Method for long time scale simulations of solids: Application to crystal growth and dopant clustering

An important challenge in theoretical chemistry is the time scale problem. Atomic motion can be simulated directly by integrating Newton's equations over a time scale of nanoseconds, but most interesting chemical reactions take place on a time scale of seconds. We have developed a methodology to bridge this time scale gap using harmonic transition state theory suitable for solid systems. Possible reactive events and their rates are found with a saddle point finding method called the dimer method. When enough events are found, a kinetic Monte Carlo algorithm is used to choose which event occurs so that the system's position can be advanced in time. This technique has two major advantages over traditional kinetic Monte Carlo -- atoms do not have to map onto lattice sites for classification and kinetic events can be arbitrarily complicated. We have studied the homoepitaxial growth of aluminum and copper using an EAM potential at 80K with experimentally relevant deposition rates of monolayers per minute using a multiple time scale approach. Atomic deposition events are simulated directly with classical dynamics for several picoseconds until the incident energy has dissipated, and the long time between deposition events is simulated with the adaptive kinetic Monte Carlo method. Our simulations indicate that the Al( 100) surface grows much smoother then Cu( 100) at temperature between 0 and 80K due in part to long range multi atom processes which enable aluminum atoms to easily descend from atop islands. The high rate of such processes is due to their low activation energy, which is supported by density functional theory calculations, and the trend that processes involving more atoms tend to have larger prefactors and be favored by entropy. The scheme is efficient enough to model the evolution of systems with ab-initio forces as well, for which I will show an example of the breakup of dopant clusters in silicon

Stress-induced phase transformation in nanocrystalline UO2

We report a stress-induced phase transfonnation in stoichiometric UO{sub 2} from fluorite to the {alpha}-PbO{sub 2} structure using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. MD simulations, performed on nanocrystalline microstructure under constant-stress tensile loading conditions, reveal a heterogeneous nucleation of the {alpha}-PbO{sub 2} phase at the grain boundaries followed by the growth of this phase towards the interior of the grain. The DFT calculations confinn the existence of the {alpha}-PbO{sub 2} structure, showing that it is energetically favored under tensile loading conditions

Barrier-free predictions of short-range ordering/clustering kinetics in binary FCC solid solutions

We present comparisons of kinetic Monte Carlo (kMC) simulations of isothermal short-range ordering (SRO) and clustering (SRC) kinetics in binary FCC alloys with a mean-field concentration wave (CW) model. We find that the CW model is able to give order-of-magnitude agreement with kMC simulations for ordering/clustering relaxation times over a wide range of temperatures and compositions. The advantage of the CW model is that it does not require parameterization of vacancy hopping energy barriers, which, for a concentrated alloy, becomes prohibitive. We assess limits in the accuracy of the model, and discuss the effect of cooling rates as well as the extension to multi-component systems. Ultimately, the simplicity and performance of the CW model compared to kMC simulations suggests that it is a useful tool to connect with models of properties dependent on SRO/SRC as well as for designing thermal treatments to control formation of SRO/SRC

Atomistic modeling of the reordering process of γ′ disordered particles in Ni-Al alloys

International audienceNi-based alloys are used in nuclear applications, including as a window material at isotope production facilities, withstanding high fluxes of different energetic particles like protons. Irradiation disorders the γ′ precipitates that in large extent confer the mechanical properties characterizing these materials. Upon disordering, the γ′ phase transforms into oversaturated γ, degrading the materials properties. Experimentally it is observed that disordering might take place at fairly low irradiation doses. Once the particles are disordered, a competition between dissolution, due to strong concentration gradients in an oversaturated solid solution, and reordering appears. Here, we examine this competition in a model Ni-Al alloy under thermal conditions for different precipitates sizes and temperatures. We observe Al interdiffusion from the supersaturated particle to the matrix. Also, stochasticity appears as an important factor in to where precipitates locate. Stress relaxation seems to modify the precipitation process, with a stronger interface effect compared to rigid lattice simulations

Defect Distributions and Transport in Nanocomposites: A Theoretical Perspective

Nanomaterials are attracting great interest for many applications, including radiation tolerance. Most work on radiation effects in nanomaterials has focused on the interfaces. Here, we examine the other aspect of nanocomposites, the dual phase nature. Solving a reaction–diffusion model of irradiated composites, we identify three regimes of steady-state behavior that depend on the defect properties in the two phases. We conclude that defect evolution in one phase depends on the defect properties in the other phase, offering a route to controlling defect evolution in these materials. These results have broad implications for nanomaterials more generally

Atomistic Simulation Informs Interface Engineering of Nanoscale LiCoO2.

Lithium-ion batteries continue to be a critical part of the search for enhanced energy storage solutions. Understanding the stability of interfaces (surfaces and grain boundaries) is one of the most crucial aspects of cathode design to improve the capacity and cyclability of batteries. Interfacial engineering through chemical modification offers the opportunity to create metastable states in the cathodes to inhibit common degradation mechanisms. Here, we demonstrate how atomistic simulations can effectively evaluate dopant interfacial segregation trends and be an effective predictive tool for cathode design despite the intrinsic approximations. We computationally studied two surfaces, {001} and {104}, and grain boundaries, Σ3 and Σ5, of LiCoO2 to investigate the segregation potential and stabilization effect of dopants. Isovalent and aliovalent dopants (Mg2+, Ca2+, Sr2+, Sc3+, Y3+, Gd3+, La3+, Al3+, Ti4+, Sn4+, Zr4+, V5+) were studied by replacing the Co3+ sites in all four of the constructed interfaces. The segregation energies of the dopants increased with the ionic radius of the dopant. They exhibited a linear dependence on the ionic size for divalent, trivalent, and quadrivalent dopants for surfaces and grain boundaries. The magnitude of the segregation potential also depended on the surface chemistry and grain boundary structure, showing higher segregation energies for the Σ5 grain boundary compared with the lower energy Σ3 boundary and higher for the {104} surface compared to the {001}. Lanthanum-doped nanoparticles were synthesized and imaged with scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) to validate the computational results, revealing the predicted lanthanum enrichment at grain boundaries and both the {001} and the {104} surfaces

Atomistic models of point defects in plutonium metal.

The aging properties of plutonium (Pu) metal and alloys are. driven by a combination of materials composit ion, p rocessing history, and self-irradiat ion effects . Understanding these driving forces requires a knowledge of both t h ermodynamic and defect properties of the material . The multiplicity of phases and the small changes in tempe rat u re, pressure, and/or stress that can induce phase changes lie at the heart of these properties . In terms of radiation damage, Pu metal represents a unique situation because of the large volume chan ges that accompany the phase changes . The most workable form of the meta l is the fcc (S-) phase, which in practice is stabi l ized by addit io n of a ll oying el eme n ts s u c h as Ga or Al. The thermodynamically stable phase at ambient conditions is the monoclinic (a-) phase, which, however, is 2 0 % lower i n volume th an the S phase . In stabilized Pu metal, there is an in t er play between th e n atu ral swe l li n g tendencies of fcc metals and the volume-contraction tendency of the u n d erlyin g thermodynamicall y stable phase. This study exp lores the point d efect pr operties that are necessary to model the long-term outcome of this interplay

How relative defect migration energies drive contrasting temperature-dependent microstructural evolution in irradiated ceramics

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