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

Description: 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 ...
Date: January 1, 2002
Creator: Henkelman, G. A. (Graeme A.); Uberuaga, B. P. (Blas Pedro) & Jónsson, Hannes,
Partner: UNT Libraries Government Documents Department

Theoretical Studies of Hydrogen Storage Alloys.

Description: Theoretical calculations were carried out to search for lightweight alloys that can be used to reversibly store hydrogen in mobile applications, such as automobiles. Our primary focus was on magnesium based alloys. While MgH{sub 2} is in many respects a promising hydrogen storage material, there are two serious problems which need to be solved in order to make it useful: (i) the binding energy of the hydrogen atoms in the hydride is too large, causing the release temperature to be too high, and (ii) the diffusion of hydrogen through the hydride is so slow that loading of hydrogen into the metal takes much too long. In the first year of the project, we found that the addition of ca. 15% of aluminum decreases the binding energy to the hydrogen to the target value of 0.25 eV which corresponds to release of 1 bar hydrogen gas at 100 degrees C. Also, the addition of ca. 15% of transition metal atoms, such as Ti or V, reduces the formation energy of interstitial H-atoms making the diffusion of H-atoms through the hydride more than ten orders of magnitude faster at room temperature. In the second year of the project, several calculations of alloys of magnesium with various other transition metals were carried out and systematic trends in stability, hydrogen binding energy and diffusivity established. Some calculations of ternary alloys and their hydrides were also carried out, for example of Mg{sub 6}AlTiH{sub 16}. It was found that the binding energy reduction due to the addition of aluminum and increased diffusivity due to the addition of a transition metal are both effective at the same time. This material would in principle work well for hydrogen storage but it is, unfortunately, unstable with respect to phase separation. A search was made for a ternary alloy of ...
Date: March 22, 2012
Creator: Jonsson, Hannes
Partner: UNT Libraries Government Documents Department