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"Molecular Dynamics Studies of Diffusion Behavior in Al-Cu metals and Alloys", Phd Thesis, Alpertunga Davutoğlu, 2003

"Non-equilibrium molecular dynamics of electromigration in aluminum and its alloys", Msc Thesis, Fatih Gürçağ Şen, 2006

With constant miniaturization of integrated circuits, the current densities experienced in interconnects in electronic circuits has been multiplied. Aluminum, which is widely used as an interconnect material, has fast diffusion kinetics under low temperatures. Unfortunately, the combination of high current density and fast diffusion at low temperatures causes the circuit to fail by electromigration (EM), which is the mass transport of atoms due to the momentum transfer between conducting electrons and diffusing atoms. In the present study, the effect of alloying elements in aluminum on the diffusion behavior is investigated using a non equilibrium molecular dynamics method (NEMD) under the effect of electromigration wind force. The electromigration force was computed by the use of a pseudopotential method in which the force depends on the imperfections on the lattice. 1.125 at% of various elements, namely Cu, Mg, Mn, Sn and Ti were added into aluminum. The electromigration force was then calculated on the alloying elements and the surrounding aluminum atoms and these forces incorporated into molecular dynamics using the non-equilibrium formalism. The jump frequencies of aluminum in these systems were then computed. Cu, Mn and Sn impurities were found to be very effective in lowering the kinetics of the diffusion under electromigration conditions. Cu was known experimentally to have such an effect on aluminum for several years, but the Mn and Sn elements are shown here for the first time that they can have a similar effect.


"First principles investigation of hydrohen storage in intermetallic systems", Msc Thesis, Alper Kınacı, 2007

In this study, total energies, hydrogen storage capacity and stability of AB (A = Al, Be, Cu, Fe, Ni, Sb, V and B = Ti) type intermetallics were investigated with the goal of spotting a potential hydrogen storage material. The relation between thermodynamic properties and the atomic and the electronic structure of hydrides are also pointed out. The principles pseudopotential method within the generalized gradient approximation (GGA) to density functional theory (DFT) was used. Calculations correctly predict experimentally determined structures except for CuTiH. Moreover, the atomic and cell parameter were found within the allowable error interval for DFT. In CuTi intermetallic, the structure having considerably lower formation energy than experimentally found mono-hydride was determined. This contradiction may be due to metastability of the experimental phase and high activation energy for the hydrogen movement in the system. It was found that AlTi and SbTi are not suitable candidates for hydrogen storage since their hydrides are too unstable. For the other intermetallic systems, the stability of the hydrides decreases in the order of VTi, CuTi, NiTi, BeTi, FeTi. For VTi, FeTi and NiTi, a change in metallic coordination around hydrogen from octahedron to tetrahedron is predicted when tetra-hydride (MTiH4) is formed. Additionally, at this composition, FeTi and NiTi have hydride structures with positive but near-zero formation energy which may be produced with appropriate alteration in chemical makeup or storage parameters. VTi is a promising intermetallic by means of storage capacity in that even VTiH6 is found to have negative formation energy but the hydrides are too stable which can be a problem during hydrogen desorption.


"Ab Initio design og novel Mg alloys for hydrogen storage", Msc Thesis, Deniz Keçik, 2008

Magnesium hydride is an outstanding compound with 7.6 wt % storage capacity, despite its slow dehydriding kinetics and high desorption temperature. In this study, bulk and surface alloys of Mg with improved hydrogen desorption characteristics were investigated. Formation energies of alloyed bulk MgH2 as well as the adsorption energies on alloyed magnesium and MgH2 surface structures were calculated by total energy pseudopotential methods. Furthermore, the effect of substitutionally placed dopants on the dissociation of hydrogen molecule at the surface of Mg was studied via Molecular Dynamics. The results displayed that 31 out of 32 selected dopants contributed to the decrease in formation energy of MgH2 within a range 37 kJ/mol-H2 where only Sr did not exhibit any such effect. The most favorable elements in this respect came out to be; P, K, Tl, Si, Sn, Ag, Pb, Au, Na, Mo, Ge and In. Also a systematical study within adsorption characteristics of hydrogen on alloyed Mg surfaces (via dynamic calculations) as well as calculations regarding adsorption energies of the impurity elements was performed. Accordingly, Mo and Ni yielded lower adsorption energies for substitutionally alloyed surfaces. MD simulations presented that Co is found to have a splitting effect on H2 in 50 fs, where the first hydrogen atom is immediately adsorbed on Mg substrate. Finally, charge density distributions were realized to verify the distinguished effects of most 3d and 4d transition metals in terms of their catalyzer effects.


"An Ab Initio surface study of FeTi for hydrogen storage applications", Msc Thesis, Afshin Izanlou, 2009

We present the effect of surface crystallography on hydrogen molecule adsorption properties in iron-titanium substrate. Using metals and their compounds is a known way for hydrogen storage. The important thing here is to find compounds that tend to adsorb hydrogen with as small as possible activation energy, which leads to an as fast as possible hydrating reaction. For example, one of the most used metals is magnesium and its compounds. Magnesium because of its low weight and cost and also the highest hydrogen content of its hydrides, is in a large field of interest. However, its high desorption temperature and slow kinetics, make drawbacks. In iron-titanium alloys, on the other hand, previous studies have shown that regardless its low hydrogen storability, it can be considered as a compound with desirable kinetic properties in order for hydrogen adsorption.



The primary data for our calculations including lattice parameters and formation energies of bulk lattices of FeTi and its two hydrides FeTiH and FeTiH2 were calculated and can be seen in our results. In surface calculations, after coming to definite surface structures, which are number of layers of each slab, number of fixed and relaxed layers, thickness of vacuum and surface structure, we must calculate the adsorption energies of hydrogen molecule or atom on pure surfaces and substitutionally adsorption energies of 3d-transition metal alloying elements. Here, Co seem to make the most stable substitutionally adsorbed structure. For hydrogen adsorption on pure surfaces, the height of the activation energy, transferring from the initial position to the final one, can be determined during climbing image nudged elastic band calculations.



Adsorption energies of H and H2 on Fe- and Ti- terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetric adsorption sites. It is shown that for (001) and (111) surfaces of both terminations, top site is the most stable site for horizontal H2 adsorption. For H adsorptions on these surfaces, however, bridge site is the most favorable. In (110) surface, the 3-fold (Ti-Ti)L-Fe hollow site and 3-fold (Ti-Ti)S-Fe hollow site, are the most stable for H2 and H adsorption, respectively. To find the most probable path for hydrogen dissociation on surfaces, we use the sites with minimum adsorption energies for both H2 and H. Relaxation and reconstruction of each surface is also fully discussed via H and H2 adsorption on the most stable sites. We also use climbing image nudged elastic band (CI-NEB) method to calculate the minimum energy path (MEP) for the dissociation of hydrogen molecule and determine the activation energy of the dissociation process to be 0.178 and 0.190 eV / H2 for Fe- and Ti- terminated (001) surfaces, respectively.