SUPERCONDUCTIVITY

SUPERCONDUCTIVITY
There are two properties that a material must possess to be considered a superconductor:
1. ? = 0
2. B = 0
Zero resistivity is observed in a superconductor at all temperatures below a critical temperature, Tc, as illustrated in Figure 3.21. At Tc the material changes from a state of normal conduction to the superconducting state. In the superconducting state an induced current will flow indefinitely: without loss.
Superconductivity has been observed in all the classes of materials: metals, ceramics, and polymers. Niobium is the element with the highest Tc, 9.2 K, whereas for tungsten Tc is only 0.0154 K. An interesting fact is that metals having the highest ?, e.g., Cu, Ag, and Au, are not superconducting even at extremely low temperatures, if at all. It is the metals that are the poorer electrical conductors that make the better superconductors, albeit still at very low temperatures.
3.14 CERAMIC SUPERCONDUCTORS
The earliest nonmetallic superconductors were NbO and NbN. Both materials have a rocksalt crystal structure. What was significant about the discovery of superconductivity in these materials (they are of no practical use) is that they linked the phenomenon to ceramics and cubic crystal structures.