Dissertations / Theses on the topic 'Conductors; Superconductors'

The excitement which followed the discovery of high-temperature superconductors in 1986 was short-lived, as it became clear that their current carrying capacity (the critical current density Jc) was limited by grain boundaries (GBs). In order to reduce their detrimental effects coated conductors have been developed, in which a superconducting thin film is deposited on a polycrystalline, textured substrate. Within certain temperature and magnetic field ranges, however, GBs still limit the overall Jc. This fact motivated the present thesis, for which the electrical properties of different types of coated conductors, and in particular their GBs, were investigated. Several GBs and a single grain were isolated in a tape produced by metal-organic deposition (MOD), using a novel approach based on electron backscatter diffraction and a focused ion beam microscope. Measurements of their critical current densities for fields swept in the film plane showed the expected decrease with increasing misorientation angle at low fields. At higher fields an angle dependent crossover was found, from a GB to grain limited Jc. In order to confirm this result and put it into broader perspective, the dependence of Jc on the width of polycrystalline tracks was studied, and then explained in terms of the behaviour of the single GBs. Investigations using low-temperature scanning laser microscopy rounded out the picture, which also showed GB dissipation at certain angles and grain limitation at others. In measurements on samples produced by metal-organic chemical vapour deposition (MOCVD) characteristic differences compared to the MOD film were found. While both conductors exhibited high values of Jc, the variation with in-plane angle was significantly stronger for the MOCVD conductor, which can be explained by its sharper texture. In a track patterned perpendicular to the tape direction the phenomenon of vicinal channelling was observed, which previously was known only from films on single crystal substrates. Finally, an isolated boundary showed very high values of Jc, consistent with its low misorientation. In order to better understand how the substrate influences the superconducting properties, measurements were carried out on otherwise identical samples grown on different substrates. A tape with grains elongated along its rolling direction showed particularly good properties at all examined field orientations. This extends the previously reported result that high aspect ratios are beneficial at fields applied perpendicular to the tape plane.

Uno de los temas de mayor interés en el ámbito de la superconductividad es la fabricación de cintas superconductoras (CCs) de YBa2Cu3O6+δ (YBCO) debido a las excelentes propiedades superconductoras que poseen y a las prometedoras perspectivas en cuanto a aplicaciones se refiere. Sin embargo, para poder generalizar el uso de dichas CCs, se requiere de un proceso de fabricación de bajo coste. En este contexto, la técnica de deposición por solución química se presenta como una alternativa muy prometedora. Las CCs pueden cumplir los requisitos actuales exigidos en diferentes aplicaciones, pero hay otras que están fuera de sus capacidades, especialmente aquellas relacionadas con aplicaciones de potencia en las que están presentes campos magnéticos de gran intensidad. El movimiento de los vórtices que tienes lugar en presencia de tales campos magnéticos hace que el YBCO sea poco efectivo en estos casos. El objetivo de este trabajo es mejorar las propiedades del YBCO de manera que se puedan satisfacer los requerimientos de estas aplicaciones de potencia. Para ello, hemos estudiado, principalmente, dos estrategias: la nanoestructuración de la matriz del YBCO añadiendo nanopartículas (NPs) obteniendo nanocomposites superconductores y la optimización del proceso de oxigenación del YBCO para conseguir aumentar todo lo posible la temperatura crítica y la densidad de corriente crítica. La preparación de los nanocomposites de YBCO se llevó a cabo siguiendo dos métodos diferentes de “deposición y crecimiento secuencial”: el método “in-situ” en el que las NPs se forman de manera espontánea durante el proceso de crecimiento, y el método “ex-situ”, que es un novedoso procedimiento desarrollado durante esta tesis en el cual las NPs se sintetizan primeramente formando una solución coloidal para luego quedar atrapadas en la matriz del YBCO durante los procesos térmicos. El uso del método “in-situ” se ha enfocado al estudio de como diferentes NPs (BaZrO3, Y2O3, Ba2YTaO6 and mezclas de éstas) afectan la microestructura del YBCO creando defectos cristalinos que incrementan la fuerza de anclaje. Hemos estudiado también la influencia de estos defectos, en particular, de las dobles cadenas Cu-O, en las propiedades finales de los nanocomposites de YBCO y GdBCO. En el caso del método “ex-situ”, el primer paso fue sintetizar diferentes soluciones coloidales de NPs magnéticas (MnFe2O4 and CoFe2O4) y no magnéticas (CeO2 and ZrO2). Se comprobó la estabilidad de las soluciones YBCO+NPs mediante medidas de DLS y de TEM para asegurar que las NPs conservaban su tamaño inicial sin formar aglomerados. El proceso de pirólisis fue optimizado para cada tipo de NPs teniendo en cuenta que los ligandos usados para estabilizar las NPs pueden influenciar de manera drástica la homogeneidad de las capas pirolizadas. Por último, el proceso de crecimiento fue también investigado para cada tipo de NPs. El estudio del proceso de oxigenación en la capas de YBCO se llevó a cabo usando medidas de resistencia “in-situ”, que permiten monitorizar la evolución de la resistencia en las capas durante los distintos procesos térmicos. Hemos estudiado como la temperatura, el flujo de gas y la presión parcial de oxigeno afectaba al proceso de difusión. De acuerdo con nuestros resultados, las reacciones que tienen lugar en la superficie de la capa antes de que en oxígeno se difunda en el interior de la misma, son el factor que limita la cinética del proceso de oxigenación. Por último El efecto de la adición de plata como catalizador y la difusión en nanocomposites también se ha estudiado.
One of the hot topics in the field of superconductivity is the YBa2Cu3O6+δ (YBCO) Coated Conductors (CCs) fabrication due to the excellent superconducting properties and promising application prospects. However, in order to spread worldwide the use of YBCO coated conductors, a low cost fabrication is required. Chemical Solution Deposition has emerged as a promising alternative that can accomplish this requirement. Despite that YBCO CCs can satisfy the requirements in many different applications, the fact is that there are other uses that are out of it reach with its current status, especially those power applications in which high magnetic fields are applied. The vortex movement which takes place at such high magnetic fields makes YBCO CCs useless for these particular applications. The aim of this work is to improve the properties of YBCO satisfying the demands of these power applications. For this, we studied, mainly, two different strategies: nanostructuration of the original YBCO matrix by adding NPs (superconducting nanocomposites) and optimization of the YBCO oxygenation process to enhance as much as possible the critical temperature and critical current density. The preparation of YBCO nanocomposites was done following two different “Sequential deposition and growth” approaches: the in-situ approach in which the NPs are spontaneously segregated during the growth process; and the ex-situ approach, a new methodology developed in this thesis in which the NPs are firstly synthesized in a colloidal solution and then embedded in the YBCO matrix. Using the in-situ approach we have made an extensive study of how different NPs (BaZrO3, Y2O3, Ba2YTaO6 and mixtures of these) affect the microstructure of the YBCO creating defects that increase the pinning properties. We have also studied the influence of these defects, in particular, the stacking faults (double chains of Cu-O), on the final properties of the YBCO and GdBCO nanocomposites. With the ex-situ approach we have started by synthesizing different colloidal solutions of both magnetic (CoFe2O4) and non-magnetic (CeO2 and ZrO2) NPs. The stability of YBCO+NPs solutions was checked using TEM and DLS analyses to ensure that the NPs are maintaining the initial size without forming agglomerates. The pyrolysis process was optimized for each type of NPs. We realized that the stabilization agents can critically influence the homogeneity of the pyrolyzed films. Finally, the growth process was also optimized for each type of NPs trying to solve different difficulties that appeared: coarsening, pushing or reactivity. The study of the oxygen diffusion process in YBCO thin films was done using in-situ resistivity measurements that allow to monitor the evolution of the resistance in the thin films in different annealing conditions. We have studied how the temperature, the gas flow and the oxygen partial pressure affect the diffusion process. According to our results, we can conclude that the surface reactions that take place before the oxygen bulk diffusion is the limiting factor for the oxygen diffusion. The effect of the silver addition to the YBCO as oxygen catalyst was also tested. Finally, the first study about the oxygen diffusion process in nanocomposite films gave an idea of how the oxygen diffusion works in this kind of materials.

