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Tungsten

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This article is about the chemical element. For other uses, see Tungsten (disambiguation).
Tungsten,  74W
Wolfram evaporated crystals and 1cm3 cube.jpg
General properties
Pronunciation/ˈtʌŋstən/ ​(TUNG-stən)
Alternative namewolfram, pronounced: /ˈwʊlfrəm/ (WUUL-frəm)
Appearancegrayish white, lustrous
Standard atomic weight (Ar, standard)183.84(1)[1]
Tungsten in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Mo
↑
W
↓
Sg
tantalum ← tungsten → rhenium
Atomic number (Z)74
Groupgroup 6
Periodperiod 6
Blockd-block
Element category  transition metal
Electron configuration[Xe] 4f14 5d4 6s2[2]
Electrons per shell
2, 8, 18, 32, 12, 2
Physical properties
Phase at STPsolid
Melting point3695 K ​(3422 °C, ​6192 °F)
Boiling point6203 K ​(5930 °C, ​10706 °F)
Density (near r.t.)19.3 g/cm3
when liquid (at m.p.)17.6 g/cm3
Heat of fusion52.31 kJ/mol[3][4]
Heat of vaporization774 kJ/mol
Molar heat capacity24.27 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 3477 3773 4137 4579 5127 5823
Atomic properties
Oxidation states−4, −2, −1, 0, +1, +2, +3, +4, +5, +6 (a mildly acidic oxide)
ElectronegativityPauling scale: 2.36
Ionization energies
  • 1st: 770 kJ/mol
  • 2nd: 1700 kJ/mol
Atomic radiusempirical: 139 pm
Covalent radius162±7 pm
Color lines in a spectral range
Spectral lines of tungsten
Other properties
Crystal structure ​body-centered cubic (bcc)
Body-centered cubic crystal structure for tungsten
Speed of sound thin rod4620 m/s (at r.t.) (annealed)
Thermal expansion4.5 µm/(m·K) (at 25 °C)
Thermal conductivity173 W/(m·K)
Electrical resistivity52.8 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic[5]
Magnetic susceptibility+59.0·10−6 cm3/mol (298 K)[6]
Young's modulus411 GPa
Shear modulus161 GPa
Bulk modulus310 GPa
Poisson ratio0.28
Mohs hardness7.5
Vickers hardness3430–4600 MPa
Brinell hardness2000–4000 MPa
CAS Number7440-33-7
History
DiscoveryCarl Wilhelm Scheele (1781)
First isolationJuan José Elhuyar and Fausto Elhuyar (1783)
Named byTorbern Bergman (1781)
Main isotopes of tungsten
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
180W 0.12% 1.8×1018 y α 176Hf
181W syn 121.2 d ε 181Ta
182W 26.50% stable
183W 14.31% stable
184W 30.64% stable
185W syn 75.1 d β− 185Re
186W 28.43% stable
| references

Tungsten, or wolfram,[7][8] is a chemical element with symbol W and atomic number 74. The name tungsten comes from the former Swedish name for the tungstate mineral scheelite, tung sten or "heavy stone".[9] Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds rather than alone. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include wolframite and scheelite.

The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, melting at 3422 °C (6192 °F, 3695 K). It also has the highest boiling point, at 5930 °C (10706 °F, 6203 K).[10] Its density is 19.3 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead.[11] Polycrystalline tungsten is an intrinsically brittle[12][13] and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw.[14]

Tungsten's many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in gas tungsten arc welding, superalloys, and radiation shielding. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are also often used as industrial catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules that are found in a few species of bacteria and archaea. It is the heaviest element known to be essential to any living organism.[15] Tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to animal life.[16][17]

Characteristics[edit]

Physical properties[edit]

In its raw form, tungsten is a hard steel-grey metal that is often brittle and hard to work. If made very pure, tungsten retains its hardness (which exceeds that of many steels), and becomes malleable enough that it can be worked easily.[14] It is worked by forging, drawing, or extruding. Tungsten objects are also commonly formed by sintering.

