Session M20: Matter at Extreme Conditions: EOS and Phase Transitions

Crystal structure and melting of dynamically compressed planetary materials

Dynamic compression experiments are important for understanding the high-pressure behavior of planetary building block materials, providing critical input for models of planetary structure, dynamics, and evolution. User facilities that couple shock and ramp compression drivers with diagnostics for x-ray diffraction and continuum velocimetry provide a means to probe materials at the conditions of deep planetary interiors. These studies can constrain equation of state, reveal crystal structure, and investigate the melting behavior of dynamically compressed materials. This talk will review recent results from National User Facilities, including the Dynamic Compression Sector of the Advanced Photon Source, the Omega Laser Facility, and the Z Machine, investigating key core and mantle materials, including iron carbide and silicate perovskite up to 600 GPa. These results provide new constraints on crystal structures, equations of state, melting behavior, and kinetics of phase transitions under extreme conditions with applications to understanding the deep interiors of the Earth and rocky exoplanets.   

Hydrogen Bond Symmetrization and High-Spin to Low-Spin Transition of ε-FeOOH at the Pressure of Earth’s Lower Mantle

ε-FeOOH may be an important carrier of water into the deep Earth and is a rich system that serves as a model for understanding two widely observed types of pressure-induced phase transitions: hydrogen bond symmetrization, and the high-spin to low-spin transition of the iron cation.  We use spin-polarized density functional theory at the PBEsol+U level to investigate structure, energetics, equation of state, phonon dispersion, elasticity, and electronic structure of this phase.  We find two distinct phase transtions: H-bond symmetrization at 37 GPa, and the high-spin to low-spin transition at 45 GPa, the latter in excellent agreement with experiment.  We find a small, but finite gap typical of semi-conductors that decreases, but does not vanish, with increasing pressure up to at least 100 GPa.  Computed vibrational spectra indicate a path towards experimental identification of the hydrogen bond symmetrization transition.  Finally, we report our recent extensions to this work that explore hydrogen bond dynamics with molecular dynamics simulations. 

    

Rock-Ice Mixtures in the Interiors of Massive Water Worlds

A water world is an exoplanet that ranges in size between Earth and Neptune that is predicted to be rich in water, lacking a massive atmosphere. The interior structure of a water world is assumed to have three layers: (1) iron core, (2) rocky mantle, and (3) water. This 3-layer model may work in smaller planets where water and rock form distinct layers with limited incorporation of water into silicates. However, in larger planets water and silicates may interact differently due to greater interior pressure and temperatures found at the rock-ice boundary. Determining the dynamics between these two materials at extreme conditions is necessary for understanding a water world's growth and evolution. In this work, we use density functional molecular dynamics (DFT-MD) simulations to investigate the miscibility and dynamics a major end-member silicate phase bridgmanite (MgSiO3), and water (H2O) at extreme conditions pertinent to the rock-ice boundary layer within water worlds. We use a heat-until-it-mixes method to explore pressures ranging from 30–120 GPa and temperatures from 500–8000 K. When the temperatures exceed the melting point of bridgmanite, we show that MgSiO3 and H2O mix in all proportions. To provide proof of concept that these conditions are met during the collisional growth of these water-rich bodies, we performed smoothed particle hydrodynamics simulations. We simulated giant impacts between water worlds of 0.7–4.7 Earth masses. This work provides theoretical evidence that many massive water worlds have mixed mantles during their collisional growth.   

Heat and charge transport of H2O in the deep interior of Uranus

Transport properties govern the pace of thermal evolution and the generation of magnetic fields in the ice giants.  We use ab initio molecular dynamics simulations and the Green-Kubo theory of linear response to determine the thermal and electrical conductivity of H₂O, leveraging recently discovered invariance principles in the numerical computation of transport coefficients, and cepstral analysis of the flux time series.  We examine the liquid, solid, and superionic phases of H₂O, the latter of which is stable over most of the pressure-temperature regime of the Uranian interior.  We use our results to build a model of the thermal evolution of Uranus that explains its hitherto poorly understood low luminosity and the evolution orbits of its moons, and which is consistent with observations of the magnetic field.    

Anharmonic phonons in high-pressure hydrogen and hydrogen-rich materials.

