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A simple and novel setup for high-frequency dielectric spectroscopy of materials has been developed using a portable vector network analyzer. The measurement principle is based on radio-frequency reflectometry, and both its capabilities and limitations are discussed. The results obtained on a typical liquid crystal prove that the device can provide reliable spectra between 107 Hz and 109 Hz, thus extending the capabilities of conventional impedance analyzers.

The martensitic transformation of Ni49.5Ti35.5Hf15 high temperature shape memory alloy is characterized by internal friction measurements. The temperatures for the reversible B2 <-> B19' transformation appear in the range 410-479 K on cooling and 447-526 K on heating. In addition, a strong anomalous pre-martensitic softening of the dynamic modulus is reported in the high-temperature phase, between 850 K and 550 K, evidencing the thermoelastic character of this family of alloys. Based on the analysis of the internal friction spectra, a theoretical and experimental methodology is proposed to measure the Clausius-Clapeyron coefficient through mechanical spectroscopy. This new non-destructive method could be extremely useful for the development of advanced prototypes of shape memory alloys.

The development of intermetallic titanium aluminides has been driven by the aeronautic and aerospace industries because of the excellent mechanical properties and low density of gamma-TiAl based alloys. Up to now, several generations of gamma-TiAl based alloys were developed with increasing complexity of the alloy systems. Nb is one of the most important alloying elements in gamma-TiAl alloys and although it is considered as a slow diffuser, its influence has not been fully quantified yet. In this work we demonstrate, through mechanical spectroscopy measurements conducted on several gamma-TiAl based alloys with different Nb content, that Nb impedes the diffusion of Ti atoms in the alpha(2)-Ti3Al phase. Internal friction measurements show a relaxation peak P(alpha(2)), which is associated with short distance diffusion of Ti atoms in the alpha(2) phase, involving stress-induced rotation of dipoles Al-V-Ti-Al, whose activation energy is dependent on the Nb content. The increase of the activation energy is quantified as Delta E-a(Ti)= 0.037 eV x at% Nb, being attributed to the next-neighbor interaction of Nb atoms with the local configuration of Ti-V-Ti. This mechanism also produces a further broadening of the relaxation peak, which is attributed to the near-next-neighbor interactions for high Nb contents. Finally, an atomic model for the mechanism responsible for this relaxation is proposed allowing to explain the observed experimental behavior

181 p., La ganancia de cuota del mercado de vehículos eléctricos es esencial si se pretenden mitigar las consecuencias del efecto invernadero causadas, entre otros factores, por las emisiones de gases de vehículos de motor térmico. Sin embargo, la tecnología encargada de suministrar energía en los vehículos eléctricos, tecnología de Li-ion, no es suficientemente competitiva debido a sus limitaciones en carga rápida, autonomía, seguridad y durabilidad. Por ello, el progreso de la tecnología de Li-ion es vital. Además, su avance asegurará la evolución de los aparatos electrónico portátiles, también alimentados por baterías de Li-ion. De entra las diferentes alternativas existentes para mejorar la densidad energética de las baterías de Li-ion, una de las estrategias más prometedores es el cambio del ánodo actual, grafito, por litio metálico. Aun así, la alta reactividad de la superficie del litio imposibilita tener una superficie estable entre el ánodo y el electrolito. En este trabajo de tesis, la superficie del litio ha sido estudiada con el objetivo de adquirir mayor conocimiento sobre su estabilidad. Para ello, primeramente, se ha analizado cómo los gases atmosféricos secos más comunes (O2, CO2 y N2) modifican la composición química y las propiedades electrónicas de la superficie del litio. Este estudio se ha realizado por medio de las técnicas espectroscópicas de fotoemisión de rayos X y de rayos ultravioleta. En el siguiente estudio, la evolución del carbonato de litio en la superficie del litio se ha analizado in situ por medio de la técnica espectroscópicas de fotoemisión de rayos X de presión ambiente. Finalmente, se ha analizado el efecto de las impurezas nativas de la superficie de una lámina de litio comercial en un sistema electroquímico. Para ello, con el fin de evitar estas impurezas, el primer paso ha sido crecer una capa fina de litio. A continuación, se ha estudiado el rendimiento electroquímico y resistencia interna de un sistema formado por electrodos simét

204 p., En esta tesis se estudian fenómenos no adiabáticos y dependientes del espín del electrón relacionados conla interacción electrón-fonón en nanoestructuras con fuertes correcciones relativistas. Se han realizadocálculos de primeros principios de estructura electrónica y dinámica vibracional basados en la teoría deldensidad del funcional, que permiten abordar el cálculo de la interacción electrón-fonón a partir de lateoría de muchos cuerpos basada en funciones de Green. En la primera parte de este trabajo se analiza elpapel que juega el espín del electrón y el acoplo espín-órbita en la interacción electrón-fonón. Se haaplicado a la monocapa de Tl sobre la superficie de Si(111). Se concluye que la polarización del espíninducida por el acoplo espín-órbita entra de una manera decisiva modulando e incluso suprimiendo lainteracción electrón-fonón en el material. En la segunda parte se estudia la emergencia de efectos noadiabáticos relacionados con la interacción electrón-fonón y regulados por la polarización del espínelectrónico tanto para electrones como para fonones. Se ha investigado la monocapa de MoS2 en funcióndel dopaje electrónico. Se concluye que los efectos no adiabáticos modifican dramáticamente ladispersión de fonones y electrones con la aparición de múltiples cuasi-partículas definidas en el planocomplejo de energía. En el caso de los fonones ópticos en el límite de longitud de onda larga se proponeun nuevo mecanismo físico capaz de entenderse por medio de sencillos modelos como el de Einstein. Enel caso de los electrones se reproducen y explican experimentos de espectroscopía de fotoemisiónrecientemente medidos, identificando tres cuasipartículas, una de las cuales tiene una vida media muylarga a pesar de la fuerte interacción electrón-fonón.

