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Polycrystalline metals have flow stress two to three orders of magnitude lower than the theoretical shear strength estimated by Frenkel model. This significant strength difference is primarily due to the presence of defects, such as dislocations and grain boundaries. However, it was experimentally found that defect-free nanoscale objects (whiskers, nanopillars, etc.) can exhibit strength close to the theoretical limit. With the development of nanotechnology, interest in the study of the theoretical strength of metals and alloys has grown significantly. It is important to find reliable criteria of lattice instability when homogeneous nucleation of defects begins during deformation of an ideal crystal lattice. Note that the Frenkel estimation does not take into account thermal vibrations of atoms and attempts are being made to take into account the effect of temperature on the theoretical strength of defect-free crystals. In this paper, using molecular dynamics simulation, we study shear deformation in the direction of for single crystals of copper and aluminum in the temperature range from 0 to 400 K. Lattice instability was evaluated using two criteria: (i) macroscopic criterion, which is related to the loss of positive definiteness of the stiffness tensor, and (ii) a microscopic criterion related to the formation of a stacking fault, which leads to a drop of the applied shear stress. It was demonstrated that both criteria are consistent at low temperatures, but the macroscopic criterion is less reliable at higher temperatures.

The effects of cation dopants in zirconia on the H2 oxidation mechanism at the pore/nickel/zirconia triple phase boundary (TPB) were theoretically examined. Y, Sc, Al, Ce, and Ca were considered as dopants, and on-boundary, O-migration, and H-migration reaction mechanisms were examined. Based on density functional theory calculations, Y as a dopant favored the on-boundary mechanism with water molecule formation within the immediate proximity of the TPB. The corresponding rate-limiting step is H transfer from the nickel surface to the boundary. In contrast, the on-boundary mechanism is not completed with the Al-, Sc-, and Ca-doped systems, due to the dissociation of water molecules at the boundary. In the Al-doped system, the O-migration mechanism is the major reaction pathway due to a low barrier for the rate-limiting step that corresponds to O transfer from zirconia to the nickel surface. The H-migration mechanism, which implies water molecule formation on the zirconia surface at a position distant from the boundary, should dominate at the Sc-, Ca-, and Ce-doped TPBs, with the lowest activation barrier at the Sc-doped TPB. The reasons for the switching of the reaction mechanisms depending on the dopant species are analyzed.

The triple phase boundary (TPB) of metal, oxide, and gas phases in the anode of solid oxide fuel cells plays an important role in determining their performance. Here we explore the TPB structures from two aspects: atomic-resolution microscopy observation and reaction dynamics simulation. Experimentally, two distinct structures are found with different contact angles of metal/oxide interfaces, metal surfaces, and pore opening sizes, which have not previously been adopted in simulations. Reaction dynamics simulations are performed using realistic models for the hydrogen oxidation reaction (HOR) at the TPB, based on extensive development of reactive force field parameters. As a result, the activity of different structures towards HOR is clarified, and a higher activity is obtained on the TPB with smaller pore opening size. Three HOR pathways are identified: two types of hydrogen diffusion processes, and one type of oxygen migration process which is a new pathway. The triple phase boundary structure in solid-oxide fuel cells largely determines the thermodynamics and kinetics of electrochemical processes therein. Here the authors use atomic-resolution microscopy and reaction dynamics simulation to reveal three discrete hydrogen oxidation reaction pathways.

This paper reports the results of a molecular dynamics (MD) study of oxide ion diffusion in cubic yttria-stabilized zirconia (YSZ) using our recently developed interatomic potential [A.M. Iskandarov et al., J. Phys.: Condens. Matter 27, 015005 (2015)], which is based on the Tangney–Scandolo dipole model. We demonstrate that our potential can reproduce oxide ion conductivity in bulk YSZ with yttria concentrations varying from 2 to 14 mol%. We confirm the significant effect of free surfaces on oxygen diffusion by direct MD calculations at elevated temperatures and nudged elastic band analyses. It was found that oxygen vacancies tend to migrate from the (111) surface and locate in the bulk region, suppressing local oxide ion diffusion at the surface. In contrast, higher oxygen vacancy concentration and activation of oxide ion jumps along the 〈110〉 direction facilitate oxygen diffusion at the (110) surface. The MD results for the (110) surface are supported by density functional theory calculations.

