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Nb, an element in Group V of the Mendeleev periodic system. Atomic number, 41; atomic weight, 92.9064. A metal with a steel-gray color. It has one natural isotope, 93Nb.
Niobium was discovered in 1801 by the English scientist C. Hatchett (1765–1847) in a mineral found in Connecticut, which he called columbium (from Columbia, the former synonym for America). In 1844 the German chemist H. Rose (1795–1864) discovered a “new” element, which he called niobium in honor of Niobe, the daughter of Tantalus, thus indicating the similarity between niobium and tantalum. Niobium was subsequently shown to be the same as columbium.
Occurrence in nature. The average niobium content of the earth’s crust is 2 × 10-3 percent by mass. Only in alkaline igneous rocks, such as nepheline syenites, does the niobium content reach 10-2–10-1 percent. Twenty-three niobium minerals and about 130 other minerals containing increased quantities of niobium have been discovered in these rocks and the related pegmatites and carbonatites, as well as in granitic pegmatites. The minerals are usually complex and simple oxides. In minerals, niobium is associated with the rare earths and with tantalum, titanium, calcium, sodium, thorium, iron, and barium (tantaloniobates, titanates, and so on). Pyrochlore and columbite are the most important of the six industrial minerals. Commercial deposits of niobium are associated with alkaline rock bodies (for example, on the Kola Peninsula) and their weathering mantles, and also with granitic pegmatites. Alluvial tantaloniobate deposits are also significant.
The geochemistry of niobium in the biosphere has been little studied. It has only been established that, in regions of niobium-enriched alkaline rock, niobium migrates in the form of compounds with organic and other complexes. Niobium minerals are known that form upon erosion of alkaline rock (murmanite and gerasimovskite). The niobium content in seawater is only about 1 × 10-9 weight.
The annual world production of niobium in the 1960’s was about 1,300 tons, which, when compared to its Clarke numbers, indicates its low consumption (less than most metals).
Physical and chemical properties. The crystal lattice of niobium is of the body-centered cubic type, with constant a = 3.294 angstroms. Density, 8.57 g/cm3 (20°C); melting point, 2500°C; boiling point, 4927°C; vapor pressures (in millimeters of mercury; 1 mm Hg = 133.3 newtons per sq m), 1 × 10-5 (2194°C), 1 × 10-4 (2355°C), 6 × 10-4 (at the melting point), and 1 × 10-3 (2539°C). Thermal conductivity at 0° and 600°C (in watts per [m · °K]), 56.2, respectively, or 0.125 and 0.156 cal/(cm · sec · °C). Volume resistivity at 0°C, 15.22 × 10-8 ohm · m (Ω · m), or 15.22 × 10-6 Ω · cm. Niobium passes into the superconducting state at 9.25°K. Electron work function, 4.01 electron volts.
Pure niobium readily undergoes cold pressure working and maintains satisfactory mechanical properties at high temperatures. Ultimate strength at 20° and 800°C, 342 and 312 mega-newtons per sq m (MN/m2), or 34.2 and 31.2 kilograms-force per sq mm (kgf/mm2), respectively. Relative elongation at 20° and 800°C, 19.2 and 20.7 percent, respectively. Brinell hardness of pure niobium, 450 MN/m2; of industrial niobium, 750–1,800 MN/m2. Impurities of a number of elements, particularly hydrogen, nitrogen, carbon, and oxygen, sharply reduce the ductility and increase the hardness of niobium.
Niobium is similar to tantalum in its chemical properties. In the cold and upon moderate heating, both elements are exceedingly resistant (tantalum more so than niobium) to the action of many corrosive media. Compact niobium oxidizes noticeably in air only above 200°C. Chlorine acts on niobium above 200°C. Liquid sodium and potassium (and their alloys), lithium, bismuth, lead, mercury, and tin from which oxygen has been removed virtually do not act on niobium; they are used as liquid-metal heat-transfer agents in atomic reactors.
Niobium is resistant to the action of many acids and salt solutions. It is impervious to aqua regia and hydrochloric and sulfuric acids at 20°C, and also to nitric, phosphoric, and perchloric acids and aqueous solutions of ammonia. Niobium is dissolved by hydrofluoric acid and its mixtures with nitric acid, and also by alkalies. In acid electrolytes, niobium becomes covered by an anodic oxide film that has good dielectric characteristics, making possible the use of niobium and its alloys with tantalum to replace expensive pure tantalum for the production of miniature high-capacitance electrolytic capacitors with low leakage currents.
The outer electron configuration of niobium atoms is 4d45s1. Pentavalent niobium compounds are the most stable, although compounds with oxidation states of +4, +3, +2, and +1 are also known. Niobium has a greater tendency to form such compounds than does tantalum. For example, in the niobium-oxygen system, the phases found are niobium pentoxide, Nb2O5 (a white compound; melting point, 1512°C); the nonstoichiometric NbO2.47 and NbO2.42; niobium dioxide, NbO2 (a black compound; melting point, 2080°C); niobium oxide, NbO (a gray compound; melting point, 1935°C); and a solid solution of oxygen in niobium. Niobium dioxide, NbO2, is a semiconductor. When niobium oxide, NbO, is smelted into ingots, it has a metallic luster and electrical conductivity of a metallic nature. It vaporizes noticeably at 1700°C and vigorously at 2300°–2350°C. This property is used for vacuum elimination of oxygen from niobium. Niobium pentoxide, Nb2O5, is acidic. Niobic acids are not isolated as definite chemical compounds, but their salts (niobates) are known.
