to 100 ns. The thermodynamic pathway of phase transition from liquid water to ice VII was investigated using temporal variation of thermodynamic state variables, power spectrum analyses of O-H bond vibration along with temporal evolution of pair correlation function between O-O, O-H and H-H atoms. Static structure factor along with pair-distribution function extended up to 20 {\AA} was calculated and compared against the ideal ice VII to get information regarding long range ordering. Bragg reflection at different crystal planes were evaluated to investigate percentage of crystallinity of the shocked sample. Specific questions answered in this work involves: What is the exact time frame after the passage of shock at certain intensity in which nucleation of solid phase can be observed? Is it a complete or partial phase transition? Are external nucleators essential for this transformation? What is the percentage of crystallinity of the nucleated phase? [Preview Abstract] |
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