Chemical element Niobium
Информация - Иностранные языки
Другие материалы по предмету Иностранные языки
on states from +5 down to -1, its most stable state is +5.
Niobium is able to form oxides with the oxidation states +5 (Nb2O5), +4 (NbO2) and +3 (Nb2O3), as well as with the rarer oxidation state +2 (NbO). The most stable oxidation state is +5, the pentoxide which, along with the dark green non-stoichiometric dioxide, is the most common of the oxides. Niobium pentoxide is used mainly in the production of capacitors, optical glass, and as starting material for several niobium compounds. The compounds are created by dissolving the pentoxide in basic hydroxide solutions or by melting it in another metal oxide. Examples are lithium niobate (LiNbO3) and lanthanum niobate (LnNbO4). In the lithium niobate, the niobate ion NbO3? is not alone but part of a perovskite-like structure, while the lanthanum niobate contains lone ions. Lithium niobate, which is a ferroelectric, is used extensively in mobile telephones and optical modulators, and for the manufacture of surface acoustic wave devices. It belongs to the ABO3 structure ferroelectrics like lithium tantalate and barium titanate.forms halogen compounds in the oxidation states of +5, +4, and +3 of the type NbX5, NbX4, and NbX3, although multi-core complexes and substoichiometric compounds are also formed. Niobium pentafluoride (NbF5) is a white solid with a melting point of 79.0 C and niobium pentachloride (NbCl5) is a yellowish-white solid (see image at left) with a melting point of 203.4 C. Both are hydrolyzed by water and react with additional niobium at elevated temperatures by forming the black and highly hygroscopic niobium tetrafluoride (NbF4) and niobium tetrachloride (NbCl4). While the trihalogen compounds can be obtained by reduction of the pentahalogens with hydrogen, the dihalogen compounds do not exist. Spectroscopically, the monochloride (NbCl) has been observed at high temperatures. The fluorides of niobium can be used after its separation from tantalum. The niobium pentachloride is used in organic chemistry as a Lewis acid in activating alkenes for the carbonyl-ene reaction and the Diels-Alder reaction. The pentachloride is also used to generate the organometallic compound niobocene dichloride ((C5H5)2NbCl2), which in turn is used as a starting material for other organoniobium compounds.binary compounds of niobium include niobium nitride (NbN), which becomes a superconductor at low temperatures and is used in detectors for infrared light, and niobium carbide, an extremely hard, refractory, ceramic material, commercially used in tool bits for cutting tools. The compounds niobium-germanium (Nb3Ge) and niobium-tin (Nb3Sn), as well as the niobium-titanium alloy, are used as a type II superconductor wire for superconducting magnets. Niobium sulfide as well as a few interstitial compounds of niobium with silicon are also known.
to estimates, niobium is 33rd on the list of the most common elements in the Earths crust with 20 ppm. The abundance on Earth should be much greater, but the missing niobium may be located in the Earths core due to the metal's high density. The free element is not found in nature, but it does occur in minerals. Minerals that contain niobium often also contain tantalum, for example, columbite ((Fe,Mn)(Nb,Ta)2O6), columbite-tantalite (or coltan, (Fe,Mn)(Ta,Nb)2O6) and pyrochlore ((Na,Ca)2Nb2O6(OH,F)). Columbite-tantalite minerals 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 such as pyrochlore and euxenite ((Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6). These large deposits of niobium have been found associated with carbonatites (carbonate-silicate igneous rocks) and as a constituent of pyrochlore.two largest deposits of pyrochlore were found in the 1950s in Brazil and Canada, and both countries are still the major producers of niobium mineral concentrates. The largest deposit is hosted within a carbonatite intrusion at Arax, Minas Gerais Brazil, owned by CBMM (Companhia Brasileira de Metalurgia e Minerao); the other deposit is located at Catalo, Gois owned by Anglo American plc (through its subsidiary Minerao Catalo), also hosted within a carbonatite intrusion. Altogether these two Brazilian mines produce around 75% of world supply. The third largest producer of niobium is the carbonatite-hosted Niobec Mine, Saint-Honor near Chicoutimi, Quebec owned by Iamgold Corporation Ltd, which produces around 7% of world supply.though unexploited resources are located in Nigeria, Democratic Republic of Congo, Malawi, Australia and in Russia.