The work in this thesis concentrates on the fabrication and characterization of MgB2 superconducting bulk wire and tape. An overview of the research on MgB2 superconductor during the last three years is also provided. High transport and magnetic critical current density values above 105 A/cm2 have been obtained for metal-clad wires and tapes. Fe-clad MgB2 tapes were fabricated using a powder-in-tube technique. The tape shows a sharp transition with a transition width ΔTc of 0.2 K and a Tc0 of 37.5 K. An high transport critical current value of 1.7 × 104 A/cm2 for both 29.5 K in 1 Tesla and 33 K in zero applied field has been achieved. The effects of sintering time and temperature on the formation and critical current densities of Feclad MgB2 wires is also investigated. MgB2 wires were sintered for different periods of time at predetermined temperatures. In contrast to the common practice of sintering for several hours, results show that there is no need for prolonged heat treatment in the fabrication of Fe/MgB2 wires. A total time in the furnace of several minutes is enough to form nearly pure MgB2. Jc of 4.5×105 A/cm2 in zero field and above 105 A/cm2 in 2 T at 15 K has been achieved for Fe/MgB2 wires sintered for a short time. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB2 wires. Ag and Cu clad MgB2 wires were also fabricated using an in-situ reaction method. The effects of a shorter than usual sintering on the critical current densities of Ag and Cu clad MgB2 wires were studied. For Ag clad wire Jc is improved by more than two times after the short period sintering process. Jc values of 1.2×105 A/cm2 in zero field and above 104 A/cm2 in 2 T at 20 K have been achieved for Ag clad MgB2 wire which is only sintered for a few minutes at 800 oC. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu2Mg and Ag3Mg for copper and silver clad wires, respectively. The results show that the short sintering causes less reaction between the magnesium and the sheath materials and markedly improves the critical current density. Our results also show that iron is still the best sheath material for MgB2 superconductor wire and tape. Sixteen-filament stainless steel/Fe/MgB2 wires were fabricated by the powder-in-tube method followed by groove rolling. Magnetic critical current densities of 3.4×105 A/cm2 in 0.5 T and about 1.9×105 A/cm2 in 1 T at 5 K were achieved. Results on transport Jc of solenoid coils up to 100 turns fabricated with Cu-sheathed MgB2 wires using a windreaction in-situ technique are reported. Despite the low density of the single core and some reaction between the Mg and the Cu-sheath, our results demonstrate that the decrease in transport Jc with increasing length of MgB2 wires is insignificant. Solenoid coils with diameters as small as 10 mm can be readily fabricated using a wind-reaction in-situ technique. The Jc of coils is essentially the same as for straight wires. Jc values of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field have been achieved for small coil wound using Cu-sheathed tape and Cu-sheathed wire respectively. The results indicate that the MgB2 wires have potential for large scale applications. The effect of chemical doping on the superconductivity and critical current density of MgB2 superconductor is investigated. Enhancements in the Jc field performance as well as the irreversibility field were obtained due to chemical doping with both C and SiC nano-particles. Doping MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10 wt% nano-SiC doped MgB2 sample, led to slight decrease in Tc and significantly enhanced Hc2, Hirr and Jc at high magnetic fields. Compared to the non-doped sample, Jc for the 10 wt% doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K, which is considered to be a benchmark operating temperature for MgB2, the best Jc for the doped sample was 2.4×105 A/cm2 at 2 T, which is comparable to Jc of the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was 36,000 A/cm2, which is an order of magnitude higher than for the Fe/MgB2 tape. Our results show that there are two distinguishable but closely related mechanisms: increase of Hc2 and improvement of flux pinning that control the performance of Jc(H) in the samples. SiC-doping introduced many nano-scale precipitates and disorders at B and Mg sites, provoking a high resistivity of ρ (40K) = 300 μΩ-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and Hirr with only minor effects on Tc. EELS and TEM analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 m and an extensive domain structure of 2-4nm domains in the doped sample which serve as strong pinning centers. The effect of nano-SiC doping on the critical current density and flux pinning of Fe/MgB2 wires is also investigated. The depression of Tc with increasing SiC doping level remained rather small. High level SiC doping resulted in a substantial enhancement in the Jc(H) performance. The transport Jc for all the wires is comparable to the magnetic Jc at higher fields despite the low density of the samples. The transport Ic for the 10 wt% doped Fe/MgB2 wire reached 675 A at 24 K and 1 T (Jc = 140,000 A/cm2) and 500 A at 20 K and 2T (Jc = 103,000A/cm2). The transport Jc for the 10wt% SiC doped MgB2 wire is 30 times higher than for the undoped wire. SiC doped MgB2 polycrystalline samples were fabricated using different grain sizes (20 nm, 100 nm, and 37 μm) of SiC and different doping levels (0, 8, 10, 12, 15 wt %) in order to investigate the effect of the particle size of the starting SiC powder on the properties of samples. Results show that grain sizes of the starting precursors of SiC have a strong effect on the critical current density and its field dependence. The smaller the SiC grains are, the better the Jc field performance is. Significant enhancement of Jc and the irreversibility field Hirr were revealed for all the SiC doped MgB2 with additions up to 15 wt%. A Jc as high as 20,000 A/cm2 in 8 T at 5 K was achieved for the sample doped with 10 wt% SiC with a grain size of 20 nm. Results indicate that the nano-inclusions and substitution inside MgB2 are responsible for the enhancement of flux pinning. Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well-controlled conditions to limit the extent of C substitution. It was found that both the a-axis lattice parameter and the Tc decreased monotonically with increasing doping level. However, for the sample doped with the highest nominal composition of x=0.4 the Tc dropped only 2.7 K. The nano-C doped samples showed an improved field dependence of the Jc compared with the undoped sample over a wide temperature range. The nhancement by C-doping is not as strong as for nano-SiC doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. A study of ac susceptibility, magnetic shielding and the sample size effect is presented in Chapter 6. Systematic ac susceptibility measurements were performed on MgB2 bulk samples. It is shown that the flux creep activation energy is a nonlinear function of the current density U , indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law (J ) ∝ J −0.2 U(B) ∝ B−1.33 , showing a steep decline in the activation energy with magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB2. Magnetic shielding was investigated by means of transport critical current measurements for Fe-sheathed MgB2 round wires. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6 T at 32 K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6 T. This was well beyond the maximum field for effective shielding of 0.2 T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between the field and the current showed that the transport current does not flow straight across the wire, but meanders between the grains. The effect of sample size on the critical current density and the flux pinning of pure and SiC doped MgB2 bulk samples has been investigated. At high fields a systematic degradation of magnetic Jc and Hirr was observed as the sample size decreased. However, Jc remarkably increased on decreasing the sample volume at low magnetic fields below 1 T. The SiC doped samples show less sample size effect than the pure samples, indicating a larger n-factor and therefore a stronger pinning effect due to SiC doping.

