Materials superconductors : claus del repte energètic des de l'exotisme quàntic

Conferència realitzada el Dissabte, 24 de Febrer de 2018En aquesta xerrada ens introduirem en el món de la superconductivitat, començant pels principis de la física quàntica que la fan possible i continuaré per la ciència de materials que ha permès al llarg dels darrers 50 anys fer-ne un ús rellevant per a la nostra societat. Vull després centrar-me en els materials superconductors d'alta temperatura que van ser descoberts ara fa 30 anys. Esmentaré les diferències que presenten respecte els principis quàntics que coneixíem de la superconductivitat fins aleshores, i exposaré els magnífics descobriments que hem dut a terme dins la ciència de materials i la nanotecnologia permeten finalment la fabricació de cintes superconductores d'alta temperatura. Finalment mencionaré les noves aplicacions, en l'àmbit de l'energia, dels materials superconductors d'alta temperatura i com aquestes poden ajudar a resoldre el repte energètic al que ens enfrontem aquest segle XXI: Xarxes elèctriques distribuïdes amb interruptors superconductors, cables superconductors que permeten evitar l'alta tensió y la incorporació de l'energia eòlica amb generadors de 10 MW en son alguns dels exemples que comentarem

Disentangling Epitaxial Growth Mechanisms of Solution Derived Functional Oxide Thin Films

Altres ajuts: CSIC E-08-2012-1321248 i E-08-2013-1028356This study investigates the mechanisms of epitaxial development and functional properties of oxide thin films (CeZrO , LaNiO, and BaSrTiO) grown on single crystal substrates (YO:ZrO, LaAlO, and SrTiO) by the chemical solution deposition approach. Rapid thermal annealing furnaces are very powerful tools in this study providing valuable information of the early stages of nucleation, the kinetics of epitaxial film growth, and the coarsening of nanocrystalline phases. Advanced transmission electron microscopies, X-ray diffraction, and atomic force microscopy are employed to investigate the film microstructure and morphology, microstrain relaxation, and epitaxial crystallization. This study demonstrates that the isothermal evolution toward epitaxial film growth follows a self-limited process driven by atomic diffusion, and surface and interface energy minimization. All investigated oxides experience a transformation from the polycrystalline to the epitaxial phase. This study unequivocally evidences that the film thickness highly influences the epitaxial crystallization rate due to the competition between heterogeneous and homogeneous nucleation barriers and the fast coarsening of polycrystalline grains as compared to epitaxial growth. The investigated films possess good functional properties, and this study successfully confirms an improvement at long annealing times that can be correlated with grain boundary healing processes. Thick epitaxial films can be crystallized by growing sequential individual epitaxial layers

Inkjet-printed chemical solution Y₂O₃ layers for planarization of technical substrates

The implementation of the Chemical Solution Deposition (CSD) methodology with 23 the Drop on Demand (DoD) inkjet printing (IJP) technology has been successfully employed to 24 develop a Solution Deposition Planarization (SDP) method. We have used nanocrystalline yttrium 25 oxide (Y₂O₃) to decrease the roughness of technical metallic substrates by filling the surface 26 imperfections and thus avoiding costly polishing steps. This alternative process represents an 27 outstanding methodology to reduce the final cost of the 2G coated conductors manufacturing. 28 Two Y₂O₃ metalorganic precursor inks formulations were successfully developed and tested to 29 obtain surfaces as smooth as possible with adequate mechanical properties to hold the internal 30 stresses developed during the subsequent layers growth. 31 By using these inks as precursors for IJP and after a proper tuning of the rheological and 32 wetting parameters, we firstly obtained short length uniform 100nm-thick Y₂O₃-SDP films on 33 unpolished stainless steel substrate from Bruker HTS. The scalability of the reel to reel (R2R)-IJP 34 process to 100m is then demonstrated on metallic substrates as well. A complete characterization of 35 the prepared short and long length SDP-Y₂O₃ inkjet-printed layers was carried out by optical 36 microscopy, FIB-SEM (Focus Ion Beam coupled to Scanning Electron Microscopy), XRD (X-Ray 37 Diffraction), AFM (Atomic Force Microscopy), reflectometry and nanoindentation techniques. Then, 38 the morphology, thickness, crystallinity and mechanical properties were evaluated, together with 39 the surface roughness in order to assess the resulting layer planarity. The impact of planarity was 40 additionally studied via growth of biaxially textured buffer layers as well as further functional 41 layers. 1.1µm-thick YSZ layers with in-plane textures better than the SS polished reference were 42 successfully deposited on top of the 100 nm SDP-Y2O3 films yielding 50% of Ic in contrast to the 43 standard reference