Porous nickel/yttria-stabilized zirconia (YSZ) composite is a common choice as an anode material for solid oxide fuel cells (SOFCs). Because of its complicated structure, there are several processes occurring in the anode that affect total performance of SOFCs. For instance, oxygen ion diffusion provides oxygen transport to triple phase boundaries (TPBs) where fuel is oxidized. Another important process is nickel sintering, which causes decreasing of the active TPB length and thus leads to degradation of SOFCs. Understanding the mechanism of each individual phenomenon in Ni/YSZ anodes can contribute to improving total performance and stability of SOFCs. Atomistic simulations can provide a deep insight into the processes taking place in Ni/YSZ anodes. While the density functional theory method is one of the most rigorous atomistic approaches used in materials science, its applicability is limited to small systems of hundreds of atoms. Larger systems can be studied by molecular dynamics (MD) and Monte-Carlo simulations, which are based on empirical potentials. However, most of the potentials for YSZ are based on simple pairwise interactions and designed in the way to reproduce specific bulk properties. Unfortunately, their validity for studying surface and interface properties of Ni/YSZ systems is usually only presumed [1], which can lead to questionable results for such processes as nickel sintering and oxygen surface diffusion. Thus, in the present paper we develop a DFT-based model that purposefully aims for studying a wide range of processes in Ni/YSZ systems, including oxygen diffusion in bulk and surface regions of YSZ, yttrium surface segregation, and nickel sintering. Our model combines three types of interatomic interactions. The first one is based on the dipole model [2] and was parameterized for bulk and surface YSZ regions in our previous work [3]. It reproduces with a good accuracy not only bulk but also surface properties of YSZ. The second describes nickel by the embedded atom method [4]. The third is for interactions between Ni and YSZ and is described by a pairwise Buckingham potential, which was parameterized individually for different Ni/YSZ interfaces. Having confirmed that the model well reproduces the bulk properties of YSZ, we employed the developed model to demonstrate that it reproduces with good accuracy the surface structures and bulk conductivity of YSZ. This allowed us to use the model to further study oxygen ion diffusion near (111) and (110) YSZ surfaces where YSZ structure can be very much distorted [5], and thus oxygen diffusion can be affected. Obtained results suggest suppressed oxygen diffusion along (111) because of the lower oxygen vacancy concentration near the surface. In contrast, oxygen vacancies tend to migrate to the (110) surface, which, together with activation of jumps along directions, facilitates diffusion along the surface. This indicates that it can be easier for oxygen ions to migrate to TPBs near the (111) surfaces from bulk regions than along the surface, while near the (110) surface ions prefers to migrate along the surface. We also demonstrate how the model performs for the nickel sintering problem, based on the structures consisting of two Ni particles being 30 Å in diameter and placed on a YSZ slab. We studied the effect of binding energy between the particles and the slab by considering different interfaces at the contact areas and using corresponding interactions. Simulations at 1250 K indicate that a relatively low binding energy of 0.5 J/m2, which corresponds to the Ni(111)/YSZ(111) interface, allows the nanoparticles to slide over the YSZ support and coalesce. When the binding energy is above 1.0 J/m2, the sliding is restricted and particle migration may occur only by means of nickel atom diffusion on nickel surface. [1] H. Lee et al., Acta Materialia 58 (2010) 2197. [2] P. Tangney et al., J. Chem. Phys. 117 (2002) 8898. [3] A. Iskandarov et al., J. Phys.: Condens. Matter 25 (2015) 015005. [4] Y. Mishin et al., Phys. Rev. B 59 (1999) 3393. [5] G. Ballabio et al., Phys. Rev. B. 70 (2004) 075417.

Molecular dynamics (MD) simulations of processes occurring in SOFC anodes require accurate description of atomic interactions. Typical SOFC anode represents nanoporous compound of Ni and yttria-stabilized zirconia (YSZ), and the main process, fuel oxidation, takes place in triple phase boundaries (TPB). Thus it is of importance, in particular, to reproduce bulk and surface properties of YSZ in order to describe interactions in TPBs with reasonable accuracy. In present paper we incorporate many-body interatomic potential based on dipole model for cubic YSZ. Parameters of the model have been determined by fitting energies, stresses and forces to their values derived from ab initio calculations. Classical MD simulations with the developed potential revealed its ability to describe YSZ surface energies and structures. The effect of temperature and yttria concentration on oxygen ions is studied and discussed.

Cation doping is often used to stabilize the cubic or tetragonal phase of zirconia for enhanced thermomechanical and electrochemical properties. In the present paper we report a combined density functional theory (DFT) and molecular dynamics study of the effect of Sc, Y, and Ce dopants on properties of Ni/[Formula: see text] interfaces and nickel sintering. First, we develop an MD model that is based on DFT data for various nickel/zirconia interfaces. Then, we employ the model to simulate Ni nanoparticles coalescing on a zirconia surface. The results show the possibility of particle migration by means of fast sliding over the surface when the work of separation is small (

We developed a new interatomic potential for yttria-stabilized zirconia (YSZ) based on the dipole model initially proposed by Tangney and Scandolo. It is demonstrated that the potential can successfully reproduce not only basic bulk properties, including interaction between point defects, but also energies and structures of clean (1 1 0) and (1 1 1) surfaces. We confirmed that the highly perturbed structure of (1 1 0) surface doped by yttria is in a good agreement with results of DFT calculations. Yttrium segregation at (1 1 1) surface was predicted and discussed by comparison with results of DFT simulations.

There have been a lot of studies dedicated to structural instability in solids. For local instability, theoretical (ideal) strength of crystals has been extensively studied withab initiocalculations. Global instability taking into account the collective motion of atoms involved in deformation has also been investigated. However, these studies have usually been done at 0 K and little has been understood about the effect of temperature. In this study, we demonstrate computational approaches to the effect of temperature on local and global instabilities. Ideal shear strength (ISS) of silicon at finite temperatures is calculated by molecular dynamics (MD) simulations with an empirical potential. ISS is obtained as a function of temperature. Our results imply that, unlike metals, the reduction in ISS by temperature cannot be estimated simply by taking into account thermal expansion of volume. In addition, global instability for dislocation nucleation in a Cu thin film model under tension is investigated. We first evaluated instability modes at 0 K with increasing strain, and then performed MD simulations at 50 K. After the nucleation of a partial dislocation, the second dislocation can be one to create a twin or one to create another partial dislocation. These different deformations can be understood as the competition of latent instability modes that have relatively small eigenvalues.