Thermal conductivity of dissociating water—anab initiostudy

Abstract

The thermal conductivity of partially dissociated and ionised water is calculated in a large-scale study using density functional theory (DFT)-based molecular dynamics (MD) simulations. In doing so, the required heat current of the nuclei is calculated by mapping the effective particle interactions from the DFT-MD simulations onto classical pair potentials. It is demonstrated that experimental and theoretical thermal conductivity data for liquid heavy water and for ice VII are well reproduced with this efficient procedure. Moreover, the approach also allows for an illustrative interpretation of the characteristics of the thermal conductivity in the dense chemically reacting fluid. The thermodynamic conditions investigated here range from densities between 0.2 and 6 g cm −3 and temperatures between 600 and 50000 K, which includes states highly relevant for understanding the interiors of water-rich planets like Uranus and Neptune and exoplanets of similar composition. During the last two decades, ab initio calculations based on finite-temperature density functional theory (FT-DFT) in combination with molecular dynamics (MD) simulations have become a reliable tool for accurate predictions of thermodynamic properties of matter under extreme conditions [12,13]. The prefix FT emphasises that the electron system is consistently treated at a nonzero (finite) temperature [14], which is typically equal to the temperature of the nuclei.The calculation of the thermal conductivity with FT-DFT-MD techniques requires a distincted treatment of the contributions from electronic heat conduction and from heat transport via the nuclei. The former is gained by evaluating the electronic Onsager coefficients using the Kohn-Sham eigenvalues and orbitals from the FT-DFT [15], which was recently accomplished also for partially ionised water [16].

Thermal Conductivity of Silicate Liquid Determined by Machine Learning Potentials

“…is the heat current, where i E is the atomic potential energy (Equation 1), v i is the velocity of atom i, ij r is the vector joining the positions of atoms i and j, ij F is the force acting upon atom i due to its interaction with atom j, N α is the number of atoms of species α (Mg, Si, O), and h α is the partial enthalpy of species α (Babaei et al, 2012;French, 2019…”

Thermal evolution of Uranus and Neptune

“…For the thermal conductivity λ, we used values for pure water as presented by French & Redmer (2017) (electronic contribution) and French (2019) (ionic contribution). They are based on ab initio DFT-MD calculations and, for the ionic contribution, have demonstrated good agreement with experimental results at and above room temperature (French 2019) where data are available. Figure 3 shows the thermal conductivity along the TBL and the inner envelope of a Uranus model during the evolution at t = 1 Myr, 1.4 Gyr, and 4.6 Gyr.…”

“…For the inner envelope, the thermal conductivity of H-C-N-O mixtures under planetary interior conditions is not well constrained by experimental data or simulations. According to French (2019), the electronic contribution to λ H2O becomes comparable to the ionic for T 6 × 10 3 K and dominant for T 10 × 10 3 K. In this regime, λ would behave roughly the same as the electrical conductivity σ. Ravasio et al (2021) find that, once fully ionised, the electrical conductivity of NH 3 is signifi- 2011) find a 10 −4 -0.1-times lower electrical conductivity in CH 4 than H 2 O at lower pressures of up to 30 GPa.…”

“…Uncertainties on the thermal state are included by varying the base thermal conductivity, its onset after formation, and its thickness. We used thermal conductivity values for water calculated via DFT-MD (French & Redmer 2017;French 2019). In Sect.…”

Heat and charge transport in H2O at ice-giant conditions from ab initio molecular dynamics simulations

“…In this paper we have reported on the first theoretically rigorous and numerically accurate evaluation of the thermal and electric conductivities of various phases of water occurring at the pressure and temperature conditions to be found in the interior of ice-giant planets, made possible by recent advances in transport theory and data analysis. In the case of the heat conductivity, our results set a reference in the wide range of values used in evolution models of Uranus and Neptune 57 or given by recent MD-based estimates on dissociating water 58 , and their moderate values point towards more efficient trapping of heat in the deep interior of these planets. These results have been instrumental in the development of a novel model of the thermal evolution of Uranus, featuring a frozen core and an anomalously low heat flow, resulting in the observed low luminosity of this planet 59 .…”