What is Superconductivity?
Superconductivity is a phenomenon where some materials exhibit no electrical resistance below certain cryogenic temperatures.
Superconducting wire is made of superconducting materials which, when cooled below its transition temperature, has zero electrical resistance (see the image to the right). Often the superconductor is in filament form or on a flat metal substrate encapsulated in a copper or aluminum matrix that carries the current should the superconductor quench (rise above critical temperature) for any reason.
High Temperature Superconducting (HTS) Wire
There are two well-recognized types of high temperature superconducting wire: BSCCO, known as first generation (1G) wire, and ReBCO, known as second generation (2G) wire. ReBCO stands for “Rare earth - Barium - Copper Oxide” for the superconducting compound. BSCCO stands for “Bismuth - Strontium - Calcium - Copper - Oxygen." Each of these processes has been refined over 20 years time and each type of coated conductor has trade-offs. The driving element that classifies each is operating temperature. Most importantly, by significantly reducing the overall system operating temperature HTS device manufacturers can realize power output increases in the magnitude of 10X.
First Generation (1G) HTS Wire
First Generation (1G) HTS Wire has been commercially available since 1990. During 1G HTS wire manufacturing, the powdered superconductive material fills silver alloy pipes. These are subsequently processed into a multi-filament HTS wire by means of a special rolling technology. The most commonly used materials in early HTS were bismuth-based, specifically Bismuth-Strontium-Calcium-Copper-Oxide (BSCCO - pronounced biss-co). These materials have been used to construct a variety of HTS power devices including transmission cable, transformers, fault current controllers, motors and generators
Though 1G HTS wire operated at higher temperatures and addressed the problem of costly cryogenics, the heavy reliance on silver as a raw material made the wire far too expensive to commercially produce. Recent improvements in 1G HTS wire performance has begun to shift this economic fault.
Second Generation (2G) HTS Wire
A majority of superconducting wire manufacturers are migrating to new Second Generation (2G) HTS materials utilizing Rare Earth, Barium-Copper-Oxide (ReBCO) compounds. 2G HTS materials are recognized as a superior superconductor by offering better performance in a magnetic field and improved mechanical properties - all at lower cost.
HTS wire manufactured with 2G HTS technology now surpasses 1G wire in electrical performance but at higher cost. Few Rare Earth compounds are recognized as 2G HTS materials options. The industry currently uses a varying of Rare Earth compounds (Yttrium, Samarium, Neodymium, Gadolinium) with Barium-Copper-Oxide (ReBCO) as the choice materials for HTS wire and HTS devices. Extensive 2G HTS wire technology R&D, pilot production and manufacturing scaling efforts are underway.
2G HTS wire offers additional benefits with its unique properties:
Medium Temperature Superconducting (MTS) Wire
HTS wire types using a Magnesium di-Boride (MgB) based process are usually produced by reaction of fine Magnesium and Boron powders, thoroughly mixed together and heated at a temperature around or above the melting point of pure Magnesium (> 600 °C).
MgB2 wires and tapes are therefore realized by means of the so-called Powder-In-Tube method (PIT). Thanks to the higher operating temperatures, MgB2 systems can be cooled by modern cryocooling devices. The main competing advantages for MgB2 based HTS wire manufacturing are low cost raw materials and relatively simple deposition techniques.
In contrast, MgB2’s low critical temperature (Tc) of 30 Kelvin is limited to applications that operate at lower temperatures (20 K). Low cost continues to be the main driver for MgB2 wire manufacturers.
However, because of its relatively simple PIT deposition approach, many believe that MgB2 may in the near term better serve applications like electronics in the form of flexible flat ribbon cables and superconducting cavities for RF applications.
MgB2 is a superconducting wire alternative operating at 20K; a temperature between LTS (4K) and HTS (65K)
Low Temperature Superconducting (LTS) Wire
Low Temperature Superconducting (LTS) technology, which operates at liquid helium temperatures (4 Kelvin), was discovered in 1911. This technology became commercially successful in the 1960’s when wire was made from LTS materials for use in superconducting electromagnets. LTS electromagnets create fields that are much stronger than conventional copper based electromagnets.
Notably, these state-of the-art LTS electromagnets enabled new technologies like Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR). LTS superconducting wire is manufactured with Niobium Titanium (Nb-Ti) or Niobium Tin (Nb3Sn) using a powder-in-tube process, embedded in a non-superconducting matrix, such as a silver alloy, somewhat similar to the way traditional wire of copper or aluminum is made. Though LTS wire can be manufactured at costs competitive with copper, LTS devices are very expensive due to the high cost of cryogenic cooling and their reliance on silver. As a result, LTS technology remains quite limited to niche and specialized applications (e.g. Hadron Collider).
In 1987, materials were discovered that exhibited superconducting properties at temperatures as high as 90 K. This class of materials was called High Temperature Superconductors or HTS. While this is still very cold, it was a significant breakthrough. These materials could now be cooled by liquid nitrogen which is much easier to work with, more readily available without supply issues and, most importantly, considerably cheaper than liquid helium.
This drastic cost reduction in cryogenic systems cost opened new opportunities for superconducting applications. HTS communication devices, Maglev transportation, superconducting power cable and superconducting motors and generators were now economically possible. As with LTS devices, many HTS devices used superconducting wire as a base technology.
Features of LTS:
Quick Fact - ReBCO materials are recognized as a superior superconductor by offering better performance in a magnetic field.
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