Date of Graduation

8-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Douglas Spearot

Committee Member

Shui-Qing Yu

Second Committee Member

Po-Hao Huang

Third Committee Member

Paul Millett

Fourth Committee Member

Uchechukwu Wejinya

Keywords

Alumina, Atomistic Simulation, Interface Structure, Surface Structure, Virtual Diffraction

Abstract

The objectives of this work are to investigate the structure and energetic stability of different alumina (Al2O3) phases using atomistic simulation and virtual diffraction characterization. To meet these objectives, this research performs molecular statics and molecular dynamics simulations employing the reactive force-field (ReaxFF) potential to model bulk, interface, and surface structures in the θ-, γ-, κ-, and α-Al2O3 system. Simulations throughout this study are characterized using a new virtual diffraction algorithm, developed and implemented for this work, that creates both selected area electron diffraction (SAED) and x-ray diffraction (XRD) line profiles without assuming prior knowledge of the crystal system. First, the transferability of the ReaxFF potential is evaluated by modelling different alumina bulk systems. ReaxFF is shown to correctly predict the energetic stability of α-Al2O3 among the crystalline alumina phases, but incorrectly predicts an even lower energy amorphous phase. Virtual XRD patterns uniquely identify each phase and validate the minimum energy bulk structures through experimental comparison. Second, stable and metastable alumina surfaces are studied at 0, 300, 500, and 700 K. ReaxFF predicts minimum energy surface structures and energies in good agreement with prior studies at 0 K; however, select surface models at 500 and 700 K undergo significant reconstructions caused by the unnatural bias for a lower-energy amorphous phase. Virtual SAED analysis performed on alumina surfaces allow advanced characterization and direct experimental validation of select models. Third, ReaxFF is used to model homophase and heterophase alumina interfaces at 0 K. Predicted minimum energy structures of α-Al2O3 interfaces show good agreement with prior works, which provides the foundation for the first atomistic study of metastable alumina grain boundaries and heterophase alumina interfaces. Virtual SAED patterns characterize select alumina interfaces and help guide the construction of low-energy heterophase alumina interfaces by providing insight into crystallographic compatibilities. Combined, the energetic data extracted from bulk, surface, and interface simulations as well as insights gained through virtual diffraction will aid the development of mesoscale predictive models of polycrystalline alumina formation during physical vapor deposition.

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