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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.

In order to rationalize the homologous nature of the Peierls stress reported in a previous paper (Y. Kamimura et al., Acta Mater. 61 (2013) 294–309), we have calculated numerically the Peierls stresses via the Peierls-Nabarro model (P-N model) of the planar core dislocation, using calculated ab-initio γ-surface for a variety of crystals: fcc metals, bcc metals, NaCl type crystals, CsCl type ionic crystals and various tetrahedrally coordinated crystals. The obtained γ-surfaces for the same group of crystals are found to have similar shape and height when normalized by materials parameters, indicating homologous nature. Except for some very soft crystals ( τ P exp / G ≪ 1 × 10−4), the calculated Peierls stresses for majority of crystals ( τ P exp / G > 1 × 10−4) are shown, for the first time, to have a correlation with experimental ones reflecting the homologous nature of the γ-surface, although the former are generally larger than the latter by half an order on the average, with an exception of bcc metal in which the strain field of the controlling screw dislocation is known to be non-planar. For very soft crystals, the calculated Peierls stresses are orders of magnitude larger than experimental ones; possible origins of this large discrepancy are discussed.

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.

Ni/YSZ cermet is a widely used SOFC anode material, particularly due to its combination of good electric conductivity, high catalytic activity and low cost. Nonetheless, Ni/YSZ cermet anodes are facing unique challenges such as nickel agglomeration and reoxidation, eventually resulting in anode degradation. A solution to these problems might be obtained via a better comprehension of the underlying manufacturing processes, particularly the final H2-induced reduction reaction of NiO to Ni. Experimental approaches have contributed immensely toward a better understanding of these phenomena, yet are generally limited in terms of spatial and temporal resolution, thus motivating us to investigate the problem from a multiscale simulation viewpoint. In this study, we developed a reactive force field (ReaxFF) for nickel/oxygen interactions, which was specifically tailored toward molecular dynamics (MD) simulations of NiO reduction in H2 environments. The force field parameters were derived by fitting the ReaxFF energies and forces of various nickel/oxygen structures to the respective values of first-principles calculations. The structures considered here include bulk NiO, Ni2O3 and Ni3O4 at various densities, NiO under shear, the NiO(100) surface, oxygen vacancies in bulk and surface NiO as well as oxygen adsorbed onto the NiO(100) surface. The developed force field was subsequently used to predict the reduction kinetics of nano-scale NiO crystals in a H2 atmosphere at different temperatures and hydrogen partial pressures. Here, we observed a significantly improved structural stability of bulk NiO at temperatures over 700 K, compared to our previous ReaxFF nickel/oxygen description [1] and similar ReaxFF parameter sets published by other authors [2]. Besides quantifying the reduction rate, further core phenomena such as the induction period before the onset of a steady state reduction process, the pore formation in NiO at lower temperatures, as well as the importance of oxygen vacancies during the reduction are discussed and compared with experimental results. Fig. 1 NiO nanocrystal after 0.2 ns of MD reduction simulation at a global temperature of 850 K and hydrogen partial pressure of 10 bars, using our previous ReaxFF parameter set (left) and the parameter set introduced in this publication (right). References [1] Oberhoffer, S., Iskandarov, A., Umeno, Y., "Reduction of NiO in a H2 Environment: A ReaxFF Molecular Dynamics Study",2016 Asian SOFC Symposium (2016.09.04-07), The University of Tokyo, Japan. [2] Zou, C., Shin, Y. K., van Duin, A. C., Fang, H., Liu, Z. K. "Molecular dynamics simulations of the effects of vacancies on nickel self-diffusion, oxygen diffusion and oxidation initiation in nickel, using the ReaxFF reactive force field.", Acta Materialia 83 (2015): 102-112. Figure 1

The electrode performance in solid oxide fuel cell depends both on electrode materials and microstructure. Therefore, it is important to breakdown the complexity of multi-physics in the porous electrode into processes in different scales. To clarify the multi-physics toward better electrode design, the use of both the advanced measurement and the simulation techniques is inevitable. For this purpose, we have taken the combined top-down and bottom-up approaches. In this manuscript, we discuss the practical issues in multi-scale, multi-physics simulation, focusing on the local activity of Ni-YSZ anode.

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.

