FellowEugen Seiler
Project NamePinning in commercial coated conductors
Host organisationInstitute of Electrical Engineering
Duration of the project01.01.2016 - 31.12.2018

Abstract
The project will investigate pinning mechanisms in commercial ReBCO coated conductors coming from different industrial producers. Different manufacturing processes incorporate various kinds of pinning centers into functional superconducting layers of these conductors and as a consequence a very diverse mixture of pinning centers with dissimilar strength dependence on temperature and magnetic field can be found. Main focus will be on the temperature and magnetic field range interesting for applications in electric machinery (electric motors, generators, transformers, etc.) – temperatures 65 K to 77 K and fields 0 to 5 T. The aim is to identify which of the pinning mechanism is the most effective in the temperature and field domain of our interest and develop experimental techniques for its classification and description. Scaling of the critical current density and of the depinning activation energy with temperature and applied magnetic field will be experimentally characterized for all the commercial ReBCO coated conductors considered. At first the experiments in a broad range of available temperatures (4.2 K to 77 K) and applied magnetic fields (0 to 14 T) will be performed, focusing in the second stage on the domain perspective for the electric machinery applications. The experimental results will be compared with the predictions of available pinning models in order to determine and distinguish the different pinning mechanisms. Project combines experimental activity with application and development of theoretical models. Basic experimental techniques will be magnetization loop measurements and magnetic relaxation measurements in the Vibrating Sample Magnetometer, combined with transport measurements of current-voltage characteristics at various orientations of applied magnetic field.

Project Summary with Interim Results

Superconductors are materials able to transport electrical current without dissipation, however only a smaller subgroup, the superconductors of the second type with pinning, are capable of transporting practically significant electrical currents. Magnetic field penetrates inside these materials in the form of flux lines (vortices) and the effective blocking of the movement of the flux lines inside the superconductor’s volume, usually at various imperfections and defects in the crystalline lattice, is called pinning.

In recent years, high temperature superconducting wires based on the YBa2Cu3Ox material (YBCO) became commercially available in long lengths from several producers worldwide. These tape-like wires consist, in simplified description, of a metallic wafer tape on top of which a thin layer of superconducting material is deposited and are usually referred to as coated conductors or coated tapes. This project aims to investigate the pinning mechanisms in commercially available coated conductors from different industrial producers, which use different deposition methods and production processes resulting in wires with different structural and superconducting properties.

First part of the project was concentrated on determination and analysis of the critical current density dependence on the magnetic field and temperature Jc(B,T). Coated conductors from the following producers were investigated: SuNAM Co., Ltd. (Korea), SuperOx company (Russia), Fujikura Ltd. (Japan) and Superpower Inc. (USA; 2 conductors). Critical current density was explored with the help of magnetization measurements in Vibrating Sample Magnetometer (VSM) and transport Jc measurements, both in magnetic field applied perpendicular to the coated conductor’s wide face. The VSM measurements were performed in a broad temperature range (5 K – 77 K), with a special focus on the 60 K – 77 K interval interesting for electric machinery applications. Transport Jc measurements were performed on full-width coated conductor samples immersed in a subcooled liquid nitrogen bath at temperatures from 65 K to 77 K. Results of magnetization and transport measurements were found to be in a very good agreement. Analysis of the magnetic field dependence of critical current density in the broad temperature range revealed that for all investigated coated conductors the experimental curves are very well described by a power function proposed by a few theoretical models. For Superox, Sunam and Fujikura coted conductors the characteristic exponents are approximately in agreement with prediction of models of strong pinning at large, sparse defects. In case of both Superpower wires, the exponents do not follow this prediction and differ significantly, especially at higher temperatures, which suggests a different effective pinning mechanism.