REBCO, BSCCO, MgB₂: A Guide to Superconductor Chemistries
Part 2 of our Superconductor 101 series. New to superconductors? Start with Part 1: What is a high-temperature superconductor?
Look down the Superconductor Chemistry column of our project tracker and you’ll see a small cast of recurring names: REBCO, BSCCO-2223, MgB₂. Each is a different material with its own critical temperature, manufacturing story, price tag and sweet spot. This article introduces the whole family, including the older low-temperature superconductors that still dominate the market, so those abbreviations mean something.
First, the baseline: low-temperature superconductors
Before high-temperature superconductors existed, two workhorse materials built the entire superconducting industry, and both are still made by the tonne.
Niobium-titanium (NbTi), with a critical temperature of about 9 K, is a ductile metal alloy that can be drawn into fine filaments and cabled just like ordinary wire. It is cheap, mechanically forgiving, and utterly proven: nearly every hospital MRI magnet and the main magnets of CERN’s Large Hadron Collider are NbTi. Its limitation is the deep cold it demands, meaning liquid helium at 4.2 K or below.
Niobium-tin (Nb₃Sn), critical temperature about 18 K, handles much stronger magnetic fields than NbTi but is brittle, so magnets are wound first and the superconductor is formed afterwards by heat treatment. It powers the highest-field accelerator and research magnets of the pre-HTS era and the main magnets of the ITER fusion project.
These two are called low-temperature superconductors (LTS). Everything below is about the materials that broke the helium barrier.
BSCCO: the first practical HTS wire
Bismuth strontium calcium copper oxide (“BSCCO”, pronounced bisko) was discovered in 1988, two years into the copper-oxide gold rush. The variant used in power engineering, Bi-2223 (the numbers count atoms in the crystal recipe), superconducts up to about 110 K, comfortably above liquid nitrogen’s 77 K.
BSCCO earned its place in history as the first HTS material anyone could manufacture as long wires, using a comparatively conventional trick: pack the ceramic powder into silver tubes, then draw and roll them into flat tape (the powder-in-tube process, covered in Part 4). Because it came first, it is called first-generation (1G) HTS wire.
The pioneering projects in the tracker are BSCCO projects: the world’s first industrial HTS cable at Southwire in Georgia (2000), the first transmission-voltage grid cable at LIPA on Long Island (2008), and Germany’s landmark AmpaCity cable in Essen (2014), which ran for seven years in a city-centre grid.
BSCCO has two significant drawbacks. The tape is roughly two-thirds silver by volume, which puts a precious-metal floor under its price, and its performance collapses quickly in magnetic fields at 77 K. It remains in production and in service, but for new projects it has largely ceded the stage to REBCO.
REBCO: the modern workhorse
REBCO stands for rare-earth barium copper oxide. The “RE” is a placeholder for a rare-earth element such as yttrium (giving YBCO, the original 1987 breakthrough material) or gadolinium (GdBCO). Different rare earths, same family, so the industry name covers them all; tracker entries listing YBCO or GdBCO are REBCO projects.
REBCO superconducts up to about 92 K and, crucially, keeps performing in strong magnetic fields far better than BSCCO does. Its problem was always manufacturability. REBCO only carries useful current when its crystal grains are aligned almost perfectly, which is impossible to achieve by melting or drawing. The solution, matured through the 2000s, is to grow a thin REBCO film with near-perfect alignment on top of a long, flexible metal ribbon. The result is second-generation (2G) HTS tape, also called coated conductor: a steel-backed tape thinner than a credit card, in which a ceramic layer a few thousandths of a millimetre thick does all the work. (Part 4 walks through how it’s made.)
REBCO’s tolerance of magnetic fields unlocked a second market beyond cables: high-field magnets. Run at 20 K instead of 77 K, REBCO carries enormous currents in fields no LTS material can survive, which is exactly what compact fusion needs. The SPARC tokamak’s 20-tesla magnets in Massachusetts are wound from REBCO tape, and fusion has become the largest single consumer of the material.
In the tracker you’ll find REBCO across the modern era: Shanghai’s 1.2 km commercial grid cable, Shenzhen’s city-centre cable, Chicago’s grid-resilience link, Munich’s planned 15 km SuperLink, VEIR’s transmission pilots, the EcoSwing wind generator and SPARC’s magnets.
MgB₂: the cheap middle ground
In January 2001, Japanese researchers announced that magnesium diboride (MgB₂), a simple and long-known compound of two abundant elements, superconducts at 39 K. That is far below the cuprates, but the material is startlingly cheap and, like BSCCO, can be made into wire by the powder-in-tube method without any silver.
MgB₂ can’t use liquid nitrogen, but it doesn’t need liquid helium either. It operates around 10–25 K, a range served by modern cryocoolers or by cold helium gas (much simpler to handle than liquid helium). Where the operating temperature can be engineered to suit the conductor, MgB₂’s economics shine. Its flagship deployment is at CERN, where flexible MgB₂ cables carrying up to 120,000 amps will feed the High-Luminosity LHC magnets; over 1,000 km of MgB₂ wire was produced for the project. It also appears in the EU’s SCARLET project, which pairs MgB₂ cables with liquid hydrogen cooling, a concept that could one day move electricity and fuel in the same pipe.
The rest of the family
Two other groups are worth knowing by name. Iron-based superconductors, discovered in 2008, superconduct up to about 55 K and tolerate high magnetic fields; they are a serious research field, with wire development furthest advanced in China, but have not yet reached commercial projects. The headline-grabbing hydride superconductors (claims of near-room-temperature superconductivity) require pressures of around a million atmospheres, so for now they are a fascinating physics story with no engineering relevance. If either family matures into real projects, they will appear in the tracker.
The materials at a glance
| Material | Critical temp. | Typical operating temp. | Form | Relative cost | Typical uses |
|---|---|---|---|---|---|
| NbTi | ~9 K | 1.9–4.2 K (liquid helium) | Ductile alloy wire | Low | MRI, accelerators |
| Nb₃Sn | ~18 K | 4.2 K (liquid helium) | Brittle wire (react after winding) | Medium | High-field magnets, ITER |
| MgB₂ | 39 K | 10–25 K (helium gas / cryocooler) | Powder-in-tube wire | Low | Accelerator links, MRI, cable concepts |
| BSCCO (Bi-2223) | ~110 K | 65–77 K (liquid nitrogen) | 1G silver-matrix tape | High (silver) | Early cables, current leads |
| REBCO (YBCO, GdBCO…) | ~92 K | 20–77 K depending on job | 2G coated-conductor tape | High (falling) | Modern cables, fusion, wind, high-field magnets |
(Temperatures are approximate. The operating point always sits well below the critical temperature to leave headroom; see Part 1 on critical limits.)
Reading the tracker’s Chemistry column
A few practical decoding rules for the project table:
- REBCO / YBCO / GdBCO / “2G” all mean modern coated-conductor tape, the default choice for new projects since the mid-2010s.
- BSCCO-2223 / “1G” / DI-BSCCO (Sumitomo’s brand) marks first-generation silver-matrix tape, dominant in projects commissioned before roughly 2015.
- MgB₂ signals a design that traded liquid-nitrogen temperatures for a cheaper conductor. Look at the Cooling System column and you’ll usually see helium gas or cryocoolers alongside it.
- An empty chemistry cell means the developer hasn’t disclosed it, which is common for early-stage projects.
The chemistry choice ripples through everything else in the row. It dictates the cooling system, influences the cable architecture, and shapes the project economics. The next two parts follow that thread: how a cable is engineered around the conductor, and how the conductor itself is manufactured.