Investigations into the Thermodynamics and Kinetics of Nanoscale Structures

This dissertation contains a series of investigations into the thermodynamics and kinetics of nanoscale structures. The first investigation focused upon understanding the nanoscale void shrinkage in copper under room-temperature ion irradiation, with the goal of validating the hypothesis that the void shrinkage at room temperature is due to a biased absorption of interstitials. Phase-field modeling was used, and the simulations revealed that void shrinkage arises from biased absorption of interstitials agreeing with the experimental findings, thus providing insights into the physical mechanisms of radiation response of nanoscale voids in metallic materials under ion irradiation. The second part of this dissertation tackles the concurrent shape change, size fluctuation, shrinkage and migration of voids at elevated temperatures. The phase field simulations predicted the spheroidization of faceted voids, void shrinkage, rapid migration of small voids, and explained the underlying mechanisms. A part of this investigation focused on the dissociation of long, pre-existing voids under heavy ion irradiation. The phase field simulations showed that the fragmentation of voids occurs due to a necking mechanism, which is controlled by competing kinetics of atoms diffusion toward and away from the necked region.

The next part of this dissertation features a combined thermodynamics and molecular dynamics investigation of the conversion of stacking fault tetrahedra to helium filled bubbles under dual (Kr, He)-ion irradiated copper. We hypothesized a previously unreported mechanism for removal of these stacking faults in irradiated copper, that helium atoms migrate into the stacking fault pushing the native atoms to one side to aggregate vacancies together then reside in these vacancies to form bubbles. This mechanism was confirmed by molecular dynamics simulations.

The last investigation focused on understanding the growth of Au interface layers around vertically aligned NiO nanoscale pillars embedded in TiN thin films, grown on top of Au pillars embedded in TiN layer. The same thin film configuration also included the formation of Ni agglomerates in the Au pillars in the lower layer. A thermodynamic investigation of various morphology and configurations confirmed that interdiffusion of Au and Ni is energetically favorable, which interprets the observed film morphology. The findings of this study are vital for understanding the formation mechanisms of complex vertically aligned nanocomposites (VANs) and future designs of new three–phase VAN structures with complex morphologies.