Dissertations / Theses on the topic 'YBCO superconductor'

In this research, we have implemented an original technique to study the electronic properties of a single electron placed in YBa2 at 300K. Using a discretized extension of Feynman's Quantum Path Integral, we have been able to characterize effective electron-phonon interactions, and electron location site probability. We find that the electron stabilizes at oxygen vacant sites in the copper-oxygen chains. In the copper-oxygen planes, the electron is unstable and moves into the chain. Upon complementing the quantum electron to a positive charge thereby simulating a hole, we then find that the hole moves into favorable sites in the copper-oxygen planes. These sites are surrounded by four oxygens and two copper ions. Next, by decoupling the electron and hole from the lattice, we obtain effective electron-phonon and hole-phonon coupling constants on the order of 30. These results indicate that the next area of research is to move toward a multi-electron system and allow for further study of the electrons near the Fermi level. Some of the difficulties associated with multi-electron systems such as "exchange", are briefly discussed.

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.

When a high-temperature superconductor transitions in the presence of a magnetic field, the resistance does not immediately go to zero. Instead, the transition is smoothed by the resistive effects of moving vortices so that perfect conductivity isn't reached until several K below Tc, where the vortices have frozen into a glass state. The properties of this glass depend mainly on the amount and correlation between impurities in the superconductor. This report describes a study of the glass transition in an YBCO thin film rich in uncorrelated impurities. The aim was to use sensitive resistive measurements to determine which of two models of this transition, the vortex glass model and the vortex molasses model, is more accurate. After evaluation, both models are found lacking and a third model is put forth. This model, which resembles the normal distribution, excellently describes the YBCO thin film case. When the new model is tested against data from similar measurements, an YBCO single crystal and a TlBaCaCuO thin film, the results are mixed. Further study is suggested to determine the model's range of validity.

Die für technische Anwendungen attraktivsten Eigenschaften der Hochtemperatur-Supraleiter beruhen auf der Wechselwirkung der Materialien mit einem externen Magnetfeld. Für hochleistungsfähige Supraleiteranwendungen werden großvolumige Funktionselemente mit eindomänigem Magnetisierungsverhalten benötigt. Zur Vergrößerung der magnetischen Domäne schmelztexturierter YBa2Cu3O7-δ-Kompaktsupraleiter wurde anhand der Multi-Seeding-Technik, die Rekristallisation unter Verwendung mehrerer SmBa2Cu3O7-δ-Keimpräparate untersucht. Als Schlüsseltechnologie zur Herstellung anwendungsorientierter Supraleitererzeugnisse wurde zu Beginn der Arbeit die Keimkristallherstellung optimiert. Gemäß den Zielvorgaben stehen zukünftig quasi-einkristalline Keimkörper definierter Form und Orientierung in hoher Stückzahl zur Verfügung. Die Supraleiter betreffend ist es gelungen, für den wechselseitigen Abstand und die Ausrichtung der Keimpräparate ein Optimum zu finden sowie tolerierbare Winkel der Verschwenkung angrenzender Kristalle zu ermitteln. Es wurde festgestellt, dass das Auftreten isolierender Korngrenzeneinschlüsse mit den magnetischen Materialeigenschaften korreliert und vom Keimabstand und der Korngrenzenorientierung abhängig ist. Mit Einsatz von bis zu 16 Keimkristallen gelang es, eindomänige Supraleiterhalbzeuge mit Höchstmaßen von (79 x 39 x 20) mm3 und Remanenzflussdichten von bis zu 1,3 T zu erzeugen. Im Chargenprozess konnten abschließend Multi-Seeding-Funktionselemente mit anwendungskonformen geometrischen und magnetischen Materialeigenschaften zum Aufbau eines hochdynamischen Supraleitermotors reproduzierbar gefertigt werden. In Leistungstests wurde für die Dynamik des Motors mit 200.000 rpm/s ein Rekordwert erzielt – nun können die Motoren in der Anwendung erprobt werden.

By allowing to measure the magnetic field distribution inside a material, muon spin rotation experiments have the potential to provide valuable information about microscopic properties of high-temperature superconductors. Nevertheless, information about the intrinsic superconducting properties of the material is masked by random thermal and static fluctuations of the magnetic field which penetrates the material in the form of vortices of quantized magnetic flux. A good understanding of the fluctuations of those vortices is needed for the correct determination of intrinsic properties, notably the coherence length ξ, and the field penetration depth λ. We develop a simulation based on the Metropolis algorithm in order to understand the effect, on the magnetic field distribution, of disorder- and thermally-induced fluctuations of the vortex lattice inside a layered superconductor.

Our model correctly predicts the melting temperatures of the YBa₂Cu₃O_{6. 95} (YBCO) superconductor but largely underestimates the observed entropy jump. Also we failed to simulate the high field disordered phase, possibly because of a finite size limitation. In addition, we found our model unable to describe the first-order transition observed in the highly anisotropic Bi₂Sr₂CaCu₂O_8+y.

Our model predicts that for YBCO, the effect of thermal fluctuations on the field distribution is indistinguishable from a change in ξ. It also confirms the usual assumption that the effect of static fluctuations at low temperature can be efficiently modeled by convolution of the field distribution with a Gaussian function. However the extraction of ξ at low fields requires a very high resolution of the field distribution because of the low vortex density.

After nearly 15 years of research effort, High Temperature Superconductors (HTS) are finding a wide range of practical applications. A clear understanding of the factors controlling the current carrying capacity of these materials is a prerequisite to their successful technological development. The critical current density (Jc) in HTS is directly dependent on the structure and pinning of the Flux Line Lattice (FLL) in these materials. This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grown on off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprate planes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO) films. By confirming, and extending, this observation, it is demonstrated that this is an intrinsic effect. The results obtained, appear to fit well with the predictions of a field angle dependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of strings and pancakes. The pinning force density for movement of strings inside the cuprate planes is considerably less than that on vortex pancake elements. When the FLL is entirely string-like this reduced pinning leads to the observed channelling minima. It is observed that anti-phase boundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings. The effect on the FLL structure cross-over of increasing anisotropy has been elucidated using de-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that the range of applied field angle for which the kinked lattice is fully developed reduces with increasing anisotropy, appears to be confirmed. Although vortex channelling cannot be observed in c-axis YBCO films, the pinning force density for vortex string channelling has been extracted by observing string dragging. By studying the effect of rotating the applied field at a constant angle to the cuprate planes, it is possible to observe the cross-over into the string pancake regime in c-axis films. In the 3D region, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model. Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+x material, grown on vicinal substrates. The absence of any flux channelling effect and clear adherence to the expected Kes-Law behaviour in the observed Jc characteristics does not provide evidence for the existence of the predicted 'crossing lattice' in Bi 2Sr2CaCu2O8+x.