Of all metals in pure form, tungsten has the highest melting point (3422 °C, 6192 °F), lowest vapor pressure (at temperatures above 1650 °C, 3000 °F), and the highest tensile strength.[18] Although carbon remains solid at higher temperatures than tungsten, carbon sublimes at atmospheric pressure instead of melting, so it has no melting point. Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons.[19] Alloying small quantities of tungsten with steel greatly increases its toughness.[11]

Tungsten exists in two major crystalline forms: α and β. The former has a body-centered cubic structure and is the more stable form. The structure of the β phase is called A15 cubic; it is metastable, but can coexist with the α phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar habit. The α phase has one third of the electrical resistivity[20] and a much lower superconducting transition temperature TC relative to the β phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermediate TC values.[21][22] The TC value can also be raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc).[23] Such tungsten alloys are sometimes used in low-temperature superconducting circuits.[24][25][26]

Isotopes[edit]

Main article: Isotopes of tungsten

Naturally occurring tungsten consists of five isotopes whose half-lives are so long that they can be considered stable. Theoretically, all five can decay into isotopes of element 72 (hafnium) by alpha emission, but only 180W has been observed to do so with a half-life of (1.8±0.2)×1018 years;[27][28] on average, this yields about two alpha decays of 180W per gram of natural tungsten per year.[29] The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be at least 4 × 1021 years.

Another 30 artificial radioisotopes of tungsten have been characterized, the most stable of which are 181W with a half-life of 121.2 days, 185W with a half-life of 75.1 days, 188W with a half-life of 69.4 days, 178W with a half-life of 21.6 days, and 187W with a half-life of 23.72 h.[29] All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes.[29] Tungsten also has 4 meta states, the most stable being 179mW (t1/2 6.4 minutes).

Chemical properties[edit]

Elemental tungsten resists attack by oxygen, acids, and alkalis.[30]

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6.[30][31] Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions, WO2−
4.

Tungsten carbides (W2C and WC) are produced by heating powdered tungsten with carbon. W2C is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride (WCl6).[11]

In aqueous solution, tungstate gives the heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, W
7O6–
24, which over time converts to the less soluble "paratungstate B" anion, H
2W
12O10–
42.[32] Further acidification produces the very soluble metatungstate anion, H
2W
12O6–
40, after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide variety of heteropoly acids, such as phosphotungstic acid H3PW12O40.

Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze.

History[edit]

In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time named tungsten).[33] Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[34] In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, at the Royal Basque Society in the town of Bergara, Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, and they are credited with the discovery of the element.[35][36][37]

The strategic value of tungsten came to notice in the early 20th century. British authorities acted in 1912 to free the Carrock mine from the German owned Cumbrian Mining Company and, during World War I, restrict German access elsewhere.[38] In World War II, tungsten played a more significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore at Panasqueira. Tungsten's desirable properties such as resistance to high temperatures, its hardness and density, and its strengthening of alloys made it an important raw material for the arms industry,[39][40] both as a constituent of weapons and equipment and employed in production itself, e.g., in tungsten carbide cutting tools for machining steel.

Etymology[edit]

The name "tungsten" (from the Swedish tung sten, "heavy stone") is used in English, French, and many other languages as the name of the element, but not in the Nordic countries. "Tungsten" was the old Swedish name for the mineral scheelite. "Wolfram" (or "volfram") is used in most European (especially Germanic and Slavic) languages and is derived from the mineral wolframite, which is the origin of the chemical symbol W.[14] The name "wolframite" is derived from German "wolf rahm" ("wolf soot" or "wolf cream"), the name given to tungsten by Johan Gottschalk Wallerius in 1747. This, in turn, derives from "lupi spuma", the name Georg Agricola used for the element in 1546, which translates into English as "wolf's froth" and is a reference to the large amounts of tin consumed by the mineral during its extraction.[41]

Occurrence[edit]

Wolframite mineral, with a scale in cm.

