Properties of B4C in the shocked state for pressures up to 1.5 TPa

Abstract

Density Functional Theory calculations using the quasi-harmonic approximation have been used to calculate the solid Hugoniot of two polytypes of boron carbide up to 100 GPa. Under the assumption that segregation into the elemental phases occurs around the pressure that the B11Cp(CBC) polytype becomes thermodynamically unstable with respect to boron and carbon, two discontinuities in the Hugoniot, one at 50 GPa and one at 90-100 GPa, are predicted. The former is a result of phase segregation, and the latter a phase transition within boron. First principles molecular dynamics (FPMD) simulations were employed to calculate the liquid Hugoniot of B4C up to 1.5 TPa, and the results are compared to recent experiments carried out at the Omega Laser Facility up to 700 GPa [Phys. Rev. B 94, 184107 (2016)]. A generally good agreement between theory and experiment was observed. Analysis of the FPMD simulations provides evidence for an amorphous, but covalently bound, fluid below 438 GPa, and an atomistic fluid at higher pressures and temperatures.

Nature of the bonded-to-atomic transition in liquid silica to TPa pressures

“…3, previously also found in MgO and MgSiO 3 in a computational study 35 ), which is eventually filled at 28,000 K or above (green and blue curves). We note that a similar behavior was found in hydrocarbons 52 and is believed to be associated with metallization of the system, and also in boron carbide (B 4 C) as a result of the disappearance of mid-range order and molecular motifs within the liquid 58 .…”

“…In this work, the initial states are estimated by starting from DFT calculations at the desired densities (2.20, 2.65, and 4.29 g/cm 3 for fused silica, α-quartz, and stishovite, respectively) 49 , then we consider each isotherm with temperature T and fit the pressure and energy data along the isotherm as functions of density by using cubic splines 50 , and the density ρ at which the energy term [E − E i ] equals the pressure term [(P + P i )(V i − V )/2] defines the Hugoniot, which has definitive values in T, ρ, P and E. We also calculate the shock velocity u s and particle velocity u p , relevant to shock experiments, by u 2 s = ξ/η and u 2 p = ξη, where ξ = (P −P i )/ρ i and η = 1−ρ i /ρ. This approach has been used previously for calculating the Hugoniot of several other materials [51][52][53][54][55][56][57] , and was found to produce consistent Hugoniots with other computational methods, such as progressive determination by running a large number of EOS calculations around the Hugoniot curve 58,59 In order to cross check the validity of the Hugoniot results based on the relatively sparse temperature-density grid, we have recalculated the Hugoniot of α-quartz by performing 2D interpolation of the pressure and energy data as functions of (T, ρ) and then determined the conditions at which the function H(ρ, T ) = E − E i + (P + P i )(V − V i )/2 equals zero. We have also made tests by using a partial set of our EOS data (by excluding the 6.62 g/cm 3 isochore).…”

Benchmarking boron carbide equation of state using computation and experiment

“…The deviation between PIMC/L2120 (and MECCA) and ACTEX/L2122 curves above 10 4 Mbar is due to the electron relativistic effect, which is considered in AC-TEX and L2122 but not in PIMC/L2120 (and not fully in MECCA). The initial sample density ρi=2.51 g/cm 3 for all the Hugoniot except that by Shamp et al36 , which is 2.529 g/cm 3 .…”

“…Fratanduono et al 35 extended the Hugoniot, sound velocities, and thermodynamic properties measurements of liquid B 4 C to 700 GPa. Shamp et al 36 performed MD calculations based on density functional theory (DFT) to determine the Hugoniot curve up to 1500 GPa, and predicted discontinuities along the Hugoniot at <100 GPa as results of phase separation and transformation in solid B 4 C. An equation of state table (LEOS 2122) based on an average atom-in-jellium model (Purgatorio) 37 has thus been developed that fits all available experimental Hugoniot data above 100 GPa 35 . However, accurate EOS at higher pressures and temperatures, in particular those corresponding to the partially ionized, warm dense state, is still unknown.…”

SQDFT: Spectral Quadrature method for large-scale parallel O(N) Kohn–Sham calculations at high temperature

“…We now consider the accuracy and efficiency of SQDFT in high-temperature Born-Oppenheimer quantum molecular dynamics (QMD) simulations. Such simulations are a cornerstone of modern warm dense matter theory [27], providing equation-of-state and shock-compression predictions of unprecedented accuracy, up to temperatures of ∼ 100 eV and pressures of 100s of Mbar; see, e.g., [36,76,77,78,79,80]. At temperatures above ∼ 100 eV, conventional Kohn-Sham methods become prohibitively expensive, and so alternative methods such as OFMD [33] and, more recently, path integral Monte Carlo (PIMC) [80] have been employed to reach temperatures of 1000s of eV and higher.…”