Hydrogen-rich materials are characterized by the presence of large quantum fluctuations affecting hydrogen motion. While the impact of nuclear quantum effects (NQEs) on the behaviour of these systems is substantial, from the theoretical sides understanding how NQEs modify their vibrational properties still represents a challenge. In this work, we introduce a framework to compute accurate phonons in atomic[1] and molecular[2] crystals even in case of strong quantum anharmonicity. This method is based on the evaluation of the static limit of the exact Matsubara phononic Green’s function. It is fully ab initio, as we obtain this quantity within density functional theory coupled with path integral molecular dynamics. We show that, by means of our approach, one can reach converged phonon frequencies at much lower computational cost and at higher precision than previously reported in literature. As application, we compute the vibrons modes of phase III and IV of high-pressure solid molecular hydrogen with unprecedented accuracy, by characterizing their nature. Finally, we show the major role played by NQEs in the phonon dispersion of the high-temperature superconducting H₃S, by coupling these results with quantum Monte Carlo calculations for the electronic part.   

Emergence of superconductivity in phase VI of solid hydrogen at high pressures

Multiple recent experiments claim to observe the transition from insulating to the metallic phase of hydrogen with increasing pressure. However, none of them detected superconductivity even under cryogenic conditions. Contradictory to this, several theoretical investigations showed that superconductivity should exist in already experimentally obtainable pressures. Here we reinvestigate the superconducting properties of solid hydrogen for a recently proposed phase diagram. We show that superconductivity emerges in the Cmca-12 molecular phase VI at pressures at least higher than 450 GPa, an estimate that is shifted to 512 GPa considering more advanced electronic interactions. Our calculations show that quantum anharmonic effects impose a large renormalization of the structure of both molecular and atomic phases, and yield extremely anomalous phonon spectral functions. For the first time ever, we calculate the superconducting critical temperature emph{ab initio} considering the full lineshape of the spectral function and show that it has a non-negligible impact on it.   

Liquid Silane at High Pressure

Compression of SiH₄ has been suggested as a possible route to achieving metallic hydrogen by way of chemical pre-compression [1]. The structures of crystalline silane and its metallization have been studied extensively, leading to several proposed solid-solid phases at high pressure and the onset of the dissociation of the tetrahedral molecules above 60 GPa. These works have focused on transitions up to room temperature while little is known about the liquid phases of silane.  Here we report results from first principles on the electronic and structural transitions in compressed liquid SiH₄predicting a rich fluid phase diagram.  Comparison between the solid and liquid phases, which exhibit both similarities and differences, are discussed.   

Ablation of carbon-based heat shield materials in Jupiter-like heating conditions

In this presentation, we report on experiments utilizing the DIII-D tokamak to investigate the ablation of various carbonaceous materials and assess their feasibility for use in thermal protection systems on spacecraft traveling to Jupiter. We recorded mass loss rates for three types of samples: porous carbon, glassy carbon, and ARJ graphite for different heating conditions. Due to inherent properties of the tokamak plasma (e.g., high temperature, rotation, and fast flows), the heat flux deposited to the material samples is comparable to that experienced by the Galileo probe during its entry into Jupiter's atmosphere. Here we discuss scaling between laboratory and space conditions, specifics of the experimental design, and calculations of material ablation as a function of incident heat flux. The validity of several analytical ablation models available in the space community is tested against mass loss rates recorded in the experiments. The experimental results are further compared to a numerical model of meteor ablation adapted for entry into Jupiter's atmosphere, which allows for scaling between laboratory and space conditions.   

The structure of liquid atomic hydrogen

Atomic hydrogen, which forms at megabar pressures, is predicted to host a number of extreme characteristics, such as room temperature superconductivity [1], possibly coexisting with superfluid order [2]. Using a model based on first-principles calculations, we investigated the structure of liquid atomic hydrogen near the predicted melting line. Up to 1 TPa, the static structure factor exhibits a marked two-peak structure, which we associate with Friedel oscillations. The distinct intensity of this feature suggests that the metallic character of liquid atomic hydrogen is much more pronounced than that of other liquid metals.