We have computed Peierls stresses of dislocations in a variety of crystals by applying a new discretized Peierls-Nabarro model (D-PN model) of the dislocation using the calculated ab-initio γ-surface. In the D-PN model, the disregistry distribution of atomic-row pairs facing at the glide plane is determined by the balance between the restoring stress due to the disregistry and the internal stress by an array of small dislocations placed at every intermediate position of atomic-row pairs, instead of continuous distribution of infinitesimal dislocations in the original PN model. Peierls stresses have been obtained by applying the shear stress to the model under which the structure is allowed to relax. The calculated Peierls stresses are largely reduced compared with those previously obtained by the original PN model (Y. Kamimura et al., Acta Mater. 148 (2018) 355-362), although the dislocation widths of the two models are almost the same. They have a good correlation with those experimentally estimated by Y. Kamimura et al. (Acta Mater. 61 (2013) 294-306) except for some very soft crystals, though the calculated Peierls stresses are generally still larger than those of experiments by a few factor.

The theoretical shear strength (τc) and its temperature dependence for a series of FCC and HCP metals have been calculated. Despite the obtained differences in temperature dependences of τc, critical deformation (γc), and shear modulus (G), the fact of a weak temperature dependence of ratio Gγc/τc relating these quantities is established for all studied materials. The deviation of Gγc/τc from the average value in the temperature ranges under consideration is no more than 12%.

We have computed Peierls stresses of dislocations in a variety of crystals by applying a simple discretized Peierls–Nabarro model of the dislocation using the calculated ab-initio γ-surface. In the model, the disregistry distribution of atomic-row pairs facing at the glide plane is determined by the balance between the restoring stress due to the disregistry and the internal stress by an array of small dislocations placed at every intermediate position of atomic-row pairs, instead of continuous distribution of infinitesimal dislocations in the original PN model. Peierls stresses have been obtained by applying the shear stress to the model under which the structure is allowed to relax. The calculated Peierls stresses are largely reduced compared with those previously obtained by the original PN model (Kamimura et al., 2018), although the dislocation widths of the two models are almost the same. As a result, the former show much better correlation with those experimentally estimated by Kamimura et al. (2013) than the latter except for some very soft crystals.

Though a number of atomistic-model studies of yttria-stabilized zirconia (YSZ) have been reported to elucidate its properties, most of them have employed simple pairwise potential functions to express interactions between atoms, which limits the transferability of the models. We have developed a Tangney-Scandolo dipole model potential for YSZ. Energy, stress and forces on atoms calculated by the ab initio (first-principles) density functional theory are provided as reference data for potential fitting. The developed potential successfully reproduces cubic-tetragonal phase transition at a range of yttria concentration relevant with SOFC application. The potential can well reproduce the barrier energy of oxygen vacancy migration. Molecular dynamics simulations of oxygen diffusion in bulk and at grain boundaries are demonstrated.

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.

B ω = ϕ +ϕ − ϕ , where i is the number of the coordination sphere. Ordering takes place due to diffusion of atoms via a vacancy mechanism with the probability of migration of an atom into a vacant node depending on growth of the energy associated with this migration, and on the temperature. The computer model was based on the solid sphere approximation and pairwise interatomic potentials. It was previously shown that for the AB stoichiometry the highest ordering rate for fixed values of i ω is attained for () i AA ϕ = () i BB ϕ and that the ordering rate decreases with growth of the difference between these two energies. It is shown that this result is quite general and that it can be generalized to alloys of the compositions 3 AB and 3 A B.

Graphene draws the attention of researchers due to its unique properties-in particular, record-high tensile strength. The time to fracture (TTF) of defect-free graphene strained by tension at a nonzero temperature has been studied by the method of molecular dynamics (MD). It is established that the time to thermoactivated fracture has a probabilistic character and obeys an exponential distribution. The mean TTF is proportional to the area of the graphene sheet and obeys the Arrhenius-Zhurkov law as a function of temperature and applied stress. The dependence of the activation energy for graphene fracture on the applied stress and sample area has been extrapolated to values of these parameters relevant for practical applications. The mechanism of graphene fracture has been analyzed.

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 (

A simple computer algorithm is proposed to describe all possible energy states of stoichiometric alloys; its application is illustrated with AB alloys having fcc, bcc, and sc lattices. This algorithm is based on the concentration-wave method, which uses the rigid-sphere approximation and takes into account pair atomic interactions in the first few coordination spheres.

