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Force Field Development and Validation for Liquid Metal Plasma-Facing Materials

Speaker: Joseph R. Vella
Series: Final Public Oral Examinations
Location: Lapidus Lounge (E-Quad A210)
Date/Time: Tuesday, February 21, 2017, 2:00 p.m. - 3:30 p.m.

Liquid metals are being considered as plasma-facing materials in tokamak fusion reactors. Liquid lithium, tin, and lithium-tin alloys are among the most promising candidates. However, an understanding of properties relevant to plasma-facing applications needs to be established. Despite the existence of some experimental studies, there is still a noticeable lack of important data. Classical molecular simulation techniques present another method for studying relevant properties and can complement existing experimental studies. Before simulation studies can take place, development and validation of liquid metal force fields needs to be done. In this thesis, embedded-atom method force fields for lithium and tin were compared to experimental and first-principles data in order to assess their ability to predict properties of liquid metal plasma-facing materials.
 
In a comparative study of several lithium force fields, it was found that the force field developed by Cui et al. [Modell. Simul. Mater. Sci. Eng. 20, 015014 (2012)] is the most robust due to its accurate prediction of the melting temperature and liquid-phase data.  This conclusion is supported by the fact that this force field accurately predicts the self-diffusivity and viscosity of liquid lithium.
 
A liquid tin force field was also developed in this work. Simulated annealing was used to construct the new force field by primarily fitting to experimental liquid data. The new force field accurately reproduces a majority of the experimental data used in the fitting procedure and accurately predicts liquid data not used in the optimization.
 
Finally, the wetting properties of liquid lithium on solid molybdenum were examined.  A lithium-molybdenum force field was developed by fitting to first-principles data. It was found that liquid lithium perfectly wets the (110) surface of molybdenum, in contrast with experimental data. This suggests the presence of oxygen and surface structure can significantly effect the ability of lithium to wet molybdenum. It was found that the lithium atoms close to the molybdenum surface exhibit solid-like behavior. This shows that lithium strongly adheres to the molybdenum surface.  This thesis highlights the importance of force field development and validation when performing molecular simulation studies, especially for systems with limited experimental data.