Transition temperatures



It was realized from the start that practical applications of superconductivity could become much more widespread if a high-temperature superconductor, that is, one with a high Tc, could be found. For instance, the only practical way to cool superconductors with transition temperatures below 20 K (?424°F) is to use liquid helium, which boils at a temperature of 4.2 K (?452°F) and which is rather expensive. On the other hand, a superconductor with a transition temperature of 100 K (?280°F) could be cooled with liquid nitrogen, which boils at 77 K (?321°F) and which is roughly 500 times less expensive than liquid helium. Another advantage of a high-Tc material is that, since many of the other superconducting properties are proportional to Tc, such a material would have enhanced properties. In 1986 the discovery of transition temperatures possibly as high as 30 K (?406°F) was reported in a compound containing barium, lanthanum, copper, and oxygen. In 1987 a compound of yttrium, barium, copper, and oxygen was shown to be superconducting above 90 K (?298°F). In 1988 researchers showed that a bismuth, strontium, calcium, copper, and oxygen compound was superconducting below 110 K (?262°F), and transition temperatures as high as 135 K (?216°F) were found in a mercury, thallium, barium, calcium, copper, and oxygen compound.

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