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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 su, El campo de la Ciencia de Materiales se ha convertido en una de las áreas más prolíficas de investigación. Dado su especial énfasis en las aplicaciones prácticas, se ha convertido en un aliado decisivo a la hora de afrontar los retos que tiene la humanidad. Hoy en día, uno de estos retos incluye el descubrimiento de nuevos materiales, así como la eficiencia de los mismos. De esta manera, la protección frente a la degradación de estos materiales toma una importancia central. En sistemas basados en metal, una interacción omnipresente en la atmósfera terrestre es la de la corrosión, consistente en el deterioro del metal a consecuencia de la oxidación y de otros ataques electroquímicos. Se trata de un problema industrial de gran importancia y de alto costo económico. De este modo, varias técnicas como el galvanizado y las aleaciones inoxidables se emplean de forma sistemática. En el caso del galvanizado o cincado, el metal a proteger se recubre con una capa de zinc. Además de mejorar su aspecto visual, el zinc, al reaccionar con el oxígeno y otros agentes corrosivos como Cl- y agua, forma una capa de óxido y otros productos derivados de la corrosión que protege el interior del metal de la oxidación y corrosión. Sin embargo, se ha encontrado que la incorporación de magnesio para formar una aleación con el zinc resulta en una creación más eficiente de la capa protectora: la formación de la capa de óxido es más rápida, y es más efectiva en el aislamiento del exterior. En particular, las composiciones Zn11Mg2 y Zn2Mg se encuentran como las más indicadas para maximizar la eficiencia de la capa protectora según la evidencia experimental. Las razones de esta cualidad no son, sin embargo, bien conocidas. El proceso de la corrosión implica procesos físicos, químicos y termodinámicos en diferentes etapas, que resultan en la capa aislante final. Este proceso intermedio de corrosión es, en consecuencia, muy complejo de estudiar y de modelizar a escala nanométrica y donde, en todo c, Escuela de Doctorado, Doctorado en Física

Las nanoaleaciones de ZnMg ricas en Zn se han posicionado como unas de las más prometedoras en la creación de materiales efectivos como revestimientos anti-corrosión. Este trabajo de fin de grado se centra en la simulación y el análisis del proceso de oxidación de una configuración previamente determinada de la nanoaleación de Zn31Mg8, para relacionarla con el fenómeno de la corrosión. Para ello, la investigación se divide en dos partes. La primera de ellas consiste en el estudio de la primera fase de la corrosión, en la que se empieza a formar la capa inicial de óxido que va a condicionar el rendimiento posterior del agregado. La búsqueda del estado fundamental se lleva a cabo mediante un método de optimización local ab-initio, basado en la teoría del funcional de la densidad y con el programa de simulación de dinámica molecular SIESTA. Las tendencias estructurales y energéticas de los estados fundamentales y excitados son analizadas tanto de forma cualitativa como cuantitativa, con el apoyo de distintos parámetros como el radio medio del agregado o su energía de adsorción, entre otros. La segunda parte de la investigación consiste en el análisis de la nanoaleación oxidada en fases más avanzadas. Para ello, se emplea un método de optimización global, basado en la utilización de potenciales de redes neuronales entrenados en la resolución de este tipo de problemas, y en el algoritmo computacional basinhopping. Los resultados obtenidos en este trabajo muestran un conjunto de tendencias positivas en la formación de la capa de óxido, y constituyen un primer paso en el estudio del comportamiento de este tipo de nanoaleaciones como protectores de la corrosión., Zn-rich ZnMg nanoalloys have positioned themselves as one of the most promising options in the development of effective materials as anti-corrosion coatings. This BSc thesis focuses on the oxide process simulation and analysis of a previously determined Zn31Mg8 nanoalloy configuration, in order to link it to corrosion phenomena. To that end, research is divided in two parts. The first one consists in an initial corrosion phase study, in which the first oxide layer begins to form, which can determine later cluster performance. Ground state configuration is calculated by an ab-initio local optimisation method, based on density functional theory and the molecular dynamic simulation program SIESTA. Structural and energetic trends of ground and excited states are qualitatively and quantitatively analysed, with the support of several parameters as cluster mean radii, adsorption energy, etc. The second part of this research consists in analysing the oxidized nanoalloy in more advanced phases. For that purpose, a global optimisation method is employed, based on neural network potentials trained to resolve this type of problems, and on the basin-hopping computational algorithm. Results achieved in this research show a positive set of tendencies related to the oxide layer formation, and constitute a first step on the efficiency study of this type of nanoalloys as corrosion protectors., Departamento de Física Teórica, Atómica y Óptica, Grado en Física

