Mühendislik Fakültesi

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Citation Count: Yıldırım, E.K., Güvenç. Z.B. (2009). A density functional study of small Li-B and Li-B-H clusters. International Journal of Hydrogen Energy , 34(11), 4797-4816. http://dx.doi.org/10.1016/j.ijhydene.2009.03.051
A density functional study of small Li-B and Li-B-H clusters
(Pergamon-Elsevier Science Ltd, 2009) Yıldırım, Erdem Kamil; Güvenç, Büşra; 1155
In this work we present the results of a detailed theoretical research for the small Li(n)B(m) clusters and their hydrogen storage properties by means of density functional theory. All calculations were performed by using Gaussian03 program. For the optimization of the clusters 6-311++G(d,p) basis set was employed. We have chosen global minimum of B(6) cluster as the starting point and replaced the boron atoms one at a time, in a stepwise manner, with Li atoms. For these new structures we have searched for the stable configurations, and calculated their energies, HOMO-LUMO energy gaps, Raman and infrared data, average boron, and lithium charge distribution, and vibrational frequencies. Our results show that as the number of Li atoms increases, stability of the clusters decreases and they become more reactive. In addition to that, there are significant charge transfers from boron atoms to lithium atoms on average. The hydrogen storage capabilities of the most stable isomers of Li(n)B(m) and B(6) clusters have also been investigated by the same methods, and the results are compared. The Li(3)B(3) has the highest hydrogen storage capacity among the clusters investigated in this study. Furthermore, formation of hydrogen molecules is observed in the vicinity of the clusters, some of which are attached to the Li atoms. In addition, as the number of hydrogen atoms increases, the boron atoms are separated from the other boron atoms, and they form satellite BH(x) (x = 3,4) clusters around the center. These are attached to the system by a bridging bond of a hydrogen or a Li atom
Citation Count: Yıldırım, E.K., Güvenç, Z.B. (2006). Differences in melting behaviours of disordered and symmetric clusters: Au(N)(N=54-56). Modelling and Simulation in Materials Science and Engineering, 14(6), 947-961. http://dx.doi.org/10.1088/0965 0393/14/6/005
Differences in melting behaviours of disordered and symmetric clusters: Au(N)(N=54-56)
(Iop Publishing ltd, 2006) Yıldırım, Erdem Kamil; Güvenç, Ziya B.; 1155
We have investigated the melting behaviours of disordered and symmetric gold clusters (Au(N), N = 54 - 56) by means of molecular dynamics simulations. We have found that there is no single isolated lowest energy structure for this size of Au clusters. Instead there are many nearly degenerate disordered low lying structures. The melting behaviours of these disordered structures showed that the melting occurs as a gradual process in which initially, behaviours of the surface and the inner atoms are quite different from each other, and they do not mix until the beginning of the melting. On the other hand, the symmetric forms of the Au(N) ( N = 54 - 56) present different melting behaviours from those of the corresponding disordered structures. Their melting occurs suddenly, i.e. over a very short temperature interval. During the heating of these symmetric forms no phase changes occur until the melting temperature at which melting occurs as a collective motion of all the atoms in the cluster. On the other hand, the phase changes in the disordered structures take place as a result of both collective motions of all the atoms in the cluster, and as local displacements of the atoms.
Citation Count: Yıldırım, E.K., Atış, M., Güvenç, Z.B. (2007). Molecular dynamics simulation of melting behaviour of small gold clusters: Au-N (N=12-14). Physica Scripta, 75(1), 111-118. http://dx.doi.org/10.1088/0031-8949/75/1/018
Molecular dynamics simulation of melting behaviour of small gold clusters: Au-N (N=12-14)
(IOP Publishing ltd, 2007) Yıldırım, Erdem Kamil; Atış, Murat; Güvenç, Ziya B.; 1155
We have investigated the melting behaviour of AuN (N = 12 - 14) clusters by means of molecular dynamics simulation on the basis of the Voter - Chen version of the embedded-atom method. The melting behaviour of the clusters is described in terms of short-time average temperatures and atomic coordination numbers of the clusters. Results have shown that during the melting process, the phase changes occur as a collective and simultaneous motion of all the atoms in a very short-time interval. Furthermore the Au-14 cluster presents a two-stage melting behaviour which is different from those of the Au-12 and Au-13 clusters. The isomer sampling probabilities are obtained from the thermal quenching of the molten clusters, and their energy-spectrum widths are investigated. The results of the isomer forming probabilities showed that the global minimum structures of these clusters are not always the most probable ones to be formed in the experiments

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