the separation from the other minerals, the mixed oxides of tantalum Ta2O5 and niobium Nb2O5 are obtained. The first step in the processing is the reaction of the oxides with hydrofluoric acid:
O5 + 14HF > 2H2[TaF7] + 5H2O, andO5 + 10HF > 2H2[NbOF5] + 3H2O
first industrial scale separation, developed by de Marignac, used the difference in solubility between the complex niobium and tantalum fluorides, dipotassium oxypentafluoroniobate monohydrate (K2[NbOF5]H2O) and dipotassium heptafluorotantalate (K2[TaF7]) 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:
[NbOF5] + 2KF > K2[NbOF5]v + 2HF, then
H2[NbOF5] + 10NH4OH > Nb2O5v + 10NH4F + 7H2O
methods are used for the reduction to metallic niobium. The electrolysis of a molten mixture of K2[NbOF5] and sodium chloride is one; the other is the reduction of the fluoride with sodium. With this method niobium with a relatively high purity can be obtained. In large scale production the reduction of Nb2O5 with hydrogen or carbon is used. In the process involving the aluminothermic reaction a mixture of iron oxide and niobium oxide is reacted with aluminum:
Nb2O5 + Fe2O3 + 12Al > 6Nb + 2Fe + 6Al2O3
enhance the reaction, small amounts of oxidizers like sodium nitrate are added. The result is aluminum oxide and ferroniobium, an alloy of iron and niobium used in the steel production. The ferroniobium contains between 60 and 70% of niobium. Without addition of iron oxide, aluminothermic process is used for the production of 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.United States Geological Survey estimates that the production increased from 38,700 metric tonnes in 2005 to 44,500 tonnes in 2006. The world wide resources are estimated to be 4,400,000 tonnes. During the ten year period between 1995 and 2005, the production more than doubled starting from 17,800 tonnes in 1995.
Applications
is estimated that out of 44,500 metric tons of niobium mined in 2006, 90% ended up in the production of high-grade structural steel, followed by its use in superalloys. The use of niobium alloys for superconductors and in electronic components account only for a small share of the production.is an effective microalloying element for steel. Adding niobium to the steel causes the formation of niobium carbide and niobium nitride within the structure of the steel. These compounds improve the grain refining, retardation of recrystallization, and precipitation hardening of the steel. These effects in turn increase the toughness, strength, formability, and weldability of the microalloyed steel. Microalloyed stainless steels have a niobium content of less than 0.1%. It is an important alloy addition to high strength low alloy steels which are widely used as structural components in modern automobiles. These niobium containing alloys are strong and are often used in pipeline construction.amounts of the element, either in its pure form or in the form of high-purity ferroniobium and nickel niobium, are used in nickel-, cobalt-, and iron-base superalloys for such applications as jet engine components, gas turbines, rocket subassemblies, and heat resisting and combustion equipment. Niobium precipitates a hardening ?''-phase within the grain structure of the superalloy. The alloys contain up to 6.5% niobium. One example of a nickel-based niobium-containing superalloy is Inconel 718, which consists of roughly 50% nickel, 18.6% chromium, 18.5% iron, 5% niobium, 3.1% molybdenum, 0.9% titanium, and 0.4% aluminum. These superalloys are used, for example, in advanced air frame systems such as those used in the Gemini program.alloy used for liquid rocket thruster nozzles, such as in the main engine of the Apollo Lunar Modules, is C103, which consists of 89% niobium, 10% hafnium and 1% titanium. Another niobium alloy was used for the nozzle of the Apollo Service Module. As niobium is oxidized at temperatures above 400 C, a protective coating is necessary for these applications to prevent the alloy from becoming brittle.becomes a superconductor when lowered to cryogenic temperatures. At atmospheric pressure, it has the highest critical temperature of the elemental superconductors: 9.2 K. Niobium has the largest magnetic penetration depth of any element. In addition, it is one of the three elemental Type II superconductors, along with vanadium and technetium. Niobium-tin and niobium-titanium alloys are used as wires for superconducting magnets capable of producing exceedingly strong magnetic fields. These superconducting magnets are used in magnetic resonance imaging and nuclear magnetic resonance instruments as well as in particle accelerators. For example, the Large