Bulletin of the American Physical Society
16th APS Topical Conference on Shock Compression of Condensed Matter
Volume 54, Number 8
Sunday–Friday, June 28–July 3 2009; Nashville, Tennessee
Session L5: PC-2: Transitions at High Pressure |
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Chair: Roger Minich, Lawrence Livermore National Laboratory Room: Cheekwood GH |
Tuesday, June 30, 2009 3:30PM - 3:45PM |
L5.00001: P. W. Bridgman's Contributions to the Foundations of Shock Physics W.J. Nellis Based on his 50-year career in static high pressure, P. W. Bridgman (PWB) is the father of high-pressure physics. What is not generally recognized is that Bridgman was also intimately connected with the foundations of shock compression as a scientific tool and he predicted major events in shock research that occurred up to 40 years after his death. In 1956 a phase transition in shocked Fe was reported at 13 GPa. PWB said a phase transition could not occur in a microsecond, thus setting off a controversy. The scientific legitimacy of shock compression resulted 5 years later when static-pressure researchers confirmed with x-ray diffraction the existence of a high-pressure Fe phase. PWB gave Altshuler the idea of using nuclear explosives to generate super high pressures, which morphed into giant lasers. PWB anticipated combining static and shock methods, which day is done with with diamond anvil cell/laser. One variation of that pre-compression method is a reverberating shock in which the first shock ``pre-compresses'' a soft sample and subsequent reverberations compress it isentropically. [Preview Abstract] |
Tuesday, June 30, 2009 3:45PM - 4:00PM |
L5.00002: Anisotropic elasto-plastic transition of MgO single crystal Tsutomu Mashimo, Mitsuru Murai, Naoto Kawayanagi The elasto-plastic transition of ceramics has not been well understood, and the yield mechanism has been further unknown. Magnesium oxide (MgO) has been used as a pressure -scale material, and the Hugoniot data have been measured on single crystal and polycrystal. In this study, we performed the Hugoniot-measurement experiments on MgO single crystals for $<$100$>$ and $<$110$>$ axis directions, etc., using the powder gun and two-stage light gas gun, to study the elasto-plastic transition and the equation of state (EOS). The Hugoniot data were measured by the inclined-mirror method combined with the long-pulsed dye laser and mirror-rotating type streak camera. The Hugoniot-elastic limits along $<$110$>$ axis were much larger than those along $<$100$>$ axis, and the difference increased with driving stress. This suggests that the dynamic strength should be carefully considered to draw the EOS of MgO. In the plastic region, the present Hugoniot data showed some differences from the previous data. [Preview Abstract] |
Tuesday, June 30, 2009 4:00PM - 4:30PM |
L5.00003: ABSTRACT WITHDRAWN |
Tuesday, June 30, 2009 4:30PM - 4:45PM |
L5.00004: Entropy-dominated Dissipation in Sapphire Compressed Dynamically from 14 to 87 GPa W.J. Nellis, G.I. Kanel, S.V. Razorenov, A.S. Savinykh, A.M. Rajendran States reached by dynamic compression are governed by free energy in which dissipative energy is --TS, where T is temperature and S is entropy. In a liquid like Ar effective pair interaction enegy is $\sim $0.01 ev. As a result Ar is relatively compressible with a shock rise time of $\sim $0.5 ps and 2.2 fold compression at a T of 14,000 K at 50 GPa. Thermal energy is $\sim $90{\%} of shock energy. Entropy changes are small in a shocked fluid and dissipative energy appears primarily as T. We have measured wave profiles of sapphire with elastic strength of $\sim $15 GPa in three different crystal orientations at shock stresses of 14, 24, and 87 GPa. At 24 GPa the rise time of the plastic wave is $\sim $300 ns, 5 orders of magnitude greater than in liquid Ar. At 50 GPa sapphire is compressed 1.1 fold to a T of $\sim $500 K. Thermal pressures are negligible and bond strengths are $\sim $1 ev, about 2 orders of magnitude greater than in Ar. Bonds in sapphire probably break over $\sim $10s of ns. This long rise time causes quasi-isentropic compression with negligible shock heating. Dissipative energy goes primarily into the entropy of disordering the strong 3-D lattice, rather than into T. [Preview Abstract] |
Tuesday, June 30, 2009 4:45PM - 5:00PM |
L5.00005: Stepwise shock compression with relation to phase diagram of C$_{70}$ fullerite Tatiana Borodina, Konstantin Khishchenko, Vladimir Milyavskiy Phase transitions of C$_{70}$ fullerite with different initial phase compositions under stepwise shock compression are experimentally studied to pressure of 52 GPa and temperature of about 1700 K. The crystalline phase of fullerite C$_{70}$ with a hexagonal close-packed structure remain practically unchanged under stepwise shock loading up to pressure of 8 GPa. Shock-induced transformation of the hexagonal phase into the face centered cubic phase is observed at pressures in the range 9 to 23.5 GPa. The amount of transformed material increases with the shock intensity. Upon further increase of the shock pressure, the destruction of C$_{70}$ molecules occurs. This destruction is accompanied by a formation of the graphite phase of carbon. Pressure--temperature history of C$_{70}$ specimens is estimated with the use of equation of state C$_{60}$ fullerite [K.V. Khishchenko et al. Diamond and Relat. Mat. 2007 (16) 1204]. Analyzing this pressure-temperature history along with the tentative phase diagram of C$_{70}$ [B. Sundqvist. Advances in Phys. 1999 (48) 1], we conclude that the observed conversion of the hexagonal to the cubic phase can not be described in the framework of this diagram. [Preview Abstract] |
Tuesday, June 30, 2009 5:00PM - 5:15PM |
L5.00006: Coupling of Atomistic and Meso-scale Phase-field Modeling of Rapid Solidification J. Belak, P.E.A. Turchi, M.R. Dorr, D.F. Richards, J.-L. Fattebert, M.E. Wickett, F.H. Streitz, M. Tang, N. Moelans Recently, phase-field models have been introduced to model the crystallography during polycrystal microstructure evolution [1,2]. Here, we assess these models with molecular dynamics and phase-field simulations that overlap in time and space. Large parallel computers have enabled MD simulations of sufficient scale to observe the formation of realistic microstructure during pressure driven solidification [3]. We compare the two methods by calculating the phase field order parameter (quaternion) from the atomic coordinates and drive the evolution with the MD. Results will be presented for the solidification of tantalum. [1] R. Kobayashi and J.A. Warren, Physica A, \textbf{356}, 127-132 (2005). [2] T. Pusztai, G. Bortel and L. Granasy, Europhys. Lett, 71, 131-137 (2005). [3] F. H. Streitz, J. N. Glosli, and M. V. Patel, Phys. Rev. Lett. 96, 225701 (2006). [Preview Abstract] |
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