106 research outputs found
Multiphase equation of state for carbon addressing high pressures and temperatures
We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: 3g/cc 100 000K (both for ρ between 3 and 12 g/cc, with select higher-ρ DFT calculations as well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.United States. Dept. of Energy (Contract DE-AC52-07NA27344
Equation of state and strength of diamond in high pressure ramp loading
Diamond is used extensively as a component in high energy density
experiments, but existing equation of state (EOS) models do not capture its
observed response to dynamic loading. In particular, in contrast with first
principles theoretical EOS models, no solid-solid phase changes have been
detected, and no general-purpose EOS models match the measured ambient
isotherm. We have performed density functional theory (DFT) calculations of the
diamond phase to ~10TPa, well beyond its predicted range of thermodynamic
stability, and used these results as the basis of a Mie-Greuneisen EOS. We also
performed DFT calculations of the elastic moduli, and calibrated an algebraic
elasticity model for use in simulations. We then estimated the flow stress of
diamond by comparison with the stress-density relation measured experimentally
in ramp-loading experiments. The resulting constitutive model allows us to
place a constraint on the Taylor-Quinney factor (the fraction of plastic work
converted to heat) from the observation that diamond does not melt on ramp
compression
A structural study of hcp and liquid iron under shock compression up to 275 GPa
We combine nanosecond laser shock compression with \emph{in-situ} picosecond
X-ray diffraction to provide structural data on iron up to 275 GPa. We
constrain the extent of hcp-liquid coexistence, the onset of total melt, and
the structure within the liquid phase. Our results indicate that iron, under
shock compression, melts completely by 258(8) GPa. A coordination number
analysis indicates that iron is a simple liquid at these pressure-temperature
conditions. We also perform texture analysis between the ambient
body-centered-cubic (bcc) , and the hexagonal-closed-packed (hcp)
high-pressure phase. We rule out the Rong-Dunlop orientation
relationship (OR) between the and phases. However, we
cannot distinguish between three other closely related ORs: Burger's,
Mao-Bassett-Takahashi, and Potter's OR. The solid-liquid coexistence region is
constrained from a melt onset pressure of 225(3) GPa from previously published
sound speed measurements and full melt (246.5(1.8)-258(8) GPa) from X-ray
diffraction measurements, with an associated maximum latent heat of melting of
623 J/g. This value is lower than recently reported theoretical estimates and
suggests that the contribution to the earth's geodynamo energy budget from heat
release due to freezing of the inner core is smaller than previously thought.
Melt pressures for these nanosecond shock experiments are consistent with gas
gun shock experiments that last for microseconds, indicating that the melt
transition occurs rapidly
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