El objetivo de este trabajo es investigar sobre la adsorción de átomos de titanio en popgrafeno, como un paso previo a su potencial para almacenar hidrógeno. Para ello se utiliza la DFT y en particular su implementación en Vienna Ab initio Simulation Package (VASP)., Departamento de Física Teórica, Atómica y Óptica, Grado en Física

Producción Científica, The accurate description of the potential energy landscape of moderate-sized nanoparticles is a formidable task, but of paramount importance if one aims to characterize, in a realistic way, their physical and chemical properties. We present here a Neural Network potential able to predict structures of pure and mixed nanoparticles with an error in energy and forces of the order of chemical accuracy as compared with the values provided by the theoretical method used in the training process, in our case the density functional theory. The neural network is integrated into a basin-hopping algorithm which dynamically feeds the training process. The main ingredients of the neural network algorithm as well as the protocol used for its implementation and training are detailed, with particular emphasis on those aspects that make it so efficient and transferable. As a first test, we have applied it to the determination of the global minimum structures of ZnMg nanoalloys with up to 52 atoms and stoichiometries corresponding to MgZn and MgZn, of special interest in the context of anticorrosive coatings. We present and discuss the structural properties, chemical order, stability and pertinent electronic indicators, and we extract some conclusions on fundamental aspects that may be at the roots of the good performance of ZnMg nanoalloys as protective coatings. Finally, we comment on the step forward that the presented machine learning approach constitutes, both in the fact that it allows to accurately explore the potential energy surface of systems that other methodologies can not, and that it opens new prospects for a variety of problems in Materials Science., Ministerio de Economía, Industria y Competitividad (project PGC2018-093745-B-I00)

Estudio de las primeras etapas de oxidación en nanoaleaciones de ZnMg ricas en Zn para explorar las razones por las cuales estos materiales son buenos anticorrosivos

El hidrógeno es una fuente de energía limpia y abundante que podría reemplazar a los combustibles fósiles en un futuro. Una de las lineras de investigación mas importantes con relación a esto es la de encontrar materiales que formen depósitos capaces de contenerlo con seguridad y eficiencia. Por otra parte, recientemente se ha descubierto un nuevo alótropo bidimensional del carbono, llamado popgrafeno. El objetivo de este trabajo consiste en explorar los posibles usos del popgrafeno como receptor de moléculas de hidrogeno. Para lo cual, se dopará con litio la estructura, ya que esto mejora considerablemente las capacidades de esta para atraer dichas moléculas. Por tanto, se diferenciarán 3 partes principales de este trabajo, que formarán la estructura del mismo. Por una parte, se pretende replicar la estructura del popgrafeno, por otra parte, se estudiará el decorado de la estructura con átomos de litio, y, por último, se estudiará las posibilidades de una estructura dopada que será seleccionada como depósito de hidrogeno., Hydrogen is a clean and abundant energy source that could perhaps replace fossil fuels in the future. One of the most important areas of research related to it consist in finding materials which are able to hold it efficiently and safely. In addition, a new bidimensional allotrope of carbon has been recently discovered, called popgraphene. The goal of this research is to explore the use of popgraphene as an hydrogen receptor. To achieve this, popgraphene will be bound with lithium, since this improves hydrogen reception. Therefore, there are three different sections in this text. On one hand, the popgraphene structure will be replicated. On the other hans, the bindings between popgraphene and lithium will be studied. Lastly we will study the odds to use popghaphene with lithium to stock hydrogen., Grado en Física

Producción Científica, We report the results of a combined empirical potential-Density Functional Theory (EP-DFT) study to assess the global minimum structures of free-standing zinc-magnesium nanoalloys of equiatomic composition and with up to 50 atoms. Within this approach, the approximate potential energy surface generated by an empirical potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. Bader charges calculated in a previous work [Corr. Sci. 124, 35 (2017)] revealed a significant transfer of electrons from Mg to Zn atoms in these nanoalloys; so the main novelty in the present work is the development of an improved EP, termed Coulomb-corrected-Gupta potential, which incorporates an explicit charge-transfer correction term onto a metallic Gupta potential description. The Coulomb correction has a many-body character and is feeded with parameterized values of the ab initio Bader charges. The potentials are fitted to a large training set containing DFT values of cluster energies and atomic forces, and the DFT results are used as benchmark data to assess the performance of Gupta and Coulomb-corrected-Gupta EP models. Quite surprisingly, the charge-transfer correction is found to represent only a 6% of the nanoalloy binding energies, yet this quantitatively small correction has a sizable benefitial effect on the predicted relative energies of homotops. Zn-Mg bulk alloys are used as sacrificial material in corrosion-protective coatings, and the long-term goal of our research is to disclose whether those corrosion-protected capabilities are enhanced at the nanoscale., Junta de Castilla y León (Ref. VA124G18), Ministerio de Economía, Industria y Competitividad ((Project PGC2018-093745-B-I00)