Ultrafast transient liquid assisted growth of high current density superconducting films

The achievement of high growth rates in YBaCuO epitaxial high-temperature superconducting films has become strategic to enable high-throughput manufacturing of long length coated conductors for energy and large magnet applications. We report on a transient liquid assisted growth process capable of achieving ultrafast growth rates (100 nm s −1) and high critical current densities (5 MA cm −2 at 77 K). This is based on the kinetic preference of Ba-Cu-O to form transient liquids prior to crystalline thermodynamic equilibrium phases, and as such is a non-equilibrium approach. The transient liquid-assisted growth process is combined with chemical solution deposition, proposing a scalable method for superconducting tapes manufacturing. Additionally, using colloidal solutions, the growth process is extended towards fabrication of nanocomposite films for enhanced superconducting properties at high magnetic fields. Fast acquisition in situ synchrotron X-ray diffraction and high resolution scanning transmission electron microscopy (STEM) become crucial measurements in disentangling key aspects of the growth process. High throughput manufacturing of long length coated conductors requires fast epitaxial growth of high-temperature superconducting films. Here, Soler et al. report an ultrafast growth rates and high critical current densities of YBaCuO films using a transient liquid-assisted growth method

Geometrically controlled ratchet effect with collective vortex motion

Rectified flux motion arising from the collective effect of many interacting vortices is obtained in a specially designed superconducting device. Ratchet structures with different asymmetric pinning potentials are generated by tuning the size, depth, and distribution of triangular blind-antidots in a high-temperature superconducting film. We experimentally and theoretically demonstrate that the amplitude and sign of the rectified vortex motion can be finely tuned with the pattern geometry. Two different dynamical regimes depending on the nature of vortices initiating the dissipation are identified, which can control the rectified vortex motion

Orientation symmetry breaking in self-assembled Ce1- : XGdxO2- y nanowires derived from chemical solutions

Understanding the growth mechanisms of nanostructures obtained from chemical solutions, a high-throughput production methodology, is essential to correlate precisely the growth conditions with the nanostructures' morphology, dimensions and orientation. It is shown that self-organized (011)-oriented Ce₀.₉Gd₀.₁O₂ᵧ (CGO) nanowires having a single in-plane orientation are achieved when an anisotropic (011)-LaAlO₃ (LAO) substrate is chosen. STEM and AFM images of the epitaxial nanowires reveal the (001)CGO[0-11](011)LAO[100] growth orientation, with the enlargement occurring along the [0-11]CGO direction with (111) lateral facets. The chosen substrate allowed us to study a unique case where the resulting biaxial strain is isotropic, while the dissimilar lateral surface energies are the key factor to obtain an energetically imbalanced and non-degenerated nanowire configuration. Rapid Thermal Annealing (RTA) has allowed sorting of experimental nucleation from coarsening and analysis of the kinetic phenomena of the nanowires. A thermodynamic driving force is shown to exist for a continuous elongation of the nanowires while the coarsening rates are found to be strongly temperature dependent and so kinetic effects are the key factors to control the size and density of the self-organized nanowire system. A remarkably fast nanowire growth rate (14-40 nm min⁻¹) is observed, which we associate with a high atomic mobility probably linked to a high concentration of oxygen vacancies, as detected by XPS. These nanowires are envisaged as model systems pushing forward the study of low energetic and highly oxygen deficient {111} lateral facets useful for catalysis, gas sensors and ionic conductivity applications

Encoding magnetic states in monopole-like configurations using superconducting dots

A large manifold of nontrivial spin textures, including the stabilization of monopole-like fields, are generated by using a completely new and versatile approach based on the combination of superconductivity and magnetism. Robust, stable, and easily controllable complex spin structures are encoded, modified, and annihilated in a continuous magnetic thin film by defining a variety of magnetic states in superconducting dots

Disentangling Epitaxial Growth Mechanisms of Solution Derived Functional Oxide Thin Films