High temperature superconductors (HTS) and solid oxide fuel cells (SOFCs) both offer the possibility for dramatic improvements in efficiency in power applications such as generation, transmission and use of electrical energy. However, production costs and energy losses prohibit widespread adoption of these technologies. This thesis investigates low-cost methods to tailor the structures of HTS wires and SOFCs to reduce these energy losses. Section I focusses on methods to produce filamentary HTS coated conductors that show reduced AC losses. This includes spark-discharge striation to pattern existing HTS tapes and inkjet printing of different filamentary architectures. The printed structures are directly deposited filaments and ‘inverse’ printed tracks where an initially deposited barrier material separates superconducting regions. Furthermore, the concept and first stages of a more complex ‘Rutherford’ cable architecture are presented. Additionally, Section I investigates how waste material produced during the manufacture of an alternative low-AC loss cable design, the Roebel cable, can be used to make trapped field magnets that produce a uniform magnetic field profile over a large area. This trapped field magnet work is extended to study self-supporting soldered stacks of HTS tape that demonstrate unprecedented magnetic field uniformity. Section II looks at how nanostructuring porous SOFC electrodes via solution infiltration of precursors can improve long-term stability and low temperature performance. Inkjet printing is utilised as a scalable, low-cost technique to infiltrate lab-scale and commercial samples. Anode infiltration via inkjet printing is demonstrated and methods to increase nanoparticle loading beyond ~1 wt% are presented. Symmetric cells with infiltrated cathodes are shown to have improved performance and stability during high temperature aging. Additionally, the sequence of solution infiltration is found to be important for samples dual-infiltrated with two different nanoparticle precursors.

