Studying Second Generation High Temperature Superconductors

Goran Majkic
Research Professor, University of Houston
Department of Mechanical Engineering
Advanced Manufacturing Institute
Texas Center for Superconductivity
Houston, TX 77204-4006

Superconductors are used in power transmission lines, energy storage devices, fault current limiters, particle accelerators, and more.  Research into the formulation, characterization, and fabrication of High Temperature Superconductors (HTS) is laying the foundation for commercial expansion of this important technology into the power grid.

I know you work in the area of materials science and more specifically in a department of mechanical engineering . Can you expand on what you are focused on in your research?

Our main research area is Second Generation High Temperature Superconductors (2G-HTS), based on RE-Ba-Cu-O (REBCO, RE = rare earth) superconductor film deposited on strong metallic flexible substrates. The main advantages of these superconductors are high operating temperatures (critical temperature >90 K), and the ability to generate very high magnetic fields, far beyond those achievable by conventional Low Temperature Superconductors (LTS).

We specialize in various aspects of 2G-HTS research, ranging from Advanced Metal Organic Chemical Vapor Deposition (MOCVD) of very thick (~ 4 μm) REBCO films, magnetic flux pinning, buffer layer development, to applications such as development of round REBCO wire (STAR® wire) for use in high energy physics and other applications.

What is important about this research today, and what are you hoping to achieve through your work?

Many applications utilizing high currents and/or magnetic fields would greatly benefit from utilization of 2G-HTS. A particularly exciting application is the development of compact fusion reactors capable of net energy production. The key enabling technology for this undertaking is the 2G-HTS conductor, as it is currently the only practical conductor capable of achieving very high magnetic fields in compact space (the compact fusion reactors being developed will operate at 20 K, 20 T). Increasing the critical current density and performance under magnetic field of 2G-HTS conductor is of vital importance for further progress not only in fusion but practically all 2G-HTS applications (e.g., MRI, NMR, Superconducting Magnetic Energy Storage and others).

How are you using micro-CT technology in your research?

Micro-CT is used for various aspects of 2G-HTS research, from studying the superconductor tape itself to studying more complex assemblies and even components of manufacturing or processing equipment used for their production, with the goal of improving the process itself. While I and my colleague, Prof. Venkat Selvamanickam, primarily focus on 2G-HTS research and development, we have also established a research program on Additive Manufacturing, and micro-CT is used extensively for both research programs.

We also offer micro-CT service to other faculty on campus for various projects ranging from batteries to crack propagation in ceramics. We have two fully trained graduate students, Nathaly Castaneda and Puskar Pathak, that perform extensive micro-CT scans and data analysis for both our needs and the needs of other research groups on campus. In addition, micro-CT is also used for collaborative efforts outside UH, such as our collaboration with Lawrence Berkeley National Laboratory (LBNL) on 2G-HTS for high energy physics applications, led by Dr. Xiaorong Wang on the LBNL side, described in more detail below.

Recently, we used micro-CT to understand failure mechanisms in round STAR® superconducting wire exposed to high currents in applied magnetic fields. In this study, we are developing the round STAR® wire specifically for use in dipoles for high energy physics applications, in particular a Canted Cosine θ dipole design under development by LBNL. This is a collaborative effort among University of Houston (UH), AMPeers LLC, and LBNL in a Small Business Innovative Research (SBIR) program funded by the U.S. Department of Energy Office of High Energy Physics. In summary, for this application, the round superconductor wire has to sustain carrying very high currents under high magnetic fields, while bent to a minimum bending radius of 15 mm. The high field testing was done in collaboration with National High Magnetic Field Laboratory (NHMFL, Tallahassee, FL) and European Magnetic Field Laboratory (EMFL), Grenoble, France. Micro-CT is instrumental in the characterization of STAR® wire for optimization purposes, as well asfor the identification of failure mechanisms under extreme transport currents and applied magnetic fields.

Another example of use of micro-CT is in the development of cable terminations for high energy physics applications, which is also a collaborative effort among UH, AMPeers LLC and LBNL. Multiple strands of round STAR® wires made by AMPeers are assembled into cables by LBNL in order to further increase the current carrying capability of this superconductor, and the cable is terminated by a copper termination block specifically designed for this purpose. The terminations are filled with solder for maximizing the current sharing among the individual strands and for providing mechanical stability against vibrations and related disturbances. We have used micro-CT to optimize the design of the termination as well as the solder impregnation conditions for maximum stability and current sharing capabilities.

Another example of use of micro-CT for superconductor research is studying the mechanisms of failure of REBCO tape itself, utilizing a tensile testing fixture for in-situ micro-CT imaging of the strands under mechanical load.

Are there any images that you could share illustrating results using micro-CT with your applications?

During the development of round STAR® superconductor wire for high energy physics applications, we have tested the STAR® wire at 4.2 K, 30 T, bent to 15 mm radius and subjected to high levels of transport current. The STAR® wire was made using our Advanced-MOCVD system, specifically developed to produce ultra-high performance conductors at thicknesses previously unachievable by other deposition methods (>4 μm), which has resulted in multiple world record performances [1-4].