Els superconductors d’alta temperatura (HTS) han liderat els programes d’investigació I desenvolupament durant tres dècades i actualment es troben a prop de ser integrats en aplicacions de gran escala, en forma de cintes superconductores (CCs), una arquitectura de cinta robusta i flexible que permet el creixement texturat d’aquest material fràgil. Un escull pendent per l’amplia implementació de les CCs és l’alt ratio de cost/prestació, essencialment controlat pel creixement complexa i descelerat de la capa de HTS. En aquesta tesis, vem agafar el repte de millorar el rendiment i cost dels mètodes de creixement convencionals a través del desenvolupament d’una nova tècnica de creixement que combina la deposició de solucions químiques (CSD), deposició del precursor inherentment de baix cost, amb l’alta velocitat de creixement, una aproximació de creixement de no equilibri que permet la formació d’una fase líquida transitòria (Ba-Cu-O) que condueix a la cristal·lització del producte final: Creixement Assistit per Líquid Transitori a través de CSD (TLAG-CSD). Hem utilitzat aquest procés nou per créixer capes primes de YBa2Cu3O7-d (YBCO), un material HTS basat en cuprats de inigualables prestacions sota camps magnètics aplicats i altes temperatures. Aquestes propietats tan impressionants sols es poden assolir si el compost, elèctricament anisotròpic, exhibeix una textura biaxial i conté suficients defectes per ancorar els vòrtexs, quantificacions de flux magnètic que necessiten ser immobilitzades per obtenir conducció de corrent elèctric sense pèrdues. Per tant, no tan sols pretenem la comprensió dels fonaments del procés TLAG-CSD, sinó també promoure formes de creixement epitaxial a grans velocitats, evitant les causes de degradació del corrent, i fomentant l’enriquiment d’un paisatge adequat per l’ancoratge de vòrtexs. Per afrontar aquests reptes, hem combinat varis mètodes avançats de caracterització: Hem realitzat experiments de creixement in-situ en una instal·lació de llum sincrotró per avaluar la cinètica de creixement a diferents pressions parcial i total d’oxigen, temperatura de creixement, rampa d’escalfament, gruix de la capa HTS i composició, així com l’addició de nanopartícules (NP). La combinació dels diferents experiments s’ha resumit satisfactòriament en els anomenats diagrames de fase cinètics, una representació visual dels processos fora de l’equilibri i un roadmap per la seva utilització. Les capes de YBCO epitaxial orientades segons l’eix c s’han estudiar minuciosament amb difracció de raigs-X, microscòpia electrònica de transmissió (TEM), experiments de transport elèctric a baixes temperatures i inducció magnètica per identificar les limitacions i oportunitats del procés. Això va incloure l’estudi de la microestructura del YBCO i les propietats elèctriques per evitar la reactivitat del líquid amb el substrat, la segregació de fases secundaries, a la vegada que asseguràvem l’eliminació del CO2 i el dopatge d’oxigen de l’estructura cristal·lina. Finalment, hem revelat que amb el mètode TLAG-CSD es poden assolir velocitats de creixement epitaxial per sobre de 100nm/s, sobrepassant dos ordres de magnitud les tècniques convencionals utilitzades en la fabricació de CC (TFA-CSD/PLD/MOCVD). Les capes superconductores van assolir un alineament epitaxial alt dels grans (Δω<0.6º i Δφ<1º), temperatures crítiques (Tc=88-92K) i densitats de corrent competitives (Jc(77K)=2-5MA/cm2). A més, classifiquem el procés TLAG-CSD com altament versàtil per enriquir el paisatge d’ancoratge de vòrtexs, no solament a través de la capacitat de promoure una alta densitat de faltes d’apilament i grans de YBCO orientats-ab molt petits (5-10nm) en les capes de YBCO, sinó per la seva compatibilitat per formar nanocompostos amb l’addició de nanopartícules preformades.
Los superconductores de alta temperatura (HTS) han liderado los programas de investigación y desarrollo durante tres décadas y actualmente se encuentran cerca de ser integrados, en aplicaciones de gran escala, en forma de cintas superconductoras (CCs), una arquitectura de cinta robusta y flexible que permite el crecimiento texturado de este material frágil. Un escollo pendiente para la amplia implementación de las CCs es el alto ratio de coste/prestación, esencialmente controlado por el crecimiento complejo y descelerado de la capa de HTS. En esta tesis, cogimos el reto de mejorar el rendimiento y coste de los métodos de crecimiento convencionales a través del desarrollo de una nueva técnica de crecimiento que combina la deposición de soluciones químicas (CSD), deposición del precursor inherentemente de bajo coste, con la alta velocidad de crecimiento, una aproximación de crecimiento de no equilibro que permite la formación de una fase líquida transitoria (Ba-Cu-O) que conduce a la cristalización del producto final: Crecimiento Asistido por Liquido Transitorio a través de CSD (TLAG-CSD). Hemos utilizado este proceso nuevo para crecer capas delgadas de YBa2Cu3O7-d (YBCO), un material HTS basado en cupratos de inigualables prestaciones bajo campos magnéticos aplicados y altas temperaturas. Estas propiedades tan impresionantes solo se pueden alcanzar si el compuesto, electrónicamente anisotrópico, exhibe una textura biaxial y contiene suficientes defectos para anclar a los vórtices. Por tanto, no solo pretendemos la comprensión de los fundamentos del proceso TLAG-CSD, sino también promover formas de crecimiento epitaxial a grandes velocidades, evitando las causas de degradación de la corriente, y fomentando el enriquecimiento de un paisaje adecuado para el anclaje de vórtices. Para afrontar estos retos, hemos combinado varios métodos avanzados de caracterización: Realizamos experimentos de crecimiento in-situ en una instalación de luz sincrotrón para evaluar la cinética de crecimiento a distintas presiones parcial y total de oxígeno, temperatura de crecimiento, rampa de calentamiento, grosor de la capa HTS y composición, así como la adición de nanopartículas (NP). La combinación de los distintos experimentos se ha resumido satisfactoriamente en los llamados diagramas de fase cinéticos, una representación visual de los procesos fuera del equilibrio y un roadmap para su utilización. Las capas de YBCO epitaxiales orientadas según el eje c se estudiaron minuciosamente con difracción de rayos-X, microscopía electrónica de transmisión (TEM), experimentos de transporte eléctrico a bajas temperaturas e inducción magnética para identificar las limitaciones y oportunidades del proceso. Ello incluyó estudiar la microstructura del YBCO y las propiedades eléctricas para evitar la reactividad del líquido con el sustrato, la segregación de fases secundarias, a la vez que asegurar la eliminación de CO2 y el dopaje de oxígeno de la estructura cristalina. Revelamos que con el método TLAG-CSD se pueden alcanzar velocidades de crecimiento epitaxial por encima de 100nm/s, sobrepasando dos órdenes de magnitud las técnicas convencionales utilizadas en la fabricación de CC (TFA-CSD/PLD/MOCVD). Las capas superconductoras alcanzaron un alineamiento epitaxial alto de los granos (Δω<0.6º y Δφ<1º), temperaturas críticas (Tc=88-92K) y densidades de corriente crítica competitivas (Jc(77K)=2-5MA/cm2). Además, clasificamos al proceso TLAG-CSD como altamente versátil para enriquecer el paisaje de anclaje de vórtices, no solo a través de la capacidad de promover una alta densidad de faltas de apilamiento y granos de YBCO orientados-ab muy pequeños (5-10nm) en las capas de YBCO, sino por su compatibilidad para formar nanocompuestos con la adición de nanopartículas preformadas.
High temperature superconductors (HTS) have been driving research and development programs for about three decades now and are on the verge of entering large scale utilization in the form of Coated Conductors (CCs), a robust and flexible tape architecture that enables textured growth of the rather brittle material class. A remaining bottleneck for widespread CC implementation is the high cost/performance ratio, essentially controlled by the complex and decelerating step of HTS layer growth. In this thesis we challenge the throughput and cost constraints of conventional growth methods through development of a novel growth technique that combines Chemical Solution Deposition (CSD), an inherently low-cost precursor deposition approach, with a high growth rate, non-equilibrium growth scheme that allows the formation of a transient (Ba-Cu-O) liquid phase prior to crystallization of the final product phase: Transient Liquid-Assisted Growth via CSD (TLAG-CSD). We employed the new process to grow YBa2Cu3O7-d (YBCO) thin films, a cuprate-based HTS material with unmatched current carrying performance in applied magnetic field conditions and at high temperatures. The unprecedented properties can only be reached if the electrically anisotropic compound exhibits biaxial texture and provides sufficient nanometric defects to pin vortices. Hence, not only is the fundamental understanding of the TLAG-CSD process required but also ways to promote epitaxial growth at high rates, avoidance of any current-degrading sources and strategies to enrich the pinning landscape. To tackle these challenges, we combined several advanced characterization methods: In-situ growth experiments at a synchrotron facility were carried out to follow growth kinetics under variation of total and oxygen partial pressures, growth temperatures, heating ramps, film thickness/composition and nanoparticle (NP) addition. The combined set of experiments was successfully summarized in the form of so-called kinetic phase diagrams, a visual representation of the out-of-equilibrium processes and a roadmap for its utilization. Epitaxial, c-axis oriented YBCO films were thoroughly studied via X-ray diffraction, transmission electron microscopy (TEM), low temperature electrical transport and inductive measurements to identify process related limitations and opportunities. This includes probing of the YBCO microstructuture and intrinsic electrical properties to avoid liquid induced substrate reactivity, segregation of secondary phases, while enabling proper CO2 elimination and oxygen doping of the crystal structure. We disclose that epitaxial layer growth rates above 100nm/s can be accomplished through the TLAG-CSD approach, surpassing conventional techniques used in CC manufacturing schemes (TFA-CSD/PLD/MOCVD) by up to two orders of magnitude. Superconducting films are demonstrated to reach high epitaxial grain alignment (Δω<0.6º and Δφ<1º), optimal critical temperatures (Tc=88-92K) and competitive critical current densities (Jc(77K)=2-5MA/cm2). We further classify the TLAG-CSD process as highly versatile in enriching the vortex pinning landscape, not only through its capability to promote a high density of stacking faults and small ab-oriented YBCO grains (5-10nm) in pristine YBCO films, but also through its compatibility with preformed nanoparticle addition in nanocomposites.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 81-83).
The twisting stacked tape cabling (TSTC) method for YBCO superconductors is very attractive for high current density, high magnetic field applications, such as nuclear fusion reactors and high energy physics experiments. Industrial scale assembling methods have been proposed, and cable samples have been tested at 77 K and 4.2 K. A new experimental device has been designed and built to measure critical current of YBCO tapes and TSTC as a function of magnetic field and temperature. The probe allows controlling the temperature between 4.2 K and 80 K within +/-1 K in liquid and gaseous helium ambient, and can be used in a 2 T magnet facility at MIT-PSFC and a 14 T magnet facility at NHMFL-FSU. Its current leads are designed to carry up to 5 kA. The device consists in a 0.9 m long, 25 x 38 mm rectangular vacuum-insulated canister. The superconducting sample and a superconducting current return lead fit inside the canister, in such a way that the Lorentz force and torque produced by the external magnetic field is cancelled. The sample temperature is controlled in a 200 mm long area inside the canister where critical current measurements are performed. Critical current measurements were performed on a single YBCO tape at self-field at temperatures between 20 K and 70 K. The results are similar to data provided by the superconductor's manufacturer. The temperature reached the set point in approximately 10 minutes, and was controlled within +/-1 K. Results of heating power required and difference between set point temperature and measured temperature as functions of set point temperature are presented for two temperature control methods.
by Franco Julio Mangiarotti.
S.M.

