S.V. Starikov

Interatomic potential for uranium in a wide range of pressures and temperatures

Using the force-matching method we develop an interatomic potential that allows us to study the structure and properties of α-U, γ-U and liquid uranium. The potential is fitted to the forces, energies and stresses obtained from ab initio calculations. The model gives a good comparison with the experimental and ab initio data for the lattice constants of α-U and γ-U, the elastic constants, the room-temperature isotherm, the normal density isochore, the bond-angle distribution functions and the vacancy formation energies. The calculated melting line of uranium at pressures up to 80 GPa and the temperature of the α-γ transition at 3 GPa agree well with the experimental phase diagram of uranium.

Fabrication of Hybrid Nanostructures via Nanoscale Laser-Induced Reshaping for Advanced Light Manipulation

Ordered hybrid nanostructures for nanophotonics applications are fabricated by a novel approach via femtosecond laser melting of asymmetric metal-dielectric (Au/Si) nanoparticles created by lithographical methods. The approach allows selective reshaping of the metal components of the hybrid nanoparticles without affecting the dielectric ones and is applied for tuning of the scattering properties of the hybrid nanostructures in the visible range.

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....

Atomistic simulation of laser ablation of gold: Effect of pressure relaxation

The process of ablation of a gold target by femto- and picosecond laser radiation pulses has been studied by numerical simulations using an atomistic model with allowance for the electron subsystem and the dependence of the ion-ion interaction potential on the electron temperature. Using this potential, it is possible to take into account the change in the physical properties of the ion subsystem as a result of heating of the electron subsystem. The results of simulations reveal a significant difference between the characteristics of metal ablation by laser pulses of various durations. For ablation with subpicosecond pulses, two mechanisms of metal fracture related to the evolution of electronic pressure in the system are established.

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.

Atomistic modeling of the self-diffusion in γ-U and γ-U-Mo

Results of investigations of the self-diffusion in gamma-uranium and metallic U-Mo alloys are presented. Calculations are performed using the method of atomistic modeling with the help of interatomic potentials based on the embedded-atom model and its modifications. Proposed potentials are verified by calculating thermodynamic and mechanical properties of uranium and U-Mo alloys. The formation energies of point defects and atomic diffusivities due to the diffusion of defects are calculated for gamma-uranium and alloy containing 9 wt % molybdenum. Self-diffusion coefficients of uranium and molybdenum are evaluated. Based on the data obtained, it has been concluded that the experimentally observed features of the self-diffusion in gamma-uranium can be explained by the prevalence of the interstitial mechanism.

New interatomic potential for computation of mechanical and thermodynamic properties of uranium in a wide range of pressures and temperatures

A new interatomic potential for uranium is proposed. The potential is constructed in terms of the embedded-atom method (EAM). As the reference data used for the optimization of potential functions, the values of forces, energies, and stresses obtained from ab initio computations have been employed. The potential has been applied for studies of properties of crystalline phases of uranium. It has been established that the lattice parameters of the α and Γ phases, the elastic moduli, the isochore, the room temperature isotherm, and the energies of vacancy formation are in good agreement with the available experimental data and calculated results in the framework of the density-functional theory. The potential suggested facilitates simulation of the first-order phase transitions between Γ uranium and liquid and between Γ and α uranium. The melting points of uranium at pressures of up to 80 GPa, and the temperature of the phase transition between the Γ phase and α phases, at ∼3 GPa have been determined. For the first time, atomistic simulation of the phase transition between α and Γ phases of uranium has been performed.

Multiscale Modeling of Uranium Mononitride: Point Defects Diffusion, Self-Diffusion, Phase Composition

Multiscale computational approach is used to evaluate microscopic parameters for description of nitride nuclear fuel. The results of atomistic simulation and thermodynamic modeling allow to estimate diffusivity and concentrations of point defects at various stoichiometric ratios of UN1+x. The diffusivities of Xe atom were calculated in various equilibrium states. In addition, we obtained the dependence of partial nitrogen pressure on x and temperature. The results of atomistic simulation were used for modeling of nuclear fuel behavior with use of mechanistic fuel codes.

Modeling of formation mechanism and optical properties of Si/Au core-shell nanoparticles

Fabrication of metal/semiconductor (“hybrid”) nanoparticles is still a challenge due to the absence of methods of a metal core coating by crystalline semiconductor shell. We propose a novel principle of formation of a core-shell nanoparticle made of liquid silicon and gold droplets. Molecular dynamics simulations of the droplets behavior demonstrates that the core-shell structure with a gold core and a silicon shell can be formed if the droplets remain in the liquid state until final material redistribution. The main driven force in this process is surface tension governed by surface energies of the droplets, where silicon tends to cover gold due to lower surface energy. Taking into account this mechanism of core-shell nanoparticles formation, we provide numerical modelling, which demonstrates the resulting nanoparticle posses enhanced local electromagnetic field, high Purcell factor and flexible power patterns of scattered light.

Features of structure and phase transitions in pure uranium and U–Mo alloys: atomistic simulation

We study structural properties of cubic and tetragonal phases of U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. For pure uranium and U-Mo alloys at low temperatures we observe body-centered tetragonal (bct) structure, which is similar to the metastable γ°-phase found in the experiments. At higher temperatures bct structure transforms to a quasi body-centered cubic (q-bcc) phase that exhibits cubic symmetry just on the scale of several interatomic spacings or when averaged over time. Instantaneous pair distribution function (PDF) differs from PDF for the time-averaged atomic coordinates corresponding to the bcc lattice. The local positions of uranium atoms in q-bcc lattice correspond to the bct structure, which is energetically favourable due to formation of short U-U bonds. Transition from bct to q-bcc could be considered as ferro-to paraelastic transition of order-disorder type. The temperature of transition depends on Mo concentration. For pure uranium it is equal to about 700 K, which is well below than the upper boundary of the stability region of the α-U phase. Due to this reason, bct phase is observed only in uranium alloys containing metals with low solubility in α-U.