High-temperature superconducting materials have found considerable technological application and still have a largely unrealised potential. The key to unlocking this potential depends on a better understanding of their properties; in particular, the maximum 'critical current density' which these materials, in the form of wires, tapes, thin-films and bulk monolithic forms, are able to support for high-current applications. The 'critical current density' in a polycrystalline high-temperature oxide superconductor system is determined by a percolation process of the super current through a three dimensional grain-boundary network. Grain-boundaries in these systems behave as Josephson junction 'weak-links' and they severely limit the critical current density in the presence of even moderate self or applied magnetic fields. In the present work, isothermal quasi-static magnetisation measurements on the polycrystalline YIBa2Cu307-x system are presented and analysed. An effective granular penetration depth in conjunction with a critical state model, which includes an approximate treatment for the percolation process, is used to obtain many of the salient physical parameters of the grain-boundary Josephson junctions and of the three-dimensional grain boundary junction network. Determination of the temperature and magnetic field dependence of several of these parameters, in particular a magnetic field-independent critical current which depends on the micro structure of the grain-boundary junction network, allows for testing and verification of models of the weak-link and network behaviour. This treatment has been carried out specifically on various silver doped polycrystalline Y1Ba2Cu30 7-x specimens in order to determine and quantify the effects of silver doping. An improvement in the critical current density with silver doping is explained in terms of silver scavenging and ridding grain-boundaries of impurities, and a proximity effect where trace amounts of silver residing in the grain-boundaries decreases the normal resistance of the grain-boundary Josephson junction. The insight gained from silver doping experimentation led to a macroscopic investigation into the joining of large single-domain YIBa2Cu307-x specimens for large-scale applications. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2006.

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references: leaf 152-160.

Thesis (Ph. D.)--Ohio State University, 2007.

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.

No description available.