Topological phases in electronic structures contain a new type of topology, called fragile, which can arise, for example, when an elementary band representation (atomic limit band) splits into a particular set of bands. For the first time, we obtain a complete classification of the fragile topological phases, which can be diagnosed by symmetry eigenvalues, to find an incredibly rich structure that far surpasses that of stable or strong topological states. We find and enumerate hundreds of thousands of different fragile topological phases diagnosed by symmetry eigenvalues, and we link the mathematical structure of these phases to that of affine monoids in mathematics. Furthermore, for the first time, we predict and calculate (hundreds of realistic) materials where fragile topological bands appear, and we showcase the very best ones.

152 p., Estudio teórico de los efectos anarmónicos en materiales termoeléctricos y bidimensionales. En el casode los materiales termoeléctricos SnSe y SnS, hemos visto que los efectos anarmónicos producentransiciones de fase de segundo orden donde la estructura cristalina se simetriza a altas temperaturas. Enla fase de alta simetría, estos materiales tienen una conductividad térmica muy reducida, lo que los lleva aser materiales termoeléctricos muy eficientes. Hemos estudiado también una monocapa de SnSe, dondehemos visto que la transición de segundo orden lleva el sistema a una fase sin centro de inversión. Ésto loconvierte en un material ferroeléctrico. Por último, hemos estudiado la estabilidad mecánica del grafeno yhemos visto que los efectos anarmónicos son clave para entender su estabilidad. También hemos vistoque los efectos anarmónicos hacen posible la transmisión del sonido en el grafeno.

KIMERA is a scientific tool for the study of mineral dissolution. It implements a reversible Kinetic Monte Carlo (KMC) method to study the time evolution of a dissolving system, obtaining the dissolution rate and information about the atomic scale dissolution mechanisms. KIMERA allows to define the dissolution process in multiple ways, using a wide diversity of event types to mimic the dissolution reactions, and define the mineral structure in great detail, including topographic defects, dislocations, and point defects. Therefore, KIMERA ensures to perform numerous studies with great versatility. In addition, it offers a good performance thanks to its parallelization and efficient algorithms within the KMC method. In this manuscript, we present the code features and show some examples of its capabilities. KIMERA is controllable via user commands, it is written in object-oriented C++, and it is distributed as open-source software.

We provide an overview of the tools directed to reversible and irreversible transformations of the magnetic structure of glass-covered microwires. The irreversible tools are the selection of the chemical composition, geometric ratio, and the stress-annealing. For reversible tuning we use the combination of magnetic fields and mechanical stresses. The studies were focused on the giant magnetoimpedance effect and the velocity of the domain walls propagation important for the technological applications. The essential increase of the giant magnetoimpedance effect and the control of the domain wall velocity were achieved as a result of the use of two types of control tools. The performed simulations reflect the real transformation of the helical domain structures experimentally found.

Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces.

Shape-memory alloys (SMAs) are the most stretchable metallic materials thanks to their superelastic behavior associated with the stress-induced martensitic transformation. This property makes SMAs of potential interest for flexible and wearable electronic technologies, provided that their properties will be retained at small scale. Nanocompression experiments on Cu-Al-Be SMA single crystals demonstrate that micro- and nanopillars, between 2 mu m and 260 nm in diameter, exhibit a reproducible superelastic behavior fully recoverable up to 8% strain, even at the nanoscale. Additionally, these micro-/nanopillars exhibit a size effect on the critical stress for superelasticity, which dramatically increases for pillars smaller than approximate to 1 mu m in diameter, scaling with a power law of exponent n = -2. The observed size effect agrees with a theoretical model of homogeneous nucleation of martensite at small scale and the universality of this scaling power law for Cu-based SMAs is proposed. These results open new directions for using SMAs as stretchable conductors and actuating devices in flexible and wearable technologies.

In this paper we analyze the spatial distribution of elements around points of interest. Based on a spatial exclusion principle we model the system by means of a Fermi–Dirac distribution defined by two easily interpretable parameters. By means of image analysis, two real cases are studied and compared to the theory: people in an open-air concert and cars in a mall parking-lot. We show that they closely obey the proposed fermion-like statistics.

In this work we study the walk-off of the beam from the interrogation spot during rotation in surface plasmon resonance experiments using prism-based coupling such as the widespread Kretschmann configuration. The impossibility of maintaining a stationary footprint on the sensing surface with a fixed rotation axis can be of high importance. This would be specially so if samples are not homogeneous such as in arrays for multiplexing. By theoretically analyzing the behavior of the walk-off during rotation around an arbitrary fixed axis, we find an optimal and simple configuration to minimize this effect. The proposed setup is experimentally tested to verify the results and to show its ease of implementation. Interestingly, the conclusions reached may also be applied to other techniques employing reflection prisms.

Carrier doping by the electric field effect has emerged as an ideal route for monitoring many-body physics in two-dimensional materials where the Fermi level is tuned so that the strength of the interactions can also be scanned. The possibility of systematic doping together with high resolution photoemission has allowed to uncover a genuinely many-body electron spectrum in single-layer MoS2 transition metal dichalcogenide, resolving three clear quasi-particle states, where only one should be expected if the electron-phonon interaction vanished. Here, we combine first-principles and consistent complex plane analytic approaches and bring into light the physical origin of the two gaps and the three quasi-particle bands which are unambiguously present in the photoemission spectrum. One of these states, though being strongly interacting with the accompanying virtual phonon cloud, presents a notably long lifetime which is an appealing property when trying to understand and take advantage of many-body interactions to modulate transport properties.

