17 research outputs found
High-pressure equations of state and phase diagrams of molecular H-C-N-O compounds
The thermodynamic description of molecular systems composed of water, ammonia, and methane at high pressure up to 1 TPa is the subject of this thesis. The focus lies on the calculation of equations of state, phase diagrams, and their characterization by employing the quantum-statistical method density functional theory molecular dynamics. In particular superionic phases are investigated using evolutionary structure searching and the linear mixing approximation is validated. The obtained simulation results serve as input for modeling the interior structure of giant planets such as Uranus.Die thermodynamische Beschreibung komplexer Gemische bestehend aus Wasser, Ammoniak und Methan unter hohem Druck bis zu 1 TPa ist Gegenstand dieser Arbeit. Dabei liegt der Fokus auf der Berechnung von Zustandsgleichungen, Phasendiagrammen und deren Charakterisierung mittels Dichtefunktionaltheorie-Molekulardynamik. Insbesondere werden superionische Phasen mit Hilfe von evolutionary structure searching untersucht und die Näherung der linearen Mischung validiert. Die erhaltenen Simulationsergebnisse dienen als Eingabeparameter für die Modellierung des inneren Aufbaus großer Planeten wie Uranus
Material Properties for the Interiors of Massive Giant Planets and Brown Dwarfs
We present thermodynamic material and transport properties for the extreme
conditions prevalent in the interiors of massive giant planets and brown
dwarfs. They are obtained from extensive \textit{ab initio} simulations of
hydrogen-helium mixtures along the isentropes of three representative objects.
In particular, we determine the heat capacities, the thermal expansion
coefficient, the isothermal compressibility, and the sound velocity. Important
transport properties such as the electrical and thermal conductivity, opacity,
and shear viscosity are also calculated. Further results for associated
quantities including magnetic and thermal diffusivity, kinematic shear
viscosity, as well as the static Love number and the equidistance are
presented. In comparison to Jupiter-mass planets, the behavior inside massive
giant planets and brown dwarfs is stronger dominated by degenerate matter. We
discuss the implications on possible dynamics and magnetic fields of those
massive objects. The consistent data set compiled here may serve as starting
point to obtain material and transport properties for other substellar H-He
objects with masses above one Jovian mass and finally may be used as input for
dynamo simulations
X-ray Thomson scattering spectra from DFT-MD simulations based on a modified Chihara formula
We study state-of-the-art approaches for calculating x-ray Thomson scattering
spectra from density functional theory molecular dynamics (DFT-MD) simulations
based on a modified Chihara formula that expresses the inelastic contribution
in terms of the dielectric function. We compare the electronic dynamic
structure factor computed from the Mermin dielectric function using an ab
initio electron-ion collision frequency to computations using a linear response
time dependent density functional theory (LR-TDDFT) framework for hydrogen and
beryllium and investigate the dispersion of free-free and bound-free
contributions to the scattering signal. A separate treatment of these
contributions in the Mermin dielectric function shows excellent agreement with
LR-TDDFT results for ambient-density beryllium, but breaks down for highly
compressed matter where the bound states become pressure ionized. LR-TDDFT is
used to reanalyze x-ray Thomson scattering experiments on beryllium
demonstrating strong deviations from the plasma conditions inferred with
traditional analytic models at small scattering angles.Comment: 14 pages, 10 figures, submitted to Physical Review
Carbon ionization at Gbar pressures: an ab initio perspective on astrophysical high-density plasmas
A realistic description of partially-ionized matter in extreme thermodynamic
states is critical to model the interior and evolution of the multiplicity of
high-density astrophysical objects. Current predictions of its essential
property, the ionization degree, rely widely on analytical approximations that
have been challenged recently by a series of experiments. Here, we propose a
novel ab initio approach to calculate the ionization degree directly from the
dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This
Density Functional Theory framework captures genuinely the condensed matter
nature and quantum effects typical for strongly-correlated plasmas. We
demonstrate this new capability for carbon and hydrocarbon, which most notably
serve as ablator materials in inertial confinement fusion experiments aiming at
recreating stellar conditions. We find a significantly higher carbon ionization
degree than predicted by commonly used models, yet validating the qualitative
behavior of the average atom model Purgatorio. Additionally, we find the carbon
ionization state to remain unchanged in the environment of fully-ionized
hydrogen. Our results will not only serve as benchmark for traditional models,
but more importantly provide an experimentally accessible quantity in the form
of the electrical conductivity.Comment: accepted for publication in Physical Review Researc
Thermodynamics of diamond formation from hydrocarbon mixtures in planets.
Hydrocarbon mixtures are extremely abundant in the Universe, and diamond formation from them can play a crucial role in shaping the interior structure and evolution of planets. With first-principles accuracy, we first estimate the melting line of diamond, and then reveal the nature of chemical bonding in hydrocarbons at extreme conditions. We finally establish the pressure-temperature phase boundary where it is thermodynamically possible for diamond to form from hydrocarbon mixtures with different atomic fractions of carbon. Notably, here we show a depletion zone at pressures above 200 GPa and temperatures below 3000 K-3500 K where diamond formation is thermodynamically favorable regardless of the carbon atomic fraction, due to a phase separation mechanism. The cooler condition of the interior of Neptune compared to Uranus means that the former is much more likely to contain the depletion zone. Our findings can help explain the dichotomy of the two ice giants manifested by the low luminosity of Uranus, and lead to a better understanding of (exo-)planetary formation and evolution
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Phase behaviours of superionic water at planetary conditions
Most water in the universe may be superionic, and its thermodynamic and transport properties are crucial for planetary science but difficult to probe experimentally or theoretically. We use machine learning and free energy methods to overcome the limitations of quantum mechanical simulations, and characterize hydrogen diffusion, superionic transitions, and phase behaviors of water at extreme conditions. We predict that a close-packed superionic phase with mixed stacking is stable over a wide temperature and pressure range, while a body-centered cubic phase is only thermodynamically stable in a small window but is kinetically favored. Our phase boundaries, which are consistent with the existing-albeit scarce-experimental observations, help resolve the fractions of insulating ice, different superionic phases, and liquid water inside of ice giants
Ab initio calculation of the reflectivity of molecular fluids under shock compression
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