RT info:eu-repo/semantics/doctoralThesis
T1 Computational modeling of metallic nanoclusters and nano-alloys for catalytic and corrosion applications.
A1 Álvarez Zapatero, Pablo
A2 Universidad de Valladolid. Escuela de Doctorado
K1 Física nuclear
K1 Ciencia de los materiales
K1 Corrosion
K1 Corrosión
K1 Reactivity
K1 Reactividad
K1 Nanosclusters
K1 Nanoagregados
K1 220 Física Atómica y Nuclear
AB The field of Material Science within the realm of Nanophysics has become one of the most thriving research areas. Indeed, due to its emphasis on practical applications, it is a decisive ally to face the challenges of the humankind. Nowadays, one of these challenges  comprises the discovery and efficiency of new materials. As such, protection against degradation becomes a fundamental part in Nanoscience.  When dealing with metals, an ubiquitous degradation of these comes in the form of their interaction with the atmosphere. Oxidation and the attack of several corroding agents imply the loss of the metallic surface. This undermines the metallic properties and can result in the collapse of the metallic structure if nothing is done to stop the electrochemical reaction. The corrosion problem entails a huge economic cost for the industry, being addressed through available control practices: techniques such as galvanization and stainless-steel alloys are used to prevent metals from rusting, and are widely used for such purpose. The galvanization is the process of applying a protective zinc coating over the metallic surface. After reacting with the atmospheric oxygen and corroding agents, it is the oxidized zinc layer and the related corrosion products the ones that protect the metal from corroding, either with oxygen or any other corroding agent. This way, the zinc layer serves as a sacrificial coating which provides barrier and galvanic protection to the steel substrates employed in industry. It has been found however, that adding magnesium to the zinc layer to form an alloy improves the protective properties of the coating. Not only the oxidized protective layer is created faster, but also the time for growing significant amounts of rust upon corrosion is longer compared to bare zinc.  More in detail, the Zn11Mg2 y Zn2Mg stoichiometries have been found to be the most suitable to optimize the protection against corrosion according to experimental evidence. The reasons for such quality are, however, not well known. The intricate physical, chemical and thermodynamical processes involved are difficult to understand in depth without a quantum-mechanical analysis. The objective of this thesis is to unveil the fundamental aspects that trigger the optimal anticorrosive properties of Zn-Mg coatings. Given the vastness of the problem, we will focus on the formation of the initial oxidized surface layer, over which the corrosion products would grow to ultimately conform the protective layer. To this aim, a detailed quantum-mechanical treatment relying on ab-initio techniques, particularly Density Functional Theory based methods, is performed. To study the complex corrosion process, we rely on cluster models. These are simple yet useful computational models for an initial study of the intricate processes that operate in the real extended surfaces. Characterisation of structures is a central task in this thesis, so the development of algorithms and protocols to seek and discover stable structures comprises the core of this work. One of these methods entails novel Machine Learning methods, such as the Neural Network potentials. These are shown to clearly outperform standard empirical potentials. Afterwards, an analysis of the initial stages of the corrosion problem by means of ab-initio methods is performed. It is found that  small amounts of Mg create a very positive synergy between Zn and Mg that increases the reactivity to oxygen while reducing, at the same time, the stress induced on the cluster substrate, both facts working in favor of promoting the growth of the oxide crust whilst protecting the core. Moreover, stoichiometries close to the Mg2Zn11 and MgZn2 compositions are found to be the best candidates to optimize the protection against corrosion in Zn-Mg alloys, in agreement with the experimental observations.
YR 2022
FD 2022
LK https://uvadoc.uva.es/handle/10324/59753
UL https://uvadoc.uva.es/handle/10324/59753
LA eng
NO Escuela de Doctorado
DS UVaDOC
RD 11-jul-2024