The equilibrium ordering states of A 1−x B 1+x alloys are investigated analytically and by computer simulation with the hard-sphere and pair-binding model. The alloy energies are calculated for the case where an excess of the B component forms a solid solution or precipitates as pure phase particles. Criteria for pair-binding energies are obtained that define the type of the resulting alloy. The analytical results are presented for an alloy of arbitrary dimensionality and for any number of coordination spheres involved in interatomic interactions. Illustrating examples are given for B2 and L10 superstructures and for a two-dimensional square lattice in view of the atomic interaction in the first two coordination spheres.

Influence of temperature on ideal shear strength (ISS), τc, of two fcc metals (Al and Cu) was studied by means of molecular dynamics simulations. To get reliable results we investigated influence of parameters of the applied Parrinello-Rahman stress control method and implemented damping of simulation cell fluctuations to avoid occurrence of structural instability assisted by too high fluctuations. The damping successfully reduces strain and stress fluctuations during simulations if the damping factor is specified properly. We also investigated simulation cell size effect to evaluate minimal number of atoms providing reliable results in order to reduce computational efforts and estimate the possibility of applying ab initio calculations. Recently developed embedded atom method (EAM) interatomic potentials for both metals were also examined to find most appropriate for our study. EAM potential developed by Zope et al. and Mishin et al. were revealed to be most suitable for Al and Cu, respectively. It is essential to choose appropriate simulation parameters and interatomic potentials for the valid evaluation of ISS at elevated temperatures. We find almost linear decrease in ideal strength with increasing temperature for [112](111) shear deformation, while critical strain decreases in a nonlinear manner. At room temperature, reduction of shear strength for Al(Cu) is less than 35%(25%) compared to that at 0 K.

Interaction of straight edge dislocation clusters with monochromatic sound wave having nonzero wavevector is investigated taking into account the dislocation mass. We report on a significant increase of drift velocities of clusters when the sound wave frequency approaches a cluster's eigenfrequency. Of practical importance is the increase of the drift velocity observed for clusters with nonzero topological charge interacting with small frequency sound waves. We also demonstrate the possibility to excite a gap discrete breather in a chain of dislocation dipoles.

In typical porous Ni/yttria-stabilized zirconia (YSZ) anode of solid oxide fuel cells (SOFCs), hydrogen oxidation reaction (HOR) occurs at the pore/nickel/zirconia triple-phase boundaries (TPBs). This anode compound i) allows gaseous hydrogen molecules to move in pores to the reaction sites, ii) makes current flow possible from the TPBs to current collector via nickel matrix, iii) allows oxide ions to transfer to the reaction sites by means of oxygen ion diffusion in zirconia. The last point is possible only when zirconia is doped by cations with oxidation states different from that of Zr4+. For instance, doping by Y3+implies creation of an oxygen vacancy for a pair of dopant cations. So, it is actually presence of the oxygen vacancies that eventually makes the oxygen diffusion possible in SOFC anode. Moreover, oxygen diffusion provides an oxygen-ion flow from cathode to anode through the solid electrolyte made of doped-zirconia. Considering high importance of the dopants in zirconia for SOFC performance, there have been intensive studies of their effect on various phenomena: oxygen diffusion, nickel sintering, phase stability, etc. However, as for details of HOR on atomic scale, all up-to-date theoretical studies considered only Ni/YSZ systems [1-3]. In the present work we perform atomistic modeling study based on density-functional theory (DFT) to see how qualitative and quantitative characteristics of HOR can be affected by selection of dopant species. The TPB structure for our study is based on the (111) zirconia and (111) Ni slabs that are attached to each other as shown in Figure 1 [4]. Two dopants are introduced instead of two zirconium cations near the TPB (sites A and B). We also introduce oxygen vacancies into zirconia slab, whose number is appropriate to formal oxidation state of the dopants and to the fact that we consider one-oxygen rich zirconia slab in reactant structures (product structures are stoichiometric). In this work we consider three trivalent (Y, Sc, Al) and two divalent dopants (Ba, Ca). Three reaction mechanisms are considered: interface reaction, O spillover, and H spillover (terms are used according to notation by Shishkin et al. [5]). All calculations were performed with Vienna Ab initio Simulation Package (VASP) [6], which can perform electronic structure calculations based on the projector-augmented wave method and the generalized gradient approximation. To evaluate transition states, we performed nudge elastic band calculations as implemented in VASP by Henkelman et al. [7]. Obtained DFT results for the total energy of the structures were processed with open-circuit voltage (OCV) correction in a similar concept given by Nørskov et al. [8]. Energy profiles built for the HOR mechanisms allow to determine the most energetically favorable mechanism for each dopant. This judgement is based on energy barriers of the rate-limiting steps (activation energy). Obtained results for Y-doped TPB agree well with literature data that predicts interface reaction mechanism to be dominant with the rate-limiting step being H transfer from the nickel surface to oxygen ion at the TPB (as indicated in Figure 1 by blue line) [2-3,5]. Qualitatively similar results were obtained for Sc and Ba. In contrast, Al and Ca dopants make O spillover mechanism favorable with the rate-limiting step being O transfer from TPB region onto the nickel surface (as indicated in Figure 1 by red line). Considering dopants promoting the same HOR mechanism, activation energies for trivalent dopants are lower, which makes them better dopant candidates. Specifically, low activation energy of O spillover mechanism in Al-doped system makes Al best candidate to promote this mechanism. Background behind switching of the reaction mechanism with respect to dopant species is analyzed and discussed in this paper. References [1] Shishkin, M.; Ziegler, T. Phys. Chem. Chem. Phys. 2014, 16, 1798 [2] Cucinotta, C. S.; Bernasconi, M.; Parrinello, M. Phys. Rev. B 2011, 107, 206103 [3] Liu, S.; Ishimoto, T.; Monder, D. S.; Koyama, M. J. Phys. Chem. C 2015, 119, 27603 [4] Tada T. ECS Transactions 2015, 68, 2875 [5] Shishkin, M.; Ziegler, T. J. Phys. Chem. C 2010, 114, 11209 [6] Kresse, G.; Hafner, J. Phys. Rev. B. 1993, 47, 558 [7] Henkelman, G.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901 [8] Nørskov, J.K; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. J. Phys. Chem. B 2004, 108, 17886 Caption: Figure 1. TPB model consisting of nickel and zirconia slabs. Nickel atoms are shown in black, oxygen - in red, zirconium - in white. Positions of cation dopants A and B are shown in green. Red line indicates O transfer step of O spillover mechanism, blue line - H transfer step of interface reaction mechanism. Figure 1