[1] N. W. Ashcroft, "Metallic Hydrogen: A High-Temperature Superconductor?" Phys. Rev. Lett. 21, 1748 (1968)

[2] Babaev, E., Sudbø, A. & Ashcroft, "N. A superconductor to superfluid phase transition in liquid metallic hydrogen." Nature 431, 666–668 (2004)   

Material Ablation During Entries into the Venusian Atmosphere

Understanding the ablation of different objects within the Venusian atmosphere is imperative both for the study of meteorite entries and the development of next-generation heat shields for spacecrafts entering the planet. Here we present the results form a numerical model solving two equations which describe the time evolution of velocity and mass as a function of entry conditions. We consider spherical ordinary chondrite impactors as a control to investigate how objects primarily composed of silica ablate when exposed to the extreme temperatures associated with the atmospheric entry process. The initial mass and velocities are varied, while assuming a 45-degree angle of entry for each simulation. The resulting mass loss rates are compared against simulation results using iron and carbon impactors and to data from carbon ablation experiments performed at the DIII-D tokamak. We conclude that silica materials should be explored in greater depths as candidate materials for future spacecraft heat shields.

 

Work supported by the United States Department of Energy under DE-SC0022554, DE-SC0021338, DE-SC0023375, and DE-FC02-04ER54698.

 

Keywords: Ablation; Heat Shield.

    

Spacecraft Heat Shield study in the DIII-D tokamak

A study of carbon ablation at high heat flux relevant to hypervelocity spacecraft entries was performed in the DIII-D tokamak. Exploration missions to the Solar System's gaseous giants and hyperbolic re-entries into the Earth's atmosphere require spacecraft heat shields that can withstand high velocity (>10 km/s) and extreme heat flux (>10 MW/m2). Conditions in DIII-D L-mode edge plasma can reproduce the flow velocity and high heat flux experienced during the Galileo probe's entry into the atmosphere of Jupiter. Three types of samples were used for the experiments: stationary graphite rods protruding from the floor of the vessel, 1-mm-diameter porous carbon spheres, and 700-micron-diameter glassy carbon spheres injected from the floor into the scrape-off layer and edge plasma. In the graphite rod experiments, the mass loss rates as a function of heat fluxes determined from an extensive array of spectroscopic measurements are found to agree with semi-empirical ablation models obtained from previous spacecraft flight data. Experimental results for the pellet trajectories and mass loss rates of the porous and glassy carbon pellets are confirmed using the UEDGE-DUSTT simulations. These pellet experiments are also compared against simulations of carbon-based meteorite atmospheric entries for different velocities, initial masses, and angles of entry.   

Unprecedented accuracy of the equation of state and band-gap closure dynamics of warm dense krypton from first-principles calculations

The thermodynamical and electric properties of warm dense krypton are crucial to accurately model the nuclear reactor process and understand the planet’s structure and dynamics. Using recently developed thermal exchange-correlation (XC) density functionals, meta-GGA level T-SCAN-L, and hybrid KDT0, we perform fully consistent density functional theory molecular dynamics (MD) simulations to explore the equation of state and optical properties of krypton in density and temperature range of 2.6 to 8.0 g/cm³ and 1 to 60 kK, respectively. Because of its high susceptibility to equation-of-state data, sound speed along the principal Hugoniot is used to validate and benchmark T-SCAN-L. We demonstrate that the inclusion of inhomogeneity and thermal XC effects via T-SCAN-L leads to much better agreement with the experimental result that has so far been unattainable by any theoretical model. The insulator-metal transition is investigated with the use of the Kubo–Greenwood formalism in combination with MD snapshotting. Reliable ionic configurations obtained from MD simulations utilizing T-SCAN-L and electronic structures with a realistic band gap predicted by KDT0 contribute to significant improvement in dc conductivity compared to previously published results.   

Effects on Pre-Applied Compression on Tensile-Strength of 3-Dimensional Graphene Foams

3D graphene foams (GrFs) are widely used in a variety of structural and functional applications because of their superior mechanical properties provided by the combination of porous structures and strong graphene sheets. A large number of simulations and experiments have been conducted in order to study the mechanical properties of GrFs using compression and tensile tests. In recent experiments it was shown that the mechanical strength of GrFs increases after they undergo pre-applied compression processes. In this study, we investigate the mechanical properties and deformation mechanisms of GrFs in the tensile test followed by compression, using molecular dynamics simulation. A 3D bonded graphene foam system is created, which consists of randomly-oriented 2D coarse-grained mesoscopic graphene flakes connected via permanently-bonded crosslinks and van-der-Waals interactions among each other. The simulation results reproduce the experimentally-observed behaviors that the tensile strength of GrF increases with pre-applied compression. The fundamental mechanisms and optimal conditions are presented through the analysis of the simulation outcomes.