Superconducting wire is wire made of superconductors. When cooled below its transition temperature, it has zero electrical resistance. Most commonly, conventional superconductors such as niobium-titanium are used, but high-Tc 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 helium or liquid nitrogen), 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 of SC wires/tapes/conductors
The construction and operating temperature will typically be chosen to maximise:
- critical temperature Tc, below this temperature the wire becomes a superconductor
- critical current density Jc, maximum super-current a superconducting wire can carry per unit cross sectional area (see images below for examples with 20 kA/cm2).
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.
The normal wire drawing process can be used for malleable alloys such as niobium-titanium.
Vanadium-gallium (V3) 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.
The powder-in-tube (PIT, or oxide powder in tube, OPIT) process is 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.
Coated superconductor tape or wire
The coated superconductor tapes are known as second generation superconductor wires. These wires are in a form of a metal tape of about 10 mm width and about 100 micrometer thickness, coated with superconductor materials such as YBCO. A few years after the discovery of High-temperature superconductivity materials such as the YBCO, it was demonstrated that epitaxial YBCO thin films grown on lattice matched single crystals such as magnesium oxide MgO, strontium titanate (SrTiO3) and sapphire had high supercritical current densities of 1–4 MA/cm2. However, a lattice-matched flexible material was needed for producing a long tape. YBCO films deposited directly on metal substrate materials exhibit poor superconducting properties. It was demonstrated that a c-axis oriented yttria-stabilized zirconia (YSZ) intermediate layer on a metal substrate can yield YBCO films of higher quality, which had still one to two orders less critical current density than that produced on the single crystal substrates.
The biaxial YSZ film acted as a lattice matched buffer layer for the epitaxial growth of the YBCO films on it. These YBCO films achieved critical current density of more than 1 MA/cm2. Other buffer layers such as cerium oxide (CeO2 and magnesium oxide (MgO) were produced using the IBAD technique for the superconductor films.
Smooth substrates with roughness in the order of 1 nm are essential for the high quality superconductor films. Initially hastelloy substrates were electro polished to create a smoothed surface. Hastelloy is a nickel based alloy capable of withstanding temperatures up to 800C without melting or heavily oxidizing. Currently a coating technique known as “spin on glass” or “ solution deposition planarization” is used to smooth the substrate surface.
CVD is used for YBCO coated tapes.
HPCVD can be used for thin-film magnesium diboride. (Bulk MgB2 can be made by PIT or reactive Mg liquid infiltration.)
- Niobium-titanium – easier to handle
- Niobium-tin – difficult to handle, higher critical field
- High-temperature superconductivity
- Residual-resistivity ratio
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