Oxygen kinetics and charge doping for high critical current ybco films

Resumen

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.