Nanostructure of Superconducting Tapes : a study by electron microscopy
27 February 2017
UAntwerp, Campus Groenenborger, V0.09 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organization / co-organization:
Department of Physics
Gustaaf Van Tendeloo
PhD defence Alexander Meledin - Faculty of Science, Department of Physics
Since the discovery of superconductivity in the beginning of the 20th century by Kamerlingh Onnes, a lot of effort has been made for its application in everyday life. At the moment, the low temperature superconductors are the most commonly used in superconducting wires. However, already the second generation of high temperature superconductors (HTS) is being developed.
Second generation HTS conductor tapes have a complicated architecture consisting of several layers. Depending on the preparation technique the final structure and morphology of each layer can vary leading to an improvement or degradation of the final product properties.
TEM is an outstanding, powerful and unique tool for a complete characterization of materials down to atomic scale. It provides techniques for the observation of the overall morphology through low magnification imaging, a study of the crystal structure and defects through diffraction and atomic resolution imaging and chemical information through X-ray and electron energy loss.
Several single layers of La2‑xGdxZr2O7 (LGZO) and double pure La2Zr2O7 (LZO) on LGZO buffer layers with different Gd concentration on Ni5%W substrates were studied. The Gd concentration had a strong influence on the roughness of the single LGZO buffer layer. The use of a LGZO seed layer also reduced the roughness of the LZO layer deposited on top. The porosity of the buffer layers was controlled as well. The double LZO/LGZO buffer layers architecture lead to a clear superconducting tape performance improvement.
We investigated the chemical solution deposition (CSD) prepared YBa2Cu3O7 (YBCO) – based nanocomposite with ex‑situ MnFe2O4 nanoparticles added to serve as pinning centers. The behavior of the particles was monitored during the preparation and for the final product. However, this procedure did not work out as expected and TEM could unravel the reasons. It helped to shed light onto the evolution of the spinel nanoparticles during the synthesis procedure.
Then a CSD YBCO–based nanocomposite with ex‑situ ZrO2 nanoparticles was studied. Using electron microscopy we were able to pinpoint the reasons of the improvement of the pinning force in the nanocomposites, in spite of the transformation of the ex-situ nanoparticles into a BaZrO3 phase.
The final part is devoted for the study of outstanding industrial tapes with high in-field currents. An elegant “bunch of flowers”-like micro- and nano‑morphology of the BaZrO3 nanocolumns in the superconducting layer was observed and correlated with the performance of the wires. Apparently the obtained morphology is responsible for the high performance. We could also point out that there is still some room for improvement and increase of the number of pinning centers.