Low-Temperature Superconducting Alloys

FIGURE 7-19 Temperature Dependence of Resistivity in Semiconductors, Metals, and Superconductors. 

Type II superconductors have a more complicated field dependence. Below a given critical temperature, they exclude the magnetic field completely. Between this first critical temperature and a second critical temperature, they allow partial penetration by the field, and above this second critical temperature they lose their superconductivity and display normal conductance behavior. In the intermediate ternperamre region, these materials seem to have a mixture of superconducting and normal regions. 

The levitation demonstration works only with Type II superconductors because the magnetic field lines that do enter the superconductor resist sideways motion and allow the balance of magnetic repulsion and gravitation to float the magnet above the superconductor. With Type I superconductors, the magnetic field lines cannot enter the superconductor at all and, because there is no resistance to sideways motion, the magnet will not remain stationary over the superconductor. 

The materials used in the coils of superconducting magnets are frequently Nb-Ti-Cu or Nb3Sn-Cu mixtures, providing a balance between ductility for easier formation into wire and Tc, which is about 10 K for these materials. 

A major goal of superconductor research is a material that is superconducting at higher temperatures, to remove the need for liquid helium and liquid nitrogen for cooling. 

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