Characterizing Superconducting Tapes and Wires

In recent years, high-temperature superconductor (HTS) tapes and wires have increased in their performance to the point where they can now be utilized in the creation of extremely high magnetic fields¹ amongst other applications.

The requirement for routine and established test techniques is now more vital. To characterize these conductors under realistic conditions, it is extremely desirable to test them as much as possible, at high currents, low temperatures, and magnetic fields where they will operate.

Most HTS conductors are a flat tape, so this testing is particularly preferable, compared to testing a small cross-section of conductor at a lower current, due to the form factor, inhomogeneities of deposition, and crystalline properties which can happen at the edges of the conductor and so decrease critical current density J_c at the edges.

Variable temperature insert (VTI) with special 1000 A test insert, used by Dr Jens Hänisch of ITEP (Institute for Technical Physics), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.

Challenge

The majority of HTS conductors are anisotropic in their current carrying capacity as characterized by J_c, this is because of the layered structure of the materials – the so-called BSCCO and REBCO materials.

Inevitably, the HTS conductors will be positioned in a background of divergent magnetic flux lines when constructing a magnet or other electrical machine (e.g. generator, motor,) containing HTS. So, to the designer, characterization of J_c at a varying incident angle of the applied magnetic field is crucial.

These problems in HTS wire and tape characterization are nothing new, and many ‘home-built’ and commercial implementations have been employed.

Dr Hänisch and team at Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany came to Oxford Instruments for an integrated solution to combat these challenges together with repeatability and high precision ^{2, 3}.

Solution

The solution is made up of three key elements:

1. A variable temperature insert (VTI) with 50 mm internal sample space diameter, permitting the sample temperature to be varied continuously, and controlled between 1.8 and 200 K – covering the complete potential temperature space for applications from pumped/forced-flow liquid helium up to liquid nitrogen temperature and beyond.
2. A sample test insert made up of a pair of 1000 A-rated current leads into the sample space of the VTI, and with a motorized rotation stage to enable the continuous angular rotation of samples within the magnetic field to a precision of 0.5 °.
3. A 6 Tesla superconducting magnet with a transverse field which is relative to the VTI (i.e. a ‘split pair’ magnet).

Combined with Oxford Instruments Mercury control electronics (VTI temperature controller and magnet power supply), the aforementioned supply highly repeatable and controllable testing conditions for HTS conductors in respect of the four key parameters of background magnetic field, applied current, temperature, and flux line angle.

Detail of sample position and magnet coils.

These jointly supply the capability to characterize an HTS conductor in its full-width state completely. According to the KIT team, this system is the only one available that can cover all these parameters at the same time.

The mechanical and thermal design of the sample stage itself by Oxford Instruments and KIT has ensured that temperature increases are much reduced during measurement. Due to interface resistances and conductor movement due to Lorenz forces, this avoids sample heating.

Outcome

The team at KIT are utilizing the system to characterize numerous HTS conductor types from different manufacturers⁴. They can also gather valuable insights into the flux-vortex-pinning mechanisms in HTS which are vital to the micro-scale engineering and the resulting potential of these materials for electromagnetic applications.

References and Further Reading 

1. “Progress in the Development and Construction of a 32-T Superconducting Magnet”, Weijers, H.W., Markiewicz, W.D., Gavrilin, A.V., Voran, A.J., Viouchkov, Y.L., Gundlach, S.R., Noyes, P.D., Abraimov, D.V., Hannahs, S.T. and Murphy, T.P., IEEE Trans. Appl. Supercond. (2016) 26 (4), 4300807; doi: 10.1109/TASC.2016.2517022
2. “High current variable temperature electrical characterization system for superconducting wires and tapes with continuous sample rotation in a split coil magnet”, Lao, M., Hänisch, J., Kauffmann-Weiss, S., Gehring, R., Fillinger, H., Drechsler, A. and Holzapfel, B., Rev. Sci. Instrum. (2019) 90, 015106; doi: 10.1063/1.5078447
3. “KIT Builds SC Conductor Measurement System”, Superconductor Week 33 (3) April 29 2019, 13-15
4. “In-field performance and flux pinning mechanism of pulsed laser deposition grown BaSnO₃/GdBa₂Cu₃O_{7– δ} nanocomposite coated conductors by SuperOx”, Lao, M., Willa, R., Meledin, A., Rijckaert, H., Chepikov, V., Lee, S., Petrykin, V., Van Driessche, I., Molodyk, A., Holzapfel, B. and Hänisch, J., Supercond. Sci. Technol. (2019) 32, 094003; doi: 10.1088/1361-6668/ab2a95

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Nanoscience.

For more information on this source, please visit Oxford Instruments Nanoscience.