Breakthrough towards highest-performing superconducting wire

Our energy future may depend on high-temperature superconducting (HTS) wires. This technology’s ability to carry electricity without resistance at temperatures higher than those required by traditional superconductors could revolutionise the electric grid and even enable commercial nuclear fusion.

Yet these large-scale applications won’t happen until HTS wires can be fabricated at a price-performance metric equal to that of the plain copper wire sold at your local hardware store.

New University at Buffalo-led research is moving us closer to that goal. In a study published in Nature Communications, researchers report that they have fabricated the world’s highest-performing HTS wire segment while making the price-performance metric significantly more favourable.

Based on rare-earth barium copper oxide (REBCO), their wires achieved the highest critical current density and pinning force — the amount of electrical current carried and ability to pin down magnetic vortices, respectively — reported to date for all magnetic fields and temperatures from 5 to 77 kelvin.

This temperature range is still extremely cold — -451 to -321°F — but higher than the absolute zero that traditional superconductors function at.

“These results will help guide industry toward further optimising their deposition and fabrication conditions to significantly improve the price-performance metric in commercial coated conductors,” said the study’s corresponding author, Amit Goyal, PhD, SUNY Distinguished Professor and SUNY Empire Innovation Professor in the Department of Chemical and Biological Engineering, within the UB School of Engineering and Applied Sciences. “Making the price-performance metric more favourable is needed to fully realise the numerous large-scale, envisioned applications of superconductors.”

HTS wires have many applications

Applications of HTS wires include energy generation, such as doubling power generated from offshore wind generators; grid-scale superconducting magnetic energy-storage systems; energy transmission, such as loss-less transmission of power in high current DC and AC transmission lines; and energy efficiency in the form of highly efficient superconducting transformers, motors and fault-current limiters for the grid.

Just one niche application of HTS wires, commercial nuclear fusion, has the potential for generation of limitless clean energy. In just the last few years, approximately 20 private companies have been founded globally to develop commercial nuclear fusion, and billions of dollars have been invested in developing HTS wires for this application alone.

Pulsed laser deposition, in which a laser beam ablates a material that is deposited as a film on a substrate, was used to fabricate the HTS wires. Image credit: University at Buffalo.

Other applications of HTS wires include next-generation MRI for medicine, next-generation nuclear magnetic resonance (NMR) for drug discovery and high-field magnets for numerous physics applications. There are also numerous defence applications, such as in the development of all-electric ships and all-electric airplanes.

Presently, most companies around the world fabricating kilometre-long, high performance HTS wires use one or more of the platform technological innovations developed previously by Goyal and his team.

These include rolling assisted biaxially textured substrates (RABiTS) technology, LMOe-enabled ion-beam assisted deposition (IBAD) MgO technology, and nanocolumnar defects at nanoscale spacings via simultaneous phase-separation and strain-driven self-assembly technology. An interview by Superconductor Week with Goyal highlights and discusses details of these technologies.

World-record critical current density and pinning force

In the present work reported in Nature Communications, Goyal’s group reports on ultra-high-performance, REBCO-based superconducting wires.

At 4.2 kelvin, the HTS wires carried 190 million amps per square centimetre without any external magnetic field, also known as self-field, and 90 million amps per square centimetre with a magnetic field of 7 tesla.

At a warmer temperature of 20 kelvin — the envisioned application temperature for commercial nuclear fusion — the wires could still carry over 150 million amps per square centimetre self-field and over 60 million amps per square centimetre at 7 tesla.

In terms of critical current, this corresponds to a 4-millimetre-wide wire segment at 4.2 kelvin having a supercurrent of 1500 amps at self-field and 700 amps at 7 tesla. At 20 kelvin, it’s 1200 amps at self-field and 500 amps at 7 tesla.

It’s worth noting that the team’s HTS film, despite being only 0.2 microns thick, can carry a current comparable to that of commercial superconducting wires with HTS film almost 10 times thicker.

As for pinning force, the wires showed a strong ability to hold magnetic vortices pinned or in place, with forces of about 6.4 teranewtons per cubic metre at 4.2 kelvin and about 4.2 teranewton per cubic metre at 20 kelvins, both under a 7 tesla magnetic field.

These are the highest values of critical current density and pinning force reported to date for all magnetic fields and operating temperatures from 5 to 77 kelvin.

“These results demonstrate that significant performance enhancements are still possible and hence the associated reduction in cost that could potentially be realised in optimised, commercial HTS wires,” Goyal said.

How high-performance wire was fabricated

The HTS wire segment was fabricated on substrates using the (IBAD) MgO technology and using the nanocolumnar defects via simultaneous phase-separation and strain-driven self-assembly technology. The self-assembly technology allows incorporation on insulating or non-superconducting nanocolumns at nanoscale spacings within the superconductor. These nanodefects can pin the superconducting vortices, allowing for higher supercurrents.

“The high critical current density was made possible by a combination of pinning effects from rare-earth doping, oxygen-point defects and insulating barium zirconate nanocolumns and their morphologies,” Goyal said.

“The HTS film was made using an advanced pulsed laser deposition system via careful control of deposition parameters,” added Rohit Kumar, postdoctoral fellow in the UB Laboratory for Heteroepitaxial Growth of Functional Materials and Devices, which Goyal leads.

In pulsed laser deposition, a laser beam impinges on a target material and ablates material that is deposited as a film on an appropriately placed substrate.

“We also conducted atomic-resolution microscopy using the most advanced microscopes at the Canadian Center for Electron Microscopy at McMaster University for characterisation of nanocolumnar and atomic-scale defects and also conducted some superconducting property measurements at the Università di Salerno in Italy,” Goyal said.

The Office of Naval Research (ONR) supported this fundamental research towards development of superior HTS wires. Goyal is principal investigator on the project.

Goyal’s research has had a significant impact on the field of HTS, both in fundamental materials science and in the transition of scientific discoveries from the laboratory to the marketplace.

Top image credit: iStock.com/aydinmutlu