96 research outputs found
Realistic time-scale fully atomistic simulations of surface nucleation of dislocations in pristine nanopillars
We use our recently proposed accelerated dynamics algorithm (Tiwary and van de Walle, 2011) to calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we achieve the fraction of a second time-scale regime. We find that the activation free energy depends significantly and non-linearly on the driving force (stress or strain) and temperature, leading to very high activation entropies. We also perform compression tests on Gold nanopillars for strain-rates varying between 7 orders of magnitudes, reaching as low as 10^3/s. Our calculations bring out the perils of high strain-rate Molecular Dynamics calculations: we find that while the failure mechanism for compression of Gold nanopillars remains the same across the entire strain-rate range, the elastic limit (defined as stress for nucleation of the first dislocation) depends significantly on the strain-rate. We also propose a new methodology that overcomes some of the limits in our original accelerated dynamics scheme (and accelerated dynamics methods in general). We lay out our methods in sufficient details so as to be used for understanding and predicting deformation mechanism under realistic driving forces for various problems
Ab initio thermodynamics of intrinsic oxygen vacancies in ceria
Nonstoichiometric ceria(CeO) is a candidate reaction medium to
facilitate two step water splitting cycles and generate hydrogen. Improving
upon its thermodynamic suitability through doping requires an understanding of
its vacancy thermodynamics. Using density functional theory(DFT) calculations
and a cluster expansion based Monte Carlo simulations, we have studied the high
temperature thermodynamics of intrinsic oxygen vacancies in ceria. The DFT+
approach was used to get the ground state energies of various vacancy
configurations in ceria, which were subsequently fit to a cluster expansion
Hamiltonian to efficiently model the configurational dependence of energy. The
effect of lattice vibrations was incorporated through a temperature dependent
cluster expansion. Lattice Monte Carlo simulations using the cluster expansion
Hamiltonian were able to detect the miscibility gap in the phase diagram of
ceria. The inclusion of vibrational and electronic entropy effects made the
agreement with experiments quantitative. The deviation from an ideal solution
model was quantified by calculating as a function of nonstoichiometry, a) the
solid state entropy from Monte Carlo simulations and b) Warren-Cowley short
range order parameters of various pair clusters
Hybrid deterministic and stochastic approach for efficient atomistic simulations at long time scales
We propose a hybrid deterministic and stochastic approach to achieve extended
time scales in atomistic simulations that combines the strengths of molecular
dynamics (MD) and Monte Carlo (MC) simulations in an easy-to-implement way. The
method exploits the rare event nature of the dynamics similar to most current
accelerated MD approaches but goes beyond them by providing, without any
further computational overhead, (a) rapid thermalization between infrequent
events, thereby minimizing spurious correlations, and (b) control over accuracy
of time-scale correction, while still providing similar or higher boosts in
computational efficiency. We present two applications of the method: (a)
Vacancy-mediated diffusion in Fe yields correct diffusivities over a wide range
of temperatures and (b) source-controlled plasticity and deformation behavior
in Au nanopillars at realistic strain rates (10^4/s and lower), with excellent
agreement with previous theoretical predictions and in situ high-resolution
transmission electron microscopy observations. The method gives several
orders-of-magnitude improvements in computational efficiency relative to
standard MD and good scalability with the size of the system.Comment: 4 pages, 2 figures. Corrected logarithm base in figures 2 and
First-Principles Calculation of the Cu-Li Phase Diagram
We present first-principles calculations of the solid-state portion of the Cu-Li phase diagram
based on the cluster expansion formalism coupled with the use of (i) bond length-dependent
transferable force constants and lattice dynamics calculations to model of vibrational disorder
and (ii) lattice gas Monte Carlo simulations to model configurational disorder. These calculations
help settle the existence of additional phases in the Cu-Li phase diagram that have been
postulated, but not yet clearly established. Our calculations predict the presence of at least one
additional phase and the associated predicted phase transitions are consistent with our
electrochemical measurements, which exhibit clear plateaus in the electromotive force-composition curve
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