Anomalous diffusion along metal/ceramic interfaces

Interface diffusion along a metal/ceramic interface present in numerous energy and electronic devices can critically affect their performance and stability. Hole formation in a polycrystalline Ni film on an α-Al2O3 substrate coupled with a continuum diffusion analysis demonstrates that Ni diffusion along the Ni/α-Al2O3 interface is surprisingly fast. Ab initio calculations demonstrate that both Ni vacancy formation and migration energies at the coherent Ni/α-Al2O3 interface are much smaller than in bulk Ni, suggesting that the activation energy for diffusion along coherent Ni/α-Al2O3 interfaces is comparable to that along (incoherent/high angle) grain boundaries. Based on these results, we develop a simple model for diffusion along metal/ceramic interfaces, apply it to a wide range of metal/ceramic systems and validate it with several ab initio calculations. These results suggest that fast metal diffusion along metal/ceramic interfaces should be common, but is not universal

Mesoscale flux-closure domain formation in single-crystal BaTiO3

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields,and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of mesoscopic dipole closure patterns in free-standing, single-crystal lamellae of BaTiO3. Formation of these patterns is a dynamical process resulting from system relaxation after the BaTiO3 has been poled with a uniform electric field. The flux-closure states are composed of shape conserving 90° stripe domains which minimize disclination stresses

Dislocation climb effects on particle bypass mechanisms

We examine the effects of dislocation climb on the mechanisms by which dislocations bypass particles. The analysis is based upon three-dimensional, level-set, dislocation dynamics simulations that include all elastic interactions, dislocation glide, cross-slip and climb, and particles that are either impenetrable or penetrable and either with or without misfit. When the particle is misfitting with respect to the matrix, the dislocation migration is strongly influenced by the elastic fields created by the misfit. An edge dislocation tends to climb towards either the top or bottom of the particle and may remain there if the stress is not too large. A screw dislocation may wrap around the particle several times, creating a helical dislocation structure at small applied stresses. If the stress is increased, these helices break into an array of loops. Without misfit, climb invariably lowers the threshold stress particle bypass. However, in the misfit case, climb can also lead to more stable dislocation structures and, hence, increases the threshold stress. This report emphasizes the detailed bypass and pinning mechanisms and provides insight into the conditions under which these mechanisms operate

Strain engineering of 2D semiconductors and graphene : from strain fields to band-structure tuning and photonic applications

202101 bcrcVersion of RecordPublishe

Nanocrystalline copper films are never flat

We used scanning tunneling microscopy to study low-angle grain boundaries at the surface of nearly planar copper nanocrystalline (111) films. The presence of grain boundaries and their emergence at the film surface create valleys composed of dissociated edge dislocations and ridges where partial dislocations have recombined. Geometric analysis and simulations indicated that valleys and ridges were created by an out-of-plane grain rotation driven by reduction of grain boundary energy. These results suggest that in general, it is impossible to form flat two-dimensional nanocrystalline films of copper and other metals exhibiting small stacking fault energies and/or large elastic anisotropy, which induce a large anisotropy in the dislocation-line energy