Atomistic Simulations of Micromixing and Segregation Phenomena in Ternary Nanoalloys

Tuesday, November 9, 2010: 4:39 PM
150 D/E Room (Salt Palace Convention Center)
Subbaraman Ramachandran, Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL and Subramanian Sankaranarayanan, Center for Nanoscale Materials, Argonne National Lab, Argonne, IL

Surface segregation or surface enrichment of transition metals such as Ni, Co etc in Pt3M alloy nanostructures is a well known phenomenon and has been observed in several experimental studies. The alloys of Pt find extensive application in catalysis and emerging energy technologies. The main advantage of using Pt alloys over pure Pt is that it enhances electrocatalytic activity due to modified electronic properties of Pt skin atoms. Since the key to all applications of nanoclusters is their small size and structure, knowledge and control of their size and shape distribution, surface composition, and crystal structure is critical to improved designs of the same. Predicting the exact microstructure in the case of ternary alloys however remains a significant challenge. There are several factors which play a critical role in determining the final atomic distribution in binary and ternary nanoalloys. These include miscibility of the compounds (characterized by their heats of solution), the surface energies, the size mismatch, and the electro-negativities of constituent elements. Computer simulations offer an effective tool to study the influence of segregation on the thermodynamic properties of nanoclusters and complement ongoing experimental efforts. Molecular dynamics simulations employing quantum Sutton-Chen potential was employed to understand the micromixing and alloy segregation characteristics. Ternary alloy clusters (Pt, Rh, Au) of various compositions were initially subjected to simulated annealing in the 0-1800 K interval in increments of 100 K. Near the melting point, the temperature increments were reduced to 10 K to account for the large temperature fluctuations. The simulations were carried out for 1ns of equilibration followed by production time of 10 ns for generating time averaged properties. The heating and cooling rates were varied to investigate the interplay of kinetics and thermodynamics in predicting the final microstructure of nanoalloys. Various thermodynamic, structural and dynamic functions and order parameters were calculated to characterize the structural deformations, melting transitions, surface segregation and atomic diffusion in these systems. Details will be presented at the AIChE 2010 meeting.

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See more of this Session: Nanoscale Materials as Catalysts III
See more of this Group/Topical: Catalysis and Reaction Engineering Division