Superconductivity occurs when a material is cooled to the extreme extent where its electric resistance disappears, allowing several hundred times more electric current to flow without energy loss than is possible via a copper wire of the same cross section. Superconductors are the ultimate material for us electric wire manufacturers. In 1986, Bednorz and Muller discovered superconducting oxides, throwing physicians and chemists the world over into a whirlwind of excitement. During the same period, we were closely watching bismuth-based copper oxides, discovered in Japan in 1988. Metal superconductors that had been previously put to practical use required expensive liquid helium, cooled to minus 269 degrees. With bismuth-based oxides, on the other hand, it became possible to achieve superconductivity at minus 163 degrees, 30 degrees above the temperature of liquid nitrogen. For this characteristic, immediately after their discovery, bismuth-based copper oxides began attracting attention as new materials with a wide range of potential applications. In reality, however, it took us more than 10 years to realize our practical application; that was a long period of continuously tackling the challenge of perfecting the synthesis of the complex and hard-to-handle material, molding it into wire and improving its performance.

In the early stage of development, our superconducting wire presented properties so unfavorable that running an electric current of only a few amperes caused superconductivity loss and electric resistance. It was also barely longer than the length of an adult’s hand. People would look at it suspiciously as if it could not be a superconductor. However, we gradually progressed and in 2002 succeeded in attaining 100A critical current by liquid nitrogen cooling, then 200A and a wire length of 2000 meters in 2006. Major factors in this improvement are superconductor purity, density and crystal orientation and superconductor-to-wire material ratio.

Bismuth-based superconducting oxide is a ceramic with an extremely complex structure made up of five elements: bismuth (Bi), strontium (Sr), calcium (Ca), copper (Cu) and oxide (O). It is an extremely hard material to synthesize, since it remains chemically stable only within a limited range of temperatures. The originally discovered material was low in purity and contained impurities with undesirable properties. We were able to enhance purity by adding a trace amount of lead and improving processing and synthesis conditions of ultra-trace powder materials. We developed a method for density improvement involving two sintering processes, between which compression is conducted; as a result, we managed to achieve a close composition with 90% density.

Thanks to these improvements, it became possible to produce the material with a high critical current, that is, the maximum amperage it can run with no resistance. It was now necessary to produce it in a long and flexible form, so that it could be used as coils and cables of various forms. How would it be possible to transform hard, brittle ceramic into soft, long wire? Our answer to this question was a multiple core structure in which very fine filaments of ceramic are embedded in a silver mold. By this method, we succeeded in producing wire over 1000 meters long. Moreover, we improved the processing method, reducing the silver content while at the same time increasing wire length, flexibility and superconductor ratio. As a result, in 2002 we completed fine wire with a cross section of 1 square millimeter and a critical current of 100A with liquid nitrogen cooling. This current density is very high, corresponding to one hundred times that of copper wire.

A major obstacle to practical application was the problem of flaw caused by ballooning, which happens when liquid nitrogen trapped in a small clearance inside the superconductor gasifies as the temperature returns to room temperature, causing the wire to swell. We handled this phenomenon, which can be considered unavoidable for ceramic, by developing technology for final sintering under a high air pressure of 300 hPa to obtain sintered elements of 100% density. This technology helped further increase the wire’s critical current. In 2006, we achieved 201A by a manufacturing method that would enable industrial mass production of long superconducting wire.

Bismuth-based high-temperature superconducting wire was developed by our group, but the technologies that went into this product were born largely out of the know-how that SEI had nurtured in its long history. For example, the method of processing and sintering trace powder raw material of ceramic is based on the know-how developed in the manufacture of ultra-hard alloys used in tools and electronic components and other sintered products. As for wire-processing technology, as a wire manufacturer SEI has rich reserves of know-how accumulated since its founding. The idea of making ceramic into fine fibers for flexibility had already been applied in optical fiber manufacturing. I believe that SEI’s strength is the availability of such a combination of know-how relating to the design and processing of a variety of materials, supporting analytical expertise that is among the best in the country, and manufacturing techniques. The bismuth-based superconducting wire has been realized, thanks to this strength, unique to SEI.

Our next goal is to achieve a critical current of 300A by 2011, the centennial of superconductor discovery. In today’s society, superconductors can play an epochal role, in revolutionizing transportation through the practical introduction of superconducting motors and linear motor high-speed trains, realizing a post-petroleum age through application to power generation, or finding solutions to global environmental problems. I hope to continue pursuing my research to establish basic technologies useful to society.