Study of multilayer deposition of superconducting YBCO thick films by MOD process

In this dissertation the effort was mainly focused on studies to increase the current carrying ability of superconducting YBCO films. Increasing the efficiency of coated conductors is essential to compensate for their high cost. Superconductor performance can be improved by increasing its flux pinning ability and by increasing its thickness while retaining its critical current density (Jc). In the current studies, possibilities in both approaches for performance improvement were explored by processing YBCO films on LaAlO 3 single crystal substrate by using Trifluoroacetic acid- Metaloorganic Deposition (TFA-MOD) technique. With regard to the improvement of infield performance of YBCO films, we have succeeded in producing Y2O 3, BaZrO3 and Sm doped YBCO films. Jc exceeding 5 MA.cm-2 at 77 K and self-field was obtained in Y 2O3 doped YBCO films. Also, it was found that YBCO films with partial substitution of 'Y' by 'Sm' have shown an improved critical current density over a wide range of magnetic fields. Jc of 0.25 MA.cm -2 at 3T and 77 K was obtained, which was more than 10% of the zero-field Jc. However the most exciting results were obtained by doping the YBCO film with 5% BaZrO3. In this film Zr enriched columnar defects were formed and the in-field Jc of the film was as high as 0.5 MA.cm-2 at 77K and 1.5T. From the results obtained, we conclude that chemical doping by TFA-MOD can be effective in improving the flux pinning in YBCO films.;In addition, efforts in this research were aimed at increasing the thickness of YBCO film using TFA-MOD method. Initially, the thickness of single-layer film was increased, and single-layer films with thickness ranging from 0.1microm to 0.5microm were processed. Jc of the 0.1microm thick film was higher than 6 MA.cm-2 but it decreased rapidly as the thickness of the film was increased beyond 0.2microm. In order to further increase the thickness of the film, multilayer deposition technique was developed. With this technique, films as thick as 1.2microm and consisting of 6 layers of 0.2microm thick YBCO film were processed. However, similar to single-layer films, Jc of multilayer films decreased with the thickness. Jc of 0.2microm-2-layer film was around 3MA.cm-2 and Jc of 0.2microm-6-layer film was 1.02 MA.cm-2 (77K, 0T). In the case of multilayer films, a model based on incremental jc was implemented to describe the results obtained. According to this model, multilayer films had a high Jc for the portion of the film near the substrate and a low J c near the surface of the film. The transition from high Jc to low Jc was sudden and occurred at around 0.3microm. In order to explain these observations, films were analyzed using XRD, SEM and TEM. Based on the results obtained, we conclude that the multilayer deposition of the films is more promising than increasing the thickness of single-layer film.