Detailed, accurate measurements of critical current density and resistivity to determine the upper critical field have been made on a technological NbsAl conductor in magnetic fields up to 15 T, temperatures from 4.2 K up to the critical temperature and in the strain range from -1.8% to 0.7%. The uncertainty in temperature above 4.2 K was equivalent to ± 100 mK with a stability during the measurements of < 5 mK up to a limiting current of 80 A and a typical noise level of 1 µ Vm(^-1).When B(_c2){T,ε) is defined at 5%pn, 50%pn or 95%/%pn, an empirical relation is found where and an approximate relation, holds. The Jε data were parameterised using F(_p) = J(_E)B = A(ε)[Bc(_2)](^n)b(^p)(1-b)(^9) where b = B/B(_c2)(T,ε). When B(_c2)(T,ε) is constrained to be the value at 50%pn or 95%pn, the scaling law for F(_p) breaks down such that p and q are strong functions of temperature and q is also a strong function of strain. However, when B(_c2)(T,ε) is defined at 5%pn, there is good scaling where p and q are constants independent of temperature and strain. F(_p) can also be approximated by a Kramer form where the Ginzburg-Landau constant is γ is the electronic density of states and is interpreted as the average B(_c2) for the bulk where percolative current flow occurs. The critical current density of Hot Isostatic Pressed (HIP'ed) and unHIP'ed Nb(_3)Sn Modified Jelly Roll wires has also been measured at 4.2 K. The critical current and upper critical field were decreased for the HIP'ed sample. The reduced upper critical field of the HIP'ed wire was found to be less sensitive to strain than the unHIP'ed wire. The exponent of B(_c2) in the flux pinning scaling law increased from 0.86 to 2.14 as a result of the HIP processing.

Ce travail a concerné la préparation des premières nanoparticules de supraconducteurs moléculaires dont la croissance en solution a pu être contrôlée par l'ajout de molécules amphiphiles neutres (amines, imines, acides carboxyliques). Notre étude s'est principalement focalisée sur les sels de Bechgaard (TMTSF)2X (TMTSF : tétraméthyltétrasélénafulvalène, X = ClO4 ou PF6) et sur le dérivé (BEDT-TTF)2I3 (BEDT-TTF : bis(éthylènedithio)tétrathiafulvalène). La taille, la morphologie et l'état de dispersion des nanoparticules ont été contrôlés par la nature et la concentration du structurant amphiphile. Ces nanoparticules ont été caractérisées par des méthodes spectroscopiques (IR, Raman, UV-visible, spectrométrie de photoélectrons), diffractométriques et microscopiques (microscopie électronique à transmission et AFM). Elles présentent une transition vers un état supraconducteur mise en évidence par des mesures de résistivité électrique ou de susceptibilité magnétique. Finalement, le pouvoir thermoélectrique des nano-objets de (BEDT-TTF)2I3 a été évalué. Des mesures préliminaires de facteur de mérite thermoélectrique (ZT) en font des candidats potentiels pour la réalisation de modules thermoélectriques organiques
In this work, we described the preparation of the first nanoparticles of molecule-based superconductors for which the growth has been controlled by the addition of neutral amphiphilic molecules (such as amines, imines, carboxylic acids). Our study focused on Bechgaard salts (TMTSF)2X (TMTSF: tetramethyltetraselenafulvalene, X = ClO4 or PF6) and (BEDT-TTF)2I3 (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). The size, the morphology and the state of dispersion of the nanoparticles have been controlled by the nature and the concentration of the amphiphilic structuring agent. The particles have been characterized by spectroscopic methods (IR, Raman, UV-visible, photoelectron spectroscopy), X-Ray diffraction and microscopy (TEM and AFM). Small particles underwent a transition to a superconducting state, as evidenced by electrical resistivity or magnetic susceptibility measurements. Finally, the thermoelectric power of nano-objects of (BEDT-TTF)2I3 has been evaluated. Preliminary measurements of the thermoelectric figure of merit (ZT) make them potential candidates for future organic-based thermoelectric generators

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung neuer Schichtsysteme für die Realisierung biaxial texturierter hochtemperatursupraleitender Bandleiter. Bisher sind eine Vielzahl von Bandleiterarchitekturen bekannt, die sowohl durch physikalische Depositionsmethoden als auch mittels Abscheidung aus der chemischen Lösung hergestellt werden können. Während die Funktion von YBCO-Bandleitern mit Hilfe physikalischer Depositionsmethoden in den letzten Jahren demonstriert werden konnte, zeigen auf chemischem Wege deponierte Bandleiter schlechtere Eigenschaften. Seitens der Industrie besteht ein starkes Interesse, die hohen Produktionskosten, die im Hinblick auf physikalische Depositionsmethoden mit einem hohen Anlagenaufwand verbunden sind, anhand der kostengünstigen chemischen Synthese von Einzelschichten oder der gesamten Bandleiterarchitektur zu senken. Gelöst wurde diese Aufgabe innerhalb der vorliegenden Arbeit durch die Entwicklung metallorganischer Vorstufenlösungen zur Deposition von CaTiO3-, SrTiO3-Pufferschichten und supraleitender YBa2Cu3O7-Schichten.

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung neuer Schichtsysteme für die Realisierung biaxial texturierter hochtemperatursupraleitender Bandleiter. Bisher sind eine Vielzahl von Bandleiterarchitekturen bekannt, die sowohl durch physikalische Depositionsmethoden als auch mittels Abscheidung aus der chemischen Lösung hergestellt werden können. Während die Funktion von YBCO-Bandleitern mit Hilfe physikalischer Depositionsmethoden in den letzten Jahren demonstriert werden konnte, zeigen auf chemischem Wege deponierte Bandleiter schlechtere Eigenschaften. Seitens der Industrie besteht ein starkes Interesse, die hohen Produktionskosten, die im Hinblick auf physikalische Depositionsmethoden mit einem hohen Anlagenaufwand verbunden sind, anhand der kostengünstigen chemischen Synthese von Einzelschichten oder der gesamten Bandleiterarchitektur zu senken. Gelöst wurde diese Aufgabe innerhalb der vorliegenden Arbeit durch die Entwicklung metallorganischer Vorstufenlösungen zur Deposition von CaTiO3-, SrTiO3-Pufferschichten und supraleitender YBa2Cu3O7-Schichten.

