Niobium

41 zirconium ← niobium → molybdenum
V
↑
Nb
↓
Ta
Nb-TableImage.png
Periodic Table - Extended Periodic Table
General
Name, Symbol, Number niobium, Nb, 41
Element category transition metals
Group, Period, Block 5, 5, d
Appearance gray metallic
Niobium crystals 1.jpg
Standard atomic weight 92.90638(2)  g·mol−1
Electron configuration [Kr] 4d4 5s1
Electrons per shell 2, 8, 18, 12, 1
Physical properties
Phase solid
Density (near r.t.) 8.57  g·cm−3
Melting point 2750 K
(2477 °C, 4491 °F)
Boiling point 5017 K
(4744 °C, 8571 °F)
Heat of fusion 30  kJ·mol−1
Heat of vaporization 689.9  kJ·mol−1
Specific heat capacity (25 °C) 24.60  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2942 3207 3524 3910 4393 5013
Atomic properties
Crystal structure cubic body centred
Oxidation states 5, 4, 3, 2,
(mildly acidic oxide)
Electronegativity 1.6 (Pauling scale)
Ionization energies
(more)		 1st:  652.1  kJ·mol−1
2nd:  1380  kJ·mol−1
3rd:  2416  kJ·mol−1
Atomic radius 145  pm
Atomic radius (calc.) 198  pm
Covalent radius 137  pm
Miscellaneous
Magnetic ordering paramagnetic
Electrical resistivity (0 °C) 152 nΩ·m
Thermal conductivity (300 K) 53.7  W·m−1·K−1
Thermal expansion (25 °C) 7.3  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3480 m/s
Young's modulus 105  GPa
Shear modulus 38  GPa
Bulk modulus 170  GPa
Poisson ratio 0.40
Mohs hardness 6.0
Vickers hardness 1320  MPa
Brinell hardness 736  MPa
CAS registry number 7440-03-1
Most-stable isotopes
Main article: Isotopes of niobium
iso NA half-life DM DE (MeV) DP
91Nb syn 6.8×102 y ε - 91Zr
91mNb syn 60.86 d IT 0.104e 91Nb
92Nb syn 10.15 d ε - 92Zr
γ 0.934 -
92Nb syn 3.47×107y ε - 92Zr
γ 0.561, 0.934 -
93Nb 100% 93Nb is stable with 52 neutrons
93mNb syn 16.13 y IT 0.031e 93Nb
94Nb syn 2.03×104 y β- 0.471 94Mo
γ 0.702, 0.871 -
95Nb syn 34.991 d β- 0.159 95Mo
γ 0.765 -
95mNb syn 3.61 d IT 0.235 95Nb
References

Niobium (pronounced /naɪˈoʊbiəm/) (Greek mythology: Niobe, daughter of Tantalus), or columbium (/kəˈlʌmbiəm/), is a chemical element with symbol Nb and atomic number 41. A rare, soft, grey, ductile transition metal, niobium is found in the minerals pyrochlore (the main source for niobium) and columbite.

Niobium has similar physical and chemical properties to another element, tantalum, and the two are therefore difficult to distinguish. The English chemist Charles Hatchett reported a new element similar to tantalum in 1801, and named it columbium. In 1809, the English chemist William Hyde Wollaston wrongly concluded that tantalum and columbium were identical. The 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. The name of the element was officially adopted as niobium in 1949.

It was not until the early 20th century that niobium was first used commercially. Brazil is the leading producer of niobium and ferroniobium, an alloy of niobium and iron. Niobium is used mostly in alloys, the largest part in special steel such as that used in gas pipelines. Although alloys contain only a maximum of 0.1%, that small percentage improves the strength of the steel. The temperature stability of niobium-containing superalloys is important for its use in jet engines and rocket engines. These superconducting alloys, also containing titanium and tin, are widely used in MRI scanners. Other applications of niobium include its use in welding, nuclear industries, electronics, optics, numismatics and jewellery. In the last two applications, niobium's low toxicity and ability to be coloured by anodisation are particular advantages.