Tungsten is found mainly in the minerals wolframite (iron–manganese tungstate (Fe,Mn)WO4, which is a solid solution of the two minerals ferberite FeWO4, and hübnerite MnWO4) and scheelite (calcium tungstate (CaWO4). Other tungsten minerals range in their level of abundance from moderate to very rare, and have almost no economical value.

Chemical compounds[edit]

See also: Category:Tungsten compounds.

Tungsten forms chemical compounds in oxidation states from -II to VI. Higher oxidation states, always as oxides, are relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with metal clusters, and very low oxidation states are typically associated with CO complexes. Mo and W chemistry shows strong similarities. The relative rarity of tungsten(III), for example, contrasts with the pervasiveness of the chromium(III) compounds. The highest oxidation state is seen in tungsten(VI) oxide (WO3).[42] The trioxide, which is volatile at high temperatures, is the precursor to virtually all other Mo compounds as well as alloys. Molybdenum has several oxidation states, the most stable being +4 and +6 (bolded in the table at left).

Tungsten(VI) oxide is soluble in aqueous base , forming tungstate (WO42−). This oxyanion condenses at lower pH values, forming polyoxotungstates.[43]

Structure of W6Cl18 ("tungsten trichloride").

The broad range of oxidation states of tungsten is reflected in it various chlorides:[42]

Organotungsten compounds are numerous and also span a range of oxidation states. Notable examples include the trigonal prismatic W(CH3)6 and octahedral W(CO)6.

Production[edit]

Tungsten mined in 2012

About 61,300 tonnes of tungsten concentrates were produced in the year 2009,[44] and in 2010, world production of tungsten was about 68,000 tonnes.[45] The main producers were as follows (data in tonnes):[46]

Major producers of tungsten[45]
Country Production (tonnes)
2009 2010 2011 2012
 China 51,000 59,000 61,800 64,000
 Russia 2,665 2,785 3,314 3,537
 Canada 1,964 420 1,966 2,194
 Bolivia 1,023 1,204 1,124 1,247
 Vietnam 725 1,150 1,635 1,050
 Portugal 823 799 819 763
 Austria 887 977 861 706
 Rwanda 380 330 520 700
 Spain 225 240 497 542
 Brazil 192 166 244 381
 Australia 33 18 15 290
 Peru 502 571 439 276
 Burundi 110 100 165 190
 Myanmar 874 163 140 140
 North Korea 100 110 110 100
 DR Congo 200 25 70 95
 Thailand 190 300 160 80
 Mongolia 39 20 13 66
 Uganda 7 44 8 21
Total 61,200 68,400 73,900 76,400
Tungsten mining in Rwanda forms an important part of the country's economy.

There is additional production in the U.S., but the amount is proprietary company information. U.S. reserves are 140,000 tonnes.[46] US industrial use of wolfram is 20,000 tonnes: 15,000 tonnes are imported and the remaining 5,000 tonnes come from domestic recycling.[47]

Tungsten is considered to be a conflict mineral due to the unethical mining practices observed in the Democratic Republic of the Congo.[48][49]

There is a large deposit of tungsten ore on the edge of Dartmoor in the United Kingdom, which was exploited during World War I and World War II as the Hemerdon Mine. With recent increases in tungsten prices, as of 2014 this mine has been reactivated.[50]

Tungsten is extracted from its ores in several stages. The ore is eventually converted to tungsten(VI) oxide (WO3), which is heated with hydrogen or carbon to produce powdered tungsten.[34] Because of tungsten's high melting point, it is not commercially feasible to cast tungsten ingots. Instead, powdered tungsten is mixed with small amounts of powdered nickel or other metals, and sintered. During the sintering process, the nickel diffuses into the tungsten, producing an alloy.