Ofrece una visión general de los cables superconductores, cómo está formado un cable superconductor además de sus diseños, los proyectos que se están dando en el mundo y los beneficios que tiene la implementación de estos cables en la red eléctrica.
Trabajo de suficiencia profesional

Presentamos los estudios no destuctivos realizados sobre cintas cuperconductoras y capas de YBCO crecidas por la técnica de MOD-TFA mediante la espectroscopía micro-Raman, complementados por otras técnicas tales como XRD, SEM, TEM, resistividad y densidad de corriente crítica. Determinamos cuantitativamente el grado de textura uniaxial y biaxial de las cintas superconductoras de YBCO e investigamos aspectos tales como la orientación del cristal, uniformidad, fases secundarias, impurezas, contenido de oxígeno y defectos de las capas crecidas mediante MOD-TFA, basados en las reglas de seleccción específicas para el YBCO y los experimentos de dispersión Raman polarizada. Además, hemos estudiado los principales parámetros de crecimiento y su relevancia en la microestructura final y en las propiedades superconductoras. Finalmente, estudiamos el rol de la fases intermedias en los mecanismos de nucleación y crecimiento del YBCO.
We present the non-destructive studies on coated conductors and YBCO TFA-MOD films by micro-Raman spectroscopy complemented by other techniques such as XRD, SEM, TEM, resistiviy and critical current density. We determined quantitatively the degree of uniaxial and biaxial texture of YBCO coated conductors and investigated aspects like crystal orientation, uniformity, secondary phases, impurities, oxygen content and defects of TFA-MOD grown films based on the specific Raman selection rules for YBCO and polarized Raman scattering experiments. Moreover, we have studied the main YBCO TFA growth parameters and their relevance in the final microstructure and superconducting properties. Finally, we studied the role of the intermediate phases on the mechanism of YBCO nucleation and growth.