Producción Científica, Putative global minimum structures for neutral CdN and singly charged Cd+ N and Cd− N clusters in the small size regime up to N = 21 atoms are reported. A global optimization approach based on the basin hopping method and a Gupta potential fitted to cluster properties is employed to generate a diverse databank of trial structures, which are then re-optimized at the densityfunctional level of theory. Novel, previously unreported, structures are found for many sizes. Our results successfully reproduce and interpret the size-dependent stabilities known from mass spectrometry, and strongly suggest that experiments aimed at determining the relative stabilities of neutral cadmium clusters are really measuring cation stabilities. We provide an in-depth analysis of electronic structure and use it to explain the gradual emergence of metallic-like behaviour as the cluster size increases., Junta de Castilla y León (Project VA124G18), Ministerio de Ciencia, Innovación y Universidades (Project PGC2018-093745- B-100)

©2018 American Meteorological Society. We thank the three anonymous reviewers for their excellent reviews providing new insight and very detailed comments that improved the clarity of the manuscript. One of the reviewers alerted us to the important role played by the vertical eddy momentum divergence in the subtropics. NOAA-ERSST data was provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, from their website at http://www.esrl.noaa.gov/psd/. We thank NOAA for this data, and the European Centre for Medium-Range Weather Forecasts for the atmospheric reanalysis data. P. Z.-G. acknowledges financial support by Grant CGL2015-72259-EXP by the Ministry of Economy and Innovation of Spain., This work investigates the covariability in the strength of the Hadley and Ferrel cells on interannual time scales using reanalysis data. A significant correlation is found in both hemispheres only during boreal winter. For other seasons, only the outermost (subtropical) part of the Hadley cell is correlated with changes in the extratropical eddy momentum fluxes, as the eddies are unable to penetrate into the deep tropics. During boreal winter, the northern Hadley cell variability is driven by extratropical planetary momentum fluxes, but the mean meridional circulation response is primarily found below the level of maximum climatological outflow. Instead, at upper levels, changes in the zonal wind dominate the response to the anomalous eddy forcing. During austral winter, the southern Hadley cell is shielded from the extratropical eddy fluxes and its variability displays some of the characteristics of the angular momentum-conserving solution., Ministerio de Economía y Competitividad (MINECO), Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

This work investigates the covariability in the strength of the Hadley and Ferrel cells on interannual time scales using reanalysis data. A significant correlation is found in both hemispheres only during boreal winter. For other seasons, only the outermost (subtropical) part of the Hadley cell is correlated with changes in the extratropical eddy momentum fluxes, as the eddies are unable to penetrate into the deep tropics. During boreal winter, the northern Hadley cell variability is driven by extratropical planetary momentum fluxes, but the mean meridional circulation response is primarily found below the level of maximum climatological outflow. Instead, at upper levels, changes in the zonal wind dominate the response to the anomalous eddy forcing. During austral winter, the southern Hadley cell is shielded from the extratropical eddy fluxes and its variability displays some of the characteristics of the angular momentum–conserving solution.

El objetivo de este Trabajo de Fin de Grado consiste en obtener las estructuras de míınima energía de agregados atómicos de cadmio, Cdq N, para tamaños comprendidos entre N = 3−21 átomos y cargas q = +1,0,−1. Para ello, utilizamos un método de primeros principios basado en la Teoría del Funcional de la Densidad para resolver el problema electrónico, junto con técnicas de optimización globales y locales para rastrear la superficie de energía potencial y determinar el mínimo absoluto sobre dicha superficie. Nuestros resultados mejoran los resultados teóricos previos publicados hasta la fecha, ya que localizamos estructuras más estables para casi todos los tamaños. Por otro lado, a partir de las energías calculadas, conseguimos reproducir e interpretar las abundancias relativas de agregados de diferentes tamaños determinadas en experimentos de espectrometría de masas. Los potenciales de ionización teóricos están también en muy buen acuerdo con medidas experimentales. La reproducción de resultados experimentales proporciona la seguridad de que todas nuestras conclusiones tienen validez cuantitativa. Finalmente, ofrecemos un análisis detallado de diversas propiedades electrónicas y de cómo se establece el comportamiento metálico en agregados de cadmio conforme aumenta su tamaño., Grado en Física