Tungsten can also be extracted by hydrogen reduction of WF6:

WF6 + 3 H2 → W + 6 HF

or pyrolytic decomposition:[51]

WF6 → W + 3 F2 (ΔHr = +)

Tungsten is not traded as a futures contract and cannot be tracked on exchanges like the London Metal Exchange. The prices are usually quoted for tungsten concentrate or WO3. If converted to the metal equivalent, they were about US$19 per kilogram in 2009.[44]

Applications[edit]

Close-up of a tungsten filament inside a halogen lamp
Tungsten carbide ring (jewelry)
1 kilogram tungsten cylinder (scale shown below)

Approximately half of the tungsten is consumed for the production of hard materials – namely tungsten carbide – with the remaining major use being in alloys and steels. Less than 10% is used in other chemical compounds.[52]

Hard materials[edit]

Tungsten is mainly used in the production of hard materials based on tungsten carbide, one of the hardest carbides, with a melting point of 2770 °C. WC is an efficient electrical conductor, but W2C is less so. WC is used to make wear-resistant abrasives, and "carbide" cutting tools such as knives, drills, circular saws, milling and turning tools used by the metalworking, woodworking, mining, petroleum and construction industries.[11] Carbide tooling is actually a ceramic/metal composite, where metallic cobalt acts as a binding (matrix) material to hold the WC particles in place. This type of industrial use accounts for about 60% of current tungsten consumption.[53]

The jewelry industry makes rings of sintered tungsten carbide, tungsten carbide/metal composites, and also metallic tungsten.[54] WC/metal composite rings use nickel as the metal matrix in place of cobalt because it takes a higher luster when polished. Sometimes manufacturers or retailers refer to tungsten carbide as a metal, but it is a ceramic.[55] Because of tungsten carbide's hardness, rings made of this material are extremely abrasion resistant, and will hold a burnished finish longer than rings made of metallic tungsten. Tungsten carbide rings are brittle, however, and may crack under a sharp blow.[56]

Alloys[edit]

Further information: Tantalum-tungsten alloys

The hardness and density of tungsten are applied in obtaining heavy metal alloys. A good example is high speed steel, which can contain as much as 18% tungsten.[57] Tungsten's high melting point makes tungsten a good material for applications like rocket nozzles, for example in the UGM-27 Polaris submarine-launched ballistic missile.[58] Tungsten alloys are used in a wide range of different applications, including the aerospace and automotive industries and radiation shielding.[59] Superalloys containing tungsten, such as Hastelloy and Stellite, are used in turbine blades and wear-resistant parts and coatings.

Quenched (martensitic) tungsten steel (approx. 5.5% to 7.0% W with 0.5% to 0.7% C) was used for making hard permanent magnets, due to its high remanence and coercivity, as noted by John Hopkinson (1849 - 1898) as early as 1886. The magnetic properties of a metal or an alloy are very sensitive to microstructure. For example, while the element tungsten is not ferromagnetic (but iron is), when present in steel in these proportions, it stabilizes the martensite phase, which has an enhanced ferromagnetism, as compared to the ferrite (iron) phase, due to its greater resistance to magnetic domain wall motion.

Tungsten's heat resistance makes it useful in arc welding applications when combined with another highly-conductive metal such as silver or copper. The silver or copper provides the necessary conductivity and the tungsten allows the welding rod to withstand the high-temperatures of the arc welding environment.

Mallory metal is proprietary name for an alloy of tungsten, with other metallic elements added to improve machining.[60]

Armaments[edit]

Tungsten, usually alloyed with nickel and iron or cobalt to form heavy alloys, is used in kinetic energy penetrators as an alternative to depleted uranium, in applications where uranium's radioactivity is problematic even in depleted form, or where uranium's additional pyrophoric properties are not desired (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, grenades and missiles, to create supersonic shrapnel. Germany used tungsten during World War II to produce shells for anti-tank gun designs using the Gerlich squeeze bore principle to achieve very high muzzle velocity and enhanced armor penetration from comparatively small caliber and light weight field artillery. The weapons were highly effective but a shortage of tungsten used in the shell core limited that effectiveness.