Las cintas superconductoras de alta temperatura (HTS) han emergido como materiales prometedores para sus uso en el campo de l'energía puesto que permiten reducir a mitad el tamaño de los equipos de energía eléctrica respecto a los convencionales, reducir las pérdidas de energía, aumentar la eficacia en la generación, la transmisión y la distribución de la misma, y así la reducir el impacto ambiental.
Sin embargo, diversamente de los conductores típicos, los materiales superconductores basados en óxido son frágiles, se dañado fácilmente y son así difíciles de procesar. Hasta ahora ha sido posible producir longitudes de un kilómetro de cables HTS de primera generación, para el uso en el trasporte de corriente eléctrica. Las cintas superconductoras de YBCO, por ejemplo, pueden soportar altas densidades de corrientes críticas y por esto representan un candidato prometedor en el trasporte de electricidad.
Uno del los substratos disponible para suportar el superconductor es un acero policristalino con una película epitaxial de YSZ en cima, YSZ(IBAD)/Stainless. El segundo que hemos utilizado es el NiO(SOE)/Ni(Rabit), una cinta de nickel previamente texturada por laminación y sucesivamente oxidata de manera controlada. Numerosas técnicas están disponible para la deposición de YBCO epitaxiale, nosotros elegimos el proceso más barato y industrialmente interesante: la técnica sol-gel. Para evitar la interacción entre el YBCO superconductor y el substrato epitaxial, evitando así la reducción de la corriente que el superconductor puede trasportar, es importante interponer un material inerte que transfiera su epitaxia al YBCO; este clase de películas se llaman capas tampón.
El objetivo principal de esta tesis ha sido optimizar el crecimiento de las capas tampón por técnica química y finalmente estudiar la deposición del YBCO por TFA sobres esas muestras optimizadas. Las capas de cerámica que hemos estudiados han sido: CeO2, BaZrO3, CaZrO3, SrZrO3, SrTiO3, BaCeO3 , y depositadas por el método químico: metal 2-4--diketone disuelto en ácido acético, o los metales isopropoxidos disuelto en metanol. Por depositar las soluciones precursoras hemos utilizado la técnica de spin coating. Controlando los diversos parámetros, velocidad, aceleración y la concentración de la solución obtuvimos películas homogéneas con diverso grosor. La fase de la cristalización se alcanza en un horno donde se controla l'atmósfera, la temperatura y la velocidad de calefacción. Durante esta investigación hemos adquirido un conocimiento total del acrecimiento de las películas delgada de MOD-CeO2. De una combinación de las análisis de TEM, de XRD y de RHEED observamos que el mecanismo de crecimiento tiene un comportamiento anómalo comparado con el otro materiales crecidos con la misma técnica.
En este proceso de síntesis de la ceria, la nucleation homogénea de hecho esta favorita debido al bajo valor de Tnuc./Tmel ceria (Tnuc./Tmel=0.21). Solamente los granos nucleados sobre el substrato resultan texturados. La dependencia del tamaño de grano con temperatura sigue una relación de tipo Arrhenius, características de un crecimiento 3D del grano. Los análisis de EELS revelaron una fracción significativa de C residual que adorna los límites de grano, es probable que el crecimiento del límite de grano se quede bloqueando debido estas impurezas.
Un proceso del recocido en aire a posteriores, ha demostrado la posibilidad de crecer las películas de CeO2 totalmente epitaxiales. Los análisis de EELS de tales muestras tratadas en oxígeno demuestran claramente que los límites de grano quedan limpios de las impurezas de C, desbloqueando así el crecimiento del grano. Después de un proceso largo de optimización de los parámetros de síntesis, podemos ahora controlar exactamente el crecimiento epitaxiale de la ceria. Se ha verificado que el óxido del cerio se puede crecer en YSZ(IBAD)/SS con solamente la orientación (00l). Para preservar la cinta del metal contra la oxidación, el proceso optimizado se ha adaptado a la deposición sobre substrato de acero inoxidable reduciendo la temperatura de síntesis a 900ºC.
Hemos optimizado también la preparación de SrTiO3(STO) y BaZrO3(BZO) sobre MgO y YSZ mono-cristales y en seguida también sobre YSZ(IBAD)/SS y NiO(SOE)/Ni.
La arquitectura más prometedora resultó ser STO/BZO/NiO(SOE)/Ni. Por ultimo depositamos YBCO por método TFA (Trifluoracéticetatos) sobre las capas tampones optimizadas. Una muestra de TFA-YBCO sobre CeO2/YSZ(IBAD)/SS preparada en aire a 900ºC en 8 h ha dato como resultado una densidad corriente crítica, Jc de 7 MA/cm2 a 5K, y 6·105A/cm2 a 77K. Estos valores están cerca de objectivo de un millón A/cm2 a 77K. Los experimentos sobre las capas tampón de BZO y de STO han demostrado la posibilidad de usar este sistema doble sobre NiO(SOE)/Ni como plantilla alternativa para el crecimiento de YBCO. Alcanzado una densidad de corriente crítica de Jc(5K) = 5·105A/cm2 con la mejor muestra de YBCO/STO/BZO/NiO(SOE)/Ni.
High-temperature superconducting (HTS) tapes have emerged as promising materials for superconducting power applications since they make possible electric power equipment that is half the size of conventional alternatives, with half energy losses, increasing the efficiency in the generation, transmission and distribution of the electric energy, and thus reducing the impact of power delivery on the environment.
However, unlike typical conductors, oxide based superconductor materials are brittle and easily damaged and thus they are difficult to process and handle, specially forming large and flexible wires.
Up to now it has been possible to produce kilometre lengths of the first generation of HTS wires for use in electrical transmission cables. YBCO coated conductors can support high critical current densities and is a promising candidate.
One of the substrate available is a polycrystalline metal substrate with an epitaxial YSZ film on it, the ion-beam assisted deposition YSZ(IBAD)/Stainless Steel. The second is the textured NiO(SOE)/Ni(Rabit).Numerous methods are available for epitaxial deposition of YBCO, including vacuum techniques but we choose the cheaper non vacuum sol-gel processes.
To avoid the interaction between the superconductor it is important to interpose a inert material that can transfer the epitaxy from the substrate to the YBCO, these kind of films are called buffer layers, avoiding than the reduction of the current that the superconductor can support.
The main aim of this thesis was optimising the buffer layer growth by chemical technique and finally studying the deposition of TFA-YBCO on those optimised templates.
The ceramic buffer layers studied:
CeO2 , BaZrO3 , CaZrO3, SrZrO3, SrTiO3,BaCeO3
The Sol-Gel system used was the Metal -diketone dissolved in Acid Acetic, or Metal isopropoxide dissolved in methanol.
The deposition step was performed by spin coating. Controlling the different parameters of rate and acceleration of the spinner and the precursor solution concentration we obtained homogenous films with different thickness. The crystallisation step is achieved in a furnace in a controlled atmosphere, temperature and heating rate.
Important knowledge on the MOD-CeO2 thin film growth has been acquired during this research. From a combination of TEM, XRD and RHEED analyses it was observed that its growth mechanism exhibits an anomalous behaviour compared with other CSD derived films. The homogeneous nucleation in fact is favoured in this MOD process due to the low Tnuc./Tmel value for ceria film (Tnuc./Tmel=0.21). Only grains nucleated on the substrate are textured as observed in XTEM images. The grain size dependence with temperature follows an Arrhenius relation: 2=otexp(-Q/kT), characteristics of 3D undergoing thermally activated grain growth . EELS analyses revealed a significant fraction of residual C decorating the grain boundaries, that very likely acts as a growth by blocking grain boundary motion. A process of post annealing or direct growth in static air, have demonstrated the possibility of growing completely epitaxial CeO2 films. EELS analyses of such samples clearly demonstrates that the oxygen clean up grain boundaries from C impurities thus unblocking grain growth. After a long process of synthesis parameter optimisation, we are now able to control exactly the epitaxial growth of ceria growth. It has been verified that Cerium oxide can be grown on YSZ(IBAD)/SS with only the (00l) orientation. The optimised process has been adapted to stainless steel substrate reducing the synthesis temperature at 900ºC in order to preserve the metal tape against oxidation. We observed an interesting phenomenon of in plane texture improvement of the ceria film with respect to the underlying YSZ(IBAD)/SS substrate, from YSZ = 8.3º and to CeO2 = 7.5º.
The solution preparation and the deposition conditions for STO and BZO on MgO and YSZ have been also optimised. After several experiments of buffer deposition on YSZ(IBAD)/SS and NiO(SOE)/Ni technical metal substrates the most promising architecture resulted to be the STO/BZO/NiO(SOE)/Ni.
We have grown YBCO by the TFA(Trifluor Acetic Acid) method on the optimized buffer layers. A sample of TFA-YBCO on a CeO2/YSZ(IBAD)/SS template prepared in air at 900ºC for 8 h has shown a critical current density, Jc has a value of 7 MA/cm2 at 5K, and 6·105A/cm2 at 77K. These values are near the target of one million A/cm2 at 77K.
The experiments on BZO and STO buffer layers have demonstrated the possibility of using the double buffer on NiO(SOE)/Ni as an alternative template for YBCO deposition. A critical current density of Jc(5K)= 5·105A/cm2 has been achieved for the best sample of YBCO/STO/BZO/NiO(SOE)/Ni.