Electrode processes in solid oxide fuel cells are intensively studied to increase the system efficiency to compete with that of the state-of-the-art LNG-fueled combined cycle thermal power plant. Because the electrode performance depends both on electrode materials and microstructure, we are obliged to breakdown the complexity of phenomena in the porous electrode into elementary processes by best utilizing both the advanced measurement and the simulation techniques. Closely examining the efforts in literature [1-7], we have taken the combined top-down and bottom-up approach. The most difficult issue has lied on the meso-scale modeling, which typically is based on microkinetic modeling [5-8]. To guarantee the predictivity of the modeling, it is inevitable to realize the simulation without arbitrary fitting parameters that are not necessarily based on realistic physics. For this purpose, we first exhaustively summarized literature data followed by the careful examinations. Next, we have made exhaustive simulations assuming different combinations of parameters in literature. Finally, we obtained a set of parameters that matches with experimental observations within reasonable differences. The details together with the results of top-down and bottom up simulations will be introduced together with future perspectives. Acknowledgements The activities of INAMORI Frontier Research Center is supported by Kyocera Co. Ltd. The activities of Advanced Automotive Research Collaborative Laboratory in Hiroshima University are supported by Mazda Corporation. The part of the research is supported by CREST, Japan Science and Technology Agency. References [1] J. Mizusaki, H. Tagawa, T. Saito, T. Yamamura, K. Kamitani, K. Hirano, S. Ehara, T. Takagi, T. Hikita, M. Ippommatsu, S. Nakagawa, K. Hashimoto, Solid State Ionics, 70/71, 52 (1994) [2] W. Yao and E. Croiset, Can. J. Chem. Eng., 93, 2157 (2015) [3] A. Bieberle, L. P. Meier, L. J. Gauckler, J. Electrochem. Soc., 148, A646 (2001) [4] A. Utz, H. Störmer, A. Leonide, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc., 157 B920 (2010) [5] W. G. Bessler, S. Gewies, M. Vogler, Electrochim. Acta, 53, 1782 (2007) [6] W. G. Bessler, Solid State Ionics, 176, 997 (2005) [7] M. Vogler, A. Bieberle-Hütter, L. Gauckler, J. Warnatz, and W. G. Bessler, J. Electrochem. Soc., 156, B663 (2009) [8] H. Kohno, S. Liu, T. Ogura, T. Ishimoto, D. S. Monder, K. Karan, M. Koyama, ECS Trans., 57(1), 2821 (2013)

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.