Orientador: Dayse Iara dos Santos
Banca: Pablo Antonio Venegas Urenda
Banca: Nelcy Della Santina Mohallem
Resumo: Heteroestruturas que combinam camadas supercondutoras (SC) com ferromagnéticas (FM), e, em alguns casos, ainda com camadas isolantes (I) entre as primeiras, possuem propriedades que não são observadas para os filmes isolados devido a interação entre elas. Este trabalho busca contribuir para a elucidação das propriedades de super-redes do tipo SC/I/FM feitas por PLD (Pulsed Laser Deposition), utilizando o YBa2Cu3O7-δ (YBCO) como supercondutor, o La2/3Ca1/3MnO3 (LCMO) como ferromagneto e o PrBa2Cu3O6+δ (PBCO), o SrTiO3 (STO) e o CeO2 (CEO) como isolantes. A sequência utilizada foi (YBCO20nm/Idi/LCMO20nm)x20, e as espessuras do isolante PBCO foram de 0,5, 2, 4, 8 e 20nm, e de 2nm para o STO e para o CEO. Também foi analisada uma amostra com a inversão da ordem de deposição do supercondutor e ferromagneto. As análises de difração de raios X confirmaram o crescimento preferencial na direção 00ℓ e parâmetros de rede comparáveis com os valores encontrados na literatura para o YBCO. A microscopia eletrônica de varredura evidenciou poros na superfície do filme de LCMO e crescimento extra de cristais na superfície do filme de YBCO. As análises de AFM mostraram que a textura da superfície depende da espessura e da natureza do isolante assim como a distribuição de cargas observada por EFM. Medidas elétricas realizadas pelo método de Quatro Pontas comprovou temperatura crítica em torno de 90K, e somente para a amostra com 20nm de PBCO foi notado grande distinção do comportamento da resist... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: Heterostructures that combine superconducting (SC) layers with ferromagnetic (FM) layers, and in, some cases, with insulating (I) layers between the first ones, have properties that are not observed in single layers due the interaction between them. This work aims to contribute for elucidation of properties of SC/I/FM superlattices made by PLD deposition, using YBa2Cu3O7-δ (YBCO) as superconductor, La2/3Ca1/3MnO3 (LCMO) as ferromagnetic and PrBa2Cu3O6+δ (PBCO), SrTiO3 (STO) and CeO2 (CEO) as insulator materials. The used sequence was (YBCO20nm/Idi/LCMO20nm)x20, and the thicknesses of the insulating PBCO were 0.5, 2, 4, 8 and 20nm, and 2nm for STO and CEO. It was also analyzed a sample with inverted deposition order of superconductor and ferromagnetic. The analysis of X-ray diffraction confirmed the preferential growth toward 00ℓ and network parameters comparable to the values found in the literature for the YBCO. The scanning electron microscopy showed pores on the surface of LCMO film and extra crystal growth on the surface of the YBCO film. The AFM analyzes showed that the surface texture depends on the thickness and nature of the insulation as well as the charge distribution observed by EFM. Electrical measurements made by Four Probe method proved critical temperature around 90K, and only for the sample with 20nm PBCO has been noted large distinction of the resistivity behavior over heating from 25K with the application of magnetic field of 0.8T, which may indicate a diff... (Complete abstract click electronic access below)
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Heteroestruturas que combinam camadas supercondutoras (SC) com ferromagnéticas (FM), e, em alguns casos, ainda com camadas isolantes (I) entre as primeiras, possuem propriedades que não são observadas para os filmes isolados devido a interação entre elas. Este trabalho busca contribuir para a elucidação das propriedades de super-redes do tipo SC/I/FM feitas por PLD (Pulsed Laser Deposition), utilizando o YBa2Cu3O7-δ (YBCO) como supercondutor, o La2/3Ca1/3MnO3 (LCMO) como ferromagneto e o PrBa2Cu3O6+δ (PBCO), o SrTiO3 (STO) e o CeO2 (CEO) como isolantes. A sequência utilizada foi (YBCO20nm/Idi/LCMO20nm)x20, e as espessuras do isolante PBCO foram de 0,5, 2, 4, 8 e 20nm, e de 2nm para o STO e para o CEO. Também foi analisada uma amostra com a inversão da ordem de deposição do supercondutor e ferromagneto. As análises de difração de raios X confirmaram o crescimento preferencial na direção 00ℓ e parâmetros de rede comparáveis com os valores encontrados na literatura para o YBCO. A microscopia eletrônica de varredura evidenciou poros na superfície do filme de LCMO e crescimento extra de cristais na superfície do filme de YBCO. As análises de AFM mostraram que a textura da superfície depende da espessura e da natureza do isolante assim como a distribuição de cargas observada por EFM. Medidas elétricas realizadas pelo método de Quatro Pontas comprovou temperatura crítica em torno de 90K, e somente para a amostra com 20nm de PBCO foi notado grande distinção do comportamento da resistividade ao longo do aquecimento a partir de 25K com a aplicação de campo magnético de 0,8T, o que pode indicar uma interação diferenciada entre as camadas de LCMO e YBCO nesta configuração. Pequenos picos de magnetorresistência foram observados sempre próximos da transição supercondutora. A intensidade e sinal do pico sofreram algumas variações com a direção do campo.
Heterostructures that combine superconducting (SC) layers with ferromagnetic (FM) layers, and in, some cases, with insulating (I) layers between the first ones, have properties that are not observed in single layers due the interaction between them. This work aims to contribute for elucidation of properties of SC/I/FM superlattices made by PLD deposition, using YBa2Cu3O7-δ (YBCO) as superconductor, La2/3Ca1/3MnO3 (LCMO) as ferromagnetic and PrBa2Cu3O6+δ (PBCO), SrTiO3 (STO) and CeO2 (CEO) as insulator materials. The used sequence was (YBCO20nm/Idi/LCMO20nm)x20, and the thicknesses of the insulating PBCO were 0.5, 2, 4, 8 and 20nm, and 2nm for STO and CEO. It was also analyzed a sample with inverted deposition order of superconductor and ferromagnetic. The analysis of X-ray diffraction confirmed the preferential growth toward 00ℓ and network parameters comparable to the values found in the literature for the YBCO. The scanning electron microscopy showed pores on the surface of LCMO film and extra crystal growth on the surface of the YBCO film. The AFM analyzes showed that the surface texture depends on the thickness and nature of the insulation as well as the charge distribution observed by EFM. Electrical measurements made by Four Probe method proved critical temperature around 90K, and only for the sample with 20nm PBCO has been noted large distinction of the resistivity behavior over heating from 25K with the application of magnetic field of 0.8T, which may indicate a different interaction between the layers of YBCO and LCMO for this configuration. Small magnetoresistance peaks were observed always close to the superconducting transition. The intensity and peak signal underwent some changes with the direction of the applied field.

L'apparition ces dernières années de supraconducteurs réalisés industriellement utilisant des composés à haute température de transition offre la possibilité de nouveaux développements en magnétisme supraconducteur. En effet ils permettent d'augmenter le champ magnétique généré en conservant une cryogénie classique à 4,2K d'une part, et ils ouvrent la voie à des développements d'aimants supraconducteurs fonctionnant entre 10 et 30K d'autre part. Les matériaux supraconducteurs à haute température critique sont alors indispensables pour dépasser les inductions magnétiques de 16 T (cas de l'insert dipolaire HTc pour le Large Hadron Collider du CERN) ou augmenter la densité spécifique d'énergie stockée dans un SMES (Superconducting Magnetic Energy Storage, cas du projet ANR SuperSMES).Les atouts incontestables (température critique, champ magnétique critique, résistance mécanique) apportés par l'utilisation des matériaux supraconducteurs à haute température critique tels que l'YBaCuO dans les aimants supraconducteurs demandent de relever quelques défis. Leur comportement est encore mal compris, surtout lors des transitions résistives. Arriver à protéger ces conducteurs requiert une réflexion nouvelle sur les systèmes de protection destinés à éviter les dégradations thermiques et mécaniques. La réponse à la question " peut-on utiliser ces matériaux de manière pérenne dans les aimants supraconducteurs ? " est incontournable.Des éléments de réponse sont donnés ici. L'utilisation des conducteurs est abordée à travers différentes études expérimentales permettant de mieux connaître le conducteur (caractérisation électrique et modélisation de la surface critique) d'une part et de définir les étapes clés de la fabrication des aimants supraconducteurs à haute température de transition (étude des jonctions entre conducteurs ou entre galettes) d'autre part. Cette étude a abouti à la réalisation de deux prototypes d'aimants ayant permis d'identifier les difficultés liées à l'utilisation des rubans d'YBaCuO. Un modèle thermoélectrique des supraconducteurs à haute température de transition est développé et un code numérique basé sur le logiciel de calcul par Eléments Finis CASTEM permet d'étudier le phénomène de transition résistive, ou quench, dans un conducteur et dans un aimant. Le code a été validé sur des essais réalisés au Laboratoire National des Champs Magnétiques Intenses de Grenoble. Les résultats obtenus ont permis la définition des conducteurs pour les deux projets liés à la thèse et la validation de la protection.