 The tapes have shown a remarkable ability to sustain extremely high currents at 30 T, and ultimately failed at >2500 A. In order to understand the failure mechanisms, failed wire was subjected to micro-CT analysis. Shown in Figure 1 is a STAR® superconductor wire mounted on a G10 holder and bent to 15 mm radius [1]. The whole assembly was tested at 4.2 K and fields up to 30 T at NHMFL, and the sample shown was subjected to transport current of >2500 A until failure.  The failure point is clearly visible in the overall view in Figure 1. The 3D reconstruction enabled us to examine the failure in much more detail, as shown in the figure. Clear signs of copper former plastic deformation, opening and flattening of individual 2G-HTS tapes and redistribution of solder are all visible in the cross section view, and was caused by local loss of superconductivity leading to shunting of over 2500 A of current into copper core and metallic parts of the STAR® wire, and thus excessive heating and subsequent softening of the metal components including the copper former. At these elevated temperatures, the exerted Lorentz force was high enough to plastically deform the structure, including the copper former.

Shown in Figure 2 is the tape segment near the damaged region but revealing the cross section further away from the damaged end, where the original round shape of the copper former and tightly packed 2G-HTS strands impregnated with solder can be seen. The outermost 2G-HTS (REBCO) strand has, however, sustained damage as evident in the partial unwinding and a sharp bend due to the Lorentz force exerting pressure on the strand against the G10 fixture. Apparently, the heat propagation from the damaged end was sufficient to soften the solder in this region and thus enable movement of the strand.

Figure 1Micro-CT images of 2.5 mm diameter round superconductor STAR® wire bent to 15 mm radius and tested at 4.2 K up to 30 T [1]. The wire carried up to ~2500 A of current before failure. Micro-CT was used to identify the associated failure mechanisms. Plastic deformation of round copper former and other features such as solder flow indicate localized heating at failure point.
Figure 2 – Three-dimensional view of round superconductor STAR® wire after failure at 4.2 K, 30 T and current of ~2500 A [1].
A sequence of cross sections near the damaged end is shown in Figure 3. The top end corresponds to the contact region with G10, where the Lorentz force has flattened the strands. Many features, including the reflow of solder, severe plastic deformation, and other details can be identified. Without micro-CT, obtaining this detailed reconstruction of the failure event would not be possible.

Figure 3 – Micro-CT images of a round 2.5 mm superconductor STAR® wire after failure during testing at 4.2 K, 30 T and carrying ~2500 A of current [1]. The center feature is copper former, the flat features are 2G-HTS superconductor strands and the brightest features are solder that was used to impregnate the assembly prior to testing. The images are located progressively closer to the failure point. Micro-CT was instrumental in identifying the failure mechanisms and providing critical information for further improvement of performance.
Another example of micro-CT use is shown in Figure 4, revealing a superconductor cable terminal interspersed with solder. The view on the left reveals the strand and the solder, while the view on the right reveals the distribution of solder. In this collaborative work with LBNL, the goal was to optimize the terminal and the solder fill technique in order to achieve full penetration of solder between the strands and the outer shell of the terminal.

Figure 4 – Micro-CT scan of superconductor cable terminal consisting of multiple round strands interspersed with solder. The view on the left reveals the strand and the solder, while the view on the right reveals the distribution of solder.

What has been the most important discovery or unexpected outcome in your research?

Probably the development of the Advanced-MOCVD process and a string of world record 2G-HTS performances we were able to achieve with it. The somewhat unexpected outcome was the ability to grow very thick (4+μm) REBCO films without performance degradation, as up to then the norm was 1-1.5 μm without a clear understanding of what limits the ability to grow thicker films with good performance, regardless of the film deposition technique used. Additionally, tape strands with such thick film REBCO (a ceramic) have been remarkably resilient even when wound to a diameter as small as 0.8 mm to fabricate STAR® wires.

Selected publications:

[1] – Eduard Galstyan, et. al., “High critical current STAR® wires with REBCO tapes by advanced MOCVD”, Supercond. Sci. Technol., 36, 055007 (2023).

[2] – Majkic, G., et al., “Over 15 MA/cm2 of critical current density in 4.8 µm thick, Zr-doped (Gd,Y)Ba2Cu3Ox superconductor at 30 K, 3T”, Sci. Rep., 8, 6982 (2018).

[3] – Majkic, G., et al., “Engineering current density over 5 kA mm−2 at 4.2 K, 14 T in thick film REBCO tapes”, Supercond. Sci. Technol., 31, 10LT01 (2018).

[4] – Majkic, G., et al., “In-field critical current performance of 4.0 μm thick film REBCO conductor with Hf addition at 4.2 K and fields up to 31.2 T”, Supercond. Sci. Technol., 33, 07LT03 (2020).

To contact Goran Majkic, please email: gmajkic@uh.edu

If you are interested in being featured in a future Researcher Spotlight, please contact ann@microphotonics.com. We love to hear how our instruments are being used in the field!

 

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