Les cintes superconductores d'alta temperatura han sorgit com un material molt prometedor per aplicacions de potència en alts camps magnètics ja que s'ha demostrat la possible utilització del material YBaCuO-123 (YBCO) per produir cintes superconductores amb prestacions de densitat de corrent crítica de l'ordre de 1MA/cm2 a 77K sota camps magnètics elevats (1-5T). Les cintes superconductores de YBCO estan formades per una estructura bi-dimensional de fronteres de gra de baix angle (FGBA) (~4º-8º) induïdes per la textura del substrat metàl·lic flexible sobre el qual estan dipositades amb previ condicionament mitjançant el dipòsit de capes tampó. La granularitat present en aquests materials representa la limitació més important al pas de corrent elèctrica i estableix la presencia de dos mecanismes de dissipació interactuants associats per una banda al pas de corrent percolativa a través de les FGBA (limitada per la densitat de corrent crítica associada a les fronteres de gra JcFG) i per altra banda a la corrent que flueix al interior dels grans superconductors (limitada per la densitat de corrent crítica associada als grans, JcG). Si be JcG esta clarament determinada per l'ancoratge dels vòrtex Abrikosov en els grans superconductors, la dissipació associada a JcFG es encara un interrogant. Estudis teòrics han predit l'existència d'una nova classe de vòrtex superconductors associats a les fronteres de gra que han estat anomenats vòrtex Abrikosov-Josephson.En aquesta tesi doctoral hem analitzat la resposta electromagnètica de les cintes superconductores de YBCO mitjançant mesures magnètiques de cicles de histèresi menors. A partir de la forma característica dels cicles de histèresi s'ha desenvolupat un formalisme que ens ha permès determinar de manera independent i simultània les densitats de corrent crítica JcG i JcFG a baix camp magnètic, en el marc del model d'estat crític. La metodologia desenvolupada ens ha permès estudiar la relació existent entre els efectes de granularitat, l'ancoratge de vòrtex Abrikosov als grans i la dissipació associada al moviment de vòrtex Abrikosov-Josephson a les FGBA i per tant ens ha permès inferir en els mecanismes físics que regeixen el transport elèctric d'aquests materials. Hem realitzat un estudi complert de la relació entre aquests mecanismes variant diferents paràmetres característics de les cintes superconductores tal com l'angle mig entre fronteres de gra, el tamany de gra, el gruix de la capa superconductora o el procés de creixement. Addicionalment, mitjançant mesures de susceptibilitat ac, hem identificat i caracteritzat diferents dominis de percolació de corrent en diverses cintes superconductores de YBCO associats a diferents valors de JcFG. Aquest estudi ens ha permès complementar l'anàlisi magnètic de la granularitat inherent a aquests materials i caracteritzar la dissipació associada. A partir de l'estudi de les corbes de susceptibilitat ac mesurades a diferent freqüència hem determinat l'energia d'activació tèrmica a diferents camps magnètics i diferents densitats de corrent. Comparant els valors obtinguts amb els valors de Jc mesurats mitjançant magnetització dc hem pogut determinar un diagrama de fases magnètic (camp magnètic/temperatura) per a les cintes superconductores de YBCO on es poden distingir diferents règims de moviment de vòrtex. / High temperature coated conductors has emerged as a very promising material for power applications at high magnetic fields since it has been demonstrated the possible utilization of YBaCuO-123 (YBCO) for performing coated conductors able to support current densities higher than 1MA/cm2 at 77K and high magnetic fields (1-5T). YBCO coated conductors consist on a bi-dimensional structure of low angle grain boundaries (LAGB) (~4º-8º) induced by the texture of the metallic tape used as a substrate, which is usually covered with several buffer layers.The granularity present in this materials represent the major limitation for the electrical current to flow and establish the presence of two different interactive dissipation mechanisms associated on one hand to the percolative current flowing through the LAGB (limited by the grain boundary critical current density, JcGB) and on the other hand to the current flowing inside the superconducting grains (limited by the grain critical current density, JcG). While JcG is clearly limited by the Abrikosov vortex pinning inside the superconducting grains, the dissipation associated to JcGB is still an open issue. Theoretical studies has predicted the existence of a new kind of vortex located at the grain boundaries called Abrikosov-Josephson vortices.In this Thesis we have analyzed the electromagnetic response of YBCO coated conductors by means of magnetic measurements of minor hysteresis loops. From the characteristic feature of the hysteresis loops we have developed a new formalism which has allowed us to determine independently and simultaneously both critical current densities, JcG i JcGB, at low applied magnetic field, in the framework of the Critical state Model. The methodology devolved has allowed us to study the relationship between granularity effects, Abrikosov vortex pinning inside the grains and the dissipation associated to the motion of Abrikosov-Josephson vortices at the LAGB and thus, it has allowed us to infer in the physical mechanisms which governing the electric transport in these materials. We have performed a complete study of the relationship between these mechanisms by changing different characteristic parameters of the YBCO coated conductors such as the average misorientation angle in the grain boundaries, the grain size, the thickness of the YBCO layer or the growing process.Additionally, by means of ac susceptibility measurements, we have identified and characterized different current percolative domains in several YBCO coated conductors associated to different values of JcGB. This study has allowed us to complement the analysis of magnetic granularity inherent at these materials and characterize the associated dissipation. By means of the study of ac susceptibility curves measured at different frequency we have determined the thermal activation energy of YBCO coated conductors at several magnetic fields and current densities. By comparing the resultant values with the values of Jc measured by means of dc magnetization we have determined a general magnetic phase diagram (magnetic field/temperature) for YBCO coated conductors where we have distinguished different vortex motion regimes.

This dissertation describes experimental studies of how a spatially alternating magnetic field can effectively pin the magnetic flux in a superconducting thin film (Pb 82 Bi18), thereby enhancing the superconductivity. The spatially alternating magnetic field was provided by a periodic array of nano-sized magnetic structures: 300 nm spacing triangular array of cobalt rods with 100 nm diameter and 300 nm height. The superconducting film deposited on top of the magnetic structures, or an embedded Ferromagnet- Superconductor Hybrids (FSH), showed enhanced critical current and critical magnetic field. The embedded FSH also showed the field matching effect, the field compensation effect, and hysteresis. This dissertation also explains how to fabricate and characterize magnetic nano- structures. Electron beam lithography and electroplating method were used to fabricate the magnetic nanostructures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the structures of the magnetic rods. Magnetic force microscopy (MFM) was used to study their magnetic properties.

No description available.

Thesis (Ph.D.)--University of Wollongong, 2006. / Typescript. Includes bibliographical references.

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Cesar Luongo, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 23, 2004). Includes bibliographical references.