Gray, Corinne
(2017)
First-Principles Investigation of Structural Defects and Energy Landscape Affecting Diffusion in Cr2O3 and Al2O3.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
In this study, chromia vacancy diffusion and alumina grain boundary structure have been investigated using the first principles density functional theory (DFT) computational methods. Comparing the predictions from the DFT and DFT+U methods on chromia, it was found that the DFT+U method describes alpha-Cr2O3 crystal better in terms of the magnetism, band gap, charge state of vacancies, and migration energies for charged vacancy diffusion than the DFT method. For Cr2O3, four distinct elementary diffusion paths were investigated for Cr and O vacancies, both with and without charge. For charged vacancies our DFT+U calculation predicted that the energy barrier for V_"O" ^(2+) diffusion varies from 1.18 to 2.98 eV and for V_"Cr" ^(3-) diffusion varies from 2.02 to 2.59 eV. In comparison, the same diffusion paths had migration energy differences as high as 1.19 eV (for O1 → O3) and 0.99 eV for (Cr1 → Cr3) when examining the neutral vacancies. These changes in migration energies indicates charge state of defects plays an important role in determining the diffusion properties through an oxide. For alumina, the fully relaxed structures of Σ11{101 ̅1}/{10(11) ̅ } grain boundary doped with one Y, one Hf, or two Hf atoms within the grain boundary have been investigated in detail. It was found that the introduction of reactive element dopants results in pinning, reducing diffusion through the grain boundary. Specifically, pinning was more strongly observed for Hf than Y. As compared to the replaced Al, Hf and Y dopants were found to change the local charge density, and introduce some defect states within the band gap. Low concentration of Hf and Y dopants causes an increase in the number of defects states near the valence band maximum, implicating an increase of hole diffusion. In contrast, increasing the concentration of Hf dopants was found to reduce the number of defect states, suggesting an effect to slow diffusion in the alumina grain boundary.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
| Creators | Email | Pitt Username | ORCID  |
|---|
| Gray, Corinne | | | |
|
| ETD Committee: |
| Title | Member | Email Address | Pitt Username | ORCID |
|---|
| Committee Chair | Wang, Guofeng | | | | | Committee Member | Jacobs, Tevis | | | | | Committee Member | Gleeson, Brian | | | | | Committee Member | Keith, John | | | |
|
| Date: |
14 June 2017 |
| Date Type: |
Publication |
| Defense Date: |
29 March 2017 |
| Approval Date: |
14 June 2017 |
| Submission Date: |
6 April 2017 |
| Access Restriction: |
2 year -- Restrict access to University of Pittsburgh for a period of 2 years. |
| Number of Pages: |
96 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Density functional theory; alumina; chromia; vacancy diffusion; |
| Date Deposited: |
14 Jun 2017 19:38 |
| Last Modified: |
14 Jun 2019 05:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/31302 |
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