Niobium forms an interstitial solid solution with hydrogen (up to 10 atomic percent hydrogen) and a hydride of composition from NbH0.7 to NbH. The solubility of hydrogen in niobium (in g/cm3) is 104 at 20°C, 74.4 at 500°C, and 4.0 at 900°C. The absorption of hydrogen is reversible; hydrogen is released upon heating, particularly under vacuum. This behavior is used to rid niobium of hydrogen, which makes it brittle, and to hydrogenate compact niobium. The brittle hydride is ground and dehy-drogenated under vacuum to yield pure niobium powder for electrolytic capacitors. The solubility of nitrogen in niobium is 0.005, 0.04, and 0.07 percent by weight at 300°, 1000°, and 1500°C, respectively. Nitrogen is eliminated from niobium under high vacuum above 1900°C or by vacuum melting. The higher nitride NbN is light gray, with a yellowish tint; it enters the superconducting state at 15.6°K.
Niobium forms three phases with carbon at 1800°–2000°C: the α-phase, which is an interstitial solid solution of carbon in niobium containing up to 2 atomic percent carbon at 2335°C; the β-phase, which is Nb2C; and the δ-phase, which is NbC. With halogens, niobium yields halides, oxyhalides, and complex salts. The most important and best studied of these are niobium pentafluoride, NbF5; niobium pentachloride, NbCl5; niobium oxytrichloride, NbOCl3; potassium niobium fluoride, K2NbF7; and potassium niobium oxyfluoride, K2NbOF7 · H2O. The small difference in the vapor pressures of NbCl5 and TaCl5 is used for their complete separation and purification by distillation.
Production and use. Niobium ores are usually polymetallic and poor in niobium, although reserves of niobium ore are much greater than those of tantalum ore. The ore concentrates contain Nb2CO5 (pyrochlore ores contain not less than 37 percent; loparitic ores, 8 percent; and columbitic ores, 30–60 percent). Most of the ores are converted by the aluminothermic or silicothermic process to ferroniobium (40–60 percent niobium) and ferrotan-talum-niobium.
Metallic niobium is obtained from ore concentrates by a complex technology in three steps: (1) opening of the concentrate, (2) separation of niobium and talantum and production of their pure chemical compounds, and (3) reduction and refining of metallic niobium and its alloys.
The main industrial methods for the production of niobium and niobium alloys are the aluminothermic, sodium-reduction, and carbothermic techniques. In the carbothermic technique, niobium carbide is first obtained from a mixture of Nb2O5 and carbon black at 1800°C in an atmosphere of hydrogen, and then the metal is produced from a mixture of the carbide and pentoxide at 1800°–1900°C under vacuum. To produce niobium alloys, oxides of the alloying metals are added to the mixture. In another version of the method, niobium is reduced directly from Nb2O5 by carbon black at high temperature under vacuum. In the sodium-reduction method, niobium is reduced by sodium from K2NbF7; in the aluminothermic method, it is reduced by aluminum from Nb2O5. The compact metal or alloy is obtained by methods of powder metallurgy by roasting bars of compressed powder under vacuum at 2300°C or by electron-beam and vacuum-arc smelting. High-purity single crystals of niobium are produced by crucibleless electron-beam zone refining.
The uses and production of niobium are expanding rapidly as a result of niobium’s combination of such properties as high melting point, low thermal neutron capture cross section (1.15 barn), capacity to form heat-resistant and superconducting alloys, corrosion resistance, getter properties, low electron work function, and good cold pressure workability and weldability.
The main areas of use of niobium are rocket construction, aerospace technology, radio, electronics, chemical instrument-making, and atomic power engineering. Pure niobium or its alloys are used in the production of aircraft parts, jackets for uranium and plutonium fuel elements, containers and pipes for liquid metals, parts for electrical condensers, “hot” fittings for electron tubes (for radar) and powerful transmitting tubes (anodes, cathodes, and grids), and corrosion-resistant equipment in chemical industry.
Niobium is used to alloy other nonferrous metals, including uranium. Niobium is used in cryotrons (superconducting elements in computers), and its stannide, Nb3Sn, and alloys of niobium with titanium and zirconium are used in the production of superconducting solenoids. Niobium and its alloys with tantalum replace tantalum in many cases, which produces great savings, since niobium is cheaper and almost twice as light as tantalum.
Ferroniobium is added to stainless chrome-nickel steel to prevent intercrystalline corrosion and failure and to other types of steel to improve their properties. Niobium compounds are also used; among them are niobium pentoxide, Nb2O5 (as a catalyst in the chemical industry and in the production of refractories, cermets, and special glasses); niobium nitride; niobium carbide; and niobates.
REFERENCESZelikman, A. N., and G. A. Meerson. Metallurgiia redkikh metallov. Moscow, 1973.
Niobii, tanta! i ikh splavy. Moscow, 1966. (Translated from English.)
Nediukha, I. M., and V. G. Chernyi. Niobii—metall kosmicheskoi ery. Kiev, 1965.
Niobii i tantal [collection of translated articles]. Edited by O. P. Kolchin. Moscow, 1961.
Filiand, M. A., and E. I. Semenova. Svoistva redkikh elementov [handbook], 2nd ed. Moscow, 1964.
O. P. KOLCHIN
coltan(COLumbite-TANtalite) A metallic ore that is critical in electronics manufacturing. A black-brown substance, niobium is extracted from the columbite for metal alloys such as stainless steel. Due to its superconducting properties, niobium is also used in particle accelerators and MRI scanners. Tantalum is extracted from the tantalite and is used as an insulator in capacitors. It is also used for neon light electrodes and rectifiers. See tantalum capacitor.
A major source of income for the Democratic Republic of the Congo (DRC), the country is considered to have the world's largest coltan reserves. Coltan mining revenue financed wars in the 1970s and 1990s.