According to Frenkel's estimation, at critical shear stress ${\ensuremath{\tau}}_{c}=G/2\ensuremath{\pi}$, where $G$ is the shear modulus, plastic deformation or fracture is initiated even in defect-free materials. In the past few decades it was realized that, if material strength is probed at the nanometer scale, it can be close to the theoretical limit, ${\ensuremath{\tau}}_{c}$. The weakening effect of the free surface and other factors has been discussed in the literature, but the effect of temperature on the ideal strength of metals has not been addressed thus far. In the present study, we perform molecular dynamics simulations to estimate the temperature effect on the ideal shear strength of two fcc metals, Al and Cu. Shear parallel to the close-packed (111) plane along the $[11\overline{2}]$ direction is studied at temperatures up to 800 K using embedded atom method potentials. At room temperature, the ideal shear strength of Al (Cu) is reduced by 25% (22%) compared to its value at 0 K. For both metals, the shear modulus, $G$, and the critical shear stress at which the stacking fault is formed, ${\ensuremath{\tau}}_{c}$, decrease almost linearly with increasing temperature. The ratio $G/{\ensuremath{\tau}}_{c}$ linearly increases with increasing temperature, meaning that ${\ensuremath{\tau}}_{c}$ decreases with temperature faster than $G$. Critical shear strain, ${\ensuremath{\gamma}}_{c}$, also decreases with temperature, but in a nonlinear fashion. The combination of parameters, $G{\ensuremath{\gamma}}_{c}/{\ensuremath{\tau}}_{c}$, introduced by Ogata et al. as a generalization of Frenkel's formula, was found to be almost independent of temperature. We also discuss the simulation cell size effect and compare our results with the results of $\mathit{ab}\phantom{\rule{4pt}{0ex}}\mathit{initio}$ calculations and experimental data.

We study the dynamics of one- and two-dimensional diatomic lattices with the interatomic Morse potentials for the initial conditions selected at the edge of the Brillouin zone of the dispersion spectrum, when only light atoms are excited with the staggered mode while all heavy atoms remain at rest (the so-called anti-Fermi-Pasta-Ulam problem). We demonstrate that modulational instability of such a nonlinear state may result in almost periodic temporal dynamics of the lattice with spatial localization and delocalization of energies. Such a recursion occurs many times with a very slow decay, especially for the initial states with low energy. The energy recursion results in the formation of highly localized, large-amplitude gap discrete breathers. For one-dimensional diatomic lattices, we describe the periodic energy recursion analytically for a simple model with the nearest-neighbor interaction and cubic anharmonicity.

A solid oxide fuel cell (SOFC) system anode material is usually composed of a mixture of Ni and other oxide ceramics, such as doped-CeO2, and Sc stabilized ZrO2, due to their high activity and structural stability. Among them the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet is widely used today as an anode material of SOFC due to its good electrocatalytic property against low price. For SOFC performance, it is essential to improve the stability of the Ni/YSZ cermet anode. Combination of Ni and YSZ has been studied experimentally, but the interaction is not well understood in the boundary between Ni and YSZ. However, there is no evidence about the atomic-scale Ni/YSZ interface dynamics and chemical changes in the structure during the operation of SOFC. Unfortunately, most classical force fields are incapable of describing chemical reactions. To overcome this difficulty, reactive force field (ReaxFF) potential was developed by van Duin and co-workers. Recently, Ni/YSZ/H2 interactions are reported by ReaxFF. But in this interaction Ni/YSZ interface stability is not taken into account. As a first step we developed our own ReaxFF potential to investigate Ni/ZrO2 interface and for determining stable Ni/YSZ interface for modeling of the triple-phase-boundary in SOFCs. Big difference is found between our developed and reported ReaxFF and that is work of separation. Using our ReaxFF potential, work of separation is in excellent agreement with literature. It is found that O around Zr4+cation is reduced in case of bulk and interface regions. Although interface region exhibited less reduction compared to bulk region. More details and Ni/YSZ interface stability through O vacancy may present at conference.

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.