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Correlation between microstructure and superconductivity in advanced coated conductors architectures based on technical templates

Applicant Dr. Ruben Hühne
Subject Area Synthesis and Properties of Functional Materials
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 416095983
 
Final Report Year 2023

Final Report Abstract

The application of high temperature superconductors in cables or coils with a high current transport capability requires materials with a strong crystallographic texture to reduce the detrimental effect of high angle grain boundaries on the current transport. This is realized by an epitaxial growth of superconducting layers on highly textured, metal-based templates, which are nowadays produced industrially in long length. The resulting coated conductors exhibit a characteristic grain boundary network, which depends both on the template used as well as on the preparation route of the layers and decisively determines the current transport in the superconductor. The aim of the project was to improve the understanding of the interplay between this grain boundary network and the local current transport in order to further optimize such coated conductors so that new fields of application might be opened through a more efficient and cheaper production or that existing materials might be replaced which are cooled with expensive helium. Therefore, epitaxial layers of superconducting cuprates (BaHfO 3 or Ag-doped YBa2Cu3O7-x, (Nd,Eu,Gd)Ba2Cu3O7-x) and selected Fe-based superconductors (BaFe2-xNixAs2, FeSe1-xTex) were prepared on two different templates and characterized in detail. Backscattered electron diffraction in a scanning electron microscope was used to analyze the grain boundary network of the films, while the local current transport was investigated using a scanning probe technique. Based on the data, individual grain boundaries were selected for a more detailed characterization using transmission electron microscopy. The investigations on the cuprate layers revealed only minor structural changes on a local level with regards to the doping content or the substitution of yttrium. In all cases, the grain boundary network was transferred from the substrate used to the superconductor layer. Investigations of the local current transport showed a homogeneous distribution on tapes prepared by ion beam assisted deposition. In contrast, significant granularity was observed on substrates, for which textured Ni-W alloys were used. In this case, both film thickness and doping reduced the influence of the granular structure on superconductivity. In comparison, it was not possible to characterize the local structure of Fe-based superconducting films due to the unfavorable surface morphology, whereas the local current distribution showed a similar behavior as observed in cuprates. The analysis of individual grain boundaries made clear that the local tilt of the template surface has a decisive influence on the structure and local current transport in addition to the grain boundary itself.

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