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Natural sulfidation of silver nanomaterials can passivate the surface, while preserving desirable optical and electrical properties, which is beneficial for limiting Ag+ release and cytotoxicity.

Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

Defect-mediated mass transport is a critical phenomenon for many material performance and degradation issues. In the area of stress corrosion cracking, a predictive model of mass transport under realistic, elevated temperatures would be instrumental in predicting crack initiation and propagation tendencies. Molecular simulations can address some of these gaps about underlying processes. In that context, this study presents a robust model that describes self-diffusion at high temperatures in Ni-xCr alloys with varying Cr concentrations. We show that by using a vacancy concentration rescaling factor, it is possible to properly estimate diffusion coefficients in Ni-5Cr, Ni-10Cr and Ni-20Cr. The model reaches its upper limit at 1600 K for Ni-20Cr, because it has been fitted on dilute solute concentrations. Extrapolations at low temperature match experimental trend. Simulations with a theoretical number of vacancies give results comparable to the diffusion coefficients with the rescaling factor. In further simulations of more complex systems, efforts should be made in interstitial formation and migration barrier energy description.

International audience; We use molecular dynamics to investigate how the structure, diffusion, and hydrodynamic properties of clay interfaces with aqueous solutions depend on the nature of the clay, the nature of the counterions, and the salt concentration in the solution. Specifically, we study water-filled nanopores between uncharged (pyrophyllite) and charged (montmorillonite and beidellite, with substitutions located in the octahedral and tetrahedral layers, respectively) clays, with sodium or cesium as counterions, in the absence and in the presence of added salt. We discuss how the balance between solvation and attraction of the cations to the surface results in various distributions between inner- and outer-sphere complexes, and how this influences the dynamics of water near the surface, as well as the hydrodynamic flow in the presence of an external force. In the latter case, the discussion based on mapping the molecular velocity profiles to a continuous description (parabolic Poiseuille flow) shows that the larger effects come from the presence/absence of charge in the mineral, as well as the localization of substitutions within the clay layer. The salt concentration and the nature of the counterions have a comparatively less important impact far from the surface—even though some differences are observed in its close vicinity, which are not properly captured by the continuous description.