El capítulo 4 está sujeto a confidencialidad por el autor. 194 p., En esta tesis doctoral titulada "Surfaces and Interfaces in Thin-Film and Conventional Li-ion Batteries" se presentan los resultados del trabajo experimental realizado en relación a materiales del estado del arte en baterías íon-litio. En particular, en una primera parte se describen los resultados obtenidos al estudiar la superficie de electrodos formados por el material Titanato de Litio (Li4Ti5O12, LTO). En esta primera parte el estudio se centra en la evolución de las fases presentes en la superficie del electrodo a lo largo del ciclado electroquímico. En una segunda parte, continuando con los electrodos basados en el material LTO, se estudia el efecto de capas protectoras depositadas en electrodos convencionales mediante la técnica de sputtering. En esta parte se describe el efecto positivo que dichas capas protectoras tienes en el comportamiento electroquímico de los electrodos, al mismo tiempo que se estudia el origen de dicha mejora. Finalmente, en un último capítulo se describe la deposición, caracterización y optimización de electrodos de capa fina del material LiMn1.5Ni0.5O4. Además de una caracterización composicional, morfológica, estructural y electroquímica de los electrodos, se dedica especial atención a las intercaras, tanto la del colector de corriente-electrodo como la del electrodo-electrolito sólido.

145 p., This thesis is based on the experimental study of spin-to-charge current conversion (SCC) phenomena that occur in systems with spin-orbit coupling: spin Hall effect (SHE) and anomalous Hall effect (AHE). The main objective is to unveil the mechanisms that give rise to SCCs, in order to show the path to enhance the conversion efficiency for plausible technological applications. In the first part, the SHE is studied in Pt and Ta using the spin absorption technique. The weight of intrinsic and extrinsic mechanisms is extracted and a clear path to enhance the conversion efficiency is shown. Next, a Pt/graphene-based spin-to-charge current converter is explored where two orders of magnitude larger output signals, originated from SCC, have been obtained. In the second part, the relation between SHE and AHE is studied in 3d ferromagnets (FMs). The results show that both effects are not related by the current spin polarization of the FM, as it is generally assumed, and lead us to suggest the presence of an extra mechanism only in the SHE of 3d FMs. Finally, we analyse an interfacial mechanism that contributes to the AHE of Co when a Bi2O3 capping layer is added on top of the FM.

178 p., Por medio de técnicas experimentales entre las que destacan el efecto Kerr magneto-óptico (MOKE) y una extensión del mismo, la elipsometría magneto-óptica generalizada (GME), investigo fenómenos relacionados con el magnetismo y la espintrónica en láminas finas metálicas fabricadas por medio de deposición catódica, recurriendo a la modelización numérica para interpretar los resultados. Por un lado, estudio la detección con GME de la acumulación de espín en la superficie de metales debido al efecto Hall de espín (SHE). Separando señales espurias de señales MOKE observo que estas últimas, relacionadas con el SHE, son mucho menores que las reportadas en la literatura previa. Por otro lado, investigo los efectos de una capa ultrafina de Ru sobre las propiedades magnéticas de Co con anisotropía magnetocristalina uniaxial en el plano. Observo que el Ru induce una rotación de la magnetización del Co fuera del plano en remanencia que explico con una modulación espacial de la interacción de Dzyaloshinskii-Moriya. Por último, exponiendo un material ferromagnético a un campo magnético externo que oscila, estudio cómo al variar el periodo del campo se da una transición de fase entre un estado dinámicamente ordenado y otro dinámicamente desordenado. Examinando el diagrama de fase de esta transición y comparándolo con el diagrama de fase de la transición de fase termodinámica convencional se observan diferencias anteriormente desconocidas

In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. The observed effects and the measured strain relaxations are in agreement with the predictions of the model proposed to explain this behavior in previous calorimetric studies. In addition, the thermal expansion coefficients of both martensite and austenite phases were measured. The neutron experiments have allowed a complete description of the strains during martensitic transformation, and the obtained conclusions can be extrapolated to other SMA systems. (c) 2019 Author(s).

Micro and nano pillars of Copper-based shape memory alloys (SMAs) with feature sizes between about 2 mu m and 250 nm are known to exhibit ultra-high mechanical damping due to the nucleation and motion of stress-induced martensite interfaces during superelastic straining. While this behavior could be extremely useful to protect micro electro-mechanical systems (MEMS) against vibrations in aggressive environments, a fundamental question must yet be answered in order to envisage further applications, namely, whether this damping is reproducible and stable over long times and many cycles, or whether the damping is a signal of accumulating damage that could compromise long-term usage. In the present paper this crucial question is answered; we show that micropillar arrays of Cu-Al-Ni SMAs exhibit a completely recoverable and reproducible superelastic response, with an ultra-high damping loss factor eta > 0.1, or even higher for sub-micrometer pillars, eta > 0.2, even after thousands of cycles (>5000) and after long times spanning more than four years. Furthermore, the first high-frequency tests on such nanoscale SMAs show that their superelastic response is very fast and relevant to ultra-high damping even at frequencies as high as 1000 Hz. This paves the way for the design of micro/nano dampers, based on SMAs, to improve the reliability of MEMS in noisy environments. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The photoelectric effect is one of the key experiments taught during first- or second-year university and high school modern physics courses. It is usually the first experiment to introduce light quantization and the concept of photons as “packets of energy.” Here, we want to point out a widespread mistake concerning the interpretation of the saturation current at constant light intensity that is found even in some classic hardback literature. Although this is usually overlooked, it can weaken the conclusions a student can draw from the correct understanding of the experiment.

beta-dicalcium silicate (beta-Ca2SiO4 or beta-C2S in cement chemistry notation) is one of the most important minerals in cement. An improvement of its hydration rate would be the key point for developing environmentally-friendly cements with lower energy consumption and CO2 emissions. However, there is a lack of fundamental understanding on the water/beta-C2S surface interactions. In this work, we aim to evaluate the water adsorption on three beta-C2S surfaces at the atomic scale using density functional theory (DFT) calculations. Our results indicate that thermodynamically favorable water adsorption takes place in several surface sites with a broad range of adsorption energies (-0.78 to -1.48 eV) depending on the particular mineral surface and adsorption site. To clarify the key factor governing the adsorption of the electronic properties of water at the surface were analyzed. The partial density of states (DOS), charge analysis, and electron density difference analyses suggest a dual interaction of water with a beta-C2S (100) surface including a nucleophilic interaction of the water oxygen lone pair with surface calcium atoms and an electrophilic interaction (hydrogen bond) of one water hydrogen with surface oxygen atoms. Despite the elucidation of the adsorption mechanism, no correlation was found between the electronic structure and the adsorption energies.