Contents

History

Charles Hatchett, discoverer of columbium

Niobium was discovered by the English chemist Charles Hatchett in 1801.[1] He found a new element in a mineral sample that had been sent to England from Massachusetts in 1734 by a John Winthrop,[2] and named the mineral columbite and the new element columbium after Columbia, the poetical name for America.[3] The columbium discovered by Hatchett was probably a mixture of a new element with tantalum.[3]

Subsequently, there was considerable confusion[4] over the difference between columbium (niobium) and the closely-related tantalum. In 1809, the English chemist William Hyde Wollaston compared the oxides derived from both columbium—columbite, with a density 5.918 g/cm3, and tantalum—tantalite, with a density 7.935 g/cm3, and concluded that the two oxides, despite the significant difference in density, were identical; thus he kept the name tantalum.[4] This conclusion was disputed in 1846 by the 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, the goddess of tears), and pelopium (from Pelops).[5][6] This confusion arose from the minimal observed differences between tantalum and niobium. Both tantalum and niobium react with chlorine and traces of oxygen, including atmospheric concentrations, with niobium forming two compounds: the white volatile niobium pentachloride (NbCl5) and the non-volatile niobium oxychloride (NbOCl3). Scientists claimed to have discovered new elements: pelopium, ilmenium and dianium,[7] which were in fact identical to niobium or mixtures of niobium and tantalum.[8]

The differences between tantalum and niobium were unequivocally demonstrated in 1864 by Christian Wilhelm Blomstrand,[8] and Henri Etienne Sainte-Claire Deville, as well as Louis J. Troost, who determined the formulas of some of the compounds in 1865[9][8] and finally by the Swiss chemist Jean Charles Galissard de Marignac,[10] in 1866, who all proved that there were only two elements. These discoveries did not stop scientists from publishing articles about ilmenium until 1871.[11] De Marignac was the first to prepare the metal in 1864, when he reduced niobium chloride by heating it in an atmosphere of hydrogen.[12]

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 first used commercially, in incandescent lamp filaments.[9] This use quickly became obsolete through the replacement of niobium with tungsten, which has a higher melting point and thus is preferable for use in incandescent lamps. The discovery that niobium improves the strength of steel was made in the 1920s, and this remains its predominant use.[9] 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,[13] making it the first material known to support the high currents and fields necessary for making useful high-power magnets and electrically powered machinery. This discovery would allow—two decades later—the production of long multi-strand cables that could be wound into coils to create large, powerful electromagnets for rotating machinery, particle accelerators, or particle detectors.[14][15]

Columbium (symbol Cb[16]) was the name originally given to this element by Hatchett, and this name remained in use in American journals—the last paper published by American Chemical Society with columbium in its title dates from 1953[17]—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.[18] 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.[18] The latter name is still sometimes used in US industry.[19] This was a compromise of sorts;[18] the IUPAC accepted tungsten instead of wolfram, in deference to North American usage; and niobium instead of columbium, in deference to European usage. Not everyone agreed, and while many leading chemical societies and government organizations refer to it by the official IUPAC name, many leading metallurgists, metal societies, and the United States Geological Survey still refer to the metal by the original "columbium".[20][21]

Characteristics

Niobium is a lustrous, grey, ductile, paramagnetic metal in group 5 of the periodic table, that takes on a bluish tinge when exposed to air at room temperature for extended periods.[22] Despite presenting a high melting point in elemental form (2,468 °C), it has a low density in comparison to other refractory metals. Furthermore, it is corrosion resistant, exhibits superconductivity properties, and forms dielectric oxide layers. These properties—especially the superconductivity—are strongly dependent on the purity of the niobium metal.[23] When very pure, it is comparatively soft and ductile, but impurities make it harder.[24]

Niobium is slightly less electropositive and smaller than its predecessor in the periodic table, zirconium, while it is virtually identical in size to the heavier tantalum as a consequence of the lanthanide contraction.[24] As a result, niobium's chemical properties are very similar to the chemical properties of tantalum, which appears directly below niobium in the periodic table.[9] Although its corrosion resistance is not as outstanding as that of tantalum, its lower price and greater availability make niobium attractive for less exact uses such as linings in chemical plants.[24]

Isotopes

Main article: Isotopes of niobium

Naturally occurring niobium is composed of one stable isotope, 93Nb.[25] As of 2003, at least 32 radioisotopes have also been synthesized, ranging in atomic mass from 81 to 113. The most stable of these is 92Nb with a half-life of 34.7 million years. One of the least stable is 113Nb, with an estimated half-life of 30 milliseconds. Isotopes that are lighter than the stable 93Nb tend to decay by β+ decay, and those that are heavier tend to decay by β- decay, with some exceptions. 81Nb, 82Nb, and 84Nb have minor β+ delayed proton emission decay paths, 91Nb decays by electron capture and positron emission, and 92Nb decays by both β+ and β- decay.[25]

At least 25 nuclear isomers have been described, ranging in atomic mass from 84 to 104. Within this range, only 96Nb, 101Nb, and 103Nb do not have isomers. The most stable of niobium's isomers is 93mNb with a half-life of 16.13 years. The least stable isotope is 84mNb with a half-life of 103 ns. All of niobium's isotopes decay by isomeric transition or beta decay except 92m1Nb, which has a minor electron capture decay path.[25]

