SUPERCONDUCTORS: PROCESSING OF HIGH-TC BULK, THIN FILM, AND WIRES

Superconductors are a class of materials possessing two unique properties: the complete loss of electrical re­sistivity below a transition temperature called the critical temperature (Tc) and the expulsion of magnetic flux from the bulk of a sample (diamagnetism) in the superconducting state. The latter property is also known as the Meissner-Ochsenfeld effect or more commonly, the Meissner effect (1). At temperatures above Tc, these materials possess electrical resistivity like ordinary conductors, although their normal state properties are unusual in many aspects. The abrupt change from normal conductivity to superconductivity occurs at a ther­modynamic phase transition determined not only by the temperature but also by the magnetic field at the surface of the material and by the current carried by the material. Several metals and metallic alloys exhibit superconductivity at temperatures below 22 K, and will be henceforth called low temperature superconductors (LTS). In 1950, superconductivity was explained as a quantum mechanical phenomenon by the London phe­nomenological theory (2). Later, the two-fluid phenomenological model explained the electronic structure of a superconductor as a mixture of superconducting and normal electrons, with the proportion of superconducting electrons ranging from zero at the onset of superconductivity to 100% at 0 K (3). In 1955, Bardeen, Cooper, and Schrieffer’s (BCS) theory explained that superconductivity was the result of the formation of electron pairs of opposite spins (known as Cooper pairs), primarily owing to electron-phonon coupling (4). The BCS theory proved to be the most complete theory for explaining the superconducting state and the normal state of LTS materials. A major development in superconductors was the discovery by Josephson in 1963 that Cooper pairs show macroscopic phase coherence, and that such pairs can tunnel through a thin insulating layer sandwiched between two superconducting layers [the superconductor-Insulator-super-conductor (SIS) junction known as the Josephson junction] (5). This effect, called the Josephson effect (5), caused a flurry of activity in the fields of high-speed computer logic and memory circuits in the 1960s and 1970s, since it can be used to make high-speed low-power switching devices. Problems were encountered in the mass fabrication of Josephson junctions for complex systems such as digital computers. Although the applications of LTS for electrical applications and electronics were demonstrated, the cost of cooling was too high for the commercial development of LTS.

The era of high-temperature superconductors (HTS) began in 1986 when two IBM Zurich researchers, K. A. Muller and J. G. Bednorz, reported the occurrence of superconductivity in a lanthanum barium copper oxide (LaBaCuO) at 30 K(6). Soon after, M. K. Wu, P. W. Chu, and their collaborators at the University of Alabama and University of Houston (7), respectively, announced the discovery of 90 K superconductivity. Since these two historic discoveries, there has been substantial progress in HTS technology. Several new families of cuprates including BiSrCaCuO (8), TlCaBaCuO (9), and HgCaBaCuO (10) have been found to be superconducting above 90 K. These discoveries make feasible electrical and electronics applications at temperatures above the boiling point of liquid nitrogen (77 K). Cuprate superconductors with a Tc value higher than 30 Khave been classified as high-temperature superconductors. The obvious advantage of using liquid nitrogen rather than liquid helium for cooling is its higher heat of vaporization, which not only simplifies the design of cryostats but also the cost of cooling. Furthermore, liquid nitrogen (at $0.25/liter) is more than an order of magnitude cheaper than liquid helium (at $5/liter). Progress made in cryocoolers has made feasible HTS applications in electrical wires,

Bulk

Thin films

Wires, tapes

Structure, electrical and magnetic properties

Epitaxial growth on single crystal substrates

Processing of silver- sheathed wires, coated conductors on metallic substrates

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1

Relationship between processing and critical superconducting properties

Large area depositions, epitaxial films on useful substrates with buffer layers

Long-length wires with high current density at high magnetic fields

HTS materials

Demonstration of applications: High current leads, power devices-transformers. motors, and large magneis

Demonstration of applications: High Q microwave components. SQUIDs. signal processing, digital circuits, and sensors

Demonstration of applications: Microwave cavities, magnetic shielding, and frictionless bearings

Fig. 1. A generalized road map for the HTSs technology. YBCO 123 compound has been the most studied material among the HTSs. Most applications have been demonstrated with the YBCO superconductor. Most applications in HTS wires have been demonstrated using BSCCO.

magnets, and electronics. The excitement and challenges posed by these HTS materials have touched multiple disciplines, such as physics, chemistry, material science, and electrical engineering. Tremendous progress has been made in the application of HTS materials in such areas as Superconducting Quantum Interference Devices (SQUIDs), passive microwave devices, and long-length wires, as illustrated in the road map for the HTS technology, shown in Fig. 1. Better-quality materials emerging from refined processing methods have made it possible to separate the intrinsic properties of HTS from its extrinsic properties. The interrelationships of processing with structural, physical, electrical, and magnetic properties continues to be an area of intensive scientific research. In this article, we provide an overview of important high-temperature superconducting materials, their properties, and promising procedures for synthesizing bulk, thin film and wire forms of HTS conductors for engineering applications.

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