Development and Application of Reliable Models for the Simulation of Metals and Oxides at the Nanoscale

Kanhaiya, Krishan

Graduate Thesis Or Dissertation

Development and Application of Reliable Models for the Simulation of Metals and Oxides at the Nanoscale Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/jm214q54g

Abstract

Corrosion of metals causes billions of dollars in annual losses and atomic scale understanding is necessary to design corrosion resistant materials. Experiments fail to monitor adsorption-desorption equilibria and the initial time scales of femtoseconds to milliseconds before reactions occur. Missing insights can be obtained by molecular dynamics simulations with accurate force fields. In this work, reliable models for FCC metals, oxides and hydroxides, and partially oxidized Al and Ni surfaces were developed using the Interface Force Field (IFF) parametrization protocol and used to investigate early stages of oxidation. First, we introduce 12-6 and 9-6 LJ parameters for classical all-atom simulations of 10 FCC metals (Ac, Ca (α), Ce (γ), Es (β), Fe (γ), Ir, Rh, Sr (α), Th (α), Yb (β)) based on earlier work, thereby expanding the IFF coverage to include all FCC metals in the periodic table. Second, non-bonded models of six oxides (NiO, CaO, MgO, α-Al₂O₃, α-Cr₂O₃ and α-Fe₂O₃) and two hydroxides (β-Ca(OH)₂ and β-Ni(OH)₂) were developed. The models are interpretable (Coulomb and LJ terms) and easy to tune. Challenges are often the search for reliable reference data, which are curated and integrated here. The IFF models outperform prior LJ models, EAM potentials, DFT and popular atomistic models in all of speed, accuracy and compatibility with other force fields (AMBER, CHARMM, COMPASS, CVFF, DREIDING, OPLS-AA, and PCFF). The models can be used to simulate nanostructures up to millions of atoms and microsecond time scales to reliably study binding mechanism, biomolecular or inorganic interfaces, properties of nanoparticles, reactivity and alloys (e.g. stainless steel).

Finally, using the IFF metals, O₂ and oxide models, the early stage oxidation was studied on Ni and Al surfaces. Investigating O₂ dynamics at this time scale has been challenging experimentally and DFT calculations often suggest erroneously instant reactions with the surface. O_{2 potential energy surfaces and} dynamics were analyzed in high resolution (0.1 kcal/mol) on 6 clean and 5 partially oxidized surfaces, including the development of models of partially oxidized surfaces using X-ray and LEED data. The computed O₂ adsorption energies, 2D diffusion coefficients and fractional residence times range from -6 to -11 kcal/mol, 5 x 10^-5 to 7 x 10^-3 cm²/s and 75% to 0.1% respectively in a temperature range from 25 °C to 300 °C on clean and partially oxidized surfaces of Ni and Al. The data is used to explain trends in observed oxidation rates, and can be used to provide information for modeling growth rates of oxide islands and oxide films.

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