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Materials that have no electrical resistance under normal ambient conditions, especially at room temperature, are considered by many to be the holy grail of solid-state physics. After the discovery of the phenomenon of so-called superconductivity in metals close to absolute temperature zero more than 100 years ago, the search for such materials with the so-called high-temperature superconductors on a ceramic basis experienced a real hype in the 1980s. However, this has now given way to clear disillusionment. Certain hydrogen-containing substances are currently the focus of interest, especially after such a compound could be synthesized, which is still superconducting at 15 ° C.

However, this was only possible under extremely high ambient pressure, which rules out practical use. In spite of this, efforts in this class of materials have given new impetus to the search for practical room temperature superconductors.

Superconductors completely lose their electrical resistance below a so-called transition temperature that is typical for them and become ideal conductors. The “conventional” metallic superconductors, which have been known since 1911, have such low transition temperatures that complex cooling with liquid helium (boiling temperature 4.2 K; 0 K = –273.15 ° C) is necessary. At the moment they are still the only superconductors used on a large scale and are used, for example, in magnetic resonance tomographs to generate high magnetic fields. The most frequently used material is still niobium or certain niobium alloys, which, due to their excellent ductility, allow the problem-free production of wires even in the length of several kilometers required for magnet construction.

Die wichtigsten Vertreter der keramischen Hochtemperatursupraleiter basieren bis heute auf sogenannten Cupraten (Kupferoxiden). Nachdem man zunächst Sprungtemperaturen über 30 K realisiert hatte, fand man innerhalb kurzer Zeit zahlreiche Stoffe, die bereits oberhalb der Siedetemperatur des flüssigen Stickstoffs (77 K) supraleitend werden. Diese zunächst rasante Entwicklung stagniert jedoch schon seit langem. Der derzeit höchste Wert für die Sprungtemperatur liegt für diese Materialklasse seit 1994 bei ungefähr 138 K. Ein weiteres wesentliches Problem für die Anwendung dieser HTSL ist die Tatsache, dass beim Supraleitungseffekt das äußere Magnetfeld und die zu tragende Stromdichte eine ebenso wichtige Rolle spielen wie die Temperatur und ebenfalls einen kritischen Wert nicht überschreiten dürfen. Kommt eine der drei Größen in die Nähe des für sie maximalen Wertes, müssen die beiden anderen gegen Null gehen. Trotzdem können Hochtemperatursupraleiter auf keramischer Basis inzwischen als Strombegrenzer in Kraftwerken praktisch eingesetzt werden. Als Leiter für elektrischen Strom stehen sie an der Schwelle zur industriellen Fertigung. So gibt es für den Stromtransport in regionalen Netzen in Deutschland bereits seit Jahren ein erfolgreiches Pilotprojekt. Das wichtigste heute etablierte Anwendungsfeld für Cuprate ist die hochgenaue Messung von Magnetfeldern z. B. im menschlichen Gehirn oder in der zerstörungsfreien Materialprüfung.

Here they are used for so-called SQUIDs, which use the magnetic field dependence of the quantum mechanical Josephson effect that occurs in superconductors.

The search for room temperature superconductors gained new momentum when, at the beginning of this millennium, high transition temperatures were theoretically predicted for certain hydrogen compounds (hydrides) that were highly compressible during cooling under laboratory conditions. The efforts to synthesize such materials led to success in various intermediate steps, with the climax so far of a material discovered in 2020 with a transition temperature of 288 K, i.e. 15 ° C.

Even if the exact crystalline structure of this material has not yet been finally clarified, one can again speak of a metallic superconductor. After compounds of two elements had previously been investigated, it is the first three-element hydride investigated in this context. It is first produced by sprinkling carbon into a sample container filled with methane and hydrogen sulfide. In a diamond press at a pressure of 2.7 million bar (!) The superconducting metal is then made from carbon, sulfur and hydrogen atoms.

It is obvious that precisely this material cannot be used in any practical application. However, research on hydrides made up of three elements is only just beginning, and further records are to be expected in the near future, possibly even at lower ambient pressures. There is also uncertainty with regard to further developments with regard to superconductivity in general. To date, this phenomenon has not been comprehensively described theoretically in all its forms. In particular, there is no theory that would rule out the existence of room temperature superconductors. However, if these are to be used in practice, they must also combine a number of other properties. This also includes high critical current densities and magnetic fields, suitable manufacturing processes and adequate mechanical properties. Last but not least, they must of course also function under other normal ambient conditions (e.g. under normal ambient pressure).

The great interest in such materials, which would profoundly change modern technology, is shown by the five Nobel Prizes that have been awarded so far in connection with this complex of topics. Until room temperature superconductors can be used in practice, one or the other Nobel Prize in this regard will certainly be added.

Jürgen Kohlhoff