Composition of the jury
- Prof. Pere Roura: Full Professor in the Girona University
- Dr. Anna Palau: Tenured Scientist, ICMAB (CSIC)
- Prof Bernhard Holzapfel: Professor at Karlsruhe Institute of Technology (KIT)
Bellaterra, 17 de Marzo de 2016
Different approaches to prepare superconducting nanocomposite film using the Chemical Solution Deposition (CSD) methodology
Complete scenario of the different phenomena (the different surface reactions and the volume diffusion) involved in the oxygen diffusion process in CSD YBCO films
One of the hot topics in the field of superconductivity is the YBa2Cu3O6+(YBCO) Coated Conductors (CCs) fabrication due to their excellent superconducting properties and promising application prospects. However, in order to spread worldwide the use of YBCO CCs, it is mandatory to use a scalable and low cost fabrication methodology. The Chemical Solution Deposition method has emerged as a promising alternative to the vacuum deposition techniques that can accomplish these requirements.
Despite the YBCO CCs can satisfy the requirements in many different applications, there are other uses that are out of their reach with their current performances, especially those power applications in which high magnetic fields are applied. The vortex movement, which takes place at such magnetic fields, makes YBCO CCs useless for these particular applications.
The aim of this work is to improve the properties of the YBCO films in order to satisfy the demands of these power applications. For this, we have studied, mainly, two different strategies: the nanostructuration of the original YBCO matrix by the addition of nanoparticles (NPs) obtaining superconducting nanocomposites and the optimization of the oxygenation process in the YBCO films studying in detail the oxygen diffusion processes.
The preparation of the YBCO nanocomposite films was carried out following two different “Sequential deposition and growth” approaches: the “in-situ” approach, in which the NPs are spontaneously segregated during the growth process, and the “ex-situ” approach, a new methodology developed in this thesis, in which the NPs are firstly synthesized in a colloidal solution and then embedded in the YBCO matrix.
Using the “in-situ” approach we have made an extensive study of how different mixtures of oxide NPs (BaZrO3, Y2O3 and Ba2YTaO6) affect the microstructure of the YBCO films creating defects that increase the pinning properties. We have also studied the influence of these defects, in particular, of the stacking faults, on the final properties of the YBCO and GdBa2Cu3O6+ nanocomposites.
In the case of the “ex-situ” approach, we have studied the structural and superconducting properties of nanocomposite films prepared with different colloidal solutions of both magnetic (CoFe2O4) and non-magnetic (CeO2 and ZrO2) NPs. We have realized that the stabilization agents can critically influence the homogeneity of the pyrolyzed films. The growth process was investigated for each type of NPs trying to solve different difficulties that have appeared as coarsening, pushing or reactivity. We could also establish a clear relationship between the features of the NPs in the initial colloidal solutions with the final performances of the films allowing us to create a general procedure to prepare high quality epitaxial YBCO nanocomposites by using the ex-situ approach.
The study of the oxygen diffusion processes in YBCO thin films was done using in-situ resistance measurements that allow to monitor the evolution of the resistance in the thin films under different annealing conditions. We have studied how the temperature, the gas flow and the oxygen partial pressure affect the oxygen diffusion processes. According to our results, we can conclude that the surface reactions that take place before the oxygen bulk diffusion are the limiting factor for the oxygen diffusion. The effect of the silver addition to the YBCO as catalyst for the oxygen diffusion was also tested. Finally, the first study about the oxygen diffusion processes in nanocomposite films gave an idea of how the oxygen diffusion works in this kind of materials.