Chemistry

See also: Category:Niobium compounds

Niobium, which belongs to group 5 in the periodic table, is in many ways similar to its predecessors in group 4. It reacts with most nonmetals at high temperatures: fluorine reacts with niobium at room temeprature, with chlorine and hydrogen at 200 °C, and with nitrogen at 400 °C, giving products that are frequently interstitial and nonstoichiometric.[24] The metal begins to oxidize in air at 200 °C,[26] and is resistant to corrosion by fused alkalis and by acids, including to aqua regia, chlorhydric, sulfuric, nitric and phosphoric acids.[24] Niobium is attacked by hot, concentrated mineral acids, such as fluorhydric acid and fluorhydric/nitric acid mixtures. Although niobium exhibits all the formal oxidation states from +5 down to -1, its most stable state is +5.[24]

Niobium is able to form oxides with the oxidation states +5 (Nb2O5), +4 (NbO2) and +3 (Nb2O3),[26] as well as with the rarer oxidation state +2 (NbO).[27] 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.[26] Niobium pentoxide is used mainly in the production of capacitors, optical glass, and as starting material for several niobium compounds.[28] 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 lanthan niobate (LnNbO4). In the lithium niobate, the niobate ion NbO3− is not alone but part of a perovskite-like structure, while the lantane niobate contains lone NbO43− ions.[26] 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.[29]

Niobium pentachloride

Niobium 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.[26][30] Niobium pentafluoride (NbF5) is a white solid with a melting point of 79.0 °C and niobium pentachloride (NbCl5) is a white solid 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.[26] Spectroscopically, the monochloride (NbCl) has been observed at high temperatures.[31] The fluorides of niobium can be used after its separation from tantalum.[32] 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.[33] 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.[34]

Other binary compounds of niobium include niobium nitride (NbN), which becomes a superconductor at low temperatures and is used in detectors for infrared light,[35] 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.[36][37] The niobium sulfide as well as a few interstitial compounds of niobium with silicon are also known.[24]

Occurrence

See also: Category:Niobium minerals

According to estimates, niobium is 33rd on the list of the most common elements in the Earth’s crust with 20 ppm.[38] The abundance on Earth should be much greater, but the “missing” niobium may be located in the Earth’s core due to the metal's high density.[20] The free element is not found in nature, but it does occur in minerals.[24] 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).[32] Less common, although they form the largest mined niobium deposits, 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 (carbon-silicate igneous rocks) and as a constituent of pyrochlore.[39]

The 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.[9] The largest deposits in Brazil are owned by CBMM (Companhia Brasileira de Metalurgia e Mineração) located in Araxá; Minas Gerais, the other deposit, is owned by Mineração Catalão located in Catalão, Goiás.[40] The third largest producer of niobium is the Niobec Inc. mine in Saint-Honoré near Chicoutimi, Quebec.[40] Yet not mined extensive ore reserves are located in Nigeria, Democratic Republic of Congo, and in Russia.

Production

Niobium producers in 2007

After 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:[32]

Ta2O5 + 14HF → 2H2[TaF7] + 5H2O, and
Nb2O5 + 10HF → 2H2[NbOF5] + 3H2O

The 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.[32] 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:[26]

H2[NbOF5] + 2KF → K2[NbOF5]↓ + 2HF, then
2H2[NbOF5] + 10NH4OH → Nb2O5↓ + 10NH4F + 3H2O

Several 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,[26] is used. In the process involving the aluminothermic reaction a mixture of iron oxide and niobium oxide is reacted with aluminium:

3Nb2O5 + Fe2O3 + 12Al → 6Nb + 2Fe + 6Al2O3

To enhance the reaction, small amounts of oxidizers like sodium nitrate are added. The result is aluminium oxide and ferroniobium, an alloy of iron and niobium used in the steel production.[41][42] The ferroniobium contains between 60 and 70% of niobium.[40] 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.[30][43]

The United States Geological Survey estimates that the production to increased from 38,700 metric tonnes in 2005 to 44,500 tonnes in 2006.[44][45] The world wide resources are estimated to be 4,400,000 tonnes.[45] In the ten years between 1995 and 2005 the production more than doubled starting from 17,800 tonnes in 1995.[46]

Applications

A niobium foil

It is estimated that of the 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.[47] The use of niobium alloys for superconductors and in electronic components account only for a small share of the production.[47]

