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Normally, electrical resistance of metals increases as temperature rises and decreases as temperature falls. For some substances, if the temperature approaches absolute zero, i.e. -273 °C, resistance falls to zero as well. The temperature threshold beyond which a sudden and significant decrease occurs in resistance is called critical temperature ( T c ).
The figure indicates the critical temperatures of various metals. The ratio of W (resistance of the metal at a given temperature) and W0 (resistance at 0 °C) is plotted on the Y- axis. The state of absolute superconductivity is reached when W/W 0 is 0. Historically, i ntermetallic compounds such as Nb 3 Sn and Nb 2 Zr were first inserted into superconductor cables because their T c (absolute temperature ) is 18 K, which is higher than that of pure metals. However, liquid helium is necessary to cool the conductors close to the absolute zero. In 1986, a ceramic superconductor made of Y-Ba-Cu-oxide was invented, which immediately attracted considerable attention. The invention was important because instead of the expensive helium cooling, it made use of the less costly liquid nitrogen cooling ( -210 °C). However, the ceramic superconductor has several disadvantages: its current density is small (superconductivity is lost at high current ), it is soluble in water and therefore it is sensitive to moisture and the strength of the magnetic field is limited. Anyway, the fact that such a superconductor could be built into PCBs suggests the foreseeable progress of thin film preparation by vapor deposition. Research for suitable solders for superconducting circuit boards will also gain importance. In the 21st century, superconductors could play an important role in projects such as power generation using superconducting coils, the construction of high-speed vessels, magnetic levitation, or storing electricity. A maglev train where the cars float above the rail lines thanks to superconductivity could reduce traveling time between Berlin and Amsterdam or Warsaw to a couple of hours while travelers could reach Budapest from Berlin within three hours.