The multistage martensitic phase transformation of a polycrystalline NiTi shape-memory alloy (50.3 at. %Ni-49.7 at. % Ti) has been studied by means of calorimetric measurements. After a conventional thermal treatment followed by successive thermal cycles, the initial two-step forward transformation splits into four-overlapping stages. However, the reverse martensitic transformation maintains the initial two-step sequence, usually assigned to the B19'-> R -> B2 transformation. The correlation between the forward and reverse steps has been established by means of selected thermal cycles together with an estimation of their enthalpy and thermal hysteresis. These results have also provided information about the storage of the elastic strain energy and the frictional works associated with the variants' nucleation. Moreover, the study around the forward transformation temperature range by means of uncompleted thermal cycles undoubtedly shows the presence of temperature memory effects in both stages.

181 p., Different methods were applied to investigate the vacancy and Al/Si order in mullite. At first the symmetry was analysed thoroughly to derive constraints on the vacancy distribution based on crystal chemical premises. On this basis a superspace model was developed that defines the polyhedra network consisting of octahedra, tetrahedral tricluster units and tetrahedral dicluster units as a function of the modulation wave vector and the vacancy concentration. Refinements of superspace models based on synchrotron single crystal X-ray diffraction measurements indicate that in the real structure the identified pattern is present, but with a decreased degree of order. Different samples exhibit different degrees of order suggesting that mainly disordered and fully ordered mullite crystals exist. The Al/Si ordering could not be derived from symmetry constraints and the occupancy of Si could not be refined. Nevertheless, an Al/Si ordering pattern could be identified from the analysis of the displacive modulation.The dependence of the satellite reflections on the chemical composition was studied with precession electron diffraction tomography and density functional theory. This allowed to characterise the structural modulation on a new level and reveal the fundamental ordering patterns that define the crystal structure of mullite in terms of vacancy, tricluster and Al/Si order. The understanding of the crystal structure forms a new basis for future research on the properties of mullite and related applications.

252 p., El presente trabajo de investigación se encuadra en la línea de "Materiales Intermetálicos para Altas Temperaturas" que se desarrolla en el Grupo de investigación en MetalurgiaFísica (GIMF) de los Dptos de Física de la Materia Condensada y Física Aplicada II dela UPV/EHU. En ese marco, el GIMF ha establecido un programa de colaboración internacional con el Dpt. of Physical Metallurgy and Materials Testing de la Universidad de Leoben, Austria, donde el Dr. H. Clemens es líder mundial en el desarrollo de lasnuevas generaciones de Materiales Intermetálicos de TiAl [Cle13]. En el marco de esta colaboración se han realizado ya varias publicaciones científicas y comunicaciones acongresos conjuntas [Kle17, Usa17].La importancia de reducir el impacto medioambiental y aumentar la seguridad y el rendimiento económico de los sistemas de transporte ha motivado intensas investigaciones durante las últimas tres décadas. El desarrollo y procesado de materiales para altas temperaturas, es la clave del avance tecnológico en áreas de la ingeniería en las cuales se deben alcanzar requerimientos extremos, como es el caso de la industria aeronáutica. Los requisitos de una mayor eficiencia, reducción de emisiones de CO2 y de consumo de combustible y disminución del peso en los motores de aviación es la fuerza motriz para el desarrollo de materiales ligeros innovadores y resistentes a altas temperaturas que puedan sustituir parcialmente los materiales empleados en la actualidad.Las aleaciones intermetálicas de TiAl muestran un gran potencial para satisfacer dichas demandas [App00][Cle13].Hasta ahora, las superaleaciones de base níquel han dominado el ámbito de los álabes de turbinas en motores de reacción debido a su capacidad para soportar las elevadas cargas mecánicas y térmicas impuestas al material. Una desventaja estas aleaciones es su altadensidad 8 g/cm3, que en el caso de los intermetálicos TiAl se ve reducida a casi la mitad(3,9-4,2 g/cm3 dependiendo de la composición y la consti

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Geopolymers are hydrated aluminosilicates with excellent binding properties. Geopolymers appeal to the construction sector as a more sustainable alternative to traditional cements, but their exploitation is limited by a poor understanding of the linkage between chemical composition and macroscopic properties. Molecular simulations can help clarify this linkage, but existing models based on amorphous or crystalline aluminosilicate structures provide only a partial explanation of experimental data on the nanoscale. This paper presents a new model for the molecular structure of geopolymers, in particular for nanoscale interfacial zones between crystalline and amorphous nanodomains, which are crucial for the overall mechanical properties of the material. For a range of Si–Al molar ratios and water contents, the proposed structures are analyzed in terms of skeletal density, ring structure, pore structure, bond-angle distribution, bond length distribution, X-ray diffraction, X-ray pair distribution function, elastic moduli, and large-strain mechanics. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with an impact on mechanical properties. This offers a new starting point for the multiscale modeling of geopolymers.

The topic of cholesteric-liquid-crystal lasers is a rapidly expanding research area in the field of soft-matter photonics. The increasing interest in this field is due to the high versatility that these lasers may possibly present and the prospects of giving rise to new miniaturized devices. However, further improvements in their operation capabilities are still required for potential applications. In this paper, we critically analyze the main strategies proposed up to now to optimize their performance. We show theoretically and experimentally that possible innovations in the device structure cannot produce lasers with threshold energies below a certain limit. This limit is determined by the light scattering and absorption losses inside the liquid crystal. Even assuming the case of samples free of defects and perfectly non-absorbing, an intrinsic light scattering, typical of mesogens, still remains. Numerical estimates of the thresholds indicate that these lasers could hardly be driven by compact light sources such as current electroluminescent or light-emitting diodes. Since the improvement possibilities regarding cell architecture seem to be exhausted, the advance must come from the use of new dye molecules. These molecules should show enhanced emission cross-sections and be efficiently integrable within the mesogenic solvent. In addition, the fluorescent systems must present very small quantum yields to triplet states if continuous-wave lasing is sought. In this respect, quantum dots are an alternative to explore for further investigations.