Steel production

Niobium 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.[20] 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.[20] Microalloyed stainless steels have a niobium content of less than 0.1%.[48] It is an important alloy addition to high strength low alloy steels which are widely used as structural components in modern automobiles.[20] These niobium containing alloys are strong and are often used in pipeline construction.[49][50]

Apollo CSM with the dark rocket nozzle made from niobium titanium alloy
A 3 tesla clinical Magnetic resonance imaging scanner using niobium-superconducting alloy

Superalloys

Appreciable 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 γ''-phase within the grain structure of the superalloy.[51] The alloys contain up to 6.5% niobium.[48] One example of a nickel-based niobium-containing superalloy is inconel 718, which consists of 18.6% chromium, 18.5% iron, 5% niobium, 3.1% molybdenum, 0.9% titanium, and 0.4% aluminium.[52][53] These superalloys are used, for example, in advanced air frame systems such as those used in the Gemini program.

An alloy used for liquid rocket thruster nozzles, such as in the main engine of the Apollo Lunar Modules, is C130, which consists of 89% niobium, 10% hafnium and 1% titanium.[54] 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.[54]

Superconducting magnets

Niobium becomes a superconductor when lowered to cryogenic temperatures. At atmospheric pressure, it has the highest critical temperature of the elemental superconductors: 9.2 K.[40] Niobium has the largest magnetic penetration depth of any element.[40] 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 for particle accelerators.[55] For example, the Large Hadron Collider uses 600 metric tons of superconducting strands, while the International Thermonuclear Experimental Reactor is estimated to use 600 metric tonnes of Nb3Sn strands and 250 metric tonnes of NbTi strands.[56] In 1992 alone, niobium-titanium wires were used to construct more than 1 billion US dollars worth of clinical magnetic resonance imaging systems.[14]

Numismatics

Niobium is used as a precious metal in commemorative coins, often with silver or gold. For example, Austria produced a series of silver niobium coins starting in 2003; the colour in these coins is created by diffraction of light by a thin oxide layer produced by anodising. In 2008, six coins are available showing a broad variety of colours in the centre of the coin: blue, green, brown, purple, violet, or yellow. Two more examples are the 2004 Austrian 25 euro 150 Years Semmering Alpine Railway commemorative coin,[57] and the 2006 Austrian 25 euro European Satellite Navigation commemorative coin.[58] Latvia produced a similar series of coins starting 2004,[59] with one following in 2007.[60]

Other uses

Niobium and some niobium alloys are used in medical devices such as pacemakers, because they are physiologically inert (and thus hypoallergenic).[61] Niobium treated with sodium hydroxide forms a porous layer that aids osseointegration.[62] Along with titanium, tantalum, and aluminium, niobium can also be electrically heated and anodized, resulting in a wide array of colours using a process known as reactive metal anodizing which is useful in making jewelry.[63][64] The fact that niobium is hypoallergenic also benefits the use in jewelry.[65]

The electrodes in some high pressure sodium vapor lamps are made from niobium, or niobium with 1% of zirconium, because niobium is resistant to the corrosive sodium vapors within the lamp.[66][67] The metal is also used in arc welding rods for some stabilized grades of stainless steel.[68]

Niobium was evaluated as a cheaper alternative to tantalum in capacitors,[69] but tantalum capacitors are still predominant. Niobium is added to glass in order to attain a higher refractive index, a property of use to the optical industry in making thinner corrective glasses. The metal has a low capture cross-section for thermal neutrons;[70] thus it is used in the nuclear industries.[71]

The Superconducting Radio Frequency (RF) cavities used in the free electron lasers TESLA and XFEL are made from pure niobium.[72]

The high sensitivity of superconducting niobium nitride bolometers make them an ideal detector for electromagnetic radiation in the THz frequency band. These detectors were tested at the Heinrich Hertz Submillimeter Telescope, the South Pole Telescope, the Receiver Lab Telescope, and at APEX and are now used in the HIFI instrument on board the Herschel Space Observatory. [73]

Precautions

Niobium has no known biological role. While niobium dust is an eye and skin irritant and a potential fire hazard, elemental niobium on a larger scale is physiologically inert (and thus hypoallergenic) and harmless. It is frequently used in jewelry and has been tested for use in some medical implants.[74][75]

Niobium-containing compounds are rarely encountered by most people, but some are toxic and should be treated with care. The short and long term exposure to niobates and niobium chloride, two chemicals that are water soluble, have been tested in rats. Rats treated with a single injection of niobium pentachloride or niobates show a median lethal dose (LD50) between 10 and 100 mg/kg.[76][77][78] For oral administration the toxicity is lower; a study with rats yielded a LD50 after seven days of 940 mg/kg.[76]

