Critical current, in superconductors

Brandstatter, G., F.M. Sauerzopf, H.W. Weber and B.W. Veal, 1998a, in Critical Currents in Superconductors for Practical Applications, eds L. Zhou, H.W. Weber, E.W Collings (World Scientific, Singapore) p. 325. 

Belous, N.A., Gabovich, A.M., Moiseev D.P. and Postnikov, V.M., Detection of the Influence of Disorder and of Frustration of Weak Links on the Critical Current in a Three-Dimensional Granular Superconductor BaPb0 7BBi0 26(V Sov. Phys. Solid State 28(9) 1615 (1986). 

Aponte, J., Abache, H., Sa-Neto, A. and Octavio, M., Temperature Dependence of the Critical Current in High-Tc Superconductors, Phys. Rev. B39 2233 (1989). 

One of the methods of rising critical currents in conventional superconductors is the introduction of defects of structure which work as the pinning centers for the vortex lattice. This method can also be applied for doped fullerite. 

This second method of destroying superconductivity is known as the Silsbee effect. This hypothesis states that when the electric current flowing in a superconductor produces a magnetic field at the surface of the material that equals or exceeds the threshold field, the normal state is restored. The current corresponding to the threshold field is called the threshold current. For example, for a long wire of diameter d, the magnetic field produced at the surface of the wire by an electric current / is = Ijnd. The critical current in this case would be... 

Single or multifilament wire with a matrix of Cu or CuNi is commonly available in several diameters. Single-filament wire is usually reserved for low current applications. Multifilament wire has a higher critical current, since supercurrents flow only in the surfaces of superconductors, and in multifilamentary wire there is more superconducting surface. 

Metallic taste, 11 565 Metallic tungsten, 25 374 Metallic Type II superconductors, critical current density value in, 23 822 Metallic vanadates, 25 513 Metalliding, 15 251 Metalliferous oxides deposits of, 17 689-690 in ocean basins, 17 693 Metalliferous sulfide deposits, 17 690-691 Metalliferous sulfides, in ocean basins, 17 693-694... 

The rather low critical current densities expected in bulk samples of these granular superconductor materials (103)(104), and their relatively low magnetic critical fields (50)(78)(105)(106), allied with the relatively low Tc s observed would appear to hinder development of superconducting applications for these materials. Even the critical current of 5 X 105 A/cm2 observed for single crystalline thin films (39) is now considered low for a superconductor at 4.2K. However, when considering the applicability of a material to a task,... 

The current transfer problem that had been identified with low temperature superconducting composites deserves additional mention for the high temperature superconductors, that in the bulk material are frequently not fully dense. Making the electrical connection in such a manner as to obtain uniform current distribution throughout the cross section of the material is difficult. The method described by Jin, et al. (24) with embedded wires or particles may provide for a significant improvement but the present techniques used to determine the critical current by a surface contact on the ceramic sample are subject to this problem. A discussion for the multifilamentary wire of NbsSn is provided by Goodrich and Fickett (30) and this discussion is likely to be similar to the high temperature materials that are not fully dense. 

In addition to a critical temperature and critical field, all superconductors have a critical current density, Jc, above which they will no longer superconduct. This limitation has important consequences. A logical application of superconductors is as current-carrying media. However, there is a limit, often a low one, to how much current they can carry before losing their superconducting capabilities. The relationship between Jc, He, and Te for a Type II superconductor is shown in Figure 6.32. Notice that the Hc-Tc portion of this plot has already been presented in Figure 6.10 for a Type I superconductor. 

Similarly, if the current in the superconductor exceeds a critical current, the superconductivity is destroyed. This is known as the Silsbee effect. The size of the critical current is dependent on the nature and geometry of the particular sample. 

More recently, other metals such as thallium, bismuth, and lead have been included in superconductor formulation. In one interesting series, the critical temperature has been found to increase with increasing n in susperconductors of the type HBa2Can CunO ,l+2 to a max,mum 122 K for n = 4 (Fig. 7.34).45 The current maximum critical temperature is 125 K for a closely related TUBa2CujO,v-... 

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