Superconducting wires are electrical wires made of superconductive material. When cooled below their transition temperatures, they have zero electrical resistance. Most commonly, conventional superconductors such as niobium–titanium are used, but high-temperature superconductors such as YBCO are entering the market.
Superconducting wire's advantages over copper or aluminum include higher maximum current densities and zero power dissipation. Its disadvantages include the cost of refrigeration of the wires to superconducting temperatures (often requiring cryogens such as liquid nitrogen or liquid helium), the danger of the wire quenching (a sudden loss of superconductivity), the inferior mechanical properties of some superconductors, and the cost of wire materials and construction.
Its main application is in superconducting magnets, which are used in scientific and medical equipment where high magnetic fields are necessary.
Important parameters
The construction and operating temperature will typically be chosen to maximise: Critical temperature Tc, the temperature below which the wire becomes a superconductor Critical current density Jc, the maximum current a superconducting wire can carry per unit cross-sectional area (see images below for examples with 20 kA/cm2).
Superconducting wires/tapes/cables usually consist of two key features: The superconducting compound (usually in the form of filaments/coating) A conduction stabilizer, which carries the current in case of the loss of superconductivity (known as quenching) in the superconductoring material.
The current sharing temperature Tcs is the temperature at which the current transported through the superconductor also starts to flow through the stabilizer. However, Tcs is not the same as the quench temperature (or critical temperature) Tc; in the former case, there is partial loss of superconductivity, while in the latter case, the superconductivity is entirely lost.
LTS wire
Low-temperature superconductor (LTS) wires are made from superconductors with low critical temperature, such as Nb3Sn (niobium–tin) and NbTi (niobium–titanium). Often the superconductor is in filament form in a copper or aluminium matrix which carries the current should the superconductor quench for any reason. The superconductor filaments can form a third of the total volume of the wire.
Preparation
Wire drawing
The normal wire-drawing process can be used for malleable alloys such as niobium–titanium.
Surface diffusion
Vanadium–gallium (V3Ga) can be prepared by surface diffusion where the high temperature component as a solid is bathed in the other element as liquid or gas. When all components remain in the solid state during high temperature diffusion this is known as the bronze process.
HTS wire
High-temperature superconductor (HTS) wires are made from superconductors with high critical temperature (high-temperature superconductivity), such as YBCO and BSCCO.
Powder-in-tube
The powder-in-tube (PIT, or oxide powder in tube, OPIT) process is an extrusion process often used for making electrical conductors from brittle superconducting materials such as niobium–tin or magnesium diboride, and ceramic cuprate superconductors such as BSCCO. It has been used to form wires of the iron pnictides. (PIT is not used for yttrium barium copper oxide as it does not have the weak layers required to generate adequate 'texture' (alignment) in the PIT process.)
This process is used because the high-temperature superconductors are too brittle for normal wire forming processes. The tubes are metal, often silver. Often the tubes are heated to react the mix of powders. Once reacted the tubes are sometimes flattened to form a tape-like conductor. The resulting wire is not as flexible as conventional metal wire, but is sufficient for many applications.
There are in situ and ex situ variants of the process, as well a 'double core' method that combines both.