Solid State Phenomena Vol. 139

Abstract: In the present study molecular dynamics simulations were carried out to investigate the deformation of pure FCC aluminum and diamond cubic silicon interfaces under shear stress. A second nearest-neighbor modified embedded atom method was used to describe the interactions between Al-Al, Si-Si and Al-Si atoms. The critical shear stress (CSS) was determined for various Al/Si and Al/Al interfaces with different alignments and orientations. Structural analyses show that the deformation is localized at approximately 10 Å thickness of the interface in Al. The critical shear stress of Al/Si interface was found to be significantly lower than the critical tensile stress due to the partial stick-slip in sliding. In addition, it has been proven that there is no explicit relationship between shear and tensile critical stresses, which is fundamentally different from isotropic materials, where the shear stress is about half of the tensile stress. The misorientation has a dramatic effect in reducing shear stress at Al/Al interfaces, but has no effect on CSS in Al/Si. As a result, it was shown that introducing Si improves the strength of the interface (and the composite material in general) for different grain orientations.

Abstract: Nanostructured composites inspired by structural biomaterials such as bone and nacre form intriguing design templates for biomimetic materials. Here we use large scale molecular dynamics to study the shock response of nanocomposites with similar nanoscopic structural features as bone, to determine whether bioinspired nanostructures provide an improved shock mitigating performance. The utilization of these nanostructures is motivated by the toughness of bone under tensile load, which is far greater than its constituent phases and greater than most synthetic materials. To facilitate the computational experiments, we develop a modified version of an Embedded Atom Method (EAM) alloy multi-body interatomic potential to model the mechanical and physical properties of dissimilar phases of the biomimetic bone nanostructure. We find that the geometric arrangement and the specific length scales of design elements at nanoscale does not have a significant effect on shock dissipation, in contrast to the case of tensile loading where the nanostructural length scales strongly influence the mechanical properties. We find that interfacial sliding between the composite’s constituents is a major source of plasticity under shock loading. Based on this finding, we conclude that controlling the interfacial strength can be used to design a material with larger shock absorption. These observations provide valuable insight towards improving the design of nanostructures in shock-absorbing applications, and suggest that by tuning the interfacial properties in the nanocomposite may provide a path to design materials with enhanced shock absorbing capability.

Abstract: We studied the atomic-level structure of a model Mg-MgH2 interface by means of the Car-Parrinello molecular dynamics method (CPMD). The interface was characterized in terms of total energy calculations, and an estimate of the work of adhesion was given, in good agreement with experimental results on similar systems. Furthermore, the interface was studied in a range of temperatures of interest for the desorption of hydrogen. We determined the diffusivity of atomic hydrogen as a function of the temperature, and give an estimate of the desorption temperature.

Abstract: We performed first-principles calculations using the projector augmented-wave (PAW) method for Au/Pd slab interface models. The calculations of relaxed configurations and energies for the thin Pd layers (3 layers) stacking on Au (111) and Au (100) slabs with an epitaxial relationship represent that Pd overlayers have a lateral expansion in both cases. This trend is in good agreement with experimental results for Pd/Au slabs and Au-Pd core-shell nanoparticles, obtained by electron microscopy, X-ray diffraction, and positron annihilation. In addition, an intermixing configuration near the Au-Pd interface was shown to be more stable than the binary separated one.

Abstract: In this paper, we investigate oxygen in-diffusion and out-diffusion with respect to a cermet composite where oxygen segregates at the interface between the metal matrix phase and the ceramic oxide phase. This phenomenological diffusion problem is treated by overlaying it with a fine-grained lattice that was addressed using a Lattice Monte Carlo method and a little-known exact expression for the lattice-based effective diffusivity in the presence of random traps. It is shown that there is very good agreement for the oxygen concentration depth profiles between the Monte Carlo results and the exact expression.

Abstract: Pt nano-particles are supported on carbon materials at the electrode catalysts of protonexchange menbrane fuel cells. Pt nano-particles are desirable to be strongly adsorbed on carbon materials for high dispersion, although strong Pt-C interactions may affect the catalytic activity of small clusters. Thus we have examined H-atom absorption on Pt clusters supported or unsupported on graphene sheets, using first-principles calculations. For Pt-atom/graphene systems, a H atom is more weakly adsorbed than for a free Pt atom, and the H-Pt interaction becomes weaker if the interaction between a Pt atom and graphene becomes stronger. For the Ptn-cluster/graphene systems (n=2-4), the H-Pt interactions are also substantially changed from those for free Pt clusters. In the Pt clusters on graphene, the Pt-Pt distances are substantially changed associated with the electronicstructure changes by the Pt-C interactions. These structural and electronic changes in the Pt clusters as well as the presence of graphene itself seem to cause the changes in the absorption energies and preferential sites of H-atom absorption.

Abstract: Atomic and electronic structures of H-adsorbed Pd overlayers on Au(100) substrates have been studied by first-principles calculations. The geometric strain effects change the electronic structure and local reactivity of the surface. The lattice strained Pd overlayers on Au surfaces have larger adsorption energies for atomic hydrogen than the unstrained Pd slabs. Adsorption energies for several adsorption sites on the models with different numbers of Pd overlayers have been analyzed from the viewpoints of strains and H-Pd and H-substrate interactions.

Abstract: Bismuth cuprate superconductor has a unique structure called a structural modulation (supercell, SC) consisting of modulated several unit cells. Strain induced by multilayered structure increases the intensity of SC modulation, while an oxygen deficient sample shows expansion of SC size. In this study, as opposed to the multilayer strain, by preparing samples with thick film thicknesses the effect of strain on crystal structure was investigated including SC structure. Epitaxial growth was verified by x-ray diffraction, and the thicker film showed other epitaxial phase rotated 32° around the surface normal with respect to the initial epitaxial phase. The SC size estimated by x-ray reciprocal space mapping was double the size of the initial epitaxial phase. Interestingly, the initial epitaxial phase became a dominant structure after further deposition. In order to evaluate the different SC size and SC modulation, a new index related with an incline of the modulation vector was proposed.

Abstract: Constant strain rate molecular dynamics simulations under the modified boundary conditions were performed to elucidate the interaction processes between the kink motion of screw dislocation and the glissile self-interstitial atom cluster loops in bcc Fe by using an EAM potential for Fe fitted to ab initio forces. The junction formation and the helical dislocation mechanisms were identified as two possible interaction processes. In the junction mechanism, the initial Burgers vector 1/2<111> of the cluster loop was transformed into <100>. In the helical dislocation mechanism first the absorption, followed by the formation of the helical dislocation and the emission of the cluster loop through Hirsch mechanism was observed. Substantial hardening was seen as result of the interactions. The stress-strain plots obtained for different loop sizes, temperature and strain rates were used to estimate the strengthening factors.