326 p., En esta tesis doctoral ha consistido en un estudio de la simetría magnética y de los grupos espaciales y superespaciales magnéticos desde un punto de vista práctico, orientado al desarrollo de herramientas teóricas que faciliten el estudio, descripción y determinación de estructuras magnéticas y sus propiedades. Esta tesis doctoral pretende demostrar la conveniencia e idoneidad del uso de la simetría magnética en la determinación de estructuras magnéticas como metodología útil y complementaria al tradicional representation method, demostrando que a menudo resulta imprescindible para un análisis completo y riguroso. Como resultado, se han creado 11 programas informáticos que incluyen bases de datos y programas interactivos que permiten, entre otras cosas, explorar todas las posibles estructuras magnéticas compatibles con los datos experimentales disponibles, haciendo uso de forma consistente de su simetría magnética, así como obtener la forma restringida por simetría de sus propiedades tensoriales y las posibles estructuras equivalentes asociadas a dominios, junto con sus propiedades tensoriales de conmutación. Es de esperar que el uso de la simetría magnética de la forma aquí descrita y la utilización de los programas desarrollados faciliten la búsqueda de materiales multiferroicos y magnetoeléctricos, cuyas propiedades son objeto de un creciente interés científico y tecnológico.

220 p., Las aleaciones con memoria de forma SMAs (Shape Memory Alloys) presentan un gran potencial para el desarrollo de nuevos MEMS (MicroElectroMechanical Systems) que puedan cubrir la creciente demanda, de sensores y actuadores sofisticados, en todos los campos de la ciencia y la tecnología. Esta tesis aborda el estudio de fabricación y caracterización a escala micro y nanométrica de la SMA de Cu-Al-Ni para su aplicación en MEMS a partir de dos aproximaciones: La fabricación y caracterización microestructural de películas delgadas, y la caracterización mecánica a escala nanométrica de micro y nanopilares monocristalinos tallados mediante bombardeo iónico focalizado (FIB). En el primer caso se presenta el método de aleado de multicapas elementales para obtener esta SMA en forma de película delgada (1 ¿m, espesor), así como su caracterización mediante In-situ-TEM. En el segundo caso, se presenta la caracterización de micro/nano pilares (2 ¿m-260 nm, diametro) mediante ensayos de nano-compresión, a partir de los cuales se ha puesto en evidencia, por primera vez, un notable efecto de tamaño sobre el comportamiento superelástico, que se ha interpretado según un modelo atomístico-elástico. También se evaluó el efecto superelástico y el amortiguamiento mecánico asociado, en función del ciclado y su reproducibilidad en el tiempo (3.5 años). Finalmente se presentan resultados de nano-compresión In-situ-SEM.

Cu-Al-based shape memory alloys are firm candidates to be used up to 473 K. The main limiting aspect is the activation of diffusion processes in the metastable austenite phase, which drive the alloy decomposition. In the present work the study of short-distance diffusion processes has been approached by internal friction. A relaxation peak has been found in the metastable beta (D0(3)) phase of a Cu-Al-Be shape memory alloy, around 500 K (at 1 Hz), with an activation energy of E-a = 138 +/- 0.05 eV. An atomic mechanism of elastic dipoles Antisite-Vacancy reorientation, involving stress-assisted short distance Cu-atoms diffusion, has been proposed. (C) 2016 Elsevier B.V.

Advanced gamma-TiAl based intermetallics Mo-bearing have been developed to obtain the fine-grained microstructure required for superplastic deformation to be used during further processing. In the present work we have studied an alloy of Ti-46.8Al-1Mo-0.2Si (at%) with two different microstructures, ascast material with a coarse grain size above 300 mu m, and the hot extruded material exhibiting a grain size smaller than 20 mu m. We have used a mechanical spectrometer especially developed for high temperature internal friction measurements to study the defect mobility processes taking place at high temperature. The internal friction spectra at different frequencies has been studied and analyzed up to 1360 K in order to characterize the relaxation processes appearing in this temperature range. A relaxation peak, with a maximum in between 900 K and 1080 K, depending on the oscillating frequency, has been attributed to Ti-atoms diffusion by the stress-induced reorientation of Al-V-Ti-Al elastic dipoles. The high temperature background in both microstructural states, as-cast and extruded, has been analyzed, measuring the apparent activation parameters, in particular the apparent energies of E-cast(IF) = 4.4 +/- 0.05 eV and E-ext(IF) = 4.75 +/- 0.05 eV respectively. These results have been compared to those obtained on the same materials by creep deformation. We may conclude that the activation parameters obtained by internal friction analysis, are consistent with the ones measured by creep. Furthermore, the analysis of the high temperature background allows establish the difference on creep resistance for both microstructural states. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd.

183 p., The main objective of this thesis is to investigate metal alloys as phase change material (PCM) for thermal energy storage (TES) applications. In particular, This work consists to analyze and characterize a set of binary and ternary metal alloys based on different compositions of Mg-Al-Zn mixtures, in order to identify new eutectic and peritectic compositions with adequate melting temperatures combined with appropriate thermophysical properties for their use as PCM storage media at high temperature for concentrated solar power (CSP) and industrial waste heat recovery applications among others. The first stage of this experimental work consists on synthesis of the selected metal alloys by using different routes of preparation. When the materials are synthesized a complete structural analysis is conducted by combining different techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM). The second stage of this work is to determine the most relevant thermophysical properties of these materials in this field such as the melting temperature, the heat capacity, the latent heat, the thermal conductivity, among others. The third stage is to investigate the long term thermal stability of the studied PCM by accelerated thermal cycling tests. The final stage consists to investigate the compatibility study between the selected alloys and different containment materials in order to identify the most suitable stainless steels that can be used in the construction of TES unit.

