V. V. Pisarev

Atomistic simulation of laser-pulse surface modification: Predictions of models with various length and time scales

In this work, the femtosecond laser pulse modification of surface is studied for aluminium (Al) and gold (Au) by use of two-temperature atomistic simulation. The results are obtained for various atomistic models with different scales: from pseudo-one-dimensional to full-scale three-dimensional simulation. The surface modification after laser irradiation can be caused by ablation and melting. For low energy laser pulses, the nanoscale ripples may be induced on a surface by melting without laser ablation. In this case, nanoscale changes of the surface are due to a splash of molten metal under temperature gradient. Laser ablation occurs at a higher pulse energy when a crater is formed on the surface. There are essential differences between Al ablation and Au ablation. In the first step of shock-wave induced ablation, swelling and void formation occur for both metals. However, the simulation of ablation in gold shows an additional athermal type of ablation that is associated with electron pressure relaxation....

Molecular dynamics simulations of the relaxation processes in the condensed matter on GPUs

Abstract We report on simulation technique and benchmarks for molecular dynamics simulations of the relaxation processes in solids and liquids using the graphics processing units (GPUs). The implementation of a many-body potential such as the embedded atom method (EAM) on GPU is discussed. The benchmarks obtained by LAMMPS and HOOMD packages for simple Lennard-Jones liquids and metals using EAM potentials are presented for both Intel CPUs and Nvidia GPUs. As an example the crystallization rate of the supercooled Al melt is computed.

Atomistic simulation of ion track formation in UO2

The atomistic simulation of track formation due to the moving of swift heavy ion is performed for uranium dioxide. The two-temperature atomistic model with an explicit account of electron pressure and electron thermal conductivity is used. This two-temperature model describes a ionic subsystem by means of molecular dynamics while the electron subsystem is considered in the continuum approach. The various mechanisms of track formation are examined. It is shown that the mechanism of surface track formation differs from the mechanism of track formation in the bulk. The threshold values of the stopping power for track formation are estimated.

A kinetic model of fracture of simple liquids

The molecular-dynamic (MD) simulation is performed of the processes of generation and growth of cavities in stretched Lennard-Jones liquid. The process of homogeneous generation of cavities in a constant-volume cell is considered. The averaging of the lifetime of the homogeneous phase over the ensemble of MD trajectories is used to determine the nucleation rate as a function of pressure and temperature. The resultant correlation is compared with the classical theory of homogeneous nucleation. The initial stage of growth of spherical cavity is simulated, and the dependence of the rate of growth on pressure is determined along two isotherms. A kinetic model is suggested of fracture of liquid upon stretching at a constant rate. This model relates the volume of pores at an arbitrary instant of time to the kinetic characteristics of their generation and growth determined in MD models for single isolated cavities. The spallation strength of liquid, calculated using this kinetic model and MD data, only slightly depends on the rate of stretching. The calculation results agree well with the experimentally obtained dependence of spallation strength of hexane on the rate of stretching.

Glass transition of an overcooled aluminum melt: A study in molecular dynamics

The glass transition of an overcooled aluminum melt upon isobaric and isochoric cooling is studied by means of molecular dynamics. The embedded-atom potential is used to model the aluminum. Three criteria of glass transition (splitting of the second peak of the pair correlation function, an increase in the number of icosahedral clusters, and a change in the activation energy of self-diffusion) are considered. It is shown that the glass transition temperatures determined by these three criteria coincide within the error range. The dependence of the glass transition temperature on the cooling rate is determined from the modeling results and agrees with the Bartenev theoretical model.

Determination of Free Energy of the Crystal-Melt Interface

The results of calculation of the free energy γ of the crystal-melt interface using the molecular dynamics method are presented. The interfacial energy is determined from analysis of the capillary fluctuations spectrum in the two-phase system. The free energy values of the crystal-melt interface are found for aluminum on the melting line in the temperature range of 935 to 1110 K. The anisotropy value of the interfacial free energy is estimated for different interface orientations. It is shown that the free energy values for the basic planes are arranged in the order γ100 > γ110 > γ111. It is revealed that the free energy of the crystalmelt interface increases with the rise in temperature along the melting line.

Atomistic Modeling and Simulation for Solving Gas Extraction Problems

Proof-of-concept results are presented on the application of molecular modeling and simulation to the gas extraction problems. Both hydrocarbon mixtures and gas hydrates in porous media are considered. Retrograde gas condensation reduces the amount of recoverable gas in reservoirs and can lead to jamming of wells. For example, the authors [1] developed a model of two-phase gas filtration through porous media that can reproduce the jamming. The model can describe gas flow in soil of reservoir if both a phase diagram of the gas mixture and permeability of pores to gaseous and liquid phases are known. Molecular dynamics simulations are used to study phase diagrams of binary hydrocarbon mixtures at temperatures between the critical points of pure components. The phase diagrams in free space and in slit pores are calculated. Effects of wall–gas interaction on the phase diagram are estimated. The data obtained from molecular simulations can be used to improve the hydrodynamic filtration model and to optimize the natural gas and gas condensate extraction conditions. Effects of pore structure on the phase stability of gas hydrates and on the diffusion of guest molecules are studied by means of molecular modeling. The anisotropic diffusion is found in hydrogen hydrates. Moreover, diffusivity of hydrogen molecules demonstrates anomalous behavior on nanosecond timescale.

Study of viscosity of aluminum melt during glass transition by molecular dynamics and Green–Kubo formula

Molecular dynamics study of shear viscosity behavior of liquid aluminum is performed. The embedded atom method potential is used at the simulation of isobaric cooling. The viscosity is calculated using the Green–Kubo formula. The stress autocorrelation functions are obtained in the range 300-1200 K. The calculated kinematic viscosity is in agreement with the experimental data for the temperatures above melting temperature. The steep change of the shear viscosity is found below 650 K which we associate with the glass transition and is in a good agreement with the temperature which is obtained using the calorimetric criterion Kolotova et al (2015 J. Non-Cryst. Solids 429 98). The viscosity coefficient can not be calculated using the direct atomistic simulations below that temperature.