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 and diagnostic MRIs. Superconducting currents generally decrease in a magnetic field, but YBCO exhibits only a small decrease in critical current in a magnetic field, and its magnetic field characteristics are considered to be extremely good. However, there was a major challenge to practical application. Because the superconducting current is interrupted at the interface between YBCO crystals, crystal control was essential to allow large currents to flow.

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 also conducted research into the ideal intermediate layer material to be formed on metal tape to realize YBCO wire. As part of research into depositing a film of zirconium sulphide (YSZ) on a heat-resistant metal as an intermediate layer, they first examined the effects of ion irradiation during deposition. Results indicated that deposition while irradiating with ions indicated alignment of the crystals. Further systematic investigation revealed that the crystals align when a specific angle and ion intensity are both satisfied, and that the optimum condition for crystal orientation is a 55° angle 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 deposited on this crystal-oriented intermediate layer was 500,000 A/cm2. This demonstrated its superior performance compared to the tens of thousands of A/cm2 achieved by a film with only one crystalline orientation, known as c-axis orientation. This increase in the current of superconducting wires from a few amperes to tens of amperes significantly contributed to the realization of practical superconducting wires. This method (Ion Beam Assisted Deposition (IBAD) method) subsequently developed into the main process for creating Y-based superconducting wires.

Biaxially fixed omniaxially oriented interlayer