Tungsten has also been used in Dense Inert Metal Explosives, which use it as dense powder to reduce collateral damage while increasing the lethality of explosives within a small radius.[61]

Chemical applications[edit]

Tungsten(IV) sulfide is a high temperature lubricant and is a component of catalysts for hydrodesulfurization.[62] MoS2 is more commonly used for such applications.[63]

Tungsten oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting. Crystal tungstates are used as scintillation detectors in nuclear physics and nuclear medicine. Other salts that contain tungsten are used in the chemical and tanning industries.[18] Tungsten oxide (WO3) is incorporated into selective catalytic reduction (SCR) catalysts found in coal-fired power plants. These catalysts convert nitrogen oxides (NOx) to nitrogen (N2) and water (H2O) using ammonia (NH3). The tungsten oxide helps with the physical strength of the catalyst and extends catalyst life.[64]

Niche uses[edit]

Applications requiring its high density include weights, counterweights, ballast keels for yachts, tail ballast for commercial aircraft, and as ballast in race cars for NASCAR and Formula One; depleted uranium is also used for these purposes, due to similarly high density. Seventy-five-kg blocks of tungsten were used as "cruise balance mass devices" on the entry vehicle portion of the 2012 Mars Science Laboratory spacecraft. It is an ideal material to use as a dolly for riveting, where the mass necessary for good results can be achieved in a compact bar. High-density alloys of tungsten with nickel, copper or iron are used in high-quality darts[65] (to allow for a smaller diameter and thus tighter groupings) or for fishing lures (tungsten beads allow the fly to sink rapidly). Tungsten has seen use recently in nozzles for 3D printing; the high wear resistance and thermal conductivity of tungsten carbide improves the printing of abrasive filaments[66]. Some cello C strings are wound with tungsten. The extra density gives this string more projection and often cellists will buy just this string and use it with three strings from a different set.[67][unreliable source?] Tungsten is used as an absorber on the electron telescope on the Cosmic Ray System of the two Voyager spacecraft.[68]

Sodium tungstate is used in Folin-Ciocalteu's reagent, a mixture of different chemicals used in the "Lowry Assay" for protein content analysis.

Gold substitution[edit]

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum.[14][69] Metallic tungsten is hypoallergenic, and is harder than gold alloys (though not as hard as tungsten carbide), making it useful for rings that will resist scratching, especially in designs with a brushed finish.

Because the density is so similar to that of gold (tungsten is only 0.36% less dense), tungsten can also be used in counterfeiting of gold bars, such as by plating a tungsten bar with gold,[70][71][72] which has been observed since the 1980s,[73] or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.[74] The densities are not exactly the same, and other properties of gold and tungsten differ, but gold-plated tungsten will pass superficial tests.[70]

Gold-plated tungsten is available commercially from China (the main source of tungsten), both in jewelry and as bars.[75]

Electronics[edit]

Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,[76] such as light bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles.[14] Its high melting point also makes tungsten suitable for aerospace and high-temperature uses such as electrical, heating, and welding applications, notably in the gas tungsten arc welding process (also called tungsten inert gas (TIG) welding).

Tungsten electrode used in a gas tungsten arc welding torch

Because of its conductive properties and relative chemical inertness, tungsten is also used in electrodes, and in the emitter tips in electron-beam instruments that use field emission guns, such as electron microscopes. In electronics, tungsten is used as an interconnect material in integrated circuits, between the silicon dioxide dielectric material and the transistors. It is used in metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or molybdenum) on silicon.[51]