Els superconductors d'alta temperatura, especialment els cuprats en forma de cintes superconductores, tenen el potencial de formar part de la propera revolució tecnològica gràcies a les seves propietats superconductores extraordinàries i inigualables. Els àtoms d'oxigen tenen un paper essencial en aquests superconductors d’alta temperatura, on la superconductivitat es regeix pel dopatge de forats. En aquesta tesi, hem estudiat extensament tots els processos on l'oxigen està implicat, des de la incorporació inicial d'oxigen a l'estructura de l‘YBa2Cu3O7-δ i el mecanisme relacionat a aquesta incorporació, fins a la influència del dopatge d'oxigen en les propietats superconductores. Una comprensió profunda de cada pas concret no només és interessant des del punt de vista acadèmic, sinó que també és necessari en l'optimització i la millora de qualsevol procés de producció comercial. Utilitzant mesures de relaxació de la conductivitat elèctrica in-situ i les mesures de difracció de raigs X in-situ, hem analitzat la cinètica d'intercanvi d'oxigen en capes primes de YBCO. S'ha estudiat una àmplia varietat de mostres i microestructures, obtingudes per diferents mètodes de creixement, amb substitució catiònica, nanocompòsits, variacions de gruix i substrat. Els nostres estudis revelen influències diferents entre la tensió macroscòpica i microscòpica sobre les energies d'activació per a l'intercanvi d'oxigen. En aquest estudi s’ha establert que la plata és un agent catalític excel·lent per a la incorporació d'oxigen, proporcionant una ruta de reacció catalítica alternativa, que permet una cinètica més ràpida d'oxigenació i una menor temperatura d'oxigenació. A més a més, hem identificat amb èxit el pas determinant en la cinètica d'intercanvi d'oxigen en capes primes d‘YBCO recobertes de plata, el qual és la recombinació dels ions d'oxigen amb vacants a la superfície de la capa. L'aprofundiment en les mesures elèctriques in-situ en combinació amb tècniques d'anàlisi ex-situ com ara mesures de XRD, STEM, SEM, resistivitat elèctrica, Hall i magnetització ens ha permès estudiar els efectes dels paràmetres de tractament tèrmic sobre la química de superfície com en la microestructura de les capes primes de YBCO . D'una banda, hem identificat la formació de defecte d'apilament (stacking faults) ja durant els tractaments a baixa temperatura. D'altra banda, hem descobert la desactivació de la cinètica d'intercanvi en superfícies per a capes no recobertes de plata, la qual cosa suposa una cinètica d’incorporació d'oxigen significativament més lenta. Les diferents velocitats de degradció de la superfície depenen del recobriment superficial i apunten cap a una modificació del pas determinant en la cinètica de reacció quan s'afegeix un recobriment de plata en capes primes d'YBCO. No només hem estudiat la incorporació d'oxigen en gran detall, sinó també el seu efecte en l'estat de dopatge del cuprat. S'ha estudiat extensament la influència de la pressió parcial d'oxigen, la temperatura d'oxigenació i el temps d'oxigenació en la densitat de portadors de càrrega i les propietats físiques normals/superconductores. Hem preparat capes primes d'YBCO superdopades per PLD (pulsed laser deposition) i CSD (chemical solution deposition). Hem mostrat que en el cas de PLD es poden acpnseguir densitats de corrent crítiques molt altes, arribant a 90 MA/cm² a 5K i auto-camp, que correspon a un terç de la densitat de corrent crítica fonamental de desaparellament. El dopatge s’ha analitzat mitjançant l'ús de mesures de resistivitat en funció temperatura, mesures d’efecte Hall i mobilitat, així com raigs-X i mesures de temperatura crítica. Mitjançant l'ús d'un model de dues bandes per al transport elèctric d'electrons i forats, hem obtingut una densitat de càrrega independent de la temperatura. Hem demostrat una correlació lineal entre la densitat de portadors de càrrega i la densitat de corrents crítica en l'estat superdopat, evidenciant així el potencial d'aquests estudis per a la millora de les propietats superconductores de les capes d'YBCO.
High temperature superconductors, especially cuprates, in the form of coated conductors have the potential to be part of the next technological revolution due to unchallenged, extraordinary superconducting properties. Oxygen plays an essential role in these cuprate high-temperature superconductors, where superconductivity is governed by hole doping. In this thesis we have intensively studied all oxygen involved processes, from the initial incorporation of oxygen into the YBa2Cu3O7-δ structure and related mechanism, up to the influence of oxygen doping on the superconducting properties. A deep understanding of each particular step is not only interesting from an academic point of view, but also necessary in the optimisation and improvement for any commercial production line. Using in situ electrical conductivity relaxation and in situ X-ray diffraction measurements we have analysed oxygen exchange kinetics in YBCO thin films. A broad variety of samples and microstructures, obtained by different growth methods, cation substitution, nanocomposites, variations in thickness and substrate, have been studied. Our studies reveal different influences of macroscopic and microscopic strain on activation energies for oxygen exchange. In this work silver was found to be an excellent catalytic agent for oxygen incorporation, by providing a catalytic alternative reaction path, which enabled faster oxygenation kinetics and lower oxygenation temperatures. Further, we have successfully identified the rate determining step (RDS) of oxygen exchange kinetics in silver coated YBCO thin films, which we have found to be the recombination of oxygen ions with surface vacancies. The thorough use of electrical in situ measurements in combination with ex situ analysis techniques as XRD, STEM, SEM, electrical resistivity, Hall and magnetisation measurements enabled us to study the effects of thermal treatment parameters on the surface chemistry and bulk microstructure of YBCO thin films. On one hand, we have identified the formation of stacking faults already during low temperature annealings . On the other hand, a deactivation of surface exchange kinetics was found for non-silver coated films, resulting in significantly slower oxygen incorporation with increasing annealing time. Surface coating dependent degeneration rates point towards a modification of the RDS upon silver coating of YBCO thin films. We have not only studied the incorporation of oxygen in great detail, but also its effect on the doping state of the cuprate material. The influence of oxygen partial pressure, oxygenation temperature and oxygenation time on the charge carrier density and normal/superconducting physical properties was intensively studied. We have prepared highly overdoped YBCO thin films grown by PLD with record-high critical current densities reaching 90 MA/cm² at 5 K and self-field, reaching a third of the depairing current density. The doping state was analysed by the use of temperature dependent resistivity, Hall and mobility measurements, as well as XRD and critical temperature measurements. By using a two band model for the electrical transport of electrons and holes, we have obtained a temperature independent charge carrier density. We have demonstrated a linear correlation between the charge carrier density and the critical current densities in the overdoped state, thus evidencing the powerfulness of these studies for the enhancement of superconducting properties of YBCO thin films.

While in conventional superconductors coupling between electrons and phonons is known to be responsible for the electron pairing, for the high temperature superconductors the pairing media remains under debates. Since the interactions of electrons with other degrees of freedom (phonons, magnetic excitations, etc) manifest themselves by an additional renormalization in the electronic dispersion, they can be investigated by means of Angle Resolved Photoelectron Spectroscopy. In the work renormalization in two families of high Tc cuprates have been studied. Along the diagonal of the two-dimensional BZ, the renormalization effects are represented by an unusual band dispersion that develops a so-called ‘‘kink’’. In the vicinity of the (pi, 0) point of the BZ, where the order parameter reaches its maximum, the renormalization is noticeably stronger and makes itself evident even in the shape of a single spectral line measured for a fixed momentum. It was shown that for the Bi-2212 samples substitution of Cu atoms in Cu-O plane changes renormalization features in ARPES spectra both in nodal and antinodal parts of the Brillouin zone. The smearing of the dip in the in the spectral line shape measured at (pi; 0) point can be well explained by coupling of electrons to the magnetic resonance mode. The effect of Zn and Ni substitution on the antinodal ARPES spectra was shown to be in good agreement with the influence of these impurities on magnetic resonance mode seen in inelastic neutron scattering experiments. This, in addition to the previous ARPES studies of temperature and doping dependence of peak-dip-hump structure, mass renormalization near antinodal region and a kink in the nodal part of Brillouin zone, provides further evidence that the coupling to magnetic excitations, rather than to phonons, is responsible for the observed unusual renormalization. Unlike the well studied Bi-2212 family of cuprates, photoemission on YBCO-123 turns out to be much more complicated. The observed spectra have a strong contribution from a heavily overdoped surface component with the hole doping level of about x~0.30, which is weakly dependent on the sample stochiometry. Absence of any signs of superconductivity in the spectra of the overdoped component was argued to result from the unusually high doping level. This conclusion is supported by the fact that the overdoped bands give rise to the Fermi surface and band structure consistent with the predictions of the LDA calculations, as well as, by the dependence of the photoemission matrix element on the excitation energy, which closely follows that of the superconducting bulk component. Specific experimental geometry was used to enhance the signal coming from the superconducting component. In particular, experiments with circularly polarized light bundled with simple theoretical considerations enabled better separation of the surface and the bulk components. This type of experiments also suggests that the overdoped component is mainly localized in the topmost CuO2 bilayer, while the next bilayers in the YBCO-123 structure already represent bulk properties and retain superconductivity. Using partially Ca substituted samples it was possible to obtain spectra with a suppressed overdoped component. The likely reason for the suppression is a shift of the most probable cleavage plane from the Ba–O interface to the Y layer. Spectra from the Ca substituted sample clearly reveal a sizable superconducting gap, and strong renormalization effects in the vicinity of the antinodal point. The fact that the renormalization vanishes above Tc and has strong momentum dependence, diminishing away from the (pi; 0)/(0; pi) point, strongly suggests that the reason for this renormalization in YBCO-123 is coupling of the electronic subsystem to spin resonance, similar to the case of Bi-2212.

