Degree Type

Dissertation

Date of Award

1993

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Kai-Ming Ho

Abstract

With the aim of understanding the anomalous phonon behavior near the martensitic phase transition in Zr, we have simulated the dynamics of atomic motion in the high temperature bcc phase of Zr using an embedded-atom potential. The calculated dynamical structure factors reproduce the strong asymmetry in the scattering cross-sections in different Brillouin zones observed in inelastic neutron scattering experiments. From the real-space atomic picture, we observed the phase fluctuations between bcc and low temperature phase hcp. It is found that the anomalous phonon behavior arises from the incompleteness of the phase fluctuations;Combining an efficient simulated annealing scheme for generating closed, hollow, spheroidal cage structures with a tight-binding molecular-dynamics method for energy optimization, we have systematically studied the ground-state structure of every even-numbered carbon fullerene from C[subscript]20to C[subscript]100. Several predicted geometric structures of large fullerenes have been confirmed by later experiments. Clusters of sizes 60, 70, and 84 are found to be energetically more stable than their neighbors. The occurrence of the more abundant fullerene cluster is related to the fragmentation stability and chemical reactivity of the clusters obtained from our calculations. As a general trend, most ground-state structures of fullerenes have relatively low symmetries. In many cases, several isomers of a fullerene are found to have competitively low energies, which suggests that a mixture of these isomers can be observed in experimentally prepared samples;To better understand the formation process of carbon fullerenes, we also simulate the collisions between fullerenes and the thermal disintegration of fullerenes. We observed three different regimes of behavior as the collisions become more and more energetic: bouncing, fusion and fragmentation. The critical energies for fusion and fragmentation as well as details of the energy transfer process during the collisions are investigated;In simulations of the thermal disintegration of fullerene cages, the most commonly observed fragments after the disintegration of the carbon cages are dimers, rings and multiple rings. The fragmentation temperature increases almost linearly with the cluster size for small cages (n ≤ 58), but becomes almost constant for larger fullerenes (n = 60 and n ≥ 70).

DOI

https://doi.org/10.31274/rtd-180813-13112

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Bulin Zhang

Language

en

Proquest ID

AAI9335040

File Format

application/pdf

File Size

138 pages

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