CoolMat Cool Materials High Tc Superconductors

Cool Materials High-Temperature Superconductors

Everything is relative and "High-Temperature" Superconductors (HTSC) are no exception. Would you say a temperature of -180ºC is high ?. One hundred and eighty degrees Celsius below the freezing point of water is a hell of a low temperature to most people. But to scientists used to deal with temperatures much lower than that, the discovery of materials presenting superconductivity at those temperatures was hot news.

In the old days (1910-1986) superconductivity was only known for a few metals and alloys and a bunch of ceramic materials. but in all cases critical temperatures were extremely low (10 to 20K, that is -263 to -253ºC)

superconductivity: a condition of certain materials which are able to conduct electrons with zero resistance. This property vanishes above a temperature characteristic of each material (critical temperature or Tc), or under a magnetic field stronger than a critical value (Hc) or when subject to a current density larger than its characteristic critical value (Jc).

In April 1986 a revolutionary change took place with the discovery of superconductivity at record high temperatures of 27K (-246ºC) in a complex oxide of copper lanthanum and barium. This was the starting point of an apparently exponential growth of Tc values. In the hectic days of 1987 the oxide YBa₂Cu₃O₇(also known as 123, YBaCuO or YBCO) was identified as the material responsible for a record-high Tc of 92K (-181ºC). This was above the boiling point of liquid nitrogen and represented the promise of a new, more accessible technology of superconducting devices. So... is that high temperature or what ?.

In general, the structure of high-Tc superconductors is like a GIANT SANDWICH.
Layers of Copper and Oxygen atoms (the meat, the essential part for superconductivity) alternate with layers of other metals (the bread).
The structure of the 123 superconductor is shown below. In fact there are two structures: the one on the left corresponds to the oxide with a smaller content of oxygen (YBa₂Cu₃O₆) which is not a superconductor. On the right, we have the same structure with just some more oxygen added (doping) leading to the superconducting YBa₂Cu₃O₇ In this case the meat are the planes of Cu-O forming the bases of the blue pyramids and the bread is also formed of copper atoms (small blue balls with cyan bonds)

YBa₂Cu₃O₆

YBa₂Cu₃O₇

Perspective views of the structures of YBa₂Cu₃O₆ (insulator) and YBa₂Cu₃O₇ (superconductor). Small yellow balls are Y³⁺ ions, larger orange balls are Ba²⁺ ions. Small blue balls Copper atoms and small red balls O^2- ions. Unit cells are outlined in yellow. Blue pyramids represent the active copper ions coordinated by five oxide ions. Cyan sticks represent bonds between copper ions (in the "bread" layers) and oxide ions.

After YBaCuO new and more complex copper oxides have been discovered and higher Tc's have been achieved. In order of increasing Tc we can mention a whole family of Bismuth-containing copper oxides, then Thallium-based copper oxides and finally the series of Mercury copper oxides. These days the record high Tc (130K, -143ºC) is held by the oxide Ca₂Ba₂Cu₃HgO₈ , the structure of which is shown below. For comparison we also include the structure of CaBa₂Cu₂HgO₆ , which can be easily compared with that of YBa₂Cu₃O₆ .

CaBa₂Cu₂HgO₆ Ca₂Ba₂Cu₃HgO₈

Perspective views of the structures of CaBa₂Cu₂HgO₆ and Ca₂Ba₂Cu₃HgO₈ (Tc =130K). Small white balls are Ca²⁺ ions, larger orange balls are Ba²⁺ ions. Small blue balls Copper atoms, small purle balls Mercury atoms and small red balls O^2- ions. Unit cells are outlined in yellow. Blue pyramids represent the active copper ions coordinated by five O² ions. Green sticks are bonds between Hg ions (in the "bread" layers) and oxide ions.

The 130K record was reached in 1993 and since then things seem to have slowed down in this respect. Does that mean that research on high-Tc superconductors is over ?. NO WAY ! . There are many other aspects of HTSC materials that need to be improved and developed. Getting higher current densities and fabricating practical devices are two major examples of this.
Concerning the search for new materials we should never rule out the possibility of getting new ones with higher Tc or other improved properties. The recent history of HTSC shows a taste for dangerous metals (Bi, Tl, Hg are all highly toxic). It would be great to change that "heavy-metal" trend and come out with new superconducting oxides with a better company for copper.

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