While in conventional superconductors coupling between electrons and phonons is known to be responsible for the electron pairing, for the high temperature superconductors the pairing media remains under debates. Since the interactions of electrons with other degrees of freedom (phonons, magnetic excitations, etc) manifest themselves by an additional renormalization in the electronic dispersion, they can be investigated by means of Angle Resolved Photoelectron Spectroscopy. In the work renormalization in two families of high Tc cuprates have been studied. Along the diagonal of the two-dimensional BZ, the renormalization effects are represented by an unusual band dispersion that develops a so-called ‘‘kink’’. In the vicinity of the (pi, 0) point of the BZ, where the order parameter reaches its maximum, the renormalization is noticeably stronger and makes itself evident even in the shape of a single spectral line measured for a fixed momentum. It was shown that for the Bi-2212 samples substitution of Cu atoms in Cu-O plane changes renormalization features in ARPES spectra both in nodal and antinodal parts of the Brillouin zone. The smearing of the dip in the in the spectral line shape measured at (pi; 0) point can be well explained by coupling of electrons to the magnetic resonance mode. The effect of Zn and Ni substitution on the antinodal ARPES spectra was shown to be in good agreement with the influence of these impurities on magnetic resonance mode seen in inelastic neutron scattering experiments. This, in addition to the previous ARPES studies of temperature and doping dependence of peak-dip-hump structure, mass renormalization near antinodal region and a kink in the nodal part of Brillouin zone, provides further evidence that the coupling to magnetic excitations, rather than to phonons, is responsible for the observed unusual renormalization. Unlike the well studied Bi-2212 family of cuprates, photoemission on YBCO-123 turns out to be much more complicated. The observed spectra have a strong contribution from a heavily overdoped surface component with the hole doping level of about x~0.30, which is weakly dependent on the sample stochiometry. Absence of any signs of superconductivity in the spectra of the overdoped component was argued to result from the unusually high doping level. This conclusion is supported by the fact that the overdoped bands give rise to the Fermi surface and band structure consistent with the predictions of the LDA calculations, as well as, by the dependence of the photoemission matrix element on the excitation energy, which closely follows that of the superconducting bulk component. Specific experimental geometry was used to enhance the signal coming from the superconducting component. In particular, experiments with circularly polarized light bundled with simple theoretical considerations enabled better separation of the surface and the bulk components. This type of experiments also suggests that the overdoped component is mainly localized in the topmost CuO2 bilayer, while the next bilayers in the YBCO-123 structure already represent bulk properties and retain superconductivity. Using partially Ca substituted samples it was possible to obtain spectra with a suppressed overdoped component. The likely reason for the suppression is a shift of the most probable cleavage plane from the Ba–O interface to the Y layer. Spectra from the Ca substituted sample clearly reveal a sizable superconducting gap, and strong renormalization effects in the vicinity of the antinodal point. The fact that the renormalization vanishes above Tc and has strong momentum dependence, diminishing away from the (pi; 0)/(0; pi) point, strongly suggests that the reason for this renormalization in YBCO-123 is coupling of the electronic subsystem to spin resonance, similar to the case of Bi-2212.

The gradual increase in global warming and environmental pollution has made low-carbon technologies an urgent need for the whole world. Superconducting technology, which is known for its extremely high conductivity and high power density, is capable enough to provide novel solutions, contributing to the future smart grid, thus aiding the power industry towards the realisation of a low-carbon and green planet. In recent decades, several industrial applications using superconducting technology have been developed. Of them, particularly in the power industry, a range of superconducting applications including superconducting magnetic energy storage (SMES), superconducting motors/generators, superconducting cables and superconducting fault current limiters (SFCLs) have been developed. Among them, SFCLs are one of the most promising and are successfully being implemented in power distribution networks. SFCLs exhibit low impedance during normal operation and gain considerable impedance under a fault condition, providing a new solution to the increasingly high fault current levels. However, most of the SFCL projects are limited to low-voltage and medium-voltage levels, there are very few successful operational trials of high voltage SFCLs. This thesis, for the first time, studies the comprehensive characteristics of solenoid type SFCLs based on second generation (2G) high temperature superconductors (HTS), which may be successfully implemented in power grids with high voltage levels. The main contributions of this work include three aspects: 1) proposing an innovative method for simulating the AC losses of the solenoid coils and an electro-magneto-thermal model for simulating the SFCL’s current limiting property; 2) comprehensive and in-depth comparison study concerning the application of the two types of non-inductive solenoid coils (braid type and non-intersecting type) in SFCLs both experimentally and numerically; and 3) the first and thorough discussion of the impact of different parameters such as pitch and radius of coils to the overall performance of braid type SFCLs and the validation of the braid type SFCL concept with a 220 V/300 A SFCL prototype. Based on these experimental and simulation works, the thesis provide strong guidance for the development of future non-inductive solenoid type SFCLs based on 2G HTS, which are promising for high voltage level power grid applications.

Aquesta tesi doctoral estudia la utilització de ceràmiques texturades monodomini de YBCO per l'aplicació de limitació de corrent. A la xarxa elèctrica hi ha curtcircuïts degut al creuament d'una o més línies. Aquests curtcicuïts representen un problema, no sol·lucionat, de cara a la interconnexió i a l'equipament de la xarxa elèctrica. Els limitadors de corrents amb superconductors es basen en l'abrupte transició entre l'estat superconductor i l'estat "nomal" d'aquests materials quan es supera el corrent crític (Jc). D'aquesta manera un limitador de corrent és transparent a la xarxa elèctrica i quan hi ha un curtciruït afegeix una alta impedància a la línia de manera que es limita el corrent de curtcircuït.

D'entre tots els materials proposats per la limitació de corrent, el YBCO és el que té millors prestacions degut a l'elevat Jc d'aquest material. En el cas dels materials ceràmics, que des del punt de vista econòmic resulten favorables, degut a la baixa conductivitat tèrmica i l'elevada corrent crítica, apareix una problemàtica degut a la transició localitzada en un petit volum del material. La zona de material transitada és petita (~mm3) i absorveix tota l'energia associada al curtcircuït de manera que el material es deteriora o destrueix.

L'objectiu d'aquesta tesi ha estat l'optimització d'aquestes ceràmiques i solucionar la problemàtica de la transició localitzada (punt calent). Disminuïnt Jc a l'hora que s'augmenta la resistivitat en l'estat normal (ro), es manté la potencialitat del material i s'aconsegueix augmentar el volum de material transitat fins al 100%. Per tal de disminuir Jc sense afegir una inhomogeneïtat al material s'ha seguit dues estratègies: el dopatge amb ions metàl·lics i el control del contingut d'oxígen del material.

El dopatge amb ions metàl·lics (Mg, Zn i Ni entre d'altres) destrueix localment el paràmetre d'ordre superconductor. El resultat d'aquest efecte és el de la disminució de Jc. En aquest treball s'ha optimitzat el procés d'obtenció de les ceràmiques texturades dopades amb ions metàl·lics de cara a reduir Jc. Gràcies a aquesta disminució controlada, s'ha aconseguit transitar el 100 % del material superconductor arribant a valors de camp elèctric de l'ordre de 2100 V/m quan en el cas del material amb alta Jc era de l'ordre de 500 V/m. Tot i aixó, no es va observar un augment de "ro" en dopar el material amb ions metàl·lics, per aixó es utilizar la segona estratègia: control del contigut d'oxígen del material.

El control del contingut d'oxígen en el material, mitjançant el tractament tèrmic adequat, permet sintonitzar Jc el valor dessitjat i a l'hora es produeix un fort augment en la rho. El valor de "ro" en el cas del material amb alta J-sub c és de l'ordre de 10 m-Omega-cm, per les mostres dopades amb ions metàl·lics no augmenta i es manté en el mateix valor. En el cas de les ceràmiques amb el contingut d'oxígen controlat la "ro" és de l'ordre de 40 m-Omega-cm. Gràcies a aquest valor de resistivitat i a la disminució de Jc, que porta a una transició del 100 % del volum del material, s'aconsegueixen valors de camp elèctric de l'ordre de 4850 V/m.

En conclusió controlant el valor de Jc s'ha aconseguit augmentar les prestacions del material en un factor 4 per les mostres dopades, i gràcies a l'elevat valor de "ro" de les mostres amb l'oxigen controlat s'arriba a un augment de fins a ~10 vegades de les prestacions del material.

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.

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.

