Niobium (Nb)

Cool niobium below 9.2 kelvin and its electrical resistance vanishes completely. That is the highest critical temperature of any element at ordinary pressure, and it is not a marginal victory — technetium and lead, the nearest rivals, sit more than a kelvin lower, and most metals that superconduct at all do so below 5 K. The consequence is that when engineers need a superconductor that can actually be manufactured into something, they reach for niobium or an alloy of it, and they have done so for sixty years.

Niobe, daughter of Tantalus

The name is a piece of chemical wit. Tantalum had already been named for Tantalus, the king condemned to stand in water he could never drink; the choice referred to the metal's refusal to absorb acid. When Heinrich Rose showed in the 1840s that columbite minerals contained a second element inseparably mixed with tantalum, he named it after Niobe, Tantalus's daughter — a name that encodes both the family resemblance and the fact that one is found inside the other.

Rose's element was not new. Charles Hatchett had found it in 1801 in a black mineral in the British Museum's collection, part of a consignment sent from Connecticut by John Winthrop the Younger more than a century earlier, and had named it columbium for the country it came from. Anders Ekeberg found tantalum in a Finnish mineral the following year. In 1809 William Hyde Wollaston compared columbite and tantalite and declared the two elements identical, and the scientific world believed him for thirty-five years. Rose's work reopened the case, and Christian Blomstrand finally reduced the metal in 1864.

The name Americans would not give up

That history left the element with two names in active use. European chemistry said niobium; American metallurgy and the mining trade said columbium, on the reasonable grounds that Hatchett was first and priority is priority. IUPAC settled the matter in 1949 in favour of niobium, in a package deal that gave the United States tungsten's spelling preferences elsewhere.

The ruling did not take everywhere. Producers, steelmakers and ASTM standards in the United States went on writing columbium for decades afterwards; the US Geological Survey used it in official commodity reports into the 2000s. Ferroniobium is still sold under both names, and the symbol Nb has to be explained to a fair number of American engineers.

Cavities polished to a mirror

The most demanding use of niobium is also the purest. A superconducting radio-frequency cavity is a hollow niobium shell, shaped like a string of beads, cooled with superfluid helium, into which microwave power is fed to accelerate a particle beam. Because the walls have no resistance, the electromagnetic field is sustained with almost no power loss, and a machine can run continuously rather than in short pulses.

Everything depends on the surface. The current flows in a layer a few tens of nanometres deep, so a single inclusion or a residue of the chemical polish will quench the cavity. Manufacturing them means high-purity niobium sheet, electron-beam welding, electropolishing and baking, all held to a standard closer to semiconductor practice than to metalwork. The European XFEL in Hamburg, LCLS-II at SLAC and the designs for future linear colliders are all built from thousands of these shells.

Alloys carry the bulk market. Niobium–titanium wire is ductile, cheap enough to buy by the tonne, and superconducting to about 9.8 K: it is what the magnets in every MRI scanner and the main dipoles of the Large Hadron Collider are wound from. For higher fields, the brittle intermetallic niobium–tin is formed in place after winding, and it is the choice for ITER's magnets and for the highest-field research magnets built anywhere.

A few grams in a tonne of steel

None of this accounts for where niobium actually goes. Roughly nine-tenths of world production is converted to ferroniobium and stirred into steel, at concentrations that sound too small to matter — often between 0.02% and 0.1%.

The mechanism is grain refinement. Niobium forms carbides and nitrides that precipitate on the grain boundaries during hot rolling and physically obstruct them from growing. A finer grain structure makes steel simultaneously stronger and tougher, which is unusual, since most strengthening mechanisms trade one for the other. The resulting high-strength low-alloy steels are what large-diameter gas pipelines are made from, and they are why a modern car body can be lighter than its predecessor without being weaker. A car contains a few tens of grams of niobium and is measurably better for it.

Niobium is also the precipitate-former in Inconel 718, the nickel superalloy that makes up a large share of the mass of a modern jet engine. And lithium niobate, a synthetic crystal with strong electro-optic and piezoelectric responses, is the standard material for the modulators that impress data onto light in fibre-optic networks and for surface acoustic wave filters in radio front ends.

One mine, most of the world

Niobium supply is the most concentrated of any major industrial metal. The pyrochlore deposit at Araxá in Minas Gerais, Brazil, worked by CBMM, supplies the large majority of world demand on its own, with a second Brazilian operation at Catalão and the Niobec mine in Quebec providing most of the remainder.

The deposit is a weathered carbonatite — an unusual igneous rock made largely of carbonate minerals — in which niobium has been concentrated to grades far above anything found elsewhere. Reserves are measured in centuries at current consumption. This is a supply risk with an unusual shape: not scarcity, since there is plenty of niobium and it is cheap, but total dependence on one geological accident and the company that owns it.

The isotope that ran out, and the one that will not

Every niobium atom in the world is niobium-93. Mononuclidic elements are uncommon, and they make precise atomic weights easy, but niobium's interest lies in two isotopes that are not around.

Niobium-92 is the extinct one. Its half-life is near 37 million years, roughly a hundred and twentieth of the age of the solar system, so it has been through more than a hundred halvings and nothing whatever remains. What it became is zirconium-92, and the pair makes a usable clock, because a few minerals — rutile above all — take up niobium far more readily than zirconium, so a grain that crystallised while niobium-92 was still live now carries a zirconium isotope pattern displaced from the ordinary one. The displacement is small and the ages it yields are relative: this grain before that one, by a few million years, which is the scale on which the early solar system did everything that mattered.

Niobium-94 has the opposite significance. It is produced when reactor components made of niobium or niobium-bearing steel sit in a neutron flux, has a half-life of about 20,000 years, and emits penetrating gamma rays. That combination makes it one of the nuclides that determines how dismantled reactor internals must be classified and disposed of, decades after the plant has stopped generating anything.