columbium

Niobium is a chemical element; it has symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has similar ductility to iron. Niobium oxidizes in Earth's atmosphere very slowly, hence its application in jewelry as a hypoallergenic alternative to nickel. Niobium is often found in the minerals pyrochlore and columbite. Its name comes from Greek mythology: Niobe, daughter of Tantalus, the namesake of tantalum. The name reflects the great similarity between the two elements in their physical and chemical properties, which makes them difficult to distinguish.

English chemist Charles Hatchett reported a new element similar to tantalum in 1801 and named it columbium. In 1809, English chemist William Hyde Wollaston wrongly concluded that tantalum and columbium were identical. German chemist Heinrich Rose determined in 1846 that tantalum ores contain a second element, which he named niobium. In 1864 and 1865, a series of scientific findings clarified that niobium and columbium were the same element (as distinguished from tantalum), and for a century both names were used interchangeably. Niobium was officially adopted as the name of the element in 1949, but the name columbium remains in current use in metallurgy in the United States.

It was not until the early 20th century that niobium was first used commercially. Niobium is an important addition to high-strength low-alloy steels. Brazil is the leading producer of niobium and ferroniobium, an alloy of 60–70% niobium with iron. Niobium is used mostly in alloys, the largest part in special steel such as that used in gas pipelines. Although these alloys contain a maximum of 0.1%, the small percentage of niobium enhances the strength of the steel by scavenging carbide and nitride. The temperature stability of niobium-containing superalloys is important for its use in jet and rocket engines.

Niobium is used in various superconducting materials. These alloys, also containing titanium and tin, are widely used in the superconducting magnets of MRI scanners. Other applications of niobium include welding, nuclear industries, electronics, optics, numismatics, and jewelry. In the last two applications, the low toxicity and iridescence produced by anodization are highly desired properties.

History

Niobium was identified by English chemist Charles Hatchett in 1801. He found a new element in a mineral sample that had been sent to England from Connecticut, United States in 1734 by John Winthrop FRS (grandson of John Winthrop the Younger) and named the mineral "columbite"" and the new element "columbium" after '' Columbia'', the poetic name for the United States. The ''columbium'' discovered by Hatchett was probably a mixture of the new element with tantalum.

Subsequently, there was considerable confusion over the difference between columbium (niobium) and the closely related tantalum. In 1809, English chemist William Hyde Wollaston compared the oxides derived from both columbium—columbite, with a density 5.918 g/cm, and tantalum— tantalite, with a density over 8 g/cm, and concluded that the two oxides, despite the significant difference in density, were identical; thus he kept the name tantalum. This conclusion was disputed in 1846 by German chemist Heinrich Rose, who argued that there were two different elements in the tantalite sample, and named them after children of Tantalus: ''niobium'' (from Niobe) and '' pelopium'' (from Pelops). This confusion arose from the minimal observed differences between tantalum and niobium. The claimed new elements ''pelopium'', '' ilmenium'', and ''dianium'' were in fact identical to niobium or mixtures of niobium and tantalum.

The differences between tantalum and niobium were unequivocally demonstrated in 1864 by Christian Wilhelm Blomstrand and Henri Étienne Sainte-Claire Deville, as well as Louis J. Troost, who determined the formulas of some of the compounds in 1865 and finally by Swiss chemist Jean Charles Galissard de Marignac in 1866, who all proved that there were only two elements. Articles on ''ilmenium'' continued to appear until 1871.

Christian Wilhelm Blomstrand was the first to prepare the metal in 1866, when he reduced niobium chloride by heating it in an atmosphere of hydrogen. Although de Marignac was able to produce tantalum-free niobium on a larger scale by 1866, it was not until the early 20th century that niobium was used in incandescent lamp filaments, the first commercial application. This use quickly became obsolete through the replacement of niobium with tungsten, which has a higher melting point. That niobium improves the strength of steel was first discovered in the 1920s, and this application remains its predominant use. In 1961, the American physicist Eugene Kunzler and coworkers at Bell Labs discovered that niobium–tin continues to exhibit superconductivity in the presence of strong electric currents and magnetic fields, making it the first material to support the high currents and fields necessary for useful high-power magnets and electrical power machinery. This discovery enabled—two decades later—the production of long multi-strand cables wound into coils to create large, powerful electromagnets for rotating machinery, particle accelerators, and particle detectors.

