Ever since superconductivity was discovered by Heike Kamerlingh Onnes in 1911, scientists have sought ways to utilize the phenomenon for the benefit of humanity. Many years transpired and a significant amount of research was carried out before superconductors were made suitable for any practical applications. In fact, many potential commercial uses of superconductors still have not been achieved, as various challenges remain that have yet to be overcome. The dream of a world filled with advanced superconductor-based products has not been as easy to achieve as many optimists anticipated in the 1980s, when great strides forward were made in the field at an amazingly rapid pace. Instead, the process of moving such products from the realm of science fiction to the laboratory to full-scale commercial production has been a gradual process, and one that is ongoing.
Superconducting Niobium Wire
One particularly important area of the superconductor field is the search for durable and efficient superconducting wire. Applications for which such wire could be used include advanced utility cables, motors, and power generators. As in other areas of the superconductor field, advances in superconducting wire have been incremental. The very first commercial superconducting wire was developed by researchers at Westinghouse in 1962. The material from which the pioneering wire was fabricated was an alloy of niobium and titanium (NbTi). Since the early achievements in the field at Westinghouse, superconducting wire has since been developed out of many other materials, produced via several different processes, and manufactured by a number of different companies.
Some superconducting wire has been produced from low temperature superconductors that require liquid helium for cooling, but the vast majority of interest has been in high temperature superconducting (HTS) wire that can be cooled much more cheaply with liquid nitrogen (a common refrigerant) since the discovery of superconductors with significantly higher critical temperatures than conventional superconductors in the 1980s. The first generation (1G) of HTS wire is available commercially today and is prepared through a powder-in-tube process somewhat similar to the way traditional wire of copper or aluminum is made. 1G HTS wire is a composite structure often containing more than 50 filaments of superconducting material embedded in a non-superconducting matrix, such as a silver alloy. It can be utilized in various motor, electric power grid, generator, and magnet applications, though its employment has thus far been limited. Improved fabrication methods may eventually improve performance and reduce costs enough that 1G HTS wire becomes more heavily depended upon for commercial applications, but many companies have been focusing more on the potential of second generation (2G) HTS wire in recent years.
Niobium-Tin in Copper Wire
Second generation HTS wire is designed to be able to replace first generation superconducting wire in form, fit, and function and is expected to surpass the earlier developed wire in electrical performance and economy. It is fabricated through a completely different process and exhibits coated conductor, rather than multifilament, architecture. The foundation of second generation HTS wire is a tape-like base upon which a thin coating of superconducting material is deposited or grown. Thus, it requires no powder or rods to be bundled with other rods, as does 1G HTS wire, and has the potential to be highly cost effective to produce. Yet, the efficient processing of satisfactory 2G HTS wire has proved itself to be a challenging endeavor.
A technique developed by IBM and adapted for superconductor use by Japanese scientists called ion-beam assisted deposition (IBAD) is central in the production of most types of second generation superconducting wire. Fujikura announced that they had employed the process to develop a new kind of HTS wire in 1991. The superconducting wire introduced was YBa2Cu3O7, which is commonly referred to as YBCO-123 or simply YBCO. The Fujikura method of producing YBCO involved depositing an insulating material (yttria-stabilized zirconia; YSZ) onto a flexible untextured metal (usually nickel alloy tape) in a vacuum while an argon ion beam was aimed at an angle to the metalís surface. This produced a YSZ film with a high level of texture. Subsequently the researchers at the company employed another vacuum process known as pulsed laser deposition, or PLD, to epitaxially deposit YBCO on the substrate.
The key to the performance of second generation HTS wire is the texturing of the superconducting material. The rationale for this is that HTS grain boundaries, which form when neighboring grains of superconducting crystalline material are out of alignment, act as obstacle that inhibit the flow of electrical current. Thus, if the texture or alignment of the grains is increased, resistance to current flow is reduced. As realized at Fujikura, however, it is much easier to align HTS grains indirectly, via epitaxial deposition on a textured substrate, than via any direct means.
Niobium-Tin in Copper Wire
When Fujikura released the performance results of its vanguard 2G HTS wire segments, those in the superconductor industry were not particularly impressed. Soon, however, great improvements were made in the processing of such wire at Los Alamos National Laboratory (LANL). In 1995, researchers at LANL achieved a superconducting critical current density of a million amperes (1MA) per square centimeter at temperatures of about 77 K. This substantial advance in performance ignited a burst of experimental work in the field as other researchers attempted to reproduce and improve on the work at LANL.
A number of factors were responsible for the sudden interest in 2G HTS wire following the release of the results at LANL. Foremost, the current density achieved at the laboratory was close to that achieved in commercial 1G HTS wire when factoring in wire width (current increases proportionally with wire width). Thus even minor improvements in 2G HTS wire would enable it to surpass the performance of the standard first generation superconducting wire, at least at 77K. Another potential advantage of 2G wire was that it appeared better suited for commercial use in applications in which it would be exposed to strong magnetic fields. This is because the irreversibility line, which delineates the critical current density of a superconductor in the magnetic field/temperature plane, of 2G wire is set at a significantly higher temperature than standard 1G wire for all magnetic field values.
Superconducting Niobium Wire
The requirements for superconducting wire intended for use with strong magnetic fields are somewhat different than those that must be met for HTS wire utilized in different environments. Since at any given temperature, current density generally is reduced as the magnetic field is increased, some means is necessary to compensate for loss of performance in magnetic fields. This compensation is achieved in 2G HTS wire designed for in-field use via a process called pinning. Magnetic flux lines can be immobilized or pinned in place in the superconducting wire by the introduction of defects in the material, allowing high levels of current flow magnetic fields.
Superconducting Niobium Wire
The defects introduced via pinning must be on a nanometer scale analogous to that of magnetic flux lines. One of the most convenient ways to achieve this is to introduce into the superconducting material in some way tiny particles known as nanodots. Different methods of nanodot introduction have been developed at different laboratories and companies and various materials are employed. The precise type of nanodot utilized as well as the orientation of the particles has an effect on the pinning along various directions of magnetic field lines. The conductive capacity of 2G HTS wire is similarly affected by the characteristics of nanodots.
While efforts with 2G HTS are still underway, and if long enough lengths of the wire can be manufactured the wire may garner widespread commercial use, a third generation (3G) of superconducting wire is already causing a stir in the field. 3G wire is still in an early phase of development, but advances with this type of superconducting wire may not require as much time as 2G HTS wire because its production hearkens back to 1G HTS wire, which has been studied and tested over the course of many years. Like the pioneering generation of high-temperature superconducting wire, 3G superconducting wire is produced via a powder-in-tube process, but its unique design encompasses what is usually described as a roller skate powder structure.
Whatever the process utilized to produce HTS wire, such wire has many inherent advantages over conventional wire. The size of machines, such as power generators, that employ superconducting wire rather than copper wire can be reduced significantly, as can their weight. At the same time, HTS machines have the potential to provide greatly improved efficiency and stability. Superconducting wire has only just begun to enter the commercial arena, but the time, anticipated since the discovery of high temperature superconductors, in which HTS is fully integrated into a wide range of technological marvels appears to be closer than ever. With three generations of superconducting wire now available for research and development, an increased opportunity exists to meet the demand for better performance, electrical stability and mechanical durability in areas such as medicine, power transmission and distribution, national defense, and transportation. Any requirements unable to be fulfilled by the types of superconducting wire currently available are sure to lead to the development of improved versions of them or to future generations of HTS wire.