Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Kai-ming Ho

Abstract

We have calculated the T0 curves for several Al-Rare Earth (RE) binary alloys and compared the results with reported observations of glass formation (T0 curve is defined as a trajectory in temperature-composition space where the liquid phase and solid phase have same Gibbs free energies), in order to assess the importance of the transport-based resistance to crystallization in the overall glass formation process. Our results show that the experimentally observed glass forming compositions for Al-(Ce,Gd,Ho,Nd,Y,Dy) alloys strongly correlate with the composition range bounded by the T0 curves associated with the relevant crystalline

phases. This agreement indicates that sluggish material transport is a key factor governing glass formation in these systems, a behavior that differs substantially from the more common oxide glasses, where directional bonding constraints may stabilize the glassy network based on topological considerations.

A jellium-passivated cluster model is developed to study the energetics of

short-range ordering in supercooled liquid and glass systems. Calculations for single atoms embedded in jellium yield results in good agreement with bulk values for the cohesive energy, atomic volume as well as angular-momentum-projected electronic density of states. The energy difference between icosahedral clusters and FCC embryos in jellium is found to correlate with the glass-forming ability of liquid Al alloys. The model will be useful for studying the short-range order tendency with minor chemical additions in metallic glass formation, without the use of large unit cell calculations.

We demonstrate an efficient and accurate first-principles method to calculate the electronic structure of a large system using a divide-and-conquer strategy based on localized quasi-atomic minimal basis set orbitals recently developed. Tight-binding Hamiltonian and overlap matrices of a big system can be constructed by extracting the matrix elements for a given pair of atoms from first-principles calculations of smaller systems that represent the local bonding environment of the particular atom pair. The approach is successfully applied to the

studies of electronic structure in graphene nano-ribbons. This provides a promising way to do the electronic simulation for big systems directly from first-principles.

We have developed a new density functional theory incorporating the correlated electronic effects into the kinetic energy via Gutzwiller approximation. All the Coulomb integrals are determined self-consistently without any adjustable parameters. In addition to the set of one-electron Schrydinger equations analogous to the standard LDA approach, we

get another set of linear equations with respect to the probabilities of local configurations as the solution of the many body problem. A preliminary Fortran90 code has been developed with an interface to VASP. We applied our method to several systems with important electron correlation effects and got encouraging results.

Copyright Owner

Yongxin Yao

Language

en

Date Available

2012-04-29

File Format

application/pdf

File Size

97 pages

Included in

Physics Commons

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