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 B2: Equation of State I: Sound Speed and Grüneisen Parameter |
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Chair: Guruswami Ravichandran, California Institute of Technology, Thomas Duffy, Princeton University Room: Grand F |
Monday, June 15, 2015 9:15AM - 9:30AM |
B2.00001: Properties of Ar at Extreme Compression Carl Greeff I will discuss results from DFT calculations on the equation of state of Ar. These include static lattice, phonon, and first principles molecular dynamics calculations on solid and liquid phases. The calculations extend beyond the metallization transition, which is thought to occur near 500 GPa. Metallization leads to, at most, weak anomalies in the EOS. Various measures of the Gr\"uneisen parameter, $\gamma$, are compared. It is found that $\gamma$ decreases rapidly under compression, and reaches values less than 1/2 at the highest densities. The high pressure melting curve will be discussed. [Preview Abstract] |
Monday, June 15, 2015 9:30AM - 9:45AM |
B2.00002: An alternative to Mie-Gr\"{u}neisen William Anderson The Mie-Gr\"{u}neisen thermal equation of state is probably the most-used EOS form in high-pressure physics, because of its simplicity and the fact that it provides a reasonable description of the thermal energy while keeping the overall EOS analytic. However, use of the Gr\"{u}neisen parameter, $\gamma $, places restrictions on forms that can be used for the specific heat, while the volume dependence often ascribed to $\gamma $ can be too simplistic. These shortcomings complicate attempts to realistically include explicit temperature dependence in models. I suggest an alternative using the Einstein thermal model with a correction term. The Einstein characteristic temperature $\theta $ can be obtained as a function of volume using the bulk modulus and Poisson ratio. Combination with any analytic and differentiable cold curve formulation results in a complete analytic EOS with volume and temperature as the natural independent variables. In the case of the quasiharmonic approximation with negligible electronic or magnetic thermal energy terms, internal energy or pressure can replace temperature as the second independent variable while maintaining analyticity. Use of this model is restricted to temperatures above 0.39$\theta $, where the Einstein and Debye specific heats are quantitatively similar. [Preview Abstract] |
Monday, June 15, 2015 9:45AM - 10:15AM |
B2.00003: Hugoniot Experiments with unsteady waves Invited Speaker: Dayne Fratandunono Recent development of transparent shock wave standard materials, such as quartz, enables continuous tracking of shock waves using optical velocimetry, thus providing information on shock wave steadiness and pressure perturbations in the target. From a first order perturbation analysis, we develop a set of analytical formulas that connect the pressure perturbations at the drive surface to the shock velocity perturbations observed in measurements. With targets that incorporate a calibrated transparent witness material, such as quartz, and with the analytical formulas describing the perturbation response, it is possible to determine the sound speed and Gruneisen coefficient of an unknown sample by using evolution of the non-steady perturbations as a probe. These formulas are used to improve the accuracy of traditional shock wave impedance match Hugoniot experiments of opaque samples driven with non-steady waves. The method is well suited for use in laser-based Hugoniot experiments where the shock waves can be unsteady, with fluctuations and/or accelerating or decelerating trends. We apply this technique to recent laser-based Hugoniot measurements and the results are presented. [Preview Abstract] |
Monday, June 15, 2015 10:15AM - 10:30AM |
B2.00004: Complete Forms of the Mie-Gruneisen Equation of State Olivier Heuze The Mie-Gr\"uneisen equation of state is often used in hydrocode simulations to model condensed materials at high pressure. It is defined in an incomplete form P(V,E) which does not allow access to temperature and entropy. We have extended it to the complete form S(V,E) which is the combination of three independent models: an isentropic or isotherm potential, Debye Temperature from which we derive the Gr\"uneisen coefficient, and a new function which gives the specific heat by derivation. Then, we access to all the thermodynamic properties. Former versions were limited by a constant heat capacity. The new function introduced here has overcome this limitation and allows now to extent to Einstein, Debye or other more accurate models for heat capacity. Moreover, its Legendre transform provides the F(V,T) form. The combinations with published models for the potential and Debye temperature are unlimited. This complete form is especially useful to build EOS with phase transitions. We have applied it to most elements of Mendeleiev table, depending on the availability of their physical data, and we reproduce their (P,T) phase diagram, up to ten phases for Bi. [Preview Abstract] |
Monday, June 15, 2015 10:30AM - 10:45AM |
B2.00005: Vibrational and Thermal Properties of $\beta$-HMX and TATB from Dispersion Corrected Density Functional Theory Aaron Landerville, Ivan Oleynik Dispersion Corrected Density Functional Theory (DFT+vdW) calculations are performed to predict vibrational and thermal properties of the bulk energetic materials (EMs) $\beta$-octahydrocyclotetramethylene-tetranitramine ($\beta$-HMX) and triaminotrinitrobenzene (TATB). DFT+vdW calculations of optimized unit cells along the hydrostatic equation of state are followed by frozen-phonon calculations of their respective vibration spectra. These are then used under the quasi-harmonic approximation to obtain zero-point and thermal free energy contributions to the pressure, resulting in PVT equations of state for each material that is in excellent agreement with experiment. Further, heat capacities, thermal expansion coefficients, and Gruneissen parameters as functions of temperature are calculated and compared with experiment. The vibrational properties, including phonon densities of states and pressure dependencies of individual modes, are also analyzed and compared with experiment. [Preview Abstract] |
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