Thesis (MEng)--Stellenbosch University, 2014.
ENGLISH ABSTRACT: High Temperature Superconductors (HTS) exhibit exceptional electrical properties that make them attractive candidates for numerous electronic devices and applications. However, constructing working devices can be challenging due to fabrication difficulties of these brittle ceramics. This thesis investigates new methods to make the fabrication of high quality YBa2Cu3O7 (YBCO) thin films easier and compatible with more materials. We present the development of a universal add-on method that can be used in situ to improve the quality of superconducting thin films deposited by Pulsed Laser deposition (PLD). We investigate the in situ application of electric fields and voltage biasing to improve the thin film growth. Considering various electrode configurations, we have developed a final electrode design that is stable and produces reproducible results. By introducing an insulated high voltage (HV) electrode into the chamber during deposition, the quality of the deposited thin films can be modulated depending on the polarity of the voltage applied. Applying a positive voltage improves the film quality obtained. Applying a negative voltage degrades the superconducting properties of the films. A simple proof-of-concept HTS dual-mode microwave filter was designed, fabricated and tested. Only the filter produced using our novel PLD technique displayed the correct filtering action upon cooling to 77K. This is attributed to the thin films better superconducting properties due to our developed technique.
AFRIKAANSE OPSOMMING: Hoë Temperatuur Supergeleiers (HTS) vertoon aantreklike elektriese eienskappe wat hulle goeie kandidate maak vir verskeie elektroniese toepassings. Om werkende toestelle te ontwikkel kan 'n uitdaging wees, as gevolg van die vervaardigings probleme wat bestaan vir hierdie bros keramiek materiaal. Hierdie tesis ondersoek nuwe metodes om die vervaardiging van 'n hoë gehalte YBa2Cu3O7 (YBCO) dun films makliker en versoenbaar te maak met verskeie materiale. Ons toon die ontwikkeling van 'n algemene metode wat maklik bygevoeg kan word om in situ die gehalte van supergeleidende dun films, wat deur gepulseerde laser deponering (PLD) gedeponeer is, te verbeter. Ons ondersoek die in situ toepassing van elektriese velde en spannings om die dun film groei te verbeter. Verder oorweeg ons verskeie elektrode konfigurasies en ontwikkel 'n finale elektrode ontwerp wat stabiel is en herhaalbare resultate produseer. Die kwaliteit van die gedeponeerde dun films kan gemoduleer word deur die byvoeging van 'n geïsoleerde hoogspannings (HV) elektrode tydens deponering, afhangende van die polariteit van die aangelegde spanning. 'n Positiewe spanning verhoog die film kwaliteit, terwyl 'n negatiewe spanning die supergeleidende eienskappe van die films verlaag. 'n Eenvoudige HTS dubbele-modus mikrogolffilter is ontwerp, vervaardig en getoets, om as toepassings voorbeeld te dien. Slegs die filter wat geproduseer was met behulp van ons nuwe PLD tegniek, vertoon die beste filter oordrag by 77K. Dit word toegeskryf aan die beter supergeleidende eienskappe van die dun film, as gevolg van die toepassing van ons ontwikkelde tegniek.

Thesis (Ph. D.)--Ohio State University, 2005.
Title from first page of PDF file. Document formatted into pages; contains xvii, 109 p.; also includes graphics (some col.). Includes bibliographical references (p. 105-109). Available online via OhioLINK's ETD Center

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.

La fabricació de capes superconductores nanocomposite d’YBa2Cu3O7-x (YBCO) mitjançant la incorporació de nanopartícules a la matriu ha demostrat haver realçat el rendiment d’ancoratge de vòrtexs sota camps magnètics aplicats i haver reduït l’anisotropia efectiva, garantint un gran potencial pel seu ús en nombroses aplicacions. Diferents concentracions, mides i processos de creixement de les nanopartícules condueixen a una rica varietat de defectes en les capes, llur efectivitat en ancoratge i relaxació de vòrtexs depèn de la temperatura i la magnitud i orientació del camp magnètic. En aquesta tesi, es presenta una amplia investigació en nanocomposites d’YBCO crescuts mitjançant la tècnica escalable i de baix cost “depòsit de solucions químiques” (CSD), en la que la incorporació de nanopartícules s’obté seguint dues estratègies diferents: nanopartícules de segregació espontània i nanopartícules prèviament formades. Mitjançant la combinació de mesures de transport elèctric amb l’anàlisi micro-estructural efectuat amb XRD o STEM, ha estat possible establir correlacions entre les propietats superconductores i el paisatge de defectes, la qual cosa ens ha permès separar contribucions d’ancoratge i relaxació de vòrtexs en les regions del diagrama camp-magnètic—temperatura i així preveure el millor paisatge per funcionar a certes condicions fins a camps magnètics molt intensos (35 T). S’ha demostrat que la incorporació de nanopartícules indueix altes densitats de falles d’apilament que afecten les contribucions d’ancoratge i relaxació de vòrtexs a qualsevol orientació. Grans forces d’ancoratge isotròpic sorgeixen a camps magnètics baixos i intermedis i a temperatures baixes i intermèdies. Per altra banda, les contribucions d’ancoratge anisotròpic es veuen altament alterades, especialment a camps magnètics intensos i temperatures altes. L’arranjament i la tipologia de les falles d’apilament induïdes per la incorporació de nanopartícules és determinant per la ponderació final de contribucions d’ancoratge. Aquí demostrem que l’ús de nanopartícules petites prèviament formades (7 nm) habilita el bon control d’una microestructura rica en falles d’apilament. S’ha identificat que un paisatge de defectes caracteritzat per una gran densitat de falles curtes i homogèniament distribuïdes és el millor dels paisatges possibles per promoure contribucions d’ancoratge isotròpic enormes atribuïdes al nanostrain localitzat a les vores de les falles d’apilament i a defectes atòmics que poden ser vacants de Cu-O situades en l'interior de les falles d’apilament. A més a més, la gran densitat de falles d’apilament és concomitant a una gran densitat de plans de macla, ambdós defectes beneficiosos per l’ancoratge anisotròpic quan el camp magnètic és paral·lel als plans-ab (H||ab) i a l’eix-c (H||c), respectivament. No obstant, la coherència dels plans de macla es trenca més sovint, cosa que fa que disminueixi la temperatura en la qual l’ancoratge anisotròpic és efectiu per H||c. Els nanocomposites gruixuts de nanopartícules prèviament formades han mostrat evitar significativament aquest trencament de la coherència i ser capaços d’assolir corrents crítiques altes a camps magnètics intensos i altes temperatures. Els falles d’apilament també han mostrat jugar un paper decisiu en l’impediment d’excitacions double-kink, que estimulen la relaxació de flux magnètic per H||c i especialment H||ab. A més a més, la contribució isotròpica de relaxació associada a les regions de nanostrain també es minimitza en el cas dels nanocomposites. En aquest treball es mostra que els nanocomposites proveeixen al mateix temps ancoratge més fort i relaxació més lenta de flux magnètic, especialment a camps magnètics baixos i intermedis i a temperatures baixes i intermèdies. La regió d’aquestes bones propietats es pot eixamplar més a camps i temperatures més alts mitjançant nano-enginyeria addicional, comptant amb que s’ha vist que diferents paisatges de defectes poden ser particularment interessants per diferents condicions donades.
The fabrication of superconducting YBa2Cu3O7-x (YBCO) nanocomposite films by the incorporation of nanoparticles in the matrix has demonstrated to strongly enhance the vortex pinning performances under applied magnetic fields and to reduce the effective anisotropy, ensuring great potential for their use in a broad number of applications. Different nanoparticle concentrations, sizes and growth process conditions lead to a rich variety of defects in the films, whose vortex pinning and vortex creep effectiveness depends on temperature and the magnitude and orientation of the magnetic field. In this thesis, it is presented an extensive research of YBCO nanocomposites grown by the scalable and low-cost chemical solution deposition (CSD) technique, where the incorporation of nanoparticles is obtained following two different approaches: spontaneous segregated nanoparticles and preformed nanoparticles. By the combination of electrical transport measurements with XRD and STEM microstructural analysis, correlation between superconducting performance and the defect landscape has been possible, allowing us to separate pinning and creep contributions in the regions of the magnetic-field--temperature diagram and therefore foresee the best landscape to operate at certain conditions up to very high magnetic fields (35 T). It has been demonstrated that the incorporation of nanoparticles induces large densities of stacking faults which strongly affect the pinning and creep contributions in all orientations. Large isotropic pinning forces arise at low-intermediate magnetic fields and at low-intermediate temperatures and anisotropic pinning contributions are strongly altered, especially at high magnetic fields and temperatures. The arrangement and the typology of the stacking faults induced by the incorporation of nanoparticles is determinant for the final balance of vortex pinning contributions. We demonstrate that the use of preformed small nanoparticles (7 nm) enables a very good control of the stacking-fault-rich microstructure. A defect landscape characterized by a large density of homogeneously distributed short stacking faults has been identified as the best one to promote huge isotropic pinning contributions, which are ascribed to the nanostrain located at the edges of stacking faults and to atomic defects which may be Cu-O vacancies hosted by stacking faults. Furthermore, the large density of stacking faults is concomitant with a large density of twin boundaries, both beneficial for the anisotropic pinning when the magnetic field orientation is parallel to the ab-planes (H||ab) and the c-axis (H||c) respectively. However, the coherence of twin boundaries is commonly broken, which reduces the temperature where anisotropic pinning is effective for H||c. Thick nanocomposites from preformed nanoparticles have shown to significantly avoid this coherence segmentation and be able to afford large critical currents at high magnetic fields and high temperatures. Stacking faults have been also found to play a decisive role for the preclusion of double kink excitations, which boost magnetic flux creep for H||c and especially H||ab. Furthermore, the isotropic flux creep contribution associated to the nanostrained regions is also reduced in nanocomposites. In this work, it is shown that nanocomposites provide simultaneously higher flux pinning and lower flux creep especially at low-intermediate temperatures and at low-intermediate magnetic fields. The region of this outstanding performance can be enlarged to larger fields and temperatures by further nanoengineering, where it has been shown that different defect landscapes can be particularly interesting for given operating conditions.