248 p., Nuevo sistema de compensación para agujas náuticas tipo 'A' destinado a sustituir al convencional constituido por: barra Flinders, esferas, imán de escora e imanes correctores longitudinales y transversales. Debido a que los nuevos sistemas técnicos de navegación desarrollados hasta hoy día, a pesar de su sofisticación para ofrecer exactitud y precisión, no han conseguido desplazar a la aguja magnética o náutica. Siendo el matemático inglés Archibald Smith, quien mayores aportaciones hizo en conseguir mediante formulación empírica el cálculo de los coeficientes que servirían para una mejora compensación de la aguja.Aún con la aparición de los girocompases, Sistemas Satelitarios Transit, Navegación inercial, etc. hasta llegar al compás satelitario, como lo más reciente para la obtención de un rumbo verdadero o de proa, los nuevos sistemas de posicionamiento, todavía en período de implementación, lo que han generado es una gran seguridad en lo que a la navegación se refiere tanto en travesías costeras, como transoceánicas.Habría que añadir que son susceptibles a interferencias y errores en las ondas electromagnéticas, poniendo en peligro tanto precisión e integridad de dichos sistemas de posicionamiento, como pueden ser las tormentas solares (de gran actividad en la época pasada muy reciente, además de manipulación subjetiva, en función de los intereses de los controladores por la potencia mundial de quien dependa) sumado a la declinación magnética terrestre y el desvío generado por el propio acero del buque o antenas que afecten a dicha aguja. De un proyecto anterior surgió el CMI compass (Compás Magnético Integral) equipo mediante el cual obtenemos en tiempo real todas las correcciones al igual que los coeficientes que aplicaremos para calcular los desvíos de cada rumbo de aguja, calculados automáticamente, amén de la tabla de desvíos, permitiendo la confección e impresión del correspondiente Certificado Oficial obligatorio de la Tablilla de Desvíos, y, que podemos

181 p., Dentro del marco industrial mundial y del País Vasco en particular el sector metalúrgico ha tenido y tiene un peso muy importante. En muchos de los materiales utilizados es vital conocer las pérdidas de calor por radiación, ya bien sea durante el proceso de obtención de éstos o mientras están en funcionamiento. Para ello es necesario estudiar las propiedades termorradiativas y ópticas de estos materiales, siendo la medida de la emisividad espectral la mejor de entender los procesos de radiación.La tesis se planteó con tres objetivos muy concretos basados en que el laboratorio de Física de la Materia Condensada de la UPV/EHU dispone de un radiómetro de alta precisión (HAIRL) que permite realizar medidas de emisión térmica espectrales y direccionales en el infrarrojo en función de la temperatura y en una atmósfera controlada.El primer objetivo era completar los análisis que se hicieron hasta ahora sobre los métodos de medida, precisión de las medidas de emisividad y temperatura, sistema y parámetros de calibración, etcétera. El segundo objetivo estaba dirigido a medidas espectrales de los metales puros entre 200 y 800 ºC y al estudio del efecto skin anómalo en metales bajo emisivos. El tercer objetivo estaba relacionado con los materiales utilizados en centrales termosolares para la absorción de la luz por un lado y para el almacenamiento térmico en otro, ya que es necesario conocer las pérdidas de calor por radiación en ambos casos y en la literatura no se encontraban estudios a temperatura de trabajo.

The normal spectral emissivity of a cutting tool based on polycrystalline cubic boron nitride (PCBN) between 400 °C and 1000 °C has been measured. Its shape shows significant differences with respect to that of pure cubic boron nitride (c-BN). Therefore, while the reflectance spectrum of pure c-BN can be fitted to a Lorentz model for linear dielectrics, the reflectance spectrum of the cutting tool (calculated from the emissivity using Kirchhoff's laws) requires a combination of a four-parameter Kurosawa model with a double-damping Drude one. A detailed study of the dependence of the emissivity spectrum with temperature is also performed. The experimental data of this paper is required for accurate temperature measurements with radiation thermometers in machining processes.

175 p. El contenido de los capítulos 3 y 5 están sujetos a confidencialidad por el autor, Las aleaciones con memoria de forma son materiales inteligentes que secaracterizan por una transformación martensítica termoelástica entre una fase austenitade alta temperatura y una fase martensita de baja temperatura. Estos materiales sonfácilmente deformables en fase martensita y son capaces de recuperar su forma inicial,mediante el efecto de memoria de forma, calentándolos hasta su fase austenita. Duranteeste proceso de recuperación de su forma original son capaces de realizar una fuerzaimportante; de hecho, son uno de los materiales que más trabajo pueden hacer porunidad de masa lo que los hace interesantes para diferentes aplicaciones industriales,especialmente en la industria aeroespacial y aeronáutica. El presente trabajo deinvestigación se ha centrado en la elaboración y caracterización termomecánica dealeaciones con memoria de forma monocristalinas capaces de trabajar en un ampliorango de temperaturas, incluido el rango de temperaturas entre 100-200ºC para el cual existe una demanda tecnológica de estos materiales. Para ello se han diseñado,construido y puesto en funcionamiento diversos hornos de crecimiento monocristalino y se ha desarrollado la metodología para producir estas aleaciones de una formareproducible. Por otra parte la transformación martensítica ha sido estudiada mediante diferentes técnicas incluidas técnicas desarrolladas en este trabajo que permiten caracterizar el efecto memoria

The total emissivity of the absorbing surfaces is a critical parameter in the calculation of the radiative thermal losses in solar thermal collectors. This is because the radiative heat losses have a significant economic impact on the final cost of the electricity produced in a solar thermal plant. This paper demonstrates the need to calculate the total emissivity from spectral emissivity measurements at the working temperature of the solar thermal collector, instead of using extrapolated values from spectral emissivities measured at room temperature. Usual uncertainties produced by the estimation of the total emissivity, in which its temperature dependence is only introduced by the Planck function, are analyzed.

