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Superconductivity Revolution

World's First "Yttrium-based Oxide Superconducting Wires" Development Story

Phase.2 Development of YBCO Wire

Ion Beam Assisted Deposition (IBAD) method by Fujikura has become a world standard.

Most of the superconducting wires are used in magnets that generate a magnetic field for devices such as linear motors and diagnostic MRI equipment. The superconducting current had a general tendency of decreasing in a magnetic field, but YBCO could be said to have a small decrease of the critical current in a magnetic field and a very good magnetic-field property. It, however, had a thorny problem to resolve to make it practically applicable. As the superconducting current was interrupted on an interface between the crystals of YBCO, crystalline control was required to carry a great current.

It first turned out that, as an YBCO crystal had a large aeolotropy, uniaxial orientation was required to align the directions of YBCO crystals. For this reason, research efforts in many wire production processes in the world were directed toward methods that deposited a film on a substrate with a vapor deposition method. Following that, it was revealed that sufficient current could not be generated with crystalline control through only one axis, and an observation was reached that only single YBCO crystals could accomplish that. This meant that a 1-cm cubic YBCO crystal could accomplish high performance, but there seemed to be no solution for anything such as superconducting wires that needed a certain length.

In those days, a complete YBCO wire was imagined to be a thin superconductive film deposited on a metallic tape with flexibility. If a film of superconducting YBCO was deposited directly on a metallic tape, however, the constituent elements of the metallic tape would diffuse and the superconductive characteristics would deteriorate. For this reason, a buffer layer was required between the tape and the film. This buffer layer was called an "intermediate layer."

Around 1990, Fujikura also made research efforts to determine what materials would be appropriate as the intermediate layer that would be deposited on a metallic tape to bring practical YBCO wires into reality. We first examined the impact of ion irradiation at the time of film deposition as a part of researches into depositing a zirconium oxide (YSZ) film as an intermediate layer material on a refractory metal. As a result, we observed a sign that the zirconium oxide crystals were aligned when depositing the film on the metal while irradiating it with ion. Continuing the examination of this aspect more systematically, we discovered that the condition under which the zirconium oxide crystals were aligned and showed the strongest crystalline orientation when a certain angle of ion irradiation and ion intensity were satisfied was 55 degrees from the refractory metal tape used as a substrate. The critical current density (the maximum current that flows per unit area of cross section in superconducting state) of the superconducting layer that was deposited as a film on this crystalline-orientated intermediate layer was 500,000 A/cm2. Since films with crystalline orientation through only one axis, or so-called c-axis oriented films, had approximately a critical current density of several tens of thousands A/cm2, the research proved how high the performance of the film was. This examination also greatly contributed to bringing practical superconducting wires to a reality by increasing the current of superconducting wires from a few A to more than 20 A. This approach (IBAD method: Ion Beam Assisted Deposition method) has continued to develop as a major process among the production processes for yttrium-based superconducting wires.

Intermediate Layer with All-axis Orientation of 2 Fixed Axes
Intermediate Layer with All-axis Orientation of 2 Fixed Axes
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