The electronic structure of tungsten makes it one of the main sources for X-ray targets,[77][78] and also for shielding from high-energy radiations (such as in the radiopharmaceutical industry for shielding radioactive samples of FDG). It is also used in gamma imaging as a material from which coded apertures are made, due to its excellent shielding properties. Tungsten powder is used as a filler material in plastic composites, which are used as a nontoxic substitute for lead in bullets, shot, and radiation shields. Since this element's thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals.[18] In addition to its high melting point, when tungsten is doped with potassium, it leads to an increased shape stability (compared to non-doped tungsten). This ensures that the filament does not sag, and no undesired changes occur.[79]

Nanowires[edit]

Through top-down nanofabrication processes, tungsten nanowires have been fabricated and studied since 2002.[80] Due to a particularly high surface to volume ratio, the formation of a surface oxide layer and the single crystal nature of such material, the mechanical properties differ fundamentally from those of bulk tungsten.[81] Such tungsten nanowires have potential applications in nanoelectronics and importantly as pH probes and gas sensors.[82] In similarity to silicon nanowires, tungsten nanowires are frequently produced from a bulk tungsten precursor followed by a thermal oxidation step to control morphology in terms of length and aspect ratio.[83] Using the Deal–Grove model it is possible to predict the oxidation kinetics of nanowires fabricated through such thermal oxidation processing.[84]

Biological role[edit]

Tungsten, at atomic number Z = 74, is the heaviest element known to be biologically functional. It is used by some bacteria and archaea,[85] but not in eukaryotes. For example, enzymes called oxidoreductases use tungsten similarly to molybdenum by using it in a tungsten-pterin complex with molybdopterin (molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms). Tungsten-using enzymes typically reduce carboxylic acids to aldehydes.[86] The tungsten oxidoreductases may also catalyse oxidations. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes.[87] One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin.[88] Acetylene hydratase is an unusual metalloenzyme in that it catalyzes a hydration reaction. Two reaction mechanisms have been proposed, in one of which there is a direct interaction between the tungsten atom and the C≡C triple bond.[89] Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.[90]

In soil, tungsten metal oxidizes to the tungstate anion. It can be selectively or non-selectively imported by some prokaryotic organisms and may substitute for molybdate in certain enzymes. Its effect on the action of these enzymes is in some cases inhibitory and in others positive.[91] The soil's chemistry determines how the tungsten polymerizes; alkaline soils cause monomeric tungstates; acidic soils cause polymeric tungstates.[92]

Sodium tungstate and lead have been studied for their effect on earthworms. Lead was found to be lethal at low levels and sodium tungstate was much less toxic, but the tungstate completely inhibited their reproductive ability.[93]

Tungsten has been studied as a biological copper metabolic antagonist, in a role similar to the action of molybdenum. It has been found that tetrathiotungstates may be used as biological copper chelation chemicals, similar to the tetrathiomolybdates.[94]

In archaea[edit]

Tungsten is essential for some archaea. The following tungsten-utilizing enzymes are known:

A wtp system is known to selectively transport tungsten in archaea:

Health factors[edit]

Because tungsten is rare[clarification needed] and its compounds are generally inert, the effects of tungsten on the environment are limited.[96] It was at first believed to be relatively inert and an only slightly toxic metal, but beginning in the year 2000, the risk presented by tungsten alloys, its dusts and particulates to induce cancer and several other adverse effects in animals as well as humans has been highlighted from in vitro and in vivo experiments.[97][98] The median lethal dose LD50 depends strongly on the animal and the method of administration and varies between 59 mg/kg (intravenous, rabbits)[99][100] and 5000 mg/kg (tungsten metal powder, intraperitoneal, rats).[101][102]

People can be exposed to tungsten in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 5 mg/m3 over an 8-hour workday and a short term limit of 10 mg/m3.[103]

Patent claim[edit]

Tungsten is unique amongst the elements in that it has been the subject of patent proceedings. In 1928, a US court rejected General Electric's attempt to patent it, overturning U.S. Patent 1,082,933 granted in 1913 to William D. Coolidge.[104][105][106]

See also[edit]

References[edit]

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External links[edit]

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