Altres ajuts: CSIC E-08-2012-1321248 i E-08-2013-1028356This study investigates the mechanisms of epitaxial development and functional properties of oxide thin films (CeZrO , LaNiO, and BaSrTiO) grown on single crystal substrates (YO:ZrO, LaAlO, and SrTiO) by the chemical solution deposition approach. Rapid thermal annealing furnaces are very powerful tools in this study providing valuable information of the early stages of nucleation, the kinetics of epitaxial film growth, and the coarsening of nanocrystalline phases. Advanced transmission electron microscopies, X-ray diffraction, and atomic force microscopy are employed to investigate the film microstructure and morphology, microstrain relaxation, and epitaxial crystallization. This study demonstrates that the isothermal evolution toward epitaxial film growth follows a self-limited process driven by atomic diffusion, and surface and interface energy minimization. All investigated oxides experience a transformation from the polycrystalline to the epitaxial phase. This study unequivocally evidences that the film thickness highly influences the epitaxial crystallization rate due to the competition between heterogeneous and homogeneous nucleation barriers and the fast coarsening of polycrystalline grains as compared to epitaxial growth. The investigated films possess good functional properties, and this study successfully confirms an improvement at long annealing times that can be correlated with grain boundary healing processes. Thick epitaxial films can be crystallized by growing sequential individual epitaxial layers

Estudio de soluciones precursoras para el crecimiento de capas superconductoras gruesas por el método de deposición de soluciones químicas

