Vortex pinning and creep in ybco nanocomposite films grown by chemical solution deposition

Abstract

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._x000D_ _x000D_ 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._x000D_ _x000D_ 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._x000D_ _x000D_ 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._x000D_ _x000D_ 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._x000D_ _x000D_ 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._x000D_ _x000D_ In this work, it is shown that nanocomposites provide simultaneously higher flux pinning_x000D_ 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.

Date of Award	7 May 2019
Original language	English
Supervisor	María Teresa Puig Molina (Director) & Ana Maria Palau Masoliver (Director)