El objetivo general de este trabajo ha sido el de obtener muestras superconductoras de alta temperatura de YBCO cuyas geometrías y dimensiones superen a las obtenidas mediante técnicas de crecimiento cristalino. Estos materiales pueden ser integrados en maquinaria eléctrica, sistemas de levitación o limitadores de corriente. Este objetivo ha promovido la investigación hacia métodos que permiten unir dos o más monodominios del material superconductor de alta temperatura YBCO mediante un proceso de solidificación por fundido utilizando agentes soldantes cuyos puntos de fusión están por debajo del punto de fusión del material que se quiere unir, el YBCO
El primer paso en obtener soldaduras superconductoras de alta temperatura del material YBCO ha sido el de conseguir un agente soldante apropiado. Hemos investigado dos materiales basados en plata como agentes soldantes: polvo de óxido de plata y una lamina de plata metálica. Análisis microestructural, junto con medidas de magnetorresistencia y magnetometria Hall se han realizado con el fin de encontrar las condiciones óptimas para conseguir soldaduras superconductoras de alta temperatura de alta calidad.
Para entender el proceso de difusión de la plata en la matriz YBCO, se ha realizado un detallado estudio de la influencia de los parámetros del proceso de la soldadura en la microestructura y las propiedades físicas de las uniones superconductoras, es decir la magnetización remanente y la densidad de corriente critica, generadas mediante este método. Se ha demostrado que estos parámetros influyen tanto en la microestructura como en las propiedades superconductoras de las uniones artificiales. Para optimizar estos parámetros se han realizado dos tipos de experimentos: enfriamiento muy rápido y crecimiento mediante enfriamiento lento. Utilizando los experimentos de enfriamiento muy rápido hemos conseguido determinar cuales son los parámetros que controlan la difusión de la plata en la matriz del material YBCO. Se ha demostrado que parámetros como: tiempo del fundido, el grosor de la lámina de plata y la configuración de la soldadura son relevenates y han sido optimizados. Por otro lado, la influencia de parámetros como: la velocidad de enfriamiento, temperatura máxima del proceso de la soldadura y la ventana de temperaturas en la microestructura de las uniones artificiales se han analizado mediante experimentos de enfriamiento lento.
La influencia de estos parámetros en las propiedades superconductoras de las soldaduras se ha estudiado a través de medidas de magnetización remanente de las uniones y del material YBCO, a la vez para su comparación. La distribución de la magnetización remanente ha sido investigada utilizando magnetometria Hall. Estas medidas nos han permitido deducir la magnitud de la densidad de corriente crítica resolviendo el problema inverso (ley Biot-Savart).
Utilizando la metodología desarrollada en este trabajo hemos conseguido obtener uniones artificiales del material YBCO cuyas microestructuras y propiedades superconductoras tienen una calidad similar a las del material inicial YBCO y por lo tanto se ha desarrollado una metodología que permitirá obtener ceramicas superconductoras con dimensiones mayores y formas complejas.
The final goal of this PhD thesis was to obtain YBa2Cu3O7- (YBCO) samples with different geometries and bigger dimensions than those obtained by the crystalline growth techniques. These materials can be integrated in electrical machinery, levitating devices or fault current limiter systems. This need has driven the investigation to welding techniques that can joint two or more YBa2Cu3O7- single-domains by a solidification process using welding agents whose melting points are below the melting point of YBa2Cu3O7- material.
The first step in achieving high temperature superconducting YBCO joints was to find a suitable welding material. We have investigated two Ag based materials as welding agents: Ag2O powder and Ag thin foil. Microstructural analysis along with magnetoresistance and in-field Hall mapping measurements have been performed to find the conditions to reach a high quality superconducting joints.
A deep study of the influence of different parameters on the microstructure and on the superconducting properties of the final joints, i.e. remanent magnetization and critical current density of the final joints, generated using a YBCO/Ag/YBCO architecture has been performed in order to understand the role of the Ag diffusion and to optimize the welding process. It has been shown that these parameters influence in one way or another the microstructure and superconducting properties of the final joints. In order to optimize these parameters, we have performed two kinds of experiments: quench experiments and slow cooling experiments. By quench experiments we have succeed in controlling the Ag diffusion process into the YBCO matrix. Parameters such as: melting time, Ag foil thickness and weld configuration have been investigated and optimized. On the other hand, the influence of parameters such as: cooling rate, processing temperature and window temperature, on the microstructure of the final joints has been analyzed by slow cooling experiments.
The influence of welding parameters on the superconducting properties of the final joints has been studied by determining the remanent magnetization profiles of each sample and calculating from them the critical current density of the joints and YBCO grains for comparison. The remanent magnetization distribution on samples joint by the welding process was investigated by using a Hall probe imaging system. These measurements allowed us to deduce the magnitude of the critical current densities by solving the inverse problem, as well as the homogeneity and spatial scale on which they flow.
We have proposed a methodology in order to determine the critical current density from the results obtained after solving the inverse problem. We have observed that the current distribution pattern obtained from this methodology agrees well with current distribution profile predicted by Bean model.
In summary, by employing the new welding methodology developed in the present work, we have been able to obtain YBCO superconducting joints having a clean and crystallographic coincident microstructure and with critical current densities through the joint similar to those of the YBCO monoliths.

In high-temperature superconductors a large region of the magnetic phase diagram is occupied by a vortex phase that displays a number of exciting phenomena. At low temperatures, vortices form a truly superconducting solid phase which at high temperatures turns into a dissipative vortex liquid. The character of the transition between these two phases depends on the amount and type of disorder present in the system. For weak point disorder the vortex solid-to-liquid transition is a first-order melting. In the presence of strong point disorder the solid is thought to be a vortex-glass and the transition into the liquid is instead of second order. When the disorder is correlated, like twin boundaries or artificially introduced columnar defects, the transition is also second order, but has essentially different properties. In this work, the transition between the solid and liquid phases of the vortex state has been studied by resistive transport measurements in mainly YBa2Cu3O7-[delta](YBCO) single crystals with different types of disorder.

The vortex-glass transition has been investigated in an extended model for the vortex-liquid resistivity close to the transition that takes into account both the temperature and magnetic field dependence of the transition line. The resistivity of samples with different properties was measured with various contact configurations at several magnetic fields and analyzed within this model. For each sample, attempts were made to scale the transition curves to one curve according to a suitable scaling variable predicted by the model. Good scaling was found in a number of different situations. The influence of increasing anisotropy and angular dependence of the magnetic field in the model were also considered.

The vortex solid-to-liquid transition was also studied in heavy-ion irradiated YBCO single crystals. The ions create columnar defects in the sample that act as correlated disorder. A magnetic field was applied at a tilt angle with respect to the direction of the columns. At the transition the resistance disappears as a power law with different exponents in the three orthogonal directions considered. This provides evidence for a new type of critical behavior with fully anisotropic critical scaling properties not previously found in any physical system.

The effect on the vortex solid-to-liquid transition of high magnetic fields applied parallel to the superconducting layers of underdoped YBCO single crystals was also studied. Some novel features were observed: a sharp kink appearing close to Tc at high magnetic fields and a triple dip in the angular dependence of the resistivity close to B||ab in some regions of the phase diagram.

I högtemperatursupraledare består en stor del av det magnetiska fasdiagrammet av en vortexfas som uppvisar ett flertal spännande fenomen. Vid låga temperaturer bildar vortexarna en fast vortexfas utan elektriskt motstånd. Vid högre temperatur övergår denna fas till en dissipativ vortexvätska. Egenskaperna hos denna fasövergång beror på oordningen i form av defekter. Vid svag punktoordning är fasomvandlingen mellan det fasta och flytande vortextillståndet en första ordningens smältövergång. Vid stark punktoordning anses den fasta fasen vara ett vortexglas och övergången till vortexvätskan är istället av andra ordningen. När oordningen är korrelerad, som för tvillinggränser eller artificiellt skapade kolumndefekter, är övergången också av andra ordningen men med väsentligt annorlunda egenskaper. I detta arbete har övergången mellan det fasta och det flytande vortextillståndet studerats med resistiva transportmätningar i framförallt enkristaller av YBa2Cu3O7-[delta] (YBCO) med olika typer av oordning.

Vortexglasövergången har undersökts i en utvidgad modell för resistansen i vortexvätskan nära fasövergången där hänsyn tas till såväl temperatur- som fältberoendet. Resistansen hos prover med olika egenskaper mättes i varierande magnetfält och i flera kontaktkonfigurationer och analyserades inom denna modell. Övergångskurvorna skalades till en kurva med en skalningsvariabel som givits av modellen. God skalning uppnåddes i flera olika fall. Effekten av ökande anisotropi och vinkelberoendet i modellen undersöktes också.

Vortexövergången mellan det fasta och det flytande vortextillståndet undersöktes även i enkristaller av YBCO bestrålade med tunga joner. Jonerna skapade kolumndefekter som fungerar som korrelerad oordning. Vinkeln mellan pålagt magnetfält och dessa kolumndefekter varierades. Vid fasövergången avtar resistansen som en potenslag med olika exponenter i de tre undersökta ortogonala riktningarna. Detta ger experimentell belägg för en ny typ av kritiskt beteende med fullständigt anisotropa kritiska skalningsegenskaper.

Egenskaparna hos på vortexövergången mellan fast och flytande fas vid höga magnetfält parallella med de supraledande lagren hos underdopade YBCO enkristaller undersöktes också. Några nya effekter observerades: en skarp knyck uppstod nära Tc vid höga magnetfält och en tredubbel dipp i den vinkelberoende resistiviteten nära B||ab i några regioner av fasdiagrammet.