In nickel-based superalloys, substitutional solute species have a strong impact on in service mechanical properties as well as on oxidation and corrosion resistances. In alloy 718, recent studies carried out by tensile tests highlighted the fact that refractory solute species are able to interact strongly with mobile dislocations during plastic deformation, generating dynamic strain ageing, and, in wide ranges of tests temperatures and strain rates, Portevin-Le Chatelier effect. The precise nature of the substitutional element responsible for such a dynamic interaction is still subject to debate. We addressed this question by means of mechanical spectroscopy studies of alloy 718 and various related alloys corresponding to monitored changes in the chemical composition. Only a single internal friction relaxation peak has been observed for all the studied alloys. By analyzing the damping behavior of these alloys at different imposed solicitation frequencies by sweeping a large temperatures range, the activation energies of the relaxation process and the type of mechanism involved have been determined. The process is a "Zener relaxation" in the alloys, i.e. a substitutional atoms dipole reorientation under applied stress. The results tend to prove that Niobium is not involved in the relaxation process whereas Molybdenum content seems to play an important role in the relaxation intensity.

An acoustic plasmon is predicted to occur, in addition to the conventional two-dimensional (2D) plasmon, as the collective motion of a system of two types of electronic carriers coexisting in the same 2D band of extrinsic (doped or gated) graphene. The origin of this novel mode stems from the anisotropy present in the graphene band structure near the Dirac points K and K'. This anisotropy allows for the coexistence of carriers moving with two distinct Fermi velocities along the Gamma K and Gamma K' directions, which leads to two modes of collective oscillation: one mode in which the two types of carriers oscillate in phase with one another (this is the conventional 2D graphene plasmon, which at long wavelengths (q -> 0) has the same dispersion, q(1/2), as the conventional 2D plasmon of a 2D free electron gas), and the other mode found here corresponds to a low-frequency acoustic oscillation (whose energy exhibits at long-wavelengths a linear dependence on the 2D wavenumber q) in which the two types of carriers oscillate out of phase. This prediction represents a realization of acoustic

We present a new efficient numerical approach for representing anisotropic physical quantities and/or matrix elements defined on the Fermi surface (FS) of metallic materials. The method introduces a set of numerically calculated generalized orthonormal functions which are the solutions of the Helmholtz equation defined on the FS. Noteworthy, many properties of our proposed basis set are also shared by the FS harmonics introduced by Philip B Allen (1976 Phys. Rev. B 13 1416), proposed to be constructed as polynomials of the cartesian components of the electronic velocity. The main motivation of both approaches is identical, to handle anisotropic problems efficiently. However, in our approach the basis set is defined as the eigenfunctions of a differential operator and several desirable properties are introduced by construction. The method is demonstrated to be very robust in handling problems with any crystal structure or topology of the FS, and the periodicity of the reciprocal space is treated as a boundary condition for our Helmholtz equation. We illustrate the method by analysing the free-electron-like lithium (Li), sodium (Na), copper (Cu), lead (Pb), tungsten (W) and magnesium diboride (MgB2)

Among the different Cu-based shape memory alloys, the Cu–Al–Be family exhibits a particular technological interest for intermediate and low temperature applications. In this work we studied the martensitic transformation behavior of Cu–Al–Be single crystalline shape memory alloys with three different concentrations. The influence of four specific thermal treatments of quenching and aging on the transformation temperatures has been studied by differential scanning calorimetry and compared with similar studies by mechanical spectroscopy. The analysis of the influence of thermal treatments on the martensitic transformation behavior allows optimize a treatment useful for all the alloys transforming between 200K and 400K, avoiding both stabilization and precipitation. In addition, an anomalous jerky behavior has been reported for the alloy with low transformation temperatures, which is discussed in terms of the possible mechanisms.

A complete experimental study of temperature dependence of the total spectral emissivity has been performed, for the first time, for absorber–reflector selective coatings used in concentrated solar power (CSP) systems for energy harvesting. The coating consist of double cermet layers of silicon oxide with different amounts of molybdenum over a silver infrared mirror layer. The experimental measurements were carried out by a high accurate radiometer (HAIRL) with controlled atmosphere in the mid-infrared and for temperatures between 150 and 600 °C. The spectral emissivity is nearly constant in this temperature range. Therefore, the temperature dependence of the total emissivity is given by Planck function. These results were compared with those obtained with the usual calculus using room temperature reflectance spectrum. Finally, the performance of the coating was analyzed by comparison of coated respect to non-coated stainless steel.

9 p., The surfaces of antimony are characterized by the presence of spin-split states within the projected bulk band gap and the Fermi contour is thus expected to exhibit a spin texture. Using spin-resolved density functional theory calculations, we determine the spin polarization of the surface bands of Sb(110). The existence of the unconventional spin texture is corroborated by the investigations of the electron scattering on this surface. The charge interference patterns formed around single scattering impurities, imaged by scanning tunneling microscopy, reveal the absence of direct backscattering signal. We identify the allowed scattering vectors and analyze their bias evolution in relation to the surface-state dispersion.

110 p., Los moléculas mesógenas con forma de banana (bent-core) dan lugar a nuevas fases cristal líquido con diversas funcionalidades de interés en aplicaciones tecnológicas. En esta tesis se han estudiado y caracterizado nuevos materiales tipo banana mediante microscopía óptica de polarización, generación de segundo armónico y rayos X. Más específicamente, los principales puntos desarrollados en este trabajo han sido los siguientes: Se han investigado las propiedades ópticas no lineales de dos nuevos materi mesógenos diseñados específicamente para este tipo de aplicación, uno tipo banana y otro con forma de H. Las eficiencias encontradas para el proceso de generación de segundo armónico son las más altas entre las publicadas hasta ahora para esta clase de materiales. Se ha estudiado en profundidad la transición de fase B1(columnar)-82(lamelar) mediante difracción de rayos X en muestras alineadas. A partir de los resultados hemos presentado un mecanismo que explica la sorprendentemente baja entalpía de transición y hemos propuesto una modificación para la estructura de la fase columnar. Por otro lado hemos deducido una nueva estructura antiferroeléctica local para la llamada fase B4 mediante el análisis del diagrama completo de rayos X. Asimismo, un material con una fase esméctica ópticamente isótropa es estudiado y se establece el tipo de razones que originan la formación de dicha estructura. Finalmente, y a partir de las propiedades observadas en algunos de los materiales estudiados, se proponen dos nuevas funcionalidades para estos mesógenos: fotoconductividad en compuestos con grupos TTF y un nuevo modo de operación LCD utllizando fases esmécticas y ópticamente isótropas.

7 p., 9 figuras, We propose a generalized gradient approximation (GGA) for the angle- and system-averaged exchange-correlation hole of a many-electron system. This hole, which satisfies known exact constraints, recovers the Perdew-Burke-Ernzerhof for solids (PBEsol) exchange-correlation energy functional, a GGA that accurately describes the equilibrium properties of densely packed solids and their surfaces. We find that our PBEsol exchange-correlation hole describes the wave-vector analysis of the jellium exchange-correlation surface energy in agreement with a sophisticated time-dependent density-functional calculation (whose three-dimensional wave-vector analysis we report here).

26 p., 7 figuras, The position-dependent exact-exchange energy per particle epsilon(x)(z) (defined as the interaction between a given electron at z and its exact-exchange hole) at metal surfaces is investigated, by using either jellium slabs or the semi-infinite (SI) jellium model. For jellium slabs, we prove analytically and numerically that in the vacuum region far away from the surface epsilon(Slab)(x)(z ->infinity)->-e(2)/2z, independent of the bulk electron density, which is exactly half the corresponding exact-exchange potential V-x(z ->infinity)->-e(2)/z [Horowitz , Phys. Rev. Lett. 97, 026802 (2006)] of density-functional theory, as occurs in the case of finite systems. The fitting of epsilon(Slab)(x)(z) to a physically motivated imagelike expression is feasible but the resulting location of the image plane shows strong finite-size oscillations every time a slab discrete energy level becomes occupied. For a semi-infinite jellium, the asymptotic behavior of epsilon(SI)(x)(z) is somehow different. As in the case of jellium slabs epsilon(SI)(x)(z ->infinity) has an imagelike behavior of the form proportional to-e(2)/z but now with a density-dependent coefficient that, in general, differs from the slab-universal coefficient 1/2. Our numerical estimates for this coefficient agree with two previous analytical estimates for the same. For an arbitrary finite thickness of a jellium slab, we find that the asymptotic limits of epsilon(Slab)(x)(z) and epsilon(SI)(x)(z) only coincide in the low-density limit (r(s)->infinity), where the density-dependent coefficient of the semi-infinite jellium approaches the slab-universal coefficient 1/2.

7 p., 3 figuras, We present it ab initio calculations of the electronic energy loss of charged particles moving outside a magnesium surface, from a realistic description of the one-electron band structure and a full treatment of the dynamical electronic response of valence electrons. Our results indicate that the finite width of the plasmon resonance, which is mainly due to the presence of band-structure effects, strongly modifies the asymptotic behaviour of the energy loss at large distances from the surface. This effect is relevant for the understanding of the interaction between charged particles and the internal surface of microcapillaries.

25 p., 9 figuras, Exact-exchange self-consistent calculations of the Kohn-Sham potential, surface energy, and work function of jellium slabs are reported in the framework of the optimized effective potential (OEP) scheme of density functional theory. In the vacuum side of the jellium surface and at a distance z that is larger than the slab thickness, the exchange-only Kohn-Sham potential is found to be imagelike (similar to-e(2)/z) but with a coefficient that differs from that of the classical image potential V-im(z)=-e(2)/4z. The three OEP contributions to the surface energy (kinetic, electrostatic, and exchange) are found to oscillate as a function of the slab thickness, as occurs in the case of the corresponding calculations based on the use of single-particle orbitals and energies obtained in the local-density approximation (LDA). The OEP work function presents large quantum size effects that are absent in the LDA and which reflect the intrinsic derivative discontinuity of the exact Kohn-Sham potential.

191 p., Recientemente, se ha comenzado a utilizar el formalismo superespacial para unificar las descripciones de los miembros de familias de estructuras de composición flexible, en un único modelo estructural. Estas estructuras son familias de compuestos cuya estequiometría depende de una variable que pude tomar valores continuos o cuasicontinuos. En la mayoría de los casos analizados hasta el momento, las estructuras de las fases pertenecientes a cada familia pueden ser descritas a partir del apilamiento de planos atómicos y los distintos miembros de la familia están formados de los mismos tipos de planos atómicos ,pero en distinta proporción y en distinto orden de apilamiento. En general, estos apilamientos dan lugar a estructuras de largo periodo. Desde el punto de vista experimental, ello implica la necesidad de una gran resolución en los experimentos de difracción para poder determinar las estructuras, puesto que las reflexiones están muy próximas entre sí. En el transcurso de esta Tesis se han estudiado las familias de compuestos FE1-xS, Sb2nTe3 y Zn1-xPdx para los que se han creado un modelo superespacial que unifica la descripción estructural de todos los miembros de cada familia. Por otro lado, se ha desarrollado una interfaz fráfica para la realización de medidas en un nuevo difractómetor de Rayos-x de alta precisión, diseñado en el dtpo.de Física de la Materia Condensada y construido por TEKNIKER, este difractómetro facilitará el estudio de estructuras de largo periodo.

The role that occupied d bands play in the inelastic lifetime of bulk and surface states in Ag is investigated from the knowledge of the quasiparticle self-energy. In the case of bulk electrons, sp bands are taken to be free-electron-like. For surface states, the surface band structure of sp states is described with the use of a realistic one-dimensional Hamiltonian. The presence of occupied d states is considered in both cases by introducing a polarizable background. We obtain inelastic lifetimes of bulk electrons that are in good agreement with first-principles band-structure calculations. Our surface-state lifetime calculations indicate that the agreement with measured lifetimes of both crystal-induced and image-potential-induced surface states on Ag(100) and Ag(111) is considerably improved when the screening of d electrons is taken into account.