Chemical compounds known for exhibiting high temperature superconductivity
Unit cell of YBCO
Rare-earth barium copper oxide (ReBCO[1]) is a family of chemical compounds known for exhibitinghigh-temperature superconductivity (HTS).[2] ReBCO superconductors have the potential to sustain stronger magnetic fields than other superconductor materials. Due to their highcritical temperature and critical magnetic field, this class of materials are proposed for use in technical applications where conventional low-temperature superconductors do not suffice. This includesmagnetic confinementfusion reactors such as theARC reactor, allowing a more compact and potentially more economical construction,[3] and superconducting magnets to use in futureparticle accelerators to come after theLarge Hadron Collider, which utilizes low-temperature superconductors.[4][5]
YBCO critical current (KA/cm2) vs absolute temperature (K), at different magnetic field (T).[9]
The most famous ReBCO isyttrium barium copper oxide, YBa2Cu3O7−x (or Y123), the first superconductor found with a critical temperature above theboiling point ofliquid nitrogen.[10] Itsmolar ratio is 1 to 2 to 3 for yttrium, barium, and copper and it has aunit cell consisting of subunits, which is the typical structure ofperovskites. In particular, the subunits are three, overlapping and containing an yttrium atom at the center of the middle one and a barium atom at the center of the others. Therefore, yttrium and barium are stacked according to the sequence [Ba-Y-Ba], along an axis conventionally denoted byc, (the vertical direction in the figure at the top right).
The resulting cell has anorthorhombic structure, unlike other superconductingcuprates that generally have atetragonal structure. All the corner sites of the unit cell are occupied by copper, which has two different coordinates, Cu(1) and Cu(2), with respect to oxygen. It offers four possible crystallographic sites for oxygen: O(1), O(2), O(3), and O(4).[11]
Because these kind of materials are brittle it was difficult to create wires from them. After 2010, industrial manufacturers started to produce tapes,[12] with different layers encapsulating the ReBCO material,[13] opening the way to commercial uses.
In September 2021Commonwealth Fusion Systems (CFS) created a test magnet with ReBCO tape that handled a current of 40,000amperes, with a magnetic field of 20tesla at 20K.[14][15] One important innovation was to avoid insulating the tape, saving space and lowering required voltages. Another was the size of the magnet: 10 tons, far larger than any prior experiment. The magnet assembly consisted of 16 plates, called pancakes, each hosting a spiral winding of tape on one side and cooling channels on the other.[16]
In June 2024, the first plasma was achieved in theHH70 tokamak, developed by the China-based fusion energy company Energy Singularity. Using ReBCO as material for the superconductors enabled the company to reduce the size ot the HH70 tokamak to two percent of conventional tokamaks.[19]
