Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session V6: Equation of State VI |
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Chair: Travis Sjostrom, Los Alamos National Laboratory Room: Regency Ballroom E |
Thursday, July 13, 2017 3:45PM - 4:00PM |
V6.00001: Uranium phase diagram from first principles Alexey Yanilkin, Ivan Kruglov, Kirill Migdal, Artem Oganov, Pavel Pokatashkin, Oleg Sergeev The work is devoted to the investigation of uranium phase diagram up to pressure of 1 TPa and temperature of 15 kK based on density functional theory. First of all the comparison of pseudopotential and full potential calculations is carried out for different uranium phases. In the second step, phase diagram at zero temperature is investigated by means of program USPEX and pseudopotential calculations. Stable and metastable structures with close energies are selected. In order to obtain phase diagram at finite temperatures the preliminary selection of stable phases is made by free energy calculation based on small displacement method. For remaining candidates the accurate values of free energy are obtained by means of thermodynamic integration method (TIM). For this purpose quantum molecular dynamics are carried out at different volumes and temperatures. Interatomic potentials based machine learning are developed in order to consider large systems and long times for TIM. The potentials reproduce the free energy with the accuracy 1-5 meV/atom, which is sufficient for prediction of phase transitions. The equilibrium curves of different phases are obtained based on free energies. Melting curve is calculated by modified Z-method with developed potential. [Preview Abstract] |
Thursday, July 13, 2017 4:00PM - 4:15PM |
V6.00002: A Four Phase DFT Based Aluminum Equation of State Scott Crockett, Travis Sjostrom, Sven Rudin We present a description of how density functional theory calculations are utilized as a constraint to basic equation of state modeling in order to generate accurate multiphase equation of state. Three solid phases (fcc, hcp, bcc) and a fluid phase are combined to generated an aluminum equation of state. Density functional calculations of the cold curves and phonons were performed for the solid phases. Quantum molecular dynamic calculations where used for the fluid region. We show comparisons of the ab initio work validated against experiment. [Preview Abstract] |
Thursday, July 13, 2017 4:15PM - 4:30PM |
V6.00003: Phase equilibria computations of multicomponent mixtures at specified internal energy and volume Philip C. Myint, Albert L. Nichols III, H. Keo Springer Hydrodynamic simulation codes for high-energy density science applications often use internal energy and volume as their working variables. As a result, the codes must determine the thermodynamic state that corresponds to the specified energy and volume by finding the global maximum in entropy. This task is referred to as the isoenergetic-isochoric flash. Solving it for multicomponent mixtures is difficult because one must find not only the temperature and pressure consistent with the energy and volume, but also the number of phases present and the composition of the phases. The few studies on isoenergetic-isochoric flash that currently exist all require the evaluation of many derivatives that can be tedious to implement. We present an alternative approach that is based on a derivative-free method: particle swarm optimization. The global entropy maximum is found by running several instances of particle swarm optimization over different sets of randomly selected points in the search space. For verification, we compare the predicted temperature and pressure to results from the related, but simpler problem of isothermal-isobaric flash. All of our examples involve the equation of state we have recently developed for multiphase mixtures of the energetic materials HMX, RDX, and TNT. [Preview Abstract] |
Thursday, July 13, 2017 4:30PM - 5:00PM |
V6.00004: Equation of State Model for Delocalization of 4$f$ Electrons in Ce Invited Speaker: Carl Greeff The elemental metal Ce has an isostructural phase transition under pressure, from the low density $\gamma$ phase to the high density $\alpha$ phase, with a line of first order transitions terminating at a critical point. This leads to anomalies under dynamic compression such as large dissipative heating and non-monotonic variation of the bulk modulus with pressure. The transition is generally understood to be associated with delocalization of the 4$f$ electron states. The thermodynamics of the low density $\gamma$ phase are accurately described by a combination of phonons, itinerant electrons, and a local 4$f$ electron on each atom. The 14-fold degeneracy of the 4$f$ state is broken by a $\sim 260$~meV spin orbit splitting and a $\sim 17$~meV crystal field splitting. I will describe our EOS model, which allows for a continuously varying degree of localization as a function of compression. I will show comparisons with static and dynamic data. Finally, I will comment on the prospects for an analogous phase transition in the liquid. [Preview Abstract] |
Thursday, July 13, 2017 5:00PM - 5:15PM |
V6.00005: Shock Compression Response of Calcium Fluoride (CaF$_2$) Seth Root The fluorite crystal structure is a textbook lattice that is observed for many systems, such as CaF$_2$, Mg$_2$Si, and CeO$_2$. Specifically, CaF$_2$ is a useful material for studying the fluorite system because it is readily available as a single crystal. Under static compression, CaF$_2$ is known to have at least three solid phases: fluorite, cotunnite, and a Ni$_2$In phase. Along the Hugoniot CaF$_2$ undergoes a fluorite to cotunnite phase transition [1], however, at higher shock pressures it is unknown whether CaF$_2$ undergoes another solid phase transition or melts directly from the cotunnite phase. In this work, we conducted planar shock compression experiments on CaF$_2$ using Sandia's Z-machine and a two-stage light gun up to 900 GPa. In addition, we use density functional theory (DFT) based quantum molecular dynamics (QMD) simulations to provide insight into the CaF$_2$ state along the Hugoniot.\\ \\$[1]$ P. Kalita et al., \textit{Dynamic XRD, Shock, and Static Compression of CaF$_2$} This conference.\\ \\In collaboration with: Michael Desjarlais, Ray Lemke, Patricia Kalita, Scott Alexander, Sandia National Laboratories.\\ \\Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energyâ€™s National Nuclear Security Administration under contract DE-AC04-94AL850 [Preview Abstract] |
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