This thesis presents theoretical investigations of triplet superfluidity (triplet superconductivity) in quasi-one-dimensional organic conductors and ultra-cold Fermi gases. Triplet superfluidity is different from its s-wave singlet counterpart since the order parameter is a complex vector and the interaction between fermions is in general anisotropic. Because of these distinctions, triplet superfluids have different physical properties in comparison to the s-wave case. The author discusses in this thesis the interplay between triplet superconductivity and spin density waves in quasi-one-dimensional organic conductors, and proposes a coexistence region of the two orders. Within the coexistence region, the interaction between the two order parameters acquires a vector structure, and induces an anomalous magnetic field effect. Furthermore, the author analyzes the matter-wave interference between two p-wave Fermi condensates, and proposes a polarization effect. For a single harmonically trapped p-wave Fermi condensate, the author also shows that the expansion upon release from the trap can be anisotropic, which reflects the anisotropy of the p-wave interaction.

Thesis advisor: Michael J. Naughton
Quasi-one dimensional (Q1D) molecular organic conductors are among the most exciting materials in condensed matter physics, exhibiting nearly every known ground state. They are highly anisotropic, structurally and electronically, and show large oscillatory phenomena in conductivity for magnetic field rotated in different crystalline planes. Several theoretical works have been published to explain these angular magnetoresistance oscillation (AMRO) effects, but the underlying physics remains illunderstood. Here, we present measurements and calculations of magnetotransport in the molecular organic (super)conductor (DMET)₂I₃ which detect and simulate all known AMRO phenomena for Q1D systems. Employing, for the first time, the true triclinic crystal structure in the calculations, these results address the mystery of the putative vanishing of the primary AMRO phenomenon, the Lebed magic angle effect, for orientations in which it is expected to be strongest. They also show a common origin for Lebed and so-called "Lee-Naughton" oscillations, and confirm the generalized nature of AMRO in Q1D systems. Furthermore, we report the temperature dependence of the upper critical magnetic field in (DMET)₂I₃, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field exhibits orbital saturation at low temperature for field in all directions, implying that superconductivity in (DMET)₂I₃ is conventional spin singlet
Thesis (PhD) — Boston College, 2010
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

Les conducteurs organiques à base du sel de transfert de charge bis(éthylènedithio)tétrathiafulvalène (BEDT-TTF ou ET) montrent un comportement électrique allant de semi-conducteur à métal supraconducteur. La pression peut induire des petits changements structuraux qui s'accompagnent de changements très important des propriétés physiques. L'étude de l'évolution de la structure de ces composés sous pression, dont les mailles sont de faible symétrie contenant plusieurs dizaines d'atomes légers, constitue un défi. Le but de ce travail est d'étudier ces changements structuraux induits par la pression en utilisant la diffraction des rayons X sur monocristaux. Les monocristaux de six conducteurs et supraconducteurs organiques (α-(BEDT-TTF)2I3, β-(BEDT-TTF)2I3, κ-(BEDT-TTF)2I3, α-(BEDT-TTF)2KHg(SCN)4, κ-(BEDT-TTF)2Cu(NCS)2, et κ-(BEDT-TTF)2Cu[N(CN)2]Br) ont été étudiés à haute pression (jusqu'à 32 GPa) dans une cellule à enclumes de diamant sur la ligne de lumière ID09A à l'ESRF. L'hélium a été utilisé comme milieu transmetteur de pression. Les compressibilités de ces six composés ainsi que leurs positions atomiques ont été déterminées sous pression. Pour la première fois, les transitions de phases structurales réversibles de α-(BEDT-TTF)2I3 vers αHP-(BEDT-TTF)2I3 à 13.6 GPa, et de κ-(BEDT-TTF)2Cu(NCS)2 vers κHP-(BEDT-TTF)2Cu(NCS)2 à 8.5 GPa ont été observées, et les structures originales à haute pression ont été décrites avec succès. Les changements structuraux observés ont été appuyés par des mesures Raman
Organic conductors based on charge-transfer salt of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) exhibit electronic properties ranging from semiconducting to metallic to superconducting. Pressure can induce subtle structural rearrangements associated with drastic changes in physical properties. Study of the pressure induced structural changes of these compounds, with low symmetry unit cells containing dozens of light atoms, is a challenge. The aim of this work is to investigate pressure-induced structural changes with single crystal X-ray diffraction. Single-crystals of six organic conductors and superconductors (α-(BEDT-TTF)2I3, β-(BEDT-TTF)2I3, κ-(BEDT-TTF)2I3, α-(BEDT-TTF)2KHg(SCN)4, κ-(BEDT-TTF)2Cu(NCS)2, and κ-(BEDT-TTF)2Cu[N(CN)2]Br) were studied at high-pressures (up to 32 GPa) in a diamond-anvil cell on the ID09A beamline at the ESRF. Helium was used as pressure transmitting-medium. The compressibilities of the six compounds are reported, and atomic positions were determined under pressure. For the first time, reversible structural phase transitions in α-(BEDT-TTF)2I3 to αHP-(BEDT-TTF)2I3 at 13.6 GPa, and in κ-(BEDT-TTF)2Cu(NCS)2 to κHP-(BEDT-TTF)2Cu(NCS)2 at 8.5 GPa have been observed, and the original high pressure structures have been described successfully. Raman measurements agree well with the observed structural changes

