No description available.

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.

YBa2Cu3O7-d (YBCO) was one of the first high temperature superconductors discovered, and its superconducting properties are strongly dependent on oxygen stoichiometry. A large amount of work has been done on the variation of stoichiometry and its effect on the superconducting properties of the material. However, in spite of all the work done, the results published in the literature are very scattered. This thesis presents a thermogravimetric study of oxygen diffusion in YBCO under isothermal and non-isothermal conditions and tries to reconcile the data available based on the results obtained and taking into account the factors that may have affected the data presented by other groups, such as the effects of the microstructure and the different diffusion coefficients measured with the techniques used. An Arrhenius expression for the chemical diffusion of oxygen has been calculated from the analysis of isothermal oxygenation data, and it has been corroborated by a study of the nonisothermal experiments carried out. This work includes the development of a macroscopic model for oxygen diffusion in YBCO based on the diffusion coefficient calculated from experimental data. The model is used to simulate for the first time oxygenations under both isothermal and non-isothermal conditions. The study of non-isothermal oxygenations has led directly to the design of novel cooling procedures that can be introduced at the end of the processing stage of YBCO samples, producing highly oxygenated specimens in shorter times than for conventional isothermal and ramped oxygenation procedures. The final section of this dissertation presents a study of the Direct Current Zoning effect. The generation of a mobile hot zone in a polycrystalline YBCO bar when passing a current across it is directly linked to the diffusion of oxygen ions in the material. A mechanism for the motion of the zone along the sample has been suggested. A computer model has been developed to reproduce this process taking into account the motion of ions due to chemical diffusion and the potential difference established. The results from this model have corroborated the mechanism suggested and give for the first time the opportunity to study this phenomenon in more detail.

A coated conductor is generally fabricated by depositing a high Tc superconducting layer onto a flexible metallic substrate, using intermediate buffer layers to prevent chemical interaction. In order for the superconductor to be capable of carrying a high current density, its grains must have good crystallographic alignment in order to avoid the presence of high angle grain boundaries. This can be ensured by transferring the texture from the substrate through epitaxial film growth. The main substrate considered in this thesis is a Ni-Fe alloy. When cold-rolled, NiFe develops a preferential orientation and upon annealing at an elevated temperature, undergoes primary recrystallisation to form grains with the cube texture {100}<001>. There crystallisation process and the texture of the tapes has been examined and various buffer layers have been fabricated. As silver does not react adversely with high temperature superconductors, it has been deposited onto Pd-buffered NiFe by DC sputtering and very sharp cube texture is obtained. Ceramic buffer layers, CeO2 and YSZ, have been deposited by RF sputtering, though an undesirable (111) oriented component accompanies the cube textured material. Also a technique has been developed to produce a suitably oriented native oxide of NiFe by a simple oxidation technique. Preliminary attempts to deposit YBCO films onto these buffer layers have shown that the quality of the metallic buffers is degraded by rapid inter-diffusion at elevated temperatures, but that cube textured material can be deposited on the oxide buffer layers. The percolative nature of current flow in such coated conductors has been considered through the development of a grain network model. As the texture of the superconducting layer is directly influenced by the underlying layers, measurements from the substrate and buffer layers are applied in order to model the orientations of the grains in a superconducting overlayer. The model calculates the critical current of coated conductors as a function of parameters such as length, width, grain size and texture, as well as examining factors such as cracks and highly misoriented grains.

This thesis describes applications of the ac Josephson effect. Firstly, results are presented from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at 72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured capacitance was less than that expected from a calculation of the parallel plate shunt capacitance. The explanation was thermal noise suppression of the hysteresis and the junction saw the shunt capacitor as a distributed impedance rather than a lumped circuit element. It was found during these investigations that the influence of the SrTiO3 substrate on the intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at 4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The dispersion relation consisted of two branches, one at low frequencies with a high resonator capacitance per unit length and a high frequency branch with a low resonator capacitance per unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3, which drops above the soft optic phonon frequency. From the dispersion relation, the permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz. The ac Josephson effect was exploited to measure the permittivity of thin films of SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators. The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the 50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz. The grain boundary capacitance was used to probe grain boundary current transport. The capacitance per unit area scaled inversely with resistance area product and increased linearly with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24° bicrystals. This behaviour could not be explained by tunneling models of grain boundary current transport, and requires current flow over a fraction of the area of the grain boundary.

No description available.

Ce travail concerne l'étude de la texture et de la microstructure en relation avec les propriétés supraconductrices Tc et Jc de supraconducteurs haute température critique SHTc : YBCO et NBCO. Les techniques de caractérisation utilisées pour la microstructure sont la diffraction de RX et la diffraction d'électrons (Electron Back-Scatter Diffraction) "EBSD". Tc et Jc ont été déduits des mesures d'aimantation et de susceptibilité magnétique au SQUID (Superconducting Quantum Interference Device) et PPMS (Physical Properties Measument System). La première partie de l'étude traite de la caractérisation de la texture de couches minces SHTc d'YBCO, et de la possibilité de déterminer les relations d'épitaxie par EBSD entre le film déposé et la couche tampon. Ce type d'échantillon est constitué d'un substrat de saphir sur lequel 3 couches successives ont été déposées dont 300 nm de YBCO, destinées aux applications électrotechniques comme limiteurs de courant. La deuxième partie concerne l'étude de matériaux massifs supraconducteurs YBCO et NBCO fabriqués par différentes techniques. L'effet du dopage d'YBCO avec de l'argent a été étudié sur des échantillons fabriqués par la technique dite de "texturation par croissance orientée" soit MTG (Melt Textured Growth) et sous une faible pression partielle d'oxygène. Le dopage jusqu'à un certain pourcentage d'argent améliore la microstructure ainsi que la densité de courant critique. Une comparaison des caractéristiques physiques et structurales d'échantillons NBCO oxygénés ex-situ et YBCO in-situ, élaborés par MTG sous champ magnétique intense a été effectuée. De même des échantillons NBCO texturés par la technique de fusion de zone (Zone Melting) oxygénés respectivement in-situ et ex-situ ont été étudiés. Ils présentent des textures similaires avec présence de macles pour NBCO oxygéné ex-situ / The aim of this work is the study of crystallographic texture and microstructure in connection with the superconducting properties (Tc and Jc) of High Temperature Superconducting (HTS) materials: YBCO and NBCO. Microstructure is studied by X-Ray Diffraction (XRD) and Electron backscatter Diffraction (EBSD). The Determination of critical temperature (Tc) and critical current density (Jc) are made with Superconducting Quantum Interference Device (SQUID) magnetometer or Physical Properties Measurement System (PPMS). The first part of this work investigates the crystallographic textures of the YBCO film by using EBSD in order to deduce the epitaxial relationship between the superconducting layer and the buffer layer. This thin film is made up of three successive deposits (among which 300 nm of YBCO), used in fault current limiters for electrical engineering applications. The second part presents the study of NdBa2Cu3O7-d (NBCO) and YBCO bulks prepared by various techniques. The effect of silver doping of YBCO has been studied on samples prepared by the Melt Textured Growth (MTG) technique under low oxygen partial pressure. The doping up to a given amount of silver enhances the microstructure and the critical current density. A comparison of the physical and structural characteristics between NBCO oxygenated exsitu and YBCO oxygenated in-situ, prepared by MTG under high magnetic field has been made. Zone-melted NBCO samples textured by zone melting method oxygenated respectively in-situ and ex-situ have been studied. These samples exhibit the same texture with the occurrence of twins for the NBCO oxygenated ex-situ

No description available.

No description available.

No description available.