Investigations Of Transport Coefficients Of Fluids And Melting Mechanism Of Nanoalloys By Molecular Dynamics Simulations

In this research, the second self-diffusion and viscosity virial coefficients of the Lennard-Jones model fluid and the melting mechanism of nanoalloys with magic stable structures have been studied using molecular dynamics methods. Accurate calculation of the second virial coefficients requires corresponding transport coefficient values with low degrees of uncertainty at low and moderate densities. These were obtained via very long simulations by increasing the number of particles and by using the knowledge of correlation functions in the Green-Kubo method in conjunction with their corresponding generalized Einstein relations. A detailed evaluation of the velocity and shear-stress autocorrelation functions has been accomplished. The values of the self-diffusion and shear viscosity coefficients have been evaluated for systems with reduced densities between0.0005and0.05and reduced temperatures from0.7to30.0. This provides a new insight into the transport coefficients beyond what can be offered by the Rainwater-Friend theory, which has not been developed for the self-diffusion coefficient. In second part, a molecular dynamics coupled to the steepest descent quenching approach has been used to study the melting mechanism of Pd24Pt14and (M=Cu, Ni, Co) nanoalloys with the anti-Mackay structures. The simulations for were found to be nonergodic. The nonergodicity has been removed by using the multiple histogram method in this thesis. The Gupta many-body model was used for interatomic potentials. The melting characteristics were determined by the analysis of variations in the potential energy, the heat capacity, the vibrational density of states, the self time-space correlation and radial distribution functions with temperatures starting from50K up to the evaporation temperatures. The calculations indicate that the melting of Pd24Pt14nanoalloy occurs at T=747K following structural transitions from the truncated-octahedral to the icosahedral basin of structures. A post-melting phenomenon was also observed at temperatures between900and1100K, which is related to the homogeneous melting. In the case of Ag32M13(M=Cu, Ni, Co) nanoalloys, it has been demonstrated that in contrast to the prediction of Kuntova et al.[Phys. Rev. B77,205431,2008], the melting of core and shell of Ag32Ni13and Ag32Co13occurs at a single stage. Even though the melting of Ag32Cu13occurs at two distinct stages, both its core and shell participate at each stage. In spite of beginning from similar excited configurations, the overall melting mechanisms of these nanoalloys are different. In addition, the analysis of the radial distributions shows that Ag is not completely miscible with Ni, Co or Cu at nanoscale. This indicates that the time correlation functions are very powerful tools to elucidate the role of the structure and composition in the melting mechanism of nanoparticles and improves the knowledge about the phase transition features of38-and45-atom nanoalloys with the magic structures.