La premiere partie est consacree a une revue des developpements de la technique experimentale de la diffraction des rayons x par des echantillons polycristallins (methode des poudres). Il est ici demontre que cette technique est aujourd'hui une methode pertinente pour l'analyse des proprietes structurales des materiaux. Dans une seconde partie, une application originale de la methode des poudres est presentee: un diffractometre de haute resolution fonctionnant entre 3 k et 470 k, a ete concu puis realise, pour l'analyse des contributions structurales dans les mecanismes regissant les transitions de phases (resolution des structures cristalline, mesure de la dilatation thermique, mise en evidence de transitions de phases/distorsions structurales). Les caracteristiques et les performances de ce diffractometre a temperature controlee sont exposees a travers plusieurs applications (transitions de phases avec deux parametres d'ordre correles, temperature de debye). La troisieme partie illustre une etude par diffraction des rayons x a toute temperature, qui a permis l'analyse du role determinant de la mise en ordre des anions c10 (a t = 24 k) sur l'etat fondamental (supraconducteur ou isolant) du compose organique unidimensionnel (tmtsf)#2c10#4. L'influence des deformations elastiques du reseau cristallin sur les proprietes geometriques de la surface fermi sont ainsi rapportees et correlees aux proprietes electroniques observees a basse temperature

碩士
國立臺灣大學
電信工程學研究所
92
In this thesis, basic theories of superconductivity are applied estimating the characteristics of high-temperature superconductor(HTS) components such as attenuation and Q factor. The properties of parallel-plate waveguide(PPWG) made of YBCO thin film and aluminum, rectilinear cacities made of YBCO thick film and copper are analyzed. The frequency-dependence of electron density is proposed to obtain reasonable attenuation of YBCO PPWG especially when the operating frequency is near critical frequency.

The present dissertation intends to contribute to the optimization process of the design and manufacture of low-resistance short-circuited superconducting coils in an architecture developed in Department of Electrical Engineering of Faculdade de Ciências e Tecnologia (DEE-FCT) of the Universidade NOVA de Lisboa. The design optimization of the previously proposed coil architecture is carried out in this work. After validation of the new design, the superconducting tapes are cut through two different methods, i.e. by water jet and punching. The critical current of the tapes is measured to assess the quality of the cut made and the preservation of the superconducting properties of the tapes. The most suitable method to perform the joint of the coils is determined by measuring the contact resistance at the joints using different solder materials and configurations. Prototypes of coils with 4 and 8 turns are constructed, and an experimental test for the determination of the critical currents through the induction of superconducting currents into the coils is carried out. It is concluded that the water jet is very aggressive to cut the tapes and difficult to adapt to large tape lengths using the current conventional machines. It is verified a better aptitude of the punching method through a cutting machine adaptable to the various moulds and lengths, conditioned by the fact that it is currently a restricted access process. Some suggestions are made for future works which do not exhaust in this dissertation.

Thesis (Ph. D.)--Florida State University, 2006.
Advisor: Justin Schwartz, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed June 16, 2006). Document formatted into pages; contains xvi, 132 pages. Includes bibliographical references.

Since the discovery of the high temperature superconductor YBa2Cu3O7-δ (YBCO), with transition temperature (Tc = 77 K), above liquid nitrogen point in 1987 many research projects have been dedicated to enhancing the high field performance of this material for practical applications. The 2nd generation YBCO-based coated conductors are believed to be the most promising approach for commercial applications including power transmission, motors, generators, and high field magnets. With the advances of nanotechnologies, different nanoengineering methods have been demonstrated to enhance the performance of YBCO thin films, include doping with 0-dimensional (0-D) self-assembled nanoparticles, 1-dimensional (1-D) nanorods, and 2-dimensional (2-D) nanolayers. Furthermore, dopants with ferromagnetic properties are also reported to provide enhanced pinning effects by Lorentz force, especially under high-applied magnetic fields. The principle of these methods is to generate high-density defects at the heterogeneous interfaces as artificial pinning centers in an effort to improve the flux-pinning properties. The morphology and dimensions of the nanoinclusions play an important role in pining enhancement. Optimized pinning structures are likely to be located at energetically favorable vortex cores, which form a triangular lattice with dimensions close to the YBCO coherence length ξ (ξab ~ 4 nm; ξc ~ 0.5 nm at 77 K.) However, it is challenging to achieve small dimensional nanodopants in the vapor deposited YBCO thin films. The purpose of this research is to utilize nanoengineering methods to produce optimized pinning structure in YBCO thin films. In this thesis, we systematically study the effects of different nanoinclusions on the flux-pinning properties of YBCO thin films. The 0-D ferromagnetic Fe2O3 and CoFe2O4 nanoparticles, 2-D CeO2 multilayers, and tunable vertically aligned nanocomposites (VAN) of (Fe2O3)x:(CeO2)1-x and (CoFe2O4)x:(CeO2)1-x systems are introduced into the YBCO matrix as artificial pinning centers. Results suggest that all nanoinclusions showed significant enhancement in the superconducting properties of YBCO. The ferromagnetic pinning centers dominate at high field and low temperature regimes, however, the defect pinning centers dominate at low field and high temperature regimes. The uniquely arranged VAN structure of alternating magnetic and non-magnetic nanophases, which incorporates both high defect density and tunable distribution of magnetic dopants, is believed to be an ideal solution for flux-pinning enhancement.