NIOBIUM

(Nb), chemical element, refractory metal of Group Vb of the periodic table, used in alloys, tools and dies, and superconductive magnets. Niobium is closely associated with tantalum in ores and in properties. Niobium was first discovered (1801) in a New England mineral by the English chemist Charles Hatchett, who called the element columbium; it was rediscovered and named (1844) by the German chemist Heinrich Rose. International agreement among chemists (about 1950) finally established the name niobium, though columbium persisted in the U.S. metallurgical industry. Niobium, more plentiful than lead and less abundant than copper in the Earth's crust, occurs dispersed except for a relatively few minerals, of which columbite and pyrochlore are the principal commercial sources. Natural niobium occurs entirely as the stable isotope niobium-93. Separation from tantalum, when necessary, is effected by solvent extraction; the niobium is then precipitated and roasted to niobium pentoxide, which is reduced to niobium powder through metallothermic and hydriding processes. The powder is consolidated and purified further by electron-beam melting. Vacuum sintering of powder is also used for consolidation. The pure metal is soft and ductile; it looks like steel or, when polished, like platinum. Although it has excellent corrosion resistance, niobium is susceptible to oxidation above about 400 C (750 F). Completely miscible with iron, it is added in the form of ferroniobium to some stainless steels to give stability on welding or heating. Niobium is used as a major alloying element in nickel-based superalloys and as a minor but important additive to high-strength, low-alloy steels. Because of its compatibility with uranium, resistance to corrosion by molten alkali-metal coolants, and low thermal-neutron cross section, it has been used alone or alloyed with zirconium in claddings for nuclear reactor cores. Cemented carbides used as hot-pressing dies and cutting tools are made harder and more resistant to shock and erosion by the presence of niobium. Niobium is useful in constructing cryogenic (low temperature) electronic devices of low power consumption. Niobium-tin (Nb3Sn) is a superconductor below 18.45 Kelvins (K), and niobium metal itself, below 9.15 K. Compounds of niobium are of relatively minor importance. Those found in nature are pentavalent, but compounds of lower valences (24) have been prepared. Tetravalent niobium, for example, in the form of the carbide, NbC, is used for making cemented carbides. atomic number41 atomic weight92.906 melting point2,468 C (4,474 F) boiling point4,927 C (8,901 F) specific gravity8.57 (20 C) valence2, 3, 4, 5 electronic config.2-8-18-12-1 or (Kr)4d45s1