UB RENEW scientists probe second-generation, high-temperature superconducting wires

A University at Buffalo-led research team is reporting new findings concerning high-temperature superconducting wires, a field of study with implications in medical imaging, transportation and numerous other sectors.

The study, published in Nature’s Scientific Reports, was led by scientists at UB’s RENEW Institute, which focuses on solving complex energy and environmental issues.

The research team – also consisting of researchers from American Superconductor, Brookhaven National Lab, and Brookhaven Technology Group – explored properties of American Superconductor’s standard second-generation high-temperature superconducting coil wire.

Researchers used a high-resolution, scanning transmission electron microscope to see atomic structures of a YBCO superconductor. The yttrium, barium, and copper atoms are labelled by yellow, red, and blue dots. The periodic arrays of atoms with spacing less than 0.24 nanometers can be identified in the undamaged area, while the disrupted periodic structure in the form of amorphous nano defects appears in areas damaged by irradiation (a) as-irradiated and (b) irradiated and annealed.

“Improving the performance of superconducting wires in applied magnetic fields is necessary to enable numerous large-scale applications,” says the study’s principal investigator, Amit Goyal, PhD, SUNY Distinguished Professor and director of UB RENEW, a university-wide institute that focuses on solving complex energy and environmental issues.

The lead author of the study is Yi Zhang, PhD, senior scientist and electron microscopist at the RENEW Institute. Additional authors include Marty Rupich, PhD, AMSC; Qiang Li, PhD, Brookhaven National Laboratory; and Vyacheslav Solovyov, PhD, Brookhaven Technology Group.

In the field of high-temperature superconductivity, a key challenge has been to significantly enhance the intragranular superconducting properties of these materials. One way to do this is by improving flux-pinning or vortex-pinning, a phenomenon where a superconductor is pinned in space above a magnet.

In the mid-2000s, Goyal developed a practical and scalable solution using phase-separation and strain-driven, self-assembly to create nanoscale columnar defects at nanoscale spacing during high-temperature superconductor film growth, which dramatically improved properties of high-temperature superconducting wires, especially in high-applied magnetic fields. Industry uses this technique on a daily basis worldwide to fabricate kilometre-long, high-performance, high-temperature superconducting wires.

Unfortunately, this phase-separation and strain-driven, self-assembly process to create nanocolumns defects at nanoscale spacings, is only possible when the superconductor film is made in-situ (in this case, the substrate is heated to over 700 degrees Celsius) and the film is deposited in a heteroepitaxial manner so that the atoms in the superconductor film align with the atoms on the substrate.

When superconducting films are fabricated using an ex-situ method, such as using chemical solution deposition (CSD), columnar defects produced by strain-driven self-assembly process are not possible. CSD is of interest as it very low-cost and easily scalable.  Hence, for CSD-derived superconducting wires, researchers must find an alternative route for defect engineering.

In this study, the research team explored this by irradiating American Superconductor’s CSD-derived, high-temperature superconducting wires using Brookhaven’s Tandem Van de Graaff accelerator followed by annealing.

“Such a low-energy, low-dose, heavy ion irradiation and post-annealing is potentially cost-effective and scalable to long-lengths and can result in a significant improvement of superconducting critical current. We investigated the defect engineering in the superconducting wires as a function of processing to realize optimization of the process,” says Goyal.

The team used high-resolution, aberration-corrected scanning transmission electron microscopy (AC-STEM) coupled with atomic resolution electron energy loss spectroscopy (EELS) for their work.

“We demonstrated that reversible oxygen migration during radiation-annealing processes leads to a significant change in the superconducting properties between an as-irradiated sample and annealed-irradiated sample,” says Goyal. “This study reveals how the low energy heavy ion irradiation and the post-annealing process can result in a remarkable improvement of the critical current of American Superconductor’s standard 2G coil wire.”

Source: State University of New York at Buffalo

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