The Superconducting Revolution

Fujikura's ion-assisted deposition method (IBAD method) has become the global standard.

Many superconducting wires are used as magnets that generate magnetic fields in linear motors, diagnostic MRIs, and other devices. Superconducting currents generally become smaller in a magnetic field, but YBCO exhibits only a small decrease in critical current in a magnetic field, and its magnetic field characteristics can be said to be extremely good. However, there was a major challenge to practical use. Because the superconducting current is interrupted at the interface between YBCO crystals, crystal control was essential to pass a large current.

First, it was discovered that YBCO crystals are highly anisotropic, and require uniaxial orientation to align the crystals. For this reason, research around the world turned to a method of forming a film on a substrate using a vapor phase method for the production of many wires. It was later discovered that sufficient current could not be obtained by controlling the crystals on only one axis, and some observed that it might be impossible without single crystals. In other words, high performance could be obtained with 1 cm square pieces, but there was no solution for things that required length, such as superconducting wires.

At the time, the image of YBCO wire was that of a thin superconducting film formed on a flexible metal tape. However, forming a superconducting film directly on the metal tape would cause the constituent elements of the metal tape to diffuse, degrading the superconducting properties, so a buffer layer was needed in between. This was called an intermediate layer.

Around 1990, Fujikura was also researching the topic of what kind of intermediate layer material should be formed on the metal tape to realize YBCO wire. First, as part of research into forming a film of zirconium oxide (YSZ) on a heat-resistant metal as an intermediate layer material, the effects of ion irradiation during film formation were examined. As a result, there were signs that the crystals would align when the film was formed while irradiating ions. Further systematic investigations revealed that the crystals would align when both a specific angle and the strength of the ions were satisfied, and that the condition for the best crystal orientation was 55° from the heat-resistant metal tape used as the substrate. The critical current density (the maximum current per unit cross-sectional area that flows in the superconducting state) of the superconducting layer formed on this crystal-oriented intermediate layer was 500,000 A/cm2. This demonstrated its high characteristics, as the current of the superconducting wire increased from a few A to a few tens of A. This technique (Ion Beam Assisted Deposition (IBAD) method) subsequently developed into the main process for creating Y-based superconducting wires.

Biaxially fixed omniaxially oriented interlayer