Practical superconductors

In 1996, a 50 m underground superconducting cable (wound from 6 km of a BSCCO ribbon) and a 150 kW superconducting electric motor were successfully demonstrated. Further development of practical superconductor devices might well depend more on the development of suitable refrigeration technology than on the preparation of new superconducting materials. 

Rediscovery. The topic and potential of superconductivity essentially was rediscovered in the late 1970s and early 1980s, during which period the research activity safely can be described as zealous. This was fueled by the scores of probable applications of superconductors, but which to date largely remain as promises. Research continues at a good pace, and the topic is much better understood as compared with a decade ago. As pointed out later m this article, much excellent technological fallout has occurred from many millions of dollars invested in research. The search for the ultimately practical superconductor, although still elusive, has reinforced tiie multidisciplinary sciences involved, notably physics and chemistry. 

The definition of the critical current of a multifilamentary composite or cable superconductor may lead to some difficulties when comparing the properties of different samples. Those difficulties can be solved by the adoption of standard definitions and measurement techniques [ ]. The maximum current that can be carried by a conductor cannot be specified by any criteria. This current is very dependent on the current-voltage characteristic of the conductor, the cooling conditions, and the operating conditions, which determine the losses during the current rise time. For the computation of hysteretic losses in a practical superconductor by means of the well-known formula, it is important to determine the actual values of the critical current density, which is very anisotropic. 

For example. He for pure Pb is 600 Gauss at 4 K. For Pb2W%In, Hci = 400 Gauss and Hci 3600 Gauss. The area under the M versus H curve is the same for both systems as illustrated in Figure 26.5, but a type-II system can be operated at much higher fields. Therefore, all practical superconductors will be type-II. 

Whether a material is type-I or type-II is determined by the ratio of the penetration depth A (the distance into the surface a magnetic field can penetrate) to the coherence length (essentially the diameter of the Cooper pair). Type-II behavior occurs when A > y/2. Most pure materials are type-I, but adding impurities such as alloying with another metal increases A and decreases so most practical superconductors are alloys or intermetallic compounds. 

There are presently four famihes of high-temperature superconductors under investigation for practical magnet appheations. Table 11-25 shows that all HTS are copper oxide ceramics even though the oxygen content may vary. However, this variation generally has little effect on the phvsical properties of importance to superconductivity. 

The most likely CVD applications of these superconductors to reach the practical stage are coatings for semiconductor and other electronic-related applications. For 1 arger current-carrying applications, a superconductor coating over a metallic conductor such as copper may also become a practical design because of its advantage over a monolithic superconductor wire. It is able to handle current excursions and has better mechanical properties. 

The high reactivity of copper superconductors is a large obstacle to their practical application. Many spectroscopic studies have been devised to investigate the surface chemistry of these materials, but electrochemical investigations can also be important. 

Solid-state (topochemical) polymerization of cyclic disulfur dinitride to poly(sulfur nitride) (or polythiazyl), -fSN, occurs on standing at ambient temperature or higher [Banister and Gorrell, 1998 Labes et al., 1979 Ray, 1978]. Disulfur dinitride is obtained by sublimation of tetrasulfur tetranitride. Polythiazyl is a potentially useful material, since it behaves like a metal. It has an electrical conductivity at room temperature about the same order of magnitude as a metal like mercury and is a superconductor at 0.3°C. Polythiazyl also has high light reflectivity and good thermal conductivity. However, it is insoluble and infusible, which prevents its practical utilization. 

The purpose of this chapter is not to address the continuing controversy about the electronic nature of the electron-doped superconductors, but rather to review the crystal chemistry of the T -Nd2Cu04 system and give practical details on how to prepare crystallographically pure electron-doped superconductors in ceramic or single-crystal form with high Meissner fractions. 

Molecular superconductors are of more immediate practical interest. Experimental evidence indicates superconductivity is incipient at 30 K, a temperature above the critical temperature for other known materials. Superconductors have a major potential use in providing two distinguishable states associated with tunneling of electron pairs through a... 

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