Orientadora: Profa. Dra. Marcia Tsuyama Escote
Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2016.
Este trabalho descreve o processo de sintese dos compostos YBa2(Cu1-xNix)3O7-¿Â (x = 0; 0,01; 0,02 e 0,04) com diferentes microestruturas. As amostras foram preparadas na forma de fios micrometricos e pos a partir do metodo de precursores polimericos e pelo metodo de eletrofiacao, respectivamente. No caso das amostras eletrofiadas, varios fatores foram avaliados como o solvente, o polimero, a viscosidade, tensao aplicada entre a agulha e a distancia anteparo e agulha. As melhores condicoes utilizadas para produzir os fios de YBa2(Cu1-xNix)3O7-¿Â foi a que utilizou como solvente uma solucao contendo acido propionico-metanol-acido acetico e o polimero PVP, que auxilia no processo de eletrofiacao. Apos a etapa de sintese, as condicoes de tratamento termico foram estimadas a partir de medidas de analise termogravimetrica e analise termica diferencial. A partir destes resultados as amostras na forma de fios e pos foram tratadas termicamente entre temperaturas variando de 350 a 925 ¿C em atmosfera de oxigenio. As propriedades fisicas das amostras foram caracterizadas por meio de microscopia eletronica de varredura (MEV), de difracao de raios X (DRX), medidas de magnetizacao e transporte eletrico. O estudo de formacao da fase cristalina YBCO tambem foi analisado por meio de DRX, que verificou os resultados previos da analise termica e indicou que as melhores caracteristicas cristalograficas foram alcancadas quando as amostras foram tratadas termicamente a temperaturas > 900 ¿C/O2. Analise dos dados de DRX indicaram que todas as amostras cristalizam em uma estrutura ortorrombica (grupo espacial Pmmm), no entanto a maioria das amostras apresentaram picos pertencentes a fases adicionais identificadas como o BaCuO2 e CuO. Apesar disto, todas foram analisadas pelo metodo de Rietveld, os dados estruturais obtidos revelaram valores de parametros de rede proximos aqueles listados na literatura. Os parameros a, b e c das amostras de fios, apresentaram valores ligeiramente menores dos aqueles obtidos para os pos, tambem notou-se uma clara reducao do volume da cela unitaria, o que acredita-se estar relacionado ao metodo de sintese. Os parametros de rede c apresentam valores proximos daqueles esperados para a estequiometria YBa2Cu3O7. As amostras na forma de fios tambem apresentaram uma menor proporcao de fase adicional. As caracteristicas microestruturais verificadas por meio de imagens de MEV revelou que as amostras produzidas a partir de fios de YBa2(Cu1-xNix)3O7 perderam a forma de fio quando tratadas termicamente, apenas a amostra com x = 0 mantem a forma de fio. Estas apenas mantem a forma se tratadas termicamente a tempos e temperaturas menores, mas nestas condicoes perdem suas caracteristicas supercondutoras. Por isso, as amostras submetidas a tratamentos termicos em temperaturas 925¿C/O2 foram caracterizadas por meio de medidas de magnetizacao e transporte eletrico. As caracterizacoes magneticas revelaram que as amostras preparadas a partir dos pos e fios apresentaram a transicao supercondutora em Tc ~ 90-92 K, sendo que a substituicao de Cu por de Ni promoveu a diminuicao de Tc. E importante observar que nos pos esta variacao foi de 92 para 88 K para amostras com x variando de 0 a 0,04, enquanto que para as amostras eletrofiadas esta variacao foi de 92 para 90,2 K. Nas medidas de transporte eletrico verificou-se que a substituicao Cu por Ni nao promoveu variacoes significativas de Tc que foi da ordem de 86-88 K tanto para as amostras obtidas de pos quanto de fio. De forma geral, este trabalho produziu pos e fios de YBCO com qualidade similar a descrita na literatura, a eletrofiacao tambem permitiu a fabricacao de amostras de YBa2(Cu1-xNix)3O7 (x= 0,01; 0,02 e 0,04) com caracteristicas estruturais e supercondutoras ligeiramente diferentes daquelas obtidas por meio de sintese convencional de po.
This work describes the synthesis process of YBa2(Cu1-xNix)3O7-ä (x = 0, 0.01, 0.02 and 0.04) compounds with different microstructure. Micrometric wires and powder samples were prepared through electrospinning and polymeric precursor method, respectively. In the case of electrospinning samples several factors were evaluated like the solvent, polymer, viscosity, electrode-needle applied voltage and the work distance. The best conditions to obtain YBa2(Cu1-xNix)3O7-ä wires were the solution with propionic acid- methanol-acetic acid solvents, and with PVP polymer, which usually helps the electrospinning process. After the synthesis, the heat treatment conditions were estimated through thermogravimetric analysis and differential thermal analysis. With these results, two baths of wires and powders samples were heat-treated in temperatures varying from 350 to 925 °C in oxygen atmosphere. All samples physical properties were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), magnetic and electrical transport properties. The YBCO crystalline phase formation was also analyzed through XRD, it showed a similar results than those from thermal analysis. These results showed that better crystallographic characteristics were reached when the samples were heat treated in temperatures > 900 °C/O2. XRD analysis indicated that all samples crystallized in orthorhombic structure (Pmmm space group symmetry), though most of them presents peaks of addition phases identified as BaCuO2 and CuO. Even though all samples were analyzed through the Rietveld method, such structural data revealed lattice parameters values close to those reported in literature. It should be noticed that a, b and c values of the wire samples were slight smaller than those obtained to the powder samples, also there is a clear reduction of the unit cell volume, which we believe is related to the synthesis method. The c lattice parameters present values close to those expected to the YBa2Cu3O7 stoichiometry. The wire samples also present a small amount of additional phases. Microstructural characteristics were verified through SEM images, which revealed that the YBa2(Cu1-xNix)3O7 wire samples lost the wire shape after the heat treatment, only the YBCO samples maintain the wire shape. Samples only maintain this shape when they were heat treated at smaller times and lower temperature, but in this case they lost their superconductor characteristic. So, only samples obtained through heat treatments at 925°C/O2 were characterized by magnetization and electric transport. Magnetic measurements of samples prepared from powder and wires revealed a superconductor transition temperature in Tc ~90-92K, whereas the Cu for Ni substitution promoted a decrease of Tc. It is important to notice that Tc values vary from 92 to 88 K for the powers samples with x varying from 0 to 0.04, though for electrospinning samples this variation is from 92 to 90.2 K. In electrical transport measurements it was verified that such Cu to Ni substitution do not promote significant variations in the Tc, which was of the order of 86-88 K for both samples produced by powder and wires. As a general remark, this work produced powders and wires of YBCO with similar quality than those described in literature. Also, the electrospinning allowed the preparation of YBa2(Cu1-xNix)3O7 (x= 0.01; 0.02 e 0.04) samples with structural and superconductors characteristic slightly different from those obtained through powder conventional synthesis.

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements.

In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.

The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa₂Cu₃O₇ (YBCO) and MgB₂ thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.

In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.

The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased.

From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

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.

High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa₂Cu₃O_7-δsingle crystals, from the superconductingfluctuations above Tcdown to the vortex matter near the onset of resistivity.Superconducting fluctuations above Tcare suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, forI||ab. ForH||c, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. ForH||ab, Δσbdepended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.

The nature of the vortex phase below Tccan be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to Tcwas strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.

The effect of anisotropy on vortex-liquid correlation infieldsH||ab, was studied byc-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with theabplane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.

Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.

Keywords:high-temperature superconductors, YBa₂Cu₃O_7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation

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.

碩士
國立中央大學
電機工程學系
105
In 1911, the scientist Heike Kamerlingh Onnes found mercury superconductivity at a low temperature of 4.2K. Superconducting materials have zero resistance, anti-magnetic characteristics, and can be applied to a wide range of levels, such as nuclear magnetic resonance (nuclear magnetic resonance, NMR). In our article, we perform performed a study of temperature dependence of thermal conductivity, Seebeck coefficient, and resistivity on YBa_2 Cu_3 O_(7-δ) sample and try to explain the trend of YBCO thermoelectric value by existing theory.

碩士
國立成功大學
材料科學及工程學系碩博士班
93
The magnitude of trapped field within bulk superconductors is proportional to the critical current density and the size of single grain. So it is necessary to improve the flux pinning ability and produce larger sized single grain REBCO, which are both key issues to practical applications. 　In this study, we focus on the enhancement of Jc value in Y-Ba-Cu-O superconductor by doping various RE(Rare Element) compound to react with matrix phase. And then we search for the optimum doping amount and particle size with these additions. The results reveal that doping 1w%~2wt% CeO2 in Y-Ba-Cu-O sample can enlarge its Jc value substantially under low-field and avoid Jc value descending down rapidly along with the increasing field. And the micro-scale CeO2 can do the more improvement than nano-scale CeO2 do. 　In the other serious research, we dope nano-scale Sm and Nd compound in Y-Ba-Cu-O superconductor. The results show the 0.1wt%nano-scale Sm211 additions can enhance the Jc value especially under high field. But in the Nd compound doped sample, the Nd atom will replace Ba atom in the superconductive crystal to destroy its superconductivity, which make its Jc value still at the low statement.