Esta tesis se centra en la síntesis de soluciones de trifluoroacetatos de itrio, bario y cobre para el crecimiento de capas epitaxiales de YBa2Cu3O7 mediante el método de descomposición metalorgánica (MOD) de deposición de soluciones químicas (CSD). Este método es barato, escalable y ha demostrado altas prestaciones en capas superconductoras delgadas (≈300nm). La fabricación de cintas epitaxiales de óxidos superconductores es un campo de gran interés debido a la gran variedad de aplicaciones de los mismos, que va desde los imanes para generar campos magnéticos altos y realizar diagnóstico médico por imagen hasta sus numerosas aplicaciones en el sector Energético (por ejemplo, almacenamiento de energía magnética por superconducción (SMES), fabricación de cables, transformadores, limitadores de corriente superconductor (FCL)…). Entre los métodos de preparación de dichas cintas se distinguen los métodos de deposición de soluciones químicas (CSD) con la variante más utilizada de descomposición metalorgánica (MOD) o por deposición química metalorgánica de vapor (MOCVD) siendo ambas una alternativa a los métodos físicos (la deposición por láser (PLD) o la evaporación por haz de electrones como más utilizados). Las metodologías por CSD son sin ninguna duda las de menor coste entre todas las alternativas. El reto en este campo es la necesidad de producir cintas epitaxiales de óxidos superconductores de forma reproducible y que transporten la máxima corriente posible. En este contexto, el primero objetivo de la tesis ha sido conseguir por métodos de deposición de soluciones químicas (CSD), soluciones estables de precursores organometálicos para el crecimiento de capas epitaxiales de muy altas prestaciones. La deposición de la solución precursora, en nuestro caso, ha sido realizada, en un monocristal de aluminato de lantano (LAO) de orientación (001). Mediante la descomposición de precursores metalorgánicos (MOD), basados en trifluoroacetatos (TFA) como precursores hemos conseguido el crecimiento de capa epitaxiales de YBCO. Usando una concentración de sales de partida de 1.5M (Y:Ba:Cu) (0.25:0.50:0.75) depositados por spin coating hemos obtenido capas epitaxiales de YBCO de 300nm±25nm sin fases secundarias, bien oxigenadas (Tc=92K) y Jc a 77K de 4MA/cm2 que equivale a una Ic=120A/(cm-w). El siguiente reto planteado se basa en conseguir que las capas superconductoras crecidas transporten la mayor corriente (Ic) posible. Esto se puede llevar a cabo por dos vías. Una de ellas es incrementando la densidad de corriente crítica (Jc) a través de la preparación de nanocompuestos con el objetivo de generar defectos los cuales sean capaces de anclar los vórtices y con ello mejorar las prestaciones superconductoras . Otro camino, es aumentando el grosor de las capas de YBCO y con ello la cantidad de material por el que podrá pasar más corriente. Nosotros abordamos la última vía, que se explora en 3 capítulos de la tesis. Múltiples estrategias se han evaluado para incrementar el grosor de las capas de YBCO, como la variación de la concentración de la solución precursora anhidra inicial, variación de los parámetros del spin coating, la realización de múltiples pasos de deposición, el uso del dip coating o de distintos aditivos. En estas estrategias el uso de múltiples pasos de deposición, permitió obtener capas de 1000nm con Jc a 77K de 2 MA/cm2 y Ic=216 A/(cm-w). Valores que son competitivos con los valores internacionales actuales reportados en la literatura. La combinación de las diferentes estrategias con las que se ha trabajado se ha presentado como la vía viable para llegar a grosores incluso mayores de 1000nm para obtener capas de YBCO capaces de transportar de 400-1000 A/ (cm-w) a 77K con CSD.This thesis focuses on the synthesis of the solutions of trifluoroacetates of yttrium, barium and copper for the growth of epitaxial layers of YBa2Cu3O7 by metal organic decomposition (MOD) by chemical solution deposition (CSD). This is a low cost and easily scalable method achieving high performance superconducting films. The production of these superconducting tapes has a great interest due to the variety of possible applications, ranging from magnets to generate high magnetic fields and perform medical imaging diagnosis to its numerous applications in the energy sector (eg magnetic energy storage by superconducting (SMES), wind generators or manufacturing cables, transformers and superconducting current limiters). The deposition of the precursor solutions can be performed by chemical solution deposition (CSD), by the metal organic decomposition method (MOD) or by metal organic chemical vapor deposition (MOCVD). CSD has the advantage of low capital investment and supply costs as compared to metal organic chemical vapor deposition (MOCVD) or physical methods as pulsed laser deposition (PLD). The challenge in this field is the need to produce epitaxial layers of oxide superconductors reproducibly and carrying the possible maximum current. In this context, the first aim of the thesis is to achieve by chemical methods, stable solutions of organometallic precursors for the growth of epitaxial layers with high performance. The deposition of the precursor solutions, in our case, has been done on a single crystal of lanthanum aluminate (LAO) orientation (001). By thermal decomposition of metalorganic precursors (MOD), based on trifluoroacetates (TFA) as starting precursors we have achieved epitaxial growth of the YBCO layer. Using a starting concentration of 1.5M of the precursors (Y: Ba: Cu) (0.25: 0.50: 0.75) deposited by spin coating, we obtained epitaxial YBCO layers of 300nm ± 25nm without secondary phases, well oxygenated (Tc = 92K) and Jc at 77K of 4MA/cm2, equivalent to Ic=120A/(cm-w). The next goal was to achieve the layers grown epitaxially exhibiting the maximum current intensity (greater than (60 A/(cm-w)) at the start of the thesis). This can be obtained in different ways. It is possible to increase the critical current density (Jc) through the introduction of nanocomposites with the aim of generating defects able to anchor the superconducting vortices and improve the performance. Other option is to increase the thickness of the YBCO layers and thus the amount of material through which current can pass. We addressed the latter route, which is explored in 3 chapters of the thesis. Multiple strategies have been evaluated to increase the thickness of the layers of YBCO, such as the variation in the initial concentration of anhydrous precursor solutions, the parameters of the spin coater, performing multiple deposition steps, using a dip coating method or the addition of other additives. The strategy of using multiple deposition steps allowed us to obtain layers with Jc at 77K 1000nm2 MA/cm2 and Ic = 216 A /(cm-w). Values that are competitive with the current international values reported in the literature. The combination of the different strategies we worked on could allow us in the future, reach YBCO layers with thicknesses (≥1000nm) and exhibit higher current density (between 400-1000A/(cm-w) at 77K) by CSD

Hybrid approach to obtain high-quality BaMO perovskite nanocrystals

A novel hybrid solvothermal approach for perovskite nanocrystal formation via accurate control of the hydrolytic process is reported. This new synthetic methodology sets a whole general route to successfully tune the sizes of high-quality BaMO (M = Ti 4+, Zr 4+, and Hf 4+) perovskite nanocrystals. Purely cubic-phase nanocrystals (stable in alcohol media) were obtained using controlled water amounts, combining the well-known aqueous sol-gel process with the classic solvothermal method. Exhaustive optimizations revealed feasibility of a fast (1 hour) and reproducible synthesis with small variations in the crystal size or agglomeration parameters. The study also reveals water content as the pivotal factor to achieve this wide range of sizes through a controlled hydrolytic step. Finally, the study of the hydrolytic process made it possible to shed some light on mechanistic insights of this synthetic route. A novel hybrid solvothermal approach for perovskite nanocrystal formation via accurate control of the hydrolytic process is reported