Superconducting sheets

Let it be assumed that a soft superconducting sheet of thickness W in a position vertical to the drawing plane is placed close to a coil C with n windings through which an... 

Since an increase in the power required by the ac coil could not be noticed when the shields were applied, screening of ac magnetic fields (50 cps) by means of superconducting sheet metals seems to be possible in practice. 

Tests carried out on transformer models of a ring type and with enclosed magnetic fields (by means of superconducting sheet metals) showed that with suitable layout current densities up to about 150 amp/mm in niobium wires (0,004 in. diameter) could be reached without destroying the superconductivity of the wires. Flux densities within the winding spaces measured up to 400 gauss. 

Superconductivity has also been discovered in rather exotic materials, including the following Buckminsterfullerene (Cgo) doped with ICI Carbon nanotubes (superconductivity in just one direction) Nickel borocarbides, which contain Ni2 B2 layers alternating with R C sheets, where R is a rare earth element such as Er and organic superconductors that contain planar organic cations and oxoanions. Chemists and physicists continue to study these and other families of superconductors. 

In these and the other cuprate superconductors, the part of the structure that leads to superconductivity is the slab of Cu02 sheets. When more than one sheet is present, they are separated by cation layers, Q (usually Ca or Y) to give a sequence Cu02-(Q-Cu02) i, which forms the superconducting layer in the material. The index n is the total number of Cu02 layers in the phase, which is equal to the formula number of Cu atoms present (Fig. 8.5). 

Figure 8.5 Superconducting planes found in cuprate superconductors (a) a single Cu02 sheet and (b) a Cu02 (Q — Cu02) i superconducting layer.

While attempts are still being made to find materials which exhibit superconductivity at temperatures as close as possible to room temperature, the main thrust of research is now directed towards superconducting materials that can withstand high current densities and are malleable and ductile (i.e. able to be prepared as thin sheets or wire). 

Many ceramics are partially polymeric in structure. These include the new superconductive materials that exist as polymeric sheets connected by metal ions similar to many of the silicate sheets. 

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