High-temperature superconductors (abbreviated high-Tc or HTS) are operatively defined as materials that behave as superconductors at temperatures above 77 K (−196.2 °C; −321.1 °F), the boiling point of liquid nitrogen, one of the simplest coolants in cryogenics.
All materials currently known to conduct at ordinary pressures become superconducting at temperatures far below ambient, and therefore require cooling. The majority of high-temperature superconductors are ceramic materials. On the other hand, Metallic superconductors usually work below −200 °C: they are then called low-temperature superconductors. Metallic superconductors are also ordinary superconductors, since they were discovered and used before the high-temperature ones.
Ceramic superconductors are now becoming suitable for some practical use, but they still have many manufacturing issues and there are very few successful practical examples of employment.
Most ceramics are brittle which makes the fabrication of wires from them very problematic.The major advantage of high-temperature ceramic superconductors is that they can be cooled by using liquid nitrogen.
On the other hand, metallic superconductors usually require more difficult coolants - mostly liquid helium. Unfortunately, none of high-temperature superconductors are coolable using only dry ice, and none of them work at room temperature and pressure (they work well below the lowest temperature recorded on Earth). All high-temperature superconductors require some type of cooling system.
The main class of high-temperature superconductors are in the class of copper oxides (only some particular copper oxides).
The second class of high-temperature superconductors in the practical classification is the class of iron-based compounds.Magnesium diboride is sometimes included in high-temperature superconductors: It is relatively simple to manufacture, but it superconducts only below −230 °C, which makes it unsuitable for liquid nitrogen cooling (approximately 30 °C below nitrogen triple point temperature). For example, it can be cooled with liquid helium, which works at much lower temperatures.
Many ceramic superconductors physically behave as superconductors of the second type.
The first high-temperature superconductor was discovered in 1986, by IBM researchers Bednorz and Müller, who were awarded the Nobel Prize in Physics in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials".Some extremely-high pressure superhydride compounds are usually categorized as high-temperature superconductors. In fact, many articles on high-temperature superconductors can be found on this research on high pressure gases, which is not suitable for practical applications. The current Tc record holder is carbonaceous sulfur hydride, beating the previous record held by lanthanum decahydride by nearly 30 °C.