A superconductor is a material that can conduct electricity or transport electrons from one atom to another with no resistance. This means no heat, sound or any other form of energy would be released from the material when it has reached "critical temperature" (T_c), or the temperature at which the material becomes superconductive. Unfortunately, most materials must be in an extremely low energy state (very cold) in order to become superconductive. Research is underway to develop compounds that become superconductive at higher temperatures. Currently, an excessive amount of energy must be used in the cooling process making superconductors inefficient and uneconomical.

Superconductors come in two different flavors: type I and type II. (1)

Type I Superconductors
A type I superconductor consists of basic conductive elements that are used in everything from electrical wiring to computer microchips. At present, type I superconductors have T_cs between 0.000325 °K and 7.8 °K at standard pressure. Some type I superconductors require incredible amounts of pressure in order to reach the superconductive state. One such material is sulfur which, requires a pressure of 9.3 million atmospheres (9.4 x 10¹¹ N/m²) and a temperature of 17 °K to reach superconductivity. Some other examples of type I superconductors include Mercury - 4.15 °K, Lead - 7.2 °K, Aluminum - 1.175 °K and Zinc - 0.85 °K. Roughly half of the elements in the periodic table are known to be superconductive. (1)

Type II Superconductors
A type II superconductor is composed of metallic compounds such as copper or lead. They reach a superconductive state at much higher temperatures when compared to type I superconductors. The cause of this dramatic increase in temperature is not fully understood. The highest T_c reached at stardard pressure, to date, is 135 °K or -138 °C by a compound (HgBa₂Ca₂Cu₃O₈) that falls into a group of superconductors known as cuprate perovskites. This group of superconductors generally has a ratio of 2 copper atoms to 3 oxygen atoms, and is considered to be a ceramic. Type II superconductors can also be penetrated by a magnetic field whereas a type I can not. (2)

References
(1) http://www.superconductors.org/Type1.htm
(2) http://www.superconductors.org/Type2.htm

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