Influenza di uno strato di germinazione sulla corrente critica di conduttori HTS rivestite

Coated conductors require excellent alignment of the superconductor coating to achieve maximum critical currents. To ensure high critical currents needed in future applications, HTS- CCs typically have a thickness of the HTS layer of several microns. Such films can be grown directly on an MgO-buffer layer, which provides the necessary biaxial texture. However, earlier investigations suggest that the stoichiometry of the HTS layer has influence on the critical current, i.e. the quality of the thick HTS layer can be changed with respect to different stoichiometry of the superconductor layers. So in our experiment, we first prepare the thin GdBCO (1μm) superconductor films with different powder composition and then measure their critical currents by Cryoscan. High Resolution Scanning Electron Microscopy (HRSEM) was used to inspect the film surfaces and cross sections. The film composition was found by Energy Dispersive X-Ray analysis (EDX) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Moreover the texture and grain alignment was achieved by performing Electron Backscatter Diffraction (EBSD). The change of HTS powder stoichiometry results in a change of critical current. Besides, the comparison of surface morphology, cross section image and fraction of precipitates for different powder compositions is achieved. There is a shift from the HTS powder stoichiometry to HTS film stoichiometry. The highest critical current is reached when the film Gd/Ba ratio is close to the ideal superconductor stoichiometry, i.e. Gd/Ba=0.5. The EBSD results show most grain boundary angle of selected tape are lower angle grain boundary, which confirm the good electrical performance of our tape. It is also known that the quality of the thick HTS can be improved if an additional thin HTS seed layer is deposited on the buffer layer. In order to find the characteristics of the seed layer that primarily influence the critical current of the thick HTS films, we first prepare the superconductor film with thin GdBCO (300nm) in the initial growth of thick HTS films. We explore the seed layer effects in two ways: firstly, by changing the powder stoichiometry of the thick layer; secondly, seed layer variation. Tapestar determines the critical current of the prepared thick films. To characterize the microstructure and texture of the prepared films a variety of high level techniques, such as HRSEM, X-ray pole figure analysis are used. With the implication of a seed layer, the critical current density is getting smaller but the engineering critical current is higher. By changing the powder stoichiometry of the thick layer, the critical current changes a lot. While the same thick layer stoichiometry with seed layer variation have less influence than expected on the critical current density. Due to the pinning effect, the films with a certain amount of Cu precipitate turned out to have a higher critical current. XRD measurement shows that the seed layer HTS films are not single crystal in the structure but strongly textured. Besides, the critical current of the thick film is increased with the decrease of in (out) plane full width half maximum, i.e. with the increase of the seed layer texture.