I. V. Zorya

Molecular dynamics simulation of hydrogen atom diffusion in crystal lattice of fcc metals

Abstract The study of diffusion of hydrogen atoms in the crystal lattice of fcc metals Pd, Ni, Al, Ag was performed by the method of molecular dynamics. The diffusion characteristics of hydrogen impurity (activation energy of hydrogen atom migration and pre-exponential factor in the Arrhenius equation) in the considered metals were calculated. It is shown that the prevailing mechanism of the over-barrier hydrogen diffusion in fcc metals consists of successive migration through octahedral and tetrahedral pores. During migration, as a rule, the hydrogen atom is not delayed in tetrahedral pores.

Formation of the Excess Free Volume in Triple Junctions during Nickel Crystallization

The formation of an excess free volume in triple junctions during crystallization has been studied by the molecular dynamics model using nickel as an example. It is shown that an excess free volume that forms during nickel crystallization in triple junctions predominantly forms as a result of the fixation of the liquid phase volume when contacting three crystallization fronts that contains, after crystallization, a high fraction of the free volume. In some cases, as the free volume is concentrated in triple junctions, a comparatively small crystalline subgrain (from one to several nanometers in diameter) forms, and the subgrain has the orientation different from those of contacting grains and exists in the extended state.

The Necessary Duration of a Molecular Dynamics Experiment for the Diffusion Coefficient Calculation

The evaluation of the necessary duration of a molecular dynamics experiment for the calculation of the diffusion coefficient at migration of different point defects in Ni (vacancy, bivacancy, self-interstitial atom, hydrogen atom) is held in the present work. It is shown that at the temperature higher than 0.6 of melting point is usually enough the simulation during of 100 ps for this. When calculating of the diffusion coefficient of impurity in the metal crystal, for example, of hydrogen, the decrease of error of mean-square displacements of impurity atoms can be achieved by introducing of a large number of the impurities.

Interaction of Hydrogen Atom with Edge Dislocation in Pd and Ni

The energy characteristics of interaction of hydrogen impurity with ½<110> edge dislocation in Pd and Ni were calculated by the method of molecular dynamics. It is shown that the dislocation is effective trap for hydrogen. At the same time the dislocation jogs increases its sorption capacity with respect to hydrogen, but reduces the diffusion mobility of hydrogen along the dislocation. The diffusion of hydrogen atoms in the dislocation region occurs mainly along the dislocation core. The energy of hydrogen migration along the dislocation, as our calculations have shown, is almost two times lower than in a defect-free crystal.

The Formation and Migration Energy of Bivacancy in fcc Metals

In the work we propose a method for determining of the formation energy of bivacancy using molecular dynamics method. The key moment of the method for determining of the formation energy of bivacancy is the use of the value ζ, the minimum work that must be spent to remove one atom to infinity from the kink in the monatomic step on the surface of the crystal, calculated indirectly through the experimental data on the formation energy of the vacancy and the sublimation energy. The energy of migration of bivacancy in the work was determined from the temperature dependence of the diffusion coefficient when one bivacancy was introduced into the calculation block.

THE STUDY OF HYDROGEN INTERACTION WITH PALLADIUM AND NICKEL NANOPARTICLES BY THE METHOD OF MOLECULAR DYNAMICS

Hydrogen interaction with Pd and Ni nanoparticles was studied by the method of molecular dynamics. The metal particle in the model was created by cutting a ball from the fcc crystal. The interaction of metal atoms with each other was described with the aid of the multiparticle Cleri-Rosato potential, constructed within the tight binding model. To describe the interactions of hydrogen atoms with metal atoms and with each other, the Morse potential was used, the parameters of which were calculated from the experimental data of absorption energy, activation energy of the above-barrier diffusion of hydrogen in the metal (at normal and high temperatures), binding energy with the vacancy and dilatations. Temperatures from 300 to 1100 K were considered. During the computer experiment the temperature in calculation block was constant. The concentration of hydrogen atoms introduced into the calculation block corresponded to a pressure of 10 and 20 MPa. The initial positions of the hydrogen atoms in the calculation block (in the metal particle or outside it) did not affect the final equilibrium distribution of hydrogen, which was established after some time of the computer experiment, depending on the temperature. As it was shown by the molecular dynamics simulation, nanoparticles are effective hydrogen accumulator having a high velocity of reversible sorption-desorption process of hydrogen. At room temperature, Pd and Ni nanoparticles sorb substantially all hydrogen which is unevenly distributed in the particle volume in an effort to form aggregates containing a few tens of hydrogen atoms. In the case of Ni particles hydrogen predominantly is located near the surface. In the Pd particles, by contrast, hydrogen strongly connected with the Pd lattice, and at increasing temperature it form larger aggregates. Intensive evaporation of hydrogen from the Pd and Ni particles occurs at temperatures above 700 K. At the same time, according to the obtained data, hydrogen is more strongly associated with the particles of Pd than with Ni particles, and the work that needs to be spent for hydrogen evacuation (desorption) in the case of Pd particles is higher than for Ni particles.