We're not sure which of two questions you're asking. One would be why superconductors quit superconducting when they're hot. The other would be why they don't heat up like ordinary conductors when current flows through them. We'll give some background, then more or less answer both questions.
All superconducting materials stop being superconductors when they are warmed up. Each superconductor has a "critical temperature", below which it superconducts, and above which it is just an ordinary material which resists electrical flow. All superconductors known before 1987 had critical temperatures below 23 degrees Kelvin (really really really cold). Newer ones have critical temperatures around or perhaps even above 125 Kelvin (just really really cold), and are made out of ceramic oxide materials.
You can always warm these materials up to room temperature or even much higher before they melt or vaporize, they just stop superconducting. People are actively searching for materials with higher critical temperatures, which would allow exciting applications (lossless power transmission, magnetic levitation) without expensive cryogenic equipment.
You can find out more background about superconductors here at howstuffworks.com. although their explanation of superconductivity misses the key point that it's a collective effect involving all of the electrons and would not occur just from separate pairs of electrons. It also distorts other aspects of the explanation- a reminder of how fallible we web-posters are.
Ordinarily, when an electric field pushes electrons through a metal it makes them (on average) gain energy, which they then lose when they bounce off atoms. That extra energy heats up the metal. In a superconductor, the electrons have fallen into a collective state, where they all lower their energy by moving in an organized fashion. No electron can pick up just a little energy from the field, then dump it back into the atoms. Instead, to leave the collective state and electron must pick up a big slug of energy, and that happens very rarely at low temperature because there's not much thermal energy around. So virtually no energy is transferred from the field to the atoms via the electrons. The current therefore doesn't heat up the superconductor.
If you warm up the superconductor, the thermal energy starts making more and more electrons hop out of the collective state. As more start to leave, there's less of a collective state left to hold the others in. So instead of the collective state just gradually getting fainter, at some temperature it disappears altogether. That's sort of like the way other collective states (say crystals) melt at a sharp temperature.
Tom and Mike
(republished on 08/02/06)