References

  1. ↑ Hatchett, Charles (1802). "Eigenschaften und chemisches Verhalten des von Charlesw Hatchett entdeckten neuen Metalls, Columbium" (in German). Annalen der Physik 11 (5): 120–122. doi:10.1002/andp.18020110507. 
  2. ↑ Griffith, William P.; Morris, Peter J. T. (2003). "Charles Hatchett FRS (1765-1847), Chemist and Discoverer of Niobium". Notes and Records of the Royal Society of London 57 (3): 299. doi:10.1098/rsnr.2003.0216. http://www.jstor.org/stable/3557720. 
  3. ↑ 3.0 3.1 Noyes, William Albert (1918). A Textbook of Chemistry. H. Holt & Co.. pp. 523. http://books.google.com/books?id=UupHAAAAIAAJ&pg=PA523&lpg=PA523&dq=columbium+discovered+by+Hatchett+was+a+mixture+of+two+elements&source=web&ots=fYAZdQEGtb&sig=awPTuKyi-Id4L06ZIu0ryIZOzw0&hl=en&sa=X&oi=book_result&resnum=7&ct=result. 
  4. ↑ 4.0 4.1 Wollaston, William Hyde (1809). "On the Identity of Columbium and Tantalum". Philosophical Transactions of the Royal Society of London 99: 246–252. doi:10.1098/rstl.1809.0017. http://www.jstor.org/stable/107264. 
  5. ↑ Rose, Heinrich (1844). "Ueber die Zusammensetzung der Tantalite und ein im Tantalite von Baiern enthaltenes neues Metall" (in German). Annalen der Physik 139 (10): 317–341. doi:10.1002/andp.18441391006. http://gallica.bnf.fr/ark:/12148/bpt6k15148n/f327.table. 
  6. ↑ Rose, Heinrich (1847). "Ueber die Säure im Columbit von Nordamérika" (in German). Annalen der Physik 146 (4): 572–577. doi:10.1002/andp.18471460410. http://gallica.bnf.fr/ark:/12148/bpt6k15155x/f586.table. 
  7. ↑ Kobell, V. (1860). "Ueber eine eigenthümliche Säure, Diansäure, in der Gruppe der Tantal- und Niob- verbindungen". Journal für Praktische Chemie 79 (1): 291–303. doi:10.1002/prac.18600790145. 
  8. ↑ 8.0 8.1 8.2 Marignac, Blomstrand, H. Deville, L. Troost und R. Hermann (1866). "Tantalsäure, Niobsäure, (Ilmensäure) und Titansäure". Fresenius' Journal of Analytical Chemistry 5 (1): 384–389. doi:10.1007/BF01302537. 
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Gupta, C. K.; Suri, A. K. (1994). Extractive Metallurgy of Niobium. CRC Press. pp. 1–16. ISBN 0849360714. 
  10. ↑ Marignac, M. C. (1866). "Recherches sur les combinaisons du niobium" (in French). Annales de chimie et de physique 4 (8): 7–75. http://gallica.bnf.fr/ark:/12148/bpt6k34818t/f4.table. 
  11. ↑ Hermann, R. (1871). "Fortgesetzte Untersuchungen über die Verbindungen von Ilmenium und Niobium, sowie über die Zusammensetzung der Niobmineralien (Further research about the compounds of ilmenium and niobium, as well as the composition of niobium minerals)" (in German). Journal für Praktische Chemie 3 (1): 373–427. doi:10.1002/prac.18710030137. 
  12. ↑ "Niobium". Universidade de Coimbra. Retrieved on 2008-09-05.
  13. ↑ Geballe et al. (1993) gives a critical point at currents of 150 kiloamperes and magnetic fields of 8.8 tesla.
  14. ↑ 14.0 14.1 Geballe, Theodore H. (October 1993). "Superconductivity: From Physics to Technology" (PDF). Physics Today 46 (10): 52–56. doi:10.1063/1.881384. http://www.physicstoday.org/vol-56/iss-10/pdf/vol46no10p52-56.pdf. 
  15. ↑ Matthias, B. T.; Geballe, T. H.; Geller, S.; Corenzwit, E. (1954). "Superconductivity of Nb3Sn". Physical Review 95: 1435–1435. doi:10.1103/PhysRev.95.1435. 
  16. ↑ Kòrösy, F. (1939). "Reaction of Tantalum, Columbium and Vanadium with Iodine". Journal of the American Chemical Society 61 (4): 838–843. doi:10.1021/ja01873a018. 
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