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Atomistic simulations are carried out to analyze the influence of oxygen environment on nickel and copper surface roughness and notch initiation. The early stages of oxidation of nickel and copper surfaces are first simulated and compared with experimental observations. Various oxygen superstructures observed on metal surfaces are reproduced as well as the nucleation of small NiO embryos. Nickel and copper surface oxidation mechanisms are different and different “oxide” nano layers are formed. None of these superficial nano layers has a major influence on the mechanical behavior of surface slips as they do not change the surface roughness fatigue evolution and micro-notch production. These atomistic results agree with experimental studies which report similar development of persistent slip band surface relief in inert and in air environment. A general model for the estimation of surface slip irreversibility is also provided and the models of environment-assisted surface relief evolution and microcrack initiation are revisited.
Simulations of TiO2(both rutile and anatase) nanoparticles with water, methanol, and formic acid were conducted using a ReaxFF reactive force field to investigate the characteristic behavior of reactivity to these organic solvents. The force field was validated by comparing water dissociative adsorption percentage and bond length between Na and O with density functional theory (DFT) and experimental results. In the simulations, 1-nm rutile and anatase nanoparticles with water, methanol, and formic acid were used, respectively. The numbers of attached hydroxyl with time and nanoparticles distortion levels are presented. We found that the rutile nanoparticle is more reactive than the anatase nanoparticle and that formic acid distorts nanoparticles more than water and methanol.
Recent developments in reactive potentials for the simulation of complex bonding and complex chemistry are reviewed. In particular, the reactive force field and charged optimized many-body methods are two paradigms that enable atoms to autonomously determine their charge state and the nature of their local bonding environments. The capabilities of these methods are illustrated by examples involving ionic-covalent systems, a metal-covalent system, a high-k dielectric gate stack, and the interaction of water with an oxide. Prospects for future development and applications are also discussed.
We report a study of dynamic cracking of a silicon single crystal in which the ReaxFF reactive force field is used for about 3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. This model has been successfully used to study crack dynamics in silicon, capable of reproducing key experimental results such as orientation dependence of crack dynamics (Buehler et al., Phys. Rev. Lett., 2006). Here we focus on crack speeds as a function of loading and crack propagation mechanisms. We find that the steady state crack speed does not increase continuously with applied load, but instead jumps to a finite value immediately after the critical load, followed by a regime of slow increase. Our results quantitatively reproduce experimental observations of crack speeds during fracture in silicon along the (111) planes, confirming the existence of lattice trapping effects. We find that the underlying reason for this behavior is formation of a 5-7-double ring defect at the tip of the crack, effectively hindering nucleation of the crack at the Griffith load. We develop a simple continuum model that explains the qualitative behavior of the fracture dynamics.
We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for ∼3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including details of crack dynamics for loading in the  orientations, such as dynamical instabilities with increasing crack velocity. We also observe formation of secondary microcracks ahead of the moving mother crack. We conclude with a study of Si(bulk)-O2 systems, showing that Si becomes more brittle in oxygen environments, as known from experiment.
In this paper, we report preliminary studies of formation of water from molecular oxygen and hydrogen. Using a series of atomistic simulations carried at finite temperature, we describe the dynamics of water formation at a Pt catalyst using a new reactive ReaxFF potential. By performing a series of studies, we obtain statistically meaningful trajectories to extract rate constants of water formation. This allows an estimate for the activation energy during water formation, which is found to be in reasonable agreement with the activation barrier calculated by restraint driven molecular dynamics simulation of water formation at the Pt surface.
Predicting the properties and behavior of materials by computer simulation from a fundamental, ab initio perspective has long been a vision of computational material scientists. The key to achieving this goal is utilizing hierarchies of paradigms and scales that connect macrosystems to first principles quantum mechanics (QM). Here we describe a new software environment, the “Computational Materials Design Facility” (CMDF), capable of simulations of complex materials studies using a variety of simulation paradigms. The CMDF utilizes a Python scripting layer to integrate different computational tools to develop multi-scale simulation applications. We have integrated DFT QM methods, the first principles ReaxFF reactive force field, empirical all atom force fields (FFs), mesoscale and continuum methods. The central data structure Extended OpenBabel (XOB) plays a critical role as glue between applications. We demonstrate the usefulness of CMDF in examples that couple complex chemistry and mechanical properties during dynamical failure processes, as for example in a study of cracking of Ni under presence of O2.
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