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En physique des particules, les grands accélérateurs permettent de sonder la matière en produisant des collisions de faisceaux à haute énergie. Dans un accélérateur circulaire, l'énergie maximale de collision dépend directement de l'intensité du champ magnétique dipolaire servant à guider les particules le long de leur orbite. Dans le large collisionneur de hadrons installé au CERN, l'énergie de collision maximale est de 14 TeV dans le centre de masse. Réaliser des collisions à plus haute énergies nécessitera alors l'utilisation de matériaux supraconducteurs à haute température critique (HTS). Dans cette perspective, les propriétés électromécaniques des différents conducteurs HTS sont caractérisées et paramétrées à 4.2 K sous des inductions allant jusqu'à 12 T. Dans les aimants supraconducteurs d'accélérateur, le câble doit être à fort ampérage et à haute densité de courant (15 kA-400 A/mm2). Il consistera en plusieurs conducteurs HTS connectés en parallèle. La caractérisation et la modélisation des performances de tels câbles dans leurs futures conditions opérationnelles sont exposées dans cette thèse. / Particle colliders are the main tool for investigating and understanding the fundamental laws of physics. The CERN Large Hadron Collider (LHC), is a circular accelerator which steers and collides two counter-rotating protons beams. It has four collision points where detectors are placed to analyze the products emerging from the collisions. In a synchrotron machine a magnetic flux density is used to guide and focus particles around the orbit. The maximum energy that a circular machine with a given geometry can achieve is limited by the maximum strength of the dipole magnetic flux density. There is therefore an interest in the particle physics community in searching for dipole magnets with higher strength. The LHC has a beam trajectory radius of 4.3 km and a collision center-of-mass energy of 14 TeV. The accelerator employs 1232 large superconducting Nb-Ti dipole magnets operated at a flux density of up to 8.3 T in a bath of superfluid helium at 1.9 K. Energies higher than that achieved with the LHC require magnets made from superconductors with higher upper critical flux density. Nb3Sn is an option for magnets operated up to about 14 T. The level of energies of the type being discussed for a potential energy upgrade of the LHC machine - 33 TeV- would require the use of high temperature superconductors (HTS). Three technical HTS are available today: YBCO, Bi-2212 and Bi-2223. At low temperature YBCO conductors present both irreversible flux density and current density in excess of those measured in Bi-2212 and Bi-2223 conductors. In addition, YBCO can be used as reacted conductor, which makes its use for applications simpler than Bi-2212, which requires heat treatment at high temperature and in oxygen atmosphere after cabling and winding. The level of currents required for application to accelerator magnets, which is above 10 kA at the nominal operating temperature and flux density, excludes the use of single strands. The high current and high current density required can be achieved with cables having several strands connected in parallel. The main objective of my work has been the study of HTS cables for high current/high current density applications, starting from the analysis and selection of suitable conductors, through the characterization of their intrinsic (e.g. critical surface, strain sensitivity and irreversible strain) and extrinsic (e.g. cabling degradation) properties, with the final objective of validating 10 kA-range cables based on HTS material for high flux density magnets. The performance of YBCO and Bi-2223 tapes at 4 K under parallel and perpendicular flux density is measured using purpose built samples holders. A complete review of the strain sensitivity of HTS materials is presented, and the measured critical current retention of HTS tapes under torsion is discussed. Expressions that describe the critical current density of HTS conductors as a function of flux density strength, flux density orientation, temperature and strain are introduced. Analytical models that provide the allowable twist and bending radius of YBCO tapes as a function of strain are elaborated and compared with measurements. The accurate expressions are then used to compute the margins of the winding pack of a 19 T dipole made with a YBCO cable. Roebel cables made of YBCO high current strands are characterized at 4.2 K and in flux densities of up to 9.6 These are the first measurements ever performed at 4.2 K and with high currents. The Roebel cables reached critical currents of up to 12 kA with engineering current density in excess of 1.1 kA/mm2 at 7.5 T. These measurements demonstrate the potential of Roebel cables for high flux density magnets. During measurements two out of four Roebel cables were irreversibly damaged. The mechanism of failures is detailed and explained. Finally the performance and current distribution of HTS cables is computed and compared with measurements

Grain boundaries form the basis of an important Josephson junction technology in the cuprates and also limit the superconducting critical currents attainable in practical, polycrystalline materials. An improved understanding of these defects is therefore important for applications. The status of the current understanding of cuprate grain boundaries is summarised and experimental investigations are presented, focusing on the less well understood high angle boundaries. Measurements of the capacitance of grain boundaries in the overdoped superconductor Y1-xCaxBa2Cu3O7-8, were performed as a function of the calcium content, using the Josephson coupling across the boundaries. Particular care was taken to eliminate the effects of heating and stray capacitance due to the substrate. The effect of thermal noise was also assessed. These measurements provide important information about the area and the width of the grain boundaries, that highlights their inhomogeneous nature. A new technique was applied to measure the normal state properties of YBa2Cu3O7-8 grain boundaries above the critical temperature. Since the resistance of the adjacent material at high temperatures is comparable to, or greater than, that of the grain boundary a compensating Wheatstonebridge structure was used. The errors involved in this technique are carefully assessed and quantified. The normal state resistance of a number of different grain boundary orientations was measured from room temperature to the critical temperature. Detailed characterisation of the grain boundaries, including measurements of the critical current and the current voltage characteristics at low temperatures, was performed. The results obtained are used to assess the validity of the various theories for the grain boundary electrical structure. A tunneling model that accounts for the band structure of the material is developed and applied to potential barriers consistent with a band bending model. This theory is shown to provide a convincing account of the experimental results presented in this thesis.

Critical currents in the latest biaxially textured 'coated conductors' are now limited by 2D networks of low angle grain boundaries (LAGBs) with misorientation θm = 1 - 10°. In order to understand and optimise current transfer in these materials it is essential to elucidate the electromagnetic behaviour of the LAGB. This work presents an investigation into the transport properties of [001]-tilt LAGBs formed by the thin film deposition of YBa2Cu3O7 onto bicrystalline substrates. Through the use of a precision two-axis goniometer, measurements of the V-I characteristic and critical current density were made as magnetic field was rotated in angles θ and φ relative to the LAGB defect. It is found that for fields applied parallel to the LAGB defect plane, dissipation is dominated by the viscous flux flow of vortices along the boundary. Clear evidence for this is found in the V-I characteristic, which displays an increased linearity indicative of the viscous regime. It is shown that the number of vortex rows involved in the flow process can discontinuously switch, leading to a V-I characteristic made up of straight segments of different gradient. For fields applied away from the LAGB defect plane a kinked vortex structure develops and the boundary critical current density, JcGB, is determined by the channelling of vortex segments still lying within the LAGB. The channelling regime is seen in angular measurements as a marked decrease in JcGB(θ,φ) as field becomes aligned to within critical angles φK or θK of the LAGB. The aligned vortices experience a reduction in dimensionality that is manifest in a reduced temperature dependence of JcGB(T). For fields applied at a sufficiently large angle from the defect plane the deleterious effects of the LAGB on current transport are mitigated considerably. In this regime a collinear vortex structure is regained and transport is controlled by the intragranular (IG) sections of the LAGB track; JcGB = δJcIG where δ ~ 0.8, a result that is independent of field, temperature and angle for a 4.9° LAGB. For rotation of the applied field within the LAGB defect plane, the presence of a pinning peak in JcGB for field aligned to the dislocation array is confirmed and modelled. The peak is found to be absent in IG track measurements and increases relative to the intrinsic peak with both increasing field and temperature. In addition, a pronounced angular hysteresis is presented, which is directly linked to a corresponding 'static' hysteresis in JcGB(B) with field. Magneto-optic measurements confirm that this effect is controlled by the flux density profile in the IG regions of the LAGB track. Finally, above a temperature, angle and sample dependent merging field, B*, the LAGB is found to be effectively transparent, as δ ~ 1. This is due to the irreversibility line, above which dissipation occurs across the whole LAGB track.

Large electrical current transport in the absence of energy losses is thekey factor in commercial applications of high temperature superconductors. This thesis demonstrates an easy and inexpensive bottom-uptechnique to produce self assembled nanorods, segmented nanorodsas well as nanoparticles in YBa2Cu3O7-δ thin films grown by pulsedlaser deposition. The structural and morphological characteristic ofthe pinning landscapes produced are investigated and correlated totheir effects on the superconducting properties of the thin films. In particular two pinning landscapes are investigated: Ba2YNbO6nanorods are grown in YBa2Cu3O7-δ thin films using a Ba2YNbO6doped YBa2Cu3O7-δ pulsed laser deposition targets andBa2(Y/Gd)(Nb/Ta)O6 segmented nanorods together with (Y/Gd)2O3nanoparticles are grown in (Y/Gd)Ba2Cu3O7-δ thin films using aBa2YNbO6 + Gd3TaO7 doped YBa2Cu3O7-δ pulsed laser depositiontargets. The Ba2YNbO6 + YBa2Cu3O7-δ is deeply characterised and the effectsof the deposition parameters are analysed. Ba2YNbO6 is demonstratedto be an interesting novel pinning addition capable to increasethe critical current and to reduce the YBa2Cu3O7-δ critical currentsangular dependencies anisotropy. The Ba2YNbO6 + Gd3TaO7 + YBa2Cu3O7-δ is found to produce anew complex pinning landscape extremely effective. At high fieldsthe synergetic combination of the different defects typology is shownto generate an interesting new feature in the critical current angulardependencies. Chapter 1 is an introduction to superconductivity, the fundamentals ofthe field are briefly presented. In chapter 2 the discussion in focused onpinning in high temperature superconductors. Cuprates and in particularYBa2Cu3O7-δ are presented. The pinning phenomenon andthe practical pinning engineering in thin films is also discussed in thischapter. Chapter 3 describes the thin films preparation methods andthe characterisation techniques used in the research work. Chapter 4and 5 are focused on the Ba2YNbO6 doped YBa2Cu3O7-δ thin films. Chapter 4 is an introduction to Ba2YNbO6 doped YBa2Cu3O7-δ, thepreliminary results obtained on Ba2YNbO6 doped YBa2Cu3O7-δ thin films are shown in this chapter. The crystalline structure, the morphologyand the superconducting properties of thin films depositedadopting different deposition parameters are analysed and discussedin chapter 5. In chapter 6 the new complex pinning landscape ofBa2(Y/Gd)(Nb/Ta)O6 and (Y/Gd)2O3 in (Y/Gd)Ba2Cu3O7-δ is presented. Concluding remarks on the research described in the workends the thesis in a brief final chapter 7.

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