Naming the element

''Columbium'' (symbol Cb) was the name originally given by Hatchett upon his discovery of the metal in 1801. The name reflected that the type specimen of the ore came from the United States of America ( Columbia). This name remained in use in American journals—the last paper published by American Chemical Society with ''columbium'' in its title dates from 1953—while ''niobium'' was used in Europe. To end this confusion, the name ''niobium'' was chosen for element 41 at the 15th Conference of the Union of Chemistry in Amsterdam in 1949. A year later this name was officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) after 100 years of controversy, despite the chronological precedence of the name ''columbium''. This was a compromise of sorts; the IUPAC accepted tungsten instead of wolfram in deference to North American usage; and ''niobium'' instead of ''columbium'' in deference to European usage. While many US chemical societies and government organizations typically use the official IUPAC name, some metallurgists and metal societies still use the original American name, "''columbium''.

Characteristics

Physical

Niobium is a lustrous, grey, ductile, paramagnetic metal in group 5 of the periodic table (see table), with an electron configuration in the outermost shells atypical for group 5. Similarly atypical configurations occur in the neighborhood of ruthenium (44) and rhodium (45).

Although it is thought to have a body-centered cubic crystal structure from absolute zero to its melting point, high-resolution measurements of the thermal expansion along the three crystallographic axes reveal anisotropies which are inconsistent with a cubic structure. Therefore, further research and discovery in this area is expected.

Niobium becomes a superconductor at cryogenic temperatures. At atmospheric pressure, it has the highest critical temperature of the elemental superconductors at 9.2  K. Niobium has the greatest magnetic penetration depth of any element. In addition, it is one of the three elemental Type II superconductors, along with vanadium and technetium. The superconductive properties are strongly dependent on the purity of the niobium metal.

When very pure, it is comparatively soft and ductile, but impurities make it harder.

The metal has a low capture cross-section for thermal neutrons; thus it is used in the nuclear industries where neutron transparent structures are desired.

Chemical

The metal takes on a bluish tinge when exposed to air at room temperature for extended periods. Despite a high melting point in elemental form (2,468 °C), it is less dense than other refractory metals. Furthermore, it is corrosion-resistant, exhibits superconductivity properties, and forms dielectric oxide layers.

Niobium is slightly less electropositive and more compact than its predecessor in the periodic table, zirconium, whereas it is virtually identical in size to the heavier tantalum atoms, as a result of the lanthanide contraction. As a result, niobium's chemical properties are very similar to those for tantalum, which appears directly below niobium in the periodic table. Although its corrosion resistance is not as outstanding as that of tantalum, the lower price and greater availability make niobium attractive for less demanding applications, such as vat linings in chemical plants.

Isotopes

Almost all of the niobium in Earth's crust is the one stable isotope, Nb. By 2003, at least 32 radioisotopes had been synthesized, ranging in atomic mass from 81 to 113. The most stable is Nb with half-life 34.7 million years. Nb, along with Nb, has been detected in refined samples of terrestrial niobium and may originate from bombardment by cosmic ray muons in Earth's crust. One of the least stable niobium isotopes is ¹¹³Nb; estimated half-life 30 milliseconds. Isotopes lighter than the stable Nb tend to β decay, and those that are heavier tend to β decay, with some exceptions. Nb, Nb, and Nb have minor β-delayed proton emission decay paths, Nb decays by electron capture and positron emission, and Nb decays by both β and β decay.

At least 25 nuclear isomers have been described, ranging in atomic mass from 84 to 104. Within this range, only Nb, Nb, and Nb do not have isomers. The most stable of niobium's isomers is Nb with half-life 16.13 years. The least stable isomer is Nb with a half-life of 103 ns. All of niobium's isomers decay by isomeric transition or beta decay except Nb, which has a minor electron capture branch.

Occurrence

Niobium is estimated to be the 33rd most abundant element in the Earth's crust, at 20  ppm. Some believe that the abundance on Earth is much greater, and that the element's high density has concentrated it in Earth's core. The free element is not found in nature, but niobium occurs in combination with other elements in minerals. Minerals that contain niobium often also contain tantalum. Examples include columbite () and columbite–tantalite (or ''coltan'', ). Columbite–tantalite minerals (the most common species being columbite-(Fe) and tantalite-(Fe), where "-(Fe)" is the Levinson suffix indicating the prevalence of iron over other elements such as manganese) that are most usually found as accessory minerals in pegmatite intrusions, and in alkaline intrusive rocks. Less common are the niobates of calcium, uranium, thorium and the rare earth elements. Examples of such niobates are pyrochlore () (now a group name, with a relatively common example being, e.g., fluorcalciopyrochlore) and euxenite (correctly named euxenite-(Y)) (). These large deposits of niobium have been found associated with carbonatites (carbonate-silicate igneous rocks) and as a constituent of pyrochlore.

The three largest currently mined deposits of pyrochlore, two in Brazil and one in Canada, were found in the 1950s, and are still the major producers of niobium mineral concentrates. The largest deposit is hosted within a carbonatite intrusion in Araxá, state of Minas Gerais, Brazil, owned by CBMM ( Companhia Brasileira de Metalurgia e Mineração); the other active Brazilian deposit is located near Catalão, state of Goiás, and owned by China Molybdenum, also hosted within a carbonatite intrusion. Together, those two mines produce about 88% of the world's supply. Brazil also has a large but still unexploited deposit near São Gabriel da Cachoeira, state of Amazonas, as well as a few smaller deposits, notably in the state of Roraima.

The third largest producer of niobium is the carbonatite-hosted Niobec mine, in Saint-Honoré, near Chicoutimi, Quebec, Canada, owned by Magris Resources. It produces between 7% and 10% of the world's supply.

Production

After the separation from the other minerals, the mixed oxides of tantalum and niobium are obtained. The first step in the processing is the reaction of the oxides with hydrofluoric acid:

:
:

The first industrial scale separation, developed by Swiss chemist de Marignac, exploits the differing solubilities of the complex niobium and tantalum fluorides, dipotassium oxypentafluoroniobate monohydrate () and dipotassium heptafluorotantalate () in water. Newer processes use the liquid extraction of the fluorides from aqueous solution by organic solvents like cyclohexanone. The complex niobium and tantalum fluorides are extracted separately from the organic solvent with water and either precipitated by the addition of potassium fluoride to produce a potassium fluoride complex, or precipitated with ammonia as the pentoxide:

:

Followed by:
:

Several methods are used for the reduction to metallic niobium. The electrolysis of a molten mixture of [] and sodium chloride is one; the other is the reduction of the fluoride with sodium. With this method, a relatively high purity niobium can be obtained. In large scale production, is reduced with hydrogen or carbon. In the aluminothermic reaction, a mixture of iron oxide and niobium oxide is reacted with aluminium:
:

Small amounts of oxidizers like sodium nitrate are added to enhance the reaction. The result is aluminium oxide and ferroniobium, an alloy of iron and niobium used in steel production. Ferroniobium contains between 60 and 70% niobium. Without iron oxide, the aluminothermic process is used to produce niobium. Further purification is necessary to reach the grade for superconductive alloys. Electron beam melting under vacuum is the method used by the two major distributors of niobium.

, CBMM from Brazil controlled 85 percent of the world's niobium production. The United States Geological Survey estimates that the production increased from 38,700 tonnes in 2005 to 44,500 tonnes in 2006. Worldwide resources are estimated to be 4.4 million tonnes. During the ten-year period between 1995 and 2005, the production more than doubled, starting from 17,800 tonnes in 1995. Between 2009 and 2011, production was stable at 63,000 tonnes per year, with a slight decrease in 2012 to only 50,000 tonnes per year.

Lesser amounts are found in Malawi's Kanyika Deposit ( Kanyika mine).

Compounds

In many ways, niobium is similar to tantalum and zirconium. It reacts with most nonmetals at high temperatures; with fluorine at room temperature; with chlorine at 150 °C and hydrogen at 200 ° C; and with nitrogen at 400 °C, with products that are frequently interstitial and nonstoichiometric. The metal begins to oxidize in air at 200 ° C. It resists corrosion by acids, including aqua regia, hydrochloric, sulfuric, nitric and phosphoric acids. Niobium is attacked by hot concentrated sulfuric acid, hydrofluoric acid and hydrofluoric/nitric acid mixtures. It is also attacked by hot, saturated alkali metal hydroxide solutions.

Although niobium exhibits all of the formal oxidation states from +5 to −1, the most common compounds have niobium in the +5 state. Characteristically, compounds in oxidation states less than 5+ display Nb–Nb bonding. In aqueous solutions, niobium only exhibits the +5 oxidation state. It is also readily prone to hydrolysis and is barely soluble in dilute solutions of hydrochloric, sulfuric, nitric and phosphoric acids due to the precipitation of hydrous Nb oxide. Nb(V) is also slightly soluble in alkaline media due to the formation of soluble polyoxoniobate species.

Oxides, niobates and sulfides

Niobium forms oxides in the oxidation states +5 ( ), +4 ( ), and the rarer oxidation state, +2 ( NbO). Most common is the pentoxide, precursor to almost all niobium compounds and alloys. Niobates are generated by dissolving the pentoxide in basic hydroxide solutions or by melting it in alkali metal oxides. Examples are lithium niobate () and lanthanum niobate (). In the lithium niobate is a trigonally distorted perovskite-like structure, whereas the lanthanum niobate contains lone ions. The layered niobium sulfide () is also known.

Materials can be coated with a thin film of niobium(V) oxide chemical vapor deposition or atomic layer deposition processes, produced by the thermal decomposition of niobium(V) ethoxide above 350 °C.

Halides

Niobium forms halides in the oxidation states of +5 and +4 as well as diverse substoichiometric compounds. The pentahalides () feature octahedral Nb centres. Niobium pentafluoride () is a white solid with a melting point of 79.0 °C and niobium pentachloride () is yellow (see image at right) with a melting point of 203.4 °C. Both are hydrolyzed to give oxides and oxyhalides, such as . The pentachloride is a versatile reagent used to generate the organometallic compounds, such as niobocene dichloride (). The tetrahalides () are dark-coloured polymers with Nb-Nb bonds; for example, the black hygroscopic niobium tetrafluoride () and dark violet niobium tetrachloride ().Macintyre, J.E.; Daniel, F.M.; Chapman and Hall; Stirling, V.M. Dictionary of Inorganic Compounds. 1992, Cleveland, OH: CRC Press, p. 2957

Anionic halide compounds of niobium are well known, owing in part to the Lewis acidity of the pentahalides. The most important is bF₇sup>2−, an intermediate in the separation of Nb and Ta from the ores. This heptafluoride tends to form the oxopentafluoride more readily than does the tantalum compound. Other halide complexes include octahedral []:
: + 2 Cl → 2 []

As with other metals with low atomic numbers, a variety of reduced halide cluster ions is known, the prime example being [].

Nitrides and carbides

Other binary compounds of niobium include niobium nitride (NbN), which becomes a superconductor at low temperatures and is used in detectors for infrared light. The main niobium carbide is NbC, an extremely hard, refractory, ceramic material, commercially used in cutting tool bits.

Applications

Out of 44,500 tonnes of niobium mined in 2006, an estimated 90% was used in high-grade structural steel. The second-largest application is superalloys. Niobium alloy superconductors and electronic components account for a very small share of the world production.

Steel production

Niobium is an effective microalloying element for steel, within which it forms niobium carbide and niobium nitride. These compounds improve the grain refining, and retard recrystallization and precipitation hardening. These effects in turn increase the toughness

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