Discovering low work function materials for thermionic energy conversion
Abstract/Contents
- Abstract
- The work function is the interfacial parameter of a surface that determines how easily electrons can escape into a vacuum or gas environment, with lower work functions generally facilitating electron emission. This thesis employs density function theory methods to a systematic approach in the discovery of new nanostructured multilayer materials with low work functions. Techniques in density functional theory can enable many promising film coating combinations to be efficiently investigated for the first time. Two main sets of screening studies are described: (1) cesiated transition metal surfaces and (2) alloyed alkali-earth oxide films on tungsten. A model is introduced for the effect of cesium adsorbates on the work function of transition metal surfaces. This model builds on the classic point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates. In addition, the model analyzes the effect of orbital overlap on the strength and orientation of electric dipoles along the adsorbate-substrate interface. This new framework improves upon earlier models in terms of agreement with the work function-coverage curves obtained via first-principles calculations based on density functional theory. All the cesiated metal surfaces have optimal coverages between 0.6 and 0.8 monolayers, in accordance with experimental data. Of all the cesiated metal surfaces considered, tungsten has the lowest minimum work function, also in accordance with experiments. The work function and stability of 570 alloyed alkali-earth oxide films on the (100) surface of tungsten have been calculated within density functional theory. Computational screening of this large phase space is enabled by implementing the virtual crystal approximation, where the degree of freedom in the chemical composition is modeled with virtual atoms of mixed calcium, strontium, and barium character. Low work functions are achieved by doping the films with scandium (1.16 eV) or lithium (~1.2 eV) and alloys containing ~15% to ~20% of calcium. In particular, lithium-doped systems with ~15% calcium also show favorable stability indicated via formation energy calculations. Identification of such film alloys outperforming any of the constituents relies on careful sampling of the chemical composition. Covalent interactions within the film limit the reduction in work function from the dipole normal to the surface. Controlling the electronic screening of these intra-film interactions by oxygen atoms is essential for the design of new low-work function materials.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2014 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Chou, Sharon Hsiao-Wei |
|---|---|
| Associated with | Stanford University, Department of Electrical Engineering. |
| Primary advisor | Howe, Roger Thomas |
| Thesis advisor | Howe, Roger Thomas |
| Thesis advisor | Noerskov, Jens |
| Thesis advisor | Pianetta, Piero |
| Advisor | Noerskov, Jens |
| Advisor | Pianetta, Piero |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Sharon H. Chou. |
|---|---|
| Note | Submitted to the Department of Electrical Engineering. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2014